<Page> ---------------------------- OMB APPROVAL ---------------------------- OMB Number: 3235-0060 Expires: May 31, 2000 Estimated average burden hours per response....5.00 ---------------------------- 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) October 10, 2001 ----------------------------- TRIANGLE PHARMACEUTICALS, INC. -------------------------------------------------------------------------------- (Exact name of registrant as specified in its charter) DELAWARE 000-21589 56-1930728 ---------------------------- ------------------------- ------------------------- (State or other jurisdiction (Commission (IRS Employer of incorporation) File Number) Identification No.) 4 University Place, 4611 University Drive, Durham, North Carolina 27707 -------------------------------------------------------------------------------- (Address of principal executive offices) (Zip Code) Registrant's telephone number, including area code (919) 493-5980 -------------------------- -------------------------------------------------------------------------------- (Former name or former address, if changed since last report) <Page> Item 5. Other Events and Regulation FD Disclosure. On October 10, 2001, Triangle Pharmaceuticals, Inc. issued a press release, a copy of which is attached hereto as Exhibit 99.1, and is incorporated herein by reference. In addition, the shareholders of Triangle approved an amendment to the Triangle's Second Restated Certificate of Incorporation. The amendment increases the number of shares of authorized capital stock to 175,000,000 shares of common stock and 10,000,000 shares of preferred stock. Item 7. Financial Statements and Exhibits. (c) Exhibits. The following documents are filed as exhibits to this Report: 4.1 Amendment to Second Restated Certificate of Incorporation. 10.1 Form of Purchase Agreement, each dated August 30, 2001, between Triangle Pharmaceuticals and each of QFinance, Inc., Caduceus Capital II, L.P., Winchester Global Trust Company Limited as Trustee for Caduceus Capital Trust, PW Eucalyptus Fund, L.L.C. and PW Eucalyptus Fund, Ltd. 99.1 Press Release, dated October 10, 2001. <Page> 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. Triangle Pharmaceuticals, Inc. --------------------------------------------------------- (Registrant) October 10, 2001 /s/ Robert F. Amundsen, Jr. -------------------- --------------------------------------------------------- Date (Signature) Name: Robert F. Amundsen, Jr. Title: Executive Vice President & Chief Financial Officer