GenVec Inc. has suspended human testing of its leading drug, a potential cancer therapy, after a patient died during a study.

The Gaithersburg biotechnology company's stock plunged 47 percent yesterday after it announced it had halted trials of TNFerade at the request of the Food and Drug Administration.

GenVec's stock closed at $1.20, down $1.06. But analysts said the link between the death and the drug has yet to be proved, making the prospects for the medicine and for GenVec unclear.

The first indication of potential problems came in a study presented at a European conference on Oct. 1.

Three of eight patients with esophageal cancer treated with the highest dose of the experimental drug, in combination with radiation, developed blood clots in their lungs and one of them died, according to a summary of the study. The death was "considered potentially related" to TNFerade, the presentation said.

Paul H. Fischer, GenVec's chief executive, said the company received the Food and Drug Administration's request to halt the trials on Tuesday afternoon and immediately complied. Fischer said GenVec will examine the results with the FDA and work with regulators to restart the studies, but he did not know how long the review would take.

"We want to make sure there is not a pattern here we should be concerned about," he said. "Our first and foremost concern is always patient safety."

The company has several drugs in the pipeline, although it has emphasized TNFerade as its leading candidate. The experimental drug is a high-concentration injection of a protein believed to shrink tumors. It is being tested on cancer of the esophagus and the pancreas.

GenVec reported a loss of $21.3 million last year on $10.5 million in revenue. Its stock had traded as high as $10 in 2001 and as low as 95 cents in April of last year.

Analysts said Wall Street investors may have overreacted yesterday.

"I think this is a very important and very critical event, but by no means do I think this is a fatal blow to TNFerade," said Michael Zasloff, an analyst of Ferris Baker Watts.

If the problem was related to the high dose, he said, the company may be able to proceed carefully at lower doses.

Mark Monane, an analyst of Needham & Co., noted that there was no evidence of problems in tests of TNFerade for pancreatic cancer, which used higher doses than in the esophageal tests.

"So the question is, are these adverse reactions due to the underlying disease or the drug?" Monane said.

GenVec, based in Gaithersburg, has several drugs under development. It halted human tests of TNFerade, a potential cancer therapy.