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): December 28, 2017

Northwest Biotherapeutics, Inc.

(Exact name of registrant as specified in its charter)

4800 Montgomery Lane, Suite 800

Bethesda, Maryland 20814

(Address of Principal Executive Offices)

(240) 497-9024

(Registrant’s telephone number, including area code)

(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 (see General Instruction A.2. below):

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. ¨

Warrant Adjustments

On March 1 through March 7, 2018, Northwest Biotherapeutics, Inc. (the “Company”) and certain investors agreed to modify the terms of outstanding warrants held by such investors. Pursuant to the agreements, the investors agreed not to exercise their warrants before a vote of the shareholders of the Company to increase the authorized capital stock of the Company is held or a predetermined date of either June 1, 2018 or four months from the date of the agreement. The modifications generally provided for a one-year extension to the expiration date of such warrants and a decrease in the exercise price of the warrants. The Company entered into a similar arrangement with an investor on December 28, 2017. In the aggregate, the Company modified warrants exercisable for up to 92,880,721 shares of common stock, with new expiration dates ranging between September 15, 2018 and June 30, 2022 and new exercise prices ranging between $0.24 and $2.50. Certain of the warrants retained their original exercise prices ranging as low as $0.175. To the extent applicable, the securities were issued pursuant to the exemption from the registration requirements afforded by Section 3(a)(9) of the Securities Act.

SIGNATURE

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.