The Interaction of Antiparasitic Drugs with the Drug-metabolising Cytochrome P450s (CYPs) and Characterisation of a CYP Variant Unique to African Populations.

The cytochrome P450 (CYP) superfamily of enzymes is now known to be an important determinant of the outcome of drug therapy. Consequently, drug-CYP interaction studies that have led to an improvement in production of safer and more effective drug therapies constitute a critical component of modern drug discovery and development programs. There is, however, a general paucity of data on the interactions of CYPs with antiparasitic drugs, most of which were discovered a long time ago and whose use is associated with numerous side effects. In addition, the enzyme kinetic profiles of CYP variants unique to African populations and important in drug metabolism are poorly characterised. In this study, therefore, the inhibitory effects of twenty-nine clinically important antiparasitic drugs on the major drug-metabolising CYPs were investigated in vitro using the well-established HPLC-based assays with human liver microsomes. Studies also involved the relatively new fluorescence-based high throughput screening (HTS) inhibition assays and human CYPs heterologously expressed in Saccharomyces cerevisiae. In addition, the inductive effects of the antiparasitic drugs on CYPs 1A1 and 1A2 were studied in vitro using the human hepatoma (HepG2) cell line as a model. The potential implications of the African specific CYP2D6*17 allele for phenotyping studies and clinical use of CYP2D6 substrate drugs in African populations were investigated with CYP2D6.17, heterologously expressed in Saccharomyces cerevisiae, by studying its enzyme kinetic profile towards CYP2D6 probe drugs and its capacity to clear CYP2D6 substrate drugs in vitro. To investigate the effects of amino acid exchanges in CYP2D6.17 on the structure of the enzyme, homology models were built using the CYP2C5 crystal structure as a template. Results from the HTS assays utilising recombinant CYPs were comparable with those from HPLC-based assays using human liver microsomes and validate inhibition data from fluorescence-based assays with recombinant CYPs. The majority of antiparasitic drugs are not expected to pose any risk for clinically significant effects based on their interactions with CYPs although some of them, however, could result in undesirable interactions with CYPs 1A1, 1A2 and 2D6. Potent inhibitors of CYP1A2 included artemisinin, niclosamide, thiabendazole, primaquine and dihydroartemisinin. Of these CYP1A2 inhibitors, thiabendazole, artemisinin and primaquine could give rise to clinically significant interactions as they were predicted to cause 98, 76 and 67 % inhibition of CYP1A2 in vivo respectively. Quinine, albendazole and primaquine induced the activities and mRNA levels of CYP1A1 and CYP1A2 with the induction by quinine and albendazole likely to be of significance in vivo. While cycloguanil, quinine, amodiaquine, desethylamodiaquine and proguanil were potent inhibitors of CYP2D6 in vitro, the inhibition is not expected to be of significance in vivo. For clinicians, knowledge of these possible antiparasitic drug-CYP interactions will be useful in designing appropriate interventions to address the effects of the drug-CYP interactions when they are encountered in the clinic. CYP2D6.17 exhibited a generally reduced capacity for clearing CYP2D6 substrates compared to CYP2D6.1 (wildtype) with the extent of reduction being dependent on the drug. This suggests the need for a general reduction in dosage of some CYP2D6 substrate drugs in African populations, particularly for drugs with narrow therapeutic indices. Results of homology modelling showed the arrangement of active-site residues in CYP2D6.17 to be different from those in CYP2D6.1 and could explain the substrate-dependent reduced capacity of CYP2D6.17 to clear CYP2D6 substrates