Six years after Merck bought out Peloton Therapeutics, the New Jersey drugmaker is not remaining stationary with the crown jewel of the deal.

On Wednesday, the FDA approved a third indication for Welireg (belzutifan), signing off on the hypoxia-inducible factor-2 alpha (HIF-2a) inhibitor to treat patients with rare pheochromocytoma or paraganglioma (PPGL) tumors in the endocrine system.

The approval covers patients 12 and older with locally advanced, unresectable or metastatic PPGL and makes Welireg the first oral treatment for advanced PPGL, per Merck.

While pheochromocytoma tumors form inside the adrenal gland, paraganglioma tumors form along but outside the adrenal gland. The tumors are commonly found along the aorta or in the neck near the carotid artery, and it can be difficult to determine whether the tumors are benign or malignant. The condition can lead to high blood pressure that can be life-threatening. Estimates cited by Merck suggest that up to 2,000 new cases of PPGL are diagnosed in the U.S. each year and 52,800 globally.

Patients with PPGL “may require specialized care due to their complexity and rare nature, often posing significant challenges for both diagnosis and treatment,” Camilo Jimenez, M.D., of the University of Texas MD Anderson Cancer Center, said in a release, adding that Welireg “could represent a change to the treatment paradigm for eligible patients,” as it is the lone non-surgical option for the disorder.

Backing the label expansion are results from the phase 2 LITESPARK-15 single-arm study of 72 patients, which showed that 26% achieved partial or complete response to treatment, meeting the primary endpoint.

The trial also met a key secondary measure with a median duration of response of 20.4 months. Additionally, 32% of the patients on baseline blood pressure medicines were able to reduce their use of the treatments by at least 50% for at least six months.

In 2019, Merck picked up belzutifan in its takeover of Peloton, just days before the Dallas-based company was set to go public. Merck paid $1.1 billion up front and committed to $1.2 billion in milestones tied to the success of the treatment.

Two years later, the FDA endorsed Welireg for von Hippel-Lindau disease, a hereditary condition that causes tumor growth in various organs of the body, most often in the kidney.

Merck gained its most impactful FDA nod for Welireg in 2023 as a treatment for the most common form of kidney cancer, renal cell carcinoma (RCC). Welireg became the first drug in its class for patients with advanced RCC and the first novel-class treatment in the indication since 2015.

As a result, sales increased from $218 million in 2023 to $509 million last year. And, another potential indication awaits, as Merck is testing Welireg as a treatment for the brain cancer glioblastoma multiforme.