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Merck & Co Inc's drug that harnesses the immune system to fight cancer showed promise in Hodgkin lymphoma patients whose disease had progressed following prior therapies, according to data from a small, early stage trial being presented at a medical meeting.

In the ongoing study of 29 patients, 66 percent had a meaningful response to the drug, Keytruda, after 24 weeks of treatment, including six patients (21 percent) who achieved complete remission.

Six other patients in the Keynote-013 study unveiled on Saturday had stable disease at the time data was analyzed, while the lymphoma progressed in four patients.

"The drug is outstanding," said Dr. Craig Moskowitz, the study's principal investigator from Memorial Sloan Kettering Cancer Center in New York.

"It may be a small patient population, but it is really remarkable," he said. "And it's really well tolerated."

There are about 9,000 new cases of Hodgkin lymphoma in the United States in 2014, according to the Leukemia and Lymphoma Society. About a quarter are likely to relapse after initially successful treatment, highlighting the need for new options.

Keytruda (pembrolizumab) belongs to a new class of drugs called PD-1 inhibitors that have generated great enthusiasm in the medical community. They work by blocking a mechanism tumors use to camouflage themselves from the immune system, allowing it to recognize and attack the cancer.

Patients in the study had either had a stem cell transplant or were ineligible for one, and all had been previously treated with the Seattle Genetics Inc drug Adcetris.

The average time to response with Keytruda was 12 weeks, according to data being presented at the American Society of Hematology meeting in San Francisco. The median duration of response had not yet been reached. Patients will stay on the drug until their cancer progresses.

Keytruda this year was approved to treat advanced melanoma - the deadliest form of skin cancer - making it the first drug from the PD-1 class to reach the U.S. market. Bristol-Myers Squibb Co is developing a rival medicine.

This marks the first Keytruda data reported against a type of blood cancer. It is also being tested in lung, breast, bladder, gastric, and head and neck cancers.

The most common side effect was shortness of breath, Moskowitz said. One patient discontinued treatment due to pneumonitis, an inflammation of the lung tissue.