Pharmacokinetic model for subcutaneous absorption of cyclosporine in the rabbit during chronic treatment

The objective of this study was to examine the concentrations in blood of cyclosporine (CyA) in rabbits during chronic subcutaneous (sc) administration and to propose a model that describes these data. Ten rabbits received sc CyA at 15 mg/kg daily for 1 week, then at 20 mg/kg twice weekly for 3 weeks, and at 15 mg/kg twice weekly for 7 weeks. Concentrations in blood were obtained weekly during the dosing period, and three to five concentrations were obtained over a 5‐week period after dosing was terminated. CyA blood concentration‐time (concentration in blood versus time) profiles could not be adequately described by absorption from a single dosing compartment. A two‐compartment, sc absorption‐site model was postulated. Steady‐state concentrations in blood from three additional rabbits that had received CyA at 16.8 μg/min as a constant‐rate intravenous infusion were added to the data set. A nonlinear mixed effects model was used to obtain the following parameter estimates (percent relative standard error): K₁₂ = 0.111 day⁻¹ (10.0), K₂₁ = 0.0109 day⁻¹ (12.6), K_a = 0.0807 day⁻¹ (11.8), CL/F = 14.6 L/day/kg (3.8), and Vd/F = 1.52 L/kg (13.4), where k₁₂ and k₂₁ are intercompartmental rate constants between sc compartments, k_a is the absorption rate constant into the sampling compartment, CL/F is the apparent blood clearance of CyA from the body (Fis bioavailability), and Vd/F is the apparent volume of distribution of CyA. The interindividual variability in CL/Fwas estimated as 20.5% (41.2), and the residual variability was 25.8% (15.6). The sc administration of CyA appears to provide slow, but very significant, absorption in rabbits.