Nonlinear pharmacokinetics and interconversion of prednisolone and prednisone in rats

The pharmacokinetics of prednisolone and prednisone were examined in 32 rats at four intravenous doses (5, 10, 25, and 50 mg/kg). Each rat was given one dose of either intravenous prednisolone or prednisone, and plasma concentrations of both compounds were measured by HPLC. Mammillary moment analysis showed the apparent clearance and volume of distribution of prednisolone and the apparent clearance of prednisone to be dose-dependent. Further diagnostic analysis using a linear interconversion model revealed modest interconversion between the two steroids and at least two saturable clearance processes: the conversion of prednisolone to prednisone and the irreversible elimination of prednisone. A comprehensive model which incorporates the nonlinear clearances of prednisolone and prednisone plus the additional feature of nonlinear tissue distribution of prednisolone was then constructed. Four differential equations describing the rate of change of each steroid in each compartment were used to numerically fit by nonlinear least squares analysis all plasma concentration-time profiles simultaneously. The final estimates from the full model only partly agreed with the results from the two moment analyses but confirmed the general structure of the model. The nonlinear tissue distribution of prednisolone was reinforced by assay of muscle tissue. This study demonstrates the utility of the model-building process where simpler models yield insights into more elaborate schemes with complex nonlinear features.