Dmd051490 1285..1294

Replacement of hydrogenwith fluorine within three pairs of structurally similar small molecule inhibitors of heat shock protein 90 (HSP90) resulted in differences in inhibition constants (Ki) in vitro as well as marked differences in rat intravenous pharmacokinetic profiles. The difference in pharmacokinetic profiles between lower and higher affinity inhibitors (LAIs and HAIs, respectively) was characterized by remarkably different estimates for steady-state volumes of distribution (Vss: 1.8–2.0 versus 10–13 l/kg) with comparable clearance estimates (3.2–3.5 l/h per kilogram). When the observed Vss estimates were compared with the values predicted with the tissue-composition-based model, the observed Vss estimates for HAIs were 4to 8-fold larger than the predicted values, whereas the Vss values for LAIs were comparable. Accordingly, a negative relationship between in vitro HSP90 Ki versus in vivo Vss estimates was observed among these inhibitors. We therefore hypothesized that pharmacokinetic profiles of these inhibitors could be characterized by a target-mediated drug disposition (TMDD) model. In vivo equilibrium dissociation constant (KD) estimates for HAIs due to target binding by TMDD model with rapid binding approximation were 1–6 nM (equivalent to 0.3–2 nM free drug), which appeared comparable to the in vitro Ki estimates (2–3 nM). In vivo KD values of LAIs were not accurately determined by the TMDDmodel, likely due to nonspecific binding-dependent tissue distribution obscuring TMDD profiles. Overall, these results suggest that the observed large Vss estimates for potent HSP90 inhibitors are likely due to pharmacological target binding.