ImmunoGen Announces Proposed Public Offering of Common Stock

ImmunoGen, Inc., (Nasdaq: IMGN), a leader in the expanding field of antibody-drug conjugates (ADCs) for the treatment of cancer, today announced that it intends to offer and sell, subject to market and other conditions, shares of its common stock in an underwritten public offering. ImmunoGen also intends to grant the underwriters a 30-day option to purchase up to an additional fifteen percent (15%) of the number of shares of common stock offered in the public offering. All of the shares of common stock to be sold in the offering are to be offered by ImmunoGen.

ImmunoGen intends to use the net proceeds of the offering, together with its existing capital, to fund its operations, including, but not limited to, clinical trial activities, supply of drug substance and drug product, pre-commercialization activities, capital expenditures, and working capital.

Jefferies, Cowen and William Blair are acting as joint book-running managers for the proposed offering.

The securities described above are being offered by ImmunoGen pursuant to a shelf registration statement that was previously filed with the Securities and Exchange Commission (SEC) and became effective upon filing. This press release does not constitute an offer to sell or a solicitation of an offer to buy the securities in this offering, nor shall there be any sale of these securities in any state or other jurisdiction in which such offer, solicitation or sale would be unlawful prior to the registration or qualification under the securities laws of any such state or other jurisdiction. A preliminary prospectus supplement and accompanying prospectus relating to the offering will be filed with the SEC and will be available on the SEC's website at www.sec.gov. Copies of the preliminary prospectus supplement, when available, may also be obtained by contacting Jefferies LLC, Attention: Equity Syndicate Prospectus Department, 520 Madison Avenue, 2nd Floor, New York, NY 10022, by e-mail at [e-mail] or by telephone at [phone]; Cowen and Company, LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY 11717, Attention: Prospectus Department, or by telephone at [phone]; or William Blair & Company, L.L.C., Attention: Prospectus Department, 150 North Riverside Plaza, Chicago, IL 60606, by e-mail at [e-mail] or by telephone at [phone].

ABOUT IMMUNOGEN ImmunoGen is developing the next generation of antibody-drug conjugates to improve outcomes for cancer patients. By generating targeted therapies with enhanced anti-tumor activity and favorable tolerability profiles, we aim to disrupt the progression of cancer and offer our patients more good days. We call this our commitment to “target a better now.”

This press release includes forward-looking statements, including statements related to the proposed public offering of securities by ImmunoGen, including the intended use of the net proceeds of the offering and existing capital. 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. Various factors could cause actual results to differ materially from those discussed or implied in the forward-looking statements, and you are cautioned not to place undue reliance on these forward-looking statements, which are current only as of the date of this release. Factors that could cause future results to differ materially from such expectations include, but are not limited to: the uncertainties related to market conditions and the completion of the public offering on the anticipated terms or at all, uncertainties inherent in the initiation of future clinical trials and such other factors more fully described in ImmunoGen's Annual Report on Form 10-K for the year ended December 31, 2018 and other reports filed with the Securities and Exchange Commission.

91592487v.6

View source version on businesswire.com: https://www.businesswire.com/news/home/20200122005762/en/

INVESTOR RELATIONS AND MEDIA

ImmunoGen Courtney O’Konek 781-895-0600 [e-mail]

OR

FTI Consulting Robert Stanislaro 212-850-5657 [e-mail]