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): November 5, 2013

ImmunoGen, Inc.

(Exact name of registrant as specified in its charter)

830 Winter Street, Waltham, MA 02451

(Address of principal executive offices)   (Zip Code)

Registrant’s telephone number, including area code: (781) 895-0600

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):

o            Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o            Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o            Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o            Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

ITEM 8.01 — OTHER EVENTS

On November 5, 2013, ImmunoGen, Inc. (the “Company”) announced its decision to stop the Phase II trial evaluating its IMGN901 product candidate as part of a combination regimen for the treatment of small-cell lung cancer.  Additional information is contained in the Company’s press release dated November 5, 2013, which is incorporated herein by reference.

ITEM 9.01. FINANCIAL STATEMENTS AND EXHIBITS

(d): The following exhibit is being furnished herewith:

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.

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Exhibit 99.1

Contacts

ImmunoGen, Inc. Announces Discontinuation of IMGN901

Study in Small-Cell Lung Cancer (SCLC)

Waltham, MA, November 5, 2013 — ImmunoGen, Inc. (Nasdaq: IMGN), a biotechnology company that develops novel anticancer therapeutics using its antibody-drug conjugate (ADC) technology, announced today the decision to stop the IMGN901 Phase II SCLC study, following the recommendation of the trial’s independent Data Monitoring Committee (DMC). ImmunoGen is in the process of updating study investigators and regulatory authorities. IMGN901 is a CD56-targeting ADC.

Based on analysis of available data, the DMC concluded that the addition of IMGN901 to etoposide/carboplatin (E/C) was unlikely to demonstrate a sufficient improvement in progression-free survival compared to E/C alone to justify continuation of the trial. As an imbalance in the rate of infection and infection-related deaths was noted between the arms, the DMC recommended that all patients discontinue IMGN901 treatment. Infection-related death is a recognized risk in SCLC trials, including trials with E/C. Among the 198 patients receiving IMGN901 as a single agent in early trials, there was one incidence of infection-related death; it was deemed possibly drug related.

“This is clearly a disappointing outcome, as there is a tremendous need for new treatment options for SCLC,” commented Dr. Charles Morris, ImmunoGen EVP and Chief Development Officer. “We will be analyzing the findings to date in this trial as part of assessing potential next steps for IMGN901.”

In the past 18 months, ImmunoGen has initiated clinical testing with three wholly owned ADC compounds: (1) IMGN853 for ovarian, endometrial, and other cancers that highly express folate receptor α; (2) IMGN529 for non-Hodgkin lymphoma; and (3) IMGN289 for lung, head and neck, and other cancers that highly express EGFR, as announced earlier today. Seven other ADCs with ImmunoGen technology are in the clinic through partnerships, the most advanced of which is the marketed product, Kadcyla®.

About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer therapeutics. The Company’s ADC technology uses a tumor-targeting engineered antibody to deliver one of ImmunoGen’s highly potent cancer-cell killing agents specifically to tumor cells. The most advanced compound with ImmunoGen’s ADC technology is Roche’s Kadcyla, which is marketed in the US by Genentech and is also gaining approvals internationally. Additional compounds are in clinical testing by ImmunoGen and through the Company’s partnerships with Amgen, Bayer HealthCare, Biotest and Sanofi. More information about ImmunoGen can be found at www.immunogen.com.

Kadcyla® is a registered trademark of Genentech, Inc., a member of the Roche Group.

This press release includes forward-looking statements. For these statements, ImmunoGen claims the protection of the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995. It should be noted that there are risks and uncertainties related to the development of novel anticancer products. A review of these risks can be found in ImmunoGen’s Annual Report on Form 10-K for the fiscal year ended June 30, 2013 and other reports filed with the Securities and Exchange Commission.

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