UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): March 23, 2018

PROTEOSTASIS THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

200 Technology Square, 4th Floor

Cambridge, MA 02139

(Address of principal executive offices, including zip code)

(617) 225-0096

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company  ☒

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.  ☒

On March 23, 2018, Proteostasis Therapeutics, Inc. (the “Company”) entered into a Sales Agreement (the “Agreement”) with Leerink Partners LLC (“Leerink”) with respect to an at-the-market offering program under which the Company may offer and sell, from time to time at its sole discretion, shares of its common stock, par value $0.001 per share (the “Common Stock”), having an aggregate offering price of up to $50,000,000 (the “Placement Shares”) through Leerink as its sales agent. The issuance and sale, if any, of the Placement Shares by the Company under the Agreement will be made pursuant to the Company’s effective registration statement on Form S-3 (Registration Statement No. 333-218545). A copy of the opinion of Goodwin Procter LLP relating to the legality of the issuance and sale of the Placement Shares is attached as Exhibit 5.1 hereto.

Leerink may sell the Placement Shares by any method permitted by law deemed to be an “at-the-market” offering as defined in Rule 415 of the Securities Act of 1933, as amended, including, without limitation, sales made through The NASDAQ Global Market (“Nasdaq”) or on any other existing trading market for the Common Stock. Leerink will use commercially reasonable efforts to sell the Placement Shares from time to time, based upon instructions from the Company (including any price, time or size limits or other customary parameters or conditions the Company may impose). The Company will pay Leerink a commission equal to 3.0 percent (3.0%) of the gross sales proceeds of any Placement Shares sold through Leerink under the Agreement, and also has provided Leerink with customary indemnification and contribution rights.

The Company is not obligated to make any sales of Common Stock under the Agreement. The Company or Leerink may suspend or terminate the offering of Shares upon notice to the other party and subject to other conditions. Leerink will act as sales agent on a commercially reasonable efforts basis consistent with its normal trading and sales practices and applicable state and federal law, rules and regulations and the rules of Nasdaq.

The foregoing description of the Agreement is not complete and is qualified in its entirety by reference to the full text of the Agreement, a copy of which is filed herewith as Exhibit 1.1 to this Current Report on Form 8-K and is incorporated herein by reference. A copy of the opinion of Goodwin Procter LLP relating to the legality of the issuance and sale of the shares in the Offering is attached as Exhibit 5.1 hereto.

This Current Report on Form 8-K shall not constitute an offer to sell or the solicitation of an offer to buy the securities discussed herein, nor shall there be any offer, solicitation, or sale of the securities in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state.

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.