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What: Shares of Onconova Therapeutics (NASDAQ:ONTX), a clinical-stage biopharmaceutical company developing small-molecule drugs to treat cancer, dropped as much as 22% after announcing the interim results of its late-stage metastatic pancreatic cancer study.

So what: According to Onconova's press release, which came out after the closing bell yesterday, Onconova is discontinuing its late-stage study of intravenous rigosertib in combination with Eli Lilly's Gemzar in treating front-line metastatic cancer. After reviewing the interim data from Onconova's Ontrak trial, the data safety monitoring board concluded that rigosertib in combination with Gemzar was unlikely to demonstrate an overall survival benefit over the placebo. There were, however, no safety issues with the drug.

Now what: Chalk up yet another failure for a metastatic pancreatic cancer drug. The good news here for Onconova is that it does have other ongoing clinical trials across three different drugs. The worrisome aspect, though, is that rigosertib is by far the most advanced of those experimental drugs being tested. The remaining trials -- as a first-line low risk and second-line high-risk treatment for myelodysplastic syndrome, and for head and neck cancer -- are as a monotherapy and not a combo drug, so there's still hope it will perform well in treating these specific cancer types. However, I'd be lying if I said I wasn't incredibly concerned about this lead drug following yesterday's announcement.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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