The principal metabolic pathway of phenytoin and phenobarbital in human is aromatic hydroxylation, catalyzed by CYP2C9 and CYP2C19. The reactive intermediate, arene oxide is deactivated by either epoxide hydrolase to dihydrodiol or by the action of glutathione (GSH) and glutathione S-transferase. The pathways of phenytoin metabolism are depicted below. Valproate metabolism by P450 CYP2C9 CYP2A6  Hypersensitivity reactions (idiosyncratic toxicity) to the aromatic antiepileptic drugs in susceptible individuals are believed to stem from the reactions of these reactive intermediates (i.e., arene oxide catechol or o-quinone) with hepatic enzymes or other cellular proteins forming covalently bonded haptens. • Both phenytoin and phenobarbital are potent liver enzyme inducers. (i) Explain why co-administering valproate with phenytoin will increase their idiosyncratic toxicities. (ii) Explain why there are cross-sensitivities in terms of idiosyncratic reactions between phenytoin, phenobarbital, and carbamazepine? (iii) Pharmacogenetic factors determine susceptibility to idiosyncratic reactions to anti-epileptic drugs. What genes may be involved in these reactions and what is the mechanism involved?

Hypersensitivity reactions (idiosyncratic toxicity) to the aromatic antiepileptic drugs in susceptible individuals are believed to stem from the reactions of these reactive intermediates (i.e., arene oxide catechol or o-quinone) with hepatic enzymes or other cellular proteins forming covalently bonded haptens. • Both phenytoin and phenobarbital are potent liver enzyme inducers. (i) Explain why co-administering valproate with phenytoin will increase their idiosyncratic toxicities. (ii) Explain why there are cross-sensitivities in terms of idiosyncratic reactions between phenytoin, phenobarbital, and carbamazepine? (iii) Pharmacogenetic factors determine susceptibility to idiosyncratic reactions to anti-epileptic drugs. What genes may be involved in these reactions and what is the mechanism involved?

Classify each of these drugs based on their ability to induce or inhibit CYP450 enzymes phenobarbital

Phenytoin works by

Phenytoin ethanol Rifampicin Carbamazepine Isoniazid enalapril barbiturate cimetidine Aspirin Atropine Pencillin VProdrug Acidic drugs Basic drug Enzyme inducers Zero order kinetics Enzyme inhibitors Prodrug Acidic drugs Basic drug Enzyme inducers Zero order kinetics Enzyme inhibitors

Human gamma-glutamyl transpeptidase (hGGT) is an enzyme involved in cysteine homeostasis, and its overexpression has been linked to asthma, cancer, and other diseases.  It catalyzes the addition of water across a bond to cleave extracellular glutathione (GSH) into glutamate and a dipeptide composed of cysteine and glycine.  It is believed to function by the mechanism shown in Figure 1.Figure 1  Proposed mechanism of hGGT-mediated GSH cleavageResearchers interested in investigating hGGT inhibitors as a treatment for pathologies related to excess hGGT activity performed the following experiments, each containing the same concentration of hGGT.Experiment 1hGGT was incubated with various concentrations of GSH in the presence or absence of 250 μM candidate compound acivicin or Compound 1.  However, both candidates were found to be unsuitable as acivicin showed significant neurotoxicity and Compound 1 acted as an activator instead of an inhibitor.Figure 2  Lineweaver-Burk plot of hGGT activity with respect to GSH in the presence or absence of 250 μM candidate inhibitorsExperiment 2Researchers hypothesized that slight variations in the structure of Compound 1 could result in hGGT inhibition and tested several derivatives, shown in Table 1.Table 1  Kinetic Parameters of hGGT in the Presence or Absence of Several Candidate Inhibitors at 250 μMExperiment 3To better understand the physiological conditions under which hGGT operates, researchers exposed a cultured cell line expressing hGGT on the cell surface to a solution that mimicked the composition of extracellular fluid, including levels of GSH.  They found that in this setup, uninhibited hGGT results in a reaction rate that is approximately ½Vmax.Adapted from Wickham, S., Regan, N., West, M.B., Thai, J., Cook, P.F., Terzyan, S.S., Li, P.K., & Hanigan, M.H. (2013). Inhibition of human ?-glutamyl transpeptidase: development of more potent, physiologically relevant, uncompetitive inhibitors. The Biochemical Journal, 450(3), 547–557. Question 24Based on the information in Experiment 3, if researchers wish to further study the inhibitor that is most effective under physiological conditions, they should choose the one that yields the lowest hGGT activity when GSH concentration is:A.orders of magnitude less than the Km of the uninhibited enzyme.B.approximately equal to the Km of the uninhibited enzyme.C.approximately four times the Km of the uninhibited enzyme.D.orders of magnitude greater than the Km of the uninhibited enzyme.