Physiologically Based Pharmacokinetic Modeling of Phosphate Prodrugs─Case Studies: Fostemsavir and Fostamatinib

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Abstract
The aim of this work was to develop a physiologically based pharmacokinetic (PBPK) model for conversion of phosphate prodrugs to active drug via intestinal alkaline phosphatase (IAP) implementing a generalized modeling strategy. Fostemsavir and fostamatinib were chosen as model drugs since there is extensive clinical pharmacokinetic data following administration of oral formulations. LUA scripting was used to develop an "in vitro" to "in vivo" extrapolation of the conversion rate of prodrugs derived from Caco2 cell lines using an absolute IAP abundance approach. The Simcyp v23 platform was modified to generate a virtual population to reflect gastric emptying rates following administration of a moderate fat meal. The PBPK model predicted the results of three different extended-release (ER) tablets of fostemsavir under fasted and fed conditions as well as for powder in capsule and tablet immediate release (IR) formulations of fostamatinib. Retrospectively, the model was also able to assess the clinical relevance of the in vitro dissolution method to rate changes of different microcrystalline cellulose-based IR tablets of fostamatinib, observed in acidic media. All predictions were within 2-fold of the observed Cmax, AUC, and Tmax, with 81% being within 1.25-fold. The developed modeling strategy can be effectively adopted to increase the confidence of using PBPK modeling to prospectively assess the in vivo performance of phosphate prodrugs and support the development of optimal oral extended-release formulations for this class of drugs.

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