Fosamprenavir (Telzir) is converted into amprenavir (Agenerase) in the gut, so it has similar properties to amprenavir. Fosamprenavir itself is not active in the body, but amprenavir is active against HIV-1 and HIV-2.

Fosamprenavir has shown similar levels of effectivness to other protease inhibitors, with or without ritonavir boosting, in patients who have not taken antiretroviral therapy before:

• A study comparing fosamprenavir to nelfinavir (Viracept), when taken with abacavir (Ziagen) and 3TC (lamivudine, Epivir) found that fosamprenavir has similar antiviral potency to nelfinavir. After 48 weeks, 58% of the fosamprenavir group and 42% of the nelfinavir group had viral load below 50 copies/ml. However, fosamprenavir showed a trend towards superiority over nelfinavir in people with viral loads above 100,000 copies/ml before starting treatment. Fosamprenavir achieved these results without ritonavir (Norvir) boosting^[1].
• These findings were supported by the results of the SOLO study, which compared once-daily ritonavir-boosted fosamprenavir with twice-daily nelfinavir. After 48 weeks of treatment, the proportion of patients with viral loads below 50 copies/ml and CD4 cell count increases were similar in both groups, although there were more treatment failures and diarrhoea in the nelfinavir arm^[2]. Data from over two years of follow-up of patients taking ritonavir-boosted fosamprenavir-based regimens demonstrated the drug’s long-term effectiveness^[3].
• Thirdly, ritonavir-boosted fosamprenavir was non-inferior to ritonavir-boosted lopinavir (Kaletra) in the KLEAN study, which included 878 patients. The protease inhibitors were taken along with abacavir and 3TC. After 48 weeks, similar proportions of patients in both arms of the study had viral loads below 400 copies/ml, with similar rates of resistance^[4]. However, it is unclear what proportions of each group achieved viral loads below 50 copies/ml, which is now viewed as the goal of first-line antiretroviral therapy.

Ritonavir-boosted fosamprenavir has also been examined in the treatment of patients with prior exposure to anti-HIV treatment. The CONTEXT study compared two doses of ritonavir-boosted fosamprenavir and Kaletra, finding evidence that ritonavir-boosted fosamprenavir was inferior to Kaletra, in terms of average viral load reductions, with a greater risk of resistance in the fosamprenavir arms. The difference between drugs was more pronounced for the once-daily fosamprenavir arm^[5].

Fosamprenavir has poor penetration into the brain, but does penetrate the male genital tract and the semen.

Fosamprenavir has not been studied in patients with liver damage, so should be used cautiously in these patients. However, a sub-study of the SOLO trial found no evidence of increased risk of side-effects in patients co-infected with hepatitis B or C, or an increased risk of liver enzyme elevations^[6].

Fosamprenavir can be used at standard doses in patients with kidney damage.