Amprenavir (Agenerase) is a protease inhibitor (PI) whose use in Canada is restricted to PHAs who have previously used anti-HIV drugs. Amprenavir is not as potent as first-line PIs, such as indinavir (Crixivan). In order to increase levels of amprenavir in the blood, and thus increase its anti-HIV activity, this drug is being tested in combination with the PI ritonavir. Researchers in the U.S. recently reported preliminary results from 104 subjects who were enrolled in a study testing two combinations of ritonavir-amprenavir.

Study details

A total of 115 subjects whose therapy was failing were enrolled in this study but data on only 104 subjects was presented. Subjects received one of two PI combinations together with the anti-HIV drugs abacavir (Ziagen) and either efavirenz (Sustiva) or tenofovir (Viread). The PI combinations tested were as follows:

• amprenavir 600 mg and ritonavir 100 mg, both twice daily
• amprenavir 900 mg and ritonavir 100 mg, both twice daily

At the start of the study, subjects had the following profile:

• average age – 43 years
• 16% female, 84% male
• average viral load – about 14,000 copies
• average CD4+ count – about 260 cells
• the most commonly used protease inhibitors before entering this study were indinavir (Crixivan) and nelfinavir (Viracept)
• on average, subjects had been taking anti-HIV therapy for about 5½ years
• testing of blood samples found that all 104 subjects had HIV that was sensitive to the anti-HIV effects of amprenavir

Results

After six months, the proportion of subjects who achieved a viral load of fewer than 200 copies was as follows:

• amprenavir 600 mg – 69%
• amprenavir 900 mg – 73%

The average increase in CD4+ cell counts seen in the study were as follows:

• amprenavir 600 mg – 72 cells
• amprenavir 900 mg – 31 cells

The researchers are not sure why this difference occurred.

Side effects

The proportion of subjects in the amprenavir 600 mg group who experienced selected side effects was as follows:

• diarrhea – 50%
• nausea – 27%
• abdominal pain/discomfort – 20%
• vomiting – 17%

The figures for the amprenavir 900 mg group were as follows:

• diarrhea – 32%
• nausea – 71%
• abdominal pain/discomfort – 21%
• vomiting – 29%

Three subjects in each group left the study because of the following side effects:

• tiredness/lack of energy
• nausea
• diarrhea
• abdominal pain
• rash

Levels of amprenavir in the blood were similar in both the 600 mg and 900 mg groups. Tenofovir and efavirenz did not appear to reduce levels of amprenavir in the blood. Efavirenz's effect on amprenavir blood levels is being confirmed in another study. The study authors suggest that amprenavir 600 mg, boosted with ritonavir 100 mg, both drugs taken twice daily, may be a better regimen than higher doses of amprenavir (also boosted with ritonavir).

REFERENCE

Schooley R, Haubrich R, Sension M, et al. Efficacy, safety and amprenavir pharmacokinetic responses of twice-daily amprenavir and low-dose ritonavir regimens in HIV-1-infected, treatment-experienced adults for 24 weeks (ESS40006). Poster 1924 - 41st annual Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), Chicago, 16–19 December 2001.

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