ImmunoGen’s ADC misses endpoint in ovarian cancer phase 3

A phase 3 trial of ImmunoGen’s antibody-drug conjugate mirvetuximab soravtansine has missed its primary endpoint. The ADC failed to extend progression-free survival by more than chemotherapy, driving investors to wipe 50% off its stock price.

ImmunoGen designed mirvetuximab soravtansine to bind to folate receptor alpha, a protein that is overexpressed by epithelial derived tumors including ovarian cancer. By using an antibody to target the receptor, ImmunoGen thought it could deliver a cytotoxic payload to ovarian tumors, thereby killing cancer cells without harming healthy tissue.

The phase 3 data cast doubt on the hypothesis. The trial enrolled 366 women with platinum-resistant ovarian cancer that expressed medium to high levels of folate receptor alpha and had received up to three prior lines of therapy. 

PFS among the two-thirds of the women who took mirvetuximab soravtansine was no better than in the cohort that received chemotherapy. Mirvetuximab soravtansine numerically outperformed the control in a prespecified high folate receptor alpha subgroup. But the failure of the overall study and statistical approach used meant the p value of 0.049 was above the threshold for success.

With mirvetuximab soravtansine improving on the overall survival and response rate achieved by  chemotherapy in the subgroup, ImmunoGen sees enough positives in the data to conduct additional analyses. ImmunoGen thinks combination studies may offer the drug a path forward, too.

A more skeptical reader of the situation could point to earlier failures of other drugs targeting folate receptors to argue that the signs of efficacy are a mirage. Eisai acquired folate receptor farletuzumab in its $325 million takeover of Morphotek only to see it fail in phase 3 ovarian cancer trials. Around the same time, Merck and Endocyte halted a phase 3 trial of vintafolide in the same indication.

ImmunoGen took a different approach to the target and moved into late-phase development on the back of a phase 2 study that achieved a 53% response rate. The response rate in the phase 3 came in well below that at 22%. 

The failure to hit the PFS endpoint wiped 50% off ImmunoGen’s stock price in premarket trading. The stock had already traded down close to 20% in the days preceding the readout.