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SAN FRANCISCO — The stock price of Trimeris Inc., a small biotechnology company, soared on stellar results from pivotal human tests on its experimental AIDS drug, which could help patients resistant to currently available treatments. 

Trimeris of Durham, N.C. is codeveloping the drug, dubbed T-20, along with the U.S. subsidiary of the Swiss health care giant Roche Holding AG. Trimeris stock price rose $11.25 a share, or nearly 29 percent, to $50.50 at the close of trading Thursday on the Nasdaq Stock Market. 

Existing AIDS drugs generally work inside cells, targeting viral enzymes involved in the replication of the virus. T-20 is the first of a new class of HIV drugs called fusion inhibitors, which stop the AIDS-causing virus before it gets into healthy cells. 

In the phase III test, patients who got a combination of T-20 and other drugs already approved by the Food and Drug Administration experienced a greater reduction of HIV in their blood than patients who were just taking the approved drugs. 

“These results are even better than the positive results of earlier studies had led us to expect,” said William M. Burns, head of pharmaceuticals at Roche. 

Using the results of that test and another currently underway, Trimeris and Roche plan to apply for FDA approval later this year and hope to have T-20 on the market next year. 

Dennis Harp, an analyst with Deutsche Bank Securities Inc., expects $600 million in annual sales in the United States alone.