Cytochrome P450 enzyme function

Isoniazid inhibits cytochrome P450 enzyme function and thus can interact with drugs that are subject to cytochrome P450 mediated metabolism like warfarin and the antiepileptic agents phenytoin and car-bamazepine. 

Generally, the liver is the center of drug metabolism, hence numerous drugs and methods to measure functional hepatic capacity are available. A host of these methods rely on the metabolic activity of cytochrome P450 enzymes, and some of the markers used include phenacetin, methacetin, trimethadione. 

Ezetimibe does not appear to be a substrate for cytochrome P450 enzymes. Experience to date reveals a low incidence of reversible impaired hepatic function with a small increase in incidence when given with a reductase inhibitor. Myositis has been reported rarely. 

Cyclophosphamide in its parent form does not have direct cytotoxic effects, and it must be activated to cytotoxic forms by microsomal enzymes (Figure 55-5). The liver microsomal cytochrome P450 mixed-function oxidase system converts cyclophosphamide to 4-hydroxycyclophosphamide, which is in equilibrium with aldophosphamide. These active metabolites are believed to be delivered by the bloodstream to both tumor and normal tissue, where nonenzymatic cleavage of aldophosphamide to the cytotoxic forms—phosphoramide mustard and acrolein—occurs. The liver appears to be protected through the enzymatic formation of the inactive metabolites 4-ketocyclophosphamide and carboxyphosphamide. 

Glaeser H, Drescher S, Eichelbaum M, et al. Influence of rifampicin on the expression and function of human intestinal cytochrome P450 enzymes. Br J Clin Pharmacol 2005 59(2) 199-206. 

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