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?
Question
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?
Solution
(i) Valproate is known to inhibit the metabolism of other drugs, including phenytoin. When valproate is co-administered with phenytoin, it can increase the concentration of phenytoin in the body by reducing its metabolism. This can lead to an increase in the formation of reactive intermediates, which can increase the risk of idiosyncratic toxicities.
(ii) Phenytoin, phenobarbital, and carbamazepine are all aromatic antiepileptic drugs. They are metabolized in the liver and can form reactive intermediates that can bind to cellular proteins and form haptens. These haptens can trigger an immune response, leading to idiosyncratic reactions. Because these drugs have similar structures and are metabolized in a similar way, there can be cross-sensitivities in terms of idiosyncratic reactions. If an individual has a hypersensitivity reaction to one of these drugs, they may also have a reaction to the others.
(iii) Pharmacogenetic factors can indeed determine susceptibility to idiosyncratic reactions to anti-epileptic drugs. The genes involved in these reactions are typically those that encode for the enzymes involved in drug metabolism, such as cytochrome P450 enzymes. Variations in these genes can affect the activity of these enzymes, which can influence the rate at which a drug is metabolized and the amount of reactive intermediates that are formed. This can affect an individual's susceptibility to idiosyncratic reactions.
Similar Questions
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?
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