ImmunoGen Announces FDA Fast Track Designation for Mirvetuximab Soravtansine in Patients with Platinum-Resistant Ovarian Cancer

Inc. (Nasdaq: IMGN), a leader in the expanding field of
antibody-drug conjugates (ADCs) for the treatment of cancer, today
announced that the U.S. Food and Drug Administration (FDA) has granted
Fast Track designation for its lead program, mirvetuximab soravtansine.
The designation is for the treatment of patients with medium to high
folate receptor alpha (FR?)-positive platinum-resistant ovarian cancer
who received at least one, but no more than three prior systemic
treatment regimens, and for whom single-agent chemotherapy is
appropriate as the next line of therapy.

“We are pleased the FDA has granted Fast Track designation for
mirvetuximab soravtansine,” said Mark Enyedy, ImmunoGen’s President and
Chief Executive Officer. “Patients with platinum-resistant ovarian
cancer have a poor prognosis and we are encouraged by the FDA’s
recognition of the significant need for new therapeutic options that may
be addressed by mirvetuximab as monotherapy. This important designation
is based on the promising safety and activity findings observed to-date
and we look forward to working closely with the FDA as we advance the
development of mirvetuximab.”

The FDA’s Fast Track Designation is intended to facilitate development
and expedite review of drugs to treat serious and life-threatening
conditions so that an approved product can reach the market

Mirvetuximab soravtansine is being evaluated in the FORWARD I Phase 3
trial. The trial is designed to randomize 333 patients 2:1 to receive
either mirvetuximab soravtansine or the physician’s choice of
single-agent chemotherapy. Eligibility criteria include patients with
platinum-resistant ovarian cancer that express medium or high levels of
FR? who have been treated with up to three prior regimens. The primary
endpoint of this study is Progression Free Survival, which is being
assessed in the entire study population and in the subset of patients
with high FR? expression. Enrollment of FORWARD I was completed ahead of
schedule in April 2018 and ImmunoGen expects to report top-line results
from the FORWARD I trial in the first half of 2019.

ImmunoGen is partnering with the Gynecologic Oncology Group Foundation
Inc., a leader in clinical research in gynecologic malignancies, on
FORWARD I, which is being conducted in North America and Europe. This
trial is intended to support full marketing approval of mirvetuximab for
patients with platinum-resistant ovarian cancer.

Mirvetuximab soravtansine is also being assessed in multiple
combinations in the FORWARD II trial. FORWARD II is a Phase 1b/2 study
of mirvetuximab in combination with Avastin® (bevacizumab),
or Keytruda® (pembrolizumab) in patients with FR?-positive
platinum-resistant ovarian cancer, primary peritoneal, or fallopian tube
tumors, as well as a triplet combination of mirvetuximab plus
carboplatin and Avastin in patients with platinum-sensitive ovarian

About Mirvetuximab Soravtansine

Mirvetuximab soravtansine (IMGN853) is the first folate receptor alpha
(FR?)-targeting ADC. It uses a humanized FR?-binding antibody to target
the ADC specifically to FR?-expressing cancer cells and a potent
anti-tumor agent, DM4, to kill the targeted cancer cells.

About Ovarian Cancer and FR?

It is estimated that 22,000 women are diagnosed annually with ovarian
cancer in the US. With more than 14,000 deaths each year, ovarian cancer
accounts for more deaths than any other cancer of the female
reproductive system.1

Standard first-line therapy for ovarian cancer is a platinum-based
combination regimen. Once the cancer becomes platinum-resistant,
treatment options include single-agent cytotoxic therapies such as
pegylated liposomal doxorubicin, paclitaxel, or topotecan, and
combination therapies that include Avastin.

There is a significant need for more effective, better-tolerated
therapies for recurrent ovarian cancer. It is estimated that
approximately 19,000 women in the US and approximately 24,000 women in
the EU have platinum-resistant ovarian cancer requiring second-line or
later treatment.2 ImmunoGen estimates that 60% of ovarian
cancer cases have medium or high FR? expression.

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.” Our lead product candidate, mirvetuximab soravtansine, is in Phase
3 study for folate receptor alpha (FR?)-positive platinum resistant
ovarian cancer, and in Phase 1b/2 testing in combination regimens. Our
novel IGN candidates for hematologic malignancies, IMGN779 and IMGN632,
are in Phase 1 studies. Learn more about who we are, what we do, and how
we do it at www.immunogen.com.

Keytruda® and Avastin® are registered trademarks
of their respective owners.

1 American Cancer Society. Cancer Facts & Figures 2018.
Atlanta, Ga: American Cancer Society; 2018.2 Decision
Resources Group Patientbase.

This press release includes forward-looking statements based on
management’s current expectations. These statements include, but are not
limited to, ImmunoGen’s ability to expand the addressable patient
population for mirvetuximab soravtansine and the regulatory and
commercial potential of mirvetuximab combinations in earlier lines of
therapy. 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. It should be noted that
there are risks and uncertainties related to the development of novel
anticancer products, including risks related to preclinical and clinical
studies, their timings and results, and the potential that earlier
clinical studies may not be predictive of future results. A review of
these risks can be found in ImmunoGen’s Annual Report on Form 10-K for
the year ended December 31, 2017 and other reports filed with the
Securities and Exchange Commission.

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