Bristol-Myers Squibb is out to expand the pool of colorectal cancer patients it can treat with Opdivo—and it’s bringing in Bayer to help.

The two companies, along with Bristol partner Ono Pharmaceutical, have joined forces in a clinical pact to test the immuno-oncology star alongside Bayer's Stivarga in patients with microsatellite stable metastatic colorectal cancer, which represents about 95% of cases.

The companies stayed tight-lipped on the collaboration’s terms, but they did note that they’ll be plotting an indication-seeking trial for the drug duo.

Right now, Opdivo bears a green light to treat a sliver of the colorectal cancer patient population. Nearly two years ago, the FDA gave the treatment a go-ahead in those with microsatellite instability-high (MSI-H) metastatic colorectal cancer, a rare tumor type that makes sufferers less likely to respond to conventional chemo.

In the broader colorectal cancer group, though, "I-O monotherapy treatment approaches have shown limited activity," Bristol noted, "thus, the need for additional treatment options including combination approaches remains high."

Meanwhile, while Stivarga already has an OK for previously treated colorectal cancer that has spread to other parts of the body, it too could benefit from another indication in the disease. While its sales grew overall in 2018, in the U.S., sales declined thanks to "a highly competitive market environment." Its competition in colorectal cancer includes Otsuka’s Lonsurf, another drug for previously treated patients whose cancer has spread.

Outside of colorectal cancer, though, it’s Opdivo itself that’s part of Stivarga’s problem. Both drugs are approved for liver cancer patients who have already received Bayer’s Nexavar, and they’re duking it out in that arena.

For its part, Bayer is looking at new cancer types for Stivarga, too. It recently opened up U.S. trial enrollment for a study program that will test the drug in glioblastoma patients.