On May 14, 2025 Merck (NYSE: MRK), known as MSD outside of the United States and Canada, reported the U.S. Food and Drug Administration (FDA) has approved WELIREG (belzutifan), Merck’s oral hypoxia-inducible factor-2 alpha (HIF-2α) inhibitor, for the treatment of adult and pediatric patients 12 years and older with locally advanced, unresectable, or metastatic pheochromocytoma or paraganglioma (PPGL) (Press release, Merck & Co, MAY 14, 2025, View Source [SID1234653067]). Pheochromocytoma and paraganglioma are rare tumors that come from the same tissue, but pheochromocytoma form in the adrenal gland and paraganglioma form outside the adrenal gland. These tumors can be caused by certain genetic syndromes or mutations. The approval is based on data from the single-arm LITESPARK-015 clinical trial, where the primary endpoint was objective response rate (ORR).
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"PPGL, sometimes referred to as pheo para, is a rare condition affecting up to 2,000 people each year in the United States. Patients with these tumors, which arise from the adrenal glands and the extra-adrenal paraganglia, may require specialized care due to their complexity and rare nature, often posing significant challenges for both diagnosis and treatment," said Dr. Camilo Jimenez, professor, department of endocrine neoplasia and hormonal disorders, The University of Texas MD Anderson Cancer Center. "This approval, which is based on objective response rate data from the LITESPARK-015 trial, introduces belzutifan as the only approved and available non-surgical option for locally advanced, unresectable, or metastatic PPGL and could represent a change to the treatment paradigm for eligible patients."
"For patients with advanced PPGL, there has been a lack of approved systemic treatment options available to help manage their disease, underscoring the importance of this approval in the U.S.," said Dr. Marjorie Green, senior vice president and head of oncology, global clinical development, Merck Research Laboratories. "This approval marks the third indication for WELIREG in the U.S. and demonstrates our company’s commitment to providing innovative cancer therapies for patients in need, including those with rare diseases."
The WELIREG label contains a boxed warning that exposure to WELIREG during pregnancy can cause embryo-fetal harm. Verify pregnancy status prior to the initiation of WELIREG. Advise patients of these risks and the need for effective non-hormonal contraception. WELIREG can render some hormonal contraceptives ineffective. WELIREG can cause severe anemia that can require a blood transfusion. Monitor for anemia before initiation of and periodically throughout treatment with WELIREG. WELIREG can cause severe hypoxia that may require discontinuation, supplemental oxygen, or hospitalization. Monitor oxygen saturation before initiation of and periodically throughout treatment with WELIREG. For more information, see "Selected Safety Information" below.
Study design
LITESPARK-015 is an open-label, multicohort Phase 2 trial (ClinicalTrials.gov, NCT04924075) evaluating the efficacy and safety of WELIREG monotherapy. The study enrolled 72 patients in a single cohort (Cohort A1) who had measurable disease verified by blinded independent central review (BICR) per RECIST v1.1, documented histopathological diagnosis of PPGL, locally advanced or metastatic disease that was not amenable to surgery or curative treatment, and adequately controlled blood pressure (defined as BP <150/90 mm Hg, <135/85 mm Hg for adolescents) with no change in antihypertensive medications for patients with concomitant hypertension for at least two weeks prior to start of study treatment. Patients with carcinomatous meningitis were excluded. Patients received WELIREG at a dose of 120 mg once daily until disease progression or unacceptable toxicity.
The major efficacy outcome measure for the treatment of advanced PPGL was ORR measured by BICR using RECIST v1.1. Additional efficacy outcome measures included duration of response and time to response.
About pheochromocytoma and paraganglioma
Pheochromocytoma and paraganglioma (PPGL), sometimes referred to as pheo para, are rare tumors that can be caused by certain genetic syndromes or mutations. It is estimated that up to 2,000 new cases of PPGL are diagnosed each year in the U.S., and up to 52,800 new cases are diagnosed each year worldwide. Pheochromocytoma form in nerve tissue in the center of the adrenal gland, whereas paraganglioma form in nerve tissue near certain blood vessels and nerves outside the adrenal glands.
About WELIREG (belzutifan) 40 mg tablets, for oral use
Indications in the U.S.
Certain von Hippel-Lindau (VHL) disease-associated tumors
WELIREG is indicated for the treatment of adult patients with von Hippel-Lindau (VHL) disease who require therapy for associated renal cell carcinoma (RCC), central nervous system (CNS) hemangioblastomas, or pancreatic neuroendocrine tumors (pNET), not requiring immediate surgery.
Advanced Renal Cell Carcinoma (RCC)
WELIREG is indicated for the treatment of adult patients with advanced renal cell carcinoma (RCC) with a clear cell component following a programmed death receptor-1 (PD-1) or programmed death ligand 1 (PD-L1) inhibitor and a vascular endothelial growth factor tyrosine kinase inhibitor (VEGF-TKI).
Pheochromocytoma or Paraganglioma (PPGL)
WELIREG is indicated for the treatment of adult and pediatric patients 12 years and older with locally advanced, unresectable, or metastatic pheochromocytoma or paraganglioma (PPGL).
Selected Safety Information for WELIREG
Warning: Embryo-Fetal Toxicity
Exposure to WELIREG during pregnancy can cause embryo-fetal harm. Verify pregnancy status prior to the initiation of WELIREG. Advise patients of these risks and the need for effective non-hormonal contraception as WELIREG can render some hormonal contraceptives ineffective.
Anemia
WELIREG can cause severe anemia that can require blood transfusion. Monitor for anemia before initiation of, and periodically throughout, treatment. Transfuse patients as clinically indicated. For patients with hemoglobin <8 g/dL, withhold WELIREG until ≥8 g/dL, then resume at the same or reduced dose or permanently discontinue WELIREG, depending on the severity of anemia. For life-threatening anemia or when urgent intervention is indicated, withhold WELIREG until hemoglobin ≥8 g/dL, then resume at a reduced dose or permanently discontinue WELIREG.
In LITESPARK-004 (N=61), decreased hemoglobin occurred in 93% of patients with VHL disease and 7% had Grade 3 events. Median time to onset of anemia was 31 days (range: 1 day to 8.4 months).
The safety of erythropoiesis-stimulating agents (ESAs) for treatment of anemia in patients with VHL disease treated with WELIREG has not been established.
In LITESPARK-005 (n=372), decreased hemoglobin occurred in 88% of patients with advanced RCC with a clear cell component and 29% had Grade 3 events. Median time to onset of anemia was 29 days (range: 1 day to 16.6 months). Of the patients with anemia, 22% received transfusions only, 20% received ESAs only, and 12% received both transfusion and ESAs.
In LITESPARK-015, anemia occurred in 96% of patients and 22% had Grade 3 events. Median time to onset of anemia was 29 days (range: 1 day to 22.1 months). Of the patients with anemia, 20% received transfusions only, 26% received ESAs only, and 6% received both transfusion and ESAs.
Hypoxia
WELIREG can cause severe hypoxia that may require discontinuation, supplemental oxygen, or hospitalization.
Monitor oxygen saturation before initiation of, and periodically throughout, treatment. For decreased oxygen saturation with exercise (e.g., pulse oximeter <88% or PaO2 ≤55 mm Hg), consider withholding WELIREG until pulse oximetry with exercise is greater than 88%, then resume at the same dose or a reduced dose. For decreased oxygen saturation at rest (e.g., pulse oximeter <88% or PaO2 ≤55 mm Hg) or when urgent intervention is indicated, withhold WELIREG until resolved and resume at a reduced dose or discontinue. For life-threatening hypoxia or recurrent symptomatic hypoxia, permanently discontinue WELIREG. Advise patients to report signs and symptoms of hypoxia immediately to a healthcare provider.
In LITESPARK-004, hypoxia occurred in 1.6% of patients.
In LITESPARK-005, hypoxia occurred in 15% of patients and 10% had Grade 3 events. Of the patients with hypoxia, 69% were treated with oxygen therapy. Median time to onset of hypoxia was 30.5 days (range: 1 day to 21.1 months).
In LITESPARK-015, hypoxia occurred in 13% of patients and 10% had Grade 3 hypoxia. Median time to onset of hypoxia was 35 days (range: 6 days to 23.9 months). Of the patients with hypoxia, 67% were treated with oxygen therapy.
Embryo-Fetal Toxicity
Based on findings in animals, WELIREG can cause fetal harm when administered to a pregnant woman.
Advise pregnant women and females of reproductive potential of the potential risk to the fetus. Advise females of reproductive potential to use effective non-hormonal contraception during treatment with WELIREG and for 1 week after the last dose. WELIREG can render some hormonal contraceptives ineffective. Advise male patients with female partners of reproductive potential to use effective contraception during treatment with WELIREG and for 1 week after the last dose.
Adverse Reactions
Adverse Reactions in LITESPARK-004
Serious adverse reactions occurred in 15% of patients, including anemia, hypoxia, anaphylaxis reaction, retinal detachment, and central retinal vein occlusion (1 patient each).
WELIREG was permanently discontinued due to adverse reactions in 3.3% of patients for dizziness and opioid overdose (1.6% each).
Dosage interruptions due to an adverse reaction occurred in 39% of patients. Those which required dosage interruption in >2% of patients were fatigue, decreased hemoglobin, anemia, nausea, abdominal pain, headache, and influenza-like illness.
Dose reductions due to an adverse reaction occurred in 13% of patients. The most frequently reported adverse reaction which required dose reduction was fatigue (7%).
The most common adverse reactions (≥25%), including laboratory abnormalities, that occurred in patients who received WELIREG were decreased hemoglobin (93%), fatigue (64%), increased creatinine (64%), headache (39%), dizziness (38%), increased glucose (34%), and nausea (31%).
Adverse Reactions in LITESPARK-005
Serious adverse reactions occurred in 38% of patients. The most frequently reported serious adverse reactions were hypoxia (7%), anemia (5%), pneumonia (3.5%), hemorrhage (3%), and pleural effusion (2.2%). Fatal adverse reactions occurred in 3.2% of patients who received WELIREG, including sepsis (0.5%) and hemorrhage (0.5%).
WELIREG was permanently discontinued due to adverse reactions in 6% of patients. Adverse reactions which resulted in permanent discontinuation (≥0.5%) were hypoxia (1.1%), anemia (0.5%), and hemorrhage (0.5%).
Dosage interruptions due to an adverse reaction occurred in 39% of patients. Of the patients who received WELIREG, 28% were 65 to 74 years, and 10% were 75 years and over. Dose interruptions occurred in 48% of patients ≥65 years of age and in 34% of younger patients. Adverse reactions which required dosage interruption in ≥2% of patients were anemia (8%), hypoxia (5%), COVID-19 (4.3%), fatigue (3.2%), and hemorrhage (2.2%).
Dose reductions due to an adverse reaction occurred in 13% of patients. Dose reductions occurred in 18% of patients ≥65 years of age and in 10% of younger patients. The most frequently reported adverse reactions which required dose reduction (≥1.0%) were hypoxia (5%) and anemia (3.2%).
The most common (≥25%) adverse reactions, including laboratory abnormalities, were decreased hemoglobin (88%), fatigue (43%), musculoskeletal pain (34%), increased creatinine (34%), decreased lymphocytes (34%), increased alanine aminotransferase (32%), decreased sodium (31%), increased potassium (29%), and increased aspartate aminotransferase (27%).
Adverse Reactions in LITESPARK-015
Serious adverse reactions occurred in 36% of patients. The most frequently reported serious adverse reactions were anemia and hypertension (4.2% each) and pyelonephritis, pneumonia, hypoxia, dyspnea and hemorrhage (2.8% each).
WELIREG was permanently discontinued due to adverse reactions in 2 patients (2.8%). Adverse reactions which resulted in permanent discontinuation were increased alanine aminotransferase and paraparesis (1.4% each).
Dosage interruptions due to an adverse reaction occurred in 40% of patients. Of the patients who received WELIREG, 13% were ≥65 years old and 4.2% were ≥75 years. Adverse reactions which required dosage interruption in >3% of patients were hypoxia, nausea and fatigue (4.2% each).
Dose reductions due to an adverse reaction occurred in 14% of patients. The most frequently reported adverse reaction which required dose reduction was hypoxia (4.2%).
The most common (≥25%) adverse reactions, including laboratory abnormalities, that occurred in patients were anemia (96%), fatigue (56%), musculoskeletal pain (56%), decreased lymphocytes (54%), increased alanine aminotransferase (51%), increased aspartate aminotransferase (42%), increased calcium (34%), dyspnea (33%), increased potassium (31%), decreased leukocytes (30%), headache (29%), increased alkaline phosphatase (25%), dizziness (26%) and nausea (25%).
Drug Interactions
Coadministration of WELIREG with inhibitors of UGT2B17 or CYP2C19 increases plasma exposure of belzutifan, which may increase the incidence and severity of adverse reactions. Monitor for anemia and hypoxia and reduce the dosage of WELIREG as recommended.
Coadministration of WELIREG with CYP3A4 substrates decreases concentrations of CYP3A4 substrates, which may reduce the efficacy of these substrates or lead to therapeutic failures. Avoid coadministration with sensitive CYP3A4 substrates. If coadministration cannot be avoided, increase the sensitive CYP3A4 substrate dosage in accordance with its Prescribing Information. Coadministration of WELIREG with hormonal contraceptives may lead to contraceptive failure or an increase in breakthrough bleeding.
Lactation
Because of the potential for serious adverse reactions in breastfed children, advise women not to breastfeed during treatment with WELIREG and for 1 week after the last dose.
Females and Males of Reproductive Potential
WELIREG can cause fetal harm when administered to a pregnant woman. Verify the pregnancy status of females of reproductive potential prior to initiating treatment with WELIREG.
Use of WELIREG may reduce the efficacy of hormonal contraceptives. Advise females of reproductive potential to use effective non-hormonal contraception during treatment with WELIREG and for 1 week after the last dose. Advise males with female partners of reproductive potential to use effective contraception during treatment with WELIREG and for 1 week after the last dose.
Based on findings in animals, WELIREG may impair fertility in males and females of reproductive potential and the reversibility of this effect is unknown.
Pediatric Use
The safety and effectiveness of WELIREG have been established in pediatric patients aged 12 years and older for the treatment of locally advanced, unresectable, or metastatic pheochromocytoma or paraganglioma.
Renal Impairment
For patients with severe renal impairment (eGFR 15-29 mL/min estimated by MDRD), monitor for increased adverse reactions and modify the dosage as recommended.
Hepatic Impairment
WELIREG has not been studied in patients with severe hepatic impairment (total bilirubin >1.5 x ULN and any AST). For patients with moderate and severe hepatic impairment, monitor for increased adverse reactions and modify the dosage as recommended.