Short Communication Potential Application of D-Optimal Designs in the Efficient Investigation of Cytochrome P450 Inhibition Kinetic Models

Correctly chosen D-optimal designs provide efficient experimental schemes when the aim of the investigation is to obtain precise estimates of parameters. In the current work, estimates of parameters refer to the enzyme kinetic parameters Vmax and Km, but they also refer to the inhibition constant Ki. In general, this experimental approach is performed on a grid of values of the design variables. However, this approach may not be very efficient, in the sense that the parameter estimates (Vmax, Km, and Ki) have unnecessarily high variances. For good estimates of parameters, the most efficient designs consist of clusters of replicates of a few sets of experimental conditions. The current study compares the application of such D-optimal designs with that of a conventional approach in assessing the competitive inhibitory potency of fluconazole and sertraline toward CYP2C9 and 2D6, respectively. In each instance, the parameter estimates, namely Vmax, Km, and Ki, were predicted well using the D-optimal design compared with those measured using the rich data sets, for both inhibitors. We show that D optimality can provide more efficient designs for estimating the model parameters, including Ki. We also show that real cost savings can be made by carefully planning studies that use the theory of optimal experimental design. Cytochrome P450 (P450) enzymes play a major role in drug metabolism, and CYP1A2, 2C9, 2C19, 2D6, and 3A4 are responsible for the majority of these reactions (Hasler et al., 1999). Altering the routes or rates of a metabolic reaction for a compound is particularly relevant with drugs that have a narrow therapeutic index, because small changes in the plasma concentration of the drug can potentially lead to an adverse effect. Such effects include reduction in efficacy or, even worse, toxicity. Inhibition of cytochromes P450 involved in the metabolic clearance of a drug, whether it is the drug itself or any coadministered drug, can potentially result in a metabolic drug-drug interaction (DDI). Inhibition of P450s results in the most frequently observed DDIs, and the associated mechanism of action can be classified as reversible, quasi-irreversible, or irreversible. Reversible inhibitors, the mechanisms of action of which can be further subdivided into competitive, noncompetitive, or uncompetitive inhibition (Houston et al., 2003), pertain to the current work. Mixed inhibition has also been reported but is less common (Houston et al., 2003). In a typical cytochrome P450 kinetic reaction, the enzyme binds substrate and metabolizes it into associated products. The binding step is reversible, whereas the catalytic step irreversible and is written as the following chemical model: