Pfizer Inc and German drugmaker Merck KGaA on Wednesday said they had begun a late-stage trial of their immuno-oncology drug avelumab in combination with standard treatment for ovarian cancer.

The 950-patient study will enroll patients with advanced ovarian cancer who have not previously been treated for the disease, which is diagnosed annually in almost 23,000 American women and has a high mortality rate.

Avelumab is a so-called checkpoint inhibitor, which works by taking the brakes off the immune system.

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"The hope is that avelumab can change the natural history of the disease and potentially take the survival rate beyond the current five-year estimate," Alise Reicin, a senior research executive at Merck KGaA said in a statement.

One group of patients in the Phase III study will receive avelumab in combination with platinum-based chemotherapy, the current standard of care. Others will receive chemotherapy alone, or avelumab a few weeks after receiving chemotherapy.

Rival drugmakers Bristol-Myers Squibb Co and Merck & Co are developing similar checkpoint inhibitors for a range of cancers, but have not yet tested them alongside standard treatments in large trials of patients with previously untreated ovarian cancer.

In one small early-stage study of Merck's Keytruda, tumor shrinkage was seen in 11.5 percent of patients with ovarian cancer, without significant safety issues. Ovarian tumors shrank by 15 percent among patients taking Bristol-Myers' Opdivo in a small trial.