Vertex Shareholders Stay Vexed

Believe it or not, that's the easy part of understanding the upcoming data. Now, we get to the complexities.

If all 20 patients in Arm D of the study were to make it through 12 weeks of treatment and 12 weeks of observation, interpreting the SVR12 data would be relatively easy. We'd just take the number of patients with undetectable virus and divide by 20 to come up with an answer.

Unfortunately, it's very possible (and probably likely) that not all 20 patients in Arm D make it through 12 weeks of treatment. (And remember, these patients are getting telaprevir, interferon alpha and ribavirin.)

According to criteria built into the Prove 1 study design, Arm D patients have to achieve undetectable levels of virus at four weeks and 10 weeks of treatment in order to stop treatment altogether at 12 weeks. If a patient doesn't meet this interim efficacy hurdle, doctors will order patients to continue receiving interferon and ribavirin for an extended period beyond 12 weeks.

Since these "nonresponding" patients will still be receiving treatment for their hepatitis C, Vertex won't be able to analyze them for SVR12 response. The denominator for the Arm D analysis, therefore, will be something less than 20 patients.

And this is where investors will have to pay close attention to how Vertex reports the data. Here's why:

Let's assume that Vertex tells us that 13 patients in Arm D achieved an SVR 12 weeks after stopping treatment with telaprevir, interferon and ribavirin. Furthermore, Vertex tells us that four patients were deemed "nonresponders" and are therefore still being treated with interferon-ribavirin beyond 12 weeks.