Constitutive and inducible hepatic cytochrome P450 isoforms in senescent male and female rats and response to low-dose phenobarbital

The Influence of Lipid Microdomain Heterogeneity on Protein-Protein Interactions: Proteomic Analysis of Co-Immunoprecipitated Binding Partners of P450 1A2 and P450 3A in Rat Liver Microsomes

Liver cytochrome P450s (CYPs) of the endoplasmic reticulum (ER) are involved in the metabolism of exogenous and endogenous chemicals. The ER is not uniform, but possesses ordered lipid microdomains containing higher levels of saturated fatty acids, sphingomyelin, and cholesterol and disordered regions containing higher levels of polyunsaturated fatty acid chains. The various forms of drug-metabolizing P450s partition to either the ordered or disordered lipid microdomains with different degrees of specificity. P450s readily form complexes with ER-resident proteins, including other forms of P450. This study aims to ascertain whether lipid microdomain localization influences protein-P450 interactions in rat liver microsomes. Thus, liver microsomes were co-immunoprecipitated with CYP1A2-specific and CYP3A-specific antibodies, and the co-immunoprecipitating proteins were identified by liquid chromatography mass spectrometry proteomic analysis. These two P450s preferentially partition to ordered and disordered microdomains, respectively. More than 100 proteins were co-immunoprecipitated with each P450. Segregation of proteins into different microdomains did not preclude their interaction. However, CYP3A interacted broadly with proteins from ordered microdomains, whereas CYP1A2 reacted with a limited subset of these proteins. This is consistent with the concept of lipid raft heterogeneity and may indicate that CYP1A2 is targeted to a specific type of lipid raft. Although many of the interacting proteins for both P450s were other-drug metabolizing enzymes, other interactions were also evident. The consistent CYP3A binding partners were predominantly involved in phase I/II drug metabolism; however, CYP1A2 interacted not only with xenobiotic metabolizing enzymes, but also with enzymes involved in diverse cellular responses such as ER stress and protein folding. SIGNIFICANCE STATEMENT: This work describes the protein interactomes in rat liver microsomes of two important cytochromes P450s (CYP1A2 and CYP3A) in drug metabolism and describes the relationship of the interacting proteins to lipid microdomain distribution.

Fungicide bromuconazole has the potential to induce hepatotoxicity at the physiological, metabolomic and transcriptomic levels in rats

Bromuconazole (BROMU), a representative triazole fungicide, has been widely used in agriculture for its low cost and highly efficiency against various fungi. BROMU residue was often detected in the environment and food chain, even though there is indication of health risk to animals, and in humans. However, the data related to the toxicity of BROMU in animals remains unclear, and the mechanism is still not fully elucidated. Here, male adult rats were exposed to 0, 13.8, 32.8 and 65.6 mg/kg/d of BROMU for 10 days by oral gavage. It was observed that short time BROMU exposure not only caused liver histological damage, including vacuolar degeneration of hepatocytes with pyknotic nuclei, but also changed the levels of some hepatic physiological parameters, including aspartate transaminase (AST), triglyceride (TG), pyruvate and total cholesterol (TC), indicating that BROMU causes hepatotoxicity in rats. In addition, according to the transcriptomics and metabolomics analysis, a total of 58 metabolites and 259 genes significantly changed in the high-dose BROMU treated group. Although several different pathways are involved, lipid metabolism- and bile acids metabolism-related pathways were highlighted in both metabolomics and transcriptomics analysis. More importantly, further validation had proven that BROMU could not only interact with peroxisome proliferator-activated receptor γ (PPAR-γ), but also significantly decrease its protein and gene expression in the liver, supporting that BROMU decreased the TG synthesis via inhibiting the PPAR-γ pathway. These results clearly showed that BROMU exposure could result in hepatotoxicity at metabolomic and transcriptomic level in rats. These observations could provide some important steps toward understanding the mechanism underlying BROMU-induced mammalian toxicity.

The endothelin receptor antagonist macitentan for the treatment of pulmonary arterial hypertension: A cross-species comparison of its cytochrome P450 induction pattern

The dual endothelin receptor antagonist macitentan was approved in 2013 for the treatment of pulmonary arterial hypertension. Macitentan is an inducer of cytochrome P450 expression in vivo in animal species but not in man. In rat and dog, changes in P450 expression manifest as autoinduction upon repeat dosing. The induction pattern, however, significantly differed between both species, and between male and female rats. While macitentan exposure steadily declined with dose in the dog, P450 induction was saturable in the rat reaching levels of 40%-60% and 60%-80% at steady-state in male and female animals, respectively. The nature and number of P450 enzymes involved in macitentan clearance were identified as a major reason for the observed species differences. In the dog, macitentan was metabolized by a single P450 enzyme, that is, Cyp3a12, whereas several members of the Cyp2c and Cyp3a families were involved in the rat. Macitentan selectively upregulated Cyp3a expression in rat, whereas the expression of the Cyp2c enzymes involved in macitentan metabolism remained mostly unchanged, eventually leading to a higher contribution of Cyp3a upon induction. Macitentan also induced CYP3A4 expression in human hepatocytes via initial activation of the human pregnane X receptor. No such induction was evident in humans at the therapeutic macitentan dose of 10 mg as shown in a clinical drug-drug interaction study with the CYP3A4 substrate sildenafil.

Age-associated changes of cytochrome P450 and related phase-2 gene/proteins in livers of rats

Cytochrome P450s (CYPs) are phase-I metabolic enzymes playing important roles in drug metabolism, dietary chemicals and endogenous molecules. Age is a key factor influencing P450s expression. Thus, age-related changes of CYP 1–4 families and bile acid homeostasis-related CYPs, the corresponding nuclear receptors and a few phase-II genes were examined. Livers from male Sprague-Dawley rats at fetus (−2 d), neonates (1, 7, and 14 d), weanling (21 d), puberty (28 and 35 d), adulthood (60 and 180 d), and aging (540 and 800 d) were collected and subjected to qPCR analysis. Liver proteins from 14, 28, 60, 180, 540 and 800 days of age were also extracted for selected protein analysis by western blot. In general, there were three patterns of their expression: Some of the drug-metabolizing enzymes and related nuclear receptors were low in fetal and neonatal stage, increased with liver maturation and decreased quickly at aging (AhR, Cyp1a1, Cyp2b1, Cyp2b2, Cyp3a1, Cyp3a2, Ugt1a2); the majority of P450s (Cyp1a2, Cyp2c6, Cyp2c11, Cyp2d2, Cyp2e1, CAR, PXR, FXR, Cyp7a1, Cyp7b1. Cyp8b1, Cyp27a1, Ugt1a1, Sult1a1, Sult1a2) maintained relatively high levels throughout the adulthood, and decreased at 800 days of age; and some had an early peak between 7 and 14 days (CAR, PXR, PPARα, Cyp4a1, Ugt1a2). The protein expression of CYP1A2, CYP2B1, CYP2E1, CYP3A1, CYP4A1, and CYP7A1 corresponded the trend of mRNA changes. In summary, this study characterized three expression patterns of 16 CYPs, five nuclear receptors, and four phase-II genes during development and aging in rat liver, adding to our understanding of age-related CYP expression changes and age-related disorders.

Effects of Postnatal Exposure to a Mixture of Polychlorinated Biphenyls, p,p′-dichlorodiphenyltrichloroethane, and p-p′-dichlorodiphenyldichloroethene in Prepubertal and Adult Female Sprague-Dawley Rats

The postnatal period is a critical phase of development and a time during which humans are exposed to higher levels of persistent organic pollutants (POPs), than during subsequent periods of life. There is a paucity of information describing effects of postnatal exposure to environmentally relevant mixtures of POPs, such as polychlorinated biphenyls (PCBs), p,p′-dichlorodiphenyltrichloroethane (DDT), and p,p′-dichlorodiphenyldichloroethene (DDE). To provide data useful for the risk assessment of postnatal exposure to POPs, mixtures containing 19 PCBs, DDT, and DDE were prepared according to their concentrations previously measured in the milk of Canadian women, and dose-response effects were tested on the proliferation of MCF7-E3 cells in vitro, and in vivo experiments. Female neonates were exposed by gavage at postnatal days (PNDs) 1, 5, 10, 15, and 20 with dosages equivalent to 10, 100, and 1000 times the estimated human exposure level over the first 24 days of life. The MCF7-E3 cells showed a 227% increase in the AlamarBlue proliferation index, suggesting estrogen-like properties of the mixture, but this was not confirmed in vivo, given the absence of uterotrophic effects at PND21. An increase (511%) in hepatic ethoxyresorufin- o-deethylase activity at the dose 100 × was the most sensitive endpoint among those measured at PND21 (organ weight, mammary gland and ovarian morphometry, hepatic enzyme inductions, serum thyroxine and pituitary hormones). In liver samples from older female rats (previously involved in a mammary tumor study [Desaulniers et al., Toxicol. Sci. 75:468–480, 2001]), hepatic metabolism of 14C-estradiol-17 β (E2) at PND55 to PND62 was significantly higher in the 1000 × compared to the control group, but hepatic detoxification enzyme activities had already returned to control values. The production of hepatic 2-hydroxy-E2 decreased, whereas that of estrone increased with age. In conclusion, the smallest dose of the mixture to induce significant effects was 100×, and mixture-induced changes in the hepatic metabolism of estrogens might be a sensitive indicator of persistent effects.

Hepatic Transcript Levels for Genes Coding for Enzymes Associated with Xenobiotic Metabolism are Altered with Age

Metabolism studies are crucial for data interpretation from rodent toxicity and carcinogenicity studies. Metabolism studies are usually conducted in 6 to 8 week old rodents. Long-term studies often continue beyond 100 weeks of age. The potential for age-related changes in transcript levels of genes encoding for enzymes associated with metabolism was evaluated in the liver of male F344/N rats at 32, 58, and 84 weeks of age. Differential expression was found between the young and old rats for genes whose products are involved in both phase I and phase II metabolic pathways. Thirteen cytochrome P450 genes from CYP families 1–3 showed alterations in expression in the older rats. A marked age-related decrease in expression was found for 4 members of the Cyp3a family that are critical for drug metabolism in the rat. Immunohistochemical results confirmed a significant decrease in Cyp3a2 and Cyp2c11 protein levels with age. This indicates that the metabolic capacity of male rats changes throughout a long-term study. Conducting multiple hepatic microarray analyses during the conduct of a long-term study can provide a global view of potential metabolic changes that might occur. Alterations that are considered crucial to the interpretation of long-term study results could then be confirmed by subsequent metabolic studies.

An expandable donor-free supply of functional hepatocytes for toxicology

The B-13 cell is a readily expandable rat pancreatic acinar-like cell that differentiates on simple plastic culture substrata into replicatively-senescent hepatocyte-like (B-13/H) cells in response to glucocorticoid exposure. B-13/H cells express a variety of liver-enriched and liver-specific genes, many at levels similar to hepatocytes in vivo. Furthermore, the B-13/H phenotype is maintained for at least several weeks in vitro, in contrast to normal hepatocytes which rapidly de-differentiate under the same simple – or even under more complex – culture conditions. The origin of the B-13 cell line and the current state of knowledge regarding differentiation to B-13/H cells are presented, followed by a review of recent advances in the use of B-13/H cells in a variety of toxicity endpoints. B-13 cells therefore offer Toxicologists a cost-effective and easy to use system to study a range of toxicologically-related questions. Dissecting the mechanism(s) regulating the formation of B-13/H cell may also increase the likelihood of engineering a human equivalent, providing Toxicologists with an expandable donor-free supply of functional rat and human hepatocytes, invaluable additions to the tool kit of in vitro toxicity tests.

Da-Chaihu-Tang alters the pharmacokinetics of nifedipine in rats and a treatment regimen to avoid this

Objectives

To investigate the influence of co-administrated Da-Chaihu-Tang (DCT; a traditional Chinese formulation) on the pharmacokinetics of nifedipine, as well as the safe optimal dosing interval to avoid the adverse interactions.

Methods

A single dose of DCT was administered with nifedipine simultaneously, 2 h before, 30 min before or 30 min after nifedipine administration. Pharmacokinetics of nifedipine with or without DCT were compared. The influences of DCT on nifedipine intestinal mucosal and hepatic metabolism were studied by using rat in-vitro everted jejunal sac model and hepatic microsomes.

Key findings

A simultaneous co-administration of DCT significantly increased the area under concentration-time curve from time zero to infinity (AUC0-inf) of nifedipine. In-vitro mechanism investigations revealed that DCT inhibited both the intestinal and the hepatic metabolism of nifedipine. Further study on the optimal dosing interval for nifedipine and DCT revealed that administration of DCT 30 min before or after nifedipine did not significantly change the AUC of nifedipine.

Conclusions

The bioavailability of nifedipine is significantly increased by a simultaneous oral co-administration of DCT. This increase is caused by the inhibitory effect of DCT on both the intestinal mucosal and the hepatic metabolism of nifedipine. The dose interval between DCT and nifedipine needs to be set for over 30 min to avoid such drug–drug interactions.

Evaluation of hepatotoxicity and cholestasis in rats treated with EtOH extract of Fructus Psoraleae

ETHNOPHARMACOLOGICAL RELEVANCE

Fructus Psoraleae (FP) has been widely used to heal skin diseases as well as osteoporosis, osteomalacia, and bone fracture. There also exist many clinical reports about FP-induced hepatotoxicity associated with acute cholestatic hepatic injury. However, the FP-induced hepatotoxicity and the underlying mechanisms remain unclear.

AIMS OF THE STUDY

The present study aims to determine the hepatotoxicity of FP in Sprague-Dawley (SD) rats and to investigate the underlying mechanisms.

MATERIALS AND METHODS

Sprague-Dawley rats of both sexes were intragastrically administered with the EtOH extract of FP (EEFP) at doses of 1.875, 1.25 and 0.625 g/kg for 28 day. Body weight, relative liver weight, biochemical analysis, histopathology, the mRNA and protein expression of Cholesterol 7α-hydroxylase (CYP7A1), farnesoid X receptor (FXR), bile-salt export pump (BSEP), multidrug resistance-associated protein 2 (MRP2), multidrug resistance-associated protein 3 (MRP3) were evaluated to study the EEFP-induced hepatotoxicity and its underlying mechanisms.

RESULTS

Many abnormalities were observed in the EEFP-treated groups including suppression of weight gain and food intake, change of some parameters in serum biochemistry, increased weight of liver, and decreased concentration of bile acid in bile. The mRNA and protein expression of CYP7A1, MRP3, MRP2, BSEP increased and the expression of FXR decreased in EEFP-treated female groups; the mRNA and protein of FXR and CYP7A1 decreased and that of the others remained the same in EEFP-treated male groups.

CONCLUSION

In conclusion, we provide evidence for the first time that EEFP can induce sex-related cholestatic hepatotoxicity, and that female rats are more sensitive to EEFP-induced hepatotoxicity, which involves the destruction of the biosynthesis and transportation of bile acid. Further investigation is still needed to uncover the mechanism of the sex-dimorphic EEFP-induced hepatotoxicity.

Quantitative shot-gun proteomics and MS-based activity assay for revealing gender differences in enzyme contents for rat liver microsome

Liver microsomes are subcellular fractions that contain many metabolizing enzymes for drugs and endogeneous compounds. Some of these enzymes are regulated by sex hormonal control and exhibit sex-dependent expression pattern and metabolizing speed. Studying these enzymes, however, are complicated by the presence of isoforms such as cytochrome P450 (CYP450), which families share more than 50% amino acid identities. In this study, we applied quantitative shot-gun proteomics approach coupled with stable-isotope dimethyl labeling, two-dimensional reversed-phase peptide separation and tandem mass spectrometry (MS) to explore the gender-dependent expression of rat liver microsomal proteins. A total of 391 proteins were identified and quantified by this approach, and 56% of quantified proteins were enzymes. Although shot-gun approach is rarely used for identifying protein isoforms, we identified 53 isoforms by at least one unique peptide including 21 isoforms of CYP450s. Moreover, by quantitative and statistics assessment, we were able to classify them into 28 male dominant enzymes including CYP2C12 CYP2C11, CYP2C13, CYP2B3, CYP2C11, CYP2C70 and CYP3A2 which are known to be male specific, 21 female dominant enzymes including CYP2A1, CYP2C7, CYP2C12, CYP2D26, alcohol dehydrogenase 1, carboxylesterase 3, glutathione S-transferase, liver carboxylesterase 4, UDP-glucuronosyltransferase 2B1, and glyceraldehyde-3-phosphate dehydrogenase which are known to be female specific; and 125 sex-independent enzymes. However, most of the sex specificities revealed from this study, such as the male specificity of CYP2D1, were novel and not yet reported. We then conducted a mass spectrometry-multiple reaction mode (MS-MRM) based enzyme activity method to determine the catalyzing rate of CYP2D1 in male and female liver microsomes using carteolol as its specific substrate. The reaction rate catalyzed by CYP2D1 in female rats was determined to differ significantly with the rate in male rats. Moreover, the ratio (female/male) of reaction rate (0.68) was found to correlate with their relative protein abundance (0.72). This study revealed novel sex dependences of many rat liver enzymes and also demonstrated a unique MS-based analytical platform that could identify novel iso-enzymes and further quantify their abundance and enzyme activity.

Cross-species comparisons of the pharmacokinetics of ibudilast

To enable clinical development of ibudilast for new indications, its pharmacokinetics were characterized in mice, rats, rabbits, dogs, cynomolgus monkeys, and minipigs. Animal pharmacokinetics were compared with a separate study in healthy volunteers. Following oral dosing, the dose-normalized area under the curve (AUC) (DN-AUC(24h)) in humans is 896 ((ng*h ml(-1))/(mg kg(-1))), and in animals ranges from 0.3 to 87. The variability among species cannot be explained by intrinsic clearance, which in intravenous dosing experiments shows only moderate interspecies variation (13-41 l h(-1) m(-2)). A portal vein rat pharmacokinetics model suggested that differences in first-pass gut clearance may explain some of the interspecies variation in oral bioavailability. Ibudilast shows auto-induction of metabolism in some animals, but not in humans. Plasma protein binding in humans and some animals is greater than or equal to 95%. The primary metabolite 6,7-dihyrdodiol-ibudilast is measurable and has been quantitated in plasma from animals and humans. Finally, biodistribution studies show that ibudilast distributes rapidly, extensively, and reversibly to the central nervous system.

Energy restriction does not compensate for the reduced expression of hepatic drug-processing genes in mice with aging

Liver is the major organ that eliminates xenobiotics from the body, a process that is accomplished by a series of drug-processing genes (DPGs). These genes encode transporters on both basolateral and apical membranes of hepatocytes, as well as phase I and II enzymes. The current study compares the expression of hepatic DPGs in adult and aged mouse livers and explores the potential effects of energy restriction (ER) on these genes during aging. Of 79 quantified hepatic DPGs, 52 were expressed lower in 24-month-old aged mice than in 12-month-old adult mice. Furthermore, the mRNA expression of multiple xenobiotic-activated transcription factors also decreased with age. Six-month ER exerted less of an effect on the hepatic DPGs than did aging. ER increased the mRNAs of two and decreased the mRNAs of nine DPGs in adult mice. In aged mice, ER increased the mRNAs of 10 and decreased the mRNAs of 5 DPGs. The only mRNA that was increased by both ER and aging was Gstm3. ER increased the mRNAs of Cyp2b10, Ugt1a9, Gsta1, and Oatp1a4 only in adult mice and decreased the mRNAs of Aldh6a1, Pon3, Ugt1a1, Sult1a1, and Atp8b1 only in aged mice. In summary, the reduced mRNA expression of hepatic DPGs in aged mice indicates decreased drug-processing capability, whereas ER did not compensate for the global reduction of hepatic DPG expression in aged mice. The hepatic transcription factors are likely to mediate the changes in hepatic DPG expression during aging and ER.

Male-specific induction of CYP3A2 in rats by zolmitriptan

We report here a novel observation that zolmitriptan induced CYP3A2 in male but not female rats. As part of our research programme to evaluate sex differences in the response to zolmitriptan, we studied the effects of zolmitriptan on CYP3A activity, protein and gene expression in male and female rats. Zolmitriptan was found to induce CYP3A activity, measured as testosterone and diazepam metabolism in-vitro, as well as midazolam pharmacokinetics in-vivo, in male but not female rats. The sex difference in response to zolmitriptan was further evaluated by analysis of CYP3A1/2 mRNA levels using real-time PCR, and CYP3A1/2 protein levels using immunoblotting. Zolmitriptan preferentially induced CYP3A2 in male but not female rats. No obvious effects on CYP3A1 were observed at any dose in either sex. Thus, we concluded that the observed sex-dependent induction of CYP3A by zolmitriptan was largely due to induction of CYP3A2 in male rats.

The elderly as a sensitive population in environmental exposures: making the case

The US population is aging. CDC has estimated that 20% of all Americans will be 65 or older by the year 2030. As a part of the aging process, the body gradually deteriorates and physiologic and metabolic limitations arise. Changes that occur in organ anatomy and function present challenges for dealing with environmental stressors of all kinds, ranging from temperature regulation to drug metabolism and excretion. The elderly are not just older adults, but rather are individuals with unique challenges and different medical needs than younger adults. The ability of the body to respond to physiological challenge presented by environmental chemicals is dependent upon the health of the organ systems that eliminate those substances from the body. Any compromise in the function of those organ systems may result in a decrease in the body's ability to protect itself from the adverse effects of xenobiotics. To investigate this issue, we performed an organ system-by-organ system review of the effects of human aging and the implications for such aging on susceptibility to drugs and xenobiotics. Birnbaum (1991) reported almost 20 years ago that it was clear that the pharmacokinetic behavior of environmental chemicals is, in many cases, altered during aging. Yet, to date, there is a paucity of data regarding recorded effects of environmental chemicals on elderly individuals. As a result, we have to rely on what is known about the effects of aging and the existing data regarding the metabolism, excretion, and adverse effects of prescription medications in that population to determine whether the elderly might be at greater risk when exposed to environmental substances. With increasing life expectancy, more and more people will confront the problems associated with advancing years. Moreover, although proper diet and exercise may lessen the immediate severity of some aspects of aging, the process will continue to gradually degrade the ability to cope with a variety of injuries and diseases. Thus, the adverse effects of long-term, low-level exposure to environmental substances will have a longer time to be manifested in a physiologically weakened elderly population. When such exposures are coupled with concurrent exposure to prescription medications, the effects could be devastating. Public health officials must be knowledgeable about the sensitivity of the growing elderly population, and ensure that the use of health guidance values (HGVs) for environmental contaminants and other substances give consideration to this physiologically compromised segment of the population.

Protein-protein interactions between rat hepatic cytochromes P450 (P450s) and UDP-glucuronosyltransferases (UGTs): evidence for the functionally active UGT in P450-UGT complex

The interaction between cytochrome P450s (CYP, P450) and UDP-glucuronosyltransferases (UGTs) was studied by co-immunoprecipitation. P450 isoform-selective antibody was used as a probe to co-precipitate UGTs with the P450s from solubilized rat liver microsomes. Antibodies toward CYP3A2, CYP2B2, CYP2C11/13 and CYP1A2 co-precipitated UGTs with corresponding P450s. However, calnexin, a type-I membrane protein, in the endoplasmic reticulum was not co-precipitated by anti-P450 antibodies. UGT activity toward 4-methylumbelliferone was detected in all co-precipitates, suggesting that UGT in the complex with P450s is functionally active. Repeated washing of co-immunoprecipitates revealed differences among P450 isoforms with regard to the affinity for UGT. Larger amounts of UGT1A1 and UGT1A6, compared with UGT2B1, were washed out from UGTs-CYP2C11/13 co-precipitates, whereas UGT-CYP3A2 and UGT-CYP2Bs complexes were resistant to thorough washing. Thus, CYP2C11/13 could associate with UGTs, but the affinity is assumed to be weaker than that of CYP2B/3As. These results suggest that there is isoform specificity in the interaction between P450s and UGTs.

o,p'-DDT elicits PXR/CAR-, not ER-, mediated responses in the immature ovariectomized rat liver

Technical-grade dichlorodiphenyltrichloroethane (DDT) is an agricultural pesticide and malarial vector control agent that has been designated a potential human hepatocarcinogen. The o,p'-enantiomer exhibits estrogenic activity that has been associated with the carcinogenicity of DDT. The temporal and dose-dependent hepatic estrogenicity of o,p'-DDT was investigated using complementary DNA microarrays in immature ovariectomized Sprague-Dawley rats with complementary histopathology and tissue-level analysis. Animals were gavaged with 300 mg/kg o,p'-DDT either once or once daily for 3 consecutive days. Liver samples were examined 2, 4, 8, 12, 18, or 24 h after a single dose or following three daily doses. For dose-response studies, a single dose of 3, 10, 30, 100, or 300 mg/kg body weight o,p'-DTT was administered for 3 consecutive days. Genes associated with drug metabolism (Cyp2b2 and Cyp3a2), the nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR), cell proliferation (Ccnd1, Ccnb1, Ccnb2, and Stmn1), and oxidative stress (Gclm and Hmox1) were significantly induced. Cyp2b2 exhibited dose-dependent regulation and was significantly induced across all time points, while cell proliferation- and oxidative stress-related genes exhibited transient induction. The induction of Cyp2b2 and Cyp3a2 mRNA levels suggest PXR/CAR activation, consistent with expression of genes associated with oxidative stress. Few genes known to be estrogen receptor (ER) regulated were differentially expressed when compared to the hepatic gene expression profile elicited by ethynyl estradiol in immature ovariectomized C57BL/6 mice using the same study design and analysis methods. These data indicate that o,p'-DDT elicits PXR/CAR-, not ER-, mediated gene expression in the rat liver. Based on the species-specific differences in CAR regulation, the extrapolation of rodent DDT hepatocarcinogenicity to humans warrants further investigation.

Metabolism of a Heterocyclic Amine Colon Carcinogen in Young and Old Rats

The incidence of colon cancer increases with age, and this may be related to altered metabolism and disposition of carcinogens. One such carcinogen implicated in colon cancer is the heterocyclic amine found in well done meat, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). The purpose of these studies was to determine whether the disposition and metabolism of IQ changes with age, comparing young (3-month) and old (22- to 24-month) male F344 rats. Animals were treated with vehicle or beta-naphthoflavone (BNF), an inducer of drug-metabolizing cytochromes P450. Disposition and metabolism of IQ were determined after i.p. injection of radiolabeled IQ. Urinary IQ metabolites were identified and quantitated by high-performance liquid chromatography and mass spectroscopy. In BNF-treated animals, total radiolabeled IQ excretion by old rats was less than half that of young rats. Binding of radiolabeled IQ metabolites by the old kidney was 10 times higher than that of the young. There were no age differences in intestinal and hepatic binding. There was a significant age-related increase in IQ conjugation to glucuronic acid and a decrease in conjugation to sulfate regardless of treatment. The induction of renal CYP1A1, a major P450 involved in IQ metabolism, by BNF did not change with age. Changes in IQ metabolism with age along with altered renal function may contribute to the decreased urinary excretion and increased renal binding of IQ and/or its metabolites seen in the old animals.

Deacetylclivorine: A Gender-Selective Metabolite of Clivorine Formed in Female Sprague-Dawley Rat Liver Microsomes

Clivorine, a naturally occurring pyrrolizidine alkaloid, causes liver toxicity via its metabolic activation to generate toxic metabolite (pyrrolic ester). Female Sprague-Dawley (SD) rats are reported to be less susceptible to clivorine intoxication than male SD rats. However, the biochemical mechanism causing such gender difference is largely unknown. The present study investigated hepatic microsomal metabolism of clivorine in female rats to delineate the mechanism of the gender difference. Two pathways, which directly metabolize clivorine, were observed. First, the metabolic activation to produce the toxic pyrrolic ester followed by formations of bound pyrroles, dehydroretronecine, 7-glutathionyldehydroretronecine, and clivoric acid were found in female rats, and CYP3A1/2 isozymes were identified to catalyze the metabolic activation. Compared with male rats ( approximately 21%), the metabolic activation in female rats was significantly lower ( approximately 4%) possibly because of significantly lower CYP3A1/2 levels expressed in female rats. Second, a direct hydrolysis to generate the novel female rat-specific metabolite deacetylclivorine was shown as the predominant pathway ( approximately 16% clivorine metabolism) in female rat liver microsomes and was determined to be mediated by microsomal hydrolase A. Furthermore, when the metabolic activation was completely inhibited by ketoconazole, the amount of deacetylclivorine formed in a 1-h incubation significantly increased from 19.44 +/- 3.00 to 54.87 +/- 9.30 nmol/mg protein, suggesting that the two pathways compete with each other. Therefore, the lower susceptibility of female SD rats to clivorine intoxication is suggested to be caused by the significantly higher extent of the direct hydrolysis and a lower degree of the metabolic activation.

Ontogeny of hepatic and plasma metabolism of deltamethrin in vitro: role in age-dependent acute neurotoxicity

Deltamethrin (DLM) is a relatively potent and widely used pyrethroid insecticide. Inefficient detoxification has been proposed to be the primary reason for the greater sensitivity of immature rats to the acute neurotoxicity of DLM. The objective of this study was to test this hypothesis by characterizing the age dependence of DLM metabolism in vitro, as well as toxic signs and blood levels of the neurotoxic parent compound following administration of 10 mg DLM/kg p.o. in glycerol formal. Metabolism was quantified in vitro by monitoring the disappearance of the parent compound from plasma [via carboxylesterases (CaEs)] and liver microsomes [via CaEs and cytochromes P450 (P450s)] obtained from 10-, 21-, and 40-day-old male Sprague-Dawley rats. Mean (+/-S.E.) intrinsic clearances (Vmax/Km) in these respective age groups by liver P450s (4.99+/-0.32, 16.99+/-1.85, and 38.45+/-7.03) and by liver CaEs (0.34+/-0.05, 1.77+/-0.38, and 2.53+/-0.19) and plasma CaEs (0.39+/-0.06, 0.80+/-0.09, and 2.28+/-0.56) increased significantly (p<or=0.05) with age, because of progressive increases in Vmax. Intrinsic clearance of DLM by plasma CaEs and liver P450s reached adult levels by 40 days, but clearance by liver CaEs did not. Hepatic P450s played the predominant role in DLM biotransformation in young and adult rats. The incidence and severity of neurotoxic effects varied inversely with age. Correspondingly, blood DLM areas under the concentration versus time curve (AUCs) and Cmax values progressively decreased with increasing age. Internal exposure to DLM (blood AUCs) was closely correlated with toxic signs (salivation and tremors). The present study provides evidence that the limited metabolic capacity of immature rats contributes to elevated systemic exposure and ensuing neurotoxic effects of DLM.

Pharmacokinetics in older persons

BACKGROUND

Physiologic changes and disease-related alterations in organ function occur with aging. These changes can affect drug pharmacokinetics in older persons.

OBJECTIVE

This article reviews age-related changes in pharmacokinetics and their clinical relevance.

METHODS

A PubMed search was conducted using the terms elderly and pharmacokinetics. Other reviews were also included for literature searching. The review includes literature in particular from 1990 through April 2004. Some articles from before 1990 were included to help illustrate principles of age-related pharmacokinetics.

RESULTS

There are minor changes in drug absorption with aging. The effect of aging on small-bowel transporter systems is not yet fully established. Bioavailability of highly extracted drugs often is increased with age. Transdermal absorption may be delayed, especially in the case of water-soluble compounds. Fat-soluble drugs may distribute more widely and water-soluble drugs less extensively in older persons. Hepatic drug metabolism shows wide interindividual variation, and in many cases, there is an age-related decline in elimination of metabolized drugs, particularly those eliminated by the cytochrome enzyme system. Any decrement in cytochrome enzyme metabolism appears nonselective. Synthetic conjugation metabolism is less affected by age. Pseudocapillarization of the sinusoidal endothelium in the liver, restricting oxygen diffusion, and the decline in liver size and liver blood flow may influence age-related changes in rate of hepatic metabolism. Frailty, physiological stress, and illness are important predictors of drug metabolism in older individuals. Inhibition of drug metabolism is not altered with aging, but induction is reduced in a minority of studies. Renal drug elimination typically declines with age, commensurate with the fall in creatinine clearance. Renal tubular organic acid transport may decline with age, while the function of the organic base transporter is preserved but may be less responsive to stimulation.

CONCLUSION

Changes in pharmacokinetics occur due to age-related physiologic perturbations. These changes contribute to altered dose requirements in older persons, particularly in the case of drugs eliminated by the kidney. Interindividual variation, disease, frailty, and stress may overshadow age-related changes.

Modulation of hepatic and renal drug metabolizing enzyme activities in rats by subchronic administration of farnesol

Farnesol demonstrates antitumor activity in several animal models for human cancer and was being considered for development as a cancer chemopreventive agent. This study was performed to characterize the effects of minimally toxic doses of farnesol on the activity of phase I and II drug metabolizing enzymes. CD((R)) rats (20/sex/group) received daily gavage exposure to farnesol doses of 0, 500, or 1000 mg/kg/day for 28 days; 10 rats/sex/group were necropsied at the termination of farnesol exposure; remaining animals were necropsied after a 28-day recovery period. No deaths occurred during the study, and farnesol had no significant effects on body weight, food consumption, clinical signs, or hematology/coagulation parameters. Modest but statistically significant alterations in several clinical chemistry parameters were observed at the termination of farnesol exposure; all clinical pathology effects were reversed during the recovery period. At the termination of dosing, the activities of CYP1A, CYP2A1-3, CYP2B1/2, CYP2C11/12, CYP2E1, CYP3A1/2, CYP4A1-3, CYP19, glutathione reductase, NADPH/quinone oxidoreductase and UDP-glucuronosyltransferase were significantly increased in the livers of farnesol-treated rats; farnesol also increased the activity of glutathione S-transferase in the kidney. The effects of farnesol on hepatic and renal enzymes were reversed during the recovery period. At the end of the dosing period, increases in absolute and relative liver and kidney weights were seen in farnesol-treated rats. These increases may be secondary to induction of drug metabolizing enzymes, since organ weight increases were not associated with histopathologic alterations and were reversed upon discontinuation of farnesol exposure. Administration of farnesol at doses of up to 1000 mg/kg/day induced reversible increases in the activities of several hepatic and renal drug metabolizing enzymes in rats, while inducing only minimal toxicity. It is concluded that non-toxic or minimally toxic doses of farnesol could alter the metabolism, efficacy, and/or toxicity of drugs with which it is co-administered.

The use of precision-cut liver slices from male Wistar rats as a tool to study age related changes in CYP3A induction and in formation of paracetamol conjugates

Precision-cut liver slices (PCLS) offer a lot of advantages because all heterogeneity and cell-cell interactions within the original tissue matrix are maintained. This in vitro model was used to study the effect of ageing on certain aspects of drug metabolism and liver function in young (3 months), adult (9 months) and old (24 months) Wistar male rats. Protein synthesis, an important liver function, was not modified in young, adult and old rats, suggesting that ageing does not impair liver functionality but it affects some specific targets. Among them, a decrease in total P450 in liver microsomes and the loss of CYP3A23 inducibility in PCLS were clearly observed in old rats as compared to adult rats. Finally, the amount of total paracetamol conjugates was not modified between 9 and 24 months but in old rats, sulfoconjugation of paracetamol, its major route of elimination, was decreased.

Effects of dimephosphone, xydiphone, and ionol on the content and activities of rat liver cytochromes P-450 during long-term treatment with phenobarbital

Effects of dimephosphone, xydiphone, and ionol administered in parallel with phenobarbital on the content of cytochromes P-450 in rat liver and on the rate of C-hydroxylation of diazepam, haloperidol, and prednisolone by rat liver microsomal enzymes were studied in vitro. Dimephosphone, xydiphone, and ionol exhibited similar inductive effects on C-hydroxylation reactions in the CYP P-450 system during treatment with phenobarbital. Xydiphone and ionol in a dose of 1 mmol/kg canceled phenobarbital-induced increase in P-450 cytochrome content in rat liver. Sex-dependent cytochromes P-450 are involved in the prednisolone and haloperidol C-hydroxylation reactions in rats.

Neonatal phenobarbital imprints overexpression of cytochromes P450 with associated increase in tumorigenesis and reduced life span

Perinatal exposure to phenobarbital produces a range of permanent reproductive, growth, locomoter, and learning dysfunctions in animals as well as humans. In addition, the affected individuals exhibit latently expressed (i.e., postpubertal) above normal activity levels of hepatic multicytochrome P450-dependent drug metabolizing enzymes. We report that in spite of apparent normal health for the better part of their lives, daily administration of therapeutic-like doses of phenobarbital to male and female rat pups during the first postpartum week reduced life expectancy by approximately 20%. Necropsy at the time of natural death revealed an associated two- to threefold increase in the incidence of tumors in barbiturate-exposed rats of both sexes and a three- to fourfold increase in urinary tract pathologies in male rats. At 2 yr of age, in agreement with an overexpression of hepatic CYP2C6 and CYP2C7, both in vitro and in vivo drug metabolism was more rapid in the phenobarbital-imprinted male and female animals. Moreover, when the senescent rats were rechallenged with a nominal dose of the barbiturate, males and females neonatally exposed to phenobarbital exhibited a dramatic overinduction of multicytochrome P450-dependent drug metabolizing enzymes as well as an overexpression of individual isoforms of cytochrome P450 implicated in enhanced susceptibility to tumorigenesis. Our findings support the growing realization that many adult diseases have their origins in early life by emphasizing that unlike adults, the new born is "plastic," and even therapeutic drugs may produce "silent" programming defects that subtly, but irrevocably, jeopardize life-long well-being.

Interpulse growth hormone secretion in the episodic plasma profile causes the sex reversal of cytochrome P450s in senescent male rats

Humans as well as other mammals experience an aging-related decline in drug metabolism as well as a diminution in growth hormone secretion. In the case of rats, these events are more pronounced in senescent males, whose expression of male-specific isoforms of cytochrome P450, the major drug-metabolizing enzymes and constituting approximately 60-70% of the total cytochrome P450 in male rat liver, is completely suppressed, whereas female-dependent isoforms are remarkably induced to female-like levels. Overlooked in these independently reported studies is the fact that "signals" inherent in the masculine episodic and female continuous growth hormone profiles regulate expression and/or suppression of the dozen or so sex-dependent cytochrome P450 isoforms in rat liver. Whereas previous studies identified profound reductions in the pulse amplitudes of the masculine growth hormone profile as the cause for the diminished hormone secretion during aging, pulse heights are not recognized by the cytochromes as regulatory signals. Instead, we have shown that just a nominal secretion of growth hormone during the usual growth hormone-devoid interpulse period in the masculine episodic profile can explain the complete repression of male-specific CYP2C11, CYP3A2, and CYP2A2 and induction of female-dependent CYP2C12, CYP2C6, and CYP2A1 observed in senescent male rats.

THE EFFECT OF AGE ON SILDENAFIL BIOTRANSFORMATION IN RAT AND MOUSE LIVER MICROSOMES

Sildenafil [SIL (Viagra); Pfizer, New York, NY] is a widely prescribed agent for erectile dysfunction in men older than 65 years. The present study evaluated experimental models to assess age-dependent changes in SIL biotransformation using hepatic microsomes from male rats and mice ranging from 6 weeks to 26 months of age. The role of specific isoforms in the conversion of SIL to its primary circulating metabolite, UK-103,320 (piperazine N-desmethyl sildenafil) in the mouse was also investigated using immunoinhibitory antibodies. Although CYP2C11 largely mediated UK-103,320 formation in the rat, UK-103,320 formation was principally inhibited by a CYP3A antibody in the mouse. An age-related decrement in metabolite formation rate was observed for both species, although this effect was more pronounced in the old rats (reduced to 7% of young) than in the old mice (reduced to 51% of young). CYP2C expression was assessed by Western blot analysis in rat and mouse livers. Age-related differences in hepatic CYP3A expression in the mouse were also compared with metabolite formation rates in the mouse model. Decrements with age in CYP2C and -3A expression in the aging rodents paralleled the decrements in SIL biotransformation, suggesting that age-related differences in SIL metabolic rate may, in part, reflect differences in expression. Although the role of specific CYP enzymes and the clearance values for this reaction may differ among species, age-related changes in these rodent models are consistent with the reduced clearance of SIL observed in human studies.