A panel of cancer experts rejected an experimental Merck drug for a rare form of cancer on Tuesday while recommending approval of a GlaxoSmithKline treatment for the same disease. Neither drug appears to help patients live longer, but panelists said Glaxo's Votrient helped delay tumor growth in the most vulnerable patients.

The FDA panel reviewed the two drugs submitted to treat sarcoma, a rare class of tumors that form in the fat, muscles and bone in the limbs and abdomen. An estimated 11,000 people in the U.S. were diagnosed with the soft tissue cancer in 2011 and 3,900 died from the disease, according to the National Cancer Institute.

Many cancer drugs approved by the FDA do not actually extend survival, but instead slow the growth of tumors or their spread to other parts of the body. In recent years, cancer experts have debated the significance of such results, particularly given the potentially dangerous side effects.

The panel voted 11-2 that the benefits of Glaxo's pill Votrient outweighed its risks, noting there are few other treatment options for patients.

"There are no drugs approved by the FDA specifically for this indication and that's what drove my decision to vote yes," said Dr. Mikkael Sekeres of the Cleveland Clinic.

GlaxoSmithKline plc, based in the U.K., studied Votrient in sarcoma patients whose cancer has spread to other parts of the body after unsuccessful treatment with chemotherapy drugs, the standard treatment for the disease. Such patients usually survive only a year to 18 months.

While patients taking Votrient didn't live longer than those taking chemotherapy alone, they did see a three-month delay in growth of their tumors, on average. Some patients experienced an even longer delay, which panelists said supported the drug's benefit.

"I feel the effect is marginal but there does appear to be a group of patients who have benefited from this for longer periods of time," said panel chair Dr. Wyndham Wilson of the National Cancer Institute.

The panel saw less potential for Merck's ridaforolimus, which the company acquired through Ariad Pharmaceuticals Inc. The group voted 13-1 against the drug, saying its significant side effects — which affected 60 percent of patients — outweighed its benefits.

Merck & Co. Inc. of Whitehouse Station, N.J., submitted the drug as a maintenance therapy, meaning it would be used to help repress sarcoma of the bone and tissue in patients whose cancer is already in remission. Since such patients are healthier than patients with active disease, panelists said they wanted to see a more dramatic benefit to justify putting patients on a drug with major side effects. The FDA has only approved a handful of cancer drugs for maintenance use.

Company trials showed no survival benefit and a meager seven-week delay in disease progression compared with patients not taking the drug. Panelists were also troubled by the high rate of side effects in patients, which led 14 percent to drop out of the study. Side effects included lung irritation, kidney failure and high blood pressure

"I didn't see anything that would indicate it should be recommended based on the information we have today," said Lee Helman of the National Cancer Institute.

The FDA is scheduled to make a decision on Glaxo's drug by May 6 and Merck's drug by June 5.