CHICAGO ( TheStreet) - -An experimental drug from Bristol-Myers Squibb ( BMY) that helps the immune system attack cancer cells was able to shrink tumors in patients with melanoma, kidney and non-small cell lung cancers, according to results from an early-stage study being presented later today at the American Society of Clinical Oncology (ASCO) annual meeting. The promising Bristol drug, BMS-936658, follows in the footsteps of Yervoy, another Bristol drug that was approved last year to treat melanoma and which also harnesses the power of a patient's immune system to fight cancer. Bristol is accelerating development of '658 and will push the drug into pivotal, phase III studies in lung cancer, melanoma and kidney cancer, based in part of the data presented at this year's ASCO meeting. The phase I study enrolled 296 patients diagnosed with solid tumors that had progressed despite treatment with currently approved therapies. After treatment with Bristol's '658, responses (partial tumor shrinkage) were observed in 28% of melanoma patients, 27% of kidney cancer patients, and 18% of lung cancer patients, including patients with squamous and non-squamous cell subtypes. Many of the patients had durable responses that lasted for 12 months or longer. "We were especially surprised to see activity in nearly 20% of patients with lung cancer, who are historically unresponsive to immune-based therapies, " said Dr. Suzanne Topalian, professor of oncology at Johns Hopkins University School of Medicine. "These findings mark what is probably the strongest anti-lung cancer activity observed to date with any immunotherapy." Bristol's '658 is an antibody that targets a cellular signal known as PD-1 which cancer cells turn on to prevent attacks from T-cells, the white blood cells that help fight infection and cancer. By blocking or turning off the PD-1 pathway, Bristol's '658 appears to re-activate the immune system and allows it to fight tumor cells. Merck ( MRK), Roche ( RHHBY) and GlaxoSmithKline ( GSK) are also developing their own PD-1 inhibitors as potential cancer therapies. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein.