Now that the best-selling Atripla has a real competitor, an important question becomes, can people switch from Atripla to the new Complera?

While switching to a new and improved drug or regimen is common, there was a concern about switching here because of the way these drugs act in the body. The Sustiva in Atripla activates the protein (or enzyme) CYP34A. This is the same enzyme that the body uses to metabolize the Edurant in Complera. Sustiva has a long half-life and therefore drops the blood levels of Edurant; because there's too much metabolism of Edurant going on, the body eliminates it.

That interaction was recognized thanks to an earlier study that found a drop in Edurant blood levels of about 25% for approximately four weeks in volunteers switching from Sustiva to Edurant.

Fortunately, an HIV study presented at both ICAAC and IDSA has now shown that patients were able to maintain undetectable viral loads when switching from Atripla to Complera, out to three months. There was only a small temporary drop in Edurant blood levels.

At ICAAC, Dr. Tony Mills of Los Angeles made a poster presentation on the 49 patients switching from Atripla to Complera because of problems with tolerability. Atripla was their first HIV regimen and they had been taking it for at least three months (the average was 2.5 years). At week 12 after the switch, all 49 still had undetectable viral loads of less than 50 copies per mL.

The study did see lower than expected blood levels of Edurant at week one, but not at the other weeks in which bloodwork was collected (2, 4, 6, 8, and 12). The study concluded that "brief [Sustiva] inductive effects on [Edurant] metabolism may not be clinically relevant in suppressed patients [those with undetectable viral loads]," since all viral loads were undetectable at 12 weeks. The study also reported that Complera was well tolerated.