Data from Confirmatory Trial to be Highlighted in Oral Presentation on Sunday, June 4

ImmunoGen, Inc. (Nasdaq: IMGN), a leader in the expanding field of antibody-drug conjugates (ADCs) for the treatment of cancer, today announced that results from the Phase 3 confirmatory MIRASOL trial (GOG 3045/ENGOT OV-55) evaluating the safety and efficacy of ELAHERE^® (mirvetuximab soravtansine-gynx) compared to chemotherapy in patients with folate receptor alpha (FRα)-positive platinum-resistant ovarian cancer, have been selected for a late-breaking oral presentation at the 2023 American Society of Clinical Oncology (ASCO) Annual Meeting taking place June 2-6, 2023 in Chicago, Illinois.

"We are honored that MIRASOL has been selected as a late-breaker presentation at ASCO, and pleased that we will be able to share the results from the trial so quickly after they became available," said Anna Berkenblit, MD, Senior Vice President and Chief Medical Officer of ImmunoGen. "Having demonstrated a statistically significant and clinically meaningful improvement in overall survival compared to investigator's choice of single-agent chemotherapy, I believe ELAHERE has the potential to be practice changing in FRα-positive, platinum-resistant ovarian cancer."

ORAL PRESENTATION

Title: Phase III MIRASOL (GOG 3045/ENGOT-ov55) Study: Initial Report of Mirvetuximab Soravtansine vs. Investigator's Choice of Chemotherapy in Platinum-Resistant, Advanced High-Grade Epithelial Ovarian, Primary Peritoneal, or Fallopian Tube Cancers with High Folate Receptor-Alpha Expression

Presenter: Dr. Kathleen Moore, Associate Director of Clinical Research and Director of the Oklahoma TSET/Sarah Cannon Phase I Program, Professor of the Section of Gynecologic Oncology at The University of Oklahoma and MIRASOL Principal Investigator

Session: Late-Breaking Abstract Session: Presentation and Discussion of LBA5507

Date: Sunday, June 4, 2023

Time: 7:30 am to 8:05 am CT / 8:30 am to 9:05 am ET

Late-breaking abstracts will be released at 7:00 am CT / 8:00 am ET on the day of the scientific presentation.

POSTER PRESENTATIONS

ImmunoGen will present two additional trial-in-progress posters at ASCO.

Title: GLORIOSA: A Randomized, Open-Label, Phase 3 Study of Mirvetuximab Soravtansine with Bevacizumab vs. Bevacizumab as Maintenance in Platinum-Sensitive Ovarian, Fallopian Tube, or Primary Peritoneal Cancer

Presenter: Dr. David O'Malley, Professor, Director of Gynecologic Oncology at the Ohio State University and the James Cancer Center

Abstract: TPS5622

Poster Board: 312a

Title: A Phase 1b/2 Study of Pivekimab Sunirine in Combination with Venetoclax/Azacitidine or Magrolimab for Patients with CD123-Positive Acute Myeloid Leukemia

Presenter: Dr. Naval Daver, Associate Professor in the Department of Leukemia at The University of Texas MD Anderson Cancer Center

Abstract: TPS7073

Poster Board: 203a

Poster abstracts will be released on Thursday, May 25, 2023 at 4:00 pm CT / 5:00 pm ET.

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

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.

ELAHERE^® is a registered trademark of ImmunoGen, Inc.

ABOUT ELAHERE

ELAHERE (mirvetuximab soravtansine-gynx) is a first-in-class ADC comprising a folate receptor alpha-binding antibody, cleavable linker, and the maytansinoid payload DM4, a potent tubulin inhibitor designed to kill the targeted cancer cells.

ELAHERE is indicated for the treatment of adult patients with folate receptor-alpha (FRα) positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, who have received one to three prior systemic treatment regimens. Select patients for therapy based on an FDA-approved test.

This indication is approved under accelerated approval based on tumor response rate and durability of response. Continued approval for this indication may be contingent upon verification and description of clinical benefit in a confirmatory trial.

Eye problems are common with ELAHERE and can be severe. ELAHERE also can cause severe or life-threatening inflammation of the lungs that may lead to death and patients may develop nerve problems called peripheral neuropathy during treatment. Please see full Prescribing Information, including Boxed Warning, and Medication Guide for ELAHERE.

FORWARD-LOOKING STATEMENTS

This press release includes forward-looking statements. These statements include, but are not limited to, the potential of ELAHERE to change how FRα-positive, platinum-resistant ovarian cancer is treated. 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 the Company's preclinical and clinical development processes; top-line data may change as more patient data become available and are subject to audit and verification procedures; the timing and outcome of the Company's preclinical and clinical development processes; the results of the ongoing MIRASOL trial may not support full approval of ELAHERE and, if so, additional studies may be required; the difficulties inherent in the development of novel pharmaceuticals, including uncertainties as to the timing, expense, and results of preclinical studies, clinical trials, and regulatory processes; the timing and outcome of the Company's anticipated interactions with regulatory authorities; the risk that the Company may not be able to obtain adequate price and reimbursement for any approved products, including the potential for delays or additional difficulties for ELAHERE in light of the FDA granting accelerated approval; risks and uncertainties associated with the scale and duration of the COVID-19 pandemic and the resulting impact on ImmunoGen's industry and business; and other factors as set forth in the Company's Annual Report on Form 10-K filed with the Securities and Exchange Commission on March 1, 2023, the Company's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commision on April 28, 2023, and other reports filed with the Securities and Exchange Commission. The forward-looking statements in this press release speak only as of the date of this press release. ImmunoGen undertakes no obligation to update any forward-looking statement, whether as a result of new information, future developments, or otherwise, except as may be required by applicable law.

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