Specificity of substrate and inhibitor probes for cytochrome P450s: evaluation ofin vitrometabolism using cDNA-expressed human P450s and human liver microsomes

Abstract

1. We evaluated the specificity of 15 substrates and 14 inhibitors of the cytochrome P450s using nine human P450 forms expressed in HepG2 cells using a recombinant vaccinia virus and also in human liver microsomes. 2. Coumarin, 7-ethoxyresorufin, 7-benzyloxyresorufin, tolbutamide, aniline and diazepam were form-selective substrates towards CYP2A6, the CYP1A subfamily, CYP2B6, the CYP2C subfamily, CYP2E1 and the CYP3A subfamily respectively. However, a selective substrate for CYP2D6 was not found among the chemicals tested. 3. SKF-525A inhibited > 40% of the metabolic activity of all substrates tested, and the inhibitory effects differed among P450 forms. Sulphaphenazole, 7,8-benzoflavone, quinidine and troleandomycin were selective inhibitors of the CYP2C subfamily (except CYP2C19), the CYP1A subfamily, CYP2D6 and the CYP3A subfamily respectively. Methoxsalen (CYP2A6 inhibitor) inhibited the metabolic activity of CYP1A2 as well as that of CYP2A6. Diethyldithiocarbamate (CYP2E1 inhibitor) inhibited the metabolic activities of CYP2A6 and CYP2C19 in addition to that of CYP2E1. 4. Our results indicated that substrates and inhibitors reported as P450 selective probes are not necessarily specific for individual human P450 forms. These results may provide useful information regarding human P450 substrates and inhibitors in vitro using human liver microsomal samples.

Pharmacokinetic and Pharmacodynamics Studies of Nicotine After Oral Administration in Mice: Effects of Methoxsalen, a CYP2A5/6 Inhibitor

“…Since the CYP2A6 gene is polymorphic, it is suggested that the inhibition of the enzyme will not have adverse effects ((Fernandez-Salguero et al, 1995). In addition, because very few therapeutically used drugs are metabolized by CYP2A6, the risk of clinically important drug interactions is also low (Ono et al, 1996).…”

Role of polymorphic human CYP2B6 in cyclophosphamide bioactivation

“…4 Further characterization of CYP2B6 revealed higher mRNA and protein levels than previously observed. [5][6][7] Meanwhile, a number of structurally diverse classes of frequently used drugs have been recognized as CYP2B6 substrates: cyclophosphamide (CPA), 8 7-ethoxy-4-trifluoromethylcoumarin, 9 bupropion, 10 ifosphamide, 8,11,12 nicotine, 13 lidocaine, 14 S-mephenytoin, 15 verapamil, 14 amitriptyline, 4 diazepam, 16 7-benzyloxyresorufin, 16 nevirapine, 4 S-mephobarbital, 17 artemisinin, 18 and propofol 19 .…”

Amifostine (WR-2721) selective protection against melphalan genotoxicity

“…In actual fact, melphalan is a substituted aniline, and aniline is a selective substrate towards various cytochrome P450s. [58][59][60] Furthermore, ring hydroxylation can increase aniline genotoxicity, with the formation of DNA adducts. 59 On the other hand, melphalan is also able to induce doserelated effects in WBC tested without S9 mix.…”