ImmunoGen announced Friday that a Phase III study of the experimental antibody-drug conjugate (ADC) mirvetuximab soravtansine in patients with folate receptor (FR) alpha-positive, platinum-resistant ovarian cancer failed to meet its primary endpoint of progression-free survival (PFS). Shares in the company plunged as much as 48 percent on the news.

The FORWARD I trial randomised 366 patients with FR alpha-positive, platinum-resistant ovarian cancer to receive either mirvetuximab soravtansine or the physician's choice of single-agent chemotherapy. ImmunoGen noted that the main goal of PFS was assessed in the entire study population and in the subset of patients with high FR alpha expression.

Top-line results showed that in the entire study population, there was not a significant difference in the primary endpoint of PFS or overall survival between mirvetuximab soravtansine and chemotherapy, although the confirmed overall response rate (ORR) in the two groups was 22 percent and 12 percent, respectively. Meanwhile, in the subset of patients with high FR alpha expression, which represented 60 percent of the study population, the company noted that PFS was longer in patients who received mirvetuximab soravtansine compared with chemotherapy, with a hazard ratio of 0.69. Further, the confirmed ORR in this subset of patients was 24 percent for mirvetuximab soravtansine and 10 percent for chemotherapy, with overall survival also longer in those given the ADC.

ImmunoGen indicated that mirvetuximab soravtansine was well tolerated in the trial, with 46 percent of patients experiencing grade 3 or greater adverse events, compared to 61 percent for chemotherapy. The company added that dose reductions and discontinuations due to drug-related adverse events were 20 percent and 5 percent, respectively, for mirvetuximab soravtansine, versus 31 percent and 8 percent, respectively, for chemotherapy. The drugmaker plans to present further results from the study at an upcoming medical meeting.

Chief medical officer Anna Berkenblit said that given the efficacy seen in the subset of patients with high FR alpha expression, ImmunoGen is "conducting additional analyses to further evaluate the potential benefit of mirvetuximab soravtansine" in this group. CEO Mark Enyedy added "we have generated encouraging data with mirvetuximab combination regimens and will evaluate our ongoing studies as an independent path forward to support a registration in ovarian cancer."

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