Effect of adjuvant-induced arthritis on hepatic drug metabolism in rats

Xenobiotic Metabolising Enzymes: Impact on Pathologic Conditions, Drug Interactions and Drug Design

Background:

The biotransformation of xenobiotics is a homeostatic defensive response of the body against bioactive invaders. Xenobiotic metabolizing enzymes, important for the metabolism, elimination and detoxification of exogenous agents, are found in most tissues and organs and are distinguished into phase I and phase II enzymes, as well as phase III transporters. The cytochrome P450 superfamily of enzymes plays a major role in the biotransformation of most xenobiotics as well as in the metabolism of important endogenous substrates such as steroids and fatty acids. The activity and the potential toxicity of numerous drugs are strongly influenced by their biotransformation, mainly accomplished by the cytochrome P450 enzymes, one of the most versatile enzyme systems.

Objective:

In this review, considering the importance of drug metabolising enzymes in health and disease, some of our previous research results are presented, which, combined with newer findings, may assist in the elucidation of xenobiotic metabolism and in the development of more efficient drugs.

Conclusion:

Study of drug metabolism is of major importance for the development of drugs and provides insight into the control of human health. This review is an effort towards this direction and may find useful applications in related medical interventions or help in the development of more efficient drugs.

Stereoselective Pharmacokinetics and Chiral Inversion of Ibuprofen in Adjuvant-induced Arthritic Rats

2-Arylpropionic acid (2-APA) nonsteroidal anti-inflammatory drugs are commonly used in racemic mixtures (rac) for clinical use. 2-APA undergoes unidirectional chiral inversion of the in vivo inactive R-enantiomer to the active S-enantiomer. Inflammation causes the reduction of metabolic activities of drug-metabolizing enzymes such as cytochrome P450 (P450) and UDP-glucuronosyltransferase. However, it is unclear whether inflammation affects the stereoselective pharmacokinetics and chiral inversion of 2-APA such as ibuprofen (IB). We examined the effects of inflammation on the pharmacokinetics of R-IB and S-IB after intravenous administration of rac-IB, R-IB, and S-IB to adjuvant-induced arthritic (AA) rats, an animal model of inflammation. The plasma protein binding of rac-IB, glucuronidation activities for R-IB and S-IB, and P450 contents of liver microsomes in AA rats were determined. Total clearance (CLtot) of IB significantly increased in AA rats, although the glucuronidation activities for IB, and P450 contents of liver microsomes decreased in AA rats. We presumed that the increased CLtot of IB in AA rats was caused by the elevated plasma unbound fraction of IB due to decreased plasma albumin levels in AA rats. Notably, CLtot of R-IB but not S-IB significantly increased in AA rats after intravenous administration of rac-IB. These results suggested that AA could affect drug efficacies after stereoselective changes in the pharmacokinetics of R-IB and S-IB.

Prediction of metabolic clearance of diclofenac in adjuvant-induced arthritis rats using a substrate depletion assay

1. The purpose of this study was to evaluate drug clearance measured by the metabolic intrinsic clearance (CL(int)) in a substrate depletion assay in comparison with the in vivo clearance (CL(tot)) observed in adjuvant-induced arthritis (AA) rats. 2. After intravenous administration of diclofenac as a model drug, CL(tot) was 2.8-fold higher in AA rats than in control rats. In two different substrate depletion assays with liver microsomes for glucuronidation and hydroxylation, the CL(int) values for glucuronidation was significantly decreased in AA rats to 60% of the value in control rats, whereas the CL(int) values for hydroxylation were similar. The unbound fraction of diclofenac in plasma (f(u, plasma)) was significantly higher (2.8-fold) in AA rats than in control rats. 3. Hepatic clearance predicted from the CL(int) values for both biotransformation pathways and f(u, plasma) was higher in AA rats than in control rats, with good consistency between predicted and observed values. The same results were obtained for experiments using hepatocytes. 4. The plasma protein-binding activities, rather than metabolic clearance, in both types of rats would be a determining factor in the pharmacokinetic behaviour differences between control and AA rats. 5. In summary, substrate depletion assays with liver microsomes and hepatocytes in combination with protein binding assessment can help to predict changes in pharmacokinetics under AA conditions.

Selective downregulation of hepatic cytochrome P450 expression and activity in a rat model of inflammatory pain

PURPOSE

This study was designed to examine the effect of Freund's complete adjuvant (FCA)-induced inflammation on liver P450 expression and activities in the first 7 days that followed a single FCA injection in the rat hindpaw.

METHODS

Rats were humanely sacrificed at regular time points, plasma and liver samples were collected, liver mRNA extracted, and liver microsomes prepared.

RESULTS

FCA injection led to the development of an acute inflammatory response evidenced by paw edema and increased alpha-1-acid glycoprotein (AGP) and total-nitrite (NOx) plasma concentrations. Plasma IL-6 levels were significantly higher in FCA-treated rats than in controls at 8 h post-FCA. Within 24 h, these changes were accompanied by a rapid decrease in total P450 contents in FCA-treated rat liver and the selective downregulation of specific CYP isoforms, as illustrated by decreased mRNA levels (CYP2B, CYP2CI1, CYP3A1, and CYP2E1), protein contents (CYP2B, CYP2C11, and CYP2E1) or catalytic activities (CYP2C6, CYP2C11, and CYP2E1). CYP3A1 mRNA levels were severely decreased by FCA administration, whereas CYP3A2 mRNA and protein levels remained unchanged.

CONCLUSIONS

These early biochemical and metabolic modifications may have pharmacokinetic and pharmacodynamic consequences when hepatically cleared drugs are administered to FCA-treated rats, especially within the first 24-72 h post-FCA.

Pharmacokinetics of meloxicam in plasma and urine of horses

OBJECTIVE

To determine pharmacokinetic parameters for meloxicam, a nonsteroidal anti-inflammatory drug, in horses.

ANIMALS

8 healthy horses.

PROCEDURE

In the first phase of the study, horses were administered meloxicam once in accordance with a 2 x 2 crossover design (IV or PO drug administration; horses fed or not fed). The second phase used a multiple-dose regimen (daily oral administration of meloxicam for 14 days), with meloxicam administered at the recommended dosage (0.6 mg/kg). Plasma and urine concentrations of meloxicam were measured by use of validated methods with a limit of quantification of 10 ng/mL for plasma and 20 ng/mL for urine.

RESULTS

Plasma clearance was low (mean +/- SD; 34 +/- 0.5 mL/kg/h), steady-state volume of distribution was limited (0.12 +/- 0.018 L/kg), and terminal half-life was 8.54 +/- 3.02 hours. After oral administration, bioavailability was nearly total regardless of feeding status (98 +/- 12% in fed horses and 85 +/- 19% in nonfed horses). During once-daily administration for 14 days, we did not detect drug accumulation in the plasma. Meloxicam was eliminated via the urine with a urine-to-plasma concentration that ranged from 13 to 18. Concentrations were detected for a relatively short period (3 days) after administration of the final daily dose.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this study support once-daily administration of meloxicam regardless of the feeding status of a horse and suggest a period of at least 3 days before urine concentrations of meloxicam reach concentrations that could be used in drug control programs.

Differences in pharmacokinetics and hepatobiliary transport of a novel anti-inflammatory agent between normal and adjuvant arthritis rats

1. The pharmacokinetics, particularly the hepatobiliary transport of T-5557 ((3-methyl-2-oxo-piperadin-3-yl)-acetic acid N'-(3-thieophen-2-yl-8-methoxy-quinazolin-1-yl)-hydrazide), a novel anti-inflammatory agent, has been examined in normal and adjuvant arthritis (AA) rats. 2. Following oral administration of T-5557, the absolute bioavailability in AA rats was increased by sixfold compared with normal rats. The extent of binding T-5557 to plasma proteins obtained from AA rats was markedly greater than in normal rats (97.0 versus 88.2%). The biliary clearance in AA rats was significantly lower than that in normal rats (1.186 versus 5.621 ml min(-1) kg(-1)), and lower intrinsic biliary clearance was also observed in AA rats (40.33 versus 69.83 ml min(-1) kg(-1)). 3. Concomitant administration of T-5557 with quinidine, a potent P-glycoprotein inhibitor, to normal rats caused a significant decrease in the biliary clearance of T-5557 by 37.9%. Moreover, the transport of T-5557 for the apical-to-basal compartment in a Caco-2 cells' monolayer was fourfold lower than that for the opposite direction, and was increased in the presence of quinidine and verapamil. 4. These results suggest that P-glycoprotein is involved in the biliary excretion of T-5557 and the decrease in the transport activity as well as the increase in plasma protein binding caused the elevated plasma concentration and bioavailability of T-5557 in AA rats.

Decreased hepatobiliary transport of methotrexate in adjuvant arthritis rats

1. We investigated the difference in hepatobiliary transport of methotrexate in normal and adjuvant arthritis (AA) rats and substantiated the expression level of multidrug resistance-associated protein 2 (MRP2) in the liver. 2. Biliary clearance of methotrexate in normal and AA rats was calculated from plasma concentrations and biliary excretion following intravenous infusion and hepatic uptake clearance was estimated from an integration plot using methotrexate concentrations in plasma and liver. 3. Biliary clearance of methotrexate in AA rats was 2.30 +/- 0.23 ml min(-1) kg(-1) (mean SD) and significantly lower than in normal rats (8.42 +/- 0.81 ml min(-1) kg(-1)). The uptake clearance of methotrexate in AA rats was also lower than in normal rats (0.138 versus 0.278 ml min(-1) g liver(-1)). 4. MRP2 in the liver was detected by fluorescein isothiocyanate-labelled antibody and visualized using a confocal laser microscope system. The expression level of MRP2 in AA rats was very low compared with normal rats, indicating a down-regulation in AA rats. 5. In conclusion, biliary clearance of methotrexate was decreased due to the lower activities in both uptake and canalicular secretion, suggesting that several active transporters in the liver, including MRP2, are down-regulated in AA rats.

Tissue-specific regulation of expression and activity of P-glycoprotein in adjuvant arthritis rats

Cyclosporine A and steroids are effective against rheumatoid arthritis and also known as substrates of P-glycoprotein (P-gp). We investigated the effect of arthritis on the hepatic and intestinal P-gp activity in rats, and substantiated the expression level of the hepatic P-gp. Doxorubicin was used as a P-gp substrate. Cumulative biliary excretion and intestinal exsorption of doxorubicin following intravenous administration were compared between adjuvant arthritis (AA) and normal rats. Intestinal P-gp activity was also investigated by intestinal everted sac method, and hepatic P-gp was detected by FITC-labeled antibody and visualized using a confocal laser microscope system. Biliary clearance of doxorubicin in AA rats was significantly decreased from that in normal rats. The expression level of the hepatic P-gp in AA rats was very low compared to normal rats, indicating down-regulation. Intestinal exsorption clearance was not different between AA and normal rats. Permeability of doxorubicin across intestinal everted sac was comparable between AA and normal rats, corresponding to in vivo study. In AA rats, hepatic P-gp activity was decreased due to the reduction of expression level, but intestinal P-gp activity was not changed. Different regulation systems may be involved in liver and intestine.

Alteration of drug biotransformation and elimination during infection and inflammation

During infection or inflammation, the expression of cytochrome P450 and its dependent biotransformation pathways are modified. This results in a change in the capacity of the liver to handle drugs and in alterations in the production and elimination of endogenous substances throughout the body. The majority of the CYP isoforms are modified at pre-translational steps in protein synthesis, and, in most cases, cytokines are involved as mediators of the response. Recent information suggests that inflammatory responses that are localized to the CNS cause a loss of CYP within the brain. This is accompanied by a parallel down-regulation of CYP in peripheral organs that is mediated by a signaling pathway between the brain and periphery. This review covers the loss that occurs in the major mammalian CYP families in response to infection/inflammation and the mediator pathways that are key to this response.

A pharmacokinetic/pharmacodynamic approach vs. a dose titration for the determination of a dosage regimen: the case of nimesulide, a Cox-2 selective nonsteroidal anti-inflammatory drug in the dog

The present experiment was designed to determine a dosage regimen (dose, interval of administration) in the dog for nimesulide, a nonsteroidal anti-inflammatory drug with in vitro selectivity for the inhibition of cyclo-oxygenase 2 (Cox-2), using a pharmacokinetic/pharmacodynamic (PK/PD) approach. The PK/PD results were compared with those obtained using a classical dose titration study. In the PK/PD experiment, 11 dogs were subjected to Freund's adjuvant arthritis characterized by permanent hyperthermia. Nimesulide (5 mg/kg, oral route) was tested during the secondary phase of the inflammatory response. In the dose titration study, nimesulide (0, 3, 6 and 9 mg/kg, oral route) was tested in eight other dogs using a reversible urate crystal arthritis in a 4-period crossover design. Different PD endpoints (including lameness assessed by force plate and hyperthermia) were regularly measured during the PK/PD experiment, and plasma samples were obtained to determine the plasma nimesulide concentration. The data were modeled using an indirect effect model. The IC50 of nimesulide for lameness was 6.26 +/- 3.01 microg/mL, which was significantly higher than the EC50 value obtained for antipyretic effect (2.72 +/- 1.29 microg/mL). The ED50 estimated from the classical dose titration study were 1.34 mg/kg (lameness) and 3.0 mg/kg (skin temperature). The PK/PD parameters were used to simulate different dosage regimens (dose, interval of administration). The antipyretic and anti-inflammatory effects were calculated from the model for the recommended dosage regimen (5 mg/kg/24 h). It was apparent from this approach, that this dosage regimen enabled 76% of the theoretical maximal drug efficacy to be obtained for pyresis and 43% for lameness. It was concluded from the comparison of in vivo and in vitro IC50, that nimesulide is a potent NSAID for which some Cox-1 inhibition is required to obtain clinically relevant efficacy.

Salicylate hydroxylation as an indicator of hydroxyl radical generation in dextran sulfate-induced colitis

Reactive oxygen and nitrogen species have been implicated as mediators of mucosal injury in inflammatory bowel disease. This study investigated hydroxyl radical (.OH) generation in the inflamed colon of dextran sulfate sodium (DSS)-induced colitis by measuring the .OH-specific product of salicylate hydroxylation, 2,3-dihydroxybenzoic acid (DHB). Colitis was induced in 6-7 week old CBA/H male mice by supplementing the drinking water with 5% DSS for 7 days. On the last day of dextran exposure, mice were injected with salicylate (SAL) (100 mg/kg i.p.) 60 min before sacrifice, and mucosal homogenates were assayed for SAL and 2,3-DHB by HPLC with fluorescence and electrochemical detection. Mucosal 2,3-DHB levels in mice exposed to 5% DSS were increased by 83% (p < .005); however, SAL levels were also elevated by 182% (p < .001). This translated to a 34% decrease in the ratio 2,3-DHB:SAL in inflamed mucosa, possibly indicating greater catabolism or decreased production of 2,3-DHB. In vitro investigation of the stability of DHBs and SAL in the presence of oxidants of inflammatory lesions revealed that 2,3-DHB and 2,5-DHB were rapidly degraded by hypochlorous acid (HOCl), with initial decomposition rates of 190 and 281 nmol/min, respectively (100microM DHB with 200microM HOCl). Methionine prevented decomposition of DHBs in vitro; however, in mice with 5% DSS-induced colitis, where mucosal myeloperoxidase activity was ten-fold control levels (p < .001), administration of methionine (up to 200 mg/kg i.p.) with SAL was ineffective at increasing the ratio 2,3-DHB:SAL. SAL was also degraded in vitro by HOCl (4.7 nmol/min) resulting in the formation of new fluorescent species which may be useful as indicators of HOCl-mediated injury. Salicylate hydroxylation was unable to provide conclusive evidence supporting a role for .OH in the tissue injury of DSS-induced colitis, as metabolic disturbances in the diseased animals other than changes in .OH generation may have altered 2,3-DHB levels. This problem is relevant to any study involving the in vivo use of trapping molecules. In particular, the susceptibility of 2,3-DHB to degradation by HOCl brings into question the usefulness of salicylate hydroxylation for measurement of .OH-generation in any neutrophilic inflammatory lesion.

Liver haem metabolism in adjuvant-induced arthritic rats

1. Adjuvant-induced arthritic (AA) rats show a striking decrease in the level of cytochrome P450, a key microsomal haemoprotein involved in electron transport and drug metabolism in the liver. In the present study, we examined the relationship between the reduction of P450 content and haem metabolism in the liver of AA rats. 2. The activities of many enzymes catalyzing the biosynthesis of haem in the liver were significantly higher in AA rats than in normal rats, whereas only coproporphyrinogen oxidase activity in AA rats was markedly lower than that in normal rats. Furthermore, the activity of haem oxygenase, a key enzyme in the haem degradative pathway, increased significantly in AA rats. In addition, the degree of increase in the activity of this enzyme was clearly higher than that in the activity of 5-aminolevulinate synthase, a key enzyme in the haem synthetic pathway. 3. These results suggest that the reduction of live P450 content in AA rats is based on the lowering of liver haem content due to the combined action of the increased haem oxygenase activity and the decreased coproporphyrinogen oxidase activity. The changes in these enzyme activities were apparently suppressed by the continuous administration of indomethacin, which improved the arthritic states of the animals.