"A combination of two BristolMyers Squibb drugs that help the immune system fight melanoma led to significantly greater tumor shrinkage than treatment with one of the medicines according to a midstage study presented on Monday.    The combination of Yervoy and the newer Opdivo also reduced the risk of disease progression by  percent compared with Yervoy alone through  months of followup in previously untreated patients with advanced melanoma researchers found.    The overall response rate defined as tumors that shrank by at least  percent was  percent for the  patients who received the combination versus  percent among the  who got Yervoy alone.    Twentytwo percent of those combination patients had a complete response meaning no sign of tumor. There were no complete responses with Yervoy alone.    The response rates and the depth of responses are quite impressive Dr. Stephen Hodi director of the Melanoma Center at DanaFarber Cancer Institute and colead author of the study said in an interview.    The median time it took for Yervoy patients to see their cancer worsen was . months. But after  months more than half of the combination group had yet to see their disease worsen. Overall survival data was not yet available.    Longer followup will be helpful to see the durability of these responses and what happens to patients who had complete responses said Hodi who presented data from the Checkpoint study at the American Association for Cancer Research meeting in Philadelphia. The results were published in the New England Journal of Medicine.    Yervoy also known as ipilimumab had been the first immunotherapy to extend survival in patients with advanced melanoma the deadliest skin cancer. It works by taking the brakes off the immune system to more efficiently attack cancer. Opdivo nivolumab belongs to a promising new class of drugs called PD inhibitors that block a mechanism tumors use to hide from the immune system.    In a separate group of  patients with a mutation of the BRAF gene involved in cancer cell growth the overall response rate was  percent for the combination. The complete response rate was  percent. That compared with  percent overall response rate and no complete responses for Yervoy alone.    The combination of the two drugs also led to far higher levels of side effects such as colitis and inflammation of lung tissue at  percent versus  percent for Yervoy alone. Patients were also more likely to stop taking the combination of drugs.    Most of the patients who stopped for toxicity continued to benefit said Fouad Namouni Bristols head of development for Yervoy and Opdivo."