Drug Interaction Report

MONITOR: Coadministration with fostemsavir may increase the plasma concentrations of drugs that are substrates of the transporters, organic anion transporting polypeptides (OATP) 1B1/1B3 and breast cancer resistance protein (BCRP). Temsavir, the active moiety of fostemsavir, is an inhibitor of OATP 1B1 and 1B3. Additionally, temsavir and two metabolites are inhibitors of BCRP. When a single 10 mg dose of rosuvastatin, an OATP 1B1/1B3 and BCRP substrate, was administered with fostemsavir 600 mg twice daily in 18 study subjects, mean rosuvastatin peak plasma concentration (Cmax) and systemic exposure (AUC) increased by 78% and 69%, respectively, compared to rosuvastatin administered alone.

MANAGEMENT: Caution is advised when fostemsavir is used concurrently with medications that are substrates of OATP 1B1/1B3 and/or BCRP, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever fostemsavir is added to or withdrawn from therapy.

GENERALLY AVOID: Moderate-to-high dietary intake of potassium, especially salt substitutes, may increase the risk of hyperkalemia in some patients who are using angiotensin II receptor blockers (ARBs). ARBs can promote hyperkalemia through inhibition of angiotensin II-induced aldosterone secretion. Patients with diabetes, heart failure, dehydration, or renal insufficiency have a greater risk of developing hyperkalemia.

MANAGEMENT: Patients should receive dietary counseling and be advised to not use potassium-containing salt substitutes or over-the-counter potassium supplements without consulting their physician. If salt substitutes are used concurrently, regular monitoring of serum potassium levels is recommended. Patients should also be advised to seek medical attention if they experience symptoms of hyperkalemia such as weakness, irregular heartbeat, confusion, tingling of the extremities, or feelings of heaviness in the legs.