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 of 32.0 (7.7) and 48.1 (10.6) microM, respectively. Chloramphenicol also weakly inhibited CYP2D6, with an apparent 50% inhibitory concentration of 375.9 (75.8) microM. 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.
Chloramphenicol is a potent inhibitor of cytochrome P450 isoforms CYP2C19 and CYP3A4 in human liver microsomes. Park JY, Kim KA, Kim SL. Antimicrob Agents Chemother. 2003 Nov;47(11):3464-9.

 

Caution is advised whenever nelfinavir is co-administered with agents that are inducers or inhibitors and/or substrates of CYP3A4; such combinations may require dose adjustment. Co-administration of nelfinavir with inhibitors of CYP2C19 may be expected to reduce the conversion of nelfinavir to its major active metabolite M8 (tert-butyl hydroxy nelfinavir) with a concomitant increase in plasma nelfinavir levels. Limited clinical trial data from patients receiving one or more of these medicinal products with nelfinavir indicated that a clinically significant effect on safety and efficacy is not expected. However, such an effect cannot be ruled out.
Viracept® (nelfinavir) Film-coated Tablets UK Summary of Product Characteristics, Roche Products Limited, updated July 2012