By Natalie Grover and Bill Berkrot

Merck & Co's immunotherapy Keytruda chalked up another approval on Tuesday as the U.S. Food and Drug Administration said the cancer medicine can be used to treat children and adults who carry a specific genetic feature regardless of where the disease originated.

It is the first time the agency has approved a cancer treatment based solely on a genetic biomarker.

"Until now, the FDA has approved cancer treatments based on where in the body the cancer started - for example, lung or breast cancers," said Richard Pazdur, head of oncology products for the FDA's Center for Drug Evaluation and Research. "We have now approved a drug based on a tumor's biomarker without regard to the tumor's original location."

The accelerated approval was for solid tumor cancers not eligible for surgery or that have spread in patients identified as having a biomarker called microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR).

Tumors with those traits are most common in colorectal, endometrial and gastrointestinal cancers, but may also appear in cancers of the breast, prostate, bladder and thyroid gland.

The approval covers patients whose cancer has progressed despite prior treatment and those who have no satisfactory alternative treatment options. It also includes patients with colorectal cancer whose disease has advanced after chemotherapy.

The FDA grants accelerated approvals to drugs for serious conditions with unmet medical needs if the treatment appears to have certain effects deemed reasonably likely to predict a clinical benefit. Merck must still conduct studies to confirm the anticipated benefit.

Keytruda belongs to a new class of drugs called PD-1 or PD-L1 inhibitors that help the immune system fight cancer by blocking a mechanism tumors use to evade detection.

It was previously approved to treat advanced melanoma, advanced non-small cell lung cancer, head and neck cancers and classical Hodgkin lymphoma.

Merck shares rose 0.8 percent to $64.55.