Patients will now be switched back to FDA-approved twice-daily dosing.

Merck & Co. is discontinuing a Phase III study evaluating its marketed HIV drug Isentress® (raltegravir) as once-daily therapy in treatment-naive adults with HIV-1. The study showed that administration of 800 mg of Isentress once daily as part of an antiretrovial therapy regimen was not as beneficial as administering 400 mg of the drug twice daily, which is the FDA-approved dosing schedule for the drug. Merck is now recommending that patients enrolled in the once-daily dosing arm of the trial be switched over to twice-daily dosing.

The now-discontinued study compared the safety and efficacy of 800 mg Isentress administered once a day with that of 400 mg Isentress twice a day, in combination with a fixed-dose combination of emtricitabine and tenofovir disoproxil fumarate. The results showed that after 48 weeks of therapy, 83.2% of patients in the once-daily Isentress cohort achieved undetectable levels of viral RNA compared with 88.9% of patients receiving the twice-daily Isentress regimen. The 5.7% difference did not meet the predefined statistical criteria for noninferiority. Merck says the overall treatment difference was primarily due to results in patients with high viral load.

Isentress is an HIV integrase inhibitor approved for use in combination with other antiretroviral agents in the treatment of HIV-1 infection. The drug made worldwide sales of $267 million in the second quarter of 2010, up 55% on Q2 2009.

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