ELAHERE® Demonstrates Overall Survival Benefit in the Phase 3 MIRASOL Trial in Patients with FRα-Positive Platinum-Resistant Ovarian Cancer Results Show Statistically Significant Improvements in PFS, ORR, and OS Compared to Chemotherapy First Medicine to Demonstrate an Overall Survival Advantage in Platinum-Resistant Ovarian Cancer Submission of MAA in Europe and sBLA in US Anticipated in H2 2023

ELAHERE^® Demonstrates Overall Survival Benefit in the Phase 3 MIRASOL Trial in Patients with FRα-Positive Platinum-Resistant Ovarian Cancer

Results Show Statistically Significant Improvements in PFS, ORR, and OS Compared to Chemotherapy

First Medicine to Demonstrate an Overall Survival Advantage in Platinum-Resistant Ovarian Cancer

Submission of MAA in Europe and sBLA in US Anticipated in H2 2023

Conference Call to be Held at 8:00 AM ET Today

Waltham, MA – May 3, 2023 – ImmunoGen Inc. (Nasdaq: IMGN), a leader in the expanding field of antibody-drug conjugates (ADCs) for the treatment of cancer, today announced positive top-line data 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 who have received one to three prior lines of therapy. Based on these data, the Company plans to submit a Marketing Authorization Application (MAA) in Europe and a supplemental Biologics License Application (sBLA) in the US for the conversion to a regular approval of ELAHERE.

"I believe the data from the confirmatory MIRASOL trial are practice-changing. They demonstrate ELAHERE's superiority to chemotherapy based on all efficacy endpoints, in particular overall survival, and build on the clinical benefit of ELAHERE previously reported in the SORAYA trial," said 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. "Last year's accelerated approval of ELAHERE was a paradigm-shifting development in the treatment landscape for this disease and I am confident that, with the MIRASOL data, ELAHERE has the potential to become the new standard of care for patients with FRα-positive, platinum-resistant ovarian cancer. FRα status is a 'must know' for all ovarian cancer patients and, for those with platinum-resistant disease who test positive, I believe ELAHERE should be their first treatment option."

MIRASOL (NCT04209855) is a randomized Phase 3 trial of ELAHERE versus investigator's choice (IC) of single-agent chemotherapy (weekly paclitaxel, pegylated liposomal doxorubicin, or topotecan). Eligibility criteria include patients with platinum-resistant ovarian cancer whose tumors express high levels of FRα, using the Ventana FOLR1 Assay, and who have been treated with up to three prior regimens. The primary endpoint of this trial is progression-free survival (PFS) by investigator assessment. Key secondary endpoints include objective response rate (ORR) and overall survival (OS).

Key Findings from MIRASOL

MIRASOL enrolled 453 patients; 14% had one prior line of therapy, 39% had two prior lines of therapy, and 47% had three prior lines of therapy. 62% of patients received prior bevacizumab; 55% received a prior PARP inhibitor. As of the data cutoff on March 6, 2023, the median follow-up time for OS was 13.1 months; 14% of patients on the ELAHERE arm remained on study drug compared to 3% on the IC chemotherapy arm.

• ELAHERE demonstrated a statistically significant and clinically meaningful improvement in OS compared to IC chemotherapy. With 204 OS events reported as of March 6, 2023, the median OS was 16.46 months in the ELAHERE arm, compared to 12.75 months in the IC chemotherapy arm, with a hazard ratio (HR) of 0.67, p=0.0046. This represents a 33% reduction in the risk of death in the ELAHERE arm in comparison to the IC chemotherapy arm.
• ELAHERE demonstrated a statistically significant and clinically meaningful improvement in PFS by investigator assessment compared to IC chemotherapy, with a hazard ratio of 0.65 (p<0.0001), which represents a 35% reduction in the risk of tumor progression or death in the ELAHERE arm compared to the IC chemotherapy arm. The median PFS in the ELAHERE arm was 5.62 months, compared to 3.98 months in the IC chemotherapy arm.
• ORR by investigator assessment in the ELAHERE arm was 42.3%, including 12 complete responses (CRs), compared to 15.9%, with no CRs, in the IC chemotherapy arm.
• PFS and ORR results by blinded independent central review were concordant with investigator assessment.
• The safety profile of ELAHERE continues to consist predominantly of low-grade ocular and gastrointestinal events. No new safety signals were identified. Compared with IC chemotherapy, ELAHERE was associated with lower rates of:
• Grade 3 or greater treatment-emergent adverse events (TEAEs) (42% vs 54%);
• Serious adverse events (24% vs 33%); and
• TEAEs leading to discontinuation of study drug (9% vs 16%).

"We are elated with the positive top-line results from MIRASOL. We believe the impressive efficacy data and consistent safety data reinforce ELAHERE's benefit for patients with platinum-resistant ovarian cancer," said Anna Berkenblit, MD, Senior Vice President and Chief Medical Officer of ImmunoGen. "Importantly, ELAHERE is the first drug to show an overall survival benefit in this patient population. These results are remarkable and we extend our appreciation to all of the patients and physicians who participated in MIRASOL. We look forward to presenting full data from the trial at a medical meeting later this year."

"These MIRASOL data show ELAHERE is a first-in-class, biomarker-driven ADC for the treatment of FRα-positive platinum-resistant ovarian cancer and mark a significant milestone for patients and our organization," said Mark Enyedy, ImmunoGen's President and Chief Executive Officer. "We believe these data will provide  the foundation for pursuing a marketing authorization in Europe and elsewhere, and seeking full approval in the US, support our goal of delivering ELAHERE to FRα-positive patients worldwide, and reinforce our conviction in our clinical development program to move this therapy into broader populations, including platinum-sensitive disease. ELAHERE’s differentiated safety and efficacy data provides further validation of our leading ADC platform and broad clinical pipeline of novel ADCs for solid tumors and hematologic malignancies."

In November 2022, the US Food and Drug Administration granted accelerated approval for ELAHERE for the treatment of adult patients with FRα-positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer who have received one to three prior systemic treatment regimens based on ORR and duration of response data from the pivotal SORAYA trial. Based on the MIRASOL results, ImmunoGen plans to submit an MAA to the European Medicines Agency and a sBLA to the FDA in the second half of this year.

CONFERENCE CALL INFORMATION

ImmunoGen will hold a conference call today at 8:00 AM. ET to discuss these results. To access the live call by phone, dial (877) 407-8835. The call may also be accessed through the Investors and Media section of the Company's website, www.immunogen.com. Following the call, a replay will be available at the same location.