Interaction between tacrolimus and chloramphenicol in a renal transplant recipient

Inhibition of voriconazole metabolism by chloramphenicol in an adolescent with central nervous system aspergillosis

For an adolescent with bacterial meningitis and subsequent cerebral aspergillosis, intravenous voriconazole dose requirements substantially decreased during coadministration with intravenous chloramphenicol and considerably rose after discontinuation of the antibiotic. In agreement with in vitro evidence, these data suggest that chloramphenicol is a rather significant inhibitor of hepatic CYP3A4 and/or CYP2C19.

Drug interactions in the hematopoietic stem cell transplant (HSCT) recipient: what every transplanter needs to know

Pharmacokinetic drug interactions among hematopoietic stem cell transplant recipients can result in either increases in serum concentrations of medications, which may lead to enhanced toxicity; or reduced serum concentrations, which can lead to treatment failure and the emergence of post transplant complications. The use of drugs that have a narrow therapeutic index, such as cyclosporine/tacrolimus (calcineurin inhibitors), increases the significance of these interactions when they occur. This report will review the clinical data evaluating the drug interactions of relevance to HSCT clinical practice, focusing on the pharmacokinetic interactions, and provides recommendations for managing these interactions to avoid both toxicity and treatment failure.

Chloramphenicol is a potent inhibitor of cytochrome P450 isoforms CYP2C19 and CYP3A4 in human liver microsomes

The inhibitory effect of chloramphenicol on human cytochrome P450 (CYP) isoforms was evaluated with human liver microsomes and cDNA-expressed CYPs. Chloramphenicol had a potent inhibitory effect on CYP2C19-catalyzed S-mephytoin 4'-hydroxylation and CYP3A4-catalyzed midazolam 1-hydroxylation, with apparent 50% inhibitory concentrations (inhibitory constant [K(i)] values are shown in parentheses) of 32.0 (7.7) and 48.1 (10.6) microM, respectively. Chloramphenicol also weakly inhibited CYP2D6, with an apparent 50% inhibitory concentration (K(i)) of 375.9 (75.8) microM. The mechanism of the drug interaction reported between chloramphenicol and phenytoin, which results in the elevation of plasma phenytoin concentrations, is clinically assumed to result from the inhibition of CYP2C9 by chloramphenicol. However, using human liver microsomes and cDNA-expressed CYPs, we showed this interaction arises from the inhibition of CYP2C19- not CYP2C9-catalyzed phenytoin metabolism. In conclusion, inhibition of CYP2C19 and CYP3A4 is the probable mechanism by which chloramphenicol decreases the clearance of coadministered drugs, which manifests as a drug interaction with chloramphenicol.

Mechanisms of clinically relevant drug interactions associated with tacrolimus

The clinical management of tacrolimus, a macrolide used as immunosuppressant after transplantation, is complicated by its narrow therapeutic index in combination with inter- and intraindividually variable pharmacokinetics. As a substrate of cytochrome P450 (CYP) 3A enzymes and P-glycoprotein, tacrolimus interacts with several other drugs used in transplantation medicine, which also are known CYP3A and/or P-glycoprotein inhibitors and/or inducers. In clinical studies, CYP3A/P-glycoprotein inhibitors and inducers primarily affect oral bioavailability of tacrolimus rather than its clearance, indicating a key role of intestinal P-glycoprotein and CYP3A. There is an almost complete overlap between the reported clinical drug interactions of tacrolimus and those of cyclosporin. However, in comparison with cyclosporin, only few controlled drug interaction studies have been carried out, but tacrolimus drug interactions have been extensively studied in vitro. These results are inconsistent and are of poor predictive value for clinical drug interactions because of false negative results. P-glycoprotein regulates distribution of tacrolimus through the blood-brain barrier into the brain as well as distribution into lymphocytes. Interaction of other drugs with P-glycoprotein may change tacrolimus tissue distribution and modify its toxicity and immunosuppressive activity. There is evidence that ethnic and gender differences exist for tacrolimus drug interactions. Therapeutic drug monitoring to guide dosage adjustments of tacrolimus is an efficient tool to manage drug interactions. In the near future, progress can be expected from studies evaluating potential pharmacokinetic interactions caused by herbal preparations and food components, the exact biochemical mechanism underlying tacrolimus toxicity, and the potential of inhibition of CYP3A and P-glycoprotein to improve oral bioavailability and to decrease intraindividual variability of tacrolimus pharmacokinetics.

Interaction of chloramphenicol and the calcineurin inhibitors in renal transplant recipients

Several case reports have described a pharmacokinetic interaction between chloramphenicol and the calcineurin inhibitors (CNIs). Based on these reports, we set out to characterize the effects of chloramphenicol on cyclosporine and tacrolimus trough concentrations in renal transplant recipients. We retrospectively evaluated daily trough CNI concentrations and compared them with baseline CNI concentrations prior to chloramphenicol. Six adult renal or pancreas/kidney transplant recipients received 11 courses of chloramphenicol. Of these, three received cyclosporine (6 episodes) and three received tacrolimus (5 episodes). The mean dose and duration of chloramphenicol was not significantly different between groups. Chloramphenicol coadministration increased mean cyclosporine troughs maximally by 41.3% on day 4, though overall differences were not significant using analysis of variance (anova). Tacrolimus trough levels increased to 99% above baseline on day 2, 151% on day 3, 161% on day 4, 191% on day 5, and to 207% on day 6 and reached statistical significance by anova (P = 0.001). These results confirm case reports and suggest that careful trough monitoring should be implemented if chloramphenicol is to be used with the CNIs.

Comparative Clinical Pharmacokinetics of Tacrolimus in Paediatric and Adult Patients

Tacrolimus is a potent immunosuppressive agent used to prevent allograft rejection. The pharmacokinetics of tacrolimus have been studied in healthy volunteers and transplant recipients, mostly by using immunoassays to measure tacrolimus in plasma or blood. However, because of the cross-reactivity for certain tacrolimus metabolites of the antibodies used, these methods often lack specificity. This should be carefully taken into account when interpreting pharmacokinetic results for tacrolimus. In adult patients, tacrolimus is generally rapidly absorbed following oral administration (the time to reach maximum concentration is 1 to 2 hours), but in some patients absorption is slow or even delayed. Because of presystemic elimination, the oral bioavailability is low (around 20%) but may vary between 4 and 89%. Tacrolimus is highly bound to erythrocytes. Its binding to plasma proteins varies between 72 and 98% depending on the methodology used. Because of the extensive partitioning of tacrolimus into erythrocytes, its apparent volume of distribution (Vd) based on blood concentrations is much lower (1.0 to 1.5 L/kg) compared with values based on plasma concentrations (about 30 L/kg). Tacrolimus is metabolised by cytochrome P450 (CYP) 3A4 to at least 10 metabolites, some of which retain significant activity. Biliary excretion is the route of elimination of the tacrolimus metabolites. Systemic plasma clearance of tacrolimus is very high (0.6 to 5.4 L/h/kg), whereas blood clearance is much lower (0.03 to 0.09 L/h/kg). The terminal elimination half-life (t1/2beta) of tacrolimus is approximately 12 hours (with a range of 3.5 to 40.5 hours). Only limited information is available on the pharmacokinetics of tacrolimus in paediatric patients. The rate and extent of tacrolimus absorption after oral administration do not seem to be altered in paediatric patients. The Vd of tacrolimus based on blood concentrations in paediatric patients (2.6 L/kg) is approximately twice the adult value. Blood clearance of tacrolimus is also approximately twice as high in paediatric (0.14 L/h/kg) compared with adult (0.06 L/h/kg) patients. Consequently, t1/2beta does not appear modified in children, but oral doses need to be generally 2-fold higher than corresponding adult doses to reach similar tacrolimus blood concentrations. More pharmacokinetic studies in paediatric patients are, however, needed to rationalise the use of therapeutic drug monitoring for optimisation of tacrolimus therapy in this patient population.