Long-term efficacy and safety of rifampin in the treatment of a patient carrying a CYP24A1 loss-of-function variant

Pharmacological therapy may be useful in the treatment of moderate to severe hypercalcemia in patients with infantile hypercalcemia-1 (HCINF1) due to pathogenic variants in the cytochrome P450 24 subfamily A member 1 (CYP24A1). Rifampin is an antituberculosis drug that is a potent inducer of cytochrome P450 3 subfamily A member 4 (CYP3A4), involved in an alternative catabolic pathway of vitamin D. The efficacy of rifampin in improving hypercalcemia was previously reported but many questions remain on the long-term efficacy and safety. Aim of the study is to test the long-term efficacy and safety of rifampin in a patient with HCINF1. We report clinical, biochemical and imaging features of a 23-year-old man affected by HCINF1 with moderate hypercalcemia (12.9 mg/dL), symptomatic nephrolithiasis, nephrocalcinosis and impaired kidney function (eGFR 60 mL/min/1.73 m2) treated with rifampin for an overall period of 24 months. Kidney, liver and adrenal function were evaluated at every follow-up visit. In 2 months, rifampin induced a normalization of serum calcium (9.6 mg/dL) associated with an improvement of kidney function (eGFR 92 mL/min/1.73 m2) stable during the treatment. After 15 months, rifampin was temporally withdrawn because of asthenia, unrelated to impairment of adrenal function. After three months, the timing of drug administration was shifted from the morning to the evening, obtaining the remission of asthenia. At the end of follow-up, the nephrolithiasis disappeared and the nephrocalcinosis was stable. Rifampin could represent an effective choice to induce a stable reduction of calcium levels in patients with HCINF1, with a good safety profile.