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): April 20, 2018

Proteostasis Therapeutics, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code (617) 225-0096

Not Applicable

(Former name or former address, if changed since last report)

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 April 20, 2018, Proteostasis Therapeutics, Inc. (the “Company”) and Astellas Pharma Inc. entered into Amendment No. 6 (“Amendment No. 6”) to their Collaboration and License Agreement dated November 4, 2014 (as amended, the “Astellas Agreement”), a collaboration formed to research and identify therapies targeting the Unfolded Protein Response pathway. Amendment No. 6, effective as of April 23, 2018, extends the Research Term for the Initial Project under the Astellas Agreement to December 4, 2018. The Research Term for the Initial Project was previously set to expire in the Company’s second quarter of 2018.

The foregoing description of Amendment No. 6 does not purport to be complete and is qualified in its entirety by reference to such agreement, which the Company intends to file as an exhibit to its next periodic report filed under the Securities Exchange Act of 1934, as amended, that corresponds to the period in which Amendment No. 6 was entered into.

The above information is not an admission as to the materiality of any information therein. The Company undertakes no duty or obligation to update or revise the information contained in this report, although it may do so from time to time as its management believes is appropriate. Any such updating may be made through the filing of other reports or documents with the SEC, through press releases or through other public disclosures.

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.