Shares of Gilead Sciences ( GILD) dropped in after-hours trading Tuesday after the company failed in a first attempt to create a new HIV pill with Bristol-Myers Squibb ( BMY).

After markets had closed, Gilead said it would reformulate the pill, which combines its Truvada HIV pill with Bristol-Myers Squibb's Sustiva HIV medication. Assuming the reformulation works, Gilead said it believed it could submit on application for the once-a-day combination drug to the Food and Drug Administration by year-end.

Gilead's stock dropped $1.80, or 4.6%, to $37.50 in after-hours trading after having lost a penny in regular trading. Shares of Bristol-Myers Squibb lost 17 cents in regular trading to close at $25.69.

The companies established a U.S. joint venture last December to develop a Truvada-Sustiva pill. Gilead said the first formulation failed because the Sustiva levels measured in patients' blood were too low.

Because blood levels of Sustiva in the combination pill were less than those of Sustiva alone, the experimental pill failed to achieve bioequivalence. The companies must meet this standard to convince the FDA to approve the new pill.

The companies say a once-a-day combination pill "may help simplify HIV therapy." Gilead's Truvada is, itself, a once-a-day combination drug containing Emtriva and Viread.

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