Role of Small Intestinal Cytochromes P450 in the Bioavailability of Oral Nifedipine

Pharmacokinetics and Bioequivalence of 2 Nifedipine Controlled-Release Tablets: A Randomized, Single-Dose, 2-Period Crossover Study in Healthy Chinese Volunteers Under Fasting and Fed Conditions

The aim of this study was to evaluate the bioequivalence of generic nifedipine controlled-release tablet compared to branded product under fasting and fed conditions. A randomized, single-dose, 2-period, crossover study with a 7-day washout period was performed in 84 healthy Chinese volunteers (fasting cohort, n = 42; fed cohort, n = 42). In each study period, volunteers were assigned to receive a single oral dose of the generic or reference product (30 mg). Blood samples were collected before dosing and up to 72 hours after administration. The plasma concentration of nifedipine was determined by a validated liquid chromatography-tandem mass spectrometry method. Pharmacokinetic parameters were obtained using a noncompartmental model and log-transformed pharmacokinetic parameters (maximum plasma concentration, area under the plasma concentration-time curve (AUC) from time 0 to the last measurable concentration, AUC from time 0 to infinity) were used to evaluate bioequivalence. The results showed that the 90% confidence interval for the geometric mean ratio of pharmacokinetic parameters of the test and reference products ranged from 80.0% to 125.0% in both the fasting and fed cohorts, meeting the criteria for bioequivalence. No serious adverse events were reported throughout the study and no adverse events led to withdrawal from the study. Food effects were found in both the test and reference products, with mean maximum plasma concentration, AUC from time 0 to the last measurable concentration, and AUC from time 0 to infinity increased by 23.7%, 20.7%, and 20.5%, respectively, for the test product and 35.2%, 13.4%, and 14.7% for the reference product after a high-fat and high-calorie breakfast.

Achieving a Deeper Understanding of Drug Metabolism and Responses Using Single-Cell Technologies

Recent advancements in single-cell technologies have enabled detection of RNA, proteins, metabolites, and xenobiotics in individual cells, and the application of these technologies has the potential to transform pharmacological research. Single-cell data has already resulted in the development of human and model species cell atlases, identifying different cell types within a tissue, further facilitating the characterization of tumor heterogeneity, and providing insight into treatment resistance. Research discussed in this review demonstrates that distinct cell populations express drug metabolizing enzymes to different extents, indicating there may be variability in drug metabolism not only between organs, but within tissue types. Additionally, we put forth the concept that single-cell analyses can be used to expose underlying variability in cellular response to drugs, providing a unique examination of drug efficacy, toxicity, and metabolism. We will outline several of these techniques: single-cell RNA-sequencing and mass cytometry to characterize and distinguish different cell types, single-cell proteomics to quantify drug metabolizing enzymes and characterize cellular responses to drug, capillary electrophoresis-ultrasensitive laser-induced fluorescence detection and single-probe single-cell mass spectrometry for detection of drugs, and others. Emerging single-cell technologies such as these can comprehensively characterize heterogeneity in both cell-type-specific drug metabolism and response to treatment, enhancing progress toward personalized and precision medicine. SIGNIFICANCE STATEMENT: Recent technological advances have enabled the analysis of gene expression and protein levels in single cells. These types of analyses are important to investigating mechanisms that cannot be elucidated on a bulk level, primarily due to the variability of cell populations within biological systems. Here, we summarize cell-type-specific drug metabolism and how pharmacologists can utilize single-cell approaches to obtain a comprehensive understanding of drug metabolism and cellular heterogeneity in response to drugs.

Intratracheally Inhalable Nifedipine-Loaded Chitosan-PLGA Nanocomposites as a Promising Nanoplatform for Lung Targeting: Snowballed Protection via Regulation of TGF-β/β-Catenin Pathway in Bleomycin-Induced Pulmonary Fibrosis

Pulmonary fibrosis is a serious ailment that may progress to lung remodeling and demolition, where the key participants in its incidence are fibroblasts responding to growth factors and cellular calcium swinging. Calcium channel blockers, like nifedipine (NFD), may represent auspicious agents in pulmonary fibrosis treatment. Unfortunately, NFD bears complicated pharmacodynamics and a diminished systemic bioavailability. Thus, the current study aimed to develop a novel, non-invasive nanoplatform for NFD for direct/effective pulmonary targeting via intratracheal instillation. A modified solvent emulsification–evaporation method was adopted for the fabrication of NFD-nanocomposites, integrating poly(D,L-lactide-co-glycolide) (PLGA), chitosan (CTS), and polyvinyl alcohol, and optimized for different physiochemical properties according to the 3² full factorial design. Additionally, the aerodynamic behavior of the nanocomposites was scrutinized through cascade impaction. Moreover, the pharmacokinetic investigations were conducted in rats. Furthermore, the optimum formulation was tested in bleomycin-induced pulmonary fibrosis in rats, wherein fibrotic and oxidative stress parameters were measured. The optimum nanocomposites disclosed a nanosized spherical morphology (226.46 nm), a high entrapment efficiency (61.81%) and a sustained release profile over 24 h (50.4%). As well, it displayed a boosted in vitro lung deposition performance with a mass median aerodynamic diameter of 1.12 µm. Pharmacokinetic studies manifested snowballed bioavailability of the optimal nanocomposites by 3.68- and 2.36-fold compared to both the oral and intratracheal suspensions, respectively. The intratracheal nanocomposites revealed a significant reduction in lung fibrotic and oxidative stress markers notably analogous to normal control besides repairing abnormality in TGF-β/β-catenin pathway. Our results conferred a compelling proof-of-principle that NFD-CTS-PLGA nanocomposites can function as a promising nanoparadigm for pulmonary fibrosis management.

Extrahepatic cytochrome P450 epoxygenases: pathophysiology and clinical significance in human gastrointestinal cancers

Cytochrome P450 (CYP) epoxygenases, a multi-gene superfamily of heme-containing enzymes, are commonly known to metabolize endogenous arachidonic acid (AA) to epoxyeicosatrienoic acids (EETs). The role of CYPs is mostly studied in liver drugs metabolism, cardiac pathophysiology, and hypertension fields. Particularly, the biological functions of these enzymes have increasingly attracted a growing interest in cancer biology. Most published studies on CYPs in cancer have been limited to their role as drug metabolizing systems. The activity of these enzymes may affect drug pharmacokinetics and bioavailability as well as exogenous compounds turnover. Some CYP isoforms are selectively highly expressed in tumors, suggesting a potential mechanistic role in promoting resistance to chemotherapy. Majority of drugs elicit their effects in extrahepatic tissues whereby their metabolism can significantly determine treatment outcome. Nonetheless, the role of extrahepatic CYPs is not fully understood and targeting these enzymes as effective anti-cancer therapies are yet to be developed. This review article summarizes an up-to-date body of information from published studies on CYP enzymes expression levels and pathophysiological functions in human normal and malignant gastrointestinal (GI) tract tissues. Specifically, we reviewed and discussed the current research initiatives by emphasizing on the clinical significance and the pathological implication of CYPs in GI malignancies of esophagus, stomach, and colon.

Proximal Tubular Vacuolization and Hypersensitivity to Drug-Induced Nephrotoxicity in Male Mice With Decreased Expression of the NADPH-Cytochrome P450 Reductase

The effect of variations in the expression of cytochrome P450 reductase (CPR or POR) is determined in mice with decreased POR expression to identify potential vulnerabilities in people with low POR expression. There is an age-dependent appearance of increasing vacuolization in the proximal tubules of the renal cortex in 4- to 9-month-old male (but not female) Cpr-low (CL) mice. These mice have low POR expression in all cells of the body and upregulation of lysosome-associated membrane protein 1 expression in the renal cortex. Vacuolization is also seen in extrahepatic CL and extrarenal CL male mice, but not in mice with tissue-specific Por deletion in liver, intestinal epithelium, or kidney. The occurrence of vacuolization is accompanied by increases in serum blood-urea-nitrogen levels. Male CL mice are hypersensitive to cisplatin- and gentamicin-induced renal toxicity at 3 months of age, before proximal tubular (PT) vacuoles are detectable. At doses that do not cause renal toxicity in wild-type mice, both drugs cause substantial increases in serum blood-urea-nitrogen levels and PT vacuolization in male but not female CL mice. The hypersensitivity to drug-induced renal toxicity is accompanied by increases in circulating drug levels. These novel findings demonstrate deficiency of renal function in mice with globally reduced POR expression and suggest that low POR expression may be a risk factor for drug-induced nephrotoxicity in humans.

Physiologically Based Pharmacokinetic and Absorption Modeling for Osmotic Pump Products

Physiologically based pharmacokinetic (PBPK) and absorption modeling approaches were employed for oral extended-release (ER) drug products based on an osmotic drug delivery system (osmotic pumps). The purpose was to systemically evaluate the in vivo relevance of in vitro dissolution for this type of formulation. As expected, in vitro dissolution appeared to be generally predictive of in vivo PK profiles, because of the unique feature of this delivery system that the in vitro and in vivo release of osmotic pump drug products is less susceptible to surrounding environment in the gastrointestinal (GI) tract such as pH, hydrodynamic, and food effects. The present study considered BCS (Biopharmaceutics Classification System) class 1, 2, and 3 drug products with half-lives ranging from 2 to greater than 24 h. In some cases, the colonic absorption models needed to be adjusted to account for absorption in the colon. C _max (maximum plasma concentration) and AUCt (area under the concentration curve) of the studied drug products were sensitive to changes in colon permeability and segmental GI transit times in a drug product-dependent manner. While improvement of the methodology is still warranted for more precise prediction (e.g., colonic absorption and dynamic movement in the GI tract), the results from the present study further emphasized the advantage of using PBPK modeling in addressing product-specific questions arising from regulatory review and drug development.

Physiologically based pharmacokinetic modeling revealed minimal codeine intestinal metabolism in first-pass removal in rats

The physiologically based model with segregated flow to the intestine (SFM-PBPK; partial, lower flow to enterocyte region vs. greater flow to serosal region) was found to describe the first-pass glucuronidation of morphine (M) to morphine-3β-glucuronide (MG) in rats after intraduodenal (i.d.) and intravenous (i.v.) administration better than the traditional model (TM), for which a single intestinal flow perfused the whole of the intestinal tissue. The segregated flow model (SFM) described a disproportionately greater extent of intestinal morphine glucuronidation for i.d. vs. i.v. administration. The present study applied the same PBPK modeling approaches to examine the contributions of the intestine and liver on the first-pass metabolism of the precursor, codeine (C, 3-methylmorphine) in the rat. Unexpectedly, the profiles of codeine, morphine and morphine-3β-glucuronide in whole blood, bile and urine, assayed by LCMS, were equally well described by both the TM-PBPK and SFM-PBPK. The fitted parameters for the models were similar, and the net formation intrinsic clearance of morphine (from codeine) for the liver was much higher, being 9- to 13-fold that of the intestine. Simulations, based on the absence of intestinal formation of morphine, correlated well with observations. The lack of discrimination of SFM and TM with the codeine data did not invalidate the SFM-PBPK model but rather suggests that the liver is the only major organ for codeine metabolism. Because of little or no contribution by the intestine to the metabolism of codeine, both the TM- and SFM-PBPK models are equally consistent with the data. Copyright © 2016 John Wiley & Sons, Ltd.

Metabolite Kinetics: The Segregated Flow Model for Intestinal and Whole Body Physiologically Based Pharmacokinetic Modeling to Describe Intestinal and Hepatic Glucuronidation of Morphine in Rats In Vivo

We used the intestinal segregated flow model (SFM) versus the traditional model (TM), nested within physiologically based pharmacokinetic (PBPK) models, to describe the biliary and urinary excretion of morphine 3β-glucuronide (MG) after intravenous and intraduodenal dosing of morphine in rats in vivo. The SFM model describes a partial (5%-30%) intestinal blood flow perfusing the transporter- and enzyme-rich enterocyte region, whereas the TM describes 100% flow perfusing the intestine as a whole. For the SFM, drugs entering from the circulation are expected to be metabolized to lesser extents by the intestine due to the segregated flow, reflecting the phenomenon of shunting and route-dependent intestinal metabolism. The poor permeability of MG crossing the liver or intestinal basolateral membranes mandates that most of MG that is excreted into bile is hepatically formed, whereas MG that is excreted into urine originates from both intestine and liver metabolism, since MG is effluxed back to blood. The ratio of MG amounts in urine/bile [Formula: see text] for intraduodenal/intravenous dosing is expected to exceed unity for the SFM but approximates unity for the TM. Compartmental analysis of morphine and MG data, without consideration of the permeability of MG and where MG is formed, suggests the ratio to be 1 and failed to describe the kinetics of MG. The observed intraduodenal/intravenous ratio of [Formula: see text] (2.55 at 4 hours) was better predicted by the SFM-PBPK (2.59 at 4 hours) and not the TM-PBPK (1.0), supporting the view that the SFM is superior for the description of intestinal-liver metabolism of morphine to MG. The SFM-PBPK model predicts an appreciable contribution of the intestine to first pass M metabolism.

Spontaneous emulsification of nifedipine-loaded self-nanoemulsifying drug delivery system

Self-nanoemulsifying drug delivery system (SNEDDS) can be used to improve dissolution of poorly water-soluble drugs. The objective of this study was to prepare SNEDDS by using ternary phase diagram and investigate their spontaneous emulsifying property, dissolution of nifedipine (NDP), as well as the pharmacokinetic profile of selected SNEDDS formulation. The results showed that the composition of the SNEDDS was a great importance for the spontaneous emulsification. Based on ternary phase diagram, the region giving the SNEDDS with emulsion droplet size of less than 300 nm after diluting in aqueous medium was selected for further formulation. The small-angle X-ray scattering curves showed no sharp peak after dilution at different percentages of water, suggesting non-ordered structure. The system was found to be robust in different dilution volumes; the droplet size was in nanometer range. In vitro dissolution study showed remarkable increase in dissolution of NDP from SNEDDS formulations compared with NDP powders. The pharmacokinetic study of selected SNEDDS formulation in male Wistar rats revealed the improved maximum concentration and area under the curve. Our results proposed that the developed SNEDDS formations could be promising to improve the dissolution and oral bioavailability of NDP.

Modeling the effects of commonly used drugs on human metabolism

Metabolism contributes significantly to the pharmacokinetics and pharmacodynamics of a drug. In addition, diet and genetics have a profound effect on cellular metabolism with respect to both health and disease. In the present study, we assembled a comprehensive, literature-based drug metabolic reconstruction of the 18 most highly prescribed drug groups, including statins, anti-hypertensives, immunosuppressants and analgesics. This reconstruction captures in detail our current understanding of their absorption, intracellular distribution, metabolism and elimination. We combined this drug module with the most comprehensive reconstruction of human metabolism, Recon 2, yielding Recon2_DM1796, which accounts for 2803 metabolites and 8161 reactions. By defining 50 specific drug objectives that captured the overall drug metabolism of these compounds, we investigated the effects of dietary composition and inherited metabolic disorders on drug metabolism and drug-drug interactions. Our main findings include: (a) a shift in dietary patterns significantly affects statins and acetaminophen metabolism; (b) disturbed statin metabolism contributes to the clinical phenotype of mitochondrial energy disorders; and (c) the interaction between statins and cyclosporine can be explained by several common metabolic and transport pathways other than the previously established CYP3A4 connection. This work holds the potential for studying adverse drug reactions and designing patient-specific therapies.

Regulation of intestinal cytochrome P450 expression by hepatic cytochrome P450: possible involvement of fibroblast growth factor 15 and impact on systemic drug exposure

Tissue-specific deletion of the gene for NADPH-cytochrome P450 (P450) reductase (CPR), the essential electron donor to all microsomal P450 enzymes, in either liver or intestine, leads to upregulation of many P450 genes in the tissue with the Cpr deletion. Here, by studying the liver-specific Cpr-null (LCN) mouse, we examined whether an interorgan regulatory pathway exists, such that a loss of hepatic CPR would cause compensatory changes in intestinal P450 expression and capacity for first-pass metabolism of oral drugs. We show for the first time that intestinal expression of CYP2B, 2C, and 3A proteins was increased in LCN mice by 2- to 3-fold compared with wild-type (WT) mice, accompanied by significant increases in small intestinal microsomal lovastatin-hydroxylase activity and systemic clearance of oral lovastatin (at 5 mg/kg). Additional studies showed that the hepatic Cpr deletion, which caused large decreases in bile acid (BA) levels in the liver, intestine, plasma, and intestinal content, led to drastic decreases in the mRNA levels of intestinal fibroblast growth factor 15 (FGF15), a target gene of the BA receptor farnesoid X receptor. Furthermore, treatment of mice with FGF19 (the human counterpart of mouse FGF15) abolished the difference between WT and LCN mice in small intestinal (SI) CYP3A levels at 6 hours after the treatment. Our findings reveal a previously unrecognized direct role of intestinal FGF15/19 in the regulation of SI P450 expression and may have profound implications for the prediction of drug exposure in patients with compromised hepatic P450 function.

Metabolism of saikosaponin a in rats: diverse oxidations on the aglycone moiety in liver and intestine in addition to hydrolysis of glycosidic bonds

The main objective of the present study was to completely characterize the metabolites of the triterpenoid saikosaponin a (SSa) in rats. To this aim, we compared the metabolites in plasma, bile, urine, and feces samples following oral and i.v. routes of administration using liquid chromatography-diode array detector coupled with hybrid ion trap-time-of-flight mass spectrometry. As a result, besides 2 known metabolites, prosaikogenin f and saikogenin f, 15 new metabolites were detected in all. It was found that SSa is metabolized mainly in phase I manner, i.e., hydration and monooxidation on the aglycone moiety and hydrolysis of the β-glucosidic bond in the liver, and sequential hydrolysis of β-glucosidic and β-fucosidic bonds followed by dehydrogenation, hydroxylation, carboxylation, and combinations of these steps on the aglycone moiety in the intestinal tract. Both the renal and biliary routes were observed for the excretion of SSa and its metabolites. Further, a clear metabolic profile in rats was proposed in detail according to the results from the in vivo animal experiment after different routes of administration. Our results update the preclinical metabolism and disposition data on SSa, which is not only helpful for the future human metabolic study of this compound but also provides basic information for better understanding of the efficacy and safety of prescriptions containing saikosaponins.

NADPH-cytochrome P450 oxidoreductase: roles in physiology, pharmacology, and toxicology

This is a report on a symposium sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the Experimental Biology 2012 meeting in San Diego, California, on April 25, 2012. The symposium speakers summarized and critically evaluated our current understanding of the physiologic, pharmacological, and toxicological roles of NADPH-cytochrome P450 oxidoreductase (POR), a flavoprotein involved in electron transfer to microsomal cytochromes P450 (P450), cytochrome b(5), squalene mono-oxygenase, and heme oxygenase. Considerable insight has been derived from the development and characterization of mouse models with conditional Por deletion in particular tissues or partial suppression of POR expression in all tissues. Additional mouse models with global or conditional hepatic deletion of cytochrome b(5) are helping to clarify the P450 isoform- and substrate-specific influences of cytochrome b(5) on P450 electron transfer and catalytic function. This symposium also considered studies using siRNA to suppress POR expression in a hepatoma cell-culture model to explore the basis of the hepatic lipidosis phenotype observed in mice with conditional deletion of Por in liver. The symposium concluded with a strong translational perspective, relating the basic science of human POR structure and function to the impacts of POR genetic variation on human drug and steroid metabolism.

Role of intestinal cytochrome p450 enzymes in diclofenac-induced toxicity in the small intestine

The aim of this study was to determine the role of small intestinal (SI) cytochrome P450 (P450) enzymes in the metabolic activation of diclofenac (DCF), a widely used nonsteroidal anti-inflammatory drug, and DCF-induced intestinal toxicity. DCF induces intestinal ulcers in humans and mice, but the underlying mechanisms, including the necessity for drug bioactivation in the target tissues and the sources and identities of reactive intermediates, are not fully understood. We found that the number of DCF-induced (at 50 mg/kg p.o.) intestinal ulcers was significantly smaller in an intestinal epithelium (IE)-specific P450 reductase (CPR) knockout (IE-Cpr-null) mouse model, which has little P450 activity in the IE, than in wild-type (WT) mice, determined at 14 h after DCF administration. The involvement of intestinal P450 enzymes was confirmed by large reductions (>80-90%) in the rates of in vitro formation, in SI microsomal reactions, of hydroxylated DCF metabolites and reactive intermediates, trapped as DCF-glutathione (GSH) conjugates, in the IE-Cpr-null, compared with WT mice. The SI levels of DCF-GSH conjugates (at 4 h after dosing) and DCF-protein adducts (at 14 h after dosing) were significantly lower in IE-Cpr-null than in WT mice. In additional experiments, we found that pretreatment of mice with grapefruit juice, which is known to inhibit SI P450 activity, ameliorated DCF-induced intestinal toxicity in WT mice. Our results not only strongly support the notion that SI P450 enzymes play an important role in DCF-induced intestinal toxicity, but also illustrate the possibility of preventing DCF-induced intestinal toxicity through dietary intervention.

Role of mammary epithelial and stromal P450 enzymes in the clearance and metabolic activation of 7,12-dimethylbenz(a)anthracene in mice

Microsomal cytochrome P450 (P450) enzymes, which are important in the metabolism of carcinogens, are expressed in both epithelial and stromal cells in the mammary gland. The aim of this study was to investigate the roles of mammary epithelial P450 enzymes in the bioactivation and disposition of 7,12-dimethylbenz(a)anthracene (DMBA), a breast carcinogen, in the mammary gland. A new mouse model (named MEpi-Cpr-null) was produced, wherein P450 activities in the mammary epithelial cells are suppressed through tissue-specific deletion of the gene for P450 reductase (Cpr), an enzyme required for the activities of all microsomal P450 enzymes. Comparisons between wild-type and MEpi-Cpr-null mice showed that the tissue-specific deletion of Cpr in the mammary epithelial cells was accompanied by significant increases in the levels of DMBA and DMBA-DNA adduct in the mammary gland following a single intraperitoneal injection of DMBA at 50mg/kg. Immunohistochemical and immunoblot analysis further revealed greater induction of CYP1B1 expression by the DMBA treatment in the mammary stroma of the MEpi-Cpr-null mice than in that of the WT mice. These findings not only demonstrate that the epithelial P450 enzymes play important roles in the clearance of DMBA, but also suggest that P450 enzymes in both mammary epithelial and stromal cells contribute to carcinogen-mediated DNA damage.

Gene expression profiling of systems involved in the metabolism and the disposition of xenobiotics: comparison between human intestinal biopsy samples and colon cell lines

Intestinal cell lines are used as in vitro models for pharmacological and toxicological studies. However, a general report of the gene expression spectrum of proteins that are involved in the metabolism and the disposition of xenobiotics in these in vitro systems is not currently available. To fill this information gap, we systematically characterized the expression profile of 377 genes encoding xenobiotic-metabolizing enzymes, transporters, and nuclear receptors and transcription factors in intestinal mucosa (ileum, ascending colon, transverse colon, descending colon, and rectum) from five healthy subjects and in five commonly used intestinal cell lines (Caco-2, C2BBe1, HT29, T84, and FHC). For this, we performed a quantitative real-time reverse transcription-polymerase chain reaction analysis using TaqMan low-density arrays and analyzed the results by different statistical approaches: Spearman correlation coefficients, hierarchical clustering, and principal component analysis (PCA). A large variation in gene expression spectra was observed between intestinal cell lines and intestinal tissues. Both hierarchical clustering and PCA showed that two distinct clusters are visible, of which one corresponds to all cultured cell lines and the other to all intestinal biopsies. The best agreement between human tissue and the representative cell line was observed for human colonic tissues and HT29 and T84 cell lines. Altogether, these data demonstrated that gene expression profiling represents a new valuable tool for investigating in vitro and in vivo expression level correlation. This study has pointed out interesting expression profiles for various colon cell lines, which will be useful for choosing the appropriate in vitro model for pharmacological and toxicological studies.

In vitro and in vivo small intestinal metabolism of CYP3A and UGT substrates in preclinical animals species and humans: species differences

Intestinal first-pass metabolism has a great impact on the bioavailability of cytochrome P450 3A4 (CYP3A) and/or uridine 5'-diphosphate (UDP)-glucoronosyltranferase (UGT) substrates in humans. In vitro and in vivo intestinal metabolism studies are essential for clarifying pharmacokinetics in animal species and for predicting the effects of human intestinal metabolism. We review species differences in intestinal metabolism both in vitro and in vivo. Based on mRNA expression levels, the major intestinal CYP3A isoform is CYP3A4 for humans, CYP3A4 (3A8) for monkeys, CYP3A9 for rats, cyp3a13 for mice, and CYP3A12 for dogs. Additionally, the intestinal-specific UGT would be UGT1A10 for humans, UGT1A8 for monkeys, and UGT1A7 for rats. In vitro and in vivo intestinal metabolism of CYP3A substrates were larger in monkeys than in humans, although a correlation in intestinal availability between monkeys and humans has been reported. Little information is available regarding species differences in in vitro and in vivo UGT activities; however, UGT-mediated in vivo intestinal metabolism has been demonstrated for raloxifene in humans and for baicalein in rats. Further assessment of intestinal metabolism, particularly for UGT substrates, is required to clarify the entire picture of species differences.

Metabolism of gambogic acid in rats: a rare intestinal metabolic pathway responsible for its final disposition

Gambogic acid (GA) is a promising natural anticancer candidate. Although the anticancer activity of GA has been well demonstrated, information regarding the metabolic fate of GA is limited. Previous studies suggested that GA is mainly excreted into intestinal tract in rats through bile after intravenous administration, whereas only traces appeared in the feces, suggesting that GA is metabolized extensively in the intestine. However, there has been no report about the intestinal metabolism of GA either in animals or humans. In this study, large amounts of two sulfonic acid metabolites of GA were found in the feces samples of rats after intravenous administration, and their structures were identified as 10-α sulfonic acid GA and 10-β sulfonic acid GA by comparison of the retention times and spectral data with those of synthesized reference substances using liquid chromatography-diode array detector-tandem mass spectrometry. This rare intestinal metabolic pathway mainly involves Michael addition of the sulfite ion to the 9,10 carbon-carbon double bond of α,β-unsaturated ketone. In addition, a more detailed metabolic profile in rats is proposed, according to the results of in vitro and in vivo studies. It was found that GA can be metabolized by a variety of routes, including monooxidation, hydration, glutathionylation, glucuronidation, and glucosidation in the liver of rats. These findings provide information on the major metabolic soft spot of GA in the intestine and liver of rats, which is not only useful in the future human metabolic study of this compound but also of value in the metabolic studies of GA analogs.

Mechanisms of olfactory toxicity of the herbicide 2,6-dichlorobenzonitrile: essential roles of CYP2A5 and target-tissue metabolic activation

The herbicide 2,6-dichlorobenzonitril (DCBN) is a potent and tissue-specific toxicant to the olfactory mucosa (OM). The toxicity of DCBN is mediated by cytochrome P450 (P450)-catalyzed bioactivation; however, it is not known whether target-tissue metabolic activation is essential for toxicity. CYP2A5, expressed abundantly in both liver and OM, was previously found to be one of the P450 enzymes active in DCBN bioactivation in vitro. The aims of this study were to determine the role of CYP2A5 in DCBN toxicity in vivo, by comparing the extents of DCBN toxicity between Cyp2a5-null and wild-type (WT) mice, and to determine whether hepatic microsomal P450 enzymes (including CYP2A5) are essential for the DCBN toxicity, by comparing the extents of DCBN toxicity between liver-Cpr-null (LCN) mice, which have little P450 activity in hepatocytes, and WT mice. We show that the loss of CYP2A5 expression did not alter systemic clearance of DCBN (at 25 mg/kg); but it did inhibit DCBN-induced non-protein thiol depletion and cytotoxicity in the OM. Thus, CYP2A5 plays an essential role in mediating DCBN toxicity in the OM. In contrast to the results seen in the Cyp2a5-null mice, the rates of systemic DCBN clearance were substantially reduced, while the extents of DCBN-induced nasal toxicity were increased, rather than decreased, in the LCN mice, compared to WT mice. Therefore, hepatic P450 enzymes, although essential for DCBN clearance, are not necessary for DCBN-induced OM toxicity. Our findings form the basis for a mechanism-based approach to assessing the potential risks of DCBN nasal toxicity in humans.

The role of small-intestinal P450 enzymes in protection against systemic exposure of orally administered benzo[a]pyrene

An intestinal epithelium-specific cytochrome P450 (P450) reductase (CPR)-knockout (IE-Cpr-null) mouse and a liver-specific CPR-knockout (liver-Cpr-null) mouse were studied for determination of the respective roles of P450 enzymes in the liver and small intestine (SI) in the clearance of orally administered benzo[a]pyrene (BaP). Pharmacokinetic analysis of blood BaP levels indicated significantly lower rates of BaP clearance in IE-Cpr-null than in wild-type (WT) mice, after oral BaP (30 mg/kg) treatment. In contrast, clearance rates for intraperitoneal BaP (45 mg/kg) were not different between IE-Cpr-null and WT mice. Furthermore, there was no significant difference between liver-Cpr-null and WT mice in BaP clearance, after either intraperitoneal or oral BaP administration. Thus, small-intestinal P450-mediated first-pass metabolism is a key determinant of the systemic bioavailability of oral BaP. In addition, we observed greater differences in the rates of clearance of oral BaP, between WT and IE-Cpr-null mice, in mice pretreated with beta-naphthoflavone, to induce CYP1A1 expression, than in untreated mice. The onset of induction (at 2 h after dosing) of CYP1A1 protein expression by oral BaP administration was earlier in the SI than in extra-gut organs analyzed; for liver, lung, and kidney, induction was not observed until 4 h after dosing. Furthermore, BaP tissue burdens in SI and extra-gut organs of IE-Cpr-null mice were greater than burdens in corresponding organs of WT mice, at 6 or 24 h after BaP administration. Taken together, these findings strongly support the concept that small-intestinal CYP1A1 induction is a critical factor in protection against systemic exposure to oral BaP.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

An intestinal epithelium-specific cytochrome P450 (P450) reductase-knockout mouse model: direct evidence for a role of intestinal p450s in first-pass clearance of oral nifedipine

To determine the in vivo function of intestinal cytochrome P450 (P450) enzymes, we have generated an intestinal epithelium (IE)-specific P450 reductase gene (Cpr) knockout mouse model (designated IE-Cpr-null). In the IE-Cpr-null mouse, CPR expression was abolished in IE cells; however, CPR expression was not altered in other tissues examined. The loss of CPR expression in the small intestine (SI) led to increased expression of several P450 proteins examined, including CYP1A1, CYP2B, CYP2C, and CYP3A. It is interesting to note that the expression of CYP1A1 was also increased in the liver, kidney, and lung of the IE-Cpr-null mice compared with wild-type (WT) littermates, a result strongly supporting the notion that SI metabolism of putative dietary CYP1A1 inducers can influence the systemic bioavailability of these inducers. The rates of SI microsomal metabolism of nifedipine (NFP) in the IE-Cpr-null mice were approximately 10% of the rates in WT littermates, despite the compensatory expression of multiple P450 enzymes in the SI. Furthermore, the area under the concentration-time curve (AUC) values for blood NFP (dosed at 10 mg/kg) levels were 1.6-fold higher in IE-Cpr-null mice than in WT littermates when NFP was given orally; in contrast, the AUC values were comparable for the two strains when NFP was given intravenously. This result directly showed that P450-catalyzed NFP metabolism in the SI plays an important role in the first-pass clearance of oral NFP. Our findings indicate that the IE-Cpr-null mouse model can be used to study the in vivo function of intestinal P450 enzymes in the clearance of oral drugs and other xenobiotics.

Nifedipine nanoparticle agglomeration as a dry powder aerosol formulation strategy

Efficient administration of drugs represents a leading challenge in pulmonary medicine. Dry powder aerosols are of great interest compared to traditional aerosolized liquid formulations in that they may offer improved stability, ease of administration, and simple device design. Particles 1-5microm in size typically facilitate lung deposition. Nanoparticles may be exhaled as a result of their small size; however, they are desired to enhance the dissolution rate of poorly soluble drugs. Nanoparticles of the hypertension drug nifedipine were co-precipitated with stearic acid to form a colloid exhibiting negative surface charge. Nifedipine nanoparticle colloids were destabilized by using sodium chloride to disrupt the electrostatic repulsion between particles as a means to achieve the agglomerated nanoparticles of a controlled size. The aerodynamic performance of agglomerated nanoparticles was determined by cascade impaction. The powders were found to be well suited for pulmonary delivery. In addition, nanoparticle agglomerates revealed enhanced dissolution of the drug species suggesting the value of this formulation approach for poorly water-soluble pulmonary medicines. Ultimately, nifedipine powders are envisioned as an approach to treat pulmonary hypertension.

Mechanism of chloroform-induced renal toxicity: non-involvement of hepatic cytochrome P450-dependent metabolism

Chloroform causes hepatic and renal toxicity in a number of species. In vitro studies have indicated that chloroform can be metabolized by P450 enzymes in the kidney to nephrotoxic intermediate, although direct in vivo evidence for the role of renal P450 in the nephrotoxicity has not been reported. This study was to determine whether chloroform renal toxicity persists in a mouse model with a liver-specific deletion of the P450 reductase (Cpr) gene (liver-Cpr-null). Chloroform-induced renal toxicity and chloroform tissue levels were compared between the liver-Cpr-null and wild-type mice at 24 h following differing doses of chloroform. At a chloroform dose of 150 mg/kg, the levels of blood urea nitrogen (BUN) were five times higher in the exposed group than in the vehicle-treated one for the liver-Cpr-null mice, but they were only slightly higher in the exposed group than in the vehicle-treated group for the wild-type mice. Severe lesions were found in the kidney of the liver-Cpr-null mice, while only mild lesions were found in the wild-type mice. At a chloroform dose of 300 mg/kg, severe kidney lesions were observed in both strains, yet the BUN levels were still higher in the liver-Cpr-null than in the wild-type mice. Higher chloroform levels were found in the tissues of the liver-Cpr-null mice. These findings indicated that loss of hepatic P450-dependent chloroform metabolism does not protect against chloroform-induced renal toxicity, suggesting that renal P450 enzymes play an essential role in chloroform renal toxicity.