Exhibit 99.3
FORM OF LETTER TO NOMINEES
SPERO THERAPEUTICS, INC.
Subscription Rights to Purchase Shares of Common Stock
February 11, 2020
To Securities Dealers, Commercial Banks
Trust Companies and Other Nominees:
This letter is being distributed to securities dealers, commercial banks, trust companies and other nominees in connection with a rights offering (the “Rights Offering”) by Spero Therapeutics, Inc. (the “Company”) to the holders of record (the “Recordholders”) of its common stock, par value $0.001 per share (the “Common Stock”), Series A convertible preferred stock, par value $0.001 per share (the “Series A Preferred Stock”), and Series B convertible preferred stock, par value $0.001 per share (“the Series B Preferred Stock”), as described in the prospectus supplement (and the accompanying prospectus) dated February 11, 2020 (the “Prospectus”). In the Rights Offering, Recordholders as of 5:00 p.m., New York time, on February 10, 2020 (the “Record Date”), are receiving, at no charge, non-transferable subscription rights (the “Rights”) to subscribe for and purchase up to an aggregate of 3,333,333 shares of Common Stock (the “Shares”) for a cash purchase price of $9.00 per share of Common Stock (the “Subscription Price”) pursuant to the Subscription Right (as defined below). Any holder, that following exercise of such holder’s Subscription Right (as defined below), would be or become a holder of greater than 9.99% of the outstanding number of shares of the Common Stock may elect to instead purchase non-voting Series C convertible preferred stock, par value $0.001 per share (the “Series C Preferred Stock”), at a purchase price of $9,000 per share (ratably adjusted for fractional shares), and any such holder so electing would have a right to purchase one one-thousandth of a share of Series C Preferred Stock for each share of Common Stock it had a right to purchase in the Rights Offering.
The Rights will expire, if not exercised prior to 5:00 p.m., New York time, on March 2, 2020, unless extended by the Company (the “Expiration Time”).
As described in the Prospectus, each beneficial owner of shares of Common Stock held through you or your nominee is entitled to 0.152 Subscription Rights for each share of Common Stock owned of record by you, or issuable upon conversion of the shares of the Company’s Series A Preferred Stock or Series B Preferred Stock owned of record by you as of the Record Date. The total number of Rights issued will be rounded down to the nearest whole number. Each whole Right will allow the holder thereof to subscribe for one share of Common Stock at the Subscription Price (or an equivalent number of shares of Series C Preferred Stock on the terms described in the Prospectus) (the “Subscription Right”).
As further described in the Prospectus, the Company has entered into an Investment Agreement with BVF Partners L.P. and its affiliates (“BVF”), pursuant to which BVF has agreed to purchase from the Company any and all Shares not subscribed for in the Rights Offering, in the form of Series C Preferred Stock, as further described in the Prospectus. BVF also will have the same right as other stockholders to subscribe for and purchase Shares under its Subscription Right. The Investment Agreement with BVF is described in more detail in the Prospectus.
The Rights will expire and be of no value, if not exercised prior to the Expiration Time.
Each holder of Rights will be required to submit payment in full for all of the Shares such holder wishes to purchase pursuant to the exercise of the Subscription Right prior to the Expiration Time.
As soon as practicable after the Expiration Time and after any and all prorations and adjustments contemplated by the terms of the Rights Offering, as described in the Prospectus, have been effected, any excess subscription payment received by Computershare Trust Company, N.A. (the “Subscription Agent”) will be returned, without interest or penalty.
