Enzalutamide and analytical interferences in digoxin assays.

We report two cases of elevated digoxin plasma levels in patients receiving enzalutamide. The first patient, an 84-year-old male treated with enzalutamide, was hospitalized due to deterioration in his general state. Atrial fibrillation was discovered and treatment with digoxin was initiated. Supratherapeutic digoxin concentrations (4 µg/L and 3.5 µg/L 3 days later) led to treatment being stopped despite the lack of clinical or biological signs of overdose. The second patient, an 84-year-old male treated with digoxin and enzalutamide, was hospitalized for the same reasons. Digoxin concentration upon admission was 2.8 μg/L. Despite stopping treatment, digoxin blood levels were observed to have increased on D3 and D7 following admission (3 and 3.6 μg/L, respectively). However, no clinical or biological findings indicated an overdose. Blood samples were sent to the Pharmacology and Toxicology Laboratory for analysis. The second patient's digoxin plasma level was determined using the chemiluminescent microparticle immunoassay (CMIA®, Abbott, Illinois) method. Enzalutamide levels were determined using HPLC-UV/DAD method. An interference study was performed using different assay methods by adding enzalutamide to control plasma at various concentrations from a Xtandi (40mg) capsule. Plasma concentration of digoxin at D7 for patient 2 was identical in both laboratories (3.5 vs. 3.6 µg/L). Enzalutamide was found in the patient's plasma (12,5 mg/L). Adding 4, 10, 20, and 40 mg/L of enzalutamide to the untreated plasma showed that the plasma concentration of digoxin was positive (from 0.35 to 3.69 µg/L) using the CMIA method. Our results highlight the analytical interferences of enzalutamide with digoxin assays using the CMIA method.