Gilead Sciences Inc's four-drug experimental HIV pill worked as well as a regimen containing protease inhibitor Reyataz in the second pivotal trial of the drug, the company said on Monday.

Although the trial succeeded, it did not show the experimental pill, called the Quad, was superior to the Reyataz regimen and Gilead shares fell less than a percentage point.

"Overall the data are good ... mostly in line with investor expectations," said Cowen and Co analyst Philip Nadeau.

"I think there might of been a little bit of expectation for statistical superiority, but only from a few outliers."

The trial found that 90 percent of patients taking the Gilead drug achieved target levels of HIV in the blood, or viral load, compared with 87 percent of patients on ritonavir-boosted Reyataz, which is sold by Bristol-Myers Squibb Co.

The four-drug pill, which combines experimental integrase inhibitor elvitegravir and boosting agent cobicistat with Truvada, a pill consisting of Gilead's older HIV drugs Emtriva and Viread, is seen as the biotechnology company's most important pipeline product.

Gilead said 5.1 percent of Reyataz patients dropped out of the trial, mainly due to elevated bilirubin levels, compared with a drop-out rate of 3.1 percent for patients treated with the Quad pill.

The company said it plans to file for U.S. regulatory approval of the Quad by the end of this year, earlier than its previous estimate of a first-quarter 2012 filing.

Gilead Chief Operating Officer John Milligan, speaking on a conference call, said the company will request a "priority review" by the Food and Drug Administration, which could clear the way for an approval as early as mid-year 2012.

Gilead shares, which fell less than one percent to close at $40.05 on Nasdaq, were slightly lower at $39.81 after hours.