Limitations of hepatocytes and liver homogenates in modelling in vivo formation of acyl glucuronide-derived drug-protein adducts

Patients with naproxen-induced liver injury display T-cell memory responses toward an oxidative (S)-O-desmethyl naproxen metabolite but not the acyl glucuronide

BACKGROUND

Exposure to nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (IBU) and naproxen (NAP) is associated with idiosyncratic drug-induced liver injury (DILI). Carboxylate bioactivation into reactive metabolites (e.g., acyl glucuronides, AG) and resulting T-cell activation is hypothesized as causal for this adverse event. However, conclusive evidence supporting this is lacking.

METHODS

In this work, we identify CD4+ and CD8+ T-cell hepatic infiltration in a biopsy from an IBU DILI patient. Lymphocyte transformation test and IFN-γ ELIspot, conducted on peripheral blood mononuclear cells (PBMCs) of patients with NAP-DILI, were used to explore drug-specific T-cell activation. T-cell clones (TCC) were generated and tested for drug specificity, phenotype/function, and pathways of T-cell activation. Cells were exposed to NAP, its oxidative metabolite 6-O-desmethyl NAP (DM-NAP), its AG or synthesized NAP-AG human-serum albumin adducts (NAP-AG adduct).

RESULTS

CD4+ and CD8+ T-cells from patients expressing a range of different Vβ receptors were stimulated to proliferate and secrete IFN-γ and IL-22 when exposed to DM-NAP, but not NAP, NAP-AG or the NAP-AG adduct. Activation of the CD4+ TCC was HLA-DQ-restricted and dependent on antigen presenting cells (APC); most TCC were activated with DM-NAP-pulsed APC, while fixation of APC blocked the T-cell response. Cross-reactivity was not observed with structurally-related drugs.

CONCLUSION

Our results confirm hepatic T-cell infiltrations in NSAID-induced DILI, and show a T-cell memory response toward DM-NAP indicating an immune-mediated basis for the adverse event. Whilst bioactivation at the carboxylate group is widely hypothesized to be pathogenic for NSAID associated DILI, we found no evidence of this with NAP.

Species differences in liver microsomal hydrolysis of acyl glucuronide in humans and rats

Acyl glucuronides (AGs) are known as one of the causes of idiosyncratic drug toxicity (IDT). Although AGs can be enzymatically hydrolysed by β-glucuronidase and esterase, much information on their characteristics and species differences is lacking. This study was aimed to clarify species differences in AG hydrolysis between human and rat liver microsomes (HLM and RLM).To evaluate the AG hydrolysis profile, and the contribution of β-glucuronidase and esterase towards AG hydrolysis in HLM and RLM, nonsteroidal anti-inflammatory drugs (NSAIDs) were used. AGs were incubated with 0.1 M Tris-HCl buffer (pH 7.4) and 0.3 mg/mL HLM or RLM in the absence or presence of β-glucuronidase inhibitor, D-saccharic acid 1,4-lactone (D-SL) and esterase inhibitor, phenylmethylsulfonyl fluoride (PMSF).AGs of mefenamic acid (MEF-AG) and etodolac (ETO-AG) showed significantly higher AG hydrolysis rates in RLM than in HLM. Esterases were found to serve as AG hydrolases dominantly in HLM, whereas both esterases and β-glucuronidase equally contribute to AG hydrolysis in RLM. However, MEF-AG and ETO-AG were hydrolysed only by β-glucuronidase.We demonstrated for the first time that the activity of AG hydrolases towards NSAID-AGs differs between humans and rats.

In vitro and in vivo studies on acyl-coenzyme A-dependent bioactivation of zomepirac in rats

Zomepirac [ZP, 5-(chlorobenzoyl)-1,4-dimethylpyrrole-2-acetic acid] was withdrawn from the market because of unpredictable allergic reactions that may have been caused by ZP-protein adducts formed by reaction of the reactive acyl glucuronide of ZP (ZP-O-G) with endogenous proteins. To test the hypothesis that the reactive ZP acyl coenzyme A thioester (ZP-CoA) was formed and potentially could contribute to formation of ZP-protein adducts, we investigated the acyl CoA-dependent metabolism of ZP in freshly isolated rat hepatocytes (1 mM) and in vivo (100 mg ZP/kg, ip) in rat livers (2 h after dose administration), rat bile (0-4 h), and rat urine (0-24 h). ZP-CoA was detected in freshly isolated hepatocytes and in vivo in rat livers by LC/MS/MS. In addition, the ZP glycine conjugate (ZP-Gly) and ZP taurine conjugates (ZP-Tau) were identified by LC/MS/MS in rat hepatocytes and in vivo in rat livers, rat urine, and rat bile. The identities of ZP-CoA, ZP-Gly, and ZP-Tau were confirmed by comparison of retention times and MS/MS spectra with those of authentic standards. Moreover, the ZP acyl carnitine ester was detected in rat urine and rat bile based upon (i) the chlorine isotope pattern, (ii) MS/MS spectra showing significant ions characteristic for carnitine (m/z 60, 144 and loss of m/z 59) and ZP (m/z 139), and (iii) accurate mass measurements with a mass accuracy of 0.2 ppm. ZP-CoA serves as an obligatory intermediate in the formation of ZP-Gly, ZP-Tau, and ZP carnitine ester, and it is therefore of mechanistic significance that these conjugates were identified. Finally, time-dependent concentration profiles obtained in experiments with rat hepatocytes and in vivo from quantitative analysis of rat livers indicate that ZP-CoA, in addition to ZP-O-G, may contribute to formation of the potentially toxic covalent ZP-protein adducts.

Acyl glucuronide reactivity in perspective: biological consequences

The metabolic conjugation of exogenous and endogenous carboxylic acid substrates with endogenous glucuronic acid, mediated by the uridine diphosphoglucuronosyl transferase (UGT) superfamily of enzymes, leads to the formation of acyl glucuronide metabolites. Since the late 1970s, acyl glucuronides have been increasingly identified as reactive electrophilic metabolites, capable of undergoing three reactions: intramolecular rearrangement, hydrolysis, and intermolecular reactions with proteins leading to covalent drug-protein adducts. This essential dogma has been accepted for over a decade. The key question proposed by researchers, and now the pharmaceutical industry, is: does or can the covalent modification of endogenous proteins, mediated by reactive acyl glucuronide metabolites, lead to adverse drug reactions, perhaps idiosyncratic in nature? This review evaluates the evidence for acyl glucuronide-derived perturbation of homeostasis, particularly that which might result from the covalent modification of endogenous proteins and other macromolecules. Because of the availability of acyl glucuronides for test tube/in vitro experiments, there is now a substantial literature documenting their rearrangement, hydrolysis and covalent modification of proteins in vitro. It is certain from in vitro experiments that serum albumin, dipeptidyl peptidase IV, tubulin and UGTs are covalently modified by acyl glucuronides. However, these in vitro experiments have been specifically designed to amplify any interference with a biological process in order to find biological effects. The in vivo situation is not at all clear. Certainly it must be concluded that all humans taking carboxylate drugs that form reactive acyl glucuronides will form covalent drug-protein adducts, and it must also be concluded that this in itself is normally benign. However, there is enough in vivo evidence implicating acyl glucuronides, which, when backed up by in vivo circumstantial and documented in vitro evidence, supports the view that reactive acyl glucuronides may initiate toxicity/immune responses. In summary, though acyl glucuronide-derived covalent modification of endogenous macromolecules is well-defined, the work ahead needs to provide detailed links between such modification and its possible biological consequences.

Tissue distribution and interindividual variation in human UDP-glucuronosyltransferase activity: relationship between UGT1A1 promoter genotype and variability in a liver bank

The variability in a liver bank and tissue distribution of three probe UDP-glucuronosyltransferase (UGT) activities were determined as a means to predict interindividual differences in expression and the contribution of extrahepatic metabolism to presystemic and systemic clearance. Formation rates of acetaminophen-O-glucuronide (APAPG), morphine-3-glucuronide (M3G), and oestradiol-3-glucuronide (E3G) as probes for UGT1A6, 2B7, and 1A1, respectively, were determined in human kidney, liver, and lung microsomes, and in microsomes from intestinal mucosa corresponding to duodenum, jejunum and ileum. While formation of E3G and APAPG were detectable in human kidney microsomes, M3G formation rates from kidney microsomes approached the levels seen in liver, indicating significant expression of UGT2B7. Interestingly, rates of E3G formation in human intestine exceeded the hepatic rates by several fold, while APAPG and M3G formation rates were low. The intestinal apparent Km value for E3G formation was essentially identical to that seen in liver, consistent with intestinal UGT1A1 expression. No UGT activities were observed in lung. Variability in APAPG and M3G activity across a bank of 20 human livers was modest (< or = 7-fold), compared to E3G formation, which varied approximately 30-fold. The E3G formation rates were found to segregate by UGT1A1 promoter genotype, with wild-type (TA)6 rates significantly greater than homozygous mutant (TA)7 individuals. Kinetic analyses were performed to demonstrate that the promoter mutation altered apparent Vmax without significantly affecting apparent Km. These results suggest that glucuronidation, and specifically UGT1A1 activity, can profoundly contribute to intestinal first pass metabolism and interindividual variability due to the expression of common allelic variants.