The drug delivery factors that influence the tissue distribution and pharmacokinetics of a series of active antiepileptic agents, the enaminones

Abstract

Early Central Nervous System (CNS) discovery generated new pharmaceuticals based on two properties of the compound, lipophilicity and molecular size. Almost all CNS pharmaceuticals presently in clinical trials possess both of these criteria and are capable of crossing the blood-brain barrier (BBB). Currently, modern methods of drug discovery utilize high throughput screening methods that may select molecules that lack both properties and will not undergo transport across the BBB. With the advent of in vitro cell culture models that mimic the BBB in vivo, researchers are now able to evaluate drug interactions at the BBB and to elucidate mechanisms at both the cellular and molecular levels pertinent for drug delivery. The utilization of both in vitro models for relatively rapid screening of permeability and related transport mechanisms, and in vivo models to assess drug pharmacokinetic distribution to the CNS provides a powerful assessment of drug delivery across the BBB to the CNS. Enaminone esters in the carbomethoxy series have previously been evaluated and shown to possess potent oral anticonvulsant activity in the mouse and rat. However, preliminary studies assessing enaminone analogs did not show expected correlations between relevant physiochemical parameters such as lipophilicity and BBB permeability. Therefore, in vitro models were utilized to assess factors associated with drug transport. An in vitro model of the BBB, Bovine Brain Microvessel Endothelial cells (BBMECs) were isolated and used to evaluated permeability and cellular mechanisms influencing enaminone transport. Results demonstrated that a multidrug resistant protein (MDR) influences enaminone permeability at the BBB. Further elucidation of possible mechanisms influencing enaminone distribution and pharmacokinetics were performed in a genetically altered mouse model [mdr1a/ b (--/--)] deficient in the expression of P-glycoprotein. Results comparing the brain distribution and partition coefficients in knockout mdr1a/b (--/--) versus wild type (+/+) counterparts demonstrated a higher accumulation of enaminones in brain tissue of knockout mice. Pharmacokinetic analysis of the tissue disposition of enaminones additionally demonstrated that the lack of P-glycoprotein in the lung and liver influence drug disposition in knockout animals. Lastly, a physiological based pharmacokinetic model was developed and found to be predictive of DM5 [methyl 4-[(4-chlorophenyl) amino]-6-methyl-2-oxocyclohex-3-en-1-oate] tissue distribution in mdr1a/b (--/--) knockout and wild type mice.