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New Boosting Drug Looks Promising

February 2010

Norvir (ritonavir) was originally approved as a protease inhibitor in HIV regimens during its first few years on the market. Abbott Laboratories discovered that adding small doses of it to a regimen with the protease inhibitor lopinavir boosted its potency because it kept higher drug levels in the blood. Other companies used the drug with their protease inhibitors to boost those levels, also making them more effective.

However, ritonavir comes with troublesome side effects and drug interactions, especially in the older higher doses. So finding an alternative booster would help ease the stomach and metabolic problems that many people face when using ritonavir. A new drug from Gilead Sciences (GS-9350) looks promising in this boosting role. In order to get approved by the FDA, GS-9350 must be at least equal to ritonavir (bioequivalent) and be studied with any HIV drugs it will be used to boost.

In February 2009, early study results presented at CROI showed that GS-9350 performed as well as ritonavir. The booster was combined with the experimental integrase inhibitor elvitegravir and Truvada. Unlike ritonavir, GS-9350 has no anti-HIV effect. Its sole purpose is to enhance the effectiveness of other HIV drugs. It is hoped that GS-9350 will not affect blood fat or sugar levels as ritonavir does, or have as many side effects or drug interactions.

Results from a small study were also presented at ICAAC in September 2009 comparing GS- 9350 + Reyataz (atazanavir) to ritonavir + Reyataz in HIV-negative people. GS-9350 performed as well as ritonavir. Another small study in HIV-positive people is underway comparing GS-9350 + Reyataz + Truvada to ritonavir + Reyataz + Truvada.

Should GS-9350 continue to show good results in larger studies, it may become available as early as 2011. Since it is being studied by Gilead to boost their new integrase inhibitor elvitegravir, the company plans to roll it out as a complete regimen with elvitegravir and Truvada, known as the "quad" pill. A second booster called SPI-452 is also under early lab study boosting Prezista (darunavir) and Reyataz and shows promising results similar to those of GS-9350.

Having effective, more tolerable boosting agents can help improve a person's adherence thereby maintaining better control of HIV. They also provide competition to ritonavir that went through a 400% price increase in 2004. They're welcomed improvements in treating HIV disease.

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