Cimetidine drug metabolism studies

Because finasteride and dutasteride are metabolized primarily by CYP3A4, the CYP3A4 inhibitors, such as ritonavir, ketoconazole, verapamil, diltiazem, cimetidine, and ciprofloxacin, may increase the drugs blood levels and, possibly, cause drug-drug interactions. Clinical drug interaction studies have shown no pharmacokinetic or pharmacodynamic interactions between dutasteride and tamsulosin or terazosin, warfarin, digoxin, and cholestyramine. 

Not understood. Studies with human plasma failed to find any evidence that cimetidine in therapeutic concentrations inhibits the metabolism of suxamethonium. However, metoclopramide may do and therefore is possibly the drug responsible for any interaction seen. In vitro studies with very high cimetidine concentrations found inhibition of plasma cholinesterase (pseudocholinesterase) activity. The cimetidine/vecuro-nium interaction is not understood, but it has been suggested that cimetidine may reduce the hepatic metabolism of vecuronium. ... 

Paclitaxel is metabolized by CYP2C8 and CYP3A4 [23, 83 ], and drugs that inhibit or induce these isoenzymes would be expected to alter the metabolism of paclitaxel. In vitro ranitidine, diphenhydramine, vincristine, vinblastine, and doxorubicin had little or no effect on the metabolism of paclitaxel, but barbiturates stimulated hydroxylation of the side-chain by induction of CYP3A isoforms [83 ]. Although cimeti-dine and famotidine have quite different CYP-modulating abilities, a clinical study showed no difference in paclitaxel clearance or associated neutropenia when cimetidine or famotidine were used as part of the premedication regimen [84 ]. 

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