Rifampicin increased metabolism of tamoxifen
Clinical evidence,mechanism, importance and management
In 10 healthy men rifampicin 600mg daily for 5 days reduced AUC of a single 80mg dose of tamoxifen by 86%, reduced peak plasma levels by 55%, and reduced half-life by 44%. Similarly, AUC of N-demethyltamoxifen was reduced by 62 % (See reference number 1).
It is likely that rifampicin induces metabolism of tamoxifen by cytochrome P450 isoenzyme CYP3A4, thereby reducing its levels. These findings suggest that efficacy of tamoxifen may be reduced by rifampicin. However,there is some in vitro evidence that suggests that tamoxifen and rifampicin have additive antineoplastic effects in pancreatic carcinoma cell lines (See reference number 2). Also,tamoxifen induces its own metabolism on long-term use (See reference number 3). Thus, further study is needed to assess clinical impact of long-term combined use of these drugs.
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