Gilead's

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second-quarter earnings surged 77% from a year ago, blowing past estimates, thanks to strong sales of its HIV-fighting drugs Viread and Truvada.

The biotech company earned $196.0 million, or 41 cents a share, in the quarter, compared with $111.5 million, or 24 cents a share, last year. Sales soared 55% from a year ago to $495.3 million. Analysts had been expecting earnings of 38 cents a share on sales of about $459 million.

Among drugs, Gilead said Viread sales were $209.1 million in the second quarter, up 6% from a year ago, while sales of Truvada were $123.1 million, up 35% from the first quarter (the drug was introduced in August 2004). Overall, Gilead's HIV product sales rose 61% from a year ago to $344.4 million.

In a separate release, Gilead said it has begun enrolling patients in a phase III trial evaluating its oral tenofovir DF in chronic hepatitis B. The company already sells Hepsera for the same disease.