Merck's HIV drug Isentress fails once-a-day study

NEW YORK Mon Nov 29, 2010 12:50pm EST

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NEW YORK (Reuters) - Merck & Co said once-daily use of its HIV drug Isentress was less effective in a late-stage study than standard twice-daily dosing among patients that had not previously been treated for the virus that causes AIDS.

Based on the disappointing initial results, the U.S. drugmaker said on Monday it was ending the Phase III study, which could have paved the way for wider use among previously untreated patients.

The product is approved for use with other HIV medicines among previously treated patients as well as those that have not yet undergone treatment. It is the only approved HIV drug that works by blocking integrase -- an enzyme that enables the virus to insert its genetic material into human DNA.

"While a once-daily approval would have been an upside event that could have resulted in more first-line usage of Isentress over time, we do not see today's news as a major negative for (Merck's) stock," J.P. Morgan analyst Chris Schott said in a research note.

Merck shares were down 1.4 percent at $34.36 in early afternoon trading on the New York Stock Exchange, amid declines in the broad stock market.

Isentress is one of Merck's fastest-growing medicines. Its sales leaped 41 percent in the third quarter to $278 million, putting it on track to become a $1 billion-a-year blockbuster.

Schott held to his forecast of annual Isentress sales of $2 billion by 2015. But he said its later peak sales will be less than they might have been, had the trial not failed.

One group in the 775-patient study received 800 milligrams of Isentress once daily, along with other HIV treatments. Another group received the approved Isentress 400-milligram dose given twice a day, also in combination with standard treatments.

After 48 weeks, HIV was driven to undetectable levels in 83.2 percent of patients receiving the once-daily regimen, which was deemed inferior to the 88.9 percent of patients who received twice-daily tablets.

(Reporting by Ransdell Pierson; Editing by Tim Dobbyn)

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