The race in immuno-oncology got a little more clarity at ASCO on Monday as both Bristol Myers (NYSE:BMY) and Merck (NYSE:MRK) presented impressive data in their ongoing trials using PD-1 inhibitors. The drugs are designed to unlock the cloaking mechanism that tumors use to avoid being detected by the immune system. While it is still early in the race, there was more clarity today. Let's take a look at the data presented.

Diving Deeper into the trial data
First up is Bristol Myers. Bristol's anti-PD-1 drug, called nivolumab, is furthest along in melanoma, lung cancer, and kidney cancer. This morning, they released more data in the ongoing phase 1b study in the nivolumab and Yervoy combination trial. Yervoy, which received FDA approval in 2011 for metastatic melanoma, is also part of Bristol's immuno-oncology portfolio and works by blocking CTLA-4 that is presented on T cells.

Bristol is hoping that combining Yervoy with nivolumab will produce even better results, and it presented new updated data from its phase 1b trial, which was split the trial into two subgroups -- one getting nivolumab and Yervoy concurrently, and one group getting the two drugs sequentially. Within the two groups, various dosing schedules were also examined. For those patients receiving 1mg/kg of nivolumab and 3mg/kg of Yervoy every three weeks, the one-year survival rate was 94%, and the two-year survival rate was 88%. This was a very small trial, so best to wait for additional data as the combination is moved into a phase 3 trial.

Bristol also announced data in its phase 3 trial of Yervoy in fully resected stage 3 patients. Bristol announced the trial did meet its primary endpoint of relapse-free survival as compared to placebo, but the toxicity was also extremely high. 46.5% of patients were recurrence-free after three years compared to only 34.8% of patients taking placebo, which resulted in a 25% reduction in the risk of recurrence or death. However, the trial also showed that nearly half (48.8%) of patients dropped out of the Yervoy arm because of toxicity.

Could Merck start leading in melanoma?
Competing directly with Bristol is Merck's PD-1 inhibitor called pembrolizumab (pembro for short), formerly known as MK-3475. Merck surprised a lot of investors earlier this year when it announced it was filing pembro in metastatic melanoma for patients that have failed Bristol's drug Yervoy.

Merck announced today at ASCO that 69% of patients were alive at one year (across 411 patients), including 74% in patients without prior Yervoy (ipilimumab) therapy. At 18 months, the estimated overall survival was 62%, and the median overall survival has not yet been reached.

Additionally, pembrolizumab presents a much more manageable side effect profile, with only 4% of patients discontinuing treatment due to related adverse events. Given this side effect profile, and the fact that long-term durable responses have been seen, it seems that Merck's pembrolizumab could take over as the preferred choice when treating later-stage patients. Merck is also testing pembro in earlier stage 3 patients as well, but again data in this group will not be known for some time.

Where to from here?
So, what should investors make of the new data? First off, the race to dominance in melanoma in the next three years looks to be primarily between Bristol and Merck. I think that Merck will have a first-mover advantage on that front if pembro is approved (PDUFA date of October 28). Bristol could ultimately win out by showing superior efficacy in its phase 3 combo trials, but it will have to prove that regimen is safe.