A population pharmacokinetic model of sildenafil in an extra-uterine system for premature lambs


Sildenafil is a phosphodiesterase 5 inhibitor with a vasodilatory and anti-remodeling effect on vascular smooth muscle cells [1]. Prenatal therapy with sildenafil has been studied for multiple indications in different animal models [2–4]. In pregnant sheep however, transplacental transfer of sildenafil is extremely low, which complicates the study of its effect on the fetus. The EXTra-uterine Environment for Neonatal Development (EXTEND) model, provides a unique opportunity to exclude the maternal-placental axis and monitor the fetal lamb over time in a non-invasive manner [5]. Our goal was to develop a population pharmacokinetic (popPK) model of sildenafil in lambs on this experimental extra-uterine system, allowing dose selection for further studies.


Healthy lambs on EXTEND received a continuous infusion of sildenafil at 0.3, 0.5 or 0.7 mg/kg/24 h. The dose was adjusted daily based on the projected body weight (BW). Sildenafil was administered to the umbilical vein and concentrations were measured in the umbilical artery. Concentrations below the limit of quantification (LLOQ) were considered to be 1 µg/L (LLOQ = 2 µg/L). PK parameters were estimated with NONMEM 7.4 using the first-order conditional estimation method with interaction and subroutine ADVAN 6 as a differential equation solver. The most parsimonious model was withheld based on standard model comparison procedures, including a drop in objective function value ≥ 3.84 points (p ≤ 0.05) and the Akaike information criterion. Goodness-of-fit was evaluated by visual inspection of diagnostic plots. No covariate testing was performed. The final model was used to simulate the concentration-time profile of three dose levels (0.3, 0.5 and 0.7 mg/kg baseline BW/24h) over 14 days. Each dose was simulated 1,000 times as a continuous infusion without correcting for BW changes over time. The initial BW were sampled from 15 values originating from 10 historical controls and the 5 study animals. The time to reach a sildenafil concentration of 47 µg/L was used as a measure of target attainment [6].


A total of 124 sildenafil concentrations were available from five lambs (average 24 samples per lamb, 6 below LLOQ). They were followed-up for 168 h to 336 h. Sildenafil PK was best described by a two-compartmental model with first-order elimination. The estimated PK parameters (typical value [relative standard error]) were volume of distribution in the central compartment (V1: 11.3 L [6.2%]), clearance (CL: 0.438 L/h [8.3%]), volume of distribution in the peripheral compartment (V2: 15.9 L [22.3%]) and intercompartmental clearance (Q: 0.632 L/h [49.8%]). An exponential error model for inter-individual variability (IIV) and a combined proportional and additive residual error was used. IIV on V2, CL, Q and covariance between CL and Q were 57.2%, 16.8%, 106.1% and 86%, respectively. Proportional and additive residual error were 0.0141 (16.7%) and 2.56 (27.3%), respectively. The goodness-of-fit plots showed that the model described the observed data adequately. The steady-state concentration (based on doses normalized for baseline BW) was 52.8 µg/L for a dose of 0.3 mg/kg/24 h, 87.8 µg/L for a dose of 0.5 mg/kg/24 h and 123.1 µg/L for 0.7 mg/kg/24 h. The volume of distribution at steady state was calculated to be 27.2 L [7]. At a dose of 0.3 mg/kg/24 h, the time to 90% probability of target attainment (% PTA) of 47 µg/L was not reached. At 0.5 mg/kg/24 h and 0.7 mg/kg/24 h, 90% PTA was reached at 143.2 h and 53.8 h, respectively. These simulations supported a dose of 0.5 or 0.7 mg/kg baseline BW/24 h for further studies.


We developed a popPK model to describe sildenafil PK in an extra-uterine system for premature lambs. By using simulations, the model supported the dosing rationale for further pharmacodynamic studies investigating the anti-remodeling effect of sildenafil in fetal lambs.


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