Merck & Co. Inc. said its immuno-oncology blockbuster Keytruda helped patients with stomach cancer live for two years at a rate comparable to chemotherapy treatments and with fewer side effects in a late-stage study.

Keytruda targets the pathway that normally protects the cells from the body's immune system — patients whose tumors express high levels of what is called PD-L1 expression are more likely to be helped by the drug, which spurs the immune system to attack the cancer cells.

Among patients with high PD-L1 levels, 39% lived for two years with Keytruda as an initial therapy in the phase 3 trial dubbed Keynote-062, which included patients with HER2-negative gastric cancer and gastroesophageal cancer. Of patients who received standard chemotherapy, 22% lived for the same amount of time.

However, when Keytruda was combined with chemotherapy in a secondary study goal, patients did not live longer than those who received only chemotherapy.

Patients taking only Keytruda showed fewer serious side effects than those taking chemotherapy. Toxic treatment-related effects were found in only 17% of Keytruda patients compared with 69% taking chemotherapy alone and 73% taking a combination of the two.

About a quarter of the 763 patients in the study had already undergone gastric surgery to remove a tumor.

Merck will present results of the study June 2 at the 2019 Annual Meeting of the American Society of Clinical Oncology.

"Chemotherapy has been our only option for many years," ASCO Senior Vice President and Chief Medical Officer Richard Schilsky said. "These results introduce a potential alternative in pembrolizumab that comes with fewer side effects, and importantly, for some it can greatly extend survival."

Merck said 27,510 new cases of stomach cancer and 11,140 deaths from the disease are expected in the U.S. in 2019.

Keytruda is approved for 19 types of cancer in the U.S., including previously treated gastric cancer with PD-L1 expression.

The 2019 American Society of Clinical Oncology meeting is expected to bring together more than 32,000 professionals from around the world, with more than 2,400 study abstracts to be presented on site and an additional 3,200 abstracts to be published online.