Studies on the reactivity of acyl glucuronides--IV. Covalent binding of diflunisal to tissues of the rat

The Burden of Gastrointestinal Complaints in Kidney Transplant Recipients Using Tacrolimus With and Without Mycophenolate Mofetil: A Randomized Controlled Study

Background

Tacrolimus (TAC) combined with mycophenolate mofetil (MMF) is the immunosuppressive regimen in the majority of solid organ transplant recipients. Gastrointestinal complaints are frequent, which is considered predominantly a side effect of MMF. However, systematic research in this field is lacking. The aim of this study is to systematically investigate the burden of gastrointestinal complaints in TAC-treated kidney transplant recipients with and without MMF.

Methods

In a single-center, open-label, randomized controlled trial, low immunological risk recipients were randomized to either TAC and MMF or to TAC monotherapy from 6 months after kidney transplantation onwards [NTR4672],. They filled in the Gastrointestinal Symptom Rating Scale questionnaire, which covers five dimensions (abdominal pain, reflux, indigestion, constipation, and diarrhea), 6, 12, and 15 months after transplantation.

Results

Seventy-nine recipients were randomized and 72 completed all questionnaires (34 TACmono and 38 TAC/MMF). At baseline, the mean age was 59 years with 72% male, mean BMI 28 kg/m2, eGFR 55 ml/min/1.73m2, mean daily dose MMF 1200 mg and TAC 5.8 mg, with trough levels of 2.1 mg/L and 7.4 ug/L. Six months after transplantation, 75% of recipients reported troublesome symptoms (score ≥3). Diarrhea was the most troublesome (mean 3.3) and discontinuing MMF significantly reduced it (mean Δ score between month 6 and 15 TAC/MMF -0.9 vs. TACmono -1.8, p=0.03). In recipients with troublesome symptoms, abdominal pain (2.7 to 1.8, p=0.003), indigestion (2.8 to 2.3, p=0.012), and reflux (2.9 to 1.7, p=0.007) significantly decreased over time, independent of MMF use.

Conclusion

The majority of kidney transplant recipients with TAC and MMF experienced troublesome gastrointestinal symptoms 6 months after transplantation. While constipation remained troublesome, indigestion, abdominal pain, and reflux improved over time by month 15. Diarrhea only improved after discontinuing MMF.

Intestinal tract injury by drugs: Importance of metabolite delivery by yellow bile road

Drug secretion into bile is typically considered a safe route of clearance. However, biliary delivery of some drugs or their reactive metabolites to the intestinal tract evokes adverse consequences due to direct toxic actions or indirect disruption of intestinal homeostasis. Biliary concentration of the chemotherapy agent 5-fluorodeoxyuridine (FUDR) and other compounds is associated with bile duct damage while enterohepatic cycling of antibiotics contributes to the disruptions of gut flora that produce diarrhea. The goal of this review is to describe key evidence that biliary delivery is an important factor in the intestinal injury caused by representative drugs. Emphasis will be given to 3 widely used drugs whose reactive metabolites are plausible causes of small intestinal injury, namely the nonsteroidal anti-inflammatory drug (NSAID) diclofenac, the immunosuppressant mycophenolic acid (MPA), and the chemotherapy agent irinotecan. Capsule endoscopy and other sensitive diagnostic techniques have documented a previously unappreciated, high prevalence of small intestinal injury among NSAID users. Clinical use of MPA and irinotecan is frequently associated such severe intestinal injury that dosage must be reduced. Observations from clinical and experimental studies have defined key events in the pathogenesis of these drugs, including roles for multidrug resistance-associated protein 2 (MRP2) and other transporters in biliary secretion and adduction of enterocyte proteins by reactive acyl glucuronide metabolites as a likely mechanism for intestinal injury. New strategies for minimizing the adverse intestinal consequences of irinotecan chemotherapy illustrate how basic information about key events in the biliary secretion of drugs and the nature of their proximate toxicants can lead to safer protocols for drugs.

Role of benoxaprofen and flunoxaprofen acyl glucuronides in covalent binding to rat plasma and liver proteins in vivo

Benoxaprofen (BNX) has been implicated in rare but serious hepatotoxicity which led to its withdrawal from the world market. Flunoxaprofen (FLX), a structural analog, appears to be less toxic. It has been postulated that the nonsteroidal antiinflammatory drugs associated toxicity may be related to covalent modification of proteins by their reactive acyl glucuronides, and the extent of covalent protein binding depends on both reactivity of the acyl glucuronide and the exposure to the reactive metabolite. The disposition of BNX and FLX in rats were compared upon intravenous administration of 20 mg/kg of BNX, FLX or their metabolites. Covalent binding of BNX and FLX to plasma and liver proteins were also determined, and an immunochemical approach was used to detect their hepatic targets. Similar concentrations of plasma protein adducts for BNX and FLX were detected even though the AUC of BNX-glucuronide (BNX-G) was almost twice that of FLX-glucuronide (FLX-G). Similar concentrations of liver protein adducts for BNX and FLX were also detected at 8 h, however, the hepatobiliary exposure of BNX-G was only 1/3rd that of FLX-G indicating that BNX-G was more reactive than FLX-G, which was in agreement with in vitro data. Proteins of 110 and 70 kDa were the major liver protein targets modified by covalent attachment of BNX and FLX. In conclusion, measuring covalent binding to tissue proteins in animals in addition to plasma adducts should be considered when evaluating and comparing carboxylic acid analogs that form reactive acyl glucuronides.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Application of quantitative structure–toxicity relationships for acute NSAID cytotoxicity in rat hepatocytes

Non-steroidal anti-inflammatory agents (NSAIDs) are widely used for pain relief. However, they have been associated with harmful and sometimes fatal side effects. Usually, the target organs are the GI tract and liver. In this study, we have investigated the physicochemical requirements of 21 NSAIDs for glucuronidation and cytotoxicity by quantitative structure-toxicity relationships (QSTRs) in isolated rat hepatocytes. Furthermore, we have investigated the contrast in physicochemical variables that correlated with NSAID-induced hepatocyte cytotoxicity when glucuronidation was inhibited with borneol. The competitive inhibition of hepatocyte p-nitrophenol glucuronidation by NSAIDs was determined by HPLC. Glucuronidation-inhibited hepatocytes were more susceptible to NSAID-induced cytotoxicity. Also, we found a parabolic correlation between lipophilicity and the inhibition of glucuronidation for a subset of NSAIDs. For NSAIDs with a benzoic acid moiety, cytotoxicity also correlated parabolically with lipophilicity, but correlated linearly with the HOMO-LUMO gap, and the first-order valence connectivity index. The cytotoxicity of NSAIDs with a phenylacetic acid (or propionic acid) substructure also correlated with lipophilicity, but not with the HOMO-LUMO gap. Our findings indicated that the inhibition of glucuronidation resulted in increased NSAID cytotoxicity, suggesting that acyl-glucuronide metabolites were acutely less cytotoxic. Also, comparative QSTRs revealed that benzoic acid NSAIDs may form cytotoxic radical metabolites (parameterized by the HOMO-LUMO gap) or alter mitochondrial respiration (parameterized by the connectivity index), whereas phenylacetic acid derived NSAIDs may form different cytotoxic metabolites, since they did not correlate with these parameters. In summary, we have used QSTRs as a tool to distinguish the cytotoxic mechanism of two groups of NSAIDs, which, if analyzed together as one group, did not reveal such mechanism-based differences.

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.

Acyl glucuronide drug metabolites: toxicological and analytical implications

Although glucuronidation is generally considered a detoxification route of drug metabolism, the chemical reactivity of acyl glucuronides has been linked with the toxic properties of drugs that contain carboxylic acid moieties. It is now well documented that such metabolites can reach appreciable concentrations in blood. Furthermore, they are labile, undergo hydrolysis and pH-dependent intramolecular acyl migration to isomeric conjugates of glucuronic acid, and may react irreversibly with plasma proteins, tissue proteins, and with nucleic acids. This stable binding causes chemical alterations that are thought to contribute to drug toxicity either through changes in the functional properties of the modified molecules or through antigen formation with subsequent hypersensitivity and other immune reactions. Whereas in vitro data on the toxicity of acyl glucuronides have steadily accumulated, direct evidence for their toxicity in vivo is scarce. Acyl glucuronides display limited stability, which is dependent on pH, temperature, nature of the aglycon, and so on. Therefore, careful sample collection, handling, and storage procedures are critical to ensure generation of reliable pharmacologic and toxicologic data during clinical studies. Acyl glucuronides can be directly quantified in biologic specimens using chromatographic procedures. Their adducts with plasma or cell proteins can be determined after electrophoretic separation, followed by blotting. ELISA techniques have been used to assess the presence of antibodies against acyl glucuronide-protein adducts. This review summarizes the most recent evidence concerning biologic and toxicologic effects of acyl glucuronide metabolites of various drugs and discusses their relevance for drug monitoring. A critical evaluation of the available methodology is included.

Stereospecific pH-dependent degradation kinetics of R- and S-naproxen-beta-l-O-acyl-glucuronide

The hydrolysis and acyl migration of biosynthetic S-naproxen-beta-l-O-acyl glucuronide (I) and R-naproxen-beta-l-O-acyl glucuronide (II) was followed by HPLC. Nine first-order kinetic rate constants for the hydrolysis and acyl migration between the beta-l-O-acyl glucuronide, its alpha/beta-2, alpha/beta-3-, alpha/beta-4-, and alpha-1-O-acyl isomers and naproxen aglycone were determined for I and II at pH 7.00, 7.40 and 8.00 at 37 degrees C by kinetic simulation. For I the 3-O-acyl isomer was the most stable isomer as the pseudo-equilibrium ratio for the major acyl-migrated isomers was 1:1.5:0.9 (2-O-acyl isomer:3-O-acyl isomer:4-O-acyl isomer). The 3- and 4-O-acyl isomers of II were equally stable as the pseudo-equilibrium ratio for the major acyl-migrated isomers was 1:1.4:1.4 (2-O-acyl isomer:3-O-acyl isomer:4-O-acyl isomer). For both I and II, the pseudo-equilibrium ratio between the major 2-O-acyl isomer and the minor alpha-l-O-acyl isomer was 10:1 (2-O-acyl isomer:alpha-l-O-acyl isomer). The pseudo-equilibrium found for the major acyl-migrated isomers of I and II in the present study corresponds with the pattern previously published for R- and S-ketoprofen-beta-l-O-acyl glucuronide acyl-migrated isomers, suggesting that these findings may be general for acyl-migrated beta-l-O-acyl glucuronides of enantiomeric 2-arylpropionic acids.

In vitro covalent binding of nafenopin-CoA to human liver proteins

Endogenous fatty acyl-CoAs play an important role in the acylation of proteins. A number of xenobiotic carboxylic acids are able to mimic fatty acids, forming CoA conjugates and acting as substrates in pathways of lipid metabolism. In this study nafenopin, a substrate for human hepatic fatty acid-CoA ligases, was chosen as a model compound to study xenobiotic acylation of human liver proteins. (3)H-nafenopin (+/- unlabeled palmitate) or (14)C-palmitate (+/- unlabeled nafenopin) were incubated for up to 120 min at 37 degrees C with ATP, CoA, and homogenate protein (1 mg/ml) from four individual human livers. Nafenopin covalently bound to proteins was detectable in all human livers and increased with time. Nafenopin adduct formation was directly proportional to nafenopin-CoA formation (r = 0.985, p < 0.05). Attachment of nafenopin to proteins involved both thioester and amide linkages with 76 and 24% of adducts formed with proteins > 100 and 50-100 kDa, respectively. Protein acylation by palmitate was also demonstrated. Palmitate significantly inhibited nafenopin-CoA formation by 29% but had no effect on nafenopin-CoA-mediated protein acylation. In contrast, nafenopin significantly inhibited protein palmitoylation by palmitoyl-CoA. This is the first study to demonstrate a direct relationship between xenobiotic-CoA formation, acylation of human liver proteins, and inhibition of endogenous palmitoylation. The ability of xenobiotics to acylate tissue proteins may have important biological consequences including perturbation of endogenous regulation of protein localization and function.

Induction of cytokine release by the acyl glucuronide of mycophenolic acid: a link to side effects?

OBJECTIVES

We have identified an acyl glucuronide (M-2) of the immunosuppressant mycophenolic acid (MPA). Acyl glucuronides have toxic potential and may contribute to drug toxicity. Whether acyl glucuronides are able to induce release of proinflammatory cytokines is unknown. Gastrointestinal disturbances have been observed during MPA therapy and may involve an inflammatory reaction. This study investigated whether M-2 can induce IL-6 and TNF-alpha release as well as gene expression of these cytokines in leukocytes.

DESIGN AND METHODS

M-2 was produced by incubation of MPA with human liver microsomes. Human mononuclear leukocytes were incubated in the presence of M-2. Concentrations of IL-6 and TNF-alpha were measured by ELISA. Expression of mRNA was determined by quantitative RT-PCR.

RESULTS

Incubation of 3 x 10(6) cells with M-2 resulted in a time and dose dependent release of cytokines, whereas MPA or its phenolic glucuronide MPAG were without effect. Cytokine liberation depended on mRNA induction. Response to M-2 showed much inter individual variability (30-fold for IL-6, 3-fold for TNF-alpha).

CONCLUSIONS

If M-2 promotes release of cytokines in vivo, these may mediate some of the toxic actions of MPA.

Endoscopic and histological features of mycophenolate mofetil colitis in patients after solid organ transplantation

BACKGROUND

Mycophenolate mofetil (MMF) is an immunosuppressive agent commonly used after organ transplantation. Gastrointestinal side effects occur in approximately 45% of patients. The spectrum of histologic features associated with MMF colitis has been well described, but data on the endoscopic features is lacking. The aim of the study was to describe the endoscopic features of MMF colitis in solid organ transplant recipients (SOTRs) as well as the frequency of histologic features and identify associated risk factors.

METHODS

A retrospective review of all SOTRs taking MMF and who underwent colonoscopy between 2000 and 2010 was performed. 36 cases of MMF colitis were identified and 361 patients served as controls. Descriptive statistics and data analysis looking for associated risk factors were performed.

RESULTS

Among SOTRs taking MMF who underwent colonoscopy, MMF colitis was diagnosed in 9%. Endoscopic findings ranged from erythema (33%) to erosions/ulcers (19%). 47% of patients had a normal colonoscopy and everyone had rectal sparing. Histological findings included acute colitis-like findings (50%), inflammatory bowel disease-like characteristics (36%), ischemia-like findings (5.6%), and graft-versus-host disease-like features (8.3%). Diarrhea occurred in 83%. Kidney transplantation was associated with a higher risk of MMF colitis (OR 5.8 [2.86-11.86], P<0.0001) whereas liver transplantation was associated with a lower risk (OR 0.06 [0.03-0.16], P<0.0001).

CONCLUSION

MMF colitis is fairly prevalent in SOTRs taking MMF who undergo colonoscopy. Diarrhea is the most common reason for colonoscopy referral (83%) and up to 47% of patients have normal colonoscopy, suggesting the need for routine biopsies to help confirm the diagnosis.

Diclofenac acyl glucuronide, a major biliary metabolite, is directly involved in small intestinal injury in rats

BACKGROUND & AIMS

Enterohepatic recirculation of nonsteroidal anti-inflammatory drugs is a critical factor in the pathogenesis of intestinal injury, but the underlying mechanism of toxicity remains obscure. The aim of this study was to examine the role of diclofenac acyl glucuronide, which is the major biliary metabolite and is chemically reactive, in the precipitation of small intestinal ulceration.

METHODS

Hepatocanalicular conjugate export pump-deficient (TR-) rats were used to selectively block diclofenac enterohepatic circulation without interrupting bile flow. Bile from diclofenac-treated normal rats was orally transferred to wild-type and TR- rats, and the extent of ulcer formation was compared with that induced by control bile containing free diclofenac. The effect of induction of hepatic diclofenac glucuronosyltransferase on the severity of diclofenac-induced ulceration was also determined.

RESULTS

TR- rats were refractory to diclofenac given either intraperitoneally or perorally. However, transfer of bile containing diclofenac glucuronide significantly increased the extent of ulcer formation in both normal and TR- rats. Moreover, induction of glucuronosyltransferase aggravated intestinal ulceration.

CONCLUSIONS

The reactive acyl glucuronide of diclofenac, or the acyl glucuronide of one of its oxidative metabolites, is directly involved in the pathogenesis of small intestinal injury.

Hepatobiliary transport of diflunisal conjugates and taurocholate by the perfused rat liver: the effect of chronic exposure of rats to diflunisal

Acyl glucuronides are reactive electrophilic metabolites of carboxylate drugs which can form covalent adducts with endogenous macromolecules such as serum albumin and hepatic proteins. Such adducts have been suggested as initiating factors in certain immune and toxic responses to acidic drugs. In the present study, pretreatment of rats with high daily doses (50 mg/kg orally) of the non-steroidal anti-inflammatory drug (NSAID) diflunisal (DF) for 35 days, followed by perfusion of the isolated liver with 3 mg DF for 3 hr, resulted in appreciable concentrations of covalent adducts of DF with hepatic tissue (3.68 microg DF/g liver). Immunoblotting using a rabbit polyclonal DF antiserum showed the major DF-modified bands at about 110, 140 and 200 kDa. A vehicle-pretreated control group achieved adduct concentrations of only 0.37 microg DF/g liver, with the 200 kDa band not detectable in immunoblots. Elimination of DF from perfusate of the isolated perfused rat liver (IPRL) preparation was the same (t1/2 about 3.4 hr) in both DF- and vehicle-pretreated groups. Appearance of the sulfate (DS) conjugate, the major metabolite in perfusate, was also similar. However, higher concentrations of the acyl glucuronide (DAG) and phenolic glucuronide (DPG) conjugates were found in perfusate at later times, though a statistically significant difference in area under the concentration-time curve was found only in the case of DAG. At 3 hr, recoveries of dose as DAG and DPG were significantly higher in perfusate, but not in bile. No significant differences in uptake and biliary excretion of taurocholate were found between the two groups. The finding of higher perfusate concentrations of DAG and DPG could signal a minor compromise to biliary excretion processes for the glucuronides, though whether such a result is simply coincident with or attributable to DAG-derived covalent DF-protein adducts in liver remains indeterminate.

Complete reversion and prevention of rectal adenomas in colectomized patients with familial adenomatous polyposis by rectal low-dose sulindac maintenance treatment. Advantages of a low-dose nonsteroidal anti-inflammatory drug regimen in reversing adenomas exceeding 33 months

PURPOSE

This nonrandomized, controlled Phase II pilot study aims at the lowest effective dose of rectally applied sulindac to achieve and maintain adenoma reversion in colectomized patients with familial adenomatous polyposis (FAP).

METHODS

The study group (n = 15) underwent proctoscopic and laboratory follow-up for polyp reversion every 6 to 12 weeks. Polyp reversion was followed by dose reduction in predefined steps. Proliferating cell nuclear antigen/cyclin (PCNA) and KI-67 proliferation indices (PI) were performed by point counting. Prostaglandin (PG)E2 and PGF2 alpha were quantified by time-resolved competitive fluorescence immunoassay.

RESULTS

All patients responded to therapy within 6 to 24 weeks. Sixty and 87 percent of patients achieved complete adenoma reversion after 48 weeks at 53 and 67 mg of sulindac per day per patient on average, respectively. Reversion was evident compared with the control group. Dose reduction by one-sixth to one-eighth of the usual oral dose was significant (Mann's trend test, P < 0.05). PCNA and KI-67 PIs of adenomatous and flat mucosa were significantly reduced (Wilcoxon's test, P < 0.05). Correlation of PCNA and KI-67 PIs indicate similar reaction of different tissue structures (Spearman's rank correlation test, P < 0.01). Nonsteroidal anti-inflammatory drug-induced redifferentiation from high-grade to low-grade dysplasia occurred in all but two patients. Tissue-PGE2 levels were greatly reduced. Unwanted, curable side effects were rare (gastritis, n = 2), and laboratory controls are within detection limits.

CONCLUSIONS

Low-dose rectal sulindac maintenance therapy is highly effective in achieving complete adenoma reversion without relapse in 87 percent of patients after 33 months. Rectal FAP phenotype should be crucial for the surgical decision. Colectomy with ileorectal anastomosis and regular chemoprevention might proceed to be a promising alternative to pouch procedures. Chemoprevention with lower incidence of FAP-related tumors via dysplasia reversion may be possible in the future.

Covalent binding of suprofen to renal tissue of rat correlates with excretion of its acyl glucuronide

1. Dosing rat with suprofen produces suprofen equivalents that are covalently bound to plasma and tissue proteins in vivo. 2. Suprofen acyl glucuronide is reactive in vitro, resulting in suprofen equivalents covalently bound to proteins of plasma and tissues in a time-dependent manner. 3. Bile duct ligation of rat increases exposure to suprofen acyl glucuronide in vivo, which leads to enhanced covalent binding of suprofen equivalents to plasma proteins and to kidney tissue. 4. Covalent binding of suprofen equivalents to kidney tissue correlates with excretion of suprofen and suprofen glucuronide by the kidney.

Rat serum albumin modified by diflunisal acyl glucuronide is immunogenic in rats

Acyl glucuronide metabolites of carboxylic acid drugs such as the salicylate derivative diflunisal (DF) have been shown to react with proteins in vitro and in vivo to produce covalent adducts. Such attachment of foreign compounds to endogenous molecules could be associated with toxic and/or immune consequences. In this study we have injected rats with rat serum albumin (RSA) modified (a) by DF using a carbodiimide reagent (-->DF-RSA-I, 4.9 micrograms DF/mg RSA) and (b) by incubation with DF acyl glucuronide (DAG) and its rearrangement isomers (iso-DAG) (-->DF-RSA-II, 0.34 micrograms DF/mg RSA). All of the six rats injected with DF-RSA-I produced antibodies reactive with DF-modified keyhole limpet hemocyanin (KLH), the coating protein used in the ELISA. Three out of six animals injected with DF-RSA-II generated similar antibodies. Cross-reactivity with other non-steroidal anti-inflammatory drugs (NSAIDs) such as naproxen and ketoprofen (as the free drugs) was not observed. This study shows that a self protein covalently modified by incubation with DAG and iso-DAG is immunogenic in rats. The data thus support the hypothesis that covalent modification of macromolecules by acyl glucuronide metabolites of acidic drugs in vivo can lead to the production of circulating antibodies which may be involved in aberrant immune responses such as drug hypersensitivity.

Studies on the reactivity of acyl glucuronides--VIII. Generation of an antiserum for the detection of diflunisal-modified proteins in diflunisal-dosed rats

Acyl glucuronide metabolites of carboxylic drugs such as the salicylate derivative diflunisal (DF) have been shown to react with proteins to produce covalent adducts. To aid in the study of the formation and distribution of these adducts in both humans and rats, we raised an antiserum against human serum albumin modified by covalent attachment of DF via an amide bond, using a carbodiimide reagent. This antiserum had wide reactivity, reacting with all types of DF-modified proteins tested and with free DF (albeit at a lower affinity). It did not cross-react with other salicylates or other non-steroidal anti-inflammatory drugs. The antiserum has been used in immunoblotting to detect proteins covalently modified by DF in the plasma and livers of rats treated with the drug for 7 days. Although some cross-reactivity was apparent on the blots, a series of DF-modified proteins was found in cytosolic, mitochondrial and mixed membrane fractions of hepatocytes, with molecular weights ranging from 28 to 130 kDa.

Idiosyncratic liver toxicity of nonsteroidal antiinflammatory drugs: molecular mechanisms and pathology

This review explores the clinical hepatic pathology associated with the use of nonsteroidal antiinflammatory drugs (NSAIDs), possible cellular and molecular mechanisms of injury, and future challenges. NSAIDs comprise a group of widely used compounds that have been associated with rare adverse reactions in the liver, including fulminant hepatitis and cholestasis. These reactions are idiosyncratic, mostly independent of the dose administered, and host-dependent. The mechanisms responsible for the initiation and perpetuation of NSAID-induced hepatotoxicity remain poorly understood and have been largely inferred from clinical manifestation. A mounting body of evidence, however, indicates that many acidic NSAIDs are metabolized to reactive acyl glucuronides that can form covalent adducts with plasma proteins and hepatocellular proteins. In hepatocytes cocultured with lymphocytes, these NSAID-altered proteins can become antigenic. Thus, long-lived, drug-altered proteins may act as immunogens and produce cytotoxic T-cell-mediated or antibody-dependent, cell-mediated toxicity in susceptible patients. Alternatively, individual abnormalities in metabolism or disposition of some NSAIDs may lead to the formation or accumulation of toxic metabolites. Additional work with transgenic animal models is needed to permit better understanding of the general and specific risk factors involved in the pathogenesis of the idiosyncratic liver injuries related to NSAIDs and other drugs.

Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity

Severe and prolonged impairment of mitochondrial beta-oxidation leads to microvesicular steatosis, and, in severe forms, to liver failure, coma and death. Impairment of mitochondrial beta-oxidation may be either genetic or acquired, and different causes may add their effects to inhibit beta-oxidation severely and trigger the syndrome. Drugs and some endogenous compounds can sequester coenzyme A and/or inhibit mitochondrial beta-oxidation enzymes (aspirin, valproic acid, tetracyclines, several 2-arylpropionate anti-inflammatory drugs, amineptine and tianeptine); they may inhibit both mitochondrial beta-oxidation and oxidative phosphorylation (endogenous bile acids, amiodarone, perhexiline and diethylaminoethoxyhexestrol), or they may impair mitochondrial DNA transcription (interferon-alpha), or decrease mitochondrial DNA replication (dideoxynucleoside analogues), while other compounds (ethanol, female sex hormones) act through a combination of different mechanisms. Any investigational molecule should be screened for such effects.

Irreversible binding of tolmetin to macromolecules via its glucuronide: binding to blood constituents, tissue homogenates and subcellular fractions in vitro

1. The degradation of tolmetin glucuronide (TG) in biological fluids and tissue homogenates appears to follow first-order kinetics and is quite rapid in plasma. TG degradation was minimized upon the addition of phenylmethylsulphonyl fluoride (PMSF) and 1,4-saccharolactone, suggesting that the majority of the degradation may be enzymatic, rather than chemical hydrolysis. 2. Irreversible binding via TG was detected in all tissue preparations examined. Upon addition of an inhibitor of esterases (PMSF) to human serum albumin (HSA) and plasma, binding was extensive (2.5%) and the extent of binding was both time- and pH-dependent. Similar extents of binding were obtained with most tissue homogenates, except for spleen and intestine which exhibited much lower binding. 3. Incubation of TG with microsomal protein from sheep and rat yielded no significant differences. Incubations of tolmetin (T) and TG with microsomes, as well as tissue homogenates, indicates that irreversible binding occurs only in the presence of TG. 4. Irreversible binding occurred in all of the blood constituents, the highest extent with haemolyzed erythrocytes. The extent of binding was 15 times higher in disrupted versus intact red blood cells, suggesting a correlation between the extent of binding and the overall exposure of TG to the macromolecules to which it may bind irreversibly.

Studies on the reactivity of acyl glucuronides--VII. Salicyl acyl glucuronide reactivity in vitro and covalent binding of salicylic acid to plasma protein of humans taking aspirin

Salicyl acyl glucuronide (SAG) is a significant metabolite of salicylic acid (SA) and aspirin. We have shown that, under physiological conditions in vitro, SAG undergoes rearrangement in a manner consistent with acyl migration to its 2-, 3- and 4-O-acyl positional isomers as the predominant pathway (T1/2 values were 1.4-1.7 hr in buffer at pH 7.4 and 37 degrees). Incubation of SAG or a mixture of its rearrangement isomers (iso-SAG) (each at approximately 50 micrograms SA equivalents/mL) with human serum albumin (HSA, at approximately 40 mg/mL) revealed the formation of covalent adducts with the protein, with peak concentrations of 1-2 micrograms SA equivalents/mL. The data support a role for the rearrangement/glycation mechanism of adduct formation. Covalent adducts of SA were also detected in the plasma of humans taking aspirin (at > or = 1200 mg/day), but the concentrations were low (<< 100 ng SA equivalents/mL). Reactivity of SAG thus provides a mechanism (though of uncertain quantitative importance) of covalent attachment of the salicyl moiety of aspirin to tissue macromolecules, which is in addition to its well-known acetylating capacity.

Studies on the reactivity of acyl glucuronides--VI. Modulation of reversible and covalent interaction of diflunisal acyl glucuronide and its isomers with human plasma protein in vitro

Acyl glucuronide conjugates are chemically reactive metabolites which can undergo hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with protein. The present study was undertaken to identify factors modulating the reactivity of diflunisal acyl glucuronide (DAG) with human serum albumin (HSA) in vitro, by comprehensively evaluating the interplay of the three pathways above when DAG and a mixture of its 2-, 3- and 4-isomers (iso-DAG) were incubated with protein. Buffer, plasma, fraction V HSA, fatty acid-free HSA, globulin-free HSA and fatty acid- and globulin-free HSA were investigated at pH 7.4 and 37 degrees, each in the absence and presence of warfarin, diazepam and diflunisal (DF) as reversible binding competitors. DAG and iso-DAG were highly reversibly bound (ca. 98-99.5%) in plasma and HSA solutions. The binding was primarily at the benzodiazepine site, since displacement occurred in the presence of diazepam and fatty acids but not warfarin. DAG degradation, via rearrangement, hydrolysis and covalent adduct formation (in that order of quantitative importance), was retarded in plasma and HSA solutions compared to buffer. The protective effect of protein was afforded by the high reversible binding to the (non-catalytic) benzodiazepine site. The warfarin site appeared to be catalytic for DAG hydrolysis, whereas rearrangement appeared to be hydroxide ion-catalysed only. In contrast to DAG, iso-DAG degradation was greatly accelerated in the presence of protein, through both covalent binding and catalysis of hydrolysis. Covalent binding via DAG was increased in the presence of warfarin but decreased in the presence of diazepam, DF and fatty acids. The opposite effects were found for covalent binding via iso-DAG. The data suggest that covalent binding of DF to HSA via DAG and iso-DAG occurs by different mechanisms (presumably transacylation and glycation, respectively) at different sites (benzodiazepine and warfarin, respectively) whereas reversible binding occurs primarily at the same site (benzodiazepine).

The spinal actions of nonsteroidal anti-inflammatory drugs and the dissociation between their anti-inflammatory and analgesic effects

The traditional classification of nonsteroidal anti-inflammatory drugs (NSAIDs) as exclusively 'peripherally acting' agents is no longer valid. For many of these agents there is a growing body of evidence in favour of an additional central mechanism for their anti-inflammatory and analgesic effects. This view is further supported by the recent discovery that a substantial component of the hyperalgesia and allodynia that characterise postinjury hypersensitivity occurs in the CNS, notably the spinal dorsal horn. An important corollary is that inhibition of central nociceptive processing may represent an important analgesic mode of action for those NSAIDs that are effective in the management of pain after tissue injury. Historically, attempts to group this heterogeneous class of compounds into a single entity are largely derived from the observation that the majority of clinically useful NSAIDs are weak organic acids (pKa 3 to 5), bind extensively to plasma albumin (= 99%), and inhibit (to varying degrees) prostaglandin synthesis. However, the significance of these various unifying features is becoming increasingly obscure. While inhibition of prostaglandin synthesis apparently remains an important analgesic mode of action for NSAIDs both in the periphery and the CNS, other mechanisms should be considered. Some NSAIDs, in addition to their effects on prostaglandin synthesis, also affect the synthesis and activity of other neuroactive substances believed to have key roles in processing nociceptive input within the dorsal horn. It has been argued that these other actions, in conjunction with inhibition of prostaglandin synthesis, may synergistically augment the effects of NSAIDs on spinal nociceptive processing. Despite much effort, it remains a formidable task to assess the significance of these differential mechanisms upon clinical pain states. In the meantime, however, it may be possible, on the basis of in vivo studies, to evaluate the impact of putative spinal analgesic mechanisms that are unrelated to inhibition of prostaglandin synthesis. This approach has recently been extended to include the identification of pharmacokinetic and clinical correlates of these derived in vivo parameters, and in this way attempt to demonstrate clinical relevance.

Glucuronidation of diflunisal by rat liver microsomes. Effect of microsomal beta-glucuronidase activity

The in vitro formation rates of the phenolic (DPG) and acyl (DAG) glucuronides of diflunisal were investigated using rat liver microsomes. Preliminary studies showed that DAG hydrolysed rapidly (T1/2 = 12 min) when incubated in the presence of rat liver microsomes at pH 7.4 and 37 degrees. DPG was much more stable under the same conditions (T1/2 = 35 hr). Hydrolysis of DAG and DPG by rat liver microsomes was inhibited by 4 mM saccharolactone, a beta-glucuronidase inhibitor. The apparent Km and Vmax values for the formation of DAG in the absence and presence of 4 mM D-saccharic acid-1,4-lactone (saccharolactone) were the following: Km = 0.05 +/- 0.02 vs 0.08 +/- 0.02 mM and Vmax = 0.20 +/- 0.06 vs 0.43 +/- 0.07 nmol/min/mg protein (0 and 4 mM saccharolactone, respectively). The significant increase in apparent Vmax for DAG formation in the presence of saccharolactone can be explained by the inhibition of beta-glucuronidase-catalysed hydrolysis of DAG. Apparent Km and Vmax values for the formation rate of DPG were not affected by addition of saccharolactone to the incubation medium. These results indicate that beta-glucuronidase-catalysed hydrolysis of certain glucuronides formed during microsomal incubations may significantly affect the apparent glucuronidation rate due to the presence of a glucuronidation-deglucuronidation cycle.

Studies on the reactivity of acyl glucuronides--V. Glucuronide-derived covalent binding of diflunisal to bladder tissue of rats and its modulation by urinary pH and beta-glucuronidase

Acyl glucuronide conjugates of acidic drugs have been shown to be reactive metabolites capable of undergoing non-enzymic hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with plasma protein. In an earlier study (King and Dickinson, Biochem Pharmacol 45: 1043-1047, 1993), we documented formation of covalent adducts of diflunisal (DF), a salicylate derivative which is metabolized in part to a reactive acyl glucuronide (DAG), with liver, kidney, skeletal muscle and small and large intestine (in addition to plasma protein) of rats given the drug i.v. twice daily at 50 mg DF/kg for 7 days. The present study shows that covalent adducts of DF were also formed with urinary bladder tissue of these rats, achieving concentrations (ca. 5 micrograms DF equivalents/g tissue) higher than those found in the other tissues noted above. After cessation of dosing, the adduct concentrations declined with an apparent T 1/2 value of ca. 20 hr. Adducts were also formed ex vivo in excised rat bladders in which DAG or a prepared mixture of its acyl migration isomers (iso-DAG) were incubated at pH 5.0, 6.5 and 8.0. After 8 hr incubation, the highest concentrations (ca. 11 micrograms DF equivalents/g) were produced with iso-DAG at pH 5.0, and the lowest (ca. 2.3 micrograms DF equivalents/g) with DAG at pH 5.0. However, a major competing reaction for DAG (at least at pH 5.0) was hydrolysis by beta-glucuronidases originating from bladder tissue. By contrast, iso-DAG was quite resistant to such hydrolysis. The phenolic glucuronide conjugate, another important metabolite of DF, was hydrolysed only slowly. Similar results were obtained in fresh rat urine adjusted to pH 5.0. The results support covalent DF adduct formation in rat bladder originating from both DAG and iso-DAG as ultimate reactants, though the extent of binding is modulated by both urinary pH and beta-glucuronidases.