An Experimental Study of Belzutifan Impact on Catecholamine Metabolism

Inclusion Criteria: * Adult patients \> 18 years * Patients must have a biochemically confirmed diagnosis of a phaeochromocytoma or paraganglioma using plasma metanephrines or 24- hour urinary metanephrines and plasma or urinary metanephrines should be at least 1.5 times the upper limit of the normal reference range. * Female patients of child-bearing potential must have a negative serum pregnancy test result within 3 days before first administration of study drug * Able to provide informed consent Exclusion Criteria: * Has hypoxia, or requires intermittent supplemental oxygen, or requires chronic supplemental oxygen. * Has clinically significant cardiac disease, including unstable angina, acute myocardial infarction, percutaneous transluminal coronary angioplasty (PTCA), or coronary artery bypass graft surgery (CABG) ≤6 months from Day 1 of study drug administration, or New York Heart Association Class III or IV congestive heart failure. Medically controlled arrhythmia stable on medication is permitted. * Has a co-existing malignancy (in addition to PPGL) * Has a Hb \< 100g/dL * Is on medications which may interfere with belzutifan pharmacokinetics and that cannot be stopped for the duration of the study and for 7 days after the study period (everolimus, omeprazole, esomeprazole, Fluconazole, fluoxetine, Voriconazole, Sirolimus) * Has a known diagnosis of HIV, hepatitis B or hepatitis C