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Project Inform
CASTLE Study Compares Reyataz to Kaletra

By Paul Dalton

October 27, 2008

A poster presented at the joint 2008 ICAAC/IDSA meeting in Washington, DC found that after 96 weeks more people taking Reyataz (atazanavir) boosted with Norvir (ritonavir) once daily had HIV levels below 50 copies compared to those taking Kaletra (lopinavir + ritonavir) twice daily. The study also found similar rates but different types of side effects for these two widely used protease inhibitors (PIs).

The CASTLE study looked at almost 900 people taking HIV drugs for the first time, who were randomly assigned to take either boosted Reyataz once a day or Kaletra twice a day, both combined with Truvada (tenofovir + emtricitabine/FTC). 31% of people were female and the average CD4 count at the start of the study was around 200. The study was designed to compare the proportion of people with HIV levels below 50 copies, as well as the frequency and types of side effects reported.

After 96 weeks, 70% of people taking boosted Reyataz had HIV levels below 50 copies compared to 63% of those on Kaletra. This difference was due to the higher rates of those who stopped taking Kaletra. When they were excluded from the analysis, 89% of those on Reyataz had undetectable HIV vs. 88% on Kaletra. People taking Kaletra had somewhat larger increases in CD4 counts, with an average gain of 290 cells compared to 268 for Reyataz.

There were similar rates of side effects reported by both groups, but the types were quite different. Overall, 14% of people taking Reyataz reported a significant side effect compared to 11% taking Kaletra. Those on Reyataz experienced elevated levels of bilirubin and jaundice more often while those taking Kaletra reported more diarrhea and other gastrointestinal (GI) symptoms.

There are two important caveats to this study. First, Kaletra is now widely used once daily when taken as part of a person's first regimen. Second, the older soft gel formulation was used by most people in the study. The newer tablet formulation might cause GI problems somewhat less frequently, although the data are not completely clear.

Reyataz is listed as a preferred option for first line therapy in the Federal Guideline and has surpassed Kaletra as the best selling PI in the US. This study supports using boosted Reyataz as part of a person's first HIV regimen. Another poster from this study was presented at this meeting.

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