Updated from 7:04 a.m. EDT

Gilead Sciences ( GILD) reported results Thursday from a phase III study showing its experimental drug darusentan successfully treated patients with high blood pressure that was not controlled by other medicines.

Investors reacted well to the initial look at the darusentan results, pushing Gilead shares higher almost 6% to $47.30 in Thursday's after-market trading session. Shares were rising 3.9% to $46.42 Friday afternoon.

The Foster City, Calif.-based company is best known for its market-leading HIV drug franchise, but more recently Gilead has sought to diversify into other disease areas, including cardiovascular disease.

Investors have been somewhat wary of this effort, so they were waiting for results from this first phase III study of darusentan to assess the new push outside of HIV. That includes Gilead's decision last month to acquire CV Therapeutics ( CVTX) and its chronic angina drug for $1 billion.

Darusentan, a once a day pill, is being developed as a treatment for resistant hypertension, defined as high blood pressure that is not alleviated by treatment with a stable of other drugs. The number of Americans currently diagnosed with resistant hypertension ranges widely from 2 million to 5 million, and analysts believe darusentan, if approved, could generate sales for Gilead in the range of $500 million to $1 billion.

In the phase III study announced by Gilead, all three doses of darusentan tested were able to reduce systolic and diastolic blood pressure compared to placebo after 14 weeks of treatment. The results were statistically significant, achieving the co-primary endpoints of the study.

Moreover, more than half the darusentan-treated patients achieved their stated blood-pressure reduction goal compared to one-quarter of patients treated with a placebo, Gilead said.

The safety and tolerability of darusentan was a bigger concern than efficacy, but here, too, the drug seems to have performed well.

Liver function tests were comparable between patients with darusentan and placebo. One death was reported in the study but the patient was treated with the placebo.

The most common adverse event reported was peripheral edema, or swelling of the limbs, which occurred in 32%, 36% and 29% of darusentan-treated patients compared to just 17% of placebo patients. Gilead said the swelling reported by patients was mild to moderate in nature and considered to be clinically manageable.

Peripheral edema is a known side effect of the other high blood pressure drugs taken by patients in the darusentan study. Moreover, the swelling reported by patients treated with the middle dose of darusentan only led 5% of them to stop taking the drug in the study. The discontinuation rate for patients in the high-dose darusentan arm was 6%. No patients treated with placebo dropped out due to swelling.

Darusentan also caused patients' hemoglobin levels (a measure of anemia) to drop compared to placebo, but Gilead said the reduction was similar to what was observed in prior phase II studies of darusentan and was not believed to be worrisome for patients.

A second phase III study of darusentan is expected to be completed by the end of the year.

Other companies with drugs to treat various forms of high blood pressure include Merck ( MRK), Pfizer ( PFE), King Pharmaceuticals ( KG) and Novartis ( NVS).

At the time of publication, Feuerstein's Biotech Select model portfolio was long Gilead.

Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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