Quantitative systems pharmacology model-based investigation of adverse gastrointestinal events associated with prolonged treatment with PI3-kinase inhibitors.

Several PI3K inhibitors are in clinical development for the treatment of various forms of cancers, including pan-PI3K inhibitors targeting all four PI3K isoforms (α, ß, γ, and δ), and isoform-selective inhibitors. Diarrhea and immune-mediated colitis are among the adverse events observed with PI3K inhibition which limits the maximal tolerated dose. A quantitative systems pharmacology model was developed to investigate PI3K-inhibitor-induced colitis. The effects of individual PI3K isoforms on relevant cellular pathways were incorporated into a mechanistic representation of mucosal inflammation. A virtual clinical population captures the observed clinical variability in the onset timing and rates of diarrhea and colitis for seven clinically tested PI3K inhibitors. Model-based analysis suggests that colitis development is governed by both the inhibition of PI3Kδ, which drives T cell differentiation and proliferation, and PI3Kα, which regulates epithelial barrier integrity. Specifically, when PI3Kα is inhibited below a given threshold, epithelial barrier dysfunction precipitates an exaggerated T effector response due to PI3Kδ-inhibition, leading to risk of diarrhea and colitis. This synergy explains why the lowest diarrhea and colitis rates are seen with the weakest PI3Kδ inhibition (alpelisib), and higher rates are seen with strong PI3Kδ inhibition if PI3Kα is even mildly inhibited (e.g., idelalisib), whereas strong PI3Kδ inhibition in the absence of PI3Kα inhibition does not result in high colitis rates (umbralisib). Thus, the model-based analysis suggests that PI3Kα and δ inhibition play unique but synergistic roles in driving colitis. Finally, we explore if and how dose-regimen might influence colitis rates for molecules that inhibit both PI3Kα and PI3Kδ.

Intrinsic and Extrinsic Pharmacokinetic Variability of Small Molecule Targeted Cancer Therapy.

Pharmacokinetic (PK) variability in cancer clinical trials may be due to heterogeneous populations and identifying sources of variability is important. Use of healthy subjects in clinical pharmacology studies together with detailed knowledge of the characteristics of patients with cancer can allow for quick identification and quantification of factors affecting PK variability. PK data and sources of variability of 40 marketed molecularly targeted oncology therapeutics were compiled from regulatory approval documents covering an 18-year period (1999-2017). Variability in PK parameters was compared and contributors to variability were identified. The results show that PK variability was ~ 16% higher for peak plasma concentration (Cmax ) and area under the concentration time curve (AUC) in patients with cancer compared with healthy subjects. Several factors were identified as major contributors to variability including hepatic/renal impairment and cytochrome P450 inhibition/induction. Lower PK variability in healthy subjects may represent an opportunity to perform rapid and robust pharmacological and PK assessments to inform subsequent studies in the development of new cancer therapies.

Racial and Ethnic Composition of Cancer Clinical Drug Trials: How Diverse Are We?

Many approved drugs demonstrate different pharmacokinetics, pharmacodynamics, and/or safety across racial and ethnic groups. The primary objective of the current study was to summarize the racial and ethnic makeup of cancer clinical drug trials using cancer drugs approved by the U.S. Food and Drug Administration (FDA) between January 1, 2010, and July 31, 2016. In clinical studies used for FDA approvals, 82.3% of participants identified as white, 10.2% as Asian, 2.3% as black, and 4.7% as Hispanic. Black participants made up 7.7% of U.S. and Canadian cancer clinical drug trials and 2.6% of global cancer clinical drug trials while Asian participants made up 13.5% of global cancer clinical drug trials but only 1.8% of U.S. and Canadian cancer clinical drug trials. The current study indicates that although cancer clinical drug trials have become more inclusive of Asian participants, other racial and ethnic minority groups remain under-represented. This may result in an inadequate understanding of drug safety and efficacy in many racial and ethnic populations.

Pharmacogenomics in the age of personalized medicine.

The aim of personalized medicine is to offer the right treatment to the right person at the right dose, thus maximizing efficacy and minimizing toxicity for each individual patient. Pharmacogenomic approaches attempt to refine the aim of personalized medicine by utilizing an individual's germline and somatic DNA signatures to guide treatment. In this review, we highlight the current use of pharmacogenomic based biomarker information in drug labeling. We also present several case studies on the implementation of pharmacogenomic strategies in drug discovery and development. Lastly, we comment on current challenges to implementing pharmacogenomic based testing in the clinic.

Contribution of Antibody Hydrodynamic Size to Vitreal Clearance Revealed through Rabbit Studies Using a Species-Matched Fab.

We have developed a tool Fab fragment of a rabbit monoclonal antibody that is useful for early evaluation in rabbit models of technologies for long acting delivery (LAD) of proteins to the eye. Using this Fab we show that vitreal clearance can be slowed through increased hydrodynamic size. Fab (G10rabFab) and Fab' (G10rabFab') fragments of a rabbit monoclonal antibody (G10rabIgG) were expressed in Chinese hamster ovary (CHO) cells and purified using antigen-based affinity chromatography. G10rabFab retains antigen-binding upon thermal stress (37 °C) for 8 weeks in phosphate-buffered saline (PBS) and can be detected in rabbit tissues using an antigen-based ELISA. Hydrodynamic radius, measured using quasi-elastic light scattering (QELS), was increased through site-specific modification of the G10rabFab' free cysteine with linear methoxy-polyethylene glycol(PEG)-maleimide of 20000 or 40000 molecular weight. Pharmacokinetic studies upon intravitreal dosing in New Zealand white rabbits were conducted on the G10rabFab and PEGylated G10rabFab'. Results of single and multidose pharmacokinetic experiments yield reproducible results and a vitreal half-life for G10rabFab of 3.2 days. Clearance from the eye is slowed through increased hydrodynamic size, with vitreal half-life showing a linear dependence on hydrodynamic radius (RH). A linear dependence of vitreal half-life on RH suggests that molecule diffusivity makes an important contribution to vitreal clearance. A method for prediction of vitreal half-life from RH measurements is proposed.

Human UGT1A4 and UGT1A3 conjugate 25-hydroxyvitamin D3: metabolite structure, kinetics, inducibility, and interindividual variability.

25-Hydroxyvitamin D3 (25OHD3) is used as a clinical biomarker for assessment of vitamin D status. Blood levels of 25OHD3 represent a balance between its formation rate and clearance by several oxidative and conjugative processes. In the present study, the identity of human uridine 5'-diphosphoglucuronyltransferases (UGTs) capable of catalyzing the 25OHD3 glucuronidation reaction was investigated. Two isozymes, UGT1A4 and UGT1A3, were identified as the principal catalysts of 25OHD3 glucuronidation in human liver. Three 25OHD3 monoglucuronides (25OHD3-25-glucuronide, 25OHD3-3-glucuronide, and 5,6-trans-25OHD3-25-glucuronide) were generated by recombinant UGT1A4/UGT1A3, human liver microsomes, and human hepatocytes. The kinetics of 25OHD3 glucuronide formation in all systems tested conformed to the Michaelis-Menten model. An association between the UGT1A4*3 (Leu48Val) gene polymorphism with the rates of glucuronide formation was also investigated using human liver microsomes isolated from 80 genotyped livers. A variant allele dose effect was observed: the homozygous UGT1A4*3 livers (GG) had the highest glucuronidation activity, whereas the wild type (TT) had the lowest activity. Induction of UGT1A4 and UGT1A3 gene expression was also determined in human hepatocytes treated with pregnane X receptor/constitutive androstane receptor agonists, such as rifampin, carbamazepine, and phenobarbital. Although UGT mRNA levels were increased significantly by all of the known pregnane X receptor/constitutive androstane receptor agonists tested, rifampin, the most potent of the inducers, significantly induced total 25OHD3 glucuronide formation activity in human hepatocytes measured after 2, but not 4 and 24 hours, of incubation. Finally, the presence of 25OHD3-3-glucuronide in both human plasma and bile was confirmed, suggesting that the glucuronidation pathway might be physiologically relevant and contribute to vitamin D homeostasis in humans.

IL-22 regulates iron availability in vivo through the induction of hepcidin.

Iron is a trace element important for the proper folding and function of various proteins. Physiological regulation of iron stores is of critical importance for RBC production and antimicrobial defense. Hepcidin is a key regulator of iron levels within the body. Under conditions of iron deficiency, hepcidin expression is reduced to promote increased iron uptake from the diet and release from cells, whereas during conditions of iron excess, induction of hepcidin restricts iron uptake and movement within the body. The cytokine IL-6 is well established as an important inducer of hepcidin. The presence of this cytokine during inflammatory states can induce hepcidin production, iron deficiency, and anemia. In this study, we show that IL-22 also influences hepcidin production in vivo. Injection of mice with exogenous mouse IgG1 Fc fused to the N terminus of mouse IL-22 (Fc-IL-22), an IL-22R agonist with prolonged and enhanced functional potency, induced hepcidin production, with a subsequent decrease in circulating serum iron and hemoglobin levels and a concomitant increase in iron accumulation within the spleen. This response was independent of IL-6 and was attenuated in the absence of the IL-22R-associated signaling kinase, Tyk2. Ab-mediated blockade of hepcidin partially reversed the effects on iron biology caused by IL-22R stimulation. Taken together, these data suggest that exogenous IL-22 regulates hepcidin production to physiologically influence iron usage.

Evaluation of UGT protein interactions in human hepatocytes: effect of siRNA down regulation of UGT1A9 and UGT2B7 on propofol glucuronidation in human hepatocytes.

Previous experiments performed in recombinant systems have suggested that protein-protein interactions occur between the UGTs and may play a significant role in modulating enzyme activity. However, evidence of UGT protein-protein interactions either in vivo or in more physiologically relevant in vitro systems has yet to be demonstrated. In this study, we examined oligomerization and its ability to affect glucuronidation in plated human hepatocytes. siRNA down regulation experiments and activity studies were used to examine changes in metabolite formation of one UGT isoform due to down regulation of a second UGT isoform. Selective siRNA directed towards UGT1A9 or UGT2B7 resulted in significant and selective decreases in their respective mRNA levels. As expected, the metabolism of the UGT1A9 substrate propofol decreased with UGT1A9 down regulation. Interestingly, UGT1A9 activity, but not UGT1A9 mRNA expression, was also diminished when UGT2B7 expression was selectively inhibited, implying potential interactions between the two isoforms. Minor changes to UGT1A4, UGT2B4 and UGT2B7 activity were also observed when UGT1A9 expression was selectively down regulated. To our knowledge, this represents the first piece of evidence that UGT protein-protein interactions occur in human hepatocytes and suggests that expression levels of UGT2B7 may directly impact the glucuronidation activity of selective UGT1A9 substrates.

Enhancement of hepatic 4-hydroxylation of 25-hydroxyvitamin D3 through CYP3A4 induction in vitro and in vivo: implications for drug-induced osteomalacia.

Long-term therapy with certain drugs, especially cytochrome P450 (P450; CYP)-inducing agents, confers an increased risk of osteomalacia that is attributed to vitamin D deficiency. Human CYP24A1, CYP3A4, and CYP27B1 catalyze the inactivation and activation of vitamin D and have been implicated in the adverse drug response. In this study, the inducibility of these enzymes and monohydroxylation of 25-hydroxyvitamin D3 (25OHD3) were evaluated after exposure to P450-inducing drugs. With human hepatocytes, treatment with phenobarbital, hyperforin, carbamazepine, and rifampin significantly increased the levels of CYP3A4, but not CYP24A1 or CYP27B1 mRNA. In addition, rifampin pretreatment resulted in an 8-fold increase in formation of the major metabolite of 25OHD3, 4ß,25(OH)2D3. This inductive effect was blocked by the addition of 6',7'-dihydroxybergamottin, a selective CYP3A4 inhibitor. With human renal proximal tubular HK-2 cells, treatment with the same inducers did not alter CYP3A4, CYP24A1, or CYP27B1 expression. 24R,25(OH)2 D3 was the predominant monohydroxy metabolite produced from 25OHD3, but its formation was unaffected by the inducers. With healthy volunteers, the mean plasma concentration of 4ß,25(OH)2D3 was increased 60% (p < 0.01) after short-term rifampin administration. This was accompanied by a statistically significant reduction in plasma 1α,25(OH)2D3 (-10%; p = 0.03), and a nonsignificant change in 24R,25(OH)2D3 (-8%; p = 0.09) levels. Further analysis revealed a negative correlation between the increase in 4ß,25(OH)2D3 and decrease in 1α,25(OH)2D3 levels. Examination of the plasma monohydroxy metabolite/25OHD3 ratios indicated selective induction of the CYP3A4-dependent 4ß-hydroxylation pathway of 25OHD3 elimination. These results suggest that induction of hepatic CYP3A4 may be important in the etiology of drug-induced osteomalacia.

Quantitative prediction of CYP2B6 induction by estradiol during pregnancy: potential explanation for increased methadone clearance during pregnancy.

There is considerable evidence that pregnancy changes the disposition of drugs in an enzyme- and gestational stage-specific manner. On the basis of probe drug studies, the activity of CYP3A4 and CYP2D6 increases and CYP1A2 decreases during human pregnancy. However, no studies of CYP2B6 activity during human pregnancy have been conducted. In rodent models and in HepG2 cells, CYP2B enzymes have been shown to be regulated by estradiol. Because estradiol concentrations increase by ∼50-fold during human pregnancy, it was hypothesized that the increasing estradiol concentrations during human pregnancy would result in induction of CYP2B6 activity. Hepatocytes from three female donors were treated with estradiol, and the EC(50) and E(max) were measured for CYP2B6 mRNA and bupropion hydroxylation activity. The measured values were used to predict the magnitude of CYP2B6 induction during human pregnancy. At 100 nM total estradiol, a concentration achievable during the third trimester of pregnancy, CYP2B6 activity was predicted to increase by 1.5-3-fold, based on increased CYP2B6 activity and mRNA. When the E(max) and EC(50) values were compared with those for carbamazepine and rifampin, estradiol was found to be as potent an inducer of CYP2B6 as rifampin and carbamazepine. These data suggest that, during human pregnancy, the increasing estradiol concentrations will result in increased clearance of drugs that have CYP2B6-mediated clearance pathways. This could in part explain the observed increase in methadone clearance during pregnancy.

In vitro hepatotoxicity and cytochrome P450 induction and inhibition characteristics of carnosic acid, a dietary supplement with antiadipogenic properties.

Carnosic acid is a phenolic diterpene isolated from rosemary (Rosmarinus officinalis), which may have anticancer, antiadipogenic, and anti-inflammatory properties. Recently, carnosic acid was shown to prevent weight gain and hepatic steatosis in a mouse model of obesity and type II diabetes. Based on these results, carnosic acid has been suggested as a potential treatment for obesity and nonalcoholic fatty liver disease; however, little is known about the safety of carnosic acid at doses needed to elicit a pharmacological effect. For this reason, hepatotoxicity and cytochrome P450 inhibition and induction studies were performed in primary human hepatocytes and microsomes. Measuring cellular ATP, carnosic acid showed a dose-dependent increase in hepatotoxicity with an EC(50) value of 94.8 ± 36.7 µM in three human hepatocyte donors without a concurrent increase in the apoptosis markers caspase-3/7. In human liver microsomes, carnosic acid did not exhibit significant time-dependent inhibition for any of the cytochrome P450 enzymes investigated, although it did inhibit CYP2C9- and CYP3A4-catalyzed reactions with K(i) values of 9.2 and 4.3 µM, respectively. Carnosic acid also induced CYP2B6 and CYP3A4 mRNA and enzyme activity in a dose-dependent manner. At 10 µM, carnosic acid increased CYP2B6 enzyme activity 61.6 and 49.3% in two donors compared with phenobarbital, and it increased CYP3A enzyme activity 82.6 and 142% compared with rifampicin. These results indicate the potential for drug interactions with carnosic acid and illustrate the need for an appropriate safety assessment before being used as a weight loss supplement.

Effects of interleukin 1ß (IL-1ß) and IL-1ß/interleukin 6 (IL-6) combinations on drug metabolizing enzymes in human hepatocyte culture.

Exposure to cytokines can down-regulate hepatic cytochrome P450 enzymes. Accordingly, relief of inflammation by cytokinetargeted drug therapy has the potential to up-regulate cytochrome P450s and thereby increase clearance of co-administered drugs. This study examined the effects of the inflammatory cytokine, interleukin 1ß (IL-1ß), and IL-1ß/interleukin 6 (IL-6) combinations on drug metabolizing enzymes in human hepatocyte culture. Treatment of hepatocytes with IL-1ß revealed suppression of mRNA expression of several clinically important cytochrome P450 isoenzymes, with EC50 values that differed by isoenzyme. Suppression of CYP1A2 activity by IL-1ß could not be measured in 3 of 5 donors due to lack of response, and in the two remaining donors the average EC50 was 450 pg/mL. CYP3A activity had an EC50 of suppression of 416 ± 454 pg/mL. Measurable EC50s were obtained for all 5 donors for CYP2C8, 3A4, 3A5, 4A11 and IL-6R mRNA with fold differences which varied between 9.5-fold (CYP2C8) to 109-fold (CYP4A11). When hepatocytes were treated with IL-1ß and IL-6 in combination at concentrations which ranged from 1-100 pg/mL, IL-6 was the main determinant of increases in acute phase response marker mRNA and of decreases in CYP3A4 mRNA. There was no synergy between IL-1ß and IL-6 in the regulation of cytochrome P450 mRNA when dosed in combination, although the effects of the two cytokines in combination were additive in certain instances. These data indicate that IL-1ß and IL-6 both suppress cytochrome P450 mRNA and enzyme levels in vitro and that, at similar physiologically-relevant concentrations in vitro, IL-6 is more potent than IL-1ß.

An inducible cytochrome P450 3A4-dependent vitamin D catabolic pathway.

Vitamin D(3) is critical for the regulation of calcium and phosphate homeostasis. In some individuals, mineral homeostasis can be disrupted by long-term therapy with certain antiepileptic drugs and the antimicrobial agent rifampin, resulting in drug-induced osteomalacia, which is attributed to vitamin D deficiency. We now report a novel CYP3A4-dependent pathway, the 4-hydroxylation of 25-hydroxyvitamin D(3) (25OHD(3)), the induction of which may contribute to drug-induced vitamin D deficiency. The metabolism of 25OHD(3) was fully characterized in vitro. CYP3A4 was the predominant source of 25OHD(3) hydroxylation by human liver microsomes, with the formation of 4ß,25-dihydroxyvitamin D(3) [4ß,25(OH)(2)D(3)] dominating (V(max)/K(m) = 0.85 ml · min(-1) · nmol enzyme(-1)). 4ß,25(OH)(2)D(3) was found in human plasma at concentrations comparable to that of 1α,25-dihydroxyvitamin D(3), and its formation rate in a panel of human liver microsomes was strongly correlated with CYP3A4 content and midazolam hydroxylation activity. Formation of 4ß,25(OH)(2)D(3) in primary human hepatocytes was induced by rifampin and inhibited by CYP3A4-specific inhibitors. Short-term treatment of healthy volunteers (n = 6) with rifampin selectively induced CYP3A4-dependent 4ß,25(OH)(2)D(3), but not CYP24A1-dependent 24R,25-dihydroxyvitamin D(3) formation, and altered systemic mineral homeostasis. Our results suggest that CYP3A4-dependent 25OHD(3) metabolism may play an important role in the regulation of vitamin D(3) in vivo and in the etiology of drug-induced osteomalacia.

Effects of interleukin-6 (IL-6) and an anti-IL-6 monoclonal antibody on drug-metabolizing enzymes in human hepatocyte culture.

The cytokine-mediated suppression of hepatic drug-metabolizing enzymes by inflammatory disease and the relief of this suppression by successful disease treatment have recently become an issue in the development of drug interaction labels for new biological products. This study examined the effects of the inflammatory cytokine interleukin-6 (IL-6) on drug-metabolizing enzymes in human hepatocyte culture and the abrogation of these effects by a monoclonal antibody directed against IL-6. Treatment of human hepatocytes with IL-6 (n = 9 donors) revealed pan-suppression of mRNA of 10 major cytochrome P450 isoenzymes, but with EC(50) values that differed by isoenzyme. Some EC(50) values were above the range of clinically relevant serum concentrations of IL-6. Marker activities for CYP1A2 and CYP3A4 enzyme were similarly suppressed by IL-6 in both freshly isolated and cryopreserved hepatocytes. IL-6 suppressed induction of CYP1A2 enzyme activity by omeprazole and CYP3A4 enzyme activity by rifampicin but only at supraphysiological concentrations of IL-6. Glycosylated and nonglycosylated IL-6 did not significantly differ in their ability to suppress CYP1A2 and CYP3A4 enzyme activity. A monoclonal antibody directed against IL-6 abolished or partially blocked IL-6-mediated suppression of CYP1A2 and CYP3A4 enzyme activity, respectively. These data indicate that experimentation with IL-6 and anti-IL-6 monoclonal antibodies in human hepatocyte primary culture can quantitatively measure cytochrome P450 suppression and desuppression and determine EC(50) values for IL-6 against individual cytochrome P450 isoenzymes. However, the complex biology of inflammatory disease may not allow for quantitative in vitro-in vivo extrapolation of these simple in vitro data.

Pregnancy decreases rat CYP1A2 activity and expression.

Pregnancy results in increased CYP3A- and CYP2D6-mediated clearance but decreases the clearance of CYP1A2 probe drugs. The aim of this study was to determine whether the decreased CYP1A2 activity during human pregnancy could be explained by decreased expression of CYP1A2 protein and mRNA using the rat as a model. Potential mechanisms leading to decreased CYP1A2 activity and expression were also investigated. Hepatic CYP1A2 activity, protein, and mRNA were measured during mid- and late gestation and compared to nonpregnant control levels. In addition, the effect of 17-ß-estradiol and progesterone on CYP1A2 mRNA levels was assessed using rat hepatocytes, and the effect of estrogens or progesterone on CYP1A2 activity in vitro was tested. CYP1A2-mediated probe clearance decreased between 48 and 62% (p < 0.05) during pregnancy, with no difference in CYP1A2 activity between mid- and late pregnancy. This decrease in probe clearance was accompanied by a 33 ± 8% (midpregnancy) and 29 ± 27% (late pregnancy) decrease in CYP1A2 protein expression (p < 0.05) and a 53% decline in methoxyresorufin O-demethylation V(max) (p < 0.05). CYP1A2 mRNA was not significantly different from controls at midpregnancy and decreased by 27 ± 20% (p < 0.05) of control during late pregnancy. Estradiol and progesterone had no effect on CYP1A2 mRNA in rat hepatocytes and did not inhibit CYP1A2 activity. These data demonstrate that pregnancy decreases CYP1A2 activity and expression with a modest effect on CYP1A2 mRNA and suggest that the rat can be used as a model to study mechanisms by which pregnancy decreases CYP1A2 activity in humans.

A comparison of the roles of peroxisome proliferator-activated receptor and retinoic acid receptor on CYP26 regulation.

The cytochrome P450 26 family is believed to be responsible for all-trans-retinoic acid (atRA) metabolism and elimination in the human fetus and adults. CYP26A1 and CYP26B1 mRNA is expressed in a tissue-specific manner, and mice in which the CPY26 isoform has been knocked out show distinct malformations and lethality. The aim of this study was to determine differences in CYP26A1 and CYP26B1 regulation and expression. Analysis of CYP26A1 and CYP26B1 expression in a panel of 57 human livers showed CYP26A1 to be the major CYP26 isoform present in the liver, and its expression to be subject to large interindividual variability between donors. CYP26A1 and retinoic acid receptor (RAR) beta were found to be greatly inducible by atRA in HepG2 cells, whereas CYP26B1, RARalpha, and RARgamma were induced to a much lesser extent. Based on treatments with RAR isoform-selective ligands, RARalpha is the major isoform responsible for CYP26A1 and RARbeta induction in HepG2 cells. Classic cytochrome P450 inducers did not affect CYP26 transcription, whereas the peroxisome proliferator-activated receptor (PPAR) gamma agonists pioglitazone and rosiglitazone up-regulated CYP26B1 transcription by as much as 209- +/- 80-fold and CYP26A1 by 10-fold. RARbeta was also up-regulated by pioglitazone and rosiglitazone. CYP26B1 induction by PPARgamma agonists was abolished by the irreversible PPARgamma antagonist 2-chloro-5-nitrobenzanilide (GW9662), whereas RARbeta and CYP26A1 induction was unaffected by GW9662. Overall, the results of this study suggest that CYP26B1 and CYP26A1 are regulated by different nuclear receptors, resulting in tissue-specific expression patterns. The fact that drugs can alter the expression of CYP26 enzymes may have toxicological and therapeutic importance.

Expression and functional characterization of cytochrome P450 26A1, a retinoic acid hydroxylase.

Retinoic acid (RA) is a critical signaling molecule that performs multiple functions required to maintain cellular viability. It is also used in the treatment of some cancers. Enzymes in the CYP26 family are thought to be responsible for the elimination of RA, and CYP26A1 appears to serve the most critical functions in this family. In spite of its importance, CYP26A1 has neither been heterologously expressed nor characterized kinetically. We expressed the rCYP26A1 in baculovirus-infected insect cells and purified the hexahistidine tagged protein to homogeneity. Heme incorporation was determined by carbon monoxide difference spectrum and a type 1 spectrum was observed with RA binding to CYP26A1. We found that RA is a tight binding ligand of CYP26A1 with low nM binding affinity. CYP26A1 oxidized RA efficiently (depletion K(m) 9.4+/-3.3nM and V(max) 11.3+/-4.3pmolesmin(-1)pmoleP450(-1)) when supplemented with P450 oxidoreductase and NADPH but was independent of cytochrome b5. 4-Hydroxy-RA (4-OH-RA) was the major metabolite produced by rCYP26A1 but two other primary products were also formed. 4-OH-RA was further metabolized by CYP26A1 to more polar metabolites and this sequential metabolism of RA occurred in part without 4-OH-RA leaving the active site of CYP26A1. The high efficiency of CYP26A1 in eliminating both RA and its potentially active metabolites supports the major role of this enzyme in regulating RA clearance in vivo. These results provide a biochemical framework for CYP26A1 function and offer insight into the role of CYP26A1 as a drug target as well as in fetal development and cell cycle regulation.

Changes in maternal liver Cyp2c and Cyp2d expression and activity during rat pregnancy.

During human pregnancy, CYP2C9, CYP2C19, and CYP2D6 activities are altered. The aim of the current study was to determine if this phenomenon can be replicated in the rat, and to evaluate the mechanisms that contribute to the changes in Cyp2c and Cyp2d activity during pregnancy. The intrinsic clearance of dextromethorphan O-demethylation, a measure of Cyp2d2 activity, was decreased 80% at both days 9 and 19 of gestation when compared to non-pregnant controls. The decreased intrinsic clearance was a result of both decreased V(max) and increased K(m)-values at both days of gestation. Quantitative RT-PCR revealed that transcripts of Cyp2d2 and Cyp2d4 were significantly decreased at day 19 of pregnancy (p<0.05) when compared to day 9 and non-pregnant controls. The decrease in Cyp2d mRNA levels correlated with a decrease in several nuclear receptor mRNA levels (RARalpha, RXRalpha, HNF1 and HNF3beta) but not with the mRNA levels of nuclear receptors usually associated with regulation of P450 enzymes (PXR, CAR and HNF4alpha). In contrast, Cyp2c12 and Cyp2c6 transcription and protein expression were not significantly altered during rat pregnancy although the intrinsic clearance of Cyp2c6 mediated diclofenac 4'-hydroxylation was increased 2-fold on day 19 of gestation when compared to non-pregnant controls. The increase in intrinsic clearance was due to a decrease in the K(m)-value for 4'-hydroxydiclofenac formation. These data show that pregnancy significantly alters the expression and activity of drug metabolizing enzymes in an enzyme and gestational stage specific manner. These changes are likely to have toxicological and therapeutic implications.

Differential time-dependent inactivation of P450 3A4 and P450 3A5 by raloxifene: a key role for C239 in quenching reactive intermediates.

The role of C239 as the active-site residue responsible for forming the covalent linkage with raloxifene during P450 3A4 time-dependent inactivation (TDI) was recently identified. The corresponding residue in CYP3A5 is S239, and when the potential for TDI in P450 3A5 was investigated, only reversible inhibition was observed against midazolam and testosterone, with median inhibitory concentration (IC50) values of 2.4 and 2.9 microM, respectively. In a similar fashion, when C239 was replaced with alanine in P450 3A4, TDI was successfully engineered out, and the reversible inhibition was characterized by IC50 values of 3.7 and 3.5 microM against midazolam and testosterone, respectively. Metabolism studies confirmed that the reactive diquinone methide intermediate required for P450 3A4 inactivation formed in all of the P450 3A enzymes investigated. Furthermore, the absence of TDI in P450 3A5 led to an increase in the formation of GSH-related adducts of raloxifene compared with that for P450 3A4. Consequently, the absence of the nucleophilic cysteine leads to differential TDI and generation of reactive metabolites in the P450 3A enzyme, providing the foundation for pharmacogenetics that contributes to individual differences in susceptibility to adverse drug reactions.