CHICAGO ( TheStreet) -- Bristol-Myers Squibb's ( BMY) immune system-boosting drug nivolumab produced long-lasting tumor shrinkage patients with advanced melanoma, according to updated results from an early-stage study. The response to Nivolumab was three times better than what's typically seen with Bristol-Myers' first-generation melanoma drug Yervoy, approved in 2011. Nivolumab belongs to a new class of drugs that work by dismantling a cloaking mechanism used by cancer cells to hide from a patient's immune system. Bristol-Myers and Merck ( MRK) are each developing drugs that target this PD-1 receptor on T cells, while Roche ( RHHBY) is working on a similar drug known as an anti-PDL-1 antibody. The potent, tumor-killing efficacy of these drugs across many different types of tumors -- lung, kidney, breast as well as skin cancer -- is generating a lot of buzz at this year's American Society of Clinical Oncology (ASCO) annual meeting. Yervoy and nivolumab could generate peak sales of $6 billion for Bristol-Myers, analysts and investor surveys indicate. The blockbuster potential for this new crop of cancer immunotherapy has been a big contributor to the 41% increase in Bristol-Myers' stock price this year. Bristol-Myers shares closed Friday at $46.01. More data on Merck's drug, known as lambrolizumab, will be released Sunday morning at the ASCO meeting. In the phase I study, 107 patients with advanced, stage IV melanoma were treated with five different doses of Bristol-Myers' nivolumab. Twenty-five percent of the patients had already been treated with three or more prior therapies, and 63% had two or more. Overall, 31% of the nivolumab-treated patients reported clinically meaningful tumor shrinkage. The median overall survival across all nivolumab doses was 16.8 months and 43% of patients were still alive at two years of follow up. The study lacked a control arm so the nivolumab survival data cannot be compared directly against another drug or a placebo. However, Yervoy typically produces tumor shrinkage rates of 5-10% and a median overall survival of 10 months in patients with advanced melanoma. Seventeen melanoma patients in the study were treated with a 3 mg of nivolumab, which Bristol-Myers has already identified as the most effective dose to use in phase III studies already underway. The overall response rate in these patients was 41% with a median overall survival of 20.3 months. Long-term follow-up results from early-stage studies of nivolumab in non-small cell lung cancer and kidney cancer are being presented on Monday at the ASCO annual meeting. Bristol-Myers is also investigating the combination of nivolumab and Yervoy. In one early-stage study, the two-drug combination produced a tumor shrinkage rate of 50 percent in melanoma patients. Merck and Roche will also be presenting updates results on their respective PD-1 and anti-PDL-1 drugs during the ASCO meeting. -- Reported by Adam Feuerstein in Chicago. Follow Adam Feuerstein on Twitter.