Potent inhibition of cytochrome P450 2B6 by sibutramine in human liver microsomes

Evaluation of small interfering RNA-dependent knockdowns of drug-metabolizing enzymes in multiwell array culture of primary human hepatocyte spheroids for estimation of fraction metabolized

The determination of the relative contribution of different drug-metabolizing enzymes to the metabolism of slowly metabolized compounds is a challenging task. The quantification of low compound turnover in standard in vitro systems, such as liver microsomes or hepatocyte suspension cultures, can be difficult. Thus, the use of long-term liver models, such as HepatoPac (BioIVT) or liver spheroids, has been suggested. Inhibitors of cytochrome P450 (P450) enzymes, the most important group of drug-metabolizing enzymes, represent the current standard to evaluate the route of drug metabolism. However, a long-term inhibition in systems such as spheroid models may be technically challenging due to limited stability of some of the commonly used inhibitors. Small interfering RNA (siRNA)-dependent knockdown of P450 enzymes in spheroid cultures of primary human hepatocytes represents a novel alternative to the established methods. In the current study, we report the successful attenuation of the CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 isoforms via siRNA on gene expression, as well as on the functional level, for at least 7 days. The analysis revealed that the knockdowns had only minor effects on the overall transcriptome. They also showed acceptable selectivity towards drug-metabolizing enzymes, except for the knockdown of CYP2C19. Applicability of the system for the determination of the fraction metabolized of low clearance substances was examined using 6 compounds metabolized by different P450s. By introducing siRNA-dependent knockdowns in phenotypically relevant primary human hepatocyte spheroid cultures, we hope to provide a novel alternative to standard systems to elucidate clearance pathways in vitro. SIGNIFICANCE STATEMENT: Small interfering RNA-mediated gene knockdowns of different cytochrome P450 enzymes were shown to be effective in long-term primary human hepatocyte spheroid cultures, representing a novel alternative for reaction phenotyping. This method has the potential to improve the assessment of pharmacokinetic variability and victim drug-drug interaction risks due to enzyme polymorphism or inhibition/induction with more confidence, particularly for low clearance drug candidates. Furthermore, minor effects of the small interfering RNA-mediated gene knockdowns for different cytochrome P450 enzymes on cell viability and the transcriptome were observed which implies that this system may be useful in deconvoluting toxicity caused by metabolites.

Inhibition and induction of CYP enzymes in humans: an update

The cytochrome P450 (CYP) enzyme family is the most important enzyme system catalyzing the phase 1 metabolism of pharmaceuticals and other xenobiotics such as herbal remedies and toxic compounds in the environment. The inhibition and induction of CYPs are major mechanisms causing pharmacokinetic drug–drug interactions. This review presents a comprehensive update on the inhibitors and inducers of the specific CYP enzymes in humans. The focus is on the more recent human in vitro and in vivo findings since the publication of our previous review on this topic in 2008. In addition to the general presentation of inhibitory drugs and inducers of human CYP enzymes by drugs, herbal remedies, and toxic compounds, an in-depth view on tyrosine-kinase inhibitors and antiretroviral HIV medications as victims and perpetrators of drug–drug interactions is provided as examples of the current trends in the field. Also, a concise overview of the mechanisms of CYP induction is presented to aid the understanding of the induction phenomena.

Site-specific oxidation of flavanone and flavone by cytochrome P450 2A6 in human liver microsomes

The roles of human cytochrome P450 (P450 or CYP) 2A6 in the oxidation of flavanone [(2R)- and (2S)-enantiomers] and flavone were studied in human liver microsomes and recombinant human P450 enzymes. CYP2A6 was highly active in oxidizing flavanone to form flavone, 2'-hydroxy-, 4'-, and 6-hydroxyflavanones and in oxidizing flavone to form mono- and di-hydroxylated products, such as mono-hydroxy flavones M6, M7, and M11 and di-hydroxy flavones M3, M4, and M5. Liver microsomes prepared from human sample HH2, defective in coumarin 7-hydroxylation activity, were very inefficient in forming 2'-hydroxyflavanone from flavanone and a mono-hydroxylated product, M6, from flavone. Coumarin and anti-CYP2A6 antibodies strongly inhibited the formation of these metabolites in microsomes prepared from liver samples HH47 and 54, which were active in coumarin oxidation activities. Molecular docking analysis showed that the C2'-position of (2R)-flavanone (3.8 Å) was closer to the iron center of CYP2A6 than the C6-position (10 Å), while distances from C2' and C6 of (2S)-flavanone to the CYP2A6 were 6.91 Å and 5.42 Å, respectively. These results suggest that CYP2A6 catalyzes site-specific oxidation of (racemic) flavanone and also flavone in human liver microsomes. CYP1A2 and CYP2B6 were also found to play significant roles in some of the oxidations of these flavonoids by human liver microsomes.

Selective inhibition of cytochrome P450 2D6 by Sarpogrelate and its active metabolite, M-1, in human liver microsomes

The present study was performed to evaluate the in vitro inhibitory potential of sarpogrelate and its active metabolite, M-1, on the activities of nine human cytochrome (CYP) isoforms. Using a cocktail assay, the effects of sarpogrelate on nine CYP isoforms and M-1 were measured by specific marker reactions in human liver microsomes. Sarpogrelate potently and selectively inhibited CYP2D6-mediated dextromethorphan O-demethylation with an IC50 (Ki) value of 3.05 μM (1.24 μM), in a competitive manner. M-1 also markedly inhibited CYP2D6 activity; its inhibitory effect with an IC50 (Ki) value of 0.201 μM (0.120 μM) was more potent than that of sarpogrelate, and was similarly potent as quinidine (Ki, 0.129 μM), a well-known typical CYP2D6 inhibitor. In addition, sarpogrelate and M-1 strongly inhibited both CYP2D6-catalyzed bufuralol 1'-hydroxylation and metoprolol α-hydroxylation activities. However, sarpogrelate and M-1 showed no apparent inhibition of the other following eight CYPs: CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2E1, or CYP3A4/5. Upon 30-minute preincubation of human liver microsomes with sarpogrelate or M-1 in the presence of NADPH, no obvious shift in IC50 was observed in terms of inhibition of the nine CYP activities, suggesting that sarpogrelate and M-1 are not time-dependent inactivators. Sarpogrelate strongly inhibited the activity of CYP2D6 at clinically relevant concentrations in human liver microsomes. These observations suggest that sarpogrelate could have an effect on the metabolic clearance of drugs possessing CYP2D6-catalyzed metabolism as a major clearance pathway, thereby eliciting pharmacokinetic drug-drug interactions.