To show drug interactions between tacrolimus and ertapenem, we retrospectively evaluated 13 renal transplant recipients who had been treated with ertapenem for urinary tract infections during prescription of a constant dose. The mean dose of tacrolimus to achieve desired therapeutic concentrations decreased significantly after beginning ertapenem. The decrease from 0.079 mg/kg to 0.043 mg/kg occurred 2 days after initiation of ertapenem (P < .005). These results suggest that ertapenem, which is not metabolized through the cytochrome (CYP) P450 3A metabolic pathway, interacts with tacrolimus by an unknown mechanism. This report recommends tacrolimus concentration monitoring and dose reductions when the two drugs are administered in combination.
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