This story has been updated.

Bristol-Myers Squibb’s Opdivo received FDA approval to treat an important segment of non-small cell lung cancer patients last week.

The drug’s indication now includes patients with non-squamous forms of the disease—extending Opdivo’s potential market significantly. Between 85% and 95% of patients have either squamous or nonsquamous forms of metastatic lung cancer, according to the American Cancer Society.

Opdivo previously received an indication in March for use in patients with squamous forms of the disease.

Merck’s competing immuno-oncology drug, Keytruda, made its foray into lung cancer in early October.

The FDA has also approved Keytruda as a treatment for metastatic lung cancer in both squamous and nonsquamous forms of the disease, but the approval came with an important caveat: a patient’s tumor must express levels of the protein PD-L1, which is determined by a companion diagnostic manufactured by Dako.

Oncologists may view that prerequisite test as an unnecessary obstacle, especially when a treatment with a similar label is available without restrictions, analysts say.

“It seems Keytruda is cursed to repeatedly have its first-mover advantage quickly snuffed out by Opdivo,” according to a report from EP Vantage, adding, “Opdivo will begin to outdistance its rival until next year.”

When asked how it sees Keytruda faring against Opdivo—given the testing requirements— Pamela Eisele, a Merck spokesperson, told MM&M: “We believe that testing for the PD-L1 biomarker can provide important information that will help guide treatment decisions for patients with metastatic non-small cell lung cancer (NSCLC). Specifically, doctors can help identify patients with metastatic NSCLC who are more likely to experience benefit from Keytruda.”

Before Opdivo received the new indication, Keytruda was the only PD-1 inhibitor that could treat non-squamous forms of the disease.

In addition, Opdivo was the first PD-1 inhibitor to market, receiving regulatory approval in March. Mark Schoenebaum, an Evercore ISI analyst, wrote in July that “about two-thirds of new second-line patients with squamous lung cancer are taking Opdivo” and characterized the uptake as very rapid.