LAUSR

INTRODUCTION: Dapsone is a commonly used alternative for the prevention of Pneumocystis jirovecii pneumonia (PJP) in immunocompromised patients However, its use is limited by its significant dose-dependent hematologic adverse effects such as dapsone-induced methemoglobinemia by inhibiting cytochrome 3A4 preventing dapsone's metabolism to its toxic hydroxylamine metabolite We report a case of dapsone-induced methemoglobinemia in a kidney transplant recipient where oral cimetidine was trialed to reduce dapsone toxicity METHODS: A 37-year-old woman presented to the emergency department (ED) with a complaint of exertional shortness of breath Her past medical history was significant for a prior living-related kidney transplant secondary to cystinosis and recent PJP treated with atovaquone She was initiated on dapsone for PJP prophylaxis five days prior (previous G6PD level was 20 7 U/g Hgb) Dapsone doses were initiated at 25 mg PO daily and uptitrated to 100 mg PO daily over the course of treatment Upon arrival to the ED, the patient's oxygen (O2) saturation was 88% on room air She was transitioned to high-flow nasal cannula and her O2 saturation remained between 83-85% She was noted to have cyanotic fingernails upon physical examination Her CT chest without contrast and V/Q scan were without abnormalities Her COVID-19 PCR was negative Her methemoglobin level was unable to be obtained Her initial dapsone serum level was 1 4 mcg/mL The patient was transferred to intensive care As the patient remained refractory to O2 in the next 24 hours, it was decided to initiate cimetidine 400 mg PO three times daily Within 24 hours of starting therapy, the patient's O2 saturation improved to 92-95% and her dapsone level downtrended to 0 88 mcg/mL Her O2 continued to be weaned until she was stable for transfer to the floor RESULTS: Dapsone-induced methemoglobinemia is rarely reported in non-deficient G6PD patients Cimetidine may be an option in patients to reduce dapsone concentrations decreasing the length and degree of toxicity Providers should closely monitor patients for dapsone toxicity despite G6PD testing as onset can occur as early as a few days after initiation