Real-Life Population Pharmacokinetics of Recombinant Factor XIII and Dosing Considerations for Preventing the Risk of Bleeding in Patients with FXIII Congenital Deficiency

Background and Objective Recombinant factor XIII (rFXIII) at the recommended dosage of 35 IU/kg every 4 weeks is currently used for prophylaxis of bleeding in patients affected by FXIII deficiency. The aim of this study was to describe the population pharmacokinetics of rFXIII in patients with FXIII deficiency being treated with rFXIII in real-life and to assess, using Monte Carlo simulations, the attainment of defined FXIII concentration thresholds associated with prevention of the risk of bleeding over time. Methods A nonlinear mixed-effects model approach was used for population analysis. Monte Carlo simulations were used to generate 10,000 FXIII concentration-time profiles associated with incremental doses of 25, 30, 35, 40, 45 and 50 IU/kg of rFXIII. The probability of target attainment (PTA) of FXIII concentrations at thresholds of > 0.05, > 0.10 and > 0.15 IU/mL were calculated weekly, from days 7 to 49. Results A total of 18 patients provided 99 FXIII concentrations; most patients (77.8%, 14/18) had severe FXIII deficiency. A two-compartment pharmacokinetic model with linear elimination from the central compartment best described rFXIII data. No covariates were associated with rFXIII disposition. Pharmacokinetic parameter estimates were 0.16 mL/h/kg for clearance, 57.35 mL/kg for volume of distribution at steady-state, and 11.72 days for elimination half-life. The standard 35 IU/kg dose resulted in PTAs of the pharmacodynamic thresholds of FXIII concentrations of > 0.05, > 0.10 and > 0.15 IU/mL at day 28 that were equal to 89.9%, 68.9% and 47.8%, respectively. Conclusions Intensive FXIII monitoring from day 14, and/or shortening the dosing interval between rFXIII administrations, should be considered to minimise the risk of bleeding.