(Reuters) - Bristol-Myers Squibb Co and Daiichi Sankyo said they were evaluating a combination of Bristol-Myers’ immuno-oncology drug, Opdivo, with the Japanese company’s experimental drug in patients with breast and bladder cancers.

Opdivo belongs to a new class of medicines called PD-1 checkpoint inhibitors, which work by taking the brakes off the immune system.

The combination therapy involves Daiichi Sankyo’s DS-8201, which delivers chemotherapy directly to target cancer cells. The drug is in early stage clinical development to treat different breast and solid cancers.

The companies said the clinical study in patients with HER2-expressing advanced breast and bladder cancer is expected to begin enrollment in the first quarter of 2018.

Bristol-Myer’s blockbuster Opdivo is already approved to treat advanced melanoma and lung cancer, and competes with Merck & Co’s Keytruda.