Quantitative Estimation of Clinical Intrasubject Variability of a Pharmacokinetic Parameter

The virtual bioequivalence (VBE) approach, which employs physiologically based pharmacokinetic (PBPK) modeling in lieu of clinical bioequivalence (BE) studies, provides a valuable alternative for the pharmaceutical industry. The VBE has the potential to substantially reduce both the time and costs associated with traditional clinical BE studies. Furthermore, this approach supports ethical principles by eliminating the need to expose healthy volunteers to investigational compounds without medical benefit. Intrasubject variability (intrasubject coefficient of variation, ICV) is a main factor for balanced cross-over design BE studies in calculating the confidence intervals for the geometric mean ratio of AUC and C_max, which determines if bioequivalence criteria are met. Thus, it is essential to employ realistic and accurate ICV values for BE assessment through PBPK modeling. Published literature indicates that the drug properties that affect oral bioavailability exhibit a good correlation with ICV. The aim of this research is to expand drug databases to reevaluate the correlation between drug properties and clinically observed ICV for C_max (ICV_{_Cmax}). The results demonstrated that the dose number (dose to solubility ratio), permeability and elimination half-life are three main factors influencing ICV_{_Cmax}. Permeability becomes a dominating factor when the value is relatively low (P_app < 0.8 × 10^-6 cm/sec). In addition, PBPK modeling tools, Gastroplus and Simcyp were evaluated for their ability to predict ICV_{_Cmax} based on drug properties and their built-in physiological variations. Both simulation tools estimated intersubject variability and ICV_{_Cmax} similarly and underestimated ICV_{_Cmax} compared with clinically observed ICV_{_Cmax}.