CHICAGO ( TheStreet) -- Merck's ( MRK - Get Report) anti-PD-1 drug labrolizumab shrank tumors in 38 percent of advanced melanoma patients. At the best dose, 52 percent of the melanoma patients treated with the immune system-boosting drug experienced significant tumor shrinkage. Updated results from the early-stage study are being presented today at the American Society of Clinical Oncology (ASCO) annual meeting and published simultaneously in the New England Journal of Medicine. Lambrolizumab is Merck's strongly competitive answer to Bristol-Myers Squibb's ( BMY - Get Report) nivolumab. Both drugs work by dismantling a cloaking mechanism known as PD-1 used by cancer cells to hide from a patient's immune system. Roche ( RHHBY) is developing a drug against a related target known as PDL-1. The tumors responses to these anti-PD-1 and PDL-1 drugs are among the strongest ever seen in melanoma clinical trials. The ASCO meeting is buzzing about the potential for this new class of drugs to dramatically improve treatment for melanoma patients. The drugs are active in lung, kidney and breast cancers. In July, Merck will start a phase III study comparing lambrolizumab against Bristol-Myers' Yervoy in newly diagnosed melanoma patients. Also in the third quarter, Merck will start a phase II/III study of lambrolizumab in non-small cell lung cancer. Bristol-Myers has phase III studies of nivolumab already underway. In the phase I study presented at ASCO today, 117 patients with advanced melanoma were treated with three different doses of lambrolizumab and had scans assessed by independent reviewers. Across all three doses, the overall, independently confirmed response rate was 38%. The median duration of response has not yet been reached at a median follow-up time of 11 months. Eighty-one percent of patients who had a response were still in the study receiving treatment with lambrolizumab at the time of the analysis. When lambrolizumab was given at a dose of 10 mg/kg every two weeks, the rate of confirmed tumor shrinkage increased to 52 percent. Ten percent of the patients in this best-performing dose group experience complete tumor shrinkage. In the study overall, tumor response rate was 37 percent for the two-thirds of melanoma patients not treated previously Bristol-Myers' Yervoy and 38% for the one-third of patients who received Yervoy before entering the study. The Merck data on lambrolizumab from Sunday compare very favorable against Bristol-Myers' nivolumab data presented also presented this weekend. With all the caveats and risks associated with comparing efficacy across separate trials of advanced melanoma patients, the best-dose response rate of 52% for lambrolizumab as a monotherapy is the same as the 53% response rate seen with the best-dose nivolumab-Yervoy combination. Again looking at best dose, lamroblizumab's 51 percent response rate tops nivolumab's 41 percent response rate. If comparing response rates across all doses, lambrolizumab (38 percent) and nivolumanb (31percent) are similar. -- Reported by Adam Feuerstein in Chicago. Follow Adam Feuerstein on Twitter.