Inhibition of human cytochrome P450 isoenzymes by a phenothiazine neuroleptic levomepromazine: An in vitro study. 
Inhibition of cytochrome P450 (CYP) isoenzymes is the most common cause of harmful drug-drug interactions.  The present study was aimed at examining the inhibitory effect of the phenothiazine neuroleptic levomepromazine on main CYP isoenzymes in human liver.  The experiment was performed in vitro using the human cDNA-expressed CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 (Supersomes).  CYP isoenzyme activities were determined using the CYP-specific reactions: caffeine 3-N-demethylation (CYP1A2), diclofenac 4'-hydroxylation (CYP2C9), perazine N-demethylation (CYP2C19), bufuralol 1'-hydroxylation (CYP2D6) and testosterone 6β-hydroxylation (CYP3A4).  The rates of the CYP-specific reactions were assessed in the absence and presence of levomepromazine (1-50 μM).  The concentrations of CYP-specific substrates and their metabolites formed by CYP isoenzymes were measured by HPLC with UV or fluorimetric detection.  Levomepromazine potently inhibited CYP2D6 (K(i) = 6 μM) in a competitive manner.  Moreover, the neuroleptic moderately diminished the activity of CYP1A2 (K(i) = 47 μM) and CYP3A4 (K(i) = 34 μM) via a mixed mechanism.  On the other hand, levomepromazine did not affect the activities of CYP2C9 and CYP2C19.  The inhibition of CYP1A2, CYP2D6 and CYP3A4 by levomepromazine, demonstrated in vitro in the present study, should also be observed in vivo (especially the CYP2D6 inhibition by levomepromazine), since the calculated K(i) values are below or close to the presumed concentration range for levomepromazine in the liver in vivo.  Therefore pharmacokinetic interactions involving levomepromazine and CYP2D6, CYP1A2 or CYP3A4 substrates are likely to occur in patients during co-administration of the above-mentioned substrates/drugs.