What's happening: Shares of the antibody-drug conjugate specialist ImmunoGen (NASDAQ:IMGN) rose by 65% today, on tremendous volume, after the company released a clinical update for its experimental ovarian cancer drug, IMGN853, at the American Society of Clinical Oncology meeting last weekend. According to the press release, patients with heavily pre-treated ovarian cancer showed a 53% response rate when receiving IMGN853 in a small early stage study.

Although the update only provided data on 17 patients, it's worth noting that this response rate is more than double average for the current host of chemotherapies used in the clinic right now. In short, IMGN853 might turn out to be a major improvement in the standard of care for this devastating disease.  

Why it's happening: Investors are obviously overjoyed that ImmunoGen's clinical pipeline is finally starting to bear fruit. The company has been plagued by clinical setbacks for more than a decade at this point. And besides partnering with Roche on the breast cancer drug Kadcyla, ImmunoGen simply hasn't had that much success at building out its product portfolio of late. Going forward, the company plans to initiate a midstage trial for IMGN853 as a treatment for advanced ovarian cancer later this year that might serve as the basis for a regulatory filing with the Food and Drug Administration.

Before buying this stock on the back of this clinical update, however, it's important to remember that early stage results, even when overwhelmingly positive, are generally poor predictors of how larger trials ultimately play out -- especially among experimental cancer drugs.

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