ImmunoGen to Present Preclinical Data on IMGN151 at AACR Virtual Annual Meeting

ImmunoGen, Inc. (Nasdaq: IMGN), a leader in the expanding field of antibody-drug conjugates (ADCs) for the treatment of cancer, today announced preclinical data for its next generation anti-folate receptor alpha (FR?) ADC, IMGN151, which is being investigated in tumors with a broad range of FR? expression. The data will be shared via poster presentation at the virtual American Association for Cancer Research (AACR) Annual Meeting II being held June 22-24, 2020.

"To address the unmet needs of additional patient populations, we sought to develop a FR?-targeting ADC active against ovarian cancer and other tumor types with a broad range of FR? expression. IMGN151 demonstrates our continued innovation in the field of ADCs, incorporating several key design elements including a novel biparatopic antibody that enhances binding with FR? and payload delivery as well as DM21, our most advanced maytansinoid derivative that, together with a peptide linker, provides improved stability and increased bystander anti-tumor activity," said Eric Westin, MD, Vice President, Clinical Development and Translational Sciences for ImmunoGen. "In preclinical models, IMGN151 showed activity not only in high FR?-expressing tumors, but also improved activity in medium and low FR?-expressing tumors, suggesting promising potential in a broad set of patients with an array of tumor types. As we continue to analyze these data, we look forward to advancing IMGN151 into preclinical development."

PRECLINICAL DATA ON IMGN151
IMGN151 comprises an asymmetric, bivalent, biparatopic antibody targeting two independent epitopes of FR?, linked to a highly potent maytansinoid derivative, DM21, via a cleavable peptide linker with enhanced stability, longer half-life, and increased bystander activity. The average drug per antibody ratio is 3.5.

Key findings include:

• IMGN151 activity was characterized against cell lines and xenograft models with a wide range of FR? expression and compared to mirvetuximab soravtansine (IMGN853). Cell lines and xenograft models originated from ovarian, endometrial, breast, and cervical cancer.
• In tumor cells with medium and high FR? expression, IMGN151 boosted antibody binding events and payload delivery by 100% and 170%, respectively.
• IMGN151 increased ADC half-life by 60 hours and conjugate exposure in vivo by 40%, as compared to IMGN853.
• In vitro, IMGN151 was up to 200 times more active against four FR?-medium cell lines. IMGN151 also had better bystander killing activity in a mixed culture of target-positive and negative cells.
• In vivo, IMGN151 induced complete tumor regressions of human tumor xenograft models with high, medium, and low FR? expression. All tested doses were well tolerated.

POSTER PRESENTATION

• Title: "IMGN151: A Next Generation Folate Receptor Alpha Targeting Antibody Drug Conjugate Active Against Tumors with Low, Medium, and High Receptor Expression"
• Day/Time: Monday, June 22, 2020 at 9:00 AM ET
• Session Category: Experimental and Molecular Therapeutics
• Session Title: Antibody Drug Conjugates
• Abstract: 2890

Additional information can be found at www.aacr.org.

ABOUT IMGN151
IMGN151 is a next-generation ADC, designed to address the unmet needs of cancer patients with tumor types expressing lower levels of folate receptor alpha (FR?). IMGN151 comprises an asymmetric, bivalent, biparatopic antibody targeting two independent epitopes of FR?, linked to a highly potent maytansinoid derivative, DM21, via a cleavable peptide linker with enhanced stability, longer half-life, and increased bystander activity.

ABOUT IMMUNOGEN
ImmunoGen is developing the next generation of antibody-drug conjugates (ADCs) 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."

Learn more about who we are, what we do, and how we do it at www.immunogen.com.

FORWARD LOOKING STATEMENTS
This press release includes forward-looking statements based on management's current expectations. These statements include, but are not limited to, ImmunoGen's expectations related to: the occurrence, timing, and outcome of potential pre-clinical, clinical, and regulatory events related to ImmunoGen's product candidates; and the presentation of pre-clinical and clinical data on ImmunoGen's product candidates. 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 ImmunoGen's 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 timing and outcome of ImmunoGen's pre-clinical and clinical development processes; the difficulties inherent in the development of novel pharmaceuticals, including uncertainties as to the timing, expense, and results of pre-clinical studies, clinical trials, and regulatory processes; ImmunoGen's ability to financially support its product programs; risks and uncertainties associated with the scale and duration of the COVID-19 pandemic and resulting impact on ImmunoGen's industry and business; and other factors more fully described in ImmunoGen's Annual Report on Form 10-K for the year ended December 31, 2019 and other reports filed with the Securities and Exchange Commission.