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A combination of two HIV drugs failed bioequivalence tests, the company says.

Gilead Sciences'

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combination of two HIV drugs failed bioequivalence tests, sending the company's shares lower after the market closed Tuesday.

Gilead shares sank nearly 5% to $40.70 after hours. According to the company, a pill combining Gilead's Truvada and

Bristol-Myers Squibb's

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Sustiva didn't provide the same medication levels in a patient's blood as separate doses of each drug would.

In April, Gilead said it

failed in its first attempt at combining the drugs. The company plans to evaluate up to three new formulations. Bioequivalence and stability studies of these new formulations will be conducted in the next few months.

Pending the results of the studies, Gilead and Bristol-Myers Squibb expect to file a new drug application with the Food and Drug Administration in the first half of 2006. If approved, the new pill would be the first complete treatment regimen of its kind available in a fixed-dose combination taken once a day.

Bristol-Myers shares were down a penny to $25.10.