Verification of a Physiologically-based Pharmacokinetic Model for Predicting CYP3A4-mediated Simvastatin and Simvastatin Acid Drug-drug Interactions.

Simvastatin is a commonly prescribed medication and a sensitive CYP3A4 substrate requiring dosage modification with CYP3A4 precipitants. Unlike other CYP3A4 substrates, grapefruit juice (GFJ) causes the largest increase in simvastatin exposure of any CYP3A4 inhibitor, leading to misunderstanding of simvastatin fraction escaping CYP3A4 metabolism in the gut (Fg, CYP3A4) and fraction metabolized by CYP3A4 in the liver (fm,CYP3A4). Simvastatin is a prodrug that is converted to the active simvastatin acid, though the mechanisms responsible for in vivo simvastatin acid formation are not well-understood. GFJ also decreases the simvastatin acid:simvastatin exposure ratio, suggesting inhibition of simvastatin acid formation. In the current work, we used a static approach to define simvastatin CYP3A4 parameters, using clinical data with index substrate midazolam; estimated simvastatin Fg,CYP3A4 and fm,CYP3A4 were ~ 0.4 and ~ 0.9, respectively. In vitro data assessing the stability of simvastatin and simvastatin acid demonstrated rapid conversion in gastric fluid that was highly pH-dependent. A simvastatin PBPK model was constructed incorporating these CYP3A4 parameters and simvastatin acid formation in stomach, intestine and plasma. The model reproduced the nonlinear pharmacokinetics of simvastatin and CYP3A4 precipitant effects, thus qualifying the model for prediction of CYP3A4-mediated interactions on both simvastatin and simvastatin acid. Simvastatin overall Fg was estimated as ~ 0.2, lower than the Fg,CYP3A4 due to additional intestinal esterase-mediated formation of simvastatin acid. The simvastatin-GFJ effect was explained by inhibition of this intestinal simvastatin acid formation, along with CYP3A4 inhibition. The unique effect of food on the simvastatin acid:simvastatin exposure ratio could also be replicated using this PBPK model.