Inhibition of proliferation of HT-29 colon adenocarcinoma cells by carboxylate NSAIDs and their acyl glucuronides

Acyl glucuronides: the good, the bad and the ugly

Acyl glucuronidation is the major metabolic conjugation reaction of most carboxylic acid drugs in mammals. The physiological consequences of this biotransformation have been investigated incompletely but include effects on drug metabolism, protein binding, distribution and clearance that impact upon pharmacological and toxicological outcomes. In marked contrast, the exceptional but widely disparate chemical reactivity of acyl glucuronides has attracted far greater attention. Specifically, the complex transacylation and glycation reactions with proteins have provoked much inconclusive debate over the safety of drugs metabolised to acyl glucuronides. It has been hypothesised that these covalent modifications could initiate idiosyncratic adverse drug reactions. However, despite a large body of in vitro data on the reactions of acyl glucuronides with protein, evidence for adduct formation from acyl glucuronides in vivo is limited and potentially ambiguous. The causal connection of protein adduction to adverse drug reactions remains uncertain. This review has assessed the intrinsic reactivity, metabolic stability and pharmacokinetic properties of acyl glucuronides in the context of physiological, pharmacological and toxicological perspectives. Although numerous experiments have characterised the reactions of acyl glucuronides with proteins, these might be attenuated substantially in vivo by rapid clearance of the conjugates. Consequently, to delineate a relationship between acyl glucuronide formation and toxicological phenomena, detailed pharmacokinetic analysis of systemic exposure to the acyl glucuronide should be undertaken adjacent to determining protein adduct concentrations in vivo. Further investigation is required to ascertain whether acyl glucuronide clearance is sufficient to prevent covalent modification of endogenous proteins and consequentially a potential immunological response.

The acyl glucuronide metabolite of mycophenolic acid induces tubulin polymerization in vitro

OBJECTIVES

The acyl glucuronide (AcMPAG) of mycophenolic acid (MPA) forms covalent protein adducts and possesses antiproliferative properties independent of IMPDH inhibition. The underlying mechanism is unknown. Disorganized tubulin polymerization prevents cell cycle progression. We investigated whether AcMPAG interacts with tubulin polymerization.

DESIGN AND METHODS

AcMPAG (1.0-100 microM) was incubated with bovine tubulin in the presence of GTP. Polymerization was followed at 340 nm. The time until onset and the extent of polymerization were determined. MPA (100 microM), phenolic glucuronide MPAG (100 microM), and paclitaxel (10 microM) served as controls.

RESULTS

MPAG was without effect. The AcMPAG effect on tubulin polymerization was dose dependent and significantly stronger (about 2.5-fold) than that of MPA (n=4; p<0.05), but weaker than paclitaxel.

CONCLUSIONS

MPA and AcMPAG can induce tubulin polymerization in the presence of GTP with AcMPAG being significantly stronger. This property of AcMPAG may contribute to its IMPDH independent antiproliferative effect.

Glucuronidation in therapeutic drug monitoring

BACKGROUND

Glucuronidation is a major drug-metabolizing reaction in humans. A pharmacological effect of glucuronide metabolites is frequently neglected and the value of therapeutic drug monitoring has been questioned. However, this may not always be true.

METHODS

In this review the impact of glucuronidation on therapeutic drug monitoring has been evaluated on the basis of a literature search and experience from the own laboratory.

RESULTS

The potential role of monitoring glucuronide metabolite concentrations to optimize therapeutic outcome is addressed on the basis of selected examples of drugs which are metabolized to biologically active/reactive glucuronides. Furthermore indirect effects of glucuronide metabolites on parent drug pharmacokinetics are presented. In addition, factors that may modulate the disposition of these metabolites (e.g. genetic polymorphisms, disease processes, age, and drug-drug interactions) are briefly mentioned and their relevance for the clinical situation is critically discussed.

CONCLUSION

Glucuronide metabolites can have indirect as well as direct pharmacological or toxicological effects. Although convincing evidence to support the introduction of glucuronide monitoring into clinical practice is currently missing, measurement of glucuronide concentrations may be advantageous in specific situations. If the glucuronide metabolite has an indirect effect on the pharmacokinetics of the parent compound, monitoring of the parent drug may be considered. Furthermore pharmacogenetic approaches considering uridine diphosphate (UDP) glucuronosyltransferases polymorphisms may become useful in the future to optimize therapy with drugs subject to glucuronidation.

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.

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.

A review of the common properties of drugs with idiosyncratic hepatotoxicity and the "multiple determinant hypothesis" for the manifestation of idiosyncratic drug toxicity

Idiosyncratic drug toxicity is generally believed to be a phenomenon that cannot be readily evaluated experimentally. Reasons for this difficulty include the following: 1. It is a rare event (<1/5,000) and therefore impossible to be studied in clinical trials; 2. It is a human-specific event not detectable in experimental animals. To aid the understanding of idiosyncratic toxicity and to develop an experimental strategy for this phenomenon, a hypothesis is proposed. The hypothesis states that the low frequency of idiosyncratic drug toxicity is due to the requirements for the occurrence of multiple critical and discrete events, with the probability for the occurrence of idiosyncratic drug toxicity as a product of the probabilities of each event. The key determinants of these critical events are proposed to be: 1. Chemical properties; 2. exposure; 3. environmental factors; and 4. genetic factors. Based on this hypothesis, idiosyncratic drug toxicity can be evaluated experimentally via studying these key determinants. The chemical properties critical to idiosyncratic drug toxicity are identified via a review of the common properties of drugs that cause idiosyncratic liver toxicity. These properties include: 1. Formation of reactive metabolites. 2. Metabolism by P450 isoforms. 3. Preponderance of P450 inducers, and 4. Occurrence of clinically significant pharmacokinetic interactions with co-administered drugs. Based on this review, it is proposed that these common properties may be useful experimental endpoints for the prediction and therefore avoidance of the selection of drug candidates with idiosyncratic drug toxicity for further development.