Darunavir (Prezista) is an effective anti-HIV drug, which must be taken with other antiretroviral drugs. It was licensed in the United States following favourable results in two ongoing phase IIB trials, POWER 1 and POWER 2. These trials are looking at the risks and benefits of darunavir in patients with substantial treatment experience. In both studies, patients took darunavir along with an optimised background regimen of two nucleoside reverse transcriptase inhibitors (NRTIs). Around half of the patients also took T-20 (enfuvirtide, Fuzeon).

The studies compared four doses of darunavir patients with experience of all three main anti-HIV drug classes. All of the patients had one or more primary protease inhibitor mutations, with a median viral load of 40,700 copies/ml and a CD4 cell count of 136 cells/mm³. Despite this, 45% of the 131 patients receiving the highest dose of 600mg twice daily, boosted with 100mg ritonavir (Norvir) twice a day achieved viral loads below 50 copies/ml after 24 weeks of treatment, with a mean viral load reduction of 1.89 log₁₀: 70% of the patients taking darunavir had a viral load reduction of 1 log₁₀ or more. CD4 cell counts rose by a mean of 92 cells/mm³ in this group.

The efficacy of darunavir was compared to a group of patients given another ritonavir-boosted protease inhibitor, chosen on the basis of resistance testing. These patients had had a mean viral load reduction of only 0.48 log₁₀ and CD4 cell count rise of 17 cells/mm³. The effectiveness of darunavir was not significantly improved by the inclusion of T-20 in the background regimen, although it is not clear whether the patients who were taking T-20 were at a similar disease stage before starting treatment to those who were not taking it^[1][2][3].

The effectiveness of darunavir has now been demonstrated in 48-week follow-up of the POWER studies. In a combined analysis, 61% of the patients randomised to the darunavir arm had a viral load decrease of at least 1 log₁₀ after 48 weeks, compared to 15% in the comparator protease inhibitor arm. Similarly, 46% of the patients taking darunavir had viral loads below 50 copies/ml, compared to 10% of the control patients. The 48-week analysis also reached similar conclusions regarding the drug's effectiveness in increasing CD4 cell counts and the impact of T-20 use^[4].

A small study has also found that ritonavir-boosted darunavir may be effective in patients with substantial treatment experience, including T-20 and tipranavir (Aptivus)^[5].

There are no available data on the safety and efficacy of darunavir in patients who have not taken any other anti-HIV drugs before. The effect of liver damage on the efficacy of darunavir is also unknown.