Metabolism Of CYP3A Substrates Research Articles

Quantitative analysis of the impact of membrane permeability on intestinal first-pass metabolism of CYP3A substrates.

The aim of this study was firstly to investigate the effect of membrane permeability on the intestinal availability (Fg ) of 10 cytochrome P450 3A4 substrates with differing permeability (Papp ) and metabolic activity (CLint ) using Madin-Darby canine kidney II (MDCKII) cells expressing human CYP3A4 (MDCKII/CYP3A4 cells), and secondly to confirm the essential factors by simulations. A membrane permeation assay using MDCKII/CYP3A4 cells showed a significant correlation between human intestinal extraction ratio (ER) (Eg (=1-Fg )) and in vitro cellular ER (r=0.834). This relationship afforded better predictability of Eg values than the relationship between Eg and CLint,HIM values obtained from human intestinal microsomes (r=0.598). An even stronger correlation was observed between 1-Fa ·Fg and ER (r=0.874). Simulation with a cellular kinetic model indicated that ER is sensitive to changes of PSpassive and CLint values, but not to the intracellular unbound fraction (fu,cell ) or P-gp-mediated efflux (PSP-gp ). It may be concluded that, based on the concentration-time profile of drugs in epithelial cells, transmembrane permeability influences Fg (or ER) and drug exposure time to metabolizing enzymes for P450 substrate.

Comparison of intestinal metabolism of CYP3A substrates between rats and humans: application of portal–systemic concentration difference method

1. Rats are frequently used in pharmacokinetic studies during drug discovery. However, there is limited information regarding species differences in intestinal availability (Fg) between rats and humans.2. Here, we directly estimated the fraction of dose absorbed in the portal vein (FaFg) of rats for nine CYP3A substrates using portal–systemic concentration difference method and compared them with human FaFg. No distinct difference in FaFg between the two species was observed, and seven of the nine compounds were within a two-fold difference. Given that their net fraction of dose absorbed (Fa) are expected to be high, this result indicates a moderate correlation in Fg between the two species.3. In contrast, the in vitro intrinsic clearance (CLint,u) in rat intestinal microsomes tended to be lower than that in humans, and the correlation between intestinal CLint,u and FaFg in rats was poor compared with that in humans.4. Our finding indicates that rats are appropriate animals for evaluation of the intestinal absorption and metabolism of CYP3A substrates. However, a degree of caution is required when estimating rat Fg from rat intestinal microsomes due to the low metabolic activity and the poor correlation between in vitro and in vivo intestinal metabolism.

A Step Closer to Personalized Chemotherapy: Consideration of the Influence of Genetic Variation in Hepatic Uptake Transporters on the Metabolism of CYP3A Substrates

Understanding the metabolic, transporter, and genetic influences on the disposition of drugs used in chemotherapy is critical to individualization of drug therapy. Recognition of the importance of transporter-enzyme interplay, in which genetic variants of drug uptake transporters can change drug metabolism when the enzymes are unchanged, is an important advance in predicting appropriate drug dosage regimens for the individual patient.

The Contributions of Cytochromes P450 3A4 and 3A5 to the Metabolism of the Phosphodiesterase Type 5 Inhibitors Sildenafil, Udenafil, and Vardenafil

The role of the genetically polymorphic CYP3A5 in the metabolism of CYP3A substrates is unclear. We investigated the contributions of the CYP3A4 and CYP3A5 isoforms to the metabolism of the phosphodiesterase type 5 inhibitors (PDE5Is) sildenafil, udenafil, and vardenafil. In vitro incubation studies of sildenafil N-demethylation, udenafil N-dealkylation, and vardenafil N-deethylation were conducted using recombinant CYP3A enzymes and 15 human liver microsome (HLM) preparations with predetermined CYP3A5 genotypes. Recombinant CYP3A4 and CYP3A5 both produced N-desalkyl metabolites of sildenafil, udenafil, and vardenafil. The catalytic efficiency (Cl(int) = V(max)/apparent K(m)) of the rCYP3A5 isoform for vardenafil N-deethylation was about 3.2-fold that of rCYP3A4, whereas the intrinsic clearance rates for N-dealkylation of both sildenafil and udenafil were similar between rCYP3A5 and rCYP3A4. The metabolite formation activity was higher in HLMs heterozygous for the CYP3A5*3 allele (n = 9) than in HLMs homozygous for CYP3A5*3 (n = 6). These findings suggest that CYP3A5 and CYP3A4 play a significant role in the metabolism of PDE5Is. The genetic polymorphism of CYP3A5 may contribute to interindividual variability in the disposition of PDE5Is, especially vardenafil. Further in vivo studies are needed to confirm the effects of CYP3A5 genotypes on the pharmacokinetics of PDE5Is.

CYP3A Probes Can Quantitatively Predict the In Vivo Kinetics of Other CYP3A Substrates and Can Accurately Assess CYP3A Induction and Inhibition

Because the rates at which therapeutics are cleared from the body can affect their effectiveness, knowing and accounting for the variables that contribute to drug clearance is of utmost importance when designing a drug dosage regimen for patients. The activities of the manifold cytochrome P450 enzymes [(CYPs), the most clinically important of which is often CYP3A] must be considered as they are essential for modification and metabolism of compounds (i.e., therapeutic, xenobiotic, etc.) prior to their excretion. To this end, much research has been expended on trying to identify and develop drug probes that accurately predict the metabolism of CYP3A substrates in individuals. Recently, Benet has written on the futility of such an enterprise; however, other researchers believe the identification of valuable predictive probes is not only possible but crucial.

Identification and phenotype characterization of two haplotypes causing different enzymatic capacity in fetal livers

The fetal liver cytochrome P450 (CYP) 3A enzymes metabolize potentially toxic and teratogenic substrates and drugs in addition to endogenous hormones and differentiation factors. CYP3A7 is the most abundant CYP in the human liver during fetal stages and the first months of postnatal age and shows a large interindividual variability of unknown molecular basis. A new variant gene (CUpsilonP3A7*2), which carries a mutation in exon 11 of CUpsilonP3A7 causing a T409R substitution, was identified by direct sequencing. Genotype analysis was performed by use of polymerase chain reaction followed by restriction enzyme analysis. CYP3A7.2 activity was assessed in heterologous expression systems and human fetal liver microsomes. The frequency of CUpsilonP3A7*2 was 8%, 17%, 28%, and 62% in white, Saudi Arabian, Chinese, and Tanzanian individuals, respectively. By use of human HEK293 cells, no significant differences in expression between CYP3A7.1 and CYP3A7.2 were found and fetal livers homozygous for CUpsilonP3A7*2 had similar or higher CYP3A7 protein contents than CUpsilonP3A7*1 livers. Kinetic studies showed that CYP3A7.2 was a functional enzyme with a significantly higher catalytic constant (kcat) as compared with CYP3A7.1 (P < .05). Interestingly, fetal livers that expressed CYP3A7.2 also expressed CYP3A5 protein, and we found a linkage disequilibrium between the CUpsilonP3A7*2 and CUpsilonP3A5*1 alleles that was subject to interethnic differences. Determination of the alprazolam 1-hydroxylation rate revealed that CYP3A5 plays a significant role in the metabolism of CYP3A substrates in the fetal liver. We have identified 2 different CYP3A phenotypes in the fetal liver--one that is the result of a CUpsilonP3A7*1/CUpsilonP3A5*3 haplotype causing CYP3A7.1 but no CYP3A5 expression and another with higher detoxification capacity, inherent in the CUpsilonP3A7*2/CUpsilonP3A5*1 haplotype, where CYP3A5 and a more active form of CYP3A7 are expressed.

A significant drug-metabolizing role for CYP3A5?

Recent research on CYP3A5 in vitro and in humans has provided discordant information on whether CYP3A5 plays a significant role in the metabolism of CYP3A substrates in vivo. For example, six separate studies have reported CYP3A5 to contribute between 2 and 60% of the total hepatic CYP3A. Suggested explanations for the reported differences in hepatic CYP3A5 levels are evaluated in this article. Furthermore, a sensitivity analysis of the contribution of CYP3A5 (in addition to CYP3A4) to the metabolism of a "midazolam"-type substrate based on recently published in vitro and clinical data is compared with the results of two in vivo studies that investigated the influence of CYP3A5 genotype on midazolam pharmacokinetics. The sensitivity analysis predicts an approximately 3-fold lower AUCoral for midazolam for those expressing the highest hepatic and intestinal levels of CYP3A5 (e.g., possessing CYP3A5*1 alleles) compared with those individuals who express insignificant amounts of CYP3A5, assuming CYP3A4 levels are the same in both groups and that CYP3A5 levels do not exceed those of CYP3A4 in CYP3A5*1 homozygotes. In contrast, the two in vivo studies show no statistically significant influence of CYP3A5 genotype on midazolam pharmacokinetics. The discordance between the prediction and the results from the two in vivo studies is discussed.

Interaction of human liver cytochromes P450 in vitro with LY307640, a gastric proton pump inhibitor.

The interactions in vitro of LY307640 with the cytochromes P450 (P450s) were studied using human liver microsomes, specific inhibitors of the P450s, and cDNA expressed enzymes. The kinetics of formation of the two major oxidative metabolites, desmethyl LY307640 and LY307640 sulfone, were determined using two human liver microsomal samples. The kinetic data indicated that high and low affinity sites were present for the production of both metabolites of LY307640. The Km(apparent) and Vmax(apparent) for desmethyl LY307640 formation by microsomes from human liver E (HL-E) for the high affinity site were 18.8 +/- 4.4 microM and 402 +/- 52 pmol product/min/mg protein. The high affinity site Km(apparent) and Vmax(apparent) for LY307640 sulfone formation by microsomes from HL-E were 4.4 +/- 2.1 microM and 81.8 +/- 18 pmol product/min/mg protein. The rates of desmethyl LY307640 and LY307640 sulfone formation by the high affinity site were determined using 14 human liver microsomal samples characterized for P450 marker catalytic activities and immunoquantified levels of the P450s. Rates of formation of desmethyl LY307640 significantly correlated with the immunoquantified levels of CYP 2C19 and the ability of the microsomes to 4'-hydroxylate S-mephenytoin. LY307640 sulfone formation significantly correlated with the immunoquantified levels of CYP 3A and the ability of the microsomes to 1'-hydroxylate midazolam. Inhibition studies and use of expressed cytochrome P450 systems confirmed the correlation data demonstrating that CYP 2C19 catalyzed the formation of desmethyl LY307640 and CYP 3A and catalyzed LY307640 sulfone formation. Further, LY307640 competitively inhibited S-mephenytoin 4'-hydroxylation and midazolam 1'-hydroxylation as did the structurally related compound, omeprazole. For the inhibition of S-mephenytoin 4'-hydroxylation and midazolam 1'-hydroxylation, LY307640 had higher Ki(apparent) values than that of omeprazole. These studies demonstrate that the high affinity enzymes which catalyze the formation of the desmethyl and sulfone metabolites of LY307640 are, respectively, CYP 2C19 and CYP 3A. In addition, the inhibition data suggest that LY307640 has less potential to inhibit the metabolism of CYP 2C19 substrates compared to omeprazole, and that LY307640 and omeprazole have a similarly low potential to inhibit the metabolism of CYP 3A substrates.

CYP3A gene expression in human gut epithelium.

CYP3A4, a major Phase I xenobiotic metabolizing enzyme present in liver, is also present in human small bowel epithelium where it appears to catalyse significant 'first pass' metabolism of some drugs. To determine whether CYP3A4 or the related enzymes CYP3A3, CYP3A5, and CYP3A7 are present in other regions of the digestive tract, we used CYP3A-specific antibodies to examine histological sections and epithelial microsomes obtained from a human organ donor. CYP3A-related proteins were detected in epithelia throughout the digestive tract and in gastric parietal cells, in pericentral hepatocytes, and in ductular cells of the pancreas. Immunoblot analysis suggested that the major CYP3A protein present in liver, jejunum, colon, and pancreas was CYP3A4 or CYP3A3, whereas CYP3A5 was the major protein present in stomach. Both CYP3A4 and CYP3A5 mRNA were detectable in all regions of the digestive tract using the polymerase chain reaction (PCR); however, only CYP3A4 could be detected by Northern blot analysis. CYP3A7 mRNA was consistently detected only in the liver by PCR and CYP3A3 mRNA was not detected in any of the tissues. We conclude that CYP3A4 and CYP3A5 are present throughout the human digestive tract and that differences in the expression of these enzymes may account for inter-organ differences in the metabolism of CYP3A substrates.