Tacrolimus, a potent immunosuppressive drug, is known to be metabolized predominantly in the liver by cytochrome P450 3A (CYP3A). In order to determine the potential of tacrolimus to inhibit the metabolism of other drugs, we have investigated its inhibitory effects on specific cytochrome reactions. Specific substrates for the seven cytochromes (CYPs) 1A2, 2A6, 2C9, 2C19, 2D6, 2E1 and 3A4/5 were incubated with human hepatic microsome preparations with or without specific inhibitors or tacrolimus and the metabolites were detected by high-pressure liquid chromatography (HPLC) or fluorimetric methods. All the specific inhibitors reduced or abolished the specific CYP activity. Tacrolimus had no effect on any CYP at concentrations below 1 microM, while at higher concentrations it had a mild inhibitory effect on CYP3A4 and 3A5. These observations suggest that tacrolimus is unlikely to potentiate the effect of coadministered drugs through inhibition of their metabolism in the liver.

A randomized multicenter study in intensive care unit (ICU) patients, evaluated the capacity of a Bayesian method to obtain an optimal first isepamicin (ISP) peak of 80 mg/L in comparison to a fixed loading dose (LD). Patients (n=236) over 18 years of age were enrolled from 6 September 1997 to 17 July 1999 and randomly assigned to received ISP in a calculated dose (CD) or a loading dose (LD) of 25 mg/kg body weight. The CD was estimated using a specific population model with Bayesian methodology implemented in the PKS program (Abbott PKS, Abbott Diagnostics, Rungis, France). The data required included age, body weight, height, gender and serum creatinine. ISP disposition is described by a one-compartment model. Blood samples were drawn 1 and 24 h after the start of infusion for fluorescence polarization immunoassay measurement of serum ISP concentrations. The predictive performance was assessed by computing bias and precision. Peak concentrations were significantly higher in CD group than the LD group (84.2 +/- 28.6 vs. 74.7 +/- 24.1 mg/L, respectively; P=0.008), but trough levels were comparable. The optimal ISP peak was attained by a significantly higher percentage of CD patients (P=0.018), and by significantly more CD patients on mechanical ventilation (P=0.025), and with simplified acute physiological scores (SAPS) > 35 (P=0.002). Pharmacokinetic parameters were similar for the two groups with large interindividual variations. Mean (+/- SD) volume of distribution of ventilated patients (72%) was significantly higher than of nonventilated patients (23.31 +/- 7.35 vs. 20.60 +/- 6.30 L, respectively; P=0.001). No relationship was found between the volume of distribution and SAPS. Total clearance was significantly correlated with estimated CLCR (creatinine clearance) (P=0.0001). Precision (RMSE) is better for CD than for LD strategy, respectively 27.96 and 28.66 mg/L. The Bayesian method was significantly more accurate and performed particularly well in ventilated patients and patients with high SAPS, compare to an LD of 25 mg/kg to obtain a first ISP peak of 80 mg/L in ICU patients. Therefore, a fixed dose of 28.5 mg/kg would be also adequate to reach a peak of 80 mg/L.

Digoxin concentrations were higher in patients with toxicity. Older age enhanced the risk of digoxin toxicity. Monitoring digoxin concentrations may help to confirm suspected digitalis toxicity.

No abstract

Clinical Pharmacology & Therapeutics (1999) 65, 136–136; doi: