As filed with the Securities and Exchange Commission on January 25, 2019

April 22, 2022

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORMS-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

ARSANIS,

X4 PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

Delaware 27-3181608
(State or other jurisdiction of
incorporation or organization)
 
(I.R.S. Employer
Identification Number)

890 Winter

61 North Beacon Street, Suite 230

Waltham, MA 02451

(781)4819-5704th

Floor

Boston, Massachusetts 02134
(857) 529-8300
(Address, Including Zip Code,including zip code, and Telephone Number, Including Area Code,telephone number, including area code of Registrant’s Principal Executive Offices)

Michael P. Gray

President and registrant’s principal executive offices)

Paula Ragan, Ph.D.
Chief Executive Officer Chief Financial Officer

Arsanis,

c/o X4 Pharmaceuticals, Inc.

890 Winter

61 North Beacon Street, Suite 230

Waltham, MA 02451

(781)4819-5704th

Floor

Boston, Massachusetts 02134
(857) 529-8300
(Name, Address, Including Zip Code,address, including zip code, and Telephone Number, Including Area Code,telephone number, including area code, of Agentagent for Service)

Copyservice)

Copies to:

Cynthia

Daniel I. Goldberg
Courtney T. Mazareas, Esq.

Mhairi C. Immermann, Esq.

Wilmer Cutler Pickering Hale and DorrThorne

Cooley LLP

60 State Street

Boston, MA 02109

(617)526-6000

55 Hudson Yards
New York, NY 1001-2157
(212) 479 6000
From time to time after the effective date of this Registration Statement
(Approximate date of commencement of proposed sale to the public: From time to time after this registration statement becomes effective.

public)

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.  

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.  

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.  

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.  




Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, anon-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company”company,” and “emerging growth company” inRule 12b-2 of the Exchange Act.

Large accelerated filer   Accelerated filer 
Non-accelerated filer   Smaller reporting company 
  Emerging growth company 

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  

CALCULATION OF REGISTRATION FEE

 

Title of Each Class of

Securities to be Registered

 

Amount

to be

Registered(1)

 

Proposed Maximum
Offering Price

Per Unit(1)

 

Proposed

Maximum Aggregate

Offering Price(1)

 Amount of
Registration Fee

Debt Securities

        

Common Stock, par value $0.001 per share

        

Preferred Stock, par value $0.001 per share

        

Units

        

Warrants

        

Total

     $150,000,000 $18,180.00

 

 

(1)

Pursuant to FormS-3 General Instruction II.D information is not required to be included. An indeterminate amount of the securities of each identified class is being registered as may from time to time be offered hereunder at indeterminate prices, along with an indeterminate number of securities that may be issued upon exercise, settlement, exchange or conversion of securities offered or sold hereunder as shall have an aggregate initial offering price not to exceed $150,000,000. Pursuant to Rule 416 under the Securities Act of 1933, as amended (the “Securities Act”),



The registrant hereby amends this registration statement also covers any additional securities that may be offered or issued in connection with any stock split, stock dividend or pursuant to anti-dilution provisions of any of the securities. Separate consideration may or may not be received for securities that are issuable upon conversion, exercise or exchange of other securities. In addition, the total amount to be registered and the proposed maximum aggregate offering price are estimated solely for the purpose of calculating the registration fee pursuant to Rule 457(o) under the Securities Act.

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrantregistrant shall file a further amendment whichthat specifically states that this Registration Statementregistration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until the Registration Statementregistration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.






The information in this prospectus is not complete and may be changed. WeThe Selling Stockholder may not sell these securities or accept an offer to buy these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities, and we areit is not soliciting offers to buy these securities in any jurisdiction where thesuch offer or sale is not permitted.

Subject

SUBJECT TO COMPLETION, DATED APRIL 22, 2022
PROSPECTUS
image_3a.jpg
1,666,666 Shares
Common Stock Offered by the Selling Stockholder
This prospectus relates to completion, dated January 25, 2019

$150,000,000

PROSPECTUS

Arsanis, Inc.

Debt Securities

Common Stock

Preferred Stock

Units

Warrants

We may offer and sell securitiesthe resale from time to time in one or more offerings of up to $150,000,0001,666,666 shares of common stock, par value $0.001 per share, of X4 Pharmaceuticals, Inc. (the “Common Stock”) by the selling stockholder listed on page 7 (the “Selling Stockholder”), including its pledgees, assignees, donees, transferees or their respective successors-in-interest, which consist of 900,000 outstanding shares of our Common Stock held by the Selling Stockholder and 766,666 shares of our Common Stock issuable upon the exercise of outstanding pre-funded warrants to purchase shares of our Common Stock held by the Selling Stockholder. We will not receive any proceeds from the sale of the shares offered by this prospectus, however, we have agreed to pay the registration expenses relating to such shares of Common Stock and we will receive the exercise price of $0.01 per share of any of the pre-funded warrants exercised for cash.

We have agreed, pursuant to a registration rights agreement that we have entered into with the Selling Stockholder, to bear all of the expenses incurred in aggregate offering price. This prospectus describesconnection with the general termsregistration of these securitiesshares. The Selling Stockholder will pay or assume discounts, commissions, fees of underwriters, selling brokers or dealer managers and similar expenses, if any, incurred for the general manner in which these securities will be offered. We will provide the specific termssale of these securities in supplements to this prospectus. shares of our Common Stock.
The prospectus supplements will also describe the specific manner in which these securities will be offered and may also supplement, update or amend information containedSelling Stockholder identified in this document. You should read this prospectus, and any applicable prospectus supplement before you invest.

Weor its pledgees, assignees, donees, transferees or their respective successors-in-interest, may offer these securities in amounts, at prices andthe shares from time to time on terms to be determined at the time of offering.sale through ordinary brokerage transactions or through any other means described in this prospectus under the caption “Plan of Distribution.” The securitiesshares may be sold directlyat fixed prices, at prevailing market prices, at prices related to prevailing market prices or at negotiated prices. For more information on the Selling Stockholder, see the section entitled “Selling Stockholder” on page 7.

We may amend or supplement this prospectus from time to time by filing amendments or supplements as required. You should read the entire prospectus and any amendments or supplements carefully before you through agents, or through underwriters and dealers. If agents, underwriters or dealers are used to sell the securities, we will name them and describe their compensation in a prospectus supplement.

make your investment decision.

Our common stockCommon Stock is listedtraded on the Nasdaq GlobalCapital Market under the symbol “ASNS”.

As of January 14, 2019,“XFOR.” On April 19, 2022, the aggregate market valuelast reported sale price of our outstanding common stock held bynon-affiliatesCommon Stock was approximately $26.0 million, which was calculated based on 7,454,904 shares of outstanding common stock held bynon-affiliates$1.47 per share. You are urged to obtain current market quotations for our Common Stock.

We are an “emerging growth company” under applicable Securities and a price per share of $3.48. PursuantExchange Commission rules and, as such, have elected to General Instruction I.B.6 ofForm S-3, in no event will we sell, pursuant to the registration statement of which this prospectus forms a part, securities in acomply with certain reduced public primary offering with a value exceedingone-third of the aggregate market value of our common stock held bynon-affiliates in any12-month period, so long as the aggregate market value of our outstanding common stock held bynon-affiliates remains below $75 million. During the 12 calendar months prior to and including the date of this prospectus, we have not offered or sold any securities pursuant to General Instruction I.B.6 ofForm S-3.

Investing in these securities involves certain risks. See “Risk Factors” on page 6 ofcompany disclosure requirements for this prospectus and future filings. See “Prospectus Summary—Implications of Being an Emerging Growth Company.”

Investing in our Common Stock involves a high degree of risk. You should carefully read and consider the section entitled “Risk Factors” on page 5 and the risk factors included in any accompanying prospectus supplement and in the documents incorporated by reference in this prospectus for a discussion of the factors you should carefully consider before deciding to purchase these securities.

Neitherour periodic reports filed with the Securities and Exchange Commission nor(“SEC”), in any state securities commission has approved or disapproved of these securities or passed uponapplicable prospectus supplement and in any other documents we file with the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

SEC.


NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

The date of this prospectus is                , 2019

2022.

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TABLE OF CONTENTS

i
ii
   1 

WHERE YOU CAN FIND MORE INFORMATION

2

2

CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

4

PROSPECTUS SUMMARY

   5 

   6 

   7 

   89 

   1711 

   2511 

   2611 

   27

PLAN OF DISTRIBUTION

29

LEGAL MATTERS

32

EXPERTS

32

SIGNATURES

II-8

SIGNATURES AND POWER OF ATTORNEY

II-812 

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ABOUT THIS PROSPECTUS

This

We urge you to read carefully this prospectus, is part of a registration statement that we filedtogether with the Securities and Exchange Commission, which we refer toinformation incorporated herein by reference as described under the “SEC,heading “Where You Can Find Additional Information,” utilizing a “shelf” registration process. Under this shelf registration process, we may from time to time sellbefore buying any combination of the securities describedbeing offered.
You should rely only on the information contained or incorporated by reference in this prospectus and the applicable prospectus supplement or in one or more offerings for an aggregate initial offering price of upany amendment to $150,000,000.

This prospectus providesthis prospectus. Neither we nor the Selling Stockholder has authorized anyone to provide you with a general descriptiondifferent information, and if anyone provides, or has provided you, with different or inconsistent information, you should not rely on it. The Selling Stockholder is offering to sell, and seeking offers to buy, shares of our Common Stock only in jurisdictions where offers and sales are permitted. The information contained in this prospectus, as well as the information filed previously with the SEC, and incorporated herein by reference, is accurate only as of the securities we may offer. Eachdate of the document containing the information, regardless of the time we sell securities underof delivery of this prospectus we will provide one or moreany applicable prospectus supplements that will contain specific information about the termssupplement or any sale of the offering. Theour Common Stock.

A prospectus supplement may also add to, update or change the information contained in this prospectus. You should read both this prospectus and the accompanyingany applicable prospectus supplement together with the additional information described below under the heading “Where You Can Find More Information” beginningAdditional Information.”
In this prospectus, references to “X4 Pharmaceuticals,” “X4,” the “Company,” the “registrant,” “we,” “us,” and “our” refer to X4 Pharmaceuticals, Inc., formerly Arsanis, Inc., and its subsidiaries. The phrase “this prospectus” refers to this prospectus and any applicable prospectus supplement, unless the context requires otherwise.



CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and any applicable prospectus supplement or free writing prospectus, including the documents incorporated by reference herein and therein, contain forward-looking statements. These are based on page 2our management’s current beliefs, expectations and assumptions about future events, conditions and results and on information currently available to us. Discussions containing these forward-looking statements may be found, among other places, in the sections entitled “Business,” “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” contained in the documents incorporated by reference herein.
Any statements in this prospectus, or incorporated herein, about our expectations, beliefs, plans, objectives, assumptions or future events or performance are not historical facts and are forward-looking statements. Within the meaning of Section 27A of the Securities Act and Section 21E of the Securities Exchange Act of 1934, as amended (“the Exchange Act”), these forward-looking statements include statements regarding:
our plans to develop and commercialize our product candidates;
the timing of our planned clinical trials for our product candidates;
the timing of and our ability to obtain and maintain regulatory approvals for our product candidates;
the clinical utility of our product candidates;
our commercialization, marketing and manufacturing capabilities and strategy;
the impact of the COVID-19 pandemic and other unforeseen events such as the armed conflict between Russia and Ukraine on our business, operations, strategy, goals and anticipated timelines;
our intellectual property position;
our plans to in-license, acquire, develop and commercialize additional product candidates;



our competitive position and the development of and projections relating to our competitors or our industry;
our ability to identify, recruit and retain key personnel;
the impact of laws and regulations;
our plans to identify additional product candidates with significant commercial potential that are consistent with our commercial objectives;
our estimates regarding future revenue, expenses and needs for additional financing; and
other risks and uncertainties, including those listed under the caption “Risk Factors” in our most recent Annual Report on Form 10-K, our most recent Quarterly Report on Form 10-Q, and other filings we make with the SEC.
In some cases, you can identify forward-looking statements by terms such as “may,” “will,” “intend,” “should,” “could,” “would,” “expect,” “plan,” “anticipate,” “believe,” “estimate,” “project,” “predict,” “potential,” “continue,” “likely,” and similar expressions (including their use in the negative) intended to identify forward-looking statements. These forward-looking statements reflect our current views with respect to future events and are based on assumptions and subject to risks and uncertainties. Given these risks and uncertainties, you should not place undue reliance on these forward-looking statements. We discuss many of these risks in greater detail under the heading “Risk Factors” in our SEC filings, and may provide additional information in any applicable prospectus supplement. Also, these forward-looking statements represent our estimates and assumptions only as of the date of the document containing the applicable statement.
We qualify all of the forward-looking statements in the foregoing documents by these cautionary statements. Unless required by law, we undertake no obligation to update or revise any forward-looking statements to reflect new information or future events or developments. Thus, you should not assume that our silence over time means that actual events are bearing out as expressed or implied in such forward-looking statements. Before deciding to purchase our Common Stock, you should carefully consider the risk factors incorporated by reference herein, in addition to the other information set forth in this prospectus and in the documents incorporated by reference herein.



PROSPECTUS SUMMARY
This summary highlights important features of this prospectus.

You should rely only onoffering and the information contained inincluded or incorporated by reference in this prospectus. This summary does not contain all of the information you should consider before investing in our Common Stock. You should carefully read this prospectus, any accompanyingapplicable prospectus supplement orand the information incorporated by reference in any related free writing prospectus filed by us with the SEC. We have not authorized anyone to provide you with different information. Thisthis prospectus and any accompanyingapplicable prospectus supplement before you invest in our Common Stock.

Company Overview
We are a late-stage clinical biopharmaceutical company focused on the research, development and commercialization of novel therapeutics for the treatment of rare diseases, with an initial focus on the treatment of people with immune system dysfunction. Our lead product candidate, mavorixafor, is a first-in-class, small molecule inhibitor of the chemokine receptor CXCR4 and is being developed as a once-daily oral therapy. Due to mavorixafor’s demonstrated ability to antagonize CXCR4 and improve the healthy maturation and trafficking of white blood cells, we believe that mavorixafor has the potential to provide therapeutic benefit across a wide variety of diseases, including primary immunodeficiencies (“PIDs”) and certain types of cancer.
In June 2019, we initiated 4WHIM, a pivotal, 52-week global Phase 3 clinical trial of mavorixafor for the treatment of patients with WHIM (Warts, Hypogammaglobulinemia, Infections, and Myelokathexis) Syndrome, a rare, inherited PID, following the completion of the dose-titration portion of a Phase 2 clinical trial whose open-label extension portion is ongoing. The U.S. Food and Drug Administration (“FDA”) has granted Breakthrough Therapy Designation for mavorixafor for the treatment of adults with WHIM. The FDA has also granted Fast Track Designation for mavorixafor for adults with WHIM and Rare Pediatric Designation for mavorixafor for the treatment of WHIM. We completed enrollment in the 4WHIM trial in the third quarter of 2021 and currently expect to report top-line data from this trial in the fourth quarter of 2022. In addition, we have begun building our commercial team in anticipation of a possible New Drug Application (“NDA”) submission to the FDA in the second half of 2023, with the goal of obtaining approval for mavorixafor for the treatment of people in the United States, aged 12 and older, with WHIM Syndrome should the final data from the 4WHIM trial support the filing of an NDA.
In November 2019, we initiated a 14-day, proof-of-concept Phase 1b clinical trial of mavorixafor in patients with chronic neutropenia disorders, and in December 2019, we initiated a Phase 1b clinical trial of mavorixafor, in combination with ibrutinib, in patients with Waldenström’s macroglobulinemia, a rare blood cancer characterized by an excess of abnormal white blood cells in the bone marrow and confirmed MYD88 and CXCR4 genetic mutations. Following announcements of positive preliminary data from both of the ongoing Phase 1b clinical trials of mavorixafor during 2021, we have continued patient enrollment. Additional data are expected from the Phase 1b clinical trial in chronic neutropenia by the third quarter of 2022 and from the Phase 1b clinical trial in Waldenström’s macroglobulinemia in the second half of 2022.
We are also investigating mavorixafor in combination with axitinib (Inlyta®) in the Phase 2a portion of an open-label Phase 1/2 clinical trial in clear cell renal cell carcinoma (“ccRCC”). We intend to pursue future development and potential commercialization of mavorixafor in ccRCC and other possible immuno-oncology indications outside of greater China only as part of a potential strategic collaboration.
In addition, we are advancing two early stage candidates: X4P-003, a second-generation CXCR4 antagonist designed to have enhanced properties relative to mavorixafor, potentially enabling broader opportunities in CXCR4-dependent disorders and primary immunodeficiencies; and X4P-002, a CXCR4 antagonist with a unique distribution profile for potential use in CXCR4-dependent diseases in the periphery, but also with the ability to cross the blood-brain barrier and provide appropriate therapeutic exposures to treat brain cancers and other diseases where exposure in brain is essential. We anticipate that we will file an Investigational New Drug application for X4P-002 in the second half of 2022.
To date, we have not generated revenue from product sales and do not constituteexpect to generate significant revenue from the sale of our products in the foreseeable future. If our development efforts for our product candidates are successful and result in regulatory approval, we may generate revenue in the future from product sales. We cannot
1


predict if, when, or to what extent we will generate revenue from the commercialization and sale of our product candidates. We may never succeed in obtaining regulatory approval for any of our product candidates.
Private Placement
Securities Purchase Agreement
On March 3, 2022, we entered into a Securities Purchase Agreement (the “Purchase Agreement”) with BCLS II Investco, LP (the “Purchaser”), an offeraffiliate of Bain Capital Life Sciences, LLC, pursuant to which we agreed to sell and issue (i) 900,000 shares of Common Stock at a purchase price of $1.80 per share, which represents the volume weighted average price per share of the Common Stock as quoted on the Nasdaq Capital Market for the thirty (30) consecutive-day trading day period ending on March 2, 2022, and (ii) pre-funded warrants to purchase 766,666 shares of Common Stock at a price of $0.01 per share, at a purchase price of $1.79 per pre-funded warrant (the “Private Placement”). On March 7, 2022, we closed the Private Placement and issued 900,000 shares of Common Stock and 766,666 pre-funded warrants to purchase Common Stock to the Purchaser.
The pre-funded warrants are exercisable at any time after their original issuance and will not expire. The pre-funded warrants provide that the holder of such pre-funded warrants will not have the right to exercise any portion of its pre-funded warrants if such holder, together with its affiliates, would beneficially own in excess of 9.99% of the number of shares of our Common Stock outstanding immediately after giving effect to such exercise (the “Beneficial Ownership Limitation”); provided, however, that the holder may increase or decrease the solicitationBeneficial Ownership Limitation by giving 61 days’ notice to us, but not to any percentage in excess of 19.99%.
The shares of Common Stock issued to the Purchaser, and the shares of Common Stock issuable upon exercise of the pre-funded warrants were not initially registered under the Securities Act or any state securities laws. We have relied on the exemption from the registration requirements of the Securities Act by virtue of Section 4(a)(2) thereof. In connection with its execution of the Purchase Agreement, the Purchaser represented to us that the Purchaser is an “accredited investor” as defined in Regulation D of the Securities Act and that the shares of Common Stock and the pre-funded warrants purchased by the Purchaser were being acquired solely for its own account and for investment purposes and not with a view to its future sale or distribution.
Registration Rights Agreement
On March 3, 2022, in connection with the Private Placement, we entered into a Registration Rights Agreement with the Purchaser (the “Registration Rights Agreement”), pursuant to which we agreed to (i) by no later than April 30, 2022, file a registration statement with the SEC to cover the resale of the shares of Common Stock, including those shares of Common Stock issuable upon exercise of the pre-funded warrants to purchase Common Stock, issued to the Purchaser pursuant to the Purchase Agreement (the “Shares”), (ii) to use commercially reasonable efforts to cause such registration statement to become effective as soon as practicable and (iii) to keep such registration statement effective until the date the Shares covered by such registration statement have been sold or may be resold pursuant to Rule 144 without restriction. In the event that such registration statement is not filed or declared effective within the timeframes set forth in the Registration Rights Agreement or, after the registration statement has been declared effective by the SEC, sales cannot be made pursuant to the registration statement for any reason including by reason of a stop order or our failure to update such registration statement, subject to certain limited exceptions, then we have agreed to make payments to the Purchaser as liquidated damages in an amount equal to 1% of the aggregate amount invested by the Investor in the Shares per 30-day period or pro rata for any portion thereof for each such month during which such event continues, subject to certain caps set forth in the registration rights agreement.
The registration statement of which this prospectus is a part relates to the offer and resale of the shares of Common Stock issued to buy any securities other than the securities describedPurchaser pursuant to the Purchase Agreement, including the shares issuable upon exercise of the pre-funded warrants to purchase Common Stock. When we refer to the Selling Stockholder in this prospectus, or such accompanying prospectus supplement or an offerwe are referring to sell or the solicitation of an offer to buy such securities in any circumstances in which such offer or solicitation is unlawful. You should assume that the information appearingPurchaser named in this prospectus as the Selling Stockholder and, as applicable,



any donees, pledgees, assignees, transferees or other successors-in-interest selling Shares received after the date of this prospectus from the Selling Stockholder as a gift, pledge, or other non-sale related transfer.
Corporate Information
Prior to March 13, 2019, we were a clinical-stage biopharmaceutical company known as Arsanis, Inc. that had historically been focused on applying monoclonal antibody immunotherapies to address serious infectious diseases. Arsanis, Inc. was originally incorporated in the State of Delaware in August 2010.
On March 13, 2019, we completed our business combination with X4 Therapeutics, Inc. (formerly X4 Pharmaceuticals, Inc.) in accordance with the terms of an Agreement and Plan of Merger, dated as of November 26, 2018, as amended on December 20, 2018 and March 8, 2019 (the “Merger Agreement”), that we entered into with X4 Therapeutics, Inc. and Artemis AC Corp., a Delaware corporation and our wholly owned subsidiary (“Merger Sub”). Pursuant to the terms of the Merger Agreement, Merger Sub merged with and into X4 Therapeutics, Inc., with X4 Therapeutics, Inc. continuing as our wholly owned subsidiary and the surviving corporation of the merger, which transaction we refer to as the Merger. At the closing of the Merger, we issued shares of our Common Stock to X4 Therapeutics, Inc. stockholders based on an agreed upon exchange ratio, and each option or warrant to purchase X4 Therapeutics, Inc. capital stock became an option or warrant, respectively, to purchase our Common Stock, subject to adjustment in accordance with the agreed upon exchange ratio. Following the closing of the Merger, we effected a 1-for-6 reverse stock split of our Common Stock, our name was changed to X4 Pharmaceuticals, Inc., the business of X4 Therapeutics, Inc. became our business, and we became a clinical-stage biopharmaceutical company focused on the discovery, development and commercialization of novel therapeutics for the treatment of rare diseases. In connection with the closing of the Merger, our stock began trading on the Nasdaq Capital Market under the symbol “XFOR” on March 14, 2019.
Our principal executive offices are located at 61 North Beacon Street, 4th Floor, Boston, Massachusetts 02134, and our telephone number is (857) 529-8300. Our website is located at http://www.x4pharma.com. We do not incorporate by reference into this prospectus the information on, or accessible through, our website, and you should not consider it as part of this prospectus.


















THE OFFERING
Common Stock Offered by the Selling Stockholder1,666,666 Shares
Use of ProceedsWe will not receive any proceeds from the sale of the Shares covered by this prospectus, except with respect to amounts received by us due to the exercise of pre-funded warrants.
Nasdaq Capital Market SymbolXFOR





RISK FACTORS

An investment in our Common Stock involves a high degree of risk. Prior to making a decision about investing in our Common Stock, you should consider carefully the specific risk factors discussed in the sections entitled “Risk Factors” contained in our most recent Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the SEC on March 17, 2022, which are incorporated in this prospectus by reference in their entirety, as well as any amendment or updates to our risk factors reflected in subsequent filings with the SEC, including any prospectus supplement hereto. These risks and uncertainties are not the documents incorporated by referenceonly risks and uncertainties we face. Additional risks and uncertainties not presently known to us, or that we currently view as immaterial, may also impair our business. If any related free writing prospectus is accurate only as of their respective dates. Ourthe risks or uncertainties described in our SEC filings or any additional risks and uncertainties actually occur, our business, financial condition, results of operations and prospects may have changedcash flow could be materially since those dates.

and adversely affected. In that case, the trading price of our Common Stock could decline and you might lose all or part of your investment.







USE OF PROCEEDS

We further note thatare filing the representations, warrantiesregistration statement of which this prospectus forms a part to permit the holder of the Shares of our Common Stock described in the section entitled “Selling Stockholder” to resell such Shares. We are not selling any securities under this prospectus, and covenants madewe will not receive any proceeds from the sale or other disposition of shares of our Common Stock held by the Selling Stockholder, except with respect to amounts received by us due to the exercise of the pre-funded warrants.

The Selling Stockholder will pay any underwriting discounts and commissions and expenses incurred by the Selling Stockholder for brokerage, accounting, tax or legal services or any other expenses incurred by the Selling Stockholder in any agreementdisposing of these Shares. We will bear all other costs, fees and expenses incurred in effecting the registration of the Shares covered by this prospectus, including, without limitation, all registration and filing fees, Nasdaq listing fees and fees and expenses of our counsel and our accountants.





SELLING STOCKHOLDER

This prospectus covers the sale or other disposition by the Selling Stockholder of up to the total number of shares of our Common Stock that is filedwere issued to the Selling Stockholder pursuant to the Purchase Agreement, plus the total number of shares of our Common Stock issuable upon exercise of the pre-funded warrants issued to the Selling Stockholder pursuant to the Purchase Agreement, without giving effect to the Beneficial Ownership Limitation described under “Prospectus Summary—Private Placement—Securities Purchase Agreement.” The table below sets forth, to our knowledge, information concerning the beneficial ownership of shares of our Common Stock by the Selling Stockholder as an exhibitof April 19, 2022. The information in the table below with respect to any document that is incorporated by referencethe Selling Stockholder has been obtained from the Selling Stockholder. When we refer to the “Selling Stockholder” in this prospectus, were made solelyor, if required, a post-effective amendment to the registration statement of which this prospectus is a part, we mean the Selling Stockholder listed in the table below as offering shares, as well as its respective pledgees, assignees, donees, transferees or successors-in-interest. Throughout this prospectus, when we refer to the shares of our Common Stock being registered on behalf of the Selling Stockholder, we are referring to the shares of our Common Stock and the shares underlying the pre-funded warrants issued to the Selling Stockholder pursuant to the Purchase Agreement, without giving effect to the Beneficial Ownership Limitation described above. The Selling Stockholder may sell all, some or none of the shares of Common Stock subject to this prospectus. See “Plan of Distribution” below as it may be supplemented and amended from time to time.

The number of shares of Common Stock beneficially owned prior to the offering for the benefitSelling Stockholder includes all shares of our Common Stock beneficially held by the Selling Stockholder as of April 19, 2022, which includes (i) all shares of our Common Stock purchased by such Selling Stockholder in the Private Placement and (ii) all shares of Common Stock issuable upon exercise of the pre-funded warrants purchased by the Selling Stockholder in the Private Placement, without giving effect to the Beneficial Ownership Limitation. The percentages of shares owned before and after the offering are based on 30,823,573 shares of Common Stock outstanding as of April 19, 2022, which includes the outstanding shares of Common Stock offered by this prospectus but does not include any shares of Common Stock offered by this prospectus that are issuable pursuant to warrants or pre-funded warrants and are deemed outstanding in the table below because they are beneficially owned by the Selling Stockholder.
Beneficial ownership is determined in accordance with the rules of the SEC and includes voting or investment power with respect to our Common Stock. Generally, a person “beneficially owns” shares of our Common Stock if the person has or shares with others the right to vote those shares or to dispose of them, or if the person has the right to acquire voting or disposition rights within 60 days. In computing the number of shares of our Common Stock beneficially owned by the Selling Stockholder and the percentage ownership of the Selling Stockholder, we deemed outstanding shares of Common Stock issuable upon the exercise of pre-funded warrants or warrants held by the Selling Stockholder that are exercisable within 60 days of April 19, 2022. The inclusion of any shares in this table does not constitute an admission of beneficial ownership for the Selling Stockholder named below.



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   Shares of
Common Stock
Beneficially Owned
Prior to Offering
  Number of
Shares of
Common Stock
Being Offered(1)
   Shares of
Common Stock
to be Beneficially
Owned After
Offering(2)
 
Name of Selling Stockholder  Number   Percentage   Number   Percentage 
BCLS II Investco, LP(3)
   3,321,147    9.99%  1,666,666        2,008,032   5.98%

(1)    The number of shares of our Common Stock in the column “Number of Shares of Common Stock Being Offered” represents all of the shares of our Common Stock that the Selling Stockholder may offer and sell from time to time under this prospectus.
(2)    We do not know when or in what amounts the Selling Stockholder may offer shares for sale. The Selling Stockholder might not sell any or might sell all of the shares offered by this prospectus. Because the Selling Stockholder may offer all or some of the shares pursuant to this offering, and because there are currently no agreements, arrangements or understandings with respect to the sale of any of the shares, we cannot estimate the number of the shares that will be held by the Selling Stockholder after completion of the offering. However, for purposes of this table, we have assumed that, after completion of the offering, none of the shares issued in the Private Placement and covered by this prospectus, including Common Stock issuable upon exercise of the pre-funded warrants, will be held by the Selling Stockholder.
(3)    The shares reported under “Shares of Common Stock Beneficially Owned Prior to the Offering” consists of (i) the following shares of Common Stock purchased by the Selling Stockholder in the Private Placement: (a) 900,000 shares of Common Stock, and (b) 766,666 shares of Common Stock issuable upon exercise of pre-funded warrants and (ii) 1,654,481 shares of Common Stock issuable upon exercise of pre-funded warrants purchased by the Selling Stockholder from us in November 2021. In addition to the foregoing shares, as of April 19, 2022, the Selling Stockholder held pre-funded warrants to purchase 353,551 additional shares of Common Stock that are not included in the shares reported under “Shares of Common Stock Beneficially Owned Prior to the Offering” because they are subject to limitations on exercisability if such exercise would result in the Selling Stockholder beneficially owning more than 9.99% of our outstanding Common Stock. Bain Capital Life Sciences Investors, LLC (“BCLSI”) is the ultimate general partner of the Selling Stockholder. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the shares of Common Stock beneficially owned by the Selling Stockholder. Additionally, the shares of Common Stock, including Common Stock issuable upon exercise of the pre-funded warrants, held by the Selling Stockholder are required to be aggregated with our other securities beneficially owned by BCLSI and held by affiliates of the Selling Stockholder for the purposes of the Beneficial Ownership Limitation. The address of the Selling Stockholder is c/o Bain Capital Life Sciences, LP, 200 Clarendon Street, Boston, MA 02116.

Relationships with Selling Stockholder
As discussed in greater detail above under the section titled “Prospectus Summary—Private Placement,” in March 2022, we entered into the Purchase Agreement with the Selling Stockholder, pursuant to which we sold and issued shares of our Common Stock and pre-funded warrants to purchase our Common Stock, and we also entered into the Registration Rights Agreement with the Selling Stockholder, pursuant to which we agreed to file a registration statement with the SEC to cover the resale by the Selling Stockholder of the shares of our Common Stock, including the shares of Common stock issuable upon exercise of the pre-funded warrants, issued pursuant to the Purchase Agreement.
The Selling Stockholder has not had a material relationship with us or any of our predecessors or affiliates within the past three years, other than as a result of the ownership of our shares of Common Stock or other securities, and the Selling Stockholder has not held any position or office with us or our affiliates within the last three years.
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PLAN OF DISTRIBUTION

The Selling Stockholder, which as used herein includes donees, pledgees, transferees or other successors-in-interest selling shares of Common Stock or interests in shares of Common Stock received after the date of this prospectus from the Selling Stockholder as a gift, pledge, partnership distribution or other transfer, may, from time to time, sell, transfer or otherwise dispose of any or all of their shares of Common Stock or interests in shares of Common Stock on any stock exchange, market or trading facility on which the shares are traded or in private transactions. These dispositions may be at fixed prices, at prevailing market prices at the time of sale, at prices related to the prevailing market price, at varying prices determined at the time of sale, or at negotiated prices.
The Selling Stockholder may use any one or more of the following methods when disposing of shares or interests therein:
ordinary brokerage transactions and transactions in which the broker-dealer solicits purchasers;
block trades in which the broker-dealer will attempt to sell the shares as agent, but may position and resell a portion of the block as principal to facilitate the transaction;
purchases by a broker-dealer as principal and resale by the broker-dealer for its account;
an exchange distribution in accordance with the rules of the applicable exchange;
privately negotiated transactions;
short sales effected after the date the registration statement of which this prospectus is a part is declared effective by the SEC;
through the writing or settlement of options or other hedging transactions, whether through an options exchange or otherwise;
broker-dealers may agree with the Selling Stockholder to sell a specified number of such shares at a stipulated price per share;
a combination of any such methods of sale; and
any other method permitted by applicable law.
The Selling Stockholder may, from time to time, pledge or grant a security interest in some or all of the shares of Common Stock owned by the Selling Stockholder and, if the Selling Stockholder defaults in the performance of its secured obligations, the pledgees or secured parties may offer and sell the shares of Common Stock, from time to time, under this prospectus, or under an amendment to this prospectus under Rule 424(b)(3) or other applicable provision of the Securities Act of 1933, as amended (the “Securities Act”), amending the selling stockholder list to include the pledgee, transferee or other successors in interest as selling stockholders under this prospectus. The Selling Stockholder also may transfer the shares of Common Stock in other circumstances, in which case the transferees, pledgees, donees or other successors in interest will be the selling beneficial owners for purposes of this prospectus.
In connection with the sale of our Common Stock or interests therein, the Selling Stockholder may enter into hedging transactions with broker-dealers or other financial institutions, which may in turn engage in short sales of the Common Stock in the course of hedging the positions they assume. The Selling Stockholder may also sell shares of our Common Stock short and deliver these securities to close out their short positions, or loan or pledge the
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Common Stock to broker-dealers that in turn may sell these securities. The Selling Stockholder may also enter into option or other transactions with broker-dealers or other financial institutions or the creation of one or more derivative securities which require the delivery to such agreement, including,broker-dealer or other financial institution of shares offered by this prospectus, which shares such broker-dealer or other financial institution may resell pursuant to this prospectus (as supplemented or amended to reflect such transaction).
The aggregate proceeds to the Selling Stockholder from the sale of the Common Stock offered by the Selling Stockholder will be the purchase price of the Common Stock less discounts or commissions, if any. The Selling Stockholder reserves the right to accept and, together with its agents from time to time, to reject, in whole or in part, any proposed purchase of Common Stock to be made directly or through agents. We will not receive any of the proceeds from this offering. Upon any exercise of the pre-funded warrants by payment of cash, however, we will receive the exercise price of $0.01 per share pursuant to the pre-funded warrants.
The Selling Stockholder also may resell all or a portion of the shares in open market transactions in reliance upon Rule 144 under the Securities Act of 1933, provided that the Selling Stockholder meets the criteria and conform to the requirements of that rule.
The Selling Stockholder and any underwriters, broker-dealers or agents that participate in the sale of the Common Stock or interests therein may be “underwriters” within the meaning of Section 2(11) of the Securities Act. Any discounts, commissions, concessions or profit they earn on any resale of the shares may be underwriting discounts and commissions under the Securities Act. A Selling Stockholder who is an “underwriter” within the meaning of Section 2(11) of the Securities Act will be subject to the prospectus delivery requirements of the Securities Act.

To the extent required, the shares of our Common Stock to be sold, the name of the Selling Stockholder, the respective purchase price and public offering price, the names of any agents, dealer or underwriter, and any applicable commissions or discounts with respect to a particular offer will be set forth in an accompanying prospectus supplement or, if appropriate, a post-effective amendment to the registration statement that includes this prospectus.
In order to comply with the securities laws of some states, if applicable, the Common Stock may be sold in these jurisdictions only through registered or licensed brokers or dealers. In addition, in some cases,states the Common Stock may not be sold unless it has been registered or qualified for sale or an exemption from registration or qualification requirements is available and is complied with.
We have advised the Selling Stockholder that the anti-manipulation rules of Regulation M under the Securities Exchange Act of 1934, as amended, may apply to sales of shares in the market and to the activities of the Selling Stockholder and its affiliates. In addition, to the extent applicable, we will make copies of this prospectus (as it may be supplemented or amended from time to time) available to the Selling Stockholder for the purpose of allocating risk amongsatisfying the partiesprospectus delivery requirements of the Securities Act. The Selling Stockholder may indemnify any broker-dealer that participates in transactions involving the sale of the shares against certain liabilities, including liabilities arising under the Securities Act.
We have agreed to indemnify the Selling Stockholder against liabilities, including liabilities under the Securities Act and state securities laws, relating to the registration of the shares offered by this prospectus.
We have agreed with the Selling Stockholder to use commercially reasonable efforts to cause the registration statement of which this prospectus constitutes a part effective and to remain continuously effective until the earlier of (1) such agreements,time as all of the shares covered by this prospectus have been disposed of pursuant to and should notin accordance with such registration statement or (2) the date on which all of the shares may be deemedsold without restriction pursuant to Rule 144 of the Securities Act.

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LEGAL MATTERS

The validity of the Shares to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as ofoffered for resale by the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

This prospectus may not be used to consummate sales of our securities unless it is accompanied by a prospectus supplement. To the extent there are inconsistencies between any prospectus supplement,Selling Stockholder under this prospectus and any documentswill be passed upon for us by Cooley LLP, New York, New York.


EXPERTS

The financial statements incorporated by reference, the document with the most recent date will control.

Unless the context otherwise indicates, references in this prospectus to “we,” “our” and “us” refer, collectively, to Arsanis, Inc., a Delaware corporation, and its consolidated subsidiaries.

We own or have rights to trademarks, service marks and trade names that we use in connection with the operation of our business, including our corporate name, logos and website names. Other trademarks, service marks and trade names appearing in this prospectus are the property of their respective owners. Solely for convenience, some of the trademarks, service marks and trade names referred to in this prospectus are listed without the® and ™ symbols, but we will assert,by reference to the fullest extent under applicable law, our rightsAnnual Report on Form 10-K for the year ended December 31, 2021 have been so incorporated in reliance on the report (which contains an explanatory paragraph relating to our trademarks, service marksthe Company’s ability to continue as a going concern as described in Note 1 to the financial statements) of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and trade names.

accounting.


WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at http://www.sec.gov. Copies of certain information filed by us with the SEC are also available on our website at www.arsanis.com. Our website is not a part of this prospectus and is not incorporated by reference in this prospectus. You may also read and copy any document we file at the SEC’s Public Reference Room, 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at

1-800-SEC-0330
for further information on the operation of the Public Reference Room.

This prospectus is part of a registration statement that we filed with the SEC. This prospectus omits some information contained in the registration statement in accordance with SEC rules and regulations. You should review the information and exhibits in the registration statement for further information about us and our consolidated subsidiaries and the securities we are offering.our securities. Statements in this prospectus concerning any document we filed as an exhibit to the registration statement or that we otherwise filed with the SEC are not intended to be comprehensive and are qualified by reference to these filings. You should review the complete document to evaluate these statements.

You can obtain a copy of the registration statement from the SEC’s website.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at http://www.sec.gov.
We make available, free of charge, through our investor relations website, our Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, statements of changes in beneficial ownership of securities and amendments to those reports and statements as soon as reasonably practicable after they are filed with the SEC. The address for our website is www.x4pharma.com. Information contained in or accessible through our website does not constitute a part of this prospectus and is not incorporated by reference in this prospectus.


INCORPORATION OF CERTAIN INFORMATION BY REFERENCE


The SEC allows us to incorporate“incorporate by reference much of thereference” information we file with the SEC,into this prospectus, which means that we can disclose important information to you by referring you to those publicly available documents.another document filed separately with the SEC. The information that we incorporateSEC file number for the documents incorporated by reference in this prospectus is considered to be part of this prospectus. Because we are incorporating by reference future filings with the SEC, this prospectus is continually updated and those future filings may modify or supersede some of the information included or incorporated in this prospectus. This means that you must look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus or in any document previously001-38295. The documents incorporated by reference have been modified or superseded. Thisinto this prospectus incorporatescontain important information that you should read about us.
The following documents are incorporated by reference the documents listed below (Fileinto this document:
No. 001-38295) and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act (in each case, other than those documents or the portions of those documents not deemed to be filed) between the date of the initial registration statement and the effectiveness of the registration statement and following the effectiveness of the registration statement until the offering of the securities under the registration statement is terminated or completed:

Our Annual Report on Form 10-K10-K for the fiscal year ended December 31, 20172021 (the “2021 Annual Report”), filed with the SEC on March 9, 2018,17, 2022, including the information specifically incorporated by reference intoin the 2021 Annual Report onForm 10-K from our definitive proxy statement for the 20182022 Annual Meeting of Stockholders;

Our Quarterly Report on Form10-Q for the fiscal quarter ended March 31, 2018Stockholders, to be filed with the SEC on May 10, 2018, our Quarterly Reportor around April 25, 2022;

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Our Current Reports on Form10-Q for the fiscal quarter ended June 30, 2018 8-K filed with the SEC on August 13, 2018, January 14, 2022, January 14, 2022, February 9, 2022 and our Quarterly Report on Form10-QMarch 3, 2022 (in each case, except for the fiscal quarter ended September 30, 2018 filed with the SEC on November 9, 2018;

information contained therein which is furnished rather than filed); and

Our Current Reports on Form8-K filed with the SEC on June 7, 2018, June 12, 2018, June 28, 2018, November 27, 2018, December 20, 2018 and January 16, 2019; and

The description of our common stock containedCommon Stock set forth in our Registration Statementthe registration statement onForm 8-A registering our Common Stock under Section 12 of the Exchange Act, which was filed with the SEC on November 15, 2017, including any amendments or reports filed for the purposepurposes of updating such description.

You may request a copy of these filings, at no cost, by writing or telephoning us atdescription, including Exhibit 4.18 to the following address or telephone number:

Arsanis, Inc.

890 Winter Street, Suite 230

Waltham, MA 02451

(781)819-57042021 Annual Report.

CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the information incorporatedWe also incorporate by reference ininto this prospectus include “forward-looking statements” withinall documents (other than current reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such form that are related to such items) that are filed by us with the meaning of Section 27A of the Securities Act of 1933, as amended,SEC pursuant to Sections 13(a), 13(c), 14 or the Securities Act, and Section 21E15(d) of the Exchange Act. These statements are based on current expectations, estimates, forecasts and projections aboutAct (i) after the industry in which we operate and the beliefs and assumptions of our management. The words “anticipate,” “believe,” “continue” “could,” “estimate,” “expect,” “intend,” “may,” “might,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “would,” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. There are a number of important risks and uncertainties that could cause our actual results to differ materially from those indicated by forward-looking statements. We may not actually achieve the plans, intentions or expectations disclosed in our forward-looking statements, and you should not place undue reliance on our forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements we make. You are cautioned that these forward-looking statements are only predictions and are subject to risks, uncertainties and assumptions that are referenced in the section of any accompanying prospectus supplement entitled “Risk Factors.” You should also carefully review the risk factors and cautionary statements described in the other documents we file from time to time with the SEC, specifically our most recent Annual Report on Form10-K, our Quarterly Reports on Form10-Q and our Current Reports onForm 8-K. We undertake no obligation to revise or update any forward-looking statements, except to the extent required by law.

PROSPECTUS SUMMARY

The following is a summary of our business and the offering of our securities under this prospectus. We urge you to read this entire prospectus, including the more detailed consolidated financial statements, notes to the consolidated financial statements and other information incorporated by reference from our other filings with the SEC or included in any applicable prospectus supplements. Investing in our securities involves risks. Therefore, carefully consider the risk factors in any prospectus supplements and in our most recent annual and quarterly filings with the SEC, as well as other information in this prospectus and any prospectus supplements and the documents incorporated by reference herein or therein, before purchasing our securities. Eachdate of the risk factors could adversely affect our business, operating results and financial condition, as well as adversely affect the valueinitial filing of an investment in our securities.

About Arsanis, Inc.

We are a clinical-stage biopharmaceutical company focused on applying monoclonal antibody, or mAb, immunotherapies to address serious infectious diseases. We possess a deep understanding of the pathogenesis of infection, paired with access to what we believe to be some of the most advanced mAb discovery techniques and platforms available today. Our pipeline is comprised of mAbs targeting multiple serious bacterial and viral pathogens, including respiratory syncytial virus.

On November 26, 2018, we entered into a merger agreement with X4 Pharmaceuticals, Inc., or X4, under which a wholly owned subsidiary of Arsanis is expected to merge with and into X4 in anall-stock transaction. The merger would result in a combined company operating under the X4 Pharmaceuticals, Inc. name that will focus on the discovery, development and commercialization of novel therapeutics for the treatment of rare diseases. The merger is currently expected to close in the first quarter of 2019, subject to the approval of the stockholders of each company and the satisfaction or waiver of other customary conditions.

As required by Rule 3-05 and Article 11 of Regulation S-X, the financial information for X4 for the years ended December 31, 2017 and 2016 and the nine months ended September 30, 2018 and 2017, and the pro forma financial information related to the proposed merger, have been filed as Exhibits 99.1 and 99.2, respectively, to the registration statement of which this prospectus is a part and are included herein.

Our principal executive offices are located at 890 Winter Street, Waltham, Massachusetts 02451 and our telephone number is (781)819-5704. We have a wholly owned subsidiary that is primarily focused on discovery research in Vienna, Austria (Arsanis Biosciences GmbH).

RISK FACTORS

Investing in our common stock involves a high degree of risk. Before you decide to invest in our common stock, you should carefully consider the risks described in the section captioned “Risk Factors” in our most recent Annual Report onForm 10-K, our most recent Quarterly Report onForm 10-Q and other filings we make with the Securities and Exchange Commission, or SEC, from time to time, which are incorporated by reference herein in their entirety, together with the other information in this prospectus and documents incorporated by reference in this prospectus. The risks described in our most recent Annual Report onForm 10-K, our most recent Quarterly Report onForm 10-Q and the other filings incorporated by reference herein are not the only ones facing our company. Additional risks and uncertainties may also impair our business operations. If any of the risks described in our most recent Annual Report onForm 10-K, our most recent Quarterly Report onForm 10-Q and the other filings incorporated by reference herein occurs, our business, financial condition, results of operations and future growth prospects could be harmed. In these circumstances, the market price of our common stock could decline, and you may lose all or part of your investment.

USE OF PROCEEDS

We intend to use the net proceeds from the sale of any securities offered under this prospectus for general corporate purposes unless otherwise indicated in the applicable prospectus supplement. General corporate purposes may include research and development expenditures, the acquisition orin-licensing of other products, product candidates, companies or businesses, repayment and refinancing of debt, working capital and capital expenditures. We may temporarily invest such net proceeds in investment-grade, interest-bearing securities until they are used for their stated purpose. We have not determined the amount of net proceeds to be used specifically for such purposes. As a result, management will retain broad discretion over the allocation of net proceeds.

DESCRIPTION OF DEBT SECURITIES

We may offer debt securities which may be senior or subordinated. We refer to the senior debt securities and the subordinated debt securities collectively as debt securities. The following description summarizes the general terms and provisions of the debt securities. We will describe the specific terms of the debt securities and the extent, if any, to which the general provisions summarized below apply to any series of debt securities in the prospectus supplement relating to the series and any applicable free writing prospectus that we authorize to be delivered. When we refer to “the Company,” “we,” “our,” and “us” in this section, we mean Arsanis, Inc. excluding, unless the context otherwise requires or as otherwise expressly stated, our subsidiaries.

We may issue senior debt securities from time to time, in one or more series under a senior indenture to be entered into between us and a senior trustee to be named in a prospectus supplement, which we refer to as the senior trustee. We may issue subordinated debt securities from time to time, in one or more series under a subordinated indenture to be entered into between us and a subordinated trustee to be named in a prospectus supplement, which we refer to as the subordinated trustee. The forms of senior indenture and subordinated indenture are filed as exhibits to the registration statement of which this prospectus forms a part. The senior indenture and the subordinated indenture are referred to individually as an indenture and together as the indentures, and the senior trustee and the subordinated trustee are referred to individually as a trustee and together as the trustees. This section summarizes some of the provisions of the indentures and is qualified in its entirety by the specific text of the indentures, including definitions of terms used in the indentures. Wherever we refer to particular sections of, or defined terms in, the indentures, those sections or defined terms are incorporated by reference in this prospectus or the applicable prospectus supplement. You should review the indentures that are filed as exhibits to the registration statement of which this prospectus forms a part for additional information.

Neither indenture will limitand prior to effectiveness of the amountregistration statement, or (ii) after the date of debt securities that we may issue. The applicable indenturethis prospectus but prior to the termination of the offering. These documents include periodic reports, such as Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, as well as proxy statements.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, without charge upon written or oral request, a copy of any or all of the documents that debt securities may be issued up to an aggregate principal amount authorized from time to time by us and may be payable in any currency or currency unit designated by us or in amounts determinedare incorporated by reference into this prospectus but not delivered with the prospectus, including exhibits that are specifically incorporated by reference into such documents. You should direct any requests for documents to an index.

General

The senior debt securities will constitute our unsecured and unsubordinated general obligations and will rank equallyX4 Pharmaceuticals, Inc., Attn: Investor Relations, 61 North Beacon Street, 4th Floor, Boston, Massachusetts 02134.

Any statement contained in right of payment with our other unsecured and unsubordinated obligations. The subordinated debt securities will constitute our unsecured and subordinated general obligations andthis prospectus or contained in a document incorporated or deemed to be incorporated by reference into this prospectus will be junior in right of paymentdeemed to our senior indebtedness (including senior debt securities), as described under the heading “—Certain Terms of the Subordinated Debt Securities—Subordination.” The debt securities will be structurally subordinated to all existing and future indebtedness and other liabilities of our subsidiaries unless such subsidiaries expressly guarantee such debt securities.

The debt securities will be our unsecured obligations. Any secured debtmodified or other secured obligations will be effectively senior to the debt securitiessuperseded to the extent of the value of the assets securing such debt or other obligations.

The applicable prospectus supplement and/or free writing prospectus will include any additional or different terms of the debt securities of any series being offered, including the following terms:

the title and type of the debt securities;

whether the debt securities will be senior or subordinated debt securities, and, with respect to any subordinated debt securities, the terms on which they are subordinated;

the initial aggregate principal amount of the debt securities;

the price or prices at which we will sell the debt securities;

the maturity date or dates of the debt securities and the right, if any, to extend such date or dates;

the rate or rates, if any, at which the debt securities will bear interest, or the method of determining such rate or rates;

the date or dates from which such interest will accrue, the interest payment dates on which such interest will be payable or the method of determination of such dates;

the right, if any, to extend the interest payment periods and the duration of that extension;

the manner of paying principal and interest and the place or places where principal and interest will be payable;

provisions for a sinking fund, purchase fund or other analogous fund, if any;

any redemption dates, prices, obligations and restrictions on the debt securities;

the currency, currencies or currency units in which the debt securities will be denominated and the currency, currencies or currency units in which principal and interest, if any, on the debt securities may be payable;

any conversion or exchange features of the debt securities;

whether the debt securities will be subject to the defeasance provisions in the indenture;

whether the debt securities will be issued in definitive or global form or in definitive form only upon satisfaction of certain conditions;

whether the debt securities will be guaranteed as to payment or performance;

any special tax implications of the debt securities;

any events of default or covenants in addition to or in lieu of those set forth in the indenture; and

any other material terms of the debt securities.

When we refer to “principal” in this section with reference to the debt securities, we are also referring to “premium, if any.”

We may from time to time, without notice to or the consent of the holders of any series of debt securities, create and issue further debt securities of any such series ranking equally with the debt securities of such series in all respects (or in all respects other than (1) the payment of interest accruing prior to the issue date of such further debt securities or (2) the first payment of interest following the issue date of such further debt securities). Such further debt securities may be consolidated and form a single series with the debt securities of such series and have the same terms as to status, redemption or otherwise as the debt securities of such series.

You may present debt securities for exchange and you may present debt securities for transfer in the manner, at the places and subject to the restrictions set forth in the debt securities and the applicable prospectus supplement. We will provide you those services without charge, although you may have to pay any tax or other governmental charge payable in connection with any exchange or transfer, as set forth in the indenture.

Debt securities may bear interest at a fixed rate or a floating rate. Debt securities bearing no interest or interest at a rate that at the time of issuance is below the prevailing market rate (original issue discount securities) may be sold at a discount below their stated principal amount. U.S. federal income tax considerations applicable to any such discounted debt securities or to certain debt securities issued at par which are treated as having been issued at a discount for U.S. federal income tax purposes will be described in the applicable prospectus supplement.

We may issue debt securities with the principal amount payable on any principal payment date, or the amount of interest payable on any interest payment date, to be determined by reference to one or more currency

exchange rates, securities or baskets of securities, commodity prices or indices. You may receive a payment of principal on any principal payment date, or a payment of interest on any interest payment date, that is greater than or less than the amount of principal or interest otherwise payable on such dates, depending on the value on such dates of the applicable currency, security or basket of securities, commodity or index. Information as to the methods for determining the amount of principal or interest payable on any date, the currencies, securities or baskets of securities, commodities or indices to which the amount payable on such date is linked and certain related tax considerations will be set forth in the applicable prospectus supplement.

Certain Terms of the Senior Debt Securities

Covenants.Unless we indicate otherwise in a prospectus supplement with respect to a particular series of senior debt securities, the senior debt securities will not contain any financial or restrictive covenants, including covenants restricting either us or any of our subsidiaries from incurring, issuing, assuming or guaranteeing any indebtedness secured by a lien on any of our or our subsidiaries’ property or capital stock, or restricting either us or any of our subsidiaries from entering into sale and leaseback transactions.

Consolidation, Merger and Sale of Assets.Unless we indicate otherwise in a prospectus supplement with respect to a particular series of senior debt securities, we may not consolidate with or merge into any other person, in a transaction in which we are not the surviving corporation, or convey, transfer or lease our properties and assets substantially as an entirety to any person, in either case, unless:

the successor entity, if any, is a U.S. corporation, limited liability company, partnership or trust;

the successor entity assumes our obligations on the senior debt securities and under the senior indenture;

immediately after giving effect to the transaction, no default or event of default shall have occurred and be continuing; and

we have delivered to the senior trustee an officer’s certificate and an opinion of counsel, each stating that the consolidation, merger, conveyance, transfer or lease and, if a supplemental indenture is required in connection with such transaction, such supplemental indenture, comply with the senior indenture and all conditions precedent provided for in the senior indenture relating to such transaction have been complied with.

The restrictions described in the bullets above do not apply (1) to our consolidation with or merging into one of our affiliates, if our board of directors determines in good faith that the purpose of the consolidation or merger is principally to change our state of incorporation or our form of organization to another form or (2) if we merge with or into a single direct or indirect wholly-owned subsidiary of ours.

The surviving business entity will succeed to, and be substituted for, us under the senior indenture and the senior debt securities and, except in the case of a lease, we shall be released from all obligations under the senior indenture and the senior debt securities.

No Protection in the Event of a Change in Control.Unless we indicate otherwise in a prospectus supplement with respect to a particular series of senior debt securities, the senior debt securities will not contain any provisions that may afford holders of the senior debt securities protection in the event we have a change in control or in the event of a highly leveraged transaction (whether or not such transaction results in a change in control).

Events of Default.Unless we indicate otherwise in a prospectus supplement with respect to a particular series of senior debt securities, the following are events of default under the senior indenture with respect to senior debt securities of each series:

failure to pay interest on any senior debt securities of such series when due and payable, if that default continues for a period of 30 days (or such other period as may be specified for such series);

failure to pay principal on the senior debt securities of such series when due and payable whether at maturity, upon redemption, by declaration or otherwise (and, if specified for such series, the continuance of such failure for a specified period);

default in the performance of or breach of any of our covenants or agreements in the senior indenture applicable to senior debt securities of such series, other than a covenant breach which is specifically dealt with elsewhere in the senior indenture, and that default or breach continues for a period of 90 days after we receive written notice from the trustee or from the holders of 25% or more in aggregate principal amount of the senior debt securities of such series;

certain events of bankruptcy or insolvency, whether or not voluntary; and

any other event of default provided for in such series of senior debt securities as may be specified in the applicable prospectus supplement.

The default by us under any other debt, including any other series of debt securities, is not a default under the senior indenture.

If an event of default other than an event of default specified in the fourth bullet point above occurs with respect to a series of senior debt securities and is continuing under the senior indenture, then, and in each such case, either the trustee or the holders of not less than 25% in aggregate principal amount of such series then outstanding under the senior indenture (each such series voting as a separate class) by written notice to us and to the trustee, if such notice is given by the holders, may, and the trustee at the request of such holders shall, declare the principal amount of and accrued interest on such series of senior debt securities to be immediately due and payable, and upon this declaration, the same shall become immediately due and payable.

If an event of default specified in the fourth bullet point above occurs and is continuing, the entire principal amount of and accrued interest on each series of senior debt securities then outstanding shall automatically become immediately due and payable.

Unless otherwise specified in the prospectus supplement relating to a series of senior debt securities originally issued at a discount, the amount due upon acceleration shall include only the original issue price of the senior debt securities, the amount of original issue discount accrued to the date of acceleration and accrued interest, if any.

Upon certain conditions, declarations of acceleration may be rescinded and annulled and past defaults may be waived by the holders of a majority in aggregate principal amount of all the senior debt securities of such series affected by the default, each series voting as a separate class. Furthermore, subject to various provisions in the senior indenture, the holders of a majority in aggregate principal amount of a series of senior debt securities, by notice to the trustee, may waive a continuing default or event of default with respect to such senior debt securities and its consequences, except a default in the payment of principal of or interest on such senior debt securities (other than any such default in payment resulting solely from an acceleration of the senior debt securities) or in respect of a covenant or provision of the senior indenture which cannot be modified or amended without the consent of the holders of each such senior debt security. Upon any such waiver, such default shall cease to exist, and any event of default with respect to such senior debt securities shall be deemed to have been cured, for every purpose of the senior indenture; but no such waiver shall extend to any subsequent or other default or event of default or impair any right consequent thereto.

The holders of a majority in aggregate principal amount of a series of senior debt securities may direct the time, method and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power conferred on the trustee with respect to such senior debt securities. However, the trustee may refuse to follow any direction that conflicts with law or the senior indenture, that may involve the trustee in personal liability or that the trustee determines in good faith may be unduly prejudicial to the rights of holders of

such series of senior debt securities not joining in the giving of such direction and may take any other action it deems proper that is not inconsistent with any such direction received from holders of such series of senior debt securities. A holder may not pursue any remedy with respect to the senior indenture or any series of senior debt securities unless:

the holder gives the trustee written notice of a continuing event of default;

the holders of at least 25% in aggregate principal amount of such series of senior debt securities make a written request to the trustee to pursue the remedy in respect of such event of default;

the requesting holder or holders offer the trustee indemnity satisfactory to the trustee against any costs, liability or expense;

the trustee does not comply with the request within 60 days after receipt of the request and the offer of indemnity; and

during such60-day period, the holders of a majority in aggregate principal amount of such series of senior debt securities do not give the trustee a direction that is inconsistent with the request.

These limitations, however, do not apply to the right of any holder of a senior debt security of any affected series to receive payment of the principal of and interest on such senior debt security in accordance with the terms of such debt security, or to bring suit for the enforcement of any such payment in accordance with the terms of such debt security, on or after the due date for the senior debt securities, which right shall not be impaired or affected without the consent of the holder.

The senior indenture requires certain of our officers to certify, on or before a fixed date in each year in which any senior debt security is outstanding, as to their knowledge of our compliance with all covenants, agreements and conditions under the senior indenture.

Satisfaction and Discharge. We can satisfy and discharge our obligations to holders of any series of debt securities if:

we have paid or caused to be paid the principal of and interest on all senior debt securities of such series (with certain limited exceptions) when due and payable; or

we deliver to the senior trustee for cancellation all senior debt securities of such series theretofore authenticated under the senior indenture (with certain limited exceptions); or

all senior debt securities of such series have become due and payable or will become due and payable within one year (or are to be called for redemption within one year under arrangements satisfactory to the senior trustee) and we deposit in trust an amount of cash or a combination of cash and U.S. government or U.S. government agency obligations (or in the case of senior debt securities denominated in a foreign currency, foreign government securities or foreign government agency securities) sufficient to make interest, principal and any other payments on the debt securities of that series on their various due dates;

and if, in any such case, we also pay or cause to be paid all other sums payable under the senior indenture, as and when the same shall be due and payable and we deliver to the senior trustee an officer’s certificate and an opinion of counsel, each stating that these conditions have been satisfied.

Under current U.S. federal income tax law, the deposit and our legal release from the debt securities would be treated as though we took back your debt securities and gave you your share of the cash and debt securities or bonds deposited in trust. In that event, you could recognize gain or loss on the debt securities you give back to us. Purchasers of the debt securities should consult their own advisers with respect to the tax consequences to them of such deposit and discharge, including the applicability and effect of tax laws other than the U.S. federal income tax law.

Defeasance.Unless the applicable prospectus supplement provides otherwise, the following discussion of legal defeasance and covenant defeasance will apply to any series of debt securities issued under the indentures.

Legal Defeasance. We can legally release ourselves from any payment or other obligations on the debt securities of any series (called “legal defeasance”) if certain conditions are met, including the following:

We deposit in trust for your benefit and the benefit of all other direct holders of the debt securities of the same series cash or a combination of cash and U.S. government or U.S. government agency obligations (or, in the case of senior debt securities denominated in a foreign currency, foreign government or foreign government agency obligations) that will generate enough cash to make interest, principal and any other payments on the debt securities of that series on their various due dates.

There is a change in current U.S. federal income tax law or an IRS ruling that lets us make the above deposit without causing you to be taxed on the debt securities any differently than if we did not make the deposit and instead repaid the debt securities ourselves when due. Under current U.S. federal income tax law, the deposit and our legal release from the debt securities would be treated as though we took back your debt securities and gave you your share of the cash and debt securities or bonds deposited in trust. In that event, you could recognize gain or loss on the debt securities you give back to us.

We deliver to the trustee a legal opinion of our counsel confirming the tax law change or ruling described above.

If we accomplish legal defeasance, as described above, you would have to rely solely on the trust deposit for repayment of the debt securities. You could not look to us for repayment in the event of any shortfall.

Covenant Defeasance. Without any change in current U.S. federal tax law, we can make the same type of deposit described above and be released from some of the covenants in the debt securities (called “covenant defeasance”). In that event, you would lose the protection of those covenants but would gain the protection of having money and securities set aside in trust to repay the debt securities. In order to achieve covenant defeasance, we must do the following (among other things):

We must deposit in trust for your benefit and the benefit of all other direct holders of the debt securities of the same series cash or a combination of cash and U.S. government or U.S. government agency obligations (or, in the case of senior debt securities denominated in a foreign currency, foreign government or foreign government agency obligations) that will generate enough cash to make interest, principal and any other payments on the debt securities of that series on their various due dates.

We must deliver to the trustee a legal opinion of our counsel confirming that under current U.S. federal income tax law we may make the above deposit without causing you to be taxed on the debt securities any differently than if we did not make the deposit and instead repaid the debt securities ourselves when due.

If we accomplish covenant defeasance, you could still look to us for repayment of the debt securities if there were a shortfall in the trust deposit. In fact, if one of the events of default occurred (such as our bankruptcy) and the debt securities become immediately due and payable, there may be such a shortfall. Depending on the events causing the default, you may not be able to obtain payment of the shortfall.

Modification and Waiver.We and the trustee may amend or supplement the senior indenture or the senior debt securities of any series without the consent of any holder:

to convey, transfer, assign, mortgage or pledge any assets as security for the senior debt securities of one or more series;

to evidence the succession of a corporation, limited liability company, partnership or trust to us, and the assumption by such successor of our covenants, agreements and obligations under the senior

indenture or to otherwise comply with the covenant relating to mergers, consolidations and sales of assets;

to comply with requirements of the SEC in order to effect or maintain the qualification of the senior indenture under the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”);

to add to our covenants such new covenants, restrictions, conditions or provisions for the protection of the holders, and to make the occurrence, or the occurrence and continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default;

to cure any ambiguity, defect or inconsistency in the senior indenture or in any supplemental indenture or to conform the senior indenture or the senior debt securities to the description of senior debt securities of such series set forthstatement contained in this prospectus or any applicable prospectus supplement;

to provide for or add guarantors with respect to the senior debt securities of any series;

to establish the form or forms or terms of the senior debt securities as permitted by the senior indenture;

to evidence and provide for the acceptance of appointment under the senior indenture by a successor trustee, or to make such changes as shall be necessary to provide for or facilitate the administration of the trusts in the senior indenture by more than one trustee;

to add to, change or eliminate any of the provisions of the senior indenture in respect of one or more series of senior debt securities, provided that any such addition, change or elimination shall (a) neither (1) apply to any senior debt security of any series created prior to the execution of such supplemental indenture and entitled to the benefit of such provision nor (2) modify the rights of the holder of any such senior debt security with respect to such provision or (b) become effective only when there is no senior debt security described in clause (a)(1) outstanding;

to make any change to the senior debt securities of any series so long as no senior debt securities of such series are outstanding; or

to make any change that does not adversely affect the rights of any holder in any material respect.

Other amendments and modifications of the senior indenture or the senior debt securities issued may be made, and our compliance with any provision of the senior indenture with respect to any series of senior debt securities may be waived, with the consent of the holders of a majority of the aggregate principal amount of the outstanding senior debt securities of each series affected by the amendment or modification (voting as separate series); provided, however, that each affected holder must consent to any modification, amendment or waiver that:

extends the final maturity of any senior debt securities of such series;

reduces the principal amount of any senior debt securities of such series;

reduces the rate, or extends the time for payment of, interest on any senior debt securities of such series;

reduces the amount payable upon the redemption of any senior debt securities of such series;

changes the currency of payment of principal of or interest on any senior debt securities of such series;

reduces the principal amount of original issue discount securities payable upon acceleration of maturity or the amount provable in bankruptcy;

waives a continuing default in the payment of principal of or interest on the senior debt securities (other than any such default in payment resulting solely from an acceleration of the senior debt securities);

changes the provisions relating to the waiver of past defaults or impairs the right of holders to receive payment or to institute suit for the enforcement of any payment or conversion of any senior debt securities of such series on or after the due date therefor;

modifies any of the provisions of these restrictions on amendments and modifications, except to increase any required percentage or to provide that certain other provisions cannot be modified or waived without the consent of the holder of each senior debt security of such series affected by the modification;

adversely affects the right to convert or exchange senior debt securities into common stock or other property in accordance with the terms of the senior debt securities; or

reduces the above-stated percentage of outstanding senior debt securities of such series whose holders must consent to a supplemental indenture or modifies or amends or waives certain provisions of or defaults under the senior indenture.

It shall not be necessary for the holders to approve the particular form of any proposed amendment,subsequently filed supplement or waiver, but it shall be sufficient if the holders’ consent approves the substance thereof. After an amendment, supplement or waiver of the senior indenture in accordance with the provisions described in this section becomes effective, the trustee must give to the holders affected thereby certain notice briefly describing the amendment, supplement or waiver. Any failure by the trustee to give such notice, or any defect therein, shall not, however, in any way impair or affect the validity of any such amendment, supplemental indenture or waiver.

No Personal Liability of Incorporators, Stockholders, Officers, Directors.The senior indenture provides that no recourse shall be had under any obligation, covenant or agreement of ours in the senior indenture or any supplemental indenture, or in any of the senior debt securities or because of the creation of any indebtedness represented thereby, against any of our incorporators, stockholders, officers or directors, past, present or future, or of any predecessor or successor entity thereof under any law, statute or constitutional provision or by the enforcement of any assessment or by any legal or equitable proceeding or otherwise. Each holder, by accepting the senior debt securities, waives and releases all such liability.

Concerning the Trustee.The senior indenture provides that, except during the continuance of an event of default, the trustee will not be liable except for the performance of such duties as are specifically set forth in the senior indenture. If an event of default has occurred and is continuing, the trustee will exercise such rights and powers vested in it under the senior indenture and will use the same degree of care and skill in its exercise as a prudent person would exercise under the circumstances in the conduct of such person’s own affairs.

The senior indenture and the provisions of the Trust Indenture Act incorporated by reference therein contain limitations on the rights of the trustee thereunder, should it become a creditor of ours or any of our subsidiaries, to obtain payment of claims in certain cases or to realize on certain property received by it in respect of any such claims, as security or otherwise. The trustee is permitted to engage in other transactions, provided that if it acquires any conflicting interest (as defined in the Trust Indenture Act), it must eliminate such conflict or resign.

We may have normal banking relationships with the senior trustee in the ordinary course of business.

Unclaimed Funds.All funds deposited with the trustee or any paying agent for the payment of principal, premium, interest or additional amounts in respect of the senior debt securities that remain unclaimed for two years after the date upon which such amounts became due and payable will be repaid to us. Thereafter, any right of any holder of senior debt securities to such funds shall be enforceable only against us, and the trustee and paying agents will have no liability therefor.

Governing Law.The senior indenture and the senior debt securities will be governed by, and construed in accordance with, the internal laws of the State of New York.

Certain Terms of the Subordinated Debt Securities

Other than the terms of the subordinated indenture and subordinated debt securities relating to subordination or otherwise as described in the prospectus supplement relating to a particular series of subordinated debt

securities, the terms of the subordinated indenture and subordinated debt securities are identical in all material respects to the terms of the senior indenture and senior debt securities.

Additional or different subordination terms may be specified in the prospectus supplement applicable to a particular series.

Subordination.The indebtedness evidenced by the subordinated debt securities is subordinate to the prior payment in full of all of our senior indebtedness, as defined in the subordinated indenture. During the continuance beyond any applicable grace period of any default in the payment of principal, premium, interest or any other payment due on any of our senior indebtedness, we may not make any payment of principal of or interest on the subordinated debt securities (except for certain sinking fund payments). In addition, upon any payment or distribution of our assets upon any dissolution,winding-up, liquidation or reorganization, the payment of the principal of and interest on the subordinated debt securities will be subordinated to the extent provided in the subordinated indenture in right of payment to the prior payment in full of all our senior indebtedness. Because of this subordination, if we dissolve or otherwise liquidate, holders of our subordinated debt securities may receive less, ratably, than holders of our senior indebtedness. The subordination provisions do not prevent the occurrence of an event of default under the subordinated indenture.

The term “senior indebtedness” of a person means with respect to such person the principal of, premium, if any, interest on, and any other payment due pursuant to any of the following, whether outstanding on the date of the subordinated indenture or incurred by that person in the future:

all of the indebtedness of that person for money borrowed;

all of the indebtedness of that person evidenced by notes, debentures, bonds or other securities sold by that person for money;

all of the lease obligations that are capitalized on the books of that person in accordance with generally accepted accounting principles;

all indebtedness of others of the kinds described in the first two bullet points above and all lease obligations of others of the kind described in the third bullet point above that the person, in any manner, assumes or guarantees or that the person in effect guarantees through an agreement to purchase, whether that agreement is contingent or otherwise; and

all renewals, extensions or refundings of indebtedness of the kinds described in the first, second or fourth bullet point above and all renewals or extensions of leases of the kinds described in the third or fourth bullet point above;

unless, in the case of any particular indebtedness, renewal, extension or refunding, the instrument creating or evidencing it or the assumption or guarantee relating to it expressly provides that such indebtedness, renewal, extension or refunding is not superior in right of payment to the subordinated debt securities. Our senior debt securities constitute senior indebtedness for purposes of the subordinated indenture.

DESCRIPTION OF COMMON STOCK AND PREFERRED STOCK

General

Our authorized capital stock consists of 200,000,000 shares of common stock, par value $0.001 per share, and 10,000,000 shares of preferred stock, par value $0.001 per share, all of which preferred stock is undesignated. The following description of our capital stock and provisions of our restated certificate of incorporation and amended and restated bylaws are summaries and are qualified by reference to our restated certificate of incorporation and our amended and restated bylaws, copies of which are filed with the SEC as exhibits to our registration statement of which this prospectus forms a part.

Common Stock

Holders of our common stock are entitled to one vote for each share held on all matters submitted to a vote of stockholders and do not have cumulative voting rights. Each election of directors by our stockholders will be determined by a plurality of the votes cast by the stockholders entitled to vote on the election. Holders of common stock are entitled to receive proportionately any dividends as may be declared by our board of directors, subject to any preferential dividend rights of outstanding preferred stock.

In the event of our liquidation or dissolution, the holders of our common stock are entitled to receive proportionately all assets available for distribution to stockholders after the payment of all debts and other liabilities and subject to the prior rights of any of our outstanding preferred stock. Holders of our common stock have nopre-emptive, subscription, redemption or conversion rights. The rights, preferences and privileges of holders of our common stock are subject to and may be adversely affected by the rights of the holders of shares of any series of our preferred stock that we may designate and issue in the future.

Preferred Stock

We are authorized to issue “blank check” preferred stock, which may be issued in one or more series upon authorization of our board of directors. Our board of directors is authorized to fix the designations, powers, preferences and the relative, participating, optional or other special rights and any qualifications, limitations and restrictions of the shares of each series of preferred stock. The authorized shares of our preferred stock are available for issuance without further action by our stockholders, unless such action is required by applicable law or the rules of any stock exchange on which our securities may be listed. If the approval of our stockholders is not required for the issuance of shares of our preferred stock, our board may determine not to seek stockholder approval. The specific terms of any series of preferred stock offered pursuant to this prospectus, will be described in the prospectus supplement relating to that series of preferred stock.

A series of our preferred stock could, depending on the terms of such series, impede the completion of a merger, tender offer or other takeover attempt. Our board of directors will make any determination to issue preferred shares based upon its judgment as to the best interests of our stockholders. Our directors, in so acting, could issue preferred stock having terms that could discourage an acquisition attempt through which an acquirer may be able to change the composition of our board of directors, including a tender offer or other transaction that some, or a majority, of our stockholders might believe to be in their best interests or in which stockholders might receive a premium for their stock over the then-current market price of the stock.

The preferred stock has the terms described below unless otherwise provided in the prospectus supplement relating to a particular series of preferred stock. You should read the prospectus supplement relating to the particular series of preferred stock being offered for specific terms, including:

the designation and stated value per share of the preferred stock and the number of shares offered;

the amount of liquidation preference per share;

the price at which the preferred stock will be issued;

the dividend rate, or method of calculation of dividends, the dates on which dividends will be payable, whether dividends will be cumulative or noncumulative and, if cumulative, the dates from which dividends will commence to accumulate;

any redemption or sinking fund provisions;

if other than the currency of the United States, the currency or currencies including composite currencies in which the preferred stock is denominated and/or in which payments will or may be payable;

any conversion provisions; and

any other rights, preferences, privileges, limitations and restrictions on the preferred stock.

The preferred stock will, when issued, be fully paid andnon-assessable. Unless otherwise specified in the prospectus supplement, each series of preferred stock will rank equally as to dividends and liquidation rights in all respects with each other series of preferred stock. The rights of holders of shares of each series of preferred stock will be subordinate to those of our general creditors.

Rank. Unless otherwise specified in the prospectus supplement, the preferred stock will, with respect to dividend rights and rights upon our liquidation, dissolution or winding up of our affairs, rank:

senior to our common stock and to all equity securities ranking junior to such preferred stock with respect to dividend rights or rights upon our liquidation, dissolution or winding up of our affairs;

on a parity with all equity securities issued by us, the terms of which specifically provide that such equity securities rank on a parity with the preferred stock with respect to dividend rights or rights upon our liquidation, dissolution or winding up of our affairs; and

junior to all equity securities issued by us, the terms of which specifically provide that such equity securities rank senior to the preferred stock with respect to dividend rights or rights upon our liquidation, dissolution or winding up of our affairs.

The term “equity securities” does not include convertible debt securities.

Dividends. Holders of the preferred stock of each series will be entitled to receive, when, as and if declared by our board of directors, cash dividends at such rates and on such dates described in the prospectus supplement. Different series of preferred stock may be entitled to dividends at different rates or based on different methods of calculation. The dividend rate may be fixed or variable or both. Dividends will be payable to the holders of record as they appear on our stock books on record dates fixed by our board of directors, as specified in the applicable prospectus supplement.

Dividends on any series of preferred stock may be cumulative or noncumulative, as described in the applicable prospectus supplement. If our board of directors does not declare a dividend payable on a dividend payment date on any series of noncumulative preferred stock, then the holders of that noncumulative preferred stock will have no right to receive a dividend for that dividend payment date, and we will have no obligation to pay the dividend accrued for that period, whether or not dividends on that series are declared payable on any future dividend payment dates. Dividends on any series of cumulative preferred stock will accrue from the date we initially issue shares of such series or such other date specified in the applicable prospectus supplement.

No dividends may be declared or paid or funds set apart for the payment of any dividends on any parity securities unless full dividends have been paid or set apart for payment on the preferred stock. If full dividends are not paid, the preferred stock will share dividends pro rata with the parity securities.

No dividends may be declared or paid or funds set apart for the payment of dividends on any junior securities unless full dividends for all dividend periods terminating on or prior to the date of the declaration or payment will have been paid or declared and a sum sufficient for the payment set apart for payment on the preferred stock.

Liquidation Preference. Upon any voluntary or involuntary liquidation, dissolution or winding up of our affairs, then, before we make any distribution or payment to the holders of any common stock or any other class or series of our capital stock ranking junior to the preferred stock in the distribution of assets upon any liquidation, dissolution or winding up of our affairs, the holders of each series of preferred stock shall be entitled to receive out of assets legally available for distribution to stockholders, liquidating distributions in the amount of the liquidation preference per share set forth in the prospectus supplement, plus any accrued and unpaid dividends thereon. Such dividends will not include any accumulation in respect of unpaid noncumulative dividends for prior dividend periods. Unless otherwise specified in the prospectus supplement, after payment of the full amount of their liquidating distributions, the holders of preferred stock will have no right or claim to any of our remaining assets. Upon any such voluntary or involuntary liquidation, dissolution or winding up, if our available assets are insufficient to pay the amount of the liquidating distributions on all outstanding preferred stock and the corresponding amounts payable on all other classes or series of our capital stock ranking on parity with the preferred stock and all other such classes or series of shares of capital stock ranking on parity with the preferred stock in the distribution of assets, then the holders of the preferred stock and all other such classes or series of capital stock will share ratably in any such distribution of assets in proportion to the full liquidating distributions to which they would otherwise be entitled.

Upon any such liquidation, dissolution or winding up and if we have made liquidating distributions in full to all holders of preferred stock, we will distribute our remaining assets among the holders of any other classes or series of capital stock ranking junior to the preferred stock according to their respective rights and preferences and, in each case, according to their respective number of shares. For such purposes, our consolidation or merger with or into any other corporation, trust or entity, or the sale, lease or conveyance of all or substantially all of our property or assets will not bedocument deemed to constitute a liquidation, dissolution or winding up of our affairs.

Redemption. If so provided in the applicable prospectus supplement, the preferred stock will be subject to mandatory redemption or redemption at our option, as a whole or in part, in each case upon the terms, at the times and at the redemption prices set forth in such prospectus supplement.

The prospectus supplement relating to a series of preferred stock that is subject to mandatory redemption will specify the number of shares of preferred stock that shall be redeemed by us in each year commencing after a date to be specified, at a redemption price per share to be specified, together with an amount equal to all accrued and unpaid dividends thereon to the date of redemption. Unless the shares have a cumulative dividend, such accrued dividends will not include any accumulation in respect of unpaid dividends for prior dividend periods. We may pay the redemption price in cash or other property, as specified in the applicable prospectus supplement. If the redemption price for preferred stock of any series is payable only from the net proceeds of the issuance of shares of our capital stock, the terms of such preferred stock may provide that, if no such shares of our capital stock shall have been issued or to the extent the net proceeds from any issuance are insufficient to pay in full the aggregate redemption price then due, such preferred stock shall automatically and mandatorily be converted into the applicable shares of our capital stock pursuant to conversion provisions specified in the applicable prospectus supplement. Notwithstanding the foregoing, we will not redeem any preferred stock of a series unless:

if that series of preferred stock has a cumulative dividend, we have declared and paid or contemporaneously declare and pay or set aside funds to pay full cumulative dividends on the preferred stock for all past dividend periods and the then current dividend period; or

if such series of preferred stock does not have a cumulative dividend, we have declared and paid or contemporaneously declare and pay or set aside funds to pay full dividends for the then current dividend period.

In addition, we will not acquire any preferred stock of a series unless:

if that series of preferred stock has a cumulative dividend, we have declared and paid or contemporaneously declare and pay or set aside funds to pay full cumulative dividends on all outstanding shares of such series of preferred stock for all past dividend periods and the then current dividend period; or

if that series of preferred stock does not have a cumulative dividend, we have declared and paid or contemporaneously declare and pay or set aside funds to pay full dividends on the preferred stock of such series for the then current dividend period.

However, at any time we may purchase or acquire preferred stock of that series (1) pursuant to a purchase or exchange offer made on the same terms to holders of all outstanding preferred stock of such series or (2) by conversion into or exchange for shares of our capital stock ranking junior to the preferred stock of such series as to dividends and upon liquidation.

If fewer than all of the outstanding shares of preferred stock of any series are to be redeemed, we will determine the number of shares that may be redeemed pro rata from the holders of record of such shares in proportion to the number of such shares held or for which redemption is requested by such holder or by any other equitable manner that we determine. Such determination will reflect adjustments to avoid redemption of fractional shares.

Unless otherwise specified in the prospectus supplement, we will mail notice of redemption at least 30 days but not more than 60 days before the redemption date to each holder of record of preferred stock to be redeemed at the address shown on our stock transfer books. Each notice shall state:

the redemption date;

the number of shares and series of preferred stock to be redeemed;

the redemption price;

the place or places where certificates for such preferred stock are to be surrendered for payment of the redemption price;

that dividends on the shares to be redeemed will cease to accrue on such redemption date;

the date on which the holder’s conversion rights, if any, as to such shares shall terminate; and

the specific number of shares to be redeemed from each such holder if fewer than all the shares of any series are to be redeemed.

If notice of redemption has been given and we have set aside the funds necessary for such redemption in trust for the benefit of the holders of any shares called for redemption, then from and after the redemption date, dividends will cease to accrue on such shares, and all rights of the holders of such shares will terminate, except the right to receive the redemption price.

Voting Rights. Holders of preferred stock will not have any voting rights, except as required by law or as indicated in the applicable prospectus supplement.

Unless otherwise provided for under the terms of any series of preferred stock, no consent or vote of the holders of shares of preferred stock or any series thereof shall be required for any amendment to our certificate of incorporation that would increase the number of authorized shares of preferred stock or the number of authorized shares of any series thereof or decrease the number of authorized shares of preferred stock or the number of authorized shares of any series thereof (but not below the number of authorized shares of preferred stock or such series, as the case may be, then outstanding).

Conversion Rights. The terms and conditions, if any, upon which any series of preferred stock is convertible into our common stock will be set forth in the applicable prospectus supplement relating thereto. Such terms will include the number of shares of common stock into which the shares of preferred stock are convertible, the conversion price, rate or manner of calculation thereof, the conversion period, provisions as to whether conversion will be at our option or at the option of the holders of the preferred stock, the events requiring an adjustment of the conversion price and provisions affecting conversion in the event of the redemption.

Transfer Agent and Registrar. The transfer agent and registrar for the preferred stock will be set forth in the applicable prospectus supplement.

Delaware Anti-Takeover Law and Certain Charter and Bylaw Provisions

Delaware Law

We are subject to Section 203 of the Delaware General Corporation Law. Subject to certain exceptions, Section 203 prevents a publicly held Delaware corporation from engaging in a “business combination” with any “interested stockholder” for three years following the date that the person became an interested stockholder, unless either the interested stockholder attained such status with the approval of our board of directors, the business combination is approved by our board of directors and stockholders in a prescribed manner or the interested stockholder acquired at least 85% of our outstanding voting stock in the transaction in which it became an interested stockholder. A “business combination” includes, among other things, a merger or consolidation involving us and the “interested stockholder” and the sale of more than 10% of our assets. In general, an “interested stockholder” is any entity or person beneficially owning 15% or more of our outstanding voting stock and any entity or person affiliated with or controlling or controlled by such entity or person.

Staggered Board; Removal of Directors

Our restated certificate of incorporation and amended and restated bylaws divide our board of directors into three classes with staggered three-year terms. In addition, our restated certificate of incorporation and our amended and restated bylaws provide that directors may be removed only for cause and only by the affirmative vote of the holders of 75% of our shares of capital stock present in person or by proxy and entitled to vote. Under our restated certificate of incorporation and amended and restated bylaws, any vacancy on our board of directors, including a vacancy resulting from an enlargement of our board of directors, may be filled only by vote of a majority of our directors then in office. Furthermore, our restated certificate of incorporation provides that the authorized number of directors may be changed only by the resolution of our board of directors. The classification of our board of directors and the limitations on the ability of our stockholders to remove directors, change the authorized number of directors and fill vacancies could make it more difficult for a third party to acquire, or discourage a third party from seeking to acquire, control of our company.

Stockholder Action; Special Meeting of Stockholders; Advance Notice Requirements for Stockholder Proposals and Director Nominations

Our restated certificate of incorporation and amended and restated bylaws provide that any action required or permitted to be taken by our stockholders at an annual meeting or special meeting of stockholders may only be taken if it is properly brought before such meeting and may not be taken by written action in lieu of a meeting. Our restated certificate of incorporation and our amended and restated bylaws also provide that, except as otherwise required by law, special meetings of the stockholders can only be called by the chairman of our board of directors, our chief executive officer or our board of directors. In addition, our amended and restated bylaws establish an advance notice procedure for stockholder proposals to be brought before an annual meeting of stockholders, including proposed nominations of candidates for election to our board of directors. Stockholders at an annual meeting may only consider proposals or nominations specified in the notice of meeting or brought before the meeting by or at the direction of our board of directors, or by a stockholder of record on the record

date for the meeting who is entitled to vote at the meeting and who has delivered timely written notice in proper form to our secretary of the stockholder’s intention to bring such business before the meeting. These provisions could have the effect of delaying until the next stockholder meeting stockholder actions that are favored by the holders of a majority of our outstanding voting securities. These provisions also could discourage a third party from making a tender offer for our common stock because even if the third party acquired a majority of our outstanding voting stock, it would be able to take action as a stockholder, such as electing new directors or approving a merger, only at a duly called stockholders meeting and not by written consent.

Super-Majority Voting

The Delaware General Corporation Law provides generally that the affirmative vote of a majority of the shares entitled to vote on any matter is required to amend a corporation’s certificate of incorporation or bylaws unless a corporation’s certificate of incorporation or bylaws, as the case may be, requires a greater percentage. Our amended and restated bylaws may be amended or repealed by a majority vote of our board of directors or the affirmative vote of the holders of at least 75% of the votes that all our stockholders would be entitled to cast in any annual election of directors. In addition, the affirmative vote of the holders of at least 75% of the votes that all our stockholders would be entitled to cast in any election of directors is required to amend or repeal or to adopt any provisions inconsistent with any of the provisions of our restated certificate of incorporation described above.

Exclusive Forum Selection

Our restated certificate of incorporation provides that, unless we consent in writing to the selection of an alternative forum, the Court of Chancery of the State of Delaware shall be the sole and exclusive forum for (1) any derivative action or proceeding brought on behalf of our company, (2) any action asserting a claim of breach of a fiduciary duty owed by any of our directors, officers, employees or stockholders to our company or our stockholders, (3) any action asserting a claim arising pursuant to any provision of the General Corporation Law of the State of Delaware or as to which the General Corporation Law of the State of Delaware confers jurisdiction on the Court of Chancery of the State of Delaware, or (4) any action asserting a claim arising pursuant to any provision of our restated certificate of incorporation or amended and restated bylaws (in each case, as they may be amended from time to time) or governed by the internal affairs doctrine. Although our restated certificate of incorporation contains the choice of forum provision described above, it is possible that a court could rule that such a provision is inapplicable for a particular claim or action or that such provision is unenforceable.

Registration Rights

Private Placement Registration Rights

Under the terms of a share purchase agreement with New Enterprise Associates 16, L.P., or NEA, we have granted NEA the right to require us to register 2,000,000 shares of our common stock under the Securities Act, at any time, subject to specified limitations set forth in such share purchase agreement. We are not obligated to file a registration statement pursuant to this provision on more than two occasions. After registration pursuant to these rights, the registrable securities will become freely tradable without restriction under the Securities Act.

Expenses

Pursuant to the share purchase agreement, we are required to pay all registration expenses, including all registration, filing and printing fees, issuer counsel and accounting fees and expenses, costs and expenses associated with clearing the shares for sale under applicable Blue Sky laws, listing fees, expenses incurred by us in connection with any “road show,” and reasonable fees, charges and disbursements of counsel to NEA, but excluding underwriting discounts or commissions and fees with respect to the shares being sold.

The share purchase agreement contains customary cross-indemnification provisions, pursuant to which we are obligated to indemnify NEA in the event of material misstatements or omissions in the registration statement attributable to us, and NEA is obligated to indemnify us for material misstatements or omissions in the registration statement attributable to NEA.

Second Amended and Restated Investors’ Rights Agreement

We have entered into a second amended and restated investors’ rights agreement, as amended, with certain holders of our common stock. Under the terms of such investors’ rights agreement, these holders have the right to require us to register their shares under the Securities Act, under specified circumstances. We refer to the shares with these registration rights as registrable securities. After registration pursuant to these rights, the registrable securities will become freely tradable without restriction under the Securities Act.

Demand andForm S-3 Registration Rights

Subject to specified limitations set forth in the investors’ rights agreement, at any time, the holders of at least 25% of the then outstanding registrable securities may demand that we register registrable securities then outstanding under the Securities Act for purposes of a public offering having an aggregate offering price to the public of not less than $10.0 million. We are not obligated to file a registration statement pursuant to this provision on more than two occasions.

In addition, subject to specified limitations set forth in the investors’ rights agreement, at any time after we become eligible to file a registration statement onForm S-3, holders of the registrable securities then outstanding may request that we register their registrable securities onForm S-3 for purposes of a public offering for which the reasonably anticipated aggregate offering price to the public would exceed $1.0 million. We are not obligated to file a registration statement pursuant to this provision on more than two occasions in any12-month period.

Incidental Registration Rights

If, at any time we propose to register for our own account any of our securities under the Securities Act, the holders of registrable securities will be entitled to notice of the registration and, subject to specified exceptions, have the right to require us to use our reasonable best efforts to register all or a portion of the registrable securities then held by them in that registration.

In the event that any registration in which the holders of registrable securities participate pursuant to our investors’ rights agreement is an underwritten public offering, we have agreed to enter into an underwriting agreement in usual and customary form and use our reasonable best efforts to facilitate such offering.

Expenses

Pursuant to the investors’ rights agreement, we are required to pay all registration expenses, including all registration and filing fees, exchange listing fees, printing expenses, fees and expenses of one counsel selected by the selling stockholders to represent the selling stockholders, state Blue Sky fees and expenses and the expense of any special audits incident to or required by any such registration, but excluding underwriting discounts, selling commissions and the fees and expenses of the selling stockholders’ own counsel (other than the counsel selected to represent all selling stockholders).

The investors’ rights agreement contains customary cross-indemnification provisions, pursuant to which we are obligated to indemnify the selling stockholders in the event of material misstatements or omissions in the registration statement attributable to us or any violation or alleged violation whether by action or inaction by us under the Securities Act, the Exchange Act, any state securities or Blue Sky law or any rule or regulation promulgated under the Securities Act, the Exchange Act or any state securities or Blue Sky law in connection with such registration statement or the qualification or compliance of the offering, and they are obligated to indemnify us for material misstatements or omissions in the registration statement attributable to them.

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is Computershare Trust Company, N.A.

Nasdaq Global Market

Our common stock is listed on the Nasdaq Global Market under the symbol “ASNS.”

DESCRIPTION OF UNITS

The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the units that we may offer under this prospectus. While the terms summarized below will apply generally to any units that we may offer, we will describe the particular terms of any series of units in more detail in the applicable prospectus supplement. If we indicate in the prospectus supplement, the terms of any units offered under that prospectus supplement may differ from the terms described below. Specific unit agreements will contain additional important terms and provisions and will be incorporated by reference as an exhibit to the registration statement that includes this prospectus.

General

We may issue units consisting of debt securities, common stock, preferred stock, or warrants for the purchase of debt securities, common stock and/or preferred stock in one or more series, in any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date.

We will describe in the applicable prospectus supplement the terms of the series of units being offered, including:

the designation and terms of the units and of the securities comprising the units, including whether and under what circumstances those securities may be held or transferred separately;

any provisions of the governing unit agreement that differ from those described below; and

any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units.

We may issue units in such amounts and in such numbers of distinct series as we determine.

The provisions described in this section, as well as those described under “Description of Common Stock and Preferred Stock” and “Description of Warrants” will apply to each unit, as applicable, and to any common stock, preferred stock or warrant included in each unit, as applicable.

Unit Agent

The name and address of the unit agent for any units we offer will be set forth in the applicable prospectus supplement.

Enforceability of Rights by Holders of Units

Each unit agent will act solely as our agent under the applicable unit agreement and will not assume any obligation or relationship of agency or trust with any holder of any unit. A single bank or trust company may act as unit agent for more than one series of units. A unit agent will have no duty or responsibility in case of any default by us under the applicable unit agreement or unit, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a unit may, without the consent of the related unit agent or the holder of any other unit, enforce by appropriate legal action its rights as holder under any security included in the unit.

DESCRIPTION OF WARRANTS

We may issue warrants to purchase common stock, preferred stock or debt securities. We may offer warrants separately or together with one or more additional warrants, common stock, preferred stock or debt securities, or any combination of those securities in the form of units, as described in the applicable prospectus supplement. If we issue warrants as part of a unit, the accompanying prospectus supplement will specify whether those warrants may be separated from the other securities in the unit prior to the expiration date of the warrants. The applicable prospectus supplement will also describe the following terms of any warrants:

the specific designation and aggregate number of, and the offering price at which we will issue, the warrants;

the currency or currency units in which the offering price, if any, and the exercise price are payable;

the date on which the right to exercise the warrants will begin and the date on which that right will expire or, if you may not continuously exercise the warrants throughout that period, the specific date or dates on which you may exercise the warrants;

whether the warrants are to be sold separately or with other securities as parts of units;

whether the warrants will be issued in definitive or global form or in any combination of these forms, although, in any case, the form of a warrant included in a unit will correspond to the form of the unit and of any security included in that unit;

any applicable material U.S. federal income tax consequences;

the identity of the warrant agent for the warrants and of any other depositaries, execution or paying agents, transfer agents, registrars or other agents;

the proposed listing, if any, of the warrants or any securities purchasable upon exercise of the warrants on any securities exchange;

the designation and terms of any equity securities purchasable upon exercise of the warrants;

the designation, aggregate principal amount, currency and terms of any debt securities that may be purchased upon exercise of the warrants;

if applicable, the designation and terms of the preferred stock with which the warrants are issued and the number of warrants issued with each security;

if applicable, the date from and after which any warrants issued as part of a unit and the related debt securities, preferred stock or common stock will be separately transferable;

the number of shares of common stock or preferred stock purchasable upon exercise of a warrant and the price at which those shares may be purchased;

if applicable, the minimum or maximum amount of the warrants that may be exercised at any one time;

information with respect to book-entry procedures, if any;

the anti-dilution provisions of, and other provisions for changes to or adjustment in the exercise price of, the warrants, if any;

any redemption or call provisions; and

any additional terms of the warrants, including terms, procedures and limitations relating to the exchange or exercise of the warrants.

FORMS OF SECURITIES

Each debt security, unit and warrant will be represented either by a certificate issued in definitive form to a particular investor or by one or more global securities representing the entire issuance of securities. Unless the applicable prospectus supplement provides otherwise, certificated securities in definitive form and global securities will be issued in registered form. Definitive securities name you or your nominee as the owner of the security, and in order to transfer or exchange these securities or to receive payments other than interest or other interim payments, you or your nominee must physically deliver the securities to the trustee, registrar, paying agent or other agent, as applicable. Global securities name a depositary or its nominee as the owner of the debt securities, units or warrants represented by these global securities. The depositary maintains a computerized system that will reflect each investor’s beneficial ownership of the securities through an account maintained by the investor with its broker/dealer, bank, trust company or other representative, as we explain more fully below.

Global Securities

We may issue the debt securities, units and warrants in the form of one or more fully registered global securities that will be deposited with a depositary or its nominee identified in the applicable prospectus supplement and registered in the name of that depositary or nominee. In those cases, one or more global securities will be issued in a denomination or aggregate denominations equal to the portion of the aggregate principal or face amount of the securities to be represented by global securities. Unless and until it is exchanged in whole for securities in definitive registered form, a global security may not be transferred except as a whole by and among the depositary for the global security, the nominees of the depositary or any successors of the depositary or those nominees.

If not described below, any specific terms of the depositary arrangement with respect to any securities to be represented by a global security will be described in the prospectus supplement relating to those securities. We anticipate that the following provisions will apply to all depositary arrangements.

Ownership of beneficial interests in a global security will be limited to persons, called participants, that have accounts with the depositary or persons that may hold interests through participants. Upon the issuance of a global security, the depositary will credit, on its book-entry registration and transfer system, the participants’ accounts with the respective principal or face amounts of the securities beneficially owned by the participants. Any dealers, underwriters or agents participating in the distribution of the securities will designate the accounts to be credited. Ownership of beneficial interests in a global security will be shown on, and the transfer of ownership interests will be effected only through, records maintained by the depositary, with respect to interests of participants, and on the records of participants, with respect to interests of persons holding through participants. The laws of some states may require that some purchasers of securities take physical delivery of these securities in definitive form. These laws may impair your ability to own, transfer or pledge beneficial interests in global securities.

So long as the depositary, or its nominee, is the registered owner of a global security, that depositary or its nominee, as the case may be, will be considered the sole owner or holder of the securities represented by the global security for all purposes under the applicable indenture, unit agreement or warrant agreement. Except as described below, owners of beneficial interests in a global security will not be entitled to have the securities represented by the global security registered in their names, will not receive or be entitled to receive physical delivery of the securities in definitive form and will not be considered the owners or holders of the securities under the applicable indenture, unit agreement or warrant agreement. Accordingly, each person owning a beneficial interest in a global security must rely on the procedures of the depositary for that global security and, if that person is not a participant, on the procedures of the participant through which the person owns its interest, to exercise any rights of a holder under the applicable indenture, unit agreement or warrant agreement. We understand that under existing industry practices, if we request any action of holders or if an owner of a beneficial interest in a global security desires to give or take any action that a holder is entitled to give or take

under the applicable indenture, unit agreement or warrant agreement, the depositary for the global security would authorize the participants holding the relevant beneficial interests to give or take that action, and the participants would authorize beneficial owners owning through them to give or take that action or would otherwise act upon the instructions of beneficial owners holding through them.

Principal, premium, if any, and interest payments on debt securities, and any payments to holders with respect to warrants or units, represented by a global security registered in the name of a depositary or its nominee will be made to the depositary or its nominee, as the case may be, as the registered owner of the global security. None of us, or any trustee, warrant agent, unit agent or other agent of ours, or any agent of any trustee, warrant agent or unit agent will have any responsibility or liability for any aspect of the records relating to payments made on account of beneficial ownership interests in the global security or for maintaining, supervising or reviewing any records relating to those beneficial ownership interests.

We expect that the depositary for any of the securities represented by a global security, upon receipt of any payment to holders of principal, premium, interest or other distribution of underlying securities or other property on that registered global security, will immediately credit participants’ accounts in amounts proportionate to their respective beneficial interests in that global security as shown on the records of the depositary. We also expect that payments by participants to owners of beneficial interests in a global security held through participants will be governed by standing customer instructions and customary practices, as is now the case with the securities held for the accounts of customers or registered in “street name,” and will be the responsibility of those participants.

If the depositary for any of the securities represented by a global security is at any time unwilling or unable to continue as depositary or ceases to be a clearing agency registered under the Exchange Act, and a successor depositary registered as a clearing agency under the Exchange Act is not appointed by us within 90 days, we will issue securities in definitive form in exchange for the global security that had been held by the depositary. Any securities issued in definitive form in exchange for a global security will be registered in the name or names that the depositary gives to the relevant trustee, warrant agent, unit agent or other relevant agent of ours or theirs. It is expected that the depositary’s instructions will be based upon directions received by the depositary from participants with respect to ownership of beneficial interests in the global security that had been held by the depositary.

PLAN OF DISTRIBUTION

We may sell securities:

through underwriters;

through dealers;

through agents;

directly to purchasers; or

through a combination of any of these methods of sale.

In addition, we may issue the securities as a dividend or distribution or in a subscription rights offering to our existing security holders. This prospectus may be used in connection with any offering of our securities through any of these methods or other methods described in the applicable prospectus supplement.

We may directly solicit offers to purchase securities, or agents may be designated to solicit such offers. We will, in the prospectus supplement relating to such offering, name any agent that could be viewed as an underwriter under the Securities Act, and describe any commissions that we must pay. Any such agent will be acting on a best efforts basis for the period of its appointment or, if indicated in the applicable prospectus supplement, on a firm commitment basis.

The distribution of the securities may be effected from time to time in one or more transactions:

at a fixed price, or prices, which may be changed from time to time;

at market prices prevailing at the time of sale;

at prices related to such prevailing market prices; or

at negotiated prices.

Each prospectus supplement will describe the method of distribution of the securities and any applicable restrictions.

The prospectus supplement with respect to the securities of a particular series will describe the terms of the offering of the securities, including the following:

the name of the agent or any underwriters;

the public offering or purchase price and the proceeds we will receive from the sale of the securities;

any discounts and commissions to be allowed orre-allowed or paid to the agent or underwriters;

all other items constituting underwriting compensation;

any discounts and commissions to be allowed orre-allowed or paid to dealers; and

any exchanges on which the securities will be listed.

If any underwriters or agents are utilized in the sale of the securities in respect of whichinto this prospectus, is delivered, we will enter into an underwriting agreementmodifies or other agreement with them at the time of sale to them, and we will set forth in the prospectus supplement relating tosupersedes such offering the names of the underwriters or agents and the terms of the related agreement with them.

If a dealer is utilized in the sale of the securities in respect of which this prospectus is delivered, we will sell such securities to the dealer, as principal. The dealer may then resell such securities to the public at varying prices to be determined by such dealer at the time of resale.

statement.

If we offer securities in a subscription rights offering to our existing security holders, we may enter into a standby underwriting agreement with dealers, acting as standby underwriters. We may pay the standby underwriters a commitment fee for the securities they commit to purchase on a standby basis. If we do not enter into a standby underwriting arrangement, we may retain a dealer-manager to manage a subscription rights offering for us.

Remarketing firms, agents, underwriters, dealers and other persons may be entitled under agreements which they may enter into with us to indemnification by us against certain civil liabilities, including liabilities under the Securities Act, and may be customers of, engage in transactions with or perform services for us in the ordinary course of business.

If so indicated in the applicable prospectus supplement, we will authorize underwriters or other persons acting as our agents to solicit offers by certain institutions to purchase securities from us pursuant to delayed delivery contracts providing for payment and delivery on the date stated in the prospectus supplement. Each contract will be for an amount not less than, and the aggregate amount of securities sold pursuant to such contracts shall not be less nor more than, the respective amounts stated in the prospectus supplement. Institutions with whom the contracts, when authorized, may be made include commercial and savings banks, insurance companies, pension funds, investment companies, educational and charitable institutions and other institutions, but shall in all cases be subject to our approval. Delayed delivery contracts will not be subject to any conditions except that:

the purchase by an institution of the securities covered under that contract shall not at the time of delivery be prohibited under the laws of the jurisdiction to which that institution is subject; and

12

if the securities are also being sold to underwriters acting as principals for their own account, the underwriters shall have purchased such securities not sold for delayed delivery. The underwriters and other persons acting as our agents will not have any responsibility in respect of the validity or performance of delayed delivery contracts.

Certain agents, underwriters and dealers, and their associates and affiliates may be customers of, have borrowing relationships with, engage in other transactions with, and/or perform services, including investment banking services, for us or one or more of our respective affiliates in the ordinary course of business.

In order to facilitate the offering of the securities, any underwriters may engage in transactions that stabilize, maintain or otherwise affect the price of the securities or any other securities the prices of which may be used to determine payments on such securities. Specifically, any underwriters may overallot in connection with the offering, creating a short position for their own accounts. In addition, to cover overallotments or to stabilize the price of the securities or of any such other securities, the underwriters may bid for, and purchase, the securities or any such other securities in the open market. Finally, in any offering of the securities through a syndicate of underwriters, the underwriting syndicate may reclaim selling concessions allowed to an underwriter or a dealer for distributing the securities in the offering if the syndicate repurchases previously distributed securities in transactions to cover syndicate short positions, in stabilization transactions or otherwise. Any of these activities may stabilize or maintain the market price of the securities above independent market levels. Any such underwriters are not required to engage in these activities and may end any of these activities at any time.

Under Rule



15c6-1
of the Exchange Act, trades in the secondary market generally are required to settle in two business days, unless the parties to any such trade expressly agree otherwise or the securities are sold by us to an underwriter in a firm commitment underwritten offering. The applicable prospectus supplement may provide that the original issue date for your securities may be more than two scheduled business days after the trade date for your securities. Accordingly, in such a case, if you wish to trade securities on any date prior to the second business day before the original issue date for your securities, you will be required, by virtue of the fact that your securities initially are expected to settle in more than two scheduled business days after the trade date for your securities, to make alternative settlement arrangements to prevent a failed settlement.

The securities may be new issues of securities and may have no established trading market. The securities may or may not be listed on a national securities exchange. We can make no assurance as to the liquidity of or the existence of trading markets for any of the securities.

In compliance with the guidelines of the Financial Industry Regulatory Authority, or FINRA, the aggregate maximum discount, commission or agency fees or other items constituting underwriting compensation to be received by any FINRA member or independent broker-dealer will not exceed 8% of the proceeds from any offering pursuant to this prospectus and any applicable prospectus supplement.


LEGAL MATTERS

Unless the applicable prospectus supplement indicates otherwise, the validity of the securities in respect of which this prospectus is being delivered will be passed upon by Wilmer Cutler Pickering Hale and Dorr LLP.

EXPERTS

The financial statements of Arsanis, Inc. incorporated in this prospectus by reference to the Annual Report on Form 10-K of Arsanis, Inc. for the year ended December 31, 2017 have been so incorporated in reliance on the report (which contains an explanatory paragraph relating to the Company’s requirement for additional financing to fund future operations as described in Note 1 to the consolidated financial statements) of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

The financial statements of X4 Pharmaceuticals, Inc. as of December 31, 2017 and 2016 and for the years then ended included in this prospectus have been so included in reliance on the report (which contains an explanatory paragraph relating to the Company’s ability to continue as a going concern as described in Note 1 to the consolidated financial statements) of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.


ARSANIS, INC.

$150,000,000

Debt Securities


1,666,666 Shares

image_3a.jpg
Common Stock

PROSPECTUS

Preferred Stock

Units

Warrants

PROSPECTUS

, 2019






PART II


INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14.    Other Expenses of Issuance and Distribution.

SetDistribution

The following table sets forth below are estimates (except in the case of the SEC registration fee and the FINRA filing fee) of the amount of fees andvarious expenses to be incurred in connection with the issuancesale and distribution of the offered securities other thanbeing registered hereby, all of which will be borne by the Registrant (except any underwriting discounts and commissions.

SEC registration fee

  $18,180 

FINRA filing fee

   23,000 

Printing and engraving

       (1)  

Accounting services

       (1)  

Legal fees of registrant’s counsel

       (1)  

Transfer agent’s, trustee’s and depositary’s fees and expenses

       (1)  

Miscellaneous

       (1)  
  

 

 

 

Total

  $ (1)  
  

 

 

 

(1)

These fees and expenses are calculated based on the securities offered and the number of issuances and accordingly are not estimated at this time and will be reflected in the applicable prospectus supplement.

commissions and expenses incurred by the Selling Stockholder for brokerage, accounting, tax or legal services or any other expenses incurred by the Selling Stockholder in disposing of the shares). All amounts shown are estimates except the SEC registration fee.

SEC registration fee  $220.94 
Accountants’ fees and expenses   15,000.00 
Legal fees and expenses   30,000.00 
Miscellaneous expenses   -
     
Total  $45,220.94 
     
Item 15.    Indemnification of DirectorsOfficers and Officers.

Directors

Subsection (a) of Section 145 of the General Corporation Law of the State of Delaware (“DGCL”(the “DGCL”) provides, generally, that, empowers a corporation shall have the power to indemnify any person who was or is a party or who is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, (except actionswhether civil, criminal, administrative or investigative (other than an action by or in the right of the corporation) by reason of the fact that suchthe person is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against all expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by suchthe person in connection with such action, suit or proceeding if suchthe person acted in good faith and in a manner suchthe person reasonably believed to be in or not opposed to the best interests of the corporation, and, with respect to any criminal action or proceeding, had no reasonable cause to believe his or herthe person’s conduct was unlawful. A
Subsection (b) of Section 145 empowers a corporation may similarlyto indemnify suchany person for expenses actually and reasonably incurred by such person in connection with the defensewho was or settlement ofis a party or is threatened to be made a party to any threatened, pending or completed action or suit by or in the right of the corporationprovided to procure a judgment in its favor by reason of the fact that the person acted in any of the capacities set forth above, against expenses (including attorneys’ fees) actually and reasonably incurred by the person in connection with the defense or settlement of such action or suit if the person acted in good faith and in a manner he or shethe person reasonably believed to be in or not opposed to the best interests of the corporation, and,except that no indemnification shall be made in the caserespect of claims, issues and mattersany claim, issue or matter as to which such person shall have been adjudged to be liable to the corporationprovided unless and only to the extent that athe Court of Chancery or the court in which such action or suit was brought shall have determined,determine upon application that, despite the adjudication of liability but in view of all of the circumstances of the case, such person is fairly and reasonably entitled to indemnity for such expenses which the Court of Chancery or such other court shall deem proper.

Section 145 further provides that to the extent a director or officer of a corporation has been successful on the merits or otherwise in the defense of any action, suit or proceeding referred to in subsections (a) and (b) of Section 145, or in defense of any claim, issue or matter therein, such person shall be indemnified against expenses (including attorneys’ fees) actually and reasonably incurred by such person in connection therewith; that indemnification provided for by Section 145 shall not be deemed exclusive of any other rights to which the indemnified party may be entitled; and the indemnification provided for by Section 145 shall, unless otherwise provided when authorized or ratified, continue as to a person who has ceased to be a director, officer, employee or agent and shall inure to the benefit of such person’s heirs, executors and administrators. Section 145 also empowers the corporation to purchase and maintain insurance on behalf of any person who is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise against any liability
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asserted against such person and incurred by such person in any such capacity, or arising out of his status as such, whether or not the corporation would have the power to indemnify such person against such liabilities under Section 145.
Section 102(b)(7) of the DGCL provides generally, that oura corporation’s certificate of incorporation may contain a provision eliminating or limiting the personal liability of a director to the corporation or its stockholders for monetary damages for breach of fiduciary duty as a director,provided that such provision mayshall not eliminate or limit the liability of a director (i) for any breach of the director’s duty of loyalty to the corporation or its stockholders, (ii) for acts or omissions not in good faith or which involve intentional misconduct or a knowing violation of law, (iii) under sectionSection 174 of the DGCL or (iv) for any transaction from which the director derived an improper personal benefit. No such provision may eliminate or limit the liability of a director for any act or omission occurring prior to the date when such provision became effective.

Our restated certificate of incorporation provides that we will indemnify each person who was or is a party or threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative (other than an action by or in the right of us), by reason of the fact

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that he or she is or was, or has agreed to become, our director or officer, or is or was serving, or has agreed to serve, at our request as a director, officer, partner, employee or trustee of, or in a similar capacity with, another corporation, partnership, joint venture, trust or other enterprise (all such persons being referred to as an Indemnitee), or by reason of any action alleged toWe have been taken or omitted in such capacity, against all expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred in connection with such action, suit or proceeding and any appeal therefrom if such Indemnitee acted in good faith and in a manner he or she reasonably believed to be in, or not opposed to, our best interests, and, with respect to any criminal action or proceeding, he or she had no reasonable cause to believe his or her conduct was unlawful.

Our restated certificate of incorporation also provides that we will indemnify any Indemnitee who was or is a party to an action or suit by or in the right of us to procure a judgment in our favor by reason of the fact that the Indemnitee is or was, or has agreed to become, our director or officer, or is or was serving, or has agreed to serve, at our request as a director, officer, partner, employee or trustee of, or in a similar capacity with, another corporation, partnership, joint venture, trust or other enterprise, or by reason of any action alleged to have been taken or omitted in such capacity, against all expenses (including attorneys’ fees) and, to the extent permitted by law, amounts paid in settlement actually and reasonably incurred in connection with such action, suit or proceeding and any appeal therefrom, if the Indemnitee acted in good faith and in a manner he or she reasonably believed to be in, or not opposed to, our best interests, except that no indemnification shall be made with respect to any claim, issue or matter as to which such person shall have been adjudged to be liable to us, unless a court determines that, despite such adjudication but in view of all of the circumstances, he or she is entitled to indemnification of such expenses. Notwithstanding the foregoing, to the extent that any Indemnitee has been successful, on the merits or otherwise, he or she will be indemnified by us against all expenses (including attorneys’ fees) actually and reasonably incurred by him or her or on his or her behalf in connection therewith. If we do not assume the defense, expenses must be advanced to an Indemnitee under certain circumstances.

We haveentered into indemnification agreements with each of our directors and executive officers, in addition to the indemnification provided for in our restated certificate, of incorporation and intend to enter into indemnification agreements with any new directors in the future.

We have purchased and intend to maintain insurance on behalf of any person who is or was a director or officer of X4 Pharmaceuticals against any loss arising from any claim asserted against him or her and incurred by him or her in any such capacity, subject to certain exclusions. Certain of ournon-employee directors may, through their relationships with their employers, be insured and/or indemnified against certain liabilities in their capacity as members of our board of directors.

The underwriting agreement

On March 13, 2019, we entered intocompleted our business combination with X4 Therapeutics, Inc. (formerly X4 Pharmaceuticals, Inc.) in connectionaccordance with the initial public offeringterms of our common stock provides that the underwriters will indemnify, under certain conditions, our directors and officers (as well as certain other persons) against certain liabilities arising in connection with such offering.

We have entered into an Agreement and Plan of Merger, with X4 Pharmaceuticals, Inc. (“X4”), dated as of November 26, 2018, andas amended as ofon December 20, 2018 and March 8, 2019 (the “Merger Agreement”), containing the termsthat we entered into with X4 Therapeutics, Inc. and conditions of the proposed business combination of us and X4. Under the Merger Agreement, Artemis AC Corp., a Delaware corporation and our wholly owned subsidiary (“Merger Sub”), is expected. Pursuant to mergethe terms of the Merger Agreement, Merger Sub merged with and into X4 Therapeutics, Inc., with X4 Therapeutics, Inc. continuing as our wholly owned subsidiary and the surviving corporation of the merger, (the “Merger”). The Merger is currently expectedwhich transaction we refer to close inas the first quarter of 2019, subject to the approval of the stockholders of each company and the satisfaction or waiver of other customary conditions.

Merger.

Under the Merger Agreement and subject to applicable law, and assuming the Merger closes, from the effective time of the Merger (the “Effective Time”) through the sixth anniversary of the date on which the Effective Time occurs, we and the corporation surviving the Merger between X4 Therapeutics, Inc. and Merger Sub (the “Surviving Corporation”) shall each, jointly and severally, indemnify and hold harmless each person who was, as of the date

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of the Merger Agreement (November 26, 2018), or has been at any time prior to such date, or who becomesbecame prior to the Effective Time, a director or officer of our company, X4 Therapeutics, Inc. or any of our or X4’sX4 Therapeutics, Inc.’s respective subsidiaries against all claims, losses, liabilities, damages, judgments, fines and reasonable fees, costs and expenses, including attorneys’ fees and disbursements, incurred in connection with any claim, action, suit, proceeding or investigation, whether civil, criminal, administrative or investigative, arising out of or pertaining to the fact that such person is or was a director, officer, employee or agent of our company, X4 Therapeutics, Inc., or any of our or X4’sX4 Therapeutics, Inc.’s respective subsidiaries (or, while a director or officer of our company, X4 Therapeutics, Inc. or any of our or X4’sX4 Therapeutics, Inc.’s respective subsidiaries, is or was serving at the request of us, X4 Therapeutics, Inc. or any of our or X4’sX4 Therapeutics, Inc.’s respective subsidiaries as a director, officer, employee or agent of another person), whether asserted or claimed prior to, at or after the Effective Time, in each case, to the fullest extent permitted by applicable law. Subject to applicable law, each such director and officer will be entitled to advancement of expenses (including attorneys’ fees) incurred in the defense of any such claim, action, suit, proceeding or investigation from each of us and the Surviving Corporation following receipt by us or the Surviving Corporation from such indemnified person of a request therefor; provided that any such person to whom expenses are advanced provides an undertaking, to the extent then required by the DGCL or the applicable governing document of us or the Surviving Corporation, as applicable, to repay such advances if it is ultimately determined that such person is not entitled to indemnification.

Further, pursuant to the Merger Agreement, for a period of six years following the closing of the Merger (the “Closing”), the provisions of the Surviving Corporation’s certificate of incorporation and bylawsby-laws will contain
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provisions at least as favorable as the provisions relating to the indemnification, advancement of expenses and elimination of liability for monetary damages set forth in the certificate of incorporation and bylawsby-laws of X4 Therapeutics, Inc. and us immediately before the Effective Time.

Pursuant to the Merger Agreement, we agreed to either (A) maintain in effect for six years after the Closing, our existing directors’ and officers’ insurance policies in place as of the date of the Merger Agreement (November 26, 2018), or (B) prior to the Closing, purchase asix-year “tail” policy under its own existing directors’ and officers’ liability insurance policy, with an effective date as of the Closing (provided, that we may substitute therefor a policy of at least the same coverage containing terms and conditions that are not less favorable in any material respect). In no event, however, will we be required to expend for an annual premium for such policies more than an amount equal to 200% of the annual premiums paid by us for such insurance as of the date of the Merger Agreement. During the term of the “tail” policy, however, neither we nor the Surviving Corporation may take any action following the Closing to cause such “tail” policy to be cancelled or any of its provisions to be amended or waived in any manner that would adversely affect in any material respect the rights of their former and current officers and directors, their heirs or representatives.

From and after the Effective Time, we agreed to pay all expenses, including reasonable attorneys’ fees, that may be incurred by indemnified persons in connection with their successful enforcement of the rights provided to such persons in the Merger Agreement. All rights to exculpation, indemnification and advancement of expenses for acts or omissions occurring at or prior to the Effective Time, whether asserted or claimed prior to, at or after the Effective Time, existing in favor of the current or former directors, officers or employees, as the case may be, of our company, X4 Therapeutics, Inc. or any of our or X4’sX4 Therapeutics, Inc.’s respective subsidiaries as provided in their respective certificates of incorporation orby-laws or other organization documents or in any agreement in existence immediately before the Effective Time, will survive the Merger and continue in full force and effect. The director and officer indemnification provisions of the Merger Agreement are intended to be in addition to the rights otherwise available to the current and former officers and directors of our company and X4 Therapeutics, Inc. by law, charter, statute, bylaw or agreement, and shall operate for the benefit of, and will be enforceable by, each of such indemnified persons, their heirs and their representatives.

In the event we or the Surviving Corporation or any of our or its respective successors or assigns (i) consolidates with or merges into any other person and is not be the continuing or surviving corporation or entity of such consolidation or merger, or (ii) transfers all or substantially all of its properties and assets to any person, then, and in each such case, proper provision will be made so that the successors and assigns of us or the

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Surviving Corporation, as the case may be, will succeed to the indemnification obligations set forth in the Merger Agreement. We will cause the Surviving Corporation to perform all of the director and officer indemnification obligations of the Surviving Corporation under the Merger Agreement.

Insofar as the forgoing provisions permit indemnification of directors, executive officers, or persons controlling us for liability arising under the Securities Act of 1933, as amended or the Securities Act,(the “Securities Act”), we have been informed that, in the opinion of the Securities and Exchange Commission, such indemnification is against public policy as expressed in the Securities Act and is therefore unenforceable.

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Item 16.    Exhibits.

Exhibits and Financial Statement Schedules

Exhibit No.

Number
  

Description

of Document
1.1*  3.1  Form of Underwriting Agreement
2.1†Agreement and Plan of Merger, dated November 26, 2018, by and among the Registrant, Artemis AC Corp. and X4 Pharmaceuticals, Inc. (incorporated by reference to Exhibit 2.1 to the Registrant’s Current Report on Form 8-K filed with the SEC on November 27, 2018 (File No. 001-38295))
2.2First Amendment to Agreement and Plan of Merger, dated December 20, 2018, by and among the Registrant, Artemis AC Corp. and X4 Pharmaceuticals, Inc. (incorporated by reference to Exhibit 2.1 to the Registrant’s Current Report on Form 8-K filed with the SEC on December 20, 2018 (File No. 001-38295))
4.1
4.2  3.2  
4.3  4.1  Reference is made to Exhibits 3.1 and 3.2.
  4.2
4.4*  4.3  
4.5  4.4  
4.6  5.1  
4.710.1  
4.8Form of Subordinated Note
4.9*Form of Warrant Agreement
4.10*Form of Unit Agreement
5.1Opinion of Wilmer Cutler Pickering Hale and Dorr LLP
23.1  
23.2  
23.3Consent of Wilmer Cutler Pickering Hale and DorrCooley LLP (included in Exhibit 5.1).

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Exhibit No.

Description

24.1  
25.1**Statement of Eligibility on FormT-1 under the Trust Indenture Act of 1939, as amended, of the Trustee under the Senior Indenture
25.2**107Statement of Eligibility on Form
T-1Filing Fee Table under the Trust Indenture Act of 1939, as amended,
Item 17.    Undertakings
The undersigned registrant hereby undertakes:
(a)    (1) To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:
(i)    To include any prospectus required by Section 10(a)(3) of the Securities Act;
(ii)    To reflect in the prospectus any facts or events arising after the effective date of the Trustee under the Subordinated Indenture99.1Consolidated Financial Statements of X4 Pharmaceuticals, Inc.99.2Unaudited Pro Forma Combined Financial Statements of Arsanis, Inc. and X4 Pharmaceuticals, Inc.

*

To be filed by amendment or by a Current Report on Form8-K.

**

To be filed pursuant to Section 305(b)(2) of the Trust Indenture Act of 1939.

All schedules (or similar attachments) have been omitted from this filing pursuant to Item 601(b)(2) of Regulation S-K. The Registrant will furnish copies of any schedules to the Securities and Exchange Commission upon request.

Item 17. Undertakings.

The undersigned Registrant hereby undertakes:

(a)(1)

To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i)

to include any prospectus required by Section 10(a)(3) of the Securities Act of 1933, as amended (the “Securities Act”);

(ii)

to reflect in the prospectus any facts or events arising after the effective date of this registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum

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offering range may be reflected in the form of prospectus filed with the SEC pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement; and
(iii)    To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;
provided, however, that the undertakings set forth in this registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement; and

(iii)

to include any material information with respect to the plan of distribution not previously disclosed in this registration statement or any material change to such information in this registration statement;

provided,however, that paragraphs (a)(1)(i), (a)(1)(ii) and (a)(1)(iii) above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the CommissionSEC by the Registrantregistrant pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), that are incorporated by reference in this registration statement or isare contained in a form of prospectus filed pursuant to Rule 424(b) that is part of this registration statement.

(2)

That, for the purposes of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at the time shall be deemed to be the initialbona fide offering thereof.

(3)

To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(2)    That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.
(3)    To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.
(4)    That, for the purpose of determining liability under the Securities Act to any purchaser:
(i)    Each prospectus filed by the registrant pursuant to Rule 424(b)(3) shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and
(ii)    Each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering made pursuant to Rule 415(a)(1)(i), (vii), or (x) for the purpose of providing the information required by Section 10(a) of the Securities Act shall be deemed to be part of and included in the registration statement as of the earlier of the date such form of prospectus is first used after effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which that prospectus relates, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such effective date.
(b)    The Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the registrant’s annual report pursuant to Section 13(a) or 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.
(c)    Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

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(4)

That, for the purpose of determining liability under the Securities Act to any purchaser:

(i)

each prospectus filed by the Registrant pursuant to Rule 424(b)(3) shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and

(ii)

each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering made pursuant to Rule 415(a)(1)(i), (vii) or (x) for the purpose of providing the information required by Section 10(a) of the Securities Act of 1933 shall be deemed to be part of and included in the registration statement as of the earlier of the date such form of prospectus is first used after effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which that prospectus relates, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof;provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such effective date.

(5)

That, for the purpose of determining liability of the Registrant under the Securities Act to any purchaser in the initial distribution of the securities, the undersigned Registrant undertakes that in a primary offering of securities of the undersigned Registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned Registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

(i)

any preliminary prospectus or prospectus of the undersigned Registrant relating to the offering required to be filed pursuant to Rule 424;

(ii)

any free writing prospectus relating to the offering prepared by or on behalf of the undersigned Registrant or used or referred to by the undersigned Registrant;

(iii)

the portion of any other free writing prospectus relating to the offering containing material information about the undersigned Registrant or its securities provided by or on behalf of the undersigned Registrant; and

(iv)

any other communication that is an offer in the offering made by the undersigned Registrant to the purchaser.

(6)

That, for purposes of determining any liability under the Securities Act:

(i)

the information omitted from the form of prospectus filed as part of the registration statement in reliance upon Rule 430A and contained in the form of prospectus filed by the Registrant pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to be part of the registration statement as of the time it was declared effective; and

(ii)

each post-effective amendment that contains a form of prospectus shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof.

(b)

The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the Registrant’s annual report pursuant to Section 13(a) or 15(d) of

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the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in this registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof.

(c)

If applicable, the undersigned Registrant hereby undertakes to supplement the prospectus, after the expiration of the subscription period, to set forth the results of the subscription offer, the transaction by the underwriters during the subscription period, the amount of unsubscribed securities to be purchased by the underwriters, and the terms of any subsequent reoffering thereof. If any public offering by the underwriters is to be made on terms differing from those set forth on the cover page of the prospectus, a post-effective amendment will be filed to set forth the terms of such offering.

(d)

The undersigned Registrant hereby undertakes to file an application for the purpose of determining the eligibility of the trustee to act under subsection (a) of Section 310 of the Trust Indenture Act in accordance with the rules and regulations prescribed by the Commission under Section 305(b)(2) of the Trust Indenture Act.

(e)

Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the Registrant pursuant to the indemnification provisions described herein, or otherwise, the Registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the Registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

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SIGNATURES


SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing onForm S-3 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Waltham,Boston, Commonwealth of Massachusetts, on the 25ththis 22nd day of January, 2019.

April, 2022.
ARSANIS,X4 PHARMACEUTICALS, INC.
By: /s/ Michael P. GrayPaula Ragan
 Name: Michael P. GrayPaula Ragan, Ph.D.
 Title: President and Chief Executive Officer Chief Financial Officer

SIGNATURES AND

POWER OF ATTORNEY

We, the undersigned officers

KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and directors of Arsanis, Inc., herebyappoints Paula Ragan, Ph.D. and Adam S. Mostafa, jointly and severally, constitute and appoint Michael P. Gray ouras his or her true and lawful attorneyagent, proxy and attorneys-in-fact, with full power toof substitution and resubstitution, for him to sign for usor her and in our nameshis or her name, place and stead, in any and all capacities, to (i) act on, sign and file with the capacities indicated below the Registration Statement on FormS-3 filed herewithSecurities and Exchange Commission any and all amendments (including post-effective amendments) to said Registration Statement, and anythis registration statement filed pursuant to Rule 462 under the Securities Act of 1933, as amended, in connection with said Registration Statement, and to file or cause to be filed the same,together with all schedules and exhibits thereto, (ii) act on, sign and file such certificates, instruments, agreements and other documents as may be necessary or appropriate in connection therewith, with the Securities(iii) act on and Exchange Commission,file any supplement to any prospectus included in this registration statement or any such amendment, and generally to do all such things in our name and on our behalf in our capacities as officers and directors to enable Arsanis, Inc. to comply with the provisions of the Securities Act of 1933, as amended,(iv) take any and all requirements of the Securitiesactions which may be necessary or appropriate to be done, as fully for all intents and Exchange Commission,purposes as he or she might or could do in person, hereby approving, ratifying and confirming all that said attorney,such agent, proxy and attorney-in-fact or any of his substitute or substitutes shallmay lawfully do or cause to be done by virtue hereof.

thereof.

Pursuant to the requirements of the Securities Act, of 1933, as amended, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

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SignatureTitleDate
/s/ Paula Ragan
Paula Ragan, Ph.D.
President, Chief Executive Officer and Director (Principal Executive Officer)
April 22, 2022
/s/ Adam S. Mostafa
Adam S. Mostafa
Chief Financial Officer and Treasurer (Principal Financial Officer and Principal Accounting Officer)
April 22, 2022
/s/ Michael S. Wyzga
Michael S. Wyzga
Chairman of the Board of DirectorsApril 22, 2022
/s/ William E. Aliski
William E. Aliski
DirectorApril 22, 2022
/s/ Gary J. Bridger
Gary J. Bridger, Ph.D.
DirectorApril 22, 2022

Signature


/s/ Françoise de Craecker
Françoise de Craecker
  

Title


Director
 

Date


April 22, 2022

/s/ Michael P. Gray

Michael P. Gray

Alison Lawton
Alison Lawton
  President and Chief Executive Officer, Chief Financial Officer, Director (Principal Executive Officer and Principal Financial Officer) January 25, 2019April 22, 2022

/s/ Tillman U. Gerngross

Tillman U. Gerngross

David McGirr
David McGirr, MBA
  Chairman of the BoardDirector January 25, 2019April 22, 2022

/s/ William Clark

William Clark

Murray W. Stewart
Murray W. Stewart, M.D.
  Director January 25, 2019

/s/ Carl Gordon

Carl Gordon

DirectorJanuary 25, 2019

/s/ David McGirr

David McGirr

DirectorJanuary 25, 2019

/s/ Terrance McGuire

Terrance McGuire

DirectorJanuary 25, 2019

/s/ Claudio Nessi

Claudio Nessi

DirectorJanuary 25, 2019April 22, 2022

II-8


Signature

Title

Date

/s/ Michael Ross

Michael Ross

DirectorJanuary 25, 2019

/s/ René Russo

René Russo

DirectorJanuary 25, 2019

/s/ Amy Schulman

Amy Schulman

DirectorJanuary 25, 2019

II-9

II-7