Gyre Therapeutics (GYRE) 424B5 Prospectus supplement for primary offering

The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been declared effective by the Securities and Exchange Commission. The preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and we are not soliciting offers to buy these securities in any state or other jurisdiction where the offer or sale is not permitted.

Filed Pursuant to Rule 424(b)(5)
Registration Numbers: 333-228970

Subject to completion, dated June 17, 2020

PRELIMINARY PROSPECTUS SUPPLEMENT

(To Prospectus dated February 14, 2019)

            Shares

Common Stock

We are offering                  shares of our common stock in this offering.

Our common stock trades on the Nasdaq Capital Market under the symbol “CBIO.” On June 16, 2020, the last reported sale price of the common stock on the Nasdaq Capital Market was $7.87 per share.

Investing in our securities involves a high degree of risk. Before making an investment decision, please read the information under the heading “Risk Factors” beginning on pageS-8 of this prospectus supplement and in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus.

	Per Share	Total
Public offering price	$	$
Underwriting discounts and commissions(1)	$	$
Proceeds, before expenses, to us	$	$

(1) See “Underwriting” beginning on page S-15 of this prospectus supplement for a description of the compensation payable to the underwriters, including reimbursement of certain expenses.

We have granted the underwriters an option for a period of 30 days from the date of this prospectus supplement to purchase up to                 additional shares of our common stock at the public offering price less underwriting discounts and commissions. If the underwriters exercise the option in full, the total underwriting discounts and commissions payable by us will be $        and the total proceeds to us, before expenses, will be $        . See “Underwriting” for more information.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense.

The underwriters expect to deliver the shares of common stock against payment on or about                , 2020.

Sole Book-Running Manager

Raymond James

Lead Manager

National Securities Corporation

The date of this prospectus supplement is                 , 2020

TABLE OF CONTENTS

PROSPECTUS SUPPLEMENT

	Page
ABOUT THIS PROSPECTUS SUPPLEMENT		S-1
PROSPECTUS SUPPLEMENT SUMMARY		S-2
THE OFFERING		S-6
RISK FACTORS		S-8
CAUTIONARY NOTE REGARDING FORWARD LOOKING INFORMATION		S-11
USE OF PROCEEDS		S-12
DILUTION		S-13
UNDERWRITING		S-15
LEGAL MATTERS		S-22
EXPERTS		S-22
WHERE YOU CAN FIND MORE INFORMATION		S-22
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE		S-23
PROSPECTUS
	Page
ABOUT THIS PROSPECTUS		1
OUR COMPANY		2
CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS		3
RISK FACTORS		5
USE OF PROCEEDS		5
DESCRIPTION OF CAPITAL STOCK		6
ADDITIONAL INFORMATION CONCERNING OUR CAPITAL STOCK		11
DESCRIPTION OF DEBT SECURITIES		14
DESCRIPTION OF WARRANTS		21
DESCRIPTION OF UNITS		23
PLAN OF DISTRIBUTION		25
LEGAL MATTERS		27
EXPERTS		27
WHERE YOU CAN FIND MORE INFORMATION		28

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is the prospectus supplement, including the documents incorporated by reference herein, which describes the specific terms of this offering and also adds to and updates the information contained in the accompanying prospectus and the documents incorporated by reference therein. The second part, the accompanying prospectus, including the documents incorporated by reference therein, provides more general information, some of which may not apply to this offering. Generally, when we refer to this prospectus, we are referring to both parts of this document combined. Before you invest, you should carefully read this prospectus supplement, the accompanying prospectus and the information incorporated by reference herein and therein, as well as the additional information described in this prospectus supplement under “Where You Can Find More Information” and “Incorporation of Certain Information By Reference.” This prospectus supplement may add, update or change information contained in the accompanying prospectus. To the extent that any statement we make in this prospectus supplement is inconsistent with statements made in the accompanying prospectus or any documents incorporated by reference therein, the statements made in this prospectus supplement will be deemed to modify or supersede those made in the accompanying prospectus and such documents incorporated by reference therein. However, if any statement in one of these documents is inconsistent with a statement in another document with a later date that is incorporated by reference herein, the statement in the document having the later date modifies and supersedes the earlier statement.

Neither we nor the underwriters have authorized anyone to provide you with any information or to make any representation, other than those contained or incorporated by reference in this prospectus supplement and the accompanying prospectus, which together we sometimes refer to generally as the prospectus, or in any free writing prospectus prepared by us or on our behalf or to which we have referred you. Neither we nor the underwriters take any responsibility for, and provide no assurance as to the reliability of, any other information that others may give you. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference in this prospectus supplement and the accompanying prospectus, and any free writing prospectus is accurate only as of the date of those respective documents. Our business, financial condition, results of operations and prospects may have changed since those dates.

We are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement must inform themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement outside the United States. This prospectus supplement does not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

Unless the context indicates otherwise, as used in this prospectus supplement and the accompanying prospectus, the terms “Company,” “Catalyst,” “we,” “us” and “our” refer to Catalyst Biosciences, Inc., a Delaware corporation, and its subsidiaries, taken as a whole, unless otherwise noted.

This prospectus supplement, the accompanying prospectus and the information incorporated herein and therein by reference includes trademarks, service marks and trade names owned by us or other companies. All trademarks, service marks and trade names included or incorporated by reference into this prospectus supplement or the accompanying prospectus are the property of their respective owners

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PROSPECTUS SUPPLEMENT SUMMARY

Company Overview

We are a fully integrated research and clinical development biopharmaceutical company with expertise in protease engineering, discovery, translational research, clinical development, and manufacturing. Currently, we are focused on advancing our protease product candidates in the fields of hemostasis and complement regulation. One of our key competitive advantages is that our systemically dosed product candidates can be delivered subcutaneously (SQ), due to the improvements we have made using our protease engineering platform. SQ dosing is less invasive, more convenient and potentially more efficacious than intravenous (IV) drugs currently on the market. Our SQ product candidates demonstrate prolonged duration of effect enabling them to provide constant or protracted therapeutic levels. In addition, these improvements allow our protease drug candidates to be dosed less frequently in ocular indications and at lower viral dose levels for gene therapy applications.

The following table summarizes our current development programs.

We are experiencing operational and other challenges as a result of the novel coronavirus disease(COVID-19) global pandemic, which could delay or halt our development programs. See “—Recent Developments” and “Risk Factors” for further discussion of the current and expected impact on our business and development programs.

We are currently focusing on the clinical development of MarzAA for Hemophilia A and B and DalcA for Hemophilia B. MarzAA is ‘Phase 3 ready’ and we recently completed a Phase 2b study of DalcA. Both product candidates have received orphan drug designation in the United States and in the European Union: MarzAA for routine prophylaxis to prevent bleeding episodes in individuals with Hemophilia A and B with inhibitors and DalcA for routine prophylaxis to prevent bleeding episodes for Hemophilia B patients. We believe MarzAA may also provide significanton-demand and prophylaxis benefits for other therapeutic areas with high unmet medical needs, such as Factor VII Deficiency, Glanzmann Thrombasthenia, Acquired Hemophilia, and SQ treatment of bleeds in Hemophilia A patients

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We control worldwide development, manufacturing and commercialization rights of both MarzAA and DalcA, except for the commercialization rights of DalcA in South Korea. We estimate the global market opportunity for MarzAA and DalcA to be approximately $4.0 billion: $2.2 billion for the Factor VIIa market and $1.8 billion for the Factor IX (FIX) market.

Moreover, we have an early stage FIX gene therapy construct—CB2679d-GT—for Hemophilia B. This gene therapy product candidate has demonstrated2-fold to3-fold higher activity compared with the Padua variant of FIX resulting in improved clotting time in a preclinical Hemophilia B mouse model. Fidanacogene elaparvovec (Pfizer/Spark),AMT-061 (uniQure) and FLT180A (Freeline) use the Padua variant as the transgene in theirAAV-based gene therapy clinical programs. Fidanacogene elaparvovec,AMT-061 and FLT180A have demonstrated encouraging FIX levels in their respective Phase 1/2 and Phase 2/3 studies with median FIX activity levels of approximately30-45%. We have licensed AAV technology from The Board of Trustees of The Leland Stanford Junior University (Stanford) and are currently optimizing the vector under a sponsored research agreement with Stanford.

Using our protein engineering platform, we have also developed a novelanti-C3 protease for dryAge-related Macular Degeneration (AMD)—CB2782-PEG. Dry AMD is estimated to have a $5.0 billion market opportunity with no approved drugs on the market. We entered into an exclusive worldwide license and collaboration agreement with Biogen International GmbH (Biogen) on December 18, 2019 to develop and commercialize CB2782-PEG and our otheranti-C3 proteases for the potential treatment of Dry AMD and other disorders (Biogen Collaboration Agreement). Under the Biogen Collaboration Agreement, we will performpre-clinical and manufacturing activities, and Biogen is solely responsible for funding thepre-clinical and manufacturing activities and performingIND-enabling activities, worldwide clinical development, and commercialization. We received a $15.0 million upfront payment from Biogen in January 2020 for the grant of an exclusive license and the relatedknow-how, and we are eligible to receive up to $340.0 million in milestone payments, along with tiered royalties for worldwide net sales of this product candidate up to low double-digits. As of March 31, 2020, we recorded the $15.0 million upfront payment in collaboration revenue.

Recent Developments

DalcA Developments

On June 15, 2020, we announced final efficacy and safety data from our Phase 2b trial of DalcA, a next-generation SQ administered FIX therapy being developed for the treatment of Hemophilia B. We presented data from the trial that showed 28 days of daily SQ dosing of DalcA achieved protective target FIX levels of >12% in all participants, with FIX levels of up to 27% and a half-life of 2.5 to 5.1 days with no bleeds, demonstrating effective prophylaxis and the potential for lower or less frequent dosing. Injection volumes were less than 1 mL. One subject withdrew on day 7 after reporting injection site reactions (ISR) from the first 3 SQ doses. No neutralizing anti-drug antibodies were detected and no serious adverse events were reported. Some subjects reported mild ISR of pain and/or redness, primarily with the initial injections. No thrombotic events occurred and blood coagulation markers ofd-dimer, prothrombin fragment 1+2, thrombin-antithrombin and fibrinogen did not show any prothrombotic signals.

The trial was designed to evaluate daily SQ dosing and the ability to maintain protective steady state FIX levels above 12% in six individuals with severe Hemophilia B. Each subject received a single intravenous dose, followed by daily SQ doses of DalcA for 28 days whereby the pharmacokinetics, pharmacodynamics, safety, tolerability and anti-drug antibody formation were monitored.

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MarzAA Developments

We are initiating a Phase 3 clinical trial to evaluate the safety and efficacy of MarzAA foron-demand treatment and control of episodic bleeding in subjects with Hemophilia A or Hemophilia B with inhibitors in 2020. The Phase 3 study will be an open-label global trial, evaluating the safety and efficacy of SQ MarzAA in the treatment of approximately 230 bleeding episodes in approximately 75 patients, compared with their prior standard of care in a similar number of bleeding episodes. The study will assess the effectiveness of SQ MarzAA, using up to three doses to treat a bleeding episode. The primary endpoint will be hemostatic efficacy using a standard four-point assessment scale. We also plan to initiate a Phase 1/2 trial of MarzAA in Factor VII deficiency, Glanzmann Thrombasthenia, and Hemlibra patients for treatment of bleeding in the fourth quarter of 2020.

Factor IX Gene Therapy Developments

In addition, we recently presented preclinical data at EAHAD in February 2020 demonstrating that a proprietary chimeric AAV capsid licensed from Stanford expressing CB2679d-GT FIX variant may significantly reduce the vector dose required of a gene therapy treatment while maintaining high factor activity levels. We expect to report data from this construct in a nonhuman primate efficacy study in Q2 2020.

SQ Systemic Complement Inhibitors

We have initiated discovery research to identify novel complement pathway regulating proteases and expect to have our first development candidate in Q4 2020.

Collaboration with Mosaic Biosciences, Inc.

We also collaborated with Mosaic Biosciences, Inc. (Mosaic) in the development of our complement product candidates, including CB2782-PEG. Previously, under the collaboration agreement, as amended in December 2019 (Mosaic Collaboration Agreement), Mosaic was entitled to a double-digit percentage of funds we receive from Biogen. Mosaic was also entitled to sublicense fees and/or research and development and commercial milestones and royalties on onenon-C3 complement product. On May 8, 2020, we further amended and restated the Mosaic Collaboration Agreement to provide for aone-time payment of $750,000, plus up to $4.0 million in future milestone payments related to regulatory and clinical development events for CB2782-PEG and an additional anti-complement product candidate, in lieu of our obligations to pay Mosaic a double-digit percentage of funds we receive from Biogen or any other amounts we receive related to sublicense fees, research and development payments, or any other commercial milestones and royalties on any other development candidates. We now own one hundred percent of all future payment streams related to these product candidates.

COVID-19 business impact

The currentCOVID-19 pandemic has presented a substantial public health and economic challenge around the world and is affecting our employees, patients, communities and business operations, as well as the U.S. economy and financial markets. The full extent to which theCOVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition will depend on future developments that are highly uncertain and cannot be accurately predicted, including new information that may emerge concerningCOVID-19, the actions taken to contain it or treat its impact and the economic impact on local, regional, national and international markets.

To date, we have instructed our employees to stay at home except as needed to ensure continuity of our operations. We have completed subject dosing and all participant activities in our DalcA Phase 2b trial and dosing of all subjects in the MarzAA Phase 1/2 PK/PD trial. As we continue to actively advance our clinical

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programs, we are in close contact with our principal investigators, clinical sites and contractors, including manufacturers, and are assessing the impact ofCOVID-19 on our current and planned trials, expected timelines and costs on an ongoing basis. We are continuing studystart-up activities where possible to allow rapid site activation and enrollment of our Phase 3 MarzAA trial at the appropriate time.

Given the focus of healthcare providers and hospitals on fightingCOVID-19, we may experience delays in the enrollment of patients in our upcoming clinical trials as well as delays in the analysis of data from our trials that have completed. We will continue to evaluate the impact of theCOVID-19 pandemic on our business and will reevaluate the timing of our anticipated clinical milestones as we learn more and the impact ofCOVID-19 on our industry becomes clearer.

Company Information

We commenced operations in 2002 and are a Delaware corporation. On August 20, 2015, we merged with Targacept, Inc. Our corporate headquarters are located at 611 Gateway Blvd., Suite 710, South San Francisco, California 94080. Our telephone number is (650)871-0761, and our website address is www.catalystbiosciences.com. The information contained on, or that can be accessed through, our website is not part of this prospectus supplement, and you should not consider information on our website to be part of this prospectus supplement.

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THE OFFERING

Common stock offered by us                 shares.

Option to purchase additional shares We have granted the underwriters an option to purchase up to                 additional shares of common stock. This option is exercisable, in whole or in part, for a period of 30 days from the date of this prospectus supplement.

Common stock to be outstanding after this offering                 shares (or approximately                 shares if the underwriters exercise their option to purchase additional                 shares of common stock in full).

Use of proceeds We expect the net proceeds from this offering to us, after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, will be approximately $     million ($     million if the underwriters exercise their option to purchase additional shares in full). We currently expect to use the net proceeds primarily for general corporate purposes, which may include research and development activities in our complement program, clinical and manufacturing activities for MarzAA and DalcA, other research and development activities, capital expenditures, selling, general and administrative costs, facilities expansion, and to meet working capital needs. We expect from time to time to evaluate the acquisition of businesses, products and technologies for which a portion of the net proceeds may be used, although we currently are not planning or negotiating any such transactions. See “Use of Proceeds” on pageS-12 of this prospectus supplement.

Risk factors Investing in our common stock involves significant risks. See “Risk Factors,” beginning on pageS-8 as well as the other information included in or incorporated by reference in this prospectus supplement and the accompanying prospectus, for a discussion of risks you should carefully consider before investing in our securities.

Nasdaq Capital Market symbol CBIO

The number of shares of our common stock to be outstanding immediately after this offering as shown above is based on 17,419,313 shares of common stock outstanding as of March 31, 2020, and excludes:

• 1,972,652 shares of common stock issuable upon exercise of options outstanding as of March 31, 2020, with a weighted average exercise price of $9.86 per share;

• 120,000 shares of common stock issuable upon exercise of options granted after March 31, 2020, with a weighted average exercise price of $5.95 per share;

• 7,857 shares of common stock issuable upon exercise of warrants outstanding as of March 31, 2020, with a weighted average exercise price of $53.61 per share;

• 596,371 shares of common stock reserved and available for future issuance as of March 31, 2020, under our equity incentive plans, consisting of (1) 276,043 shares of common stock reserved and

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available for issuance under our 2018 Omnibus Incentive Plan as of March 31, 2020, and (2) 320,328 shares of common stock reserved for issuance under our 2018 Employee Stock Purchase Plan as of March 31, 2020; and

• 1,300,000 additional shares of common stock reserved and available for future issuance after March 31, 2020 under our 2018 Omnibus Incentive Plan.

Except as otherwise indicated, all information in this prospectus supplement assumes no exercise of outstanding options and warrants and no exercise of the underwriters’ option to purchase additional shares of common stock.

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RISK FACTORS

An investment in our securities involves a high degree of risk. Prior to making a decision about investing in our securities, you should carefully consider the risk factors described below together with all of the risks, uncertainties and assumptions described in our Annual Report on Form10-K for the year ended December 31, 2019 and our Quarterly Report on Form10-Q for the quarter ended March 31, 2020, which are incorporated herein by reference, and may be amended, supplemented or superseded from time to time by other reports we file with the Securities and Exchange Commission, or SEC, in the future. If any of the risks incorporated by reference or set forth below occurs, our business, operations and financial condition could suffer significantly. As a result, you could lose some or all of your investment in our common stock. The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business, operations and financial condition, or cause the value of our common stock to decline.

Risks related toCOVID-19

The outbreak ofCOVID-19 could adversely impact our business, including our preclinical studies and clinical trials.

The globalCOVID-19 pandemic has resulted in widespread requirements for individuals to stay in their homes and strained medical facilities worldwide. It is too early to assess the full impact of theCOVID-19 outbreak on our business, including our trials for MarzAA and DalcA and our development activities in our anti-complement program butCOVID-19 may affect our ability to complete recruitment and data analysis for our clinical trials and our ability to conduct research and development of our complement programs in our planned timeframe. The extent to whichCOVID-19 impacts our operations will depend on future developments, which are highly uncertain and cannot be predicted with confidence, including the duration and severity of the outbreak, and the actions that may be required to containCOVID-19 or treat its impact. In particular, as a result of theCOVID-19 pandemic, we may experience disruptions that could severely impact our business, preclinical studies, drug manufacturing and clinical trials including:

• delays or difficulties in enrolling patients in our clinical trials;

• delays or difficulties in clinical site initiation, including difficulties in recruiting clinical site investigators and clinical site staff;

• diversion of healthcare resources away from the conduct of clinical trials, including the diversion of hospitals serving as our clinical trial sites and hospital staff supporting the conduct of our clinical trials;

• interruption of key clinical trial activities, such as clinical trial site data monitoring, due to limitations on travel imposed or recommended by federal or state governments, employers and others or interruption of clinical trial subject visits and study procedures, which may impact the integrity of subject data and clinical study endpoints;

• interruption or delays in the operations of the Food and Drug Administration, European Medicines Agency or other regulatory authorities, which may impact review and approval timelines;

• interruption of, or delays in receiving, supplies of our product candidates from our contract manufacturing organizations due to staffing shortages, production slowdowns or stoppages and disruptions in delivery systems;

• interruptions in preclinical studies due to restricted or limited operations at laboratory facilities;

• limitations on employee resources that would otherwise be focused on the conduct of our preclinical studies and clinical trials, including because of sickness of employees or their families or the desire of employees to avoid contact with large groups of people; and

• material delays and complications with respect to our research and development programs.

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In addition, the trading prices for our common stock and other biopharmaceutical companies have been highly volatile as a result of theCOVID-19 pandemic. As a result, we may face difficulties raising capital through sales of our common stock or such sales may be on unfavorable terms.

Risks related to this offering

We have broad discretion in the use of the net proceeds from this offering and may not use them effectively.

Our management has broad discretion in the application of the net proceeds from this offering, and you will not have the opportunity as part of your investment decision to assess whether the net proceeds are being used appropriately. Our management could spend the net proceeds from this offering in ways that do not improve our results of operations or enhance the value of our common stock. The failure by our management to apply these funds effectively could result in financial losses that could have a material adverse effect on our business, cause the price of our common stock to decline and delay the development of our product candidates. Pending their use, we may invest the net proceeds from this offering in a manner that does not produce income or that loses value.

If you purchase shares of common stock sold in this offering you will experience immediate and substantial dilution in your investment. You will experience further dilution if we issue additional equity securities in the future.

Since the price per share of our common stock being offered is higher than the net tangible book value per share of our common stock, you will suffer dilution with respect to the net tangible book value of the shares of common stock you purchase in this offering. Based on a public offering price of $             per share and our net tangible book value as of March 31, 2020, if you purchase shares of common stock in this offering, you will suffer immediate dilution of $             per share with respect to the net tangible book value of the common stock. Furthermore, if outstanding options or warrants are exercised, you could experience further dilution. See “Dilution” for a more detailed discussion of the dilution you will incur if you purchase shares of common stock in this offering.

Future sales of our common stock in the public market could cause the market price of our common stock to drop significantly, even if our business is doing well.

Sales of a substantial number of shares of our common stock in the public market, or the perception in the market that the holders of a large number of shares intend to sell shares, could reduce the market price of our common stock and make it more difficult for you to sell your common stock at a time and price that you deem appropriate.

We may raise money through additional public or private offerings of our equity securities or equity-linked securities. Any sales of our equity or equity-linked securities could have a material adverse effect on the market price of our common stock.

In connection with this offering, we and our directors and executive officers have entered intolock-up agreements for a period of 90 days following this offering. Thelock-up agreements are subject to various exceptions, and we and our directors and executive officers may be released from thelock-up agreements prior to the expiration of thelock-up period at the sole discretion of the representative. See “Underwriting.” Upon expiration or earlier release of thelock-up agreements, we and our directors and executive officers may sell shares into the market, which could adversely affect the market price of shares of our common stock.

We have registered under the Securities Act of 1933, as amended (Securities Act) shares of common stock that we may issue under our equity compensation plans. In addition, we have a significant number of stock options outstanding, and may also choose to issue additional common stock, or securities convertible into or

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exchangeable for common stock, in the future in connection with a financing, an acquisition, a litigation settlement, employee arrangements or otherwise. In the event that the outstanding options are exercised, or that we make additional issuances of common stock or other convertible or exchangeable securities, you could experience additional dilution. Furthermore, we cannot assure you that we will be able to issue shares or other securities in any other offering at a price per share that is equal to or greater than the price per share paid by investors in this offering, and investors purchasing our securities in the future may have rights superior to investors purchasing shares in this offering.

We do not anticipate paying dividends on our common stock in the foreseeable future.

We currently plan to invest all available funds, including the proceeds from this offering, and future earnings, if any, in the development and growth of our business. We currently do not anticipate paying any cash dividends on our common stock in the foreseeable future. As a result, a rise in the market price of our common stock, which is uncertain and unpredictable, will be your sole source of potential gain in the foreseeable future, and you should not rely on an investment in our common stock for dividend income.

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CAUTIONARY NOTE REGARDING FORWARD LOOKING INFORMATION

This prospectus supplement, the accompanying prospectus, and documents incorporated by reference herein and therein contain “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. These forward-looking statements involve a number of risks and uncertainties. We caution readers that any forward-looking statement is not a guarantee of future performance and that actual results could differ materially from those contained in the forward-looking statement. These statements are based on current expectations of future events. Such statements include, but are not limited to, statements about future financial and operating results, plans, objectives, expectations and intentions, costs and expenses, outcome of contingencies, financial condition, results of operations, liquidity, objectives of management, business strategies, financing, the timing, plans and expected results of our current and future clinical trials, the extent to which we will be able to advance development of our product candidates using the proceeds of this offering together with our existing cash resources; our focus on specific product candidates, the progress, outcomes, scope or duration of the development of product candidates or programs, the benefits that may be derived from product candidates or the commercial or market opportunity in any target indication, the progress of our third-party collaborations, including estimated milestones, the advancement of our technologies and our product candidates, approvals and commercialization of product candidates, the impacts of theCOVID-19 pandemic and other statements that are not historical facts. You can find many of these statements by looking for words like “believes,” “expects,” “anticipates,” “estimates,” “may,” “might,” “should,” “will,” “could,” “plan,” “intend,” “project,” “seek” or similar expressions in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein and any free writing prospectus. We intend that such forward-looking statements be subject to the safe harbors created thereby.

These forward-looking statements are based on the current beliefs and expectations of our management and are subject to significant risks and uncertainties. If underlying assumptions prove inaccurate or unknown risks or uncertainties materialize, actual results may differ materially from current expectations and projections. Factors that might cause such a difference include those discussed in the Risk Factors sections of our Annual Report on Form10-K for the year ended December 31, 2019 and our Quarterly Report on Form10-Q for the quarter ended March 31, 2020, as well as those discussed in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein and any free writing prospectus. You are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date made.

All subsequent written or oral forward-looking statements attributable to us or any person acting on our behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this section. We do not undertake any obligation to release publicly any revisions to these forward-looking statements to reflect events or circumstances after the date of this prospectus supplement or to reflect the occurrence of unanticipated events, except as may be required under applicable U.S. securities laws. If we do update one or more forward-looking statements, no inference should be drawn that we will make additional updates with respect to those or other forward-looking statements.

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USE OF PROCEEDS

We expect that the net proceeds of this offering, after deducting underwriting discounts and commissions and estimated offering expenses, will be approximately $         million, or approximately $         million if the underwriters exercise their over-allotment option in full.

We intend to use the net proceeds from the sale of the shares of common stock under this prospectus supplement for general corporate purposes, which may include research and development activities in our complement program, clinical and manufacturing activities for MarzAA and DalcA, other research and development activities, capital expenditures, selling, general and administrative costs, facilities expansion, and to meet working capital needs. We expect from time to time to evaluate the acquisition of businesses, products and technologies for which a portion of the net proceeds may be used, although we currently are not planning or negotiating any such transactions.

As of the date of this prospectus supplement, we cannot specify with certainty all of the particular uses of the proceeds from this offering. The amounts and timing of our actual expenditures will depend on numerous factors, including the progress of our clinical trials and other development efforts and other factors described under “Risk factors” in this prospectus supplement and the documents incorporated by reference herein. As a result, our management will have broad discretion over the uses of the net proceeds, if any, we receive in connection with securities offered pursuant to this prospectus supplement and investors will be relying on the judgment of our management regarding the application of the proceeds. Pending these uses, we intend to invest the net proceeds in short-term, interest-bearing obligations, investment-grade instruments, certificates of deposit or guaranteed obligations of the U.S. government.

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DILUTION

If you invest in our common stock, your interest will be diluted immediately to the extent of the difference between the public offering price per share of our common stock and the as adjusted net tangible book value per share of common stock after this offering.

Our net tangible book value as of March 31, 2020 was approximately $95.7 million, or approximately $5.50 per share. Net tangible book value per share is determined by dividing our total tangible assets, less total liabilities, by the number of shares of our common stock outstanding as of March 31, 2020.

After giving effect to our sale of shares of our common stock in this offering at the public offering price of $         per share, and after deducting the underwriting discount and commissions and the estimated offering expenses payable by us, our as adjusted net tangible book value as of March 31, 2020 would have been approximately $         million, or $         per share of common stock. This represents an immediate decrease in net tangible book value of $         per share to existing stockholders and an immediate dilution in net tangible book value of $         per share to investors participating in this offering. The following table illustrates this dilution on a per share basis:

Public offering price per share of common stock			$
Net tangible book value per share as of March 31, 2020	$	5.50
Increase in net tangible book value per share attributable to investors in this offering
As adjusted net tangible book value per share as of March 31, 2020 after giving effect to the offering
Dilution in net tangible book value per share to new investors			$

If the underwriters exercise in full their option to purchase          additional shares of common stock at the public offering price of $         per share, the as adjusted net tangible book value after this offering would be approximately $         per share, representing a decrease in net tangible book value of approximately $         per share to existing stockholders and immediate dilution in net tangible book value of approximately $         per share to investors participating in this offering at the public offering price.

The above discussion and table are based on 17,419,313 shares of common stock outstanding as of March 31, 2020, and excludes as of such date:

• 1,972,652 shares of common stock issuable upon exercise of options outstanding as of March 31, 2020, with a weighted average exercise price of $9.86 per share;

• 120,000 shares of common stock issuable upon exercise of options granted after March 31, 2020, with a weighted average exercise price of $5.95 per share;

• 7,857 shares of common stock issuable upon exercise of warrants outstanding as of March 31, 2020, with a weighted average exercise price of $53.61 per share; and

• 596,371 shares of common stock reserved and available for future issuance as of March 31, 2020, under our equity incentive plans, consisting of (1) 276,043 shares of common stock reserved and available for issuance under our 2018 Omnibus Incentive Plan as of March 31, 2020, and (2) 320,328 shares of common stock reserved for issuance under our 2018 Employee Stock Purchase Plan as of March 31, 2020; and

• 1,300,000 additional shares of common stock reserved and available for future issuance after March 31, 2020 under our 2018 Omnibus Incentive Plan.

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To the extent that outstanding options have been or may be exercised or other shares are issued, investors purchasing our common stock in this offering may experience further dilution. In addition, we may choose to issue additional common stock, or securities convertible into or exchangeable for common stock, in the future. The issuance of these securities could result in further dilution for investors purchasing our common stock in this offering.

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UNDERWRITING

We and the underwriters named below have entered into an underwriting agreement, dated the date of this prospectus supplement, with respect to the shares being offered. Subject to certain conditions, each underwriter has severally agreed to purchase the respective number of common shares shown opposite its name in the following table. Raymond James & Associates, Inc. is the representative of the underwriters.

Underwriters	Number of Shares
Raymond James & Associates, Inc.
National Securities Corporation
Total

The underwriters are committed to take and pay for all of the shares of common stock being offered, if any are taken, other than the shares of common stock covered by the option described below unless and until that option is exercised. If an underwriter fails or refuses to purchase any of its committed shares, the purchase commitments of thenon-defaulting underwriters may be increased, or the offering may be terminated.

The underwriters have an option to buy up to an additional          shares of common stock from us to cover sales by the underwriters of a greater number of shares than the total number set forth in the table above. They may exercise this option for 30 days. If any shares are purchased pursuant to this option, the underwriters will severally purchase shares in approximately the same proportion as set forth in the table above, and the underwriters will offer the additional shares of common stock on the same terms as those on which the shares of common stock are being offered.

The underwriters propose to offer the shares of common stock directly to the public at the public offering price set forth on the cover of this prospectus supplement and to certain dealers at such offering price less a concession not in excess of $         per share. After the initial public offering of the common stock, the offering price and the selling concession may be changed by the underwriters.

The following table shows the per share and total underwriting discounts and commissions to be paid by us to the underwriters assuming both no exercise and full exercise of the underwriters’ option to purchase additional shares of common stock.

	Per share		Total
	Without Option	With Option	Without Option	With Option
Public offering price of common stock	$	$	$	$
Underwriting discounts and commissions	$	$	$	$
Proceeds before expenses, to us	$	$	$	$

We estimate that the total expenses of this offering, including registration, filing and listing fees, printing fees and legal and accounting expenses, but excluding underwriting discounts and commissions, will be approximately $        , all of which will be paid by us and which includes up to $100,000 that we have agreed to reimburse the underwriters for the fees and expenses incurred by them in connection with the offering.

We expect to deliver the common stock against payment for the shares on or about                  , 2020.

The underwriters will require all our directors and officers to agree not to offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, any shares of common stock or any securities convertible into or exchangeable or exercisable for shares of common stock, or, collectively, theLock-Up Securities, without the prior written consent of the representative for a period of 90 days from the date of this prospectus supplement. These restrictions are subject to certain exceptions, including if the transfer (i) does not trigger any filing or reporting requirement or obligation or result in any other voluntary or mandatory public disclosure, including but

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not limited to Form 4 of Section 16 of the Exchange Act, (ii) is a gift, (iii) is by process of will or intestacy, (iv) is a distribution to partners, members or shareholders of the undersigned, (v) is to a settlor of a trust or to the estate of a beneficiary of a trust, (vi) is pursuant to apre-existing trading plan, (vii) is in satisfaction of tax withholding obligations, (viii) is pursuant to a liquidation, tender offer, merger, consolidation, binding share exchange or other similar transaction involving a change in control of the Company, (ix) is pursuant to a court order or settlement agreement related to the distribution of assets in connection with the dissolution of marriage or civil union, or (x) relates to the exercise of any option, warrant or other right to acquire shares of common stock, the settlement of any stock-settled stock appreciation rights, restricted stock or restricted stock units, including through a “net” or cashless exercise, granted and outstanding as of the date of thelock-up agreement or at the closing date of the offering contemplated by this prospectus supplement, or the conversion of any convertible security of the Company in accordance with its terms; provided, however, that it shall be a condition to the transfer pursuant to clauses (i), (ii), (iii), (iv), (v), (viii) and (ix) that the transferee execute an agreement stating that the transferee is receiving and holding the securities subject to the provisions of suchlock-up agreement.

We have agreed, subject to certain exceptions, that for a period of 90 days from the date of this prospectus supplement, we will not, without the prior written consent of the representative, directly or indirectly offer, pledge, sale, contract to sell, short sale or otherwise dispose of any shares of common stock or other securities convertible into or exchangeable or exercisable for shares of common stock or derivative of common stock. These restrictions do not apply among others to (i) the sale of shares of common stock to the underwriters; (ii) the issuance of shares of common stock upon exercise, conversion or exchange of options or upon the conversion or exchange of other securities outstanding on the date of this prospectus supplement; (iii) the filing of one or more registration statements registering the secondary sale of shares of common stock or securities convertible into shares of common stock; (iv) the sale of shares of common stock, options to purchase shares of common stock, restricted stock units or similar equity securities pursuant to any stock option, stock bonus or other stock plan or arrangement described in this prospectus supplement and the accompanying prospectus; (v) the issuance of shares of common stock, options, restricted stock units or similar equity securities to our directors as compensation for their services as directors of the company; (vi) agreements providing for the issuance of shares of common stock or any securities convertible into or exchangeable or exercisable for shares of common stock in connection with the acquisition by the Company or any of its subsidiaries of the securities, business, property or other assets of another person or entity (including pursuant to an employee benefit plan assumed by the Company in connection with such acquisition), and the issuance of any such securities pursuant to any such agreement, and (vii) agreements providing for the issuance of shares of common stock or any security convertible into or exercisable for shares of common stock in connection with joint ventures, commercial relationships, debt financing or other strategic transactions, and issuance of any such securities pursuant to any such agreements; provided that in the case of clauses (vi) and (vii), the aggregate number of shares of common stock that the Company may sell or issue or agree to sell or issue pursuant to clauses (vi) and (vii), taken together, shall not exceed 2.5% of the total number of shares of common stock issued and outstanding immediately subsequent to the completion of the transactions contemplated by this prospectus supplement.

In connection with the offering, the underwriters may purchase and sell our common shares in the open market. These transactions may include short sales, stabilizing transactions and purchases to cover positions created by short sales. Short sales involve the sale by the underwriters of a greater number of shares than they are required to purchase in the offering, and a short position represents the amount of such sales that have not been covered by subsequent purchases. A “covered short position” is a short position that is not greater than the amount of additional shares for which the underwriters’ option described above may be exercised. The underwriters may cover any covered short position by either exercising their option to purchase additional shares or purchasing shares in the open market. In determining the source of shares to cover the covered short position, the underwriters will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which they may purchase additional shares pursuant to the option described above. “Naked” short sales are any short sales that create a short position greater than the amount of additional shares for which the option described above may be exercised. The underwriters must cover any such naked short

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position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the common shares in the open market after pricing that could adversely affect investors who purchase in the offering. Stabilizing transactions consist of various bids for or purchases of common shares made by the underwriters in the open market prior to the completion of the offering.

The underwriters may also impose a penalty bid. This occurs when a particular underwriter repays to the underwriters a portion of the underwriting discount received by it because the representatives have repurchased shares sold by or for the account of such underwriter in stabilizing or short covering transactions.

Purchases to cover a short position and stabilizing transactions, as well as other purchases by the underwriters for their own accounts, may have the effect of preventing or retarding a decline in the market price of our shares, and together with the imposition of the penalty bid, may stabilize, maintain or otherwise affect the market price of the common shares. As a result, the price of our common shares may be higher than the price that otherwise might exist in the open market. The underwriters are not required to engage in these activities and may end any of these activities at any time. These transactions may be effected on the Nasdaq Capital Market, in theover-the- counter market or otherwise.

Our common stock trades on the Nasdaq Capital Market under the symbol “CBIO.”

In connection with this offering, the underwriters may engage in passive market making transactions in the common shares on the Nasdaq Capital Market in accordance with Rule 103 of Regulation M under the Exchange Act during a period before the commencement of offers or sales of common shares and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker’s bid, that bid must then be lowered when specified purchase limits are exceeded. Passive market making may cause the price of our common shares to be higher than the price that otherwise would exist in the open market in the absence of those transactions. The underwriters are not required to engage in passive market making and may end passive market making activities at any time.

The underwriters do not expect sales to discretionary accounts to exceed five percent of the total number of shares offered.

We have agreed to indemnify the several underwriters against certain liabilities, including liabilities under the Securities Act and to contribute to payments that the underwriters may be required to make for these liabilities.

A prospectus supplement in electronic format may be made available on websites maintained by one or more underwriters, or selling group members, if any, participating in this offering. The representatives may agree to allocate a number of our common shares to underwriters for sale to their online brokerage account holders. Internet distributions will be allocated by the representatives to underwriters that may make Internet distributions on the same basis as other allocations.

The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage and other financial andnon- financial activities and services. Certain of the underwriters and their respective affiliates have provided, and may in the future provide, a variety of these services to the issuer and to persons and entities with relationships with the issuer, for which they received or will receive customary fees and expenses. Oppenheimer & Co. Inc. served as an underwriter in our underwritten public offering of our common shares in December 2018, for which they received customary fees and expenses.

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In the ordinary course of their various business activities, the underwriters and their respective affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively traded securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate to assets, securities and/or instruments of the issuer (directly, as collateral securing other obligations or otherwise) and/or persons and entities with relationships with the issuer. The underwriters and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such assets, securities or instruments and may at any time hold, or recommend to clients that they should acquire, long and/or short positions in such assets, securities and instruments.

Notice toNon-U.S. Investors

Investors are advised to contact their legal, financial or tax advisers to obtain an independent assessment of the financial and tax consequences of an investment in common stock.

Belgium

The offering is exclusively conducted under applicable private placement exemptions and therefore it has not been and will not be notified to, and this document or any other offering material relating to the common stock has not been and will not be approved by, the Belgian Banking, Finance and Insurance Commission (“Commission bancaire, financière et des assurances/Commissie voor het Bank, Financie en Assurantiewezen”). Any representation to the contrary is unlawful.

Each underwriter has undertaken not to offer sell, resell, transfer or deliver directly or indirectly, any common stock, or to take any steps relating/ancillary thereto, and not to distribute or publish this document or any other material relating to the common stock or to the offering in a manner which would be construed as: (a) a public offering under the Belgian Royal Decree of 7 July 1999 on the public character of financial transactions; or (b) an offering of securities to the public under Directive 2003/71/EC which triggers an obligation to publish a prospectus in Belgium. Any action contrary to these restrictions will cause the recipient and us to be in violation of the Belgian securities laws.

France

Neither this prospectus supplement nor any other offering material relating to the common stock has been submitted to the clearance procedures of the Autorité des marchés financiers in France. The common stock have not been offered or sold and will not be offered or sold, directly or indirectly, to the public in France. Neither this prospectus supplement nor any other offering material relating to the common stock has been or will be: (a) released, issued, distributed or caused to be released, issued or distributed to the public in France; or (b) used in connection with any offer for subscription or sale of the common stock to the public in France. Such offers, sales and distributions will be made in France only: (i) to qualified investors (investisseurs qualifiés) and/or to a restricted circle of investors (cercle restreint d’investisseurs), in each case investing for their own account, all as defined in and in accordance with ArticlesL.411-2,D.411-1,D.411-2,D.734-1,D.744-1,D.754-1 andD.764-1 of the French Code monétaire et financier; (ii) to investment services providers authorised to engage in portfolio management on behalf of third parties; or (iii) in a transaction that, in accordance with articleL.411-2-II-1°-or-2°-or 3° of the French Code monétaire et financier and article211-2 of the General Regulations (Règlement Général) of the Autorité des marchés financiers, does not constitute a public offer (appel public à l’épargne). Such common stock may be resold only in compliance with ArticlesL.411-1,L.411-2,L.412-1 andL.621-8 throughL.621-8-3 of the French Code monétaire et financier.

United Kingdom / Germany / Norway / The Netherlands

In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a “Relevant Member State”) an offer to the public of the common stock which is the subject of

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the offering contemplated by this prospectus supplement may not be made in that Relevant Member State other than the offers contemplated in this prospectus supplement in name(s) of Member State(s) where prospectus will be approved or passported for the purposes of anon-exempt offer once this prospectus supplement has been approved by the competent authority in such Member State and published and passported in accordance with the Prospectus Directive as implemented in name(s) of relevant Member State(s) except that an offer to the public in that Relevant Member State of any common stock may be made at any time under the following exemptions under the Prospectus Directive, if they have been implemented in that Relevant Member State:

(a) to legal entities which are authorised or regulated to operate in the financial markets or, if not so authorised or regulated, whose corporate purpose is solely to invest in securities;

(b) to any legal entity which has two or more of (1) an average of at least 250 employees during the last financial year; (2) a total balance sheet of more than €43,000,000 and (3) an annual net turnover of more than €50,000,000, as shown in its last annual or consolidated accounts;

(c) by the representative to fewer than 100 natural or legal persons (other than qualified investors as defined in the Prospectus Directive); or

(d) in any other circumstances falling within Article 3(2) of the Prospectus Directive, provided that no such offer of common stock shall result in a requirement for the publication by the Company or any underwriter of a prospectus pursuant to Article 3 of the Prospectus Directive.

For the purposes of this provision, the expression an “offer to the public” in relation to any common stock in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and any common stock to be offered so as to enable an investor to decide to purchase any common stock, as the same may be varied in that Member State by any measure implementing the Prospectus Directive in that Member State and the expression “Prospectus Directive” means Directive 2003/71/EC and includes any relevant implementing measure in each Relevant Member State.

This prospectus supplement and any other material in relation to the common stock is only being distributed to, and is only directed at, persons in the United Kingdom that are qualified investors within the meaning of Article 2(1)(e) of the Prospective Directive (“qualified investors”) that also (i) have professional experience in matters relating to investments falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended, or the Order, (ii) who fall within Article 49(2)(a) to (d) of the Order or (iii) to whom it may otherwise lawfully be communicated (all such persons together being referred to as “relevant persons”). The common stock is only available to, and any invitation, offer or agreement to purchase or otherwise acquire such common stock will be engaged in only with, relevant persons. This prospectus supplement and its contents are confidential and should not be distributed, published or reproduced (in whole or in part) or disclosed by recipients to any other person in the United Kingdom. Any person in the United Kingdom that is not a relevant person should not act or rely on this prospectus supplement or any of its contents.

Israel

In the State of Israel, the common stock offered hereby may not be offered to any person or entity other than the following:

(a) a fund for joint investments in trust (i.e., mutual fund), as such term is defined in the Law for Joint Investments in Trust, 5754-1994, or a management company of such a fund;

(b) a provident fund as defined in Section 47(a)(2) of the Income Tax Ordinance of the State of Israel, or a management company of such a fund;

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(c) an insurer, as defined in the Law for Oversight of Insurance Transactions, 5741-1981, (d) a banking entity or satellite entity, as such terms are defined in the Banking Law (Licensing), 5741-1981, other than a joint services company, acting for their own account or for the account of investors of the type listed in Section 15A(b) of the Securities Law 1968;

(d) a company that is licensed as a portfolio manager, as such term is defined in Section 8(b) of the Law for the Regulation of Investment Advisors and Portfolio Managers, 5755-1995, acting on its own account or for the account of investors of the type listed in Section 15A(b) of the Securities Law 1968;

(e) a company that is licensed as an investment advisor, as such term is defined in Section 7(c) of the Law for the Regulation of Investment Advisors and Portfolio Managers, 5755-1995, acting on its own account;

(f) a company that is a member of the Tel Aviv Stock Exchange, acting on its own account or for the account of investors of the type listed in Section 15A(b) of the Securities Law 1968;

(g) an underwriter fulfilling the conditions of Section 56(c) of the Securities Law, 5728-1968;

(h) a venture capital fund (defined as an entity primarily involved in investments in companies which, at the time of investment, (i) are primarily engaged in research and development or manufacture of new technological products or processes and (ii) involve above-average risk);

(i) an entity primarily engaged in capital markets activities in which all of the equity owners meet one or more of the above criteria; and

(j) an entity, other than an entity formed for the purpose of purchasing common stock in this offering, in which the shareholders equity (including pursuant to foreign accounting rules, international accounting regulations and U.S. generally accepted accounting rules, as defined in the Securities Law Regulations (Preparation of Annual Financial Statements), 1993) is in excess of NIS 250 million.

Any offeree of the common stock offered hereby in the State of Israel shall be required to submit written confirmation that it falls within the scope of one of the above criteria. This prospectus supplement will not be distributed or directed to investors in the State of Israel who do not fall within one of the above criteria.

Italy

The offering of the common stock offered hereby in Italy has not been registered with the Commissione Nazionale per la Società e la Borsa (“CONSOB”) pursuant to Italian securities legislation and, accordingly, the common stock offered hereby cannot be offered, sold or delivered in the Republic of Italy (“Italy”) nor may any copy of this prospectus supplement or any other document relating to the common stock offered hereby be distributed in Italy other than to professional investors (operatori qualificati) as defined in Article 31, second paragraph, of CONSOB Regulation No. 11522 of 1 July, 1998 as subsequently amended. Any offer, sale or delivery of the common stock offered hereby or distribution of copies of this prospectus supplement or any other document relating to the common stock offered hereby in Italy must be made:

(a) by an investment firm, bank or intermediary permitted to conduct such activities in Italy in accordance with Legislative Decree No. 58 of 24 February 1998 and Legislative Decree No. 385 of 1 September 1993 (the “Banking Act”);

(b) in compliance with Article 129 of the Banking Act and the implementing guidelines of the Bank of Italy; and

(c) in compliance with any other applicable laws and regulations and other possible requirements or limitations which may be imposed by Italian authorities.

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Sweden

This prospectus supplement has not been nor will it be registered with or approved by Finansinspektionen (the Swedish Financial Supervisory Authority). Accordingly, this prospectus supplement may not be made available, nor may the common stock offered hereunder be marketed and offered for sale in Sweden, other than under circumstances which are deemed not to require a prospectus under the Financial Instruments Trading Act (1991: 980).

Switzerland

The common stock being offered pursuant to this prospectus supplement will not be offered, directly or indirectly, to the public in Switzerland and this prospectus supplement does not constitute a public offering prospectus as that term is understood pursuant to art. 652a or art. 1156 of the Swiss Federal Code of Obligations. We have not applied for a listing of the common stock being offered pursuant to this prospectus supplement on the SWX Swiss Exchange or on any other regulated securities market, and consequently, the information presented in this prospectus supplement does not necessarily comply with the information standards set out in the relevant listing rules. The common stock being offered pursuant to this prospectus supplement have not been registered with the Swiss Federal Banking Commission as foreign investment funds, and the investor protection afforded to acquirers of investment fund certificates does not extend to acquirers of common stock.

Canada

Notice to Canadian Residents

This document constitutes an “exempt offering document” as defined in and for the purposes of applicable Canadian securities laws. No prospectus has been filed with any securities commission or similar regulatory authority in Canada in connection with the offer and sale of the shares of common stock described herein. No securities commission or similar regulatory authority in Canada has reviewed or in any way passed upon this document or on the merits of the shares of common stock and any representation to the contrary is an offence.

Canadian investors are advised that this document has been prepared in reliance on section 3A.3 of National Instrument33-105Underwriting Conflicts (“NI33-105”). Pursuant to section 3A.3 of NI33-105,this document is exempt from the requirement to provide investors with certain conflicts of interest disclosure pertaining to “connected issuer” and/or “related issuer” relationships as would otherwise be required pursuant to subsection 2.1(1) of NI33-105.

Resale Restrictions

The offer and sale of the shares of common stock in Canada is being made on a private placement basis only and is exempt from the requirement to prepare and file a prospectus under applicable Canadian securities laws. Any resale of the shares of common stock acquired by a Canadian investor in this offering must be made in accordance with applicable Canadian securities laws, which may vary depending on the relevant jurisdiction, and which may require resales to be made in accordance with Canadian prospectus requirements, a statutory exemption from the prospectus requirements, in a transaction exempt from the prospectus requirements or otherwise under a discretionary exemption from the prospectus requirements granted by the applicable local Canadian securities regulatory authority. These resale restrictions may under certain circumstances apply to resales of the shares of the common stock outside of Canada.

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LEGAL MATTERS

The validity of the securities being offered by this prospectus will be passed upon by Orrick, Herrington & Sutcliffe LLP, New York, New York. The underwriters are being represented in connection with this offering by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., New York, New York.

EXPERTS

The consolidated balance sheets of Catalyst Biosciences, Inc. as of December 31, 2019 and 2018, and the related consolidated statements of operations, comprehensive loss, and stockholders’ equity, and cash flows for each of the years then ended, have been audited by EisnerAmper LLP, independent registered public accounting firm, as stated in their reports which are incorporated herein by reference, which reports (i) express an unqualified opinion on financial statements and (ii) express an unqualified opinion on the effectiveness of internal control over financial reporting as of December 31, 2019. Such financial statements have been incorporated herein by reference in reliance on the reports of such firm given upon their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We have filed with the SEC a registration statement on FormS-3 under the Securities Act with respect to the securities offered hereby. This prospectus supplement, which constitutes a part of the registration statement, does not contain all of the information set forth in the registration statement or the exhibits filed therewith. For further information about us and the securities offered hereby, reference is made to the accompanying prospectus and registration statement of which it is a part and the exhibits filed therewith. Statements contained in this prospectus supplement regarding the contents of any contract or any other document that is filed as an exhibit to the accompanying prospectus and the registration statement of which it is a part are not necessarily complete, and in each instance we refer you to the copy of such contract or other document filed as an exhibit to the registration statement or the exhibits to the reports or other documents incorporated by reference in this prospectus for a copy of such contract or other document.

We are subject to the informational requirements of the Exchange Act and are required to file annual, quarterly and other reports, proxy statements and other information with the SEC. The SEC maintains an Internet site (http://www.sec.gov) that contains reports, proxy and information statements, and various other information about us. You may also inspect the documents described herein at our principal executive offices, 611 Gateway Blvd., Suite 710, South San Francisco, California 94080, during normal business hours.

Information about us is also available at our website at www.catalystbiosciences.com. However, the information on our website is not a part of this prospectus supplement and is not incorporated by reference into this prospectus supplement.

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INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” the information we file with the SEC, which means we can disclose important information to you by referring you to those documents. The information we incorporate by reference is an important part of this prospectus, and certain information that we will later file with the SEC will automatically update and supersede this information. We incorporate by reference the documents listed below as well as any future filings made with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus until we sell all of the securities under this prospectus, except that we do not incorporate any document or portion of a document that is “furnished” to the SEC, but not deemed “filed.” The following documents filed with the SEC are incorporated by reference in this prospectus:

• our Annual Report on Form10-K for the year ended December 31, 2019 filed with the SEC onFebruary 20, 2020;

• our Quarterly Report on Form10-Q for the quarter ended March 31, 2020 filed with the SEC onMay 11, 2020;

• our Current Reports on Form8-K filed with the SEC onJanuary 17, 2020,January 29, 2020,February  7, 2020 (Item 8.01),February 14, 2020,March  20, 2020 andJune 16, 2020; and

• the description of our common stock in our Registration Statement on Form8-A (Commission FileNo. 000-51173), filed with the SEC onApril 6, 2006, including any subsequent amendment or any report filed for the purpose of updating such description.

We will furnish without charge to you, on written or oral request, a copy of any or all of such documents that has been incorporated herein by reference (other than exhibits to such documents unless such exhibits are specifically incorporated by reference into the documents that this prospectus incorporates). Written or oral requests for copies should be directed Catalyst Biosciences, Inc., Attn: Investor Relations, 611 Gateway Blvd., Suite 710, South San Francisco, California 94080, telephone number (650)871-0761. See the section of this prospectus supplement entitled “Where You Can Find More Information” for information concerning how to read and obtain copies of materials that we file with the SEC at the SEC’s public offices.

Any statement contained in this prospectus supplement, or in a document all or a portion of which is incorporated by reference, shall be modified or superseded for purposes of this prospectus supplement to the extent that a statement contained in this prospectus supplement or any document incorporated by reference modifies or supersedes such statement. Any such statement so modified or superseded shall not, except as so modified or superseded, constitute a part of this prospectus.

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PROSPECTUS

$200,000,000

Common Stock

Preferred Stock

Debt Securities

Warrants

Units

From time to time, we may offer and sell, in one or more offerings, in amounts, at prices and on terms determined at the time of any such offering, common stock, preferred stock, debt securities, warrants, either individually or in units, with a total value of up to $200,000,000. We refer to our common stock, preferred stock, debt securities, warrants and units in this prospectus as “securities.”

Our common stock trades on The Nasdaq Capital Market under the symbol “CBIO.” On December 20, 2018, the last reported sale price of the common stock on The Nasdaq Capital Market was $7.10 per share.

This prospectus provides you with a general description of the securities we may offer. Each time we offer securities using this prospectus, we will provide the specific terms of the securities and the offering in one or more supplements to this prospectus. We may also authorize one or more free writing prospectuses to be provided to you in connection with these offerings. The prospectus supplement and any related free writing prospectus will also describe the specific manner in which we will offer the securities and may also supplement, update or amend information contained in this prospectus. You should read this prospectus, any accompanying prospectus supplement and any related free writing prospectus carefully before you purchase any of our securities.

We may offer the securities in amounts, at prices and on terms determined at the time of offering. We may sell the securities directly to you, through agents designated from time to time or to or through underwriters or dealers, on a continuous or delayed basis. If we use agents, underwriters or dealers to sell the securities, we will name them and describe their compensation in a prospectus supplement. You can find additional information about our plan of distribution for the securities under the heading “Plan of Distribution” in this prospectus. We will also describe the plan of distribution for any particular offering of these securities in the prospectus supplement. This prospectus may not be used to offer and sell securities unless accompanied by a prospectus supplement.

INVESTING IN OUR SECURITIES INVOLVES A HIGH DEGREE OF RISK. YOU SHOULD CAREFULLY READ AND CONSIDER THE RISKS AND UNCERTAINTIES DESCRIBED IN THIS PROSPECTUS, ANY ACCOMPANYING PROSPECTUS SUPPLEMENT AND ANY RELATED FREE WRITING PROSPECTUS AND IN THE DOCUMENTS INCORPORATED BY REFERENCE INTO THIS PROSPECTUS, ANY ACCOMPANYING PROSPECTUS SUPPLEMENT AND ANY RELATED FREE WRITING PROSPECTUS. SEE “RISK FACTORS” BEGINNING ON PAGE 5.

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 February 14, 2019

TABLE OF CONTENTS

Caption	Page
ABOUT THIS PROSPECTUS		1
OUR COMPANY		2
CAUTIONARY NOTE REGARDING FORWARD-LOOKING INFORMATION		3
RISK FACTORS		5
USE OF PROCEEDS		5
DESCRIPTION OF CAPITAL STOCK		6
ADDITIONAL INFORMATION CONCERNING OUR CAPITAL STOCK		11
DESCRIPTION OF DEBT SECURITIES		14
DESCRIPTION OF WARRANTS		21
DESCRIPTION OF UNITS		23
PLAN OF DISTRIBUTION		25
LEGAL MATTERS		27
EXPERTS		27
WHERE YOU CAN FIND MORE INFORMATION		28

You should rely only on the information contained or incorporated by reference in this prospectus, in any accompanying prospectus supplement or in any related free writing prospectus filed by us with the U.S. Securities and Exchange Commission (the “SEC”). We have not authorized any other person to provide you with different information. If anyone provides you with additional, different or inconsistent information, you should not rely on it. You should not assume that the information appearing in this prospectus, any applicable prospectus supplement, any related free writing prospectus or any documents incorporated by reference herein or therein is accurate as of any date other than the dates of those documents. Our business, financial condition, results of operations and prospects may have changed since those dates. Any information contained on or accessible through our Internet site is not incorporated herein and does not constitute part of this prospectus.

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement on FormS-3 that we filed with the U.S. Securities and Exchange Commission, or the SEC, using a “shelf” registration process. Under this shelf registration process, we may, from time to time, issue, offer and sell to the public any part of the securities described in this prospectus, together or separately, in one or more offerings, for a maximum aggregate offering price not to exceed $200,000,000.

This prospectus provides you with a general description of the securities we may offer. Each time we sell any securities under this prospectus, we will provide a prospectus supplement containing more specific information about the terms of that offering, including the specific amounts, prices and terms of the securities offered. Any such prospectus supplement may include a discussion of risks or other special considerations applicable to us or the securities offered thereby. Any such prospectus supplement may also add, update or change information in this prospectus or in documents incorporated by reference in this prospectus. To the extent that any statement that we make in a prospectus supplement is inconsistent with statements made in this prospectus or in documents incorporated by reference in this prospectus, the statements made or incorporated by reference in this prospectus will be deemed modified or superseded by those made in the prospectus supplement.

This prospectus and any applicable prospectus supplement contain and incorporate by reference market data, industry statistics and other data that have been obtained or compiled from information made available by third parties. These data, to the extent they contain estimates or projections, involve a number of assumptions and limitations, and you are cautioned not to give undue weight to such estimates or projections. Industry publications and other reports we have obtained from independent parties generally state that the data contained in these publications or other reports have been obtained in good faith or from sources considered to be reliable, but they do not guarantee the accuracy or completeness of such data. We urge you to carefully read this prospectus, any applicable prospectus supplement and any related free writing prospectus, any documents that we incorporate by reference in this prospectus, any applicable prospectus supplement and any related free writing prospectus, and the additional information described below under “Where You Can Find More Information” before making an investment decision.

This document may only be used where it is legal to sell these securities. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any jurisdiction whether the offer or sale is not permitted.

THIS PROSPECTUS MAY NOT BE USED TO OFFER AND SELL SECURITIES UNLESS IT IS ACCOMPANIED BY A PROSPECTUS SUPPLEMENT.

For purposes of this prospectus, references to the terms “Catalyst,” the “Company,” “we,” “us” and “our” refer to Catalyst Biosciences, Inc., a Delaware corporation, unless the context otherwise requires.

The registration statement containing this prospectus, including exhibits to the registration statement, provides additional information about us and the securities offered under this prospectus. The registration statement can be read at the SEC’s website or at the SEC offices mentioned under the heading “Where You Can Find More Information.”

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OUR COMPANY

We are a clinical-stage biopharmaceutical company focused on developing novel medicines to address serious medical conditions for individuals who need new or better treatment options. We are focusing our product development efforts in the field of hemostasis (the process that regulates bleeding) and have a mission to develop valuable therapies for individuals with hemophilia. We used a scientific approach to engineer several protease-based therapeutic candidates that regulate blood clotting.

Additional details of our development or clinical programs and related strategic agreements are contained in our Annual Report on Form10-K for the fiscal year ended December 31, 2017 and our Quarterly Reports on Form10-Q for the quarterly periods ended March 31, 2018, June 30, 2018 and September 30, 2018.

We commenced operations in 2002 and are a Delaware corporation. Our corporate headquarters are located at South San Francisco, California 94080. Our telephone number is (650)871-0761, and our website address is www.catalystbiosciences.com. The information on or accessible through our website does not constitute part of this prospectus or any accompanying prospectus supplement and should not be relied upon in connection with making any investment in our securities. Further, our reference to the URL for the website is intended to be an inactive textual reference only. We make our annual reports on Form10-K, quarterly reports on Form10-Q, current reports on Form8-K, and amendments to these reports available free of charge on our website as soon as reasonably practicable after we file these reports with the SEC. Our Code of Ethics can be found on our website.

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CAUTIONARY NOTE REGARDING FORWARD-LOOKING INFORMATION

This prospectus and any accompanying prospectus supplement, as well as the documents we incorporate by reference herein or therein, contains 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”). All statements, other than statements of historical facts, included or incorporated in this prospectus regarding our strategy, future operations, future financial position, future revenues, projected costs, prospects, plans, intentions and objectives of management are forward-looking statements. We may, in some cases, use words such as “believe,” “anticipate,” “should,” “could,” “intend,” “plan,” “may,” “will,” “seek,” “estimate,” “predict,” “project,” “potentially,” “continue,” “expect” or the negative of these terms or similar expressions, although not all forward-looking statements contain these identifying words. These statements are based on our management’s assumptions and assessments in light of information currently available to our management, its experience and its perception of historical trends, current conditions, expected future developments and other factors our management believes to be appropriate. These forward-looking statements are subject to a number of risks and uncertainties, including those risks described under “Risk Factors” and in other sections in this prospectus and in our current and periodic reports, and other filings, filed from time to time with the SEC that are incorporated by reference into this prospectus, that could cause actual results to differ materially from those anticipated in the forward-looking statements. See “Where You Can Find More Information” below and for information about how to obtain copies of those documents.

Forward-looking statements included or incorporated by reference in this prospectus include, for example, statements about:

• the strategies, prospects, plans, expectations or objectives of management for future operations;

• our focus on specific product candidates;

• the progress, scope or duration of the development of product candidates or programs, clinical trial plans, timelines and potential results;

• the benefits that may be derived from product candidates or the commercial or market opportunity in any target indication;

• our ability to protect intellectual property rights;

• our anticipated operations, financial position, revenues, costs or expenses, statements regarding future economic conditions or performance, statements of belief and any statement of assumptions underlying any of the foregoing;

• potential regulatory filings for or approval of any of our product candidates;

• the progress of our third-party collaborations, including estimated milestones;

• our intention to seek, and the ability to enter into, strategic alliances and collaborations;

• the responsibilities of our collaborators, including the responsibility to make cost reimbursement, milestone, royalty and other payments to us, and our expectations regarding our collaborators’ plans with respect to our products;

• our responsibilities to our collaborators, including our responsibilities to conduct research and development, clinical trials and manufacture products;

• the results and timing of clinical trials and the possible commencement of future clinical trials;

• conditions for obtaining regulatory approval of our product candidates;

• submission and timing of applications for regulatory approval;

• the impact of the U.S. Food and Drug Administration (FDA) and other government regulations on our business;

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• uncertainties associated with obtaining and protecting patents and other intellectual property rights, as well as avoiding the intellectual property rights of others;

• products and companies that will compete with the products we license to third-party collaborators;

• the possibility we may commercialize our own products and build up our commercial, sales and marketing capabilities and other required infrastructure;

• our employees, including the number of employees and the continued service of key management, technical and scientific personnel;

• our future performance and obligations under agreements we have entered into, such as the definitive agreement related to the termination of the Pfizer Agreement;

• our future performance and our expectations regarding our ability to achieve profitability;

• requirements for us to purchase supplies and raw materials from third parties, and the ability of third parties to provide us with required supplies and raw materials;

• sufficiency of our cash resources, anticipated capital requirements and capital expenditures and our need for additional financing;

• the composition of future revenues;

• accounting policies and estimates, including revenue recognition policies; and

• statements of belief and any statement of assumptions underlying any of the foregoing.

Any such forward-looking statements are not guarantees of future performance and are subject to certain risks and uncertainties that could cause actual results to differ materially from those contemplated by such forward-looking statements. All such forward-looking statements are made only as of the date of the document in which they are contained, based on our management’s beliefs and assumptions and information available to us as of the date of that document, and we caution you not to place undue reliance on forward-looking statements in light of the risks and uncertainties associated with them. We undertake no obligation, and disclaim any duty, to update or revise any forward-looking statements in light of future developments. You should also carefully consider other information set forth in reports or other documents that we file with the SEC.

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RISK FACTORS

Our business is subject to certain risks and uncertainties. Before you invest in our securities, in addition to the other information, documents or reports incorporated by reference in this prospectus and any prospectus supplement or other offering materials, you should carefully consider the risk factors in this section, the section entitled “Risk Factors” in any prospectus supplement as well as our most recent Annual Report on Form10-K, and in our Quarterly Reports on Form10-Q filed subsequent to the Annual Report on Form10-K, which are incorporated by reference into this prospectus and any prospectus supplement in their entirety, as the same may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future. Each of the risks described in these sections and documents could materially and adversely affect our business, financial condition, results of operations and prospects, and could result in a partial or complete loss of your investment.

USE OF PROCEEDS

Unless otherwise indicated in a prospectus supplement, the net proceeds from the sale of securities offered by this prospectus will be used for general corporate purposes, which may include clinical trials, research and development activities, capital expenditures, selling, general and administrative costs, facilities expansion, and to meet working capital needs. We expect from time to time to evaluate the acquisition of businesses, products and technologies for which a portion of the net proceeds may be used, although we currently are not planning or negotiating any such transactions. Pending such uses, we may invest the net proceeds in investment grade interest-bearing securities.

The amounts actually expended for each purpose may vary significantly depending upon numerous factors, including the amount and timing of the proceeds from this offering and progress with the commercial development of our products as well as our clinical development programs. Expenditures will also depend upon the establishment of collaborative arrangements with other companies, the availability of additional financing and other factors. Investors will be relying on the judgment of our management regarding the application of the proceeds of any sale of securities.

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DESCRIPTION OF CAPITAL STOCK

The following description of our common stock and preferred stock, together with the additional information we include in any applicable prospectus supplements, summarizes the material terms and provisions of the common stock and preferred stock that we may offer under this prospectus. It may not contain all the information that is important to you. For the complete terms of our common stock and preferred stock, please refer to our Fourth Amended and Restated Certificate of Incorporation, as amended (the “restated certificate of incorporation”) and our amended and restated bylaws, which are incorporated by reference into the registration statement which includes this prospectus. The Delaware General Corporation Law (“DGCL”) may also affect the terms of these securities. While the terms we have summarized below will apply generally to any future common stock and preferred stock that we may offer, we will describe the particular terms of these securities in more detail in the applicable prospectus supplement. If we so indicate in a prospectus supplement, the terms of any security we offer under that prospectus supplement may differ from the terms we describe below.

Common Stock

Under our restated certificate of incorporation, we have authority to issue 100,000,000 shares of our common stock, par value $0.001 per share. As of December 20, 2018, 11,947,738 shares of our common stock were issued and outstanding. All shares of our common stock will, when issued, be duly authorized, fully paid and nonassessable.

Dividends.Subject to preferential dividend rights of any other class or series of stock, the holders of shares of our common stock are entitled to receive dividends, including dividends of our stock, as and when declared by our board of directors, subject to any limitations imposed by law and to the rights of the holders, if any, of our preferred stock. We have never paid cash dividends on our common stock, except with respect to a cash dividend paid in connection with the closing of our business combination with Targacept, Inc. We do not anticipate paying periodic cash dividends on our common stock for the foreseeable future. Any future determination about the payment of dividends will be made at the discretion of our board of directors and will depend upon our earnings, if any, capital requirements, operating and financial conditions and on such other factors as the board of directors deems relevant.

Liquidation. In the event we are liquidated, dissolved or our affairs are wound up, after we pay or make adequate provision for all of our known debts and liabilities, each holder of our common stock will be entitled to share ratably in all assets that remain, subject to any rights that are granted to the holders of any class or series of preferred stock.

Voting Rights. For all matters submitted to a vote of stockholders, each holder of our common stock is entitled to one vote for each share registered in his or her name. Except as may be required by law and in connection with some significant actions, such as mergers, consolidations, or amendments to our restated certificate of incorporation that affect the rights of stockholders, holders of our common stock vote together as a single class. There is no cumulative voting in the election of our directors, which means that, subject to any rights to elect directors that are granted to the holders of any class or series of preferred stock, a plurality of the votes cast at a meeting of stockholders at which a quorum is present is sufficient to elect a director.

Other Rights and Restrictions. Subject to the preferential rights of any other class or series of stock, all shares of our common stock have equal dividend, distribution, liquidation and other rights, and have no preference, appraisal or exchange rights, except for any appraisal rights provided by Delaware law. Furthermore, holders of our common stock have no conversion, sinking fund or redemption rights, or preemptive rights to subscribe for any of our securities. Our restated certificate of incorporation and our bylaws do not restrict the ability of a holder of our common stock to transfer his or her shares of our common stock.

The rights, powers, preferences and privileges of holders of our common stock are subject to, and may be adversely affected by, the rights of holders of shares of any series of preferred stock which we may designate and issue in the future.

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Listing. Our common stock is listed on the Nasdaq Capital Market under the symbol “CBIO.”

Transfer Agent and Registrar.The transfer agent for our common stock is American Stock Transfer &Trust Company, LLC. Its address is 6201 15th Avenue, Brooklyn, NY 11219.

Preferred Stock

Under our restated certificate of incorporation, we have authority, subject to any limitations prescribed by law and without further stockholder approval, to issue from time to time up to 5,000,000 shares of preferred stock, par value $0.001 per share, in one or more series. As of December 20, 2018, the Company had no shares of preferred stock issued and outstanding.

Pursuant to our restated certificate of incorporation, we are authorized to issue “blank check” preferred stock, which may be issued from time to time in one or more series upon authorization by our board of directors. Our board of directors, without further approval of the stockholders, is authorized to fix the designation, powers, preferences, relative, participating optional or other special rights, and any qualifications, limitations and restrictions applicable to each series of the preferred stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes could, among other things, adversely affect the voting power or rights of the holders of our common stock and, under certain circumstances, make it more difficult for a third party to gain control of us, discourage bids for our common stock at a premium or otherwise adversely affect the market price of the common stock.

The preferred stock will have the terms described below unless otherwise provided in the prospectus supplement relating to a particular series of the preferred stock. You should read the prospectus supplement relating to the particular series of the 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 any dividend, the dates on which dividends will be payable, whether dividends will be cumulative or noncumulative and, if cumulative, the dates from which dividends will 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, qualifications, limitations and restrictions on the preferred stock.

The preferred stock will, when issued, be duly authorized, fully paid and nonassessable. Unless otherwise specified in the prospectus supplement, each series of the preferred stock will rank equally as to dividends and liquidation rights in all respects with any 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 all classes or series of 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;

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• 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 the 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.

Liquidation Preference.Upon any voluntary or involuntary liquidation, dissolution or winding up of our affairs, 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 applicable 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.

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.

Voting Rights. Holders of preferred stock will have voting rights as required by law or as indicated in the applicable prospectus supplement.

Conversion Rights.The terms and conditions, if any, upon which any series of preferred stock is convertible into 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

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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. Any transfer agent and registrar for the preferred stock will be set forth in the applicable prospectus supplement.

Series A Convertible Preferred Stock

As of December 20, 2018, the Company had no shares of preferred stock issued and outstanding.

Liquidation Preference. In the event of a liquidation, the holders of Series A Convertible Preferred Stock will be entitled to participate on anas-converted-to-common-stock basis with holders of the common stock in any distribution of assets of the Company to the holders of the common stock.

Dividends. The Series A Certificate of Designation provides, among other things, that we shall not pay any dividends on shares of common stock (other than dividends in the form of common stock) unless and until such time as we pay dividends on each share of Series A Convertible Preferred Stock on anas-converted basis. Other than as set forth in the previous sentence, the Series A Certificate of Designation provides that no other dividends shall be paid on shares of Series A Convertible Preferred Stock and that we shall pay no dividends (other than dividends in the form of common stock) on shares of common stock unless we simultaneously comply with the previous sentence. The Series A Certificate of Designation does not provide for any restriction on the repurchase of Series A Convertible Preferred Stock by us while there is any arrearage in the payment of dividends on the Series A Convertible Preferred Stock. There are no sinking fund provisions applicable to the Series A Convertible Preferred Stock.

Voting Rights. With certain exceptions, as described in the Series A Certificate of Designation, the Series A Convertible Preferred Stock has no voting rights. However, as long as any shares of Series A Convertible Preferred Stock remain outstanding, the Series A Certificate of Designation provides that we shall not, without the affirmative vote of holders of a majority of the then-outstanding shares of Series A Convertible Preferred Stock, (a) alter or change adversely the powers, preferences or rights given to the Series A Convertible Preferred Stock or alter or amend the Series A Certificate of Designation, (b) increase the number of authorized shares of Series A Convertible Preferred Stock or (c) effect a stock split or reverse stock split of the Series A Convertible Preferred Stock or any like event.

Conversion. Each share of Series A Convertible Preferred Stock is convertible at any time at the holder’s option into a number of shares of Common Stock equal to $1,000.00 per share divided by the Conversion Price. The “Conversion Price” is initially $5.00, subject to adjustment for reverse and forward stock splits, stock dividends, stock combinations and other similar transactions as specified in the Certificate of Designation. Notwithstanding the foregoing, the Series A Certificate of Designation provides further that we shall not effect any conversion of the Series A Convertible Preferred Stock, with certain exceptions, to the extent that, after giving effect to an attempted conversion, the holder of Series A Convertible Preferred Stock (together with such holder’s affiliates, and any persons acting as a group together with such holder or any of such holder’s affiliates) would beneficially own a number of shares of Common Stock in excess of 4.99% (or, at the election of the purchaser, 9.99%) of the shares of our common stock then outstanding after giving effect to such exercise (the “Preferred Stock Beneficial Ownership Limitation”).

Call Right. Additionally, subject to certain exceptions, at any time prior to the three year anniversary of the issuance of the Series A Convertible Preferred Stock, subject to the Preferred Stock Beneficial Ownership Limitation, we will have the right to cause each holder of the Series A Convertible Preferred Stock to convert all or part of such holder’s Series A Convertible Preferred Stock in the event that (i) the volume weighted average price of our common stock for 30 consecutive trading days (the “Measurement Period”) exceeds 300% of the

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conversion price of the Series A Convertible Preferred Stock issued (subject to adjustment for forward and reverse stock splits, recapitalizations, stock dividends and similar transactions), (ii) the average daily trading volume for such Measurement Period exceeds $500,000 per trading day and (iii) the holder is not in possession of any information that constitutes or might constitute, materialnon-public information which was provided by the Company and subject to the Preferred Beneficial Ownership Limitation. Our right to cause each holder of the Series A Convertible Preferred Stock to convert all or part of such holder’s Series A Convertible Preferred Stock shall be exercised ratably among the holders of the then outstanding Series A Convertible Preferred Stock.

Listing. We have not applied for listing of the Series A Convertible Preferred Stock on any securities exchange or other trading system.

Transfer Agent. The transfer agent for our Series A Convertible Preferred Stock is American Stock Transfer & Trust Company, LLC.

Certain Effects of Authorized but Unissued Stock

We have shares of common stock and preferred stock available for future issuance without stockholder approval. We may issue these additional shares for a variety of corporate purposes, including future public or private offerings to raise additional capital or to facilitate corporate acquisitions or for payment as a dividend on our capital stock. The existence of unissued and unreserved preferred stock may enable our board of directors to issue shares of preferred stock with terms that could render more difficult or discourage a third-party attempt to obtain control of us by means of a merger, tender offer, proxy contest or otherwise, thereby protecting the continuity of our management. In addition, if we issue preferred stock, the issuance could adversely affect the voting power of holders of common stock and the likelihood that holders of common stock will receive dividend payments or payments upon liquidation.

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ADDITIONAL INFORMATION CONCERNING OUR CAPITAL STOCK

Anti-Takeover Effects of Provisions of Our Charter Documents

Our restated certificate of incorporation provides for our board of directors to be divided into three classes serving staggered terms. Approximatelyone-third of our board of directors will be elected each year. The provision for a classified board could prevent a party who acquires control of a majority of the outstanding voting stock from obtaining control of the board of directors until the second annual stockholders meeting following the date the acquirer obtains the controlling stock interest. The classified board provision could discourage a potential acquirer from making a tender offer or otherwise attempting to obtain control of Catalyst and could increase the likelihood that incumbent directors will retain their positions. Our restated certificate of incorporation provides that directors may be removed with or without cause by the affirmative vote of the holders of at least 66 2/3% of the voting power of all outstanding stock.

Our restated certificate of incorporation requires that certain amendments to the restated certificate of incorporation and amendments by the stockholders of our bylaws require the affirmative vote of at least 66 2/3% of the voting power of all outstanding stock. These provisions could discourage a potential acquirer from making a tender offer or otherwise attempting to obtain control of the Company and could delay changes in management.

Our amended and restated bylaws establish an advance notice procedure for stockholder proposals to be brought before an annual stockholders meeting, including proposed nominations of persons for election to our board of directors. At an annual stockholders meeting, stockholders 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. Stockholders may also consider a proposal or nomination by a person who was a stockholder of record on the record date for the meeting, who is entitled to vote at the meeting and who has given to the Secretary of the Company timely written notice, in proper form, of his or her intention to bring that business before the annual stockholders meeting. The amended and restated bylaws do not give our board of directors the power to approve or disapprove stockholder nominations of candidates or proposals regarding other business to be conducted at a special or annual meeting of the stockholders. However our bylaws may have the effect of precluding the conduct of business at a meeting if the proper procedures are not followed. These provisions may also discourage or deter a potential acquirer from conducting a solicitation of proxies to elect the acquirer’s own slate of directors or otherwise attempting to obtain control of the Company.

Our amended and restated bylaws provide that only our board of directors, the chairperson of the board, the President or the Chief Executive Officer may call a special meeting of stockholders. Because our stockholders do not have the right to call a special meeting, a stockholder could not force stockholder consideration of a proposal over the opposition of our board of directors by calling a special meeting of stockholders prior to such time as a majority of our board of directors, the chairperson of the board, the President or the Chief Executive Officer believed the matter should be considered or until the next annual meeting provided that the requestor met the notice requirements. The restriction on the ability of stockholders to call a special meeting means that a proposal to replace the board also could be delayed until the next annual stockholders meeting.

Our restated certificate of incorporation does not allow stockholders to act by written consent without a meeting. Without the availability of stockholder’s actions by written consent, a holder controlling a majority of our capital stock would not be able to amend our bylaws or remove directors without holding a stockholders’ meeting.

Anti-Takeover Effects of Provisions of Delaware Law

We are subject to the provisions of Section 203 of the DGCL, or Section 203. Under Section 203, we would generally be prohibited from engaging in any business combination with any interested stockholder for a period of three years following the time that this stockholder became an interested stockholder unless:

• prior to this time, our board of directors approved either the business combination or the transaction that resulted in the stockholder becoming an interested stockholder;

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• upon completion of the transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding shares owned by persons who are directors and also officers, and by employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

• at or subsequent to such time, the business combination is approved by our board of directors and authorized at a special or annual stockholders meeting, and not by written consent, by the affirmative vote of at least 66 2/3% of the outstanding voting stock that is not owned by the interested stockholder.

Under Section 203, a “business combination” includes:

• any merger or consolidation involving the corporation and the interested stockholder;

• any sale, transfer, pledge or other disposition of 10% or more of the assets of the corporation involving the interested stockholder;

• any transaction that results in the issuance or transfer by the corporation of any stock of the corporation to the interested stockholder, subject to limited exceptions;

• any transaction involving the corporation that has the effect of increasing the proportionate share of the stock of any class or series of the corporation beneficially owned by the interested stockholder; or

• the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits provided by or through the corporation.

In general, Section 203 defines an interested stockholder as an entity or person beneficially owning 15% or more of the outstanding voting stock of the corporation and any entity or person affiliated with or controlling or controlled by such entity or person.

Limitation of Liability and Indemnification

Our restated certificate of incorporation provides that our directors shall not be personally liable to us or our stockholders for monetary damages for any breach of fiduciary duty as a director, except for liability for breach of the director’s duty of loyalty to us or our stockholders, for acts or omissions not in good faith or involving intentional misconduct or a knowing violation of law, for payment of dividends or approval of stock purchases or redemptions that are prohibited by the DGCL, or for any transaction from which the director derived an improper personal benefit. Under the DGCL, our directors have a fiduciary duty to us that is not eliminated by this provision of the restated certificate of incorporation and, in appropriate circumstances, equitable remedies such as injunctive or other forms ofnon-monetary relief will remain available. This provision also does not affect our directors’ responsibilities under any other laws, such as federal securities laws or state or federal environmental laws.

Section 145 of the DGCL empowers a corporation to indemnify its directors and officers against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlements actually and reasonably incurred by them in connection with any action, suit or proceeding brought by third parties by reason of the fact that they were or are directors or officers of the corporation, if they acted in good faith, in a manner they 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 that their conduct was unlawful. The DGCL provides further that the indemnification permitted thereunder shall not be deemed exclusive of any other rights to which the directors and officers may be entitled under the corporation’s bylaws, any agreement, a vote of stockholders or otherwise. Our restated certificate of incorporation provides that, to the fullest extent permitted by Section 145 of the DGCL, we shall indemnify any person who is or was a director or officer of us, or is or was serving at our request as a director, officer or trustee of another corporation, partnership, joint venture, trust, employee benefit

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plan or other enterprise, against the expenses, liabilities or other matters referred to in or covered by Section 145 of the DGCL. Our amended and restated bylaws provide that we will indemnify any person who was or is a party or threatened to be made a party to any proceeding by reason of the fact that such person is or was a director or officer of us or is or was serving at our request as a director, officer or trustee of another corporation, partnership, joint venture, trust, employee benefit plan or other enterprise to the fullest extent permitted by the DGCL.

In addition, we have entered into indemnification agreements with each of our directors and with certain of our executive officers. Pursuant to the indemnification agreements, we has agreed to indemnify and hold harmless these directors and officers to the fullest extent permitted by the DGCL. The agreements generally cover expenses that a director or officer incurs or amounts that a director or officer becomes obligated to pay because of any proceeding to which he or she is made or threatened to be made a party or participant by reason of his or her service as a current or former director, officer, employee or agent of the Company. The agreements also provide for the advancement of expenses to the directors and officers subject to specified conditions. There are certain exceptions to our obligation to indemnify the directors and officers, including any intentional malfeasance or act where the director or officer did not in good faith believe he or she was acting in our best interests, with respect to “short-swing” profit claims under Section 16(b) of the 1934 Act and, with certain exceptions, with respect to proceedings that he or she initiates.

Section 145 of the DGCL also empowers a corporation to purchase insurance for its officers and directors for such liabilities. We maintain liability insurance for our officers and directors.

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DESCRIPTION OF DEBT SECURITIES

The following description, together with the additional information we include in any applicable prospectus supplements, summarizes the material terms and provisions of the debt securities that we may offer under this prospectus. While the terms we have summarized below will apply generally to any future debt securities we may offer under this prospectus, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. The terms of any debt securities we offer under a prospectus supplement may differ from the terms we describe below.

The following summary description, together with the additional information we may include in any applicable prospectus supplements does not purport to be complete and is subject to, and qualified in its entirety by reference to, the form of indenture filed as an exhibit to the registration statement of which this prospectus is part, as it may be supplemented, amended or modified from time to time, as well as the notes and supplemental agreements relating to each series of debt securities that will be incorporated by reference as exhibits to the registration statement that includes this prospectus or as exhibits to a current report on Form8-K if we offer debt securities.

We will issue senior debt securities under one or more senior indentures that we will enter into with a trustee named in the relevant senior indenture. We will issue subordinated debt securities under one or more subordinated indentures that we will enter into with a trustee named in the relevant subordinated indenture. We have filed a form of indenture as an exhibit to the registration statement of which this prospectus is a part. We use the terms “indenture” and “indentures” in this prospectus to refer to both the senior indenture and the subordinated indenture.

The indentures will be qualified under the Trust Indenture Act of 1939, as amended. We use the term “debenture trustee” to refer to either the trustee under the senior indenture or the trustee under the subordinated indenture, as applicable.

The following summaries of material provisions of the senior debt securities, the subordinated debt securities and the indentures are subject to, and qualified in their entirety by reference to, all the provisions of the indenture applicable to a particular series of debt securities. We urge you to read the applicable prospectus supplements related to the debt securities that we sell under this prospectus, as well as the indenture that would contain the terms of the debt securities. Except as we may otherwise indicate, the terms of the senior indenture and the subordinated indenture would be identical.

General

We will describe in each prospectus supplement the following terms relating to a series of debt securities:

• the title;

• the principal amount being offered, and if a series, the total amount authorized and the total amount outstanding;

• any limit on the amount that may be issued;

• whether or not we will issue the series of debt securities in global form, the terms and who the depositary will be;

• the maturity date;

• whether and under what circumstances, if any, we will pay additional amounts on any debt securities held by a person who is not a U.S. person for tax purposes, and whether we can redeem the debt securities if we have to pay such additional amounts;

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• the annual interest rate, which may be fixed or variable, or the method for determining the rate and the date interest will begin to accrue, the dates interest will be payable and the regular record dates for interest payment dates or the method for determining such dates;

• whether or not the debt securities will be secured or unsecured, and the terms of any secured debt;

• the terms of the subordination of any series of subordinated debt;

• the place where payments will be payable;

• restrictions on transfer, sale or other assignment, if any;

• our right, if any, to defer payment of interest and the maximum length of any such deferral period;

• the date, if any, after which, and the price at which, we may, at our option, redeem the series of debt securities pursuant to any optional or provisional redemption provisions and the terms of those redemptions provisions;

• the date, if any, on which, and the price at which we are obligated, pursuant to any mandatory sinking fund or analogous fund provisions or otherwise, to redeem, or at the holder’s option to purchase, the series of debt securities and the currency or currency unit in which the debt securities are payable;

• whether the indenture will restrict our ability and/or the ability of our subsidiaries to:

• incur additional indebtedness;

• issue additional securities;

• create liens;

• pay dividends and make distributions in respect of our capital stock and the capital stock of our subsidiaries;

• redeem capital stock;

• place restrictions on our subsidiaries’ ability to pay dividends, make distributions or transfer assets;

• make investments or other restricted payments;

• sell or otherwise dispose of assets;

• enter into sale-leaseback transactions;

• engage in transactions with stockholders and affiliates;

• issue or sell stock of our subsidiaries; or

• effect a consolidation or merger;

• whether the indenture will require us to maintain any interest coverage, fixed charge, cash flow-based, asset-based or other financial ratios;

• a discussion of any material or special U.S. federal income tax considerations applicable to the debt securities;

• information describing any book-entry features;

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

• whether the debt securities are to be offered at a price such that they will be deemed to be offered at an “original issue discount” as defined in paragraph (a) of Section 1273 of the Internal Revenue Code;

• the denominations in which we will issue the series of debt securities, if other than denominations of $1,000 and any integral multiple thereof; and

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• any other specific terms, preferences, rights or limitations of, or restrictions on, the debt securities, including any additional events of default or covenants provided with respect to the debt securities, and any terms that may be required by us or advisable under applicable laws or regulations.

Conversion or Exchange Rights

We will set forth in the prospectus supplement the terms on which a series of debt securities may be convertible into or exchangeable for our common stock or our other securities. We will include provisions as to whether conversion or exchange is mandatory, at the option of the holder or at our option. We may include provisions pursuant to which the number of shares of our common stock or our other securities that the holders of the series of debt securities receive would be subject to adjustment.

Consolidation, Merger or Sale

Any successor to or acquiror of the indentures must assume all of our obligations under the indentures or the debt securities, as appropriate. If the debt securities are convertible for our other securities or securities of other entities, the person with whom we consolidate or merge or to whom we sell all of our property must make provisions for the conversion of the debt securities into securities that the holders of the debt securities would have received if they had converted the debt securities before the consolidation, merger or sale.

Events of Default Under the Indenture

Unless otherwise provided in any applicable prospectus supplement, documents incorporated by reference or free writing prospectus, the following will be events of default under the indenture with respect to each series of debt securities issued thereunder:

(a) if we fail to pay interest when due and payable and our failure continues for 30 days, or within such other time period as may be specified in the applicable indenture, and the time for payment has not been extended or deferred;

(b) if we fail to pay the principal, premium or sinking fund payment, if any, when due and payable and the time for payment has not been extended or delayed;

(c) if specified events of bankruptcy, insolvency or reorganization occur; and

(d) if we fail to observe or perform any other covenant contained in the debt securities or the indentures, other than a covenant specifically relating to another series of debt securities, and our failure continues for 60 days, or within such other time period as may be specified in the applicable indenture, after we receive notice from the debenture trustee or holders of at least a majority in principal amount of the outstanding debt securities of an affected series, or such other percentage as may be specified in the applicable indenture, in aggregate principal amount of the outstanding debt securities of the applicable series.

If an event of default with respect to debt securities of any series occurs and is continuing, other than an event of default specified in the last bullet point above, the debenture trustee or the holders of at least 25%, or such other percentage as may be specified in the applicable indenture, in aggregate principal amount of the outstanding debt securities of that series, by notice to us in writing, and to the debenture trustee if notice is given by such holders, may declare the unpaid principal of, premium, if any, and accrued interest, if any, due and payable immediately. If an event of default specified in the last bullet point above occurs with respect to us, the principal amount of and accrued interest, if any, of each issue of debt securities then outstanding shall be due and payable without any notice or other action on the part of the debenture trustee or any holder.

The holders of a majority in principal amount of the outstanding debt securities of an affected series may waive any default or event of default with respect to the series and its consequences, except defaults or events of default regarding payment of principal, premium, if any, or interest, unless we have cured the default or event of default in accordance with the indenture. Any waiver shall cure the default or event of default.

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Subject to the terms of the indentures, if an event of default under an indenture shall occur and be continuing, the debenture trustee will be under no obligation to exercise any of its rights or powers under such indenture at the request or direction of any of the holders of the applicable series of debt securities, unless such holders have offered the debenture trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the debenture trustee, or exercising any trust or power conferred on the debenture trustee, with respect to the debt securities of that series, provided that:

• the direction so given by the holder is not in conflict with any law or the applicable indenture; and

• subject to its duties under the Trust Indenture Act of 1939, the debenture trustee need not take any action that might involve it in personal liability or might be unduly prejudicial to the holders not involved in the proceeding.

A holder of the debt securities of any series will only have the right to institute a proceeding under the indentures or to appoint a receiver or trustee, or to seek other remedies if:

• the holder has given written notice to the debenture trustee of a continuing event of default with respect to that series;

• the holders of at least 25% (or, in the case of a default of the type described under subsection (d), above, a majority in principal amount of the outstanding debt securities of an affected series), or such other percentage as may be specified in the applicable indenture, in aggregate principal amount of the outstanding debt securities of that series have made written request, and such holders have offered reasonable indemnity to the debenture trustee to institute the proceeding as trustee; and

• the debenture trustee does not institute the proceeding, and does not receive from the holders of a majority in aggregate principal amount of the outstanding debt securities of that series other conflicting directions within 60 days, or within such other time period as may be specified in the applicable indenture, after the notice, request and offer of indemnity.

These limitations do not apply to a suit instituted by a holder of debt securities if we default in the payment of the principal, premium, if any, or interest on, the debt securities.

We will periodically file statements with the debenture trustee regarding our compliance with specified covenants in the indentures.

Modification of Indenture; Waiver

We and the debenture trustee may change an indenture without the consent of any holders with respect to specific matters:

• to evidence the succession of another corporation to us and the assumption by any such successor of our covenants in such indenture and in the debt securities issued thereunder;

• to add to our covenants or to surrender any right or power conferred on us pursuant to the indenture;

• to establish the form and terms of debt securities issued thereunder;

• to evidence and provide for a successor trustee under such indenture with respect to one or more series of debt securities issued thereunder or to provide for or facilitate the administration of the trusts under such indenture by more than one trustee;

• to cure any ambiguity, to correct or supplement any provision in the indenture that may be defective or inconsistent with any other provision of the indenture or to make any other provisions with respect to matters or questions arising under such indenture; provided that no such action adversely affects the interests of the holders of any series of debt securities issued thereunder in any material respect;

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• to add to, delete from or revise the conditions, limitations and restrictions on the authorized amount, terms or purposes of issue, authentication and delivery of securities under the indenture;

• to add any additional events of default with respect to all or any series of debt securities;

• to supplement any of the provisions of the indenture as may be necessary to permit or facilitate the defeasance and discharge of any series of debt securities, provided that such action does not adversely affect the interests of any holder of an outstanding debt security of such series or any other security in any material respect;

• to make provisions with respect to the conversion or exchange rights of holders of debt securities of any series;

• to pledge to the trustee as security for the debt securities of any series any property or assets;

• to add guarantees in respect of the debt securities of one or more series;

• to change or eliminate any of the provisions of the indenture, provided that any such change or elimination becomes effective only when there is no security of any series outstanding created prior to the execution of such supplemental indenture which is entitled to the benefit of such provision;

• to provide for certificated securities in addition to or in place of global securities;

• to qualify such indenture under the Trust Indenture Act of 1939, as amended;

• with respect to the debt securities of any series, to conform the text of the indenture or the debt securities of such series to any provision of the description thereof in our offering memorandum or prospectus relating to the initial offering of such debt securities, to the extent that such provision, in our good faith judgment, was intended to be a verbatim recitation of a provision of the indenture or such securities; or

• to make any other change that does not adversely affect the rights of holders of any series of debt securities issued thereunder in any material respect.

In addition, under the indentures, the rights of holders of a series of debt securities may be changed by us and the debenture trustee with the written consent of the holders of at least a majority in aggregate principal amount of the outstanding debt securities of each series that is affected. However, we and the debenture trustee may only make the following changes with the consent of each holder of any outstanding debt securities affected:

• extending the fixed maturity of the series of debt securities; or

• reducing the principal amount, reducing the rate of or extending the time of payment of interest, or reducing any premium payable upon the redemption of any debt securities; or

• reducing the percentage of debt securities, the holders of which are required to consent to any amendment, supplement, modification or waiver; or

• make any change that adversely affects the right to convert or exchange any security into or for common stock or other securities, cash or other property in accordance with the terms of the applicable debt security.

Each indenture provides that we can elect to be discharged from our obligations with respect to one or more series of debt securities, except for specified obligations, including obligations to:

• register the transfer or exchange of debt securities of the series;

• replace stolen, lost or mutilated debt securities of the series;

• maintain paying agencies;

• hold monies for payment in trust;

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• recover excess money held by the debenture trustee;

• compensate and indemnify the debenture trustee; and

• appoint any successor trustee.

In order to exercise our rights to be discharged, we must deposit with the debenture trustee money or government obligations sufficient to pay all the principal of, any premium, if any, and interest on, the debt securities of the series on the dates payments are due.

Form, Exchange and Transfer

Each debt security will be represented by either one or more global securities registered in the name of The Depository Trust Company, as depositary, or a nominee (which we refer to, in the case of any debt security represented by a global debt security, as a “book-entry debt security”), or a certificate issued in definitive registered form (which we refer to, in the case of any debt security represented by a certificated security, as a “certificated debt security”) as set forth in the applicable prospectus supplement. Except as set forth in the applicable prospectus supplement, book-entry debt securities will not be issuable in certificated form.

You may transfer or exchange certificated debt securities at any office we maintain for this purpose in accordance with the terms of the indenture. No service charge will be made for any registration of transfer or exchange of certificated debt securities, but we may require payment of a sum sufficient to cover all taxes, assessments or other governmental charges that may be imposed in connection with a transfer or exchange.

You may effect the transfer of certificated debt securities and the right to receive the principal of, and any premium and interest on, certificated debt securities only by surrendering the certificate representing those certificated debt securities and either execution by us, and authentication and delivery by the debenture trustee, of the certificate to the new holder or execution by us, and authentication and delivery by the debenture trustee, of a new certificate to the new holder.

If we elect to redeem the debt securities of any series, we will not be required to:

• issue, register the transfer of, or exchange any debt securities of that series during a period beginning at the opening of business 15 days, or within such other time period as may be specified in the applicable indenture, before the day of mailing of a notice of redemption of any debt securities that may be selected for redemption and ending at the close of business on the day of the mailing; or

• register the transfer of or exchange any debt securities so selected for redemption, in whole or in part, except the unredeemed portion of any debt securities we are redeeming in part.

Repurchases on the Open Market

We, or an affiliate of ours, may at any time or from time to time repurchase any debt security in the open market or otherwise. Such debt securities may, at our option (or our affiliate’s option), be held, resold or surrendered to the trustee for cancellation.

Discharge of Debt Securities

When all outstanding debt securities of any series will become due and payable within one year of their stated maturity and we have deposited with the debenture trustee cash sufficient to pay and discharge all outstanding debt securities of such series on the date of their stated maturity, then we may discharge our obligations under the relevant indenture with respect to such debt securities while they remain outstanding.

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Information Concerning the Debenture Trustee

The debenture trustee, other than during the occurrence and continuance of an event of default under an indenture, undertakes to perform only those duties as are specifically set forth in the applicable indenture. Upon an event of default under an indenture, the debenture trustee must use the same degree of care as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the debenture trustee is under no obligation to exercise any of the powers given him or her by the indentures at the request of any holder of debt securities unless he or she is offered reasonable security and indemnity against the costs, expenses and liabilities that he or she might incur.

Payment and Paying Agents

Unless we otherwise indicate in the applicable prospectus supplement, we will make payment of the interest on any debt securities on any interest payment date to the person in whose name the debt securities, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

We will pay principal of and any premium and interest on the debt securities of a particular series at the office of the paying agents designated by us, except that unless we otherwise indicate in the applicable prospectus supplement, we will make interest payments by check that we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in a prospectus supplement, we will designate the corporate trust office of the debenture trustee in the City of New York as our sole paying agent for payments with respect to debt securities of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the debt securities of a particular series. We will maintain a paying agent in each place of payment for the debt securities of a particular series.

All money we pay to a paying agent or the debenture trustee for the payment of the principal of or any premium or interest on any debt securities that remains unclaimed at the end of two years after such principal, premium or interest has become due and payable will be repaid to us, and the holder of the debt security thereafter may look only to us for payment thereof.

Governing Law

The indentures and the debt securities will be governed by and construed in accordance with the laws of the State of New York, except to the extent that the Trust Indenture Act of 1939 is applicable.

Subordination of Subordinated Debt Securities

The subordinated debt securities will be unsecured and will be subordinate and junior in priority of payment to certain of our other indebtedness to the extent described in a prospectus supplement. The subordinated indenture does not limit the amount of subordinated debt securities that we may issue. It also does not limit us from issuing any other secured or unsecured debt.

Outstanding Debt Securities

As of December 20, 2018, the Company had no outstanding debt securities.

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DESCRIPTION OF WARRANTS

The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the warrants that we may offer under this prospectus and the related warrant agreements and warrant certificates. While the terms summarized below will apply generally to any warrants that we may offer, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. The terms of any warrants offered under that prospectus supplement may differ from the terms described below.

The following summary description, together with the additional information we may include in any applicable prospectus supplements does not purport to be complete and is subject to, and qualified in its entirety by reference to, the form of warrant agreement and form of warrant certificate relating to each series of warrants that will be incorporated by reference as an exhibit to the registration statement that includes this prospectus or as an exhibit to a current report on Form8-K if we offer warrants.

General

We will describe in the applicable prospectus supplement the terms of the series of warrants, including:

• the offering price and aggregate number of warrants offered;

• the currency for which the warrants may be purchased;

• if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;

• if applicable, the date on and after which the warrants and the related securities will be separately transferable;

• in the case of warrants to purchase common stock, the number of shares of common stock purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;

• the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreements and the warrants;

• the terms of any rights to redeem or call the warrants;

• any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;

• the dates on which the right to exercise the warrants will commence and expire;

• the manner in which the warrant agreements and warrants may be modified;

• federal income tax consequences of holding or exercising the warrants;

• the terms of the securities issuable upon exercise of the warrants; and

• any other specific terms, preferences, rights or limitations of or restrictions on the warrants.

Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including the right to receive dividends, if any, or, payments upon our liquidation, dissolution or winding up or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. Unless we otherwise

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specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to 5:00 P.M. Eastern Time on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

Holders of the warrants may exercise the warrants by delivering the warrant certificate representing the warrants to be exercised together with specified information, and paying the required amount to the warrant agent in immediately available funds, as provided in the applicable prospectus supplement. We will set forth on the reverse side of the warrant certificate and in the applicable prospectus supplement the information that the holder of the warrant will be required to deliver to the warrant agent.

Upon receipt of the required payment and the warrant certificate properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will issue and deliver the securities purchasable upon such exercise. If fewer than all of the warrants represented by the warrant certificate are exercised, then we will issue a new warrant certificate for the remaining amount of warrants. If we so indicate in the applicable prospectus supplement, holders of the warrants may surrender securities as all or part of the exercise price for warrants.

Enforceability of Rights by Holders of Warrants

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

Form, Exchange, and Transfer

We may issue the warrants in registered form or bearer form. Warrants issued in registered form, i.e., book-entry form, will be represented by a global security registered in the name of a depository, which will be the holder of all the warrants represented by the global security. Those investors who own beneficial interests in a global warrant will do so through participants in the depository’s system, and the rights of these indirect owners will be governed solely by the applicable procedures of the depository and its participants. In addition, we may issue warrants innon-global form, i.e., bearer form. If any warrants are issued innon-global form, warrant certificates may be exchanged for new warrant certificates of different denominations, and holders may exchange, transfer, or exercise their warrants at the warrant agent’s office or any other office indicated in the applicable prospectus supplement or other offering material.

Outstanding Warrants

As of December 20, 2018, we have outstanding warrants to purchase common stock as follows: (i) at any time until the5-year anniversary of the original date of issuance in 2014, warrants to purchase an aggregate of 2,313 shares of our common stock at an exercise price of $499.05 per share, (ii) at any time until the5-year anniversary of the original date of issuance in 2015, as applicable, warrants to purchase an aggregate of 7,772 shares of our common stock at an exercise price of $49.91 per share and warrants to purchase an aggregate of 24 shares of our common stock at an exercise price of $499.05 per share, and (iii) at any time until the7-year anniversary of the original date of issuance in 2012, warrants to purchase an aggregate of 85 shares of our common stock at an exercise price of $392.70 per share. Effective November 13, 2018, Healthcare Ventures VIII, L.P. agreed to cancel outstanding warrants representing the right to purchase up to an aggregate 1,845 shares of the Company’s common stock (as adjusted for the 15:1 reverse stock split effected February 10, 2017).

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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 we have summarized below will apply generally to any units that we may offer under this prospectus, we will describe the particular terms of any series of units in more detail in the applicable prospectus supplement. The terms of any units offered under a prospectus supplement may differ from the terms described below.

The following summary description, together with the additional information we may include in any applicable prospectus supplements does not purport to be complete and is subject to, and qualified in its entirety by reference to, the form of unit agreement and form of unit certificate relating to each series of units that will be incorporated by reference as an exhibit to the registration statement that includes this prospectus or as an exhibit to a current report on Form8-K if we offer units.

General

We may issue units comprised of common stock, preferred stock, debt securities, debt obligations of third parties, including U.S. treasury securities, warrants or any combination thereof. 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, 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.

The provisions described in this section, as well as those described under “Description of Capital Stock,” “Description of Debt Securities” and “Description of Warrants” will apply to each unit and to any common stock, preferred stock, debt security or warrants included in each unit, respectively.

Issuance in Series

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

Enforceability of Rights by Holders of Units

Any 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.

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Title

We, any unit agents and any of their agents may treat the registered holder of any unit certificate as an absolute owner of the units evidenced by that certificate for any purpose and as the person entitled to exercise the rights attaching to the units so requested, despite any notice to the contrary. See “Legal Ownership of Securities” below.

Outstanding Units

We have no outstanding units.

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PLAN OF DISTRIBUTION

We may sell the securities being offered by this prospectus separately or together through any of the following methods:

• to or through one or more underwriters or dealers in a public offering and sale by them;

• directly to investors;

• through agents;

• through block trades in which the broker or dealer engaged to handle the block trade will attempt to sell the securities as agent, but may position and resell a portion of the block as principal to facilitate the transaction;

• through any combination if these methods of sale; or

• in any manner, as provided in the applicable prospectus supplement.

We may distribute securities from time to time in one or more transactions:

• at a fixed price or prices, which may be changed;

• at market prices prevailing at the times of sale;

• at prices related to such prevailing market prices; or

• at negotiated prices.

We will describe the method of distribution of the securities in the applicable prospectus supplement. We may also determine the price or other terms of the securities offered under this prospectus by use of an electronic auction. We will describe how any auction will determine the price or any other terms, how potential investors may participate in the auction and the nature of the obligations of the underwriter, dealer or agent in the applicable prospectus supplement.

Unless otherwise specified in the applicable prospectus supplement, each class or series of securities will be a new issue with no established trading market, other than our common stock, which is traded on The Nasdaq Capital Market. We may elect to list any other class or series of securities on any exchange, but we are not obligated to do so. It is possible that one or more underwriters may make a market in a class or series of securities, but the underwriters will not be obligated to do so and may discontinue any market making at any time without notice. We cannot give any assurance as to the liquidity of the trading market for any of the securities.

Underwriters, dealers or agents may receive compensation in the form of discounts, concessions or commissions from us or our purchasers (as their agents in connection with the sale of the securities). In addition, underwriters may sell the securities to or through dealers, and those dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters and/or commissions from the purchasers for whom they act as agent. These underwriters, dealers or agents may be considered to be underwriters under the Securities Act. As a result, discounts, commissions, or profits on resale received by the underwriters, dealers or agents may be treated as underwriting discounts and commissions. The prospectus supplement will identify any such underwriter, dealer or agent, and describe any compensation received by them from us. Only underwriters named in the prospectus supplement are underwriters of the securities offered by the prospectus supplement. Any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

We may sell the securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions we will pay the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

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We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

Underwriters, dealers and agents may be entitled to indemnification by us against certain civil liabilities, including liabilities under the Securities Act, or to contribution with respect to payments made by the underwriters, dealers or agents, under agreements between us and the underwriters, dealers and agents.

Any person participating in the distribution of common stock registered under the registration statement that includes this prospectus will be subject to applicable provisions of the Exchange Act, and the applicable SEC rules and regulations, including, among others, Regulation M, which may limit the timing of purchases and sales of any of our common stock by any such person. Furthermore, Regulation M may restrict the ability of any person engaged in the distribution of our common stock to engage in market-making activities with respect to our common stock. These restrictions may affect the marketability of our common stock and the ability of any person or entity to engage in market-making activities with respect to our common stock.

We may grant underwriters who participate in the distribution of the securities an option to purchase additional securities to cover overallotments, if any, in connection with the distribution. Any underwriter may engage in overallotment, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M that stabilize, maintain or otherwise affect the price of the offered securities. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the common stock in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the common stock originally sold by the dealer is purchased in a covering transaction to cover short positions. Those activities may cause the price of the common stock to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time. If any such activities will occur, they will be described in the applicable prospectus supplement.

Underwriters or agents and their associates may be customers of, engage in transactions with or perform services for us in the ordinary course of business and any such relationships will be described in the applicable prospectus supplement.

To the extent required, this prospectus may be amended or supplemented from time to time to describe a specific plan of distribution.

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LEGAL MATTERS

The validity of the securities being offered by this prospectus will be passed upon by Morrison & Foerster LLP of Palo Alto, California. Additional legal matters may be passed upon for us or any underwriters, dealers or agents, by counsel that we will name in an applicable prospectus supplement.

EXPERTS

The consolidated balance sheets of Catalyst Biosciences, Inc. as of December 31, 2017 and 2016, and the related consolidated statements of operations, comprehensive loss, convertible preferred stock and stockholders’ equity and cash flows for each of the years then ended, have been audited by EisnerAmper LLP, independent registered public accounting firm, as stated in their report which is incorporated herein by reference. Such financial statements have been incorporated herein by reference in reliance on the report of such firm given upon their authority as experts in accounting and auditing.

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WHERE YOU CAN FIND MORE INFORMATION

We are a reporting company and file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission, or the SEC. You may read and copy these reports, proxy statements and other information at the SEC’s public reference rooms at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. You can request copies of these documents by writing to the SEC and paying a fee for the copying cost. Please call the SEC at1-800-SEC-0330 for more information about the operation of the public reference rooms. Our SEC filings are also available at the SEC’s website at www.sec.gov and our website at www.catalystbiosciences.com. We have not incorporated by reference into this prospectus the information contained on our website and you should not consider it to be part of this prospectus. In addition, you can read and copy our SEC filings at the office of the National Association of Securities Dealers, Inc. at 1735 K Street, Washington, D.C. 20006.

We have filed a registration statement on FormS-3 with the SEC relating to the securities covered by this prospectus. This prospectus is a part of the registration statement and does not contain all of the information in the registration statement. You may review a copy of the registration statement at the SEC’s public reference room in Washington, D.C., as well as through the SEC’s Internet site at www.sec.gov.

The SEC allows us to “incorporate by reference” the information we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus. In addition, information we file with the SEC in the future will automatically update and supersede information contained in this prospectus and any accompanying prospectus supplement.

This prospectus incorporates by reference the documents set forth below that we have previously filed with the SEC:

• our Annual Report onForm10-K for the fiscal year ended December 31, 2017 filed with the SEC on March 19, 2018;

• our Quarterly Reports on Form10-Q for the quarterly periods endedMarch 31, 2018,June 30, 2018andSeptember  30, 2018 filed with the SEC on May 3, 2018, August 2, 2018 and November 1, 2018, respectively; and

• our Current Reports on Form8-K filed with the SEC onApril 13, 2018,June 14, 2018,June  18, 2018,July 18, 2018,August  15, 2018,August 31, 2018 andDecember 18, 2018.

All filings filed by us pursuant to the Exchange Act after the date of the initial filing of the registration statement of which this prospectus is a part and prior to effectiveness of the registration statement shall be deemed to be incorporated by reference into this prospectus.

We also incorporate by reference into this prospectus additional documents that we may file with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, excluding, in each case, information deemed furnished and not filed until we sell all of the securities we are offering. Any statements contained in a previously filed document incorporated by reference into this prospectus is deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus, or in a subsequently filed document also incorporated by reference herein, modifies or supersedes that statement.

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We will provide to you at no cost a copy of any and all of the information incorporated by reference into the registration statement of which this prospectus is a part. You may make a request for copies of this information in writing or by telephone. Requests should be directed to:

Catalyst Biosciences, Inc.

611 Gateway Blvd. Suite 710

South San Francisco, CA 94080

Attn: Fletcher Payne, Chief Financial Officer

(650)871-0761

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Shares

Common Stock

PROSPECTUS SUPPLEMENT

Sole Book-Running Manager

Raymond James

Lead Manager

National Securities Corporation

, 2020