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What: Shares of Isis Pharmaceuticals (NASDAQ:IONS) dropped by nearly 10% today after the company announced top-line results from a mid-stage study for its type 2 diabetes drug candidate called ISIS-PTP1B Rx. The study's primary objective was to examine the average change in glycated hemoglobin (HbA1c) -- a commonly used measure of glucose control -- in obese patients after 27 weeks.

Although Isis reported a significant change in HbA1c in patients receiving the drug compared to those taking a placebo, a closer look at the press release shows that the average reduction in HbA1c was assessed at 36 weeks, not 27.

Isis didn't offer any explanation of the discrepancy. But the Vice President of clinical development Sanjay Bhanot, M.D., Ph.D. did note on the press release that the company is "encouraged that the improvements in glucose control and body weight continued to increase through the treatment period." Put simply, it doesn't look like the drug met its primary outcome measure within the specified time frame.  

So what: Drug companies are often reticent to release negative trial results for top clinical candidates because the market generally doesn't react well. So it's not uncommon to see press releases that focus on any positive aspects of studies that have essentially failed, such as this one for ISIS-PTP1B Rx. The problem moving forward, though, is that future trial results are probably going to receive extra scrutiny.

Now what: Isis stated in its press release that it plans on furthering the development of ISIS-PTP1B Rx as a potential therapy for the control of blood sugar in type 2 diabetics. Until the details of this midstage study are released at an upcoming medical meeting, though, the market appears content on doubting the drug's potential for this indication going forward, despite the reported significant change in average HbA1c reduction 9 weeks past the study's pre-specified cutoff date.

That said, Isis does have one of the deepest clinical pipelines in the industry that is expected to produce multiple data readouts this year -- which should help to dampen any lingering long-term effects from today's news.

George Budwell owns shares of Isis Pharmaceuticals. The Motley Fool recommends Isis Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.