Enhanced elimination of cyclophosphamide by high cut-off haemodialysis: single-dose pharmacokinetics in a patient with cast nephropathy

High cut-off (HCO) haemodialysis removes free light chains in patients with multiple myeloma. This is possible as HCO dialysers allow clearance of molecules up to a molecular weight of 65 kDa. In contrast, high-flux dialysers, which are used in routine haemodialysis, only remove molecules up to a molecular weight of 20 kDa. Even though patients with advanced myeloma frequently need dialysisand alkylating agents, drug dosing recommendations in this patient population are scarce at best or absent as for cyclophosphamidedosing in patients with myeloma undergoing HCO dialysis. Therefore, we aimed to determine pharmacokinetics of cyclophosphamidein a 52-year-old man (height 172 cm, weight 80 kg) with cast nephropathy. Intermittent 4-hour haemodialysis was started ~6 hours after the end of a 70 min cyclophosphamideinfusion containing 1700 mg of this drug. Blood/dialysate flow rates were 300/500 mL/hour, respectively. Peak concentration of cyclophosphamidewas 24.7 mg/L. Using HCO dialysis, plasma concentration of cyclophosphamidedecreased from 10.8 mg/L to 3.7 mg/L during the treatment. The calculated whole blood dialyser clearance was 166 mL/min. HCO dialysisled to a marked decrease of cyclophosphamideresulting in a a 50% reduction in half-life as compared with the half-life before dialysis. This removal has to be accounted for in dosing cyclophosphamide.