BOULDER, Colo. (AP) _ Synergen Inc. reported Thursday promising results of preliminary trials of Antril, its drug for the treatment of sepsis syndrome.

A 28-day study indicated Antril could reduce the mortality rate of patients by 64 percent, said Synergen spokeswoman Debra Bannister.

The announcement sent Synergen's stock surging $5.50 to $63.25 a share in over-the-counter trading.

Antril is one of three drugs biotechnology companies are developing to treat sepsis syndrome, a dangerous blood infection.

''In a nutshell, I don't think the (Synergen) data could have been any better,'' said Robert Peterson, a research analyst with Denver-based Hanifen, Imhoff Inc.

In the study, Synergen tested three dose strengths of Antril and a placebo on 99 patients. Eleven of the 25 patients who received placebos died after 28 days, said Bannister. In the group that received the biggest dosage of Antril, the mortality rate was four out of 25.

Dr. Charles J. Fisher, director of the Center for Critical Care Research at University Hospitals of Cleveland, presented the results of the Synergen trial at a meeting of American College of Chest Physicians in San Francisco Thursday.

The next step for Antril is a trial in which up to 1,000 patients will be studied, Bannister said. The company hopes to enroll patients in the United States, Canada and Europe in the study, which will begin in 1992.

Sepsis syndrome affects 500,000 people a year in the United States, with 70,000 dying from the disease. It generally is treated with antibiotic therapy and blood-pressure medication, Bannister said.

Other drugs being developed to battle sepsis syndrome are Centoxin, from Centocor Inc. of Malvern, Pa., and E-5, developed by Xoma Inc. of Berkeley, Calif.

Synergen also said Thursday it planned to sell 3 million new shares of common stock. It plans to use the proceeds from the sale for working capital and corporate purposes, Bannister said.