Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Puma Biotechnology (NYSE:PBYI), a clinical-stage biopharmaceutical company focused on developing therapies to treat cancer, plunged as much as 25% after reporting phase 2 data on PB272 (also known as neratinib) at the American Society for Clinical Oncology's annual meeting over the weekend.

So what: Puma is actually being dealt a double blow today with investors souring on its reported data as well as punishing the company for a rival's failure.

Beginning with Puma's phase 2 results, the company reported data only from its first cohort of 40 patients which have metastatic breast cancer that had metastasized to the brain and received PB272 as a monotherapy. As the results show, a number of these patients experienced grade 3 diarrhea associated with PB272: 29% of patients not taking any therapy meant to curb neratinib-related diarrhea, and 21% of patients given a low-dose loperamide to counter neratinib-related diarrhea. Perhaps more damaging was that despite the data being described as "positive" by Puma, only three of 40 patients had a partial response, while four demonstrated prolonged stable disease (more than six months), and 12 experienced stable disease equating to six months or less. Median progression-free survival was 1.9 months, while median overall survival was 8.7 months. Clearly, investors were expected partial response figures to be more robust.

In addition, GlaxoSmithKline (NYSE:GSK) reported the failure of its metastatic breast cancer therapy, Tykerb, to meet its primay endpoint of improved disease-free survival in a phase 3 study in combination with Herceptin for HER2-positve early stage breast cancer. The thought here from investors is that if Glaxo's Tykerb is having difficulty in the neoadjuvant setting for metastatic breast cancer, then PB272 may have just as difficult a time.

Now what: Today is certainly a batten down the hatches kind of day if you're a Puma Biotechnology shareholder – but it's also not the end of the world, either. Keep in mind that Puma is actually running three mid-stage studies on PB272 and only had data at ASCO to present on one, so there's still plenty of time left for it to potentially demonstrate that PB272 is better than existing therapies on the market. I'd be lying if I said I wasn't disappointed with the low partial response rate in its 40-patient study, but brain metastases are inherently difficult to fight to begin with. The factor I would watch more would be those grade 3 diarrhea-related events. Safety is paramount to the Food and Drug Administration, and a considerably higher degree of adverse events could knock PB272 down the totem the pole. Moving forward I'd suggest keeping Puma on your watchlist but sticking the sidelines for the time being.

Puma may offer plenty of potential, but even it's unlikely to be able to keep pace with this top stock over the long run
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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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