An experimental cancer drug shrank prostate tumors dramatically and more than doubled survival in 70 percent to 80 percent of patients with aggressive cancers, British researchers reported Tuesday.

Although the study published in the Journal of Clinical Oncology covered only 21 patients, the drug is being tested in more than 250 men with what appear to be similar results, experts said.

"There is a general sense in the prostate cancer community that this agent is extremely promising and is very likely to have an important role in the management of prostate cancer patients," said Dr. Howard Sandler, a radiation oncologist at the University of Michigan who is a spokesman for the American Society of Clinical Oncology.

Experts expect the new drug, called abiraterone, to be widely available by 2011. It could find use among most of the 28,000 U.S. men diagnosed each year with the most aggressive, and almost always fatal, type of prostate cancer.

The trial was sponsored by Cougar Biotechnology Inc. of Los Angeles, which holds the patent rights to abiraterone.

The drug is also being tested in breast cancer, but no results have been released.

The key to the excitement is the drug's unusual mechanism of action.

Prostate cancer is fueled by the male hormone testosterone. One of the key methods of treatment is known as hormonal deprivation or "chemical castration," in which drugs are used to block production of the hormone by the testes and the adrenal gland.

Yet many tumors continue to grow. Recent research has shown that the tumor cells themselves produce testosterone and that this production is not blocked by existing drugs.

Researchers at the Institute of Cancer Research in London developed abiraterone to block an enzyme called cytochrome P17, which is crucial in converting cholesterol to testosterone. The key point, said Sandler, is that the drug blocks testosterone production everywhere in the body, including inside the tumor cells themselves.