By Bill Berkrot

April 9 (Reuters) - An experimental drug that spurs the immune system to fight cancer appeared to be safe and demonstrated anti-tumor activity against a variety of cancers in a small early stage study, researchers said on Tuesday.

The drug, called MPDL320A, was discovered and is being developed by Roche's Genentech unit. The Phase 1 trial of the drug in 30 patients with advanced cancer was designed as a dose escalation study to test for toxicity at higher doses.

But researchers were also pleased to find early signs of effectiveness of the drug.

"We saw clear evidence of anti-tumor activity in a broad range of diagnoses, including lung cancer, kidney cancer, colon cancer and gastric cancer," said Dr. Michael Gordon, who presented the data on Tuesday at the American Association for Cancer Research meeting in Washington, D.C.

The drug is an engineered antibody that targets a protein called PD-L1, for programmed death-ligand 1, and enables T cells of the immune system to more effectively attack cancer cells. PD-L1 is found on the surface of many cancer cells and impairs the immune system's ability to fight the disease.

The drug was administered intravenously every three weeks, beginning with a dose of just 0.01 milligrams per kilogram of weight and increased up to a maximum dose of 20mg/kg.

"There were no dose limiting toxicities," Gordon, research director at Pinnacle Oncology Hematology in Scottsdale, Arizona, said in a telephone interview. "We achieved our highest deliverable dose without any undue toxicities."

Researchers and Genentech believe the PD-L1 approach may be more selective and safer than a similar promising class of immunotherapy called PD-1 inhibitors being developed by several other companies. The PD-1 drugs impact not only the intended target but potentially another receptor on healthy cells called PD-L2 and may cause lung inflammation that was not seen with the Roche drug, researchers said.

Although the study was very small, Gordon said he was impressed by the lasting effect of the drug in some of the advanced cancer patients in the trial, who were still alive for more than a year.

"The responses have been durable," he said. "In at least two cases there were near complete responses and patients are sustaining those responses in the absence of continued therapy and doing very well."

Roche is working on developing a diagnostic aimed at helping to better identify the patients most likely to respond to the PD-L1 drug. "It's efficacy and use will be defined by future clinical trials," Gordon said.

While the value of the Roche drug must also be determined in much larger clinical trials, Gordon was excited about the use of new immunotherapies in the war against cancer.

Bristol-Myers Squibb's Yervoy, which was approved in 2011 and uses a different mechanism to help the immune system fight cancer, was the first drug to significantly extend survival in patients with advanced melanoma, the most deadly form of skin cancer.

"I think the field of oncology is going to change dramatically with regard to their inclusion and incorporation in cancer care," Gordon said.


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