Isoniazid (INH) is the first-line anti-tubercular drug that is used both for the prophylaxis as well as the treatment of tuberculosis (TB). The patients with TB are more vulnerable to secondary infections and other health complications, hence, they are usually administered a cocktail of drugs. This increases the likelihood of drug-drug interactions (DDIs). INH is clinically proven to interact with drugs like phenytoin, carbamazepine, diazepam, triazolam, acetaminophen, etc. Most of such clinical observations have been supported by in vitro inhibition studies involving INH and cytochrome P450 (CYP) enzymes. A few published in vitro studies have explored the CYP2E1 inhibition potential of INH to explain its interactions with acetaminophen and other CY2E1 substrates, such as chlorzoxazone, but none of them were able to demonstrate any significant inhibition of the enzyme by the drug. It was reported that metabolites of INH, such as acetylhydrazine and hydrazine, were bioactivated by CYP2E1, highlighting that perhaps the drug metabolites were responsible for the mechanism based inhibition (MBI) of the enzyme. Therefore, the purpose of this investigation was to explore CYP2E1 enzyme inhibition potential of INH and its four major metabolites, viz., acetylisoniazid, isonicotinic acid, acetylhydrazine and hydrazine, using human liver microsomes (HLM). Additionally, we determined the fraction unbound in microsomal incubation (fumic) for all the five compounds using equilibrium dialysis assay. We observed that INH and its metabolites had lower propensity for microsomal binding, and the metabolites also lacked the potential to inhibit CYP2E1 enzyme, either by direct inhibition or through MBI. This suggests involvement of some other mechanism to explain interactions of INH with CY2E1 substrates, signifying need of further exploration.
Keywords: CYP2E1; Drug-drug interactions; Inhibition; Isoniazid; Metabolites; Microsomal binding.
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