What happened

Shares of ImmunoGen (NASDAQ:IMGN) are down 14.9% at 11:34 a.m. EDT after the biotech reported data from the Forward II trial testing its drug mirvetuximab soravtansine, combined with Merck's (NYSE:MRK) Keytruda, in ovarian cancer patients. Unfortunately, the data, which was presented at the European Society for Medical Oncology (ESMO) 2018 Congress, didn't look nearly as good as the findings for the initial 14 patients that were presented previously.

Merck was trading flat on the day. The big pharma certainly isn't counting on this combination to drive sales of Keytruda. And Merck had multiple presentations of its own at ESMO that are more likely to set its valuation.

Doctor talking to a female patient in front of a window.

Image source: Getty Images.

So what

Most patients, 45 of 54, or 83%, to be exact, saw their tumors shrink, but most of those didn't reach the level to be considered responders. All told, there were 16 confirmed partial responses, for a confirmed overall response rate (ORR) of just 30%. Previously, in the initial 14 patients, the ORR was 43%.

Looking at the patients with medium or high FRalpha expression -- the target of mirvetuximab -- the confirmed ORR was 31%, down substantially from the 63% ORR seen in the initial eight patients.

It should be noted that these are still initial data, so there are some unconfirmed responses that could turn into confirmed responses. And a response rate of 30% is higher than some have seen with ovarian cancer patients who have failed multiple previous treatments. While it's hard to compare across trials, the patients in ImmunoGen's trial had previously received a median of three therapies.

But this is still a prime example of why investors should always be cautious of interpreting interim data from small populations. Depending on who was enrolled initially, the results could be off substantially from the true response rate.

Now what

The lower response rate is certainly bad news for the potential long-term sales of mirvetuximab but probably doesn't say much about its short-term prospects, which are tied to Forward I, a phase III study testing mirvetuximab as a monotherapy, which is scheduled to read out next year.

Investors who can't wait that long will get some data for a pair of ImmunoGen's phase I blood cancer drugs, IMGN779 and IMGN632, at the American Society of Hematology meeting in December.

Brian Orelli has no position in any of the stocks mentioned. The Motley Fool recommends ImmunoGen. The Motley Fool has a disclosure policy.