PT  - JOURNAL ARTICLE
AU  - Vivaswath S. Ayyar
AU  - Dawei Song
AU  - Debra C. DuBois
AU  - Richard R. Almon
AU  - William J. Jusko
TI  - Modeling Corticosteroid Pharmacokinetics and Pharmacodynamics, Part I: Determination and Prediction of Dexamethasone and Methylprednisolone Tissue Binding in the Rat
AID  - 10.1124/jpet.119.257519
DP  - 2019 Aug 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 318--326
VI  - 370
IP  - 2
4099  - http://jpet.aspetjournals.org/content/370/2/318.short
4100  - http://jpet.aspetjournals.org/content/370/2/318.full
SO  - J Pharmacol Exp Ther2019 Aug 01; 370
AB  - The plasma and tissue binding properties of two corticosteroids, dexamethasone (DEX) and methylprednisolone (MPL), were assessed in the rat in anticipation of developing physiologically based pharmacokinetic and pharmacokinetic/pharmacodynamic models. The tissue-to-plasma partition coefficients (KP) of DEX and MPL were measured in liver, muscle, and lung in vivo after steady-state infusion and bolus injection in rats. Since KP is often governed by reversible binding to macromolecules in blood and tissue, an attempt was made to assess KP values of DEX and MPL by in vitro binding studies using rat tissue homogenates and to compare these estimates to those obtained from in vivo kinetics after dosing. The KP values of both steroids were also calculated in rat tissues using mechanistic tissue composition–based equations. The plasma binding of DEX and MPL was linear with moderate binding (60.5% and 82.5%) in male and female rats. In vivo estimates of steroid uptake appeared linear across the tested concentrations and KP was highest in liver and lowest in muscle for both steroids. Assessment of hepatic binding of MPL in vitro was severely affected by drug loss at 37°C in male liver homogenates, whereas DEX was stable in both male and female liver homogenates. With the exception of MPL in liver, in vitro–derived KP estimates reasonably agreed with in vivo values. The mechanistic equations modestly underpredicted KP for both drugs. Tissue metabolism, saturable tissue binding, and active uptake are possible factors that can complicate assessments of in vivo tissue binding of steroids when using tissue homogenates.SIGNIFICANCE STATEMENT Assuming the free hormone hypothesis, the ratio of the unbound drug fraction in plasma and in tissues defines the tissue-to-plasma partition coefficient (KP), an important parameter in physiologically based pharmacokinetic modeling that determines total drug concentrations within tissues and the steady-state volume of distribution. This study assessed the plasma and tissue binding properties of the synthetic corticosteroids, dexamethasone and methylprednisolone, in rats using ultrafiltration and tissue homogenate techniques. In vitro–in vivo and in silico–in vivo extrapolation of KP was assessed for both drugs in liver, muscle, and lung. Although the extrapolation was fairly successful across the tissues, in vitro homogenate studies severely underpredicted the KP of methylprednisolone in liver, partly attributable to the extensive hepatic metabolism.