The results of a new study comparing the efficacy of Isentress (raltegravir) and Sustiva (efavirenz) were presented on Sunday at the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy.

Results of the study showed that the efficacy of Isentress was similar to the efficacy of Sustiva in treatment-naïve patients. Isentress reduced HIV RNA levels in two percent more patients than Sustiva, and, on average, Isentress raised CD4 levels higher. However, the RNA levels and CD4 counts were not statistically different between the patients receiving Isentress and the patients taking Sustiva.

CD4 cells are white blood cells that help protect the body from foreign pathogens. When HIV infects humans, it mainly targets the CD4 cells, and decreases their quantity, making it harder to fight infections.

The study followed 563 previously untreated HIV infected individuals for 96 weeks in a Phase 3 study called STARTMRK. Isentress or Sustiva was administered in combination with Truvada (emtricitabine/tenofovir). Isentress was administered to participants in a 400 mg pill taken twice daily, and Sustiva was administered in a 600 mg pill once daily.

“Results from the 96 week analysis of STARTMRK showed that Isentress in combination therapy was as effective as [Sustiva] at suppressing HIV viral load and increasing immune system function,” said Edwin de Jesus, M.D., F.A.C.P., medical director of the Orlando Immunology Center in Orlando, Florida.

Isentress is different than any other antiretroviral treatment that is currently on the market. It is an integrase inhibitor, whereas all other available treatments are protease, reverse transcriptase, or fusion inhibitors. Integrase inhibitors prevent HIV genetic material from being integrated into DNA of CD4 cells, therefore inhibiting viral replication.

Isentress was approved by the United States Food and Drug Administration (FDA) in 2007 for the treatment of HIV patients who have undergone previous therapy. On July 28, the FDA approved Isentress for patients who have not had any prior HIV treatment, based on preliminary results from this trial.

Sustiva is a reverse transcriptase inhibitor, which was approved by the FDA in 1998. Sustiva blocks the conversion of HIV RNA into DNA, which disrupts further replication.

In this study, side effects were reported for 47 percent of participants treated with Isentress compared to 78 percent of patients treated with Sustiva. The most common side effects were headache, nausea, weakness, and fatigue.

Merck recently launched a program in the U.S. offering reduced costs to patients taking Isentress who are insured and have co-payments or coinsurance from $30 to $400 per month. This discount is valid for up to one year.

For more information, see the press release on the Merck Web site.