"The complete and near-complete response rates we're seeing are unprecedented for an immunotherapy in melanoma. We were particularly impressed that the drugs work together so well," said Dr. Jedd Wolchok, of the Memorial Sloan-Kettering Cancer Center. "Melanoma researchers have been hopeful that combination regimens would increase the effectiveness of single-agent immunotherapies, and now we have confirmation that such an approach has significant potential."

Serious adverse events related to the combination therapy were reported by 59% of patients and included swelling inside the eye, inflammation of the colon and reversible lab abnormalities.

Bristol-Myers is planning to start a phase III study of the Yervoy-nivolumab combination in melanoma patients next month.

Phase III studies of nivolumab as a stand-alone therapy in lung cancer, kidney cancer and melanoma are already underway.

The Roche phase I study of MPDL3280A enrolled 140 patients with advanced solid tumors. The overall rate of tumor shrinkage was 21%, with the highest number of responses seen in lung cancer (22%) and melanoma (31%.) Responses have been durable, with only three responders reporting tumor re-growth following MPDL3280A treatment so far.

MPDL3280A targets the cellular receptor PDL-1, and in a subgroup of patients with tumors positive for PDL-1, the overall response rate was 36%. Conversely, the response rate in patients PDL-1-negative tumors was 13%. Roche is developing a diagnostic test to better identify and define PDLI-1 positive tumors.

Thirteen percent of patients reported serious adverse events considered to be related to MPDL3208, but immune-related adverse events were reported by only 2% of patients.

"We are impressed with the frequency and duration of the responses in these patients with very difficult-to-treat tumors. So far, almost none of the patients that have had tumor shrinkage have progressed," said Dr. Roy Herbst of the Yale Cancer Center.

Roche is starting a phase III study of MPDL3280A in non-small cell lung cancer, which will enroll patients with PDL-1-positive tumors, detected by the companion diagnostic. Additional, follow-on studies of MPDL3208A in other cancers are being considered.

Given the striking efficacy observed with all these drugs, even in early stage trials, investors essentially assume approval. The big questions that remain are which company -- Bristol-Myers, Roche or Merck -- has the most effective PD-1 and PDL-1 drugs, and how fast can the new therapies be approved.

ISI Group analyst Mark Schoenebaum expects Bristol-Myers' nivolumab to be approved first, perhaps late next year in lung cancer, followed by Merck's lambrolizumab in late 2015. Roche's approval timeline for MPDL3208A is relatively unknown.

-- Reported by Adam Feuerstein in Boston.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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