Interspecies variations in small intestinal and hepatic drug hydroxylation and glucuronidation

Effect of oral administration of robenacoxib on inhibition of paracentesis-induced blood-aqueous barrier breakdown in healthy cats

OBJECTIVE To determine the effect of oral administration of robenacoxib on inhibition of anterior chamber paracentesis (ACP)-induced breakdown of the blood-aqueous barrier (BAB) and assess whether robenacoxib can cross an intact BAB in healthy cats. ANIMALS 12 healthy adult domestic shorthair cats. PROCEDURES Cats received robenacoxib (6-mg tablet in a treat, PO; n = 6) or a control treatment (treat without any drug, PO; 6) once daily for 3 days, beginning 1 day before ACP. One eye of each cat served as an untreated control, whereas the other underwent ACP, during which a 30-gauge needle was used to aspirate 100 μL of aqueous humor for determination of robenacoxib concentration. Both eyes of each cat underwent anterior chamber fluorophotometry at 0 (immediately before), 6, 24, and 48 hours after ACP. Fluorescein concentration and percentage fluorescein increase were used to assess extent of ACP-induced BAB breakdown and compared between cats that did and did not receive robenacoxib. RESULTS Extent of BAB breakdown induced by ACP did not differ significantly between cats that did and did not receive robenacoxib. Low concentrations of robenacoxib were detected in the aqueous humor (mean, 5.32 ng/mL; range, 0.9 to 16 ng/mL) for 5 of the 6 cats that received the drug. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that oral administration of robenacoxib did not significantly decrease extent of BAB breakdown in healthy cats. Detection of low robenacoxib concentrations in the aqueous humor for most treated cats indicated that the drug can cross an intact BAB.

Coadministration of lapatinib increases exposure to docetaxel but not doxorubicin in the small intestine of mice

Combination therapy is increasingly being utilized for the treatment of metastatic breast cancer. However, coadministration of drugs, particularly agents that are substrates for or inhibitors of p-glycoprotein, can result in increased tissue toxicity. Unfortunately, determination of levels of chemotherapeutics in human tissues is challenging, and plasma drug concentrations are not always indicative of tissue toxicokinetics or toxicodynamics, especially when tissue penetration is altered. The aim of the present work was to determine whether concomitant administration of compounds currently being combined in clinical trials for metastatic breast cancer treatment alters plasma and tissue pharmacokinetics in mice if both agents are p-glycoprotein substrates and/or inhibitors. Accordingly, we investigated the pharmacokinetic interactions of the classic cytotoxics and p-glycoprotein substrates docetaxel and doxorubicin when administered concurrently with the targeted agent and p-glycoprotein inhibitor lapatinib. Our time-course plasma and tissue distribution studies showed that coadministration of lapatinib with doxorubicin did not appreciably alter the pharmacokinetics of this anthracycline in the plasma or six tissues evaluated in mice, presumably because, at doses relevant to human exposure, lapatinib inhibition of p-glycoprotein did not significantly alter doxorubicin transport out of these tissue compartments. However, combining lapatinib with docetaxel significantly increased intestinal exposure to this chemotherapeutic, which has clinical implications for enhancing gastrointestinal toxicity. The significant lapatinib-docetaxel interaction is likely CYP3A4-mediated, suggesting that caution should be exercised when this combination is administered, particularly to patients with compromised CYP3A activity, and recipients should be monitored closely for enhanced toxicity, particularly for adverse effects on the intestine.

Physiologically based pharmacokinetic model of lapatinib developed in mice and scaled to humans

Lapatinib is an oral 4-anilinoquinazoline derivative that dually inhibits epidermal growth factor receptor and human epidermal growth factor receptor 2 (HER2). This drug is a mere decade old and has only been approved by the FDA for the treatment of breast cancer since 2007. Consequently, the intricacies of the pharmacokinetics are still being elucidated. In the work presented herein, we determined the biodistribution of orally administered lapatinib in mouse plasma, brain, heart, lung, kidney, intestine, liver, muscle and adipose tissue. Using this data, we subsequently developed a physiologically based pharmacokinetic (PBPK) model of lapatinib in mice that accurately predicted the tissue concentrations after doses of 30, 60 and 90 mg/kg. By taking into account interspecies differences in physiology and physiochemistry, we then extrapolated the mouse PBPK model to humans. Our model predictions closely reflected lapatinib plasma pharmacokinetics in healthy subjects. Additionally, we were also able to simulate the pharmacokinetics of this drug in the plasma of patients with solid malignancies by incorporating a decrease in liver metabolism into the model. Finally, our PBPK model also facilitated the estimation of various human tissue exposures to lapatinib, which harmonize with the organ-specific toxicities observed in clinical trials. This first-generation PBPK model of lapatinib can be further improved with a greater understanding of lapatinib absorption, distribution, metabolism and excretion garnered from subsequent in vitro and in vivo studies and expanded to include other pharmacokinetic determinants, including efflux transporters, metabolite generation, combination dosing, etc., to better predict lapatinib disposition in both mouse and man.

Comparative oxidative metabolic profiles of clomipramine in cats, rats and dogs: preliminary results from an in vitro study

The objectives of this in vitro study were to describe cytochrome-dependent metabolism of clomipramine in canine and feline microsomes, compare metabolic profiles between cats, rats and dogs, and investigate a potential gender-related difference in metabolic activity between male and female cats. Pooled liver microsomes were incubated with clomipramine, where species and gender-specific reactions were initiated by the addition of a nicotinamide adenine dinucleotide phosphate regenerating system and quenched with methanol at 0, 5, 15, 30, 45 and 60 min, and 0, 30, 60, 90, 120, 180, 240 and 360 min respectively. Liquid chromatography tandem mass spectrometry was used to measure clomipramine and its metabolites. Preliminary results showed that cat microsomes biotransformed clomipramine slower and less efficiently than rat and dog microsomes. Moreover, gender differences in metabolic profiles suggested that male cat microsomes may be less efficient demethylators and hydroxylators than female cat microsomes. As gender metabolic differences may carry clinical significance for this antidepressant, further studies are warranted.

Nonsteroidal anti-inflammatory drugs in cats: a review

OBJECTIVE

To review the evidence regarding the use of nonsteroidal anti-inflammatory drugs (NSAIDs) in cats.

DATABASES USED

PubMed, CAB abstracts.

CONCLUSIONS

Nonsteroidal anti-inflammatory drugs should be used with caution in cats because of their low capacity for hepatic glucuronidation, which is the major mechanism of metabolism and excretion for this category of drugs. However, the evidence presented supports the short-term use of carprofen, flunixin, ketoprofen, meloxicam and tolfenamic acid as analgesics in cats. There were no data to support the safe chronic use of NSAIDs in cats.

Intestinal UDP-glucuronosyltransferase activities in rat and rabbit

1. Tissues other than the liver can contribute significantly to the drug-metabolizing capacity of an animal. In the current study, the glucuronidation of several aglycones in microsomes from the small intestinal mucosa of rat and rabbit has been investigated and compared with glucuronidation in liver microsomes. 2. UDP-glucuronosyltransferase activities in intestinal microsomes were generally higher in rabbit when compared with rat, ranging from 200 to 300% for 1-naphthol, 2-naphthol, 4-methylumbelliferone, 2-hydroxybiphenyl and 4-hydroxybiphenyl. 3. In contrast, hepatic activities were much higher in rat than in rabbit, ranging from 300 to 400% for 1-naphthol, 2-naphthol, 4-methylumbelliferone, 2-hydroxybiphenyl and testosterone; and from 150 to 250% for 4-nitrophenol and diclofenac. 4. In rabbit, activities in the small intestinal mucosa were comparable (70-100%) with hepatic activities for most aglycones. In rat, intestinal mucosa activities were much lower than in liver, with activities toward 1-naphthol, 2-naphthol, 4-nitrophenol, 4-methylumbelliferone, 2-hydroxybiphenyl and 4-hydroxybiphenyl in the small intestine representing 5-15% of hepatic activities. 5. With a higher intestine:liver activity ratio, intestinal UDP-glucuronosyltransferases could be anticipated to contribute more to overall drug glucuronidation in rabbit as compared with rat, thereby contributing more to reducing drug bioavailability.

Effects of microsomal enzyme inducers upon UDP-glucuronic acid concentration and UDP-glucuronosyltransferase activity in the rat intestine and liver

This study was conducted to evaluate UDP-glucuronosyl-transferase (UDP-GT) activity, UDP-glucuronic acid (UDP-GA) concentration, and UDP-glucose (UDPG) concentration in the rat intestine and liver following oral administration of butylated hydroxyanisole (BHA), benzo[a]pyrene (BaP), 3-methylcholanthrene (3MC), phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), or trans-stilbene oxide (TSO). Microsomal UDP-GT activity was assayed in vitro with acetaminophen (AA), harmol (HA), and 1-naphthol (NA) as the aglycones. Intestinal HA and AA glucuronidation were enhanced by BHA, BaP, and TSO, whereas 3MC, PB, PCN, and TCDD augmented hepatic HA-glucuronide formation and BHA, PB, PCN, TCDD, and TSO significantly increased hepatic AA glucuronidation. All inducing agents except PB and PCN markedly increased both intestinal and hepatic NA glucuronidation. PB, PCN, and TCDD paradoxically decreased intestinal glucuronidation of AA and HA. A similar effect upon hepatic glucuronidation was not observed with any of the agents studied. Hepatic UDP-GA concentration was increased significantly by all inducers studied except PCN and TCDD, whereas hepatic UDPG concentration was increased only by BHA. In the intestine, significant increases in UDP-GA concentration were produced only by BHA and BaP, which also elevated intestinal UDPG. These results demonstrate that microsomal enzyme inducers evoke different effects upon intestinal and hepatic glucuronidation. These differences are manifested with regard to induced changes in UDP-GT activity as well as treatment-induced alterations in UDP-GA content. Thus, the present study further underscores the marked variance of intestinal and hepatic xenobiotic glucuronidation.

Review: first-pass metabolism by the gastrointestinal mucosa

The bioavailability of orally administered drugs may be reduced due to presystemic elimination. The first-pass effect can occur in the gastrointestinal tract, the liver and lung. Although the liver is the main drug metabolizing organ in the body, the gut wall can play an important role in the first-pass metabolism of certain drugs. Both phase I (preconjugation) and phase II (conjugation) reactions have been described. However, while the oxidative metabolic capacity of the intestinal mucosa is considerably smaller than that of the liver, the activity of conjugation reactions in the gut may be close to that of the liver, and in some cases may exceed it. Sulphate conjugation is particularly important for steriod hormones such as ethinyloestradiol, and for the beta-adrenoceptor stimulants isoprenaline and isoetharine. Glucuronidation has been demonstrated to occur in man for morphine, paracetamol and oestrogens. Significant drug--drug interactions have been described involving drugs undergoing sulphate conjugation. The study of intestinal metabolism in vivo is difficult in man since direct methods (for example, hepatic portal vein catheterization) is justified in only a small number of patients. Therefore, much of our present understanding has been derived from various in-vitro studies involving intestinal sheets, mucosal biopsies, isolated enterocytes and microsomal preparations.

Carcinogen metabolizing enzymes in nude mice

Hepatic and cutaneous microsomal hydroxylating and conjugating enzymes from female and male NMRI nu/nu mice were analyzed, and the response of these enzymes to repeated exposures with 3-methylcholanthrene were studied. Sex differences were observed in basal activities of hydroxylating enzymes. These differences were not the same in the liver as in the skin and were reversed in female and male. Activity ratios of hepatic and cutaneous hydroxylating enzymes were between 40-200. The ratios between hydroxylating and conjugating enzymes were much lower in the skin than in liver. Furthermore the ratios were depending on hydroxylating enzymes. 3-Methylcholanthrene treatment increased both hydroxylating and conjugating enzymes in the liver and in the skin. Again, there were sex differences in the induction pattern, and also the induction in the liver was unrelated to that in the skin. There was no correlation in induction between hydroxylating and conjugating enzymes.

Dietary influences on rat hepatic and intestinal DT-diaphorase activity

DT-diaphorase (DTD) is a flavoprotein that catalyses the two-electron reduction of various redox dyes and quinones such as menadione and phylloquinone. It has been proposed that this enzyme may have a protective effect against cancer, as the two-electron reduction prevents the formation of toxic oxygen metabolites that may be generated as a result of the one-electron reduction catalysed by enzymes such as NADPH-cytochrome P-450 reductase. The effects of a purified diet supplemented with either 25% Brussels sprouts, phylloquinone (2.5 or 25 ppm) or 250 ppm indole-3-carbinol on hepatic and intestinal DTD activity in adult male Sprague-Dawley rats have been determined. One group was fed on the purified diet and dosed with 3-methylcholanthrene (20 mg/kg), 24 hr before being killed. Hepatic DTD activity was increased 3-fold in the indole-3-carbinol group, 4.4-fold in the sprouts-fed animals and 8.2-fold in the 3-methylcholanthrene-treated animals. Neither level of phylloquinone affected hepatic DTD activity. Intestinal DTD activity was increased 2.1-fold in the indole-3-carbinol group, 3.7-fold in the sprouts-fed animals and 4.3-fold in the 3-methylcholanthrene group. In animals given 25 ppm phylloquinone, intestinal enzyme activity was 60% of the control level, while no effect was noted in those given 2.5 ppm phylloquinone. Although increases in the activities of intestinal xenobiotic-metabolizing enzymes resulting from dietary influences are well documented, the increase in hepatic DTD activity seen in response to vegetable consumption has not been reported. The significance of these results in relation to the possible protective effects of dietary cruciferous vegetables against cancer is discussed.

The effects of dietary brussels sprouts and Schizandra chinensis on the xenobiotic-metabolizing enzymes of the rat small intestine

After an initial equilibration period of 7 days on a semi-synthetic basal diet, male Sprague-Dawley rats were fed for 2 wk on either the basal diet (controls), the basal diet containing 5% Schizandra chinensis or 25% Brussels sprouts, or on rat chow. One group of chow-fed rats was pretreated with 20 mg 3-methylcholanthrene (3-MC)/kg body weight, 24 hr before they were killed. Microsomal and cytosolic fractions were prepared from small intestine mucosa. Microsomes were assayed for cytochrome P-450, aryl hydrocarbon hydroxylase (AHH), ethoxycoumarin O-deethylase (ECD) and epoxide hydrolase (EH) activities, and for metabolism of benzo[a]pyrene (BaP), Cytosols were assayed for glutathione S-transferase (GST) activity. The largest increase in intestinal mixed-function oxidase activity over levels in the controls was seen in the 3-MC-treated group. However, EH and GST activities in these animals were not significantly increased. Increases in cytochrome P-450 levels and significant increases in AHH, ECD, EH and GST activities occurred in the rats fed Brussels sprouts. Rats in the S. chinensis group showed inhibition of AHH activity relative to controls, but increased activity of ECD, EH and GST. In the rats fed chow there were significant increases in the activities of all the enzymes assayed except GST. The percentage conversion of BaP to metabolites reflected the results of the AHH assay and the groups were ranked in the following order: 3-MC greater than Brussels sprouts greater than rat chow greater than basal diet greater than S. chinensis. The profile of BaP metabolites showed a larger proportion of the BaP diols and 3-hydroxybenzo[a]pyrene, and a smaller proportion of BaP-4,5-epoxide and the BaP quinones, for the Brussels sprouts- and S. chinensis-fed groups. The significance of these results is discussed in regard to the role of the small intestine as a mediator of toxicity induced by ingested chemicals.

Structural and biotransformational membrane changes in the liver and intestine during chronic ethanol administration

The binding of a fluorescent probe 1-anilinonaphthalene-8-sulphonic acid (1,8-ANS) to liver microsomal membranes was markedly increased after chronic ethanol administration while the binding of a non-ionised probe phenylnaphthylamine (PNA) was not altered. The increase in 1,8-ANS binding is in accordance with the simultaneous increase of the ethoxycoumarin O-de-ethylase activity and cytochrome P-450 concentration. Also the intestinal ethyoxycoumarin O-de-ethylase activity and cytochrome P-450 concentration were increased. No changes in the aryl hydrocarbon hydroxylase or UDP-glucuronosyltransferase activities were found. The chronic ehtanol administration increased the phospholipid amount in the liver microsomes and altered the fatty acid composition of microsomal phospholipids by decreasing the amount of oleic acid and increasing linoleic acid proportion. The data suggest that chronic ethanol administration may effect the biotransformation enzyme activities by changing the structural properties of the membranes as well as increasing the cytochrome P-450 concentration.

Intestinal absorption and metabolism of xenobiotics

There are five possible processes of intestinal absorption of xenobiotics. These are active transport, passive diffusions, pinocytosis, filtration through "pores," and lymphatic absorption. The passive diffusion is major process for transport of foreign chemicals across the intestine. Though the lymphatic absorption of drugs is not of any major therapeutic significance, the uptake of toxic chemicals such as 3-MC, benzpyrene, and DDT through lymphatics may enhance their toxicity, since they are distributed to other organ systems in the body without being metabolized by liver. A number of factors such as diet, motility of intestine, interference with gastrointestinal flora, changes in the rate of gastric emptying, age of the animal, and dissolution rate of xenobiotic can alter the rate of absorption of chemicals. Liver is the major site of metabolism of xenobiotics, but the contribution of intestinal metabolism of xenobiotic can influence the overall bioavailability of chemicals. The xenobiotic metabolizing enzymes located in endoplasmic reticulum of intestine possess biochemical characteristics similar to that of liver. In general, the rate of metabolism of xenobiotics by intestinal microsomal preparation is lower than that observed with similar hepatic microsomal preparations. The in vitro intestinal metabolism of xenobiotics is affected by several factors including age, sex, diurnal variations, species, and nutritional status of the animal. The intestinal xenobiotic metabolizing enzymes are stimulated by the pretreatment of animals with foreign chemicals, but this depends on the route of administration of chemicals, drug substrate and the animal species used. Rabbit intestinal drug metabolizing enzymes seem to be resistant to induction by foreign chemicals.

Species difference and characterization of intestinal esterase on the hydrolizing activity of ester-type drugs

The ability of the esterase from intestine was studied for hydrolysis of ester-type drugs during absorption. The intestinal esterase is present in the absorption sites in the intestine and hydrolyzes to a large extent during the absorption. In a study of the dietary effect on intestinal esterase, the esterase activity increased in rats fed a high-fat diet, decreased in those fasted or fed a fat-free diet, whereas the esterase activity in the rat treated with phenobarbital showed no marked change. Thus the esterase from intestinal mucosa appears to be characteristically quite different from hepatic esterase. The esterase from human intestine was characterized and compared with esterase from rats, mice, rabbits, guinea pigs and dogs. There was a difference in the substrate specificity of the esterase and there were significant species differences in the electrophoretic behavior of the enzyme among the species tested. These results indicate that intestinal esterase from humans differs characteristically from esterases in experimental animals.

Effect of administration route on DDT on acute toxicity and on drug biotransformation in various rodents

DDT was administered to the guinea pig, mouse and rat either ig or ip and to the hamster ig in order to investigate variations in the response of hepatic and duodenal drug-metabolizing enzymes to DDT. The intragastric dose (160 mg/kg) was found to produce gastric bleeding and severe tremor in rats and mice but not in other rodents. The hepatic aryl hydrocarbon hydroxylase activity and cytochrome P-450 concentration decreased after the ig administration of DDT to rats, mice and guinea pigs but in hamsters the activiy of aryl hydrocarbon hydroxylase and cytochrome P-450 concentration increased 12 hr after the dosage. The aryl hydrocarbon hydroxylase activity decreased also in the duodenal mucosa of the rat after the ig administration of DDT. The ip dose had no effects on the hepatic or duodenal monooxygenase system in 12 hr. The UDPglucuronosyltransferase activity was slightly lowered in hepatic microsomes of the rat and mouse after the ig dose of DDT, but the decrease was more profound when measured after in vitro trypsin digestion of microsomes. The trypsin digestion activated the hepatic UDPglucuronosyltransferase in all the species studied, i.e., guinea pig, hamster, mouse and rat (3-, 3-, 5-, and 8-fold, respectively). The duodenal UDPglucuronosyltransferase activity was not affected by DDT administration in any of the species studied. The results suggest that the acute toxic effects of DDT are species-dependent and the administration route is important in DDT toxicity. The hydroxylation step in drug metabolism is more sensitive to DDT than the glucuronidation step.

Dietary fats and properties of endoplasmic reticulum: II. Dietary lipid induced changes in activities of drug metabolizing enzymes in liver and duodenum of rat

Rats were fed cholesterol, cacao butter, or olive oil diets to determine the effect of dietary lipids on the rate of drug biotransformation in the liver and duodenum. The cholesterol rich diet maintained the hepatic aryl hydrocarbon hydroxylase activity at the same level as did the standard diet. Rats fed olive oil and cacao butter diets showed lower hepatic aryl hydrocarbon hydrorylase activity. The p-nitroanisole O-demethylase activity was doubled in hepatic microsomes of rats fed the high cholesterol diet when compared to rats fed the standard diet. The hepatic uridine diphosphate glucuronosyltransferase activity showed different patterns depending on the in vitro treatment of the microsomal membranes. If the enzyme activity was assayed from the native, untreated microsomes, an increase in the measurable uridine diphosphate glucuronosyl transferase activity was found in rats having cholesterol rich diet. After the in vitro activation of membrane-bound uridine diphosphate glucuronosyltransferase by trypsin, the increase in measurable activity was 10 fold in the group fed the standard diet, 6 fold in group fed cholesterol, 4 fold in group fed cacao butter, and 3 fold in group fed olive oil. Trypsin digestion of microsomes increased the measurable uridine diphosphate glucuronosyltransferase activity less in rats fed diets rich in neutral fats than those fed the standard diet. In the duodenal mucosa, lipid diets decreased the activities of drug hydroxylation and glucuronidation.

Mucosal and hepatic metabolism during the spontaneous disappearance of salicylate-induced gastric erosions

Salicylate-induced gastric erosions have been shown to disappear despite continuing salicylic acid administration in the rat. On the other hand, numerous drugs are able to change the capacity of the gastric mucosa to conjugate xenobiotics, which gives reason to follow gastric resistance to salicylic acid and to correlate it with changes in mucosal rate of drug biotransformation reactions. Gastric and duodenal UDP glucuronyltransferase activity decreased markedly within 12 hours after a single dose of salicylic acid. when continuing salicylic acid administration, macroscopic gastric lesions disappeared within 3 days and mucosal UDP glucuronyltransferase activity increased above control level. In 2 weeks the activity returned to control level. In spite of the fact that salicylates markedly inhibited gastroduodenal glucuronidation in vitro, there was no substrate effect of salicylic acid present at the time rats were killed. Duodenal 3,4-benzpyrene hydroxylase activity was not affected by salicylic acid administration. The gastric activity of benzpyrene hydroxylase in controls and in rats treated with salicylic acid was below the sensitivity of the method. Hepatic detoxification capacity was quite stable. A slight depression of 3,4-benzpyrene hydroxylase activity did, however, take place within 2 weeks. Gastric and duodenal protein contents decreased after a single salicylic acid administration, but returned to control level in 5 days in the duodenum, and in 2 weeks in the stomach, when the administration was prolonged. The results suggest that mucosal detoxification capacity may have a role in the pathogenesis of drug-induced gastric erosions. Gastric mucosa adapts to repeated salicylic acid administration, having reduced susceptibility to drug-induced erosions.