Ovarian cancer patients with tumors expressing high levels of a specific protein on their surface respond best to treatment with ImmunoGen's (IMGN) - Get Report "smart bomb" cancer drug, according to an update of an ongoing, early-stage clinical trial presented Sunday.

The overall response rate attributed to ImmunoGen's drug mirvetuximab soravtansin in the study of ovarian cancer patients is now 50%, slightly lower than the 53% response rate reported last June.

However, the response rate increased to 90% for a subset of patients with tumors most visible to ImmunoGen's drug. The tumors in these best responders are crowded with folate receptors on their surface, which is the primary target of mirvetuximab soravtansin.

Mirvetuximab soravtansin (formerly known as IMGN853) consists of a monoclonal antibody designed to seek out and attach itself to tumors with large quantities of folate receptor alpha, on their surface. The antibody carries a toxic chemotherapy payload, which is released into the tumor once attached.

Sunday's update of ImmunoGen's phase I study was presented at AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics. The study enrolled women with folate-positive ovarian cancer that had grown resistant to standard, platinum-based chemotherapy.

Among 20 evaluable patients, the overall response rate to mirvetuximab soravtansin was 50% encompassing two complete responses and eight partial responses.

Last June, the response rate was 53% among 17 evaluable patients. The earlier results were presented at the American Society of Clinical Oncology annual meeting.

On Sunday, ImmunoGen presented a new analysis which divided the 20 ovarian cancer patients based on the amount of folate receptors present on the surface of the tumor cells.

Ten patients had "high" levels of folate expression and nine of them responded to treatment with mirvetuximab soravtansin. Two of these patients had complete responses, seven patients had partial responses. Six patients with "medium" folate expression resulted in a single partial response. None of the four patients with "low" folate expression responded to mirvetuximab soravtansin.

Based on these new findings, ImmunoGen is planning a phase II study of mirvetuximab soravtansin which will enroll ovarian cancer patients with tumors containing medium and high levels of folate receptors. The study is expected to start before the end of the year.

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