Welireg (belzutifan)
An approved treatment for Paraganglioma and Pheochromocytoma.
The same compound appears under different names depending on the context. Here is how to identify Belzutifan wherever you encounter it, plus the key facts at a glance.
- Generic name
- Belzutifan
- Brand name
- Welireg
- Development codes
- MK-6482, PT2977
- Drug class
- HIF-2α inhibitor
- Manufacturer
- Merck
- How it's taken
- Taken by mouth once daily, with or without food.
The first FDA-approved oral therapy for advanced pheochromocytoma or paraganglioma (PPGL). For PPGL it is approved for adults and children 12 years and older whose tumors are locally advanced, cannot be removed by surgery, or have spread (metastatic). Belzutifan blocks HIF-2α, a transcription factor that drives the abnormal blood vessel growth and hormone secretion seen in these tumors.
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How Belzutifan works
Pheochromocytomas and paragangliomas often have mutations (such as in SDHB or VHL) that leave the HIF-2α protein switched on even when oxygen is plentiful. HIF-2α then turns on genes that drive new blood vessel growth, tumor metabolism, and catecholamine production, the hormones responsible for the high blood pressure, racing heart, and sweating that PPGL patients experience. Belzutifan binds directly to HIF-2α and prevents it from teaming up with its partner protein, shutting down that downstream cascade.
Mechanism: Oral small-molecule inhibitor of hypoxia-inducible factor 2 alpha (HIF-2α)
Side effects and safety
The most common side effects are anemia and fatigue. Belzutifan can cause severe anemia requiring transfusion, and serious hypoxia (low oxygen levels) requiring supplemental oxygen. Welireg has a boxed warning: it can harm an unborn baby. Pregnancy must be ruled out before starting, and people who can become pregnant need effective non-hormonal birth control, because Welireg can make some hormonal birth control stop working. Other common effects include musculoskeletal pain, nausea, headache, and elevated blood pressure. Blood counts and oxygen saturation must be monitored regularly.
This is not a complete list of side effects. Talk to your doctor or pharmacist about what to expect and when to seek medical attention.
Taking Belzutifan
Taken by mouth once daily, with or without food. For PPGL the dose is 120 mg once daily, or 80 mg once daily for children 12 and older who weigh less than 40 kg. Tablets should be swallowed whole. Treatment continues until disease progression or unacceptable toxicity. Anemia and low oxygen are managed by pausing treatment or lowering the dose, and Welireg may need to be stopped for good depending on how severe they are. It must be stopped permanently for life-threatening low oxygen or for low oxygen with symptoms that keeps coming back.
Help paying for Welireg
Pick your insurance to see which help fits. Drugmaker copay cards can't be used with Medicare, Medicaid or TRICARE; charity funds are the usual route there.
- Copay help
Eligible privately insured patients may save on out-of-pocket costs. Not valid for uninsured patients or those with Medicare or other government insurance.
For: private insurance · source - Insurance and case manager help
The Merck Access Program answers questions about insurance coverage, co-pay options and eligibility for free medicine. Mon-Fri 8 AM-8 PM ET.
The official page does not say who qualifies. Ask the program. · source - Free medicine program
Merck Patient Assistance Program gives Welireg free for up to 1 year to eligible US residents without drug coverage who meet income limits; hardship exceptions possible.
For: no insurance · source
Good to know: Patients who get Welireg through a specialty pharmacy (Biologics or Onco360) are told to ask that pharmacy about coverage and financial help. Free drug is through the separate Merck Patient Assistance Program, Inc.; income limits apply. Savings card questions: 877-264-2454.
No charity fund for this condition is open right now. Funds reopen when they receive new donations; the foundations let you sign up to be told when one opens.
Clinical trial results
LITESPARK-015 (NCT04924075) was the open-label Phase 2 trial that supported approval. In the 72 PPGL patients with locally advanced or metastatic disease, the objective response rate was 26% and the median duration of response was 20.4 months, with 53% of responders maintaining response at least 12 months. Patients also showed reductions in catecholamine-related symptoms and antihypertensive medication burden.
Development history
Belzutifan was originally developed by Peloton Therapeutics as PT2977, with Merck acquiring Peloton in 2019. The drug was first approved in August 2021 for VHL-associated tumors, then expanded to advanced renal cell carcinoma in December 2023. In 2026 it was also approved with pembrolizumab as add-on treatment after kidney surgery for some people with clear cell kidney cancer. The PPGL indication was approved on May 14, 2025, making it the first oral systemic therapy for advanced PPGL, previous options were limited to surgery, MIBG radiotherapy, and chemotherapy.
Explore Paraganglioma and Pheochromocytoma trials
Common questions about Belzutifan
▸What is Belzutifan (Welireg)?
The first FDA-approved oral therapy for advanced pheochromocytoma or paraganglioma (PPGL). For PPGL it is approved for adults and children 12 years and older whose tumors are locally advanced, cannot be removed by surgery, or have spread (metastatic). Belzutifan blocks HIF-2α, a transcription factor that drives the abnormal blood vessel growth and hormone secretion seen in these tumors.
▸How does Belzutifan work?
Pheochromocytomas and paragangliomas often have mutations (such as in SDHB or VHL) that leave the HIF-2α protein switched on even when oxygen is plentiful. HIF-2α then turns on genes that drive new blood vessel growth, tumor metabolism, and catecholamine production, the hormones responsible for the high blood pressure, racing heart, and sweating that PPGL patients experience. Belzutifan binds directly to HIF-2α and prevents it from teaming up with its partner protein, shutting down that downstream cascade.
▸What are the side effects of Belzutifan?
The most common side effects are anemia and fatigue. Belzutifan can cause severe anemia requiring transfusion, and serious hypoxia (low oxygen levels) requiring supplemental oxygen. Welireg has a boxed warning: it can harm an unborn baby. Pregnancy must be ruled out before starting, and people who can become pregnant need effective non-hormonal birth control, because Welireg can make some hormonal birth control stop working. Other common effects include musculoskeletal pain, nausea, headache, and elevated blood pressure. Blood counts and oxygen saturation must be monitored regularly.
▸How is Belzutifan taken?
Taken by mouth once daily, with or without food. For PPGL the dose is 120 mg once daily, or 80 mg once daily for children 12 and older who weigh less than 40 kg. Tablets should be swallowed whole. Treatment continues until disease progression or unacceptable toxicity. Anemia and low oxygen are managed by pausing treatment or lowering the dose, and Welireg may need to be stopped for good depending on how severe they are. It must be stopped permanently for life-threatening low oxygen or for low oxygen with symptoms that keeps coming back.
▸Is Belzutifan FDA approved?
Yes, Belzutifan (Welireg) is FDA approved (2025) for the treatment of Paraganglioma and Pheochromocytoma.
▸What is the difference between pheochromocytoma and paraganglioma?
Pheochromocytomas are catecholamine-secreting tumors that arise inside the adrenal gland. Paragangliomas arise from the same type of nerve tissue but outside the adrenal gland, anywhere along the sympathetic and parasympathetic nervous system chains. They are managed together as PPGL because they share biology, genetics, and treatment options.
▸How does belzutifan compare to MIBG radiotherapy?
Azedra (high-specific-activity MIBG) was a radioactive IV therapy for PPGL whose tumors take up MIBG, but its maker Lantheus stopped making it in March 2024, so it is no longer available in the US. Belzutifan is an oral daily pill that does not require MIBG-avid tumors. They have not been compared head-to-head, so choice depends on patient factors and center capabilities.
▸Does belzutifan work for all genetic subtypes of PPGL?
LITESPARK-015 enrolled patients with advanced PPGL, but none of the 72 patients in the PPGL group had VHL disease. Results for specific genetic subtypes are limited. Because HIF-2α activation is a shared downstream driver across many PPGL genetic backgrounds, the mechanism is biologically plausible across subtypes, though sample sizes for individual mutations are small.
▸Who should not take belzutifan?
Pregnant patients cannot take it due to embryo-fetal toxicity. Patients with severe baseline anemia or hypoxia need careful evaluation. Hormonal contraceptives can be made less effective by belzutifan, so a non-hormonal backup is recommended for patients who can become pregnant.
▸How is response monitored?
Tumor response is followed by CT or MRI imaging at regular intervals. In PPGL, biochemical markers (plasma or urine metanephrines) and blood pressure are also tracked to gauge whether the tumor is producing fewer catecholamines. Many patients see a reduction in antihypertensive medication needs as catecholamine secretion falls.