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

Abstract

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.

FIG. 1.

Inhibitory effect of chloramphenicol on CYP-catalyzed reactions in human liver microsomes. Effects of chloramphenicol (0 to 250 μM) on CYP1A2-catalyzed phenacetin O-deethylation (○), CYP2C8-catalyzed paclitaxel 6α-hydroxylation (•), CYP2C9-catalyzed S-warfarin 7-hydroxylation (▵), CYP2C19-catalyzed S-mephenytoin 4′-hydroxylation (▴), CYP2D6-catalyzed dextromethorphan O-demethylation (▿), CYP2E1-catalyzed chlorzoxazone 6-hydroxylation (▾), and CYP3A4-catalyzed midazolam 1-hydroxylation (□). Each data point indicates the average value calculated from three different liver microsomal preparations.

FIG. 2.

(A) Representative Dixon plot for the inhibition by chloramphenicol (0 to 100 μM) of CYP2C19-catalyzed S-mephenytoin 4′-hydroxylation in human liver microsomes with 25 (•), 50 (○), 75 (▴), or 100 (▵) μM S-mephenytoin. (B) Lineweaver-Burk plot of CYP2C19-catalyzed S-mephenytoin 4′-hydroxylation in the absence (▪) or presence of 10 (▵), 20 (▴), 50 (○), or 100 (•) μM chloramphenicol. (C) Secondary plot of slopes taken from Lineweaver-Burk plots versus chloramphenicol concentration. Each data point represents the average of duplicate measurements.

FIG. 3.

(A) Representative Dixon plot for the inhibition by chloramphenicol (0 to 100 μM) of CYP3A4-catalyzed midazolam 1-hydroxylation in human liver microsomes with 5 (•), 10 (○), 25 (▴), or 50 (▵) μM midazolam. (B) Lineweaver-Burk plot of CYP3A4-catalyzed midazolam 1-hydroxylation in the absence (▪) or presence of 10 (▵), 20 (▴), 50 (○), or 100 (•) μM chloramphenicol. (C) Secondary plot of slopes taken from Lineweaver-Burk plots versus chloramphenicol concentration. Each data point represents the average of duplicate measurements.

FIG. 4.

Inhibitory effect of chloramphenicol on phenytoin p-hydroxylation in human liver microsomes (A) and the inhibitory effect of chloramphenicol on HPPH formation from phenytoin (25 μM) in incubations with cDNA-expressed CYP2C9 (B) and CYP2C19 (C). Data are the averages of duplicate determinations.

FIG. 4.

Inhibitory effect of chloramphenicol on phenytoin p-hydroxylation in human liver microsomes (A) and the inhibitory effect of chloramphenicol on HPPH formation from phenytoin (25 μM) in incubations with cDNA-expressed CYP2C9 (B) and CYP2C19 (C). Data are the averages of duplicate determinations.

FIG. 4.

Inhibitory effect of chloramphenicol on phenytoin p-hydroxylation in human liver microsomes (A) and the inhibitory effect of chloramphenicol on HPPH formation from phenytoin (25 μM) in incubations with cDNA-expressed CYP2C9 (B) and CYP2C19 (C). Data are the averages of duplicate determinations.