The benefit for breast cancer patients treated with Puma's neratinib was "awfully small" for a drug that causes "a lot of diarrhea," said Dr. Harold Burstein, a breast cancer expert from the Dana-Farber Cancer Institute.
Puma shares are down more than 9% to $177.04 in Monday trading.
Burstein spoke to me following presentation of results from the neratinib phase III "ExteNet" study at the American Society of Clinical Oncology (ASCO) annual meeting Monday. Puma is developing neratinib for the treatment of HER2-positive breast cancer patients in the extended adjuvant setting.
The absolute disease-free survival for neratinib was 93.9% compared to 91.6% for placebo -- a difference of 2.3 percentage points. The benefit, while statistically significant, is tough to call clinically meaningful for breast cancer patients given the drug's toxicity, Burstein said.
Forty percent of patients treated with neratinib experienced grade 3 diarrhea, which is defined as seven more stools per day, incontinence and hospitalization.
"This is not someone simply eating too many chili peppers and having a single bout of diarrhea," said Burstein.
After the study was conducted, Puma implemented a protocol in which patients are treated with anti-diarrhea medicines before taking neratinib. The presenter of the ExteNet study data on Monday noted that this strategy lowered the incidence of grade 3 diarrhea but it still occurred in up to 17% of patients.
Burstein was also concerned that two years of follow up in the study was too short to conclude with certainty the drug's real benefit for patients. This may cause FDA regulators to ask for more data.