What happened

Shares of Puma Biotechnology (NASDAQ:PBYI) are up 44% at 12:12 p.m., having been up as much as 84% today, as investors digest the Food and Drug Administration documents posted ahead of Wednesday's advisory committee meeting to review the approvability of Puma's breast cancer drug neratinib.

So what

Today's jump wasn't because of surprisingly good data but rather a relative lack of negative opinions of neratinib by the FDA reviewers.

Puma Biotechnology is trying to get neratinib approved as an extended treatment for breast cancer after the patient has received surgery followed by Herceptin and chemotherapy. The clinical trial showed that adding neratinib to that standard of care decreased the reoccurrence of breast cancer compared to placebo after a year of treatment with neratinib. But the drug causes severe diarrhea in many patients, which leads to patients reducing the dose or stopping the drug altogether.

Good efficacy with poor tolerability has led investors to worry about the approvability of the drug. The company's negotiating with the FDA over which patients to include in the analysis didn't help, although that issue appears to be mostly worked out. Today's review was fairly neutral, with the documents for the advisory committee noting that the efficacy analysis was basically the same using either set of data, "supporting an effect of neratinib" while also pointing out that "the tolerability of neratinib in this patient population is a concern."

Doctor looking at a mammogram.

Image source: Getty Images.

Now what

The lack of a negative review is good news for Puma Biotechnology, but given the fairly neutral stance, the committee of outside experts could have a big influence on the FDA's final decision when they meet on Wednesday. The biggest thing for investors to watch is the committee's opinion of the diarrhea side effect and whether the members think treating with an antidiarrheal prophylaxis is an acceptable way to combat it. The doctors' opinions on neratinib relative to other post-surgery treatments will also be important because a negative opinion relative to the other treatments could foretell poor neratinib sales even if the drug is approved.

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