Synthesis and in vitro reactivity of 3-carbamoyl-2-phenylpropionaldehyde and 2-phenylpropenal: putative reactive metabolites of felbamate

Identifying chemicals based on receptor binding/bioactivation/mechanistic explanation associated with potential to elicit hepatotoxicity and to support structure activity relationship-based read-across

The liver is the most common target organ in toxicology studies. The development of chemical structural alerts for identifying hepatotoxicity will play an important role in in silico model prediction and help strengthen the identification of analogs used in structure activity relationship (SAR)- based read-across. The aim of the current study is development of an SAR-based expert-system decision tree for screening of hepatotoxicants across a wide range of chemistry space and proposed modes of action for clustering of chemicals using defined core chemical categories based on receptor-binding or bioactivation. The decision tree is based on ∼ 1180 different chemicals that were reviewed for hepatotoxicity information. Knowledge of chemical receptor binding, metabolism and mechanistic information were used to group these chemicals into 16 different categories and 102 subcategories: four categories describe binders to 9 different receptors, 11 categories are associated with possible reactive metabolites (RMs) and there is one miscellaneous category. Each chemical subcategory has been associated with possible modes of action (MOAs) or similar key structural features. This decision tree can help to screen potential liver toxicants associated with core structural alerts of receptor binding and/or RMs and be used as a component of weight of evidence decisions based on SAR read-across, and to fill data gaps.

Assessment of metabolic activation of felbamate in chimeric mice with humanized liver in combination with in vitro metabolic assays

Felbamate (FBM) is an antiepileptic drug that has minimal toxicity in preclinical toxicological species but has a serious idiosyncratic drug toxicity (IDT) in humans. The formation of reactive metabolites is common among most drugs associated with IDT, and 2-phenylpropenal (2-PP) is believed to be the cause of IDT by FBM. It is important to consider the species difference in susceptibility to IDT between experimental animals and humans. In the present study, we used an in vitro and in vivo model system to reveal species difference in IDT of FBM. Human cytochrome P450 (CYP) and carboxylesterase (CES) expressing microsomes were used to clarify the isozymes involved in the metabolism of FBM. The remaining amount of FBM was significantly reduced in incubation with microsomes expressing human CYP2C8, 2C9, 2E1, and CES1c isozymes. Chimeric mice with humanized liver are expected to predict IDT in humans. Therefore, metabolite profiles in chimeric mice with humanized liver were investigated after administration of FBM. Metabolites after glutathione (GSH) conjugation of 2-phenylpropenal (2-PP), which is the reactive metabolite responsible for FBM-induced IDT, were detected in chimeric mice plasma and liver homogenate. Mass spectrometry imaging (MSI) visualizes distribution of FBM and endogenous GSH, and GSH levels in human hepatocyte were decreased after administration of FBM. In this study, we identified CYP and CES isozymes involved in the metabolism of FBM and confirmed reactive metabolite formation and subsequent decrease in GSH using humanized animal model. These results would provide useful information for the susceptibility to IDT between experimental animals and humans.

Evolving Role of Pharmacogenetic Biomarkers to Predict Drug-Induced Hematological Disorders

ABSTRACT

Drug-induced hematological disorders constitute up to 30% of all blood dyscrasias seen in the clinic. Hematologic toxicity from drugs may range from life-threatening marrow aplasia, agranulocytosis, hemolysis, thrombosis to mild leukopenia, and thrombocytopenia. Pathophysiologic mechanisms underlying these disorders vary from an extension of the pharmacological effect of the drug to idiosyncratic and immune-mediated reactions. Predicting these reactions is often difficult, and this makes clinical decision-making challenging. Evidence supporting the role of pharmacogenomics in the management of these disorders in clinical practice is rapidly evolving. Despite the Clinical Pharmacology Implementation Consortium and Pharmacogenomics Knowledge Base recommendations, few tests have been incorporated into routine practice. This review aims to provide a comprehensive summary of the various drugs which are implicated for the hematological adverse events, their underlying mechanisms, and the current evidence and practical recommendations to incorporate pharmacogenomic testing in clinical care for predicting these disorders.

The ups and downs of alkyl-carbamates in epilepsy therapy: How does cenobamate differ?

Since 1955, several alkyl-carbamates have been developed for the treatment of anxiety and epilepsy, including meprobamate, flupirtine, felbamate, retigabine, carisbamate, and cenobamate. They have each enjoyed varying levels of success as antiseizure drugs; however, they have all been plagued by the emergence of serious and sometimes life-threatening adverse events. In this review, we compare and contrast their predominant molecular mechanisms of action, their antiseizure profile, and where possible, their clinical efficacy. The preclinical, clinical, and mechanistic profile of the prototypical γ-aminobutyric acidergic (GABAergic) modulator phenobarbital is included for comparison. Like phenobarbital, all of the clinically approved alkyl-carbamates share an ability to enhance inhibitory neurotransmission through modulation of the GABA_A receptor, although the specific mechanism of interaction differs among the different drugs discussed. In addition, several alkyl-carbamates have been shown to interact with voltage-gated ion channels. Flupirtine and retigabine share an ability to activate K⁺ currents mediated by KCNQ (Kv7) K⁺ channels, and felbamate, carisbamate, and cenobamate have been shown to block Na⁺ channels. In contrast to other alkyl-carbamates, cenobamate seems to be unique in its ability to preferentially attenuate the persistent rather than transient Na⁺ current. Results from recent randomized controlled clinical trials with cenobamate suggest that this newest antiseizure alkyl-carbamate possesses a degree of efficacy not witnessed since felbamate was approved in 1993. Given that ceno-bamate's mechanistic profile is unique among the alkyl-carbamates, it is not clear whether this impressive efficacy reflects an as yet undescribed mechanism of action or whether it possesses a unique synergy between its actions at the GABA_A receptor and on persistent Na⁺ currents. The high efficacy of cenobamate is, however, tempered by the risk of serious rash and low tolerability at higher doses, meaning that further safety studies and clinical experience are needed to determine the true clinical value of cenobamate.

A Predominant Oxidative Renal Metabolite of Empagliflozin in Male Mice Is Cytotoxic in Mouse Renal Tubular Cells but not Genotoxic

In a previously reported CD-1 mouse 2-year carcinogenicity study with the sodium glucose cotransporter-2 inhibitor empagliflozin, an increased incidence of renal tubular adenomas and carcinomas was identified only in the male high-dose group. Follow-up investigative studies have shown that the renal tumors in male high-dose mice were preceded by a number of renal degenerative/regenerative findings. Prior cross-species in vitro metabolism studies using microsomes identified an oxidative metabolite (M466/2) predominantly formed in the male mouse kidney and which spontaneously degrades to a metabolite (M380/1) and reactive 4-OH crotonaldehyde (CTA). In order to further evaluate potential modes of action for empagliflozin-associated male mouse renal tumors, we report here a series of in vitro investigative toxicology studies conducted to evaluate the cytotoxic and genotoxic potential of empagliflozin and M466/2. To assess the cytotoxic potential of empagliflozin and M466/2, a primary mouse renal tubular epithelial (mRTE) cell model was used. In mRTE cells, M466/2-derived in vitro 4-OH CTA exposure was cytotoxic, while empagliflozin was not cytotoxic or mitogenic. Empagliflozin and M466/2 were not genotoxic, supporting an indirect mode of action for empagliflozin-associated male mouse renal tumorigenesis. In conclusion, these in vitro data show that M466/2-derived 4-OH CTA exposure is associated with cytotoxicity in renal tubule cells and may be involved in promoting compound-related in vivo renal metabolic stress and chronic low-level renal injury, in turn supporting a nongenotoxic mode of tumor pathogenesis specific to the male mouse.

Methanol adducts leading to the identification of a reactive aldehyde metabolite of CPAQOP in human liver microsomes by ultra-high-performance liquid chromatography/mass spectrometry

RATIONALE

The incubation of CPAQOP (1-[(2R)-2-[[4-[3-chloro-4-(2-pyridyloxy)anilino]quinazolin-5-yl]oxymethyl]-1-piperidyl]-2-hydroxy) with human liver microsomes generated several metabolites that highlighted the hydroxyacetamide side chain was a major site of metabolism for the molecule. The metabolites were derived predominantly from oxidative biotransformations; however, two unexpected products were detected by liquid chromatography/ultraviolet/mass spectrometry (LC/UV/MS) and identified as methanol adducts. This observation prompted further LC/MS investigations into their formation.

METHODS

Three separate incubations of CPAQOP were conducted in human liver microsomes; Naïve, fortified with methoxyamine and fortified with glutathione. Separation was achieved via ultra-high-performance liquid chromatography with either methanol or acetonitrile gradients containing formic acid. MS analysis was conducted by electrospray ionisation LTQ Orbitrap mass spectrometry acquiring accurate mass full scan, data-dependent MS² and all ion fragmentation.

RESULTS

No methanol adducts were detected by MS when acetonitrile was used in the mobile phase instead of methanol, verifying that a metabolite was reacting with methanol on column. Although this reactive metabolite could not be isolated or structurally characterised by LC/MS directly, product ion spectra of the methanol adducts confirmed addition of methanol on the hydroxyacetamide side chain. Additional experiments using methoxyamine showed the disappearance of the two methanol adducts and appearance of a methoxyamine adduct, confirming the presence of an aldhyde. Product ion spectra of the methoxyamine adduct confirmed addition of methoxyamine to the hydroxyacetamide side chain.

CONCLUSIONS

The proposed bioactivation of CPAQOP occurred via the reactive aldehyde intermediate, which readily reacted with methanol in the mobile phase to form a pair of isomeric hemiacetal methanol adducts. In acidified methanol the equilibrium favoured the methanol adduct and in acidified acetonitrile it favoured the hydrate; therefore, the reactive aldehyde metabolite was not detected and could not be structurally characterised directly. Copyright © 2016 John Wiley & Sons, Ltd.

Qualitative structure-metabolism relationships in the hydrolysis of carbamates

The aims of this review were 1) to compile a large number of reliable literature data on the metabolic hydrolysis of medicinal carbamates and 2) to extract from such data a qualitative relation between molecular structure and lability to metabolic hydrolysis. The compounds were classified according to the nature of their substituents (R³OCONR¹R²), and a metabolic lability score was calculated for each class. A trend emerged, such that the metabolic lability of carbamates decreased (i.e., their metabolic stability increased), in the following series: Aryl-OCO-NHAlkyl >> Alkyl-OCO-NHAlkyl ~ Alkyl-OCO-N(Alkyl)₂ ≥ Alkyl-OCO-N(endocyclic) ≥ Aryl-OCO-N(Alkyl)₂ ~ Aryl-OCO-N(endocyclic) ≥ Alkyl-OCO-NHAryl ~ Alkyl-OCO-NHAcyl >> Alkyl-OCO-NH₂ > Cyclic carbamates. This trend should prove useful in the design of carbamates as drugs or prodrugs.

Hepatotoxicity associated with antiepileptic drugs

BACKGROUND

Drug-induced liver injury associated with antiepileptic drugs (AED) is well recognized. The frequency of the most common AED is rare but the consequences can be very serious leading to death or liver transplantation due to acute liver failure induced by these drugs.

CLINICAL FEATURES

Hypersensitivity features are found in more than 70% of patients with phenytoin-induced liver injury, whereas this is only observed in 30% of carbamazepine-associated hepatotoxicity and very rarely with valproate (VPA)-induced liver injury.

PATHOPHYSIOLOGY

The underlying mechanisms behind hepatotoxicity induced by AED are not clear. Reactive metabolites from AED can, in some cases, lead to direct cytotoxicity and liver cell necrosis, whereas in other cases this may lead to neoantigen formation inducing immunoallergic mechanisms.

TREATMENT

No specific therapy is of proved value in severe hepatotoxicity due to AED. However, N-acetylcystein is an appropriate treatment in patients with clinically significant liver injury due to phenytoin and carbamazepine. In patients with VPA-associated liver injury, carnitine that is an important co-factor in the mitochondrial beta-oxidation of fatty acids is the recommended treatment. Early referral of patients with severe liver reactions and coagulopathy to liver transplant centers before encephalopathy can be the difference between liver transplantation and death.

Role of Metabolism in Drug-Induced Idiosyncratic Hepatotoxicity

Rare adverse reactions to drugs that are of unknown etiology, or idiosyncratic reactions, can produce severe medical complications or even death in patients. Current hypotheses suggest that metabolic activation of a drug to a reactive intermediate is a necessary, yet insufficient, step in the generation of an idiosyncratic reaction. We review evidence for this hypothesis with drugs that are associated with hepatotoxicity, one of the most common types of idiosyncratic reactions in humans. We identified 21 drugs that have either been withdrawn from the U.S. market due to hepatotoxicity or have a black box warning for hepatotoxicity. Evidence for the formation of reactive metabolites was found for 5 out of 6 drugs that were withdrawn, and 8 out of 15 drugs that have black box warnings. For the other drugs, either evidence was not available or suitable studies have not been carried out. We also review evidence for reactive intermediate formation from a number of additional drugs that have been associated with idiosyncratic hepatotoxicity but do not have black box warnings. Finally, we consider the potential role that high dosages may play in these adverse reactions.

Can in vitro metabolism-dependent covalent binding data in liver microsomes distinguish hepatotoxic from nonhepatotoxic drugs? An analysis of 18 drugs with consideration of intrinsic clearance and daily dose

In vitro covalent binding assessments of drugs have been useful in providing retrospective insights into the association between drug metabolism and a resulting toxicological response. On the basis of these studies, it has been advocated that in vitro covalent binding to liver microsomal proteins in the presence and the absence of NADPH be used routinely to screen drug candidates. However, the utility of this approach in predicting toxicities of drug candidates accurately remains an unanswered question. Importantly, the years of research that have been invested in understanding metabolic bioactivation and covalent binding and its potential role in toxicity have focused only on those compounds that demonstrate toxicity. Investigations have not frequently queried whether in vitro covalent binding could be observed with drugs with good safety records. Eighteen drugs (nine hepatotoxins and nine nonhepatotoxins in humans) were assessed for in vitro covalent binding in NADPH-supplemented human liver microsomes. Of the two sets of nine drugs, seven in each set were shown to undergo some degree of covalent binding. Among hepatotoxic drugs, acetaminophen, carbamazepine, diclofenac, indomethacin, nefazodone, sudoxicam, and tienilic acid demonstrated covalent binding, while benoxaprofen and felbamate did not. Of the nonhepatotoxic drugs evaluated, buspirone, diphenhydramine, meloxicam, paroxetine, propranolol, raloxifene, and simvastatin demonstrated covalent binding, while ibuprofen and theophylline did not. A quantitative comparison of covalent binding in vitro intrinsic clearance did not separate the two groups of compounds, and in fact, paroxetine, a nonhepatotoxin, showed the greatest amount of covalent binding in microsomes. Including factors such as the fraction of total metabolism comprised by covalent binding and the total daily dose of each drug improved the discrimination between hepatotoxic and nontoxic drugs based on in vitro covalent binding data; however, the approach still would falsely identify some agents as potentially hepatotoxic.

Biotransformation of 3-amino-5,6,7,8-tetrahydro-2-{4-[4-(quinolin-2-yl)piperazin-1-yl]butyl}quinazolin-4(3H)-one (TZB-30878), a novel 5-hydroxytryptamine (5-HT)1A agonist/5-HT3 antagonist, in human hepatic cytochrome P450 enzymes

3-Amino-5,6,7,8-tetrahydro-2-{4-[4-(quinolin-2-yl)piperazin-1-yl]-butyl}quinazolin-4(3H)-one (TZB-30878), a novel 5-hydroxytryptamine (5-HT)(1A) agonist/5-HT(3) antagonist, is currently under development for the treatment of irritable bowel syndrome. The objective of this investigation was to obtain information on the biotransformation of TZB-30878. This compound has quinazoline, piperazine, and quinoline rings. Metabolites of [quinazoline-2-(14)C]TZB-30878 were determined using radio high-performance liquid chromatography on samples obtained after incubation with human hepatic microsomes. Eight metabolites were detected in the microsomal incubation mixture, and their structures were proposed by mass spectrometry techniques using TZB-30878 and two stable labeled TZB-30878 analogs, [quinoline-deuterium (D)(6)]TZB-30878 and [piperazin-D(8)]TZB-30878. Liquid chromatography/tandem mass spectrometry analyses suggested that the eight metabolites consisted of a cyclic metabolite (M6), four hydroxylated metabolites (M1, M2, M3, and M4) (three on quinoline ring and one on quinazoline ring), a deaminated metabolite (M5), and two metabolites (M7 and M8) that were presumably intermediates leading to the formation of the cyclic metabolite M6. Hydroxylation sites in the quinoline and quinazoline rings were predicted by electron density calculations and confirmed by comparison with authentic standards. To the best of our knowledge, N-deamination by microsomes leading to the formation of M5 appears to be novel. In addition, in vitro experiments in human liver microsomes with cytochrome P450 (P450)-specific inhibitors revealed that CYP3A4 was the major enzyme responsible for the metabolism of TZB-30878. Other P450 enzymes, such as a CYP2D6, played a minor role in its metabolism.

An Interaction Study Between the New Antiepileptic and CNS Drug Carisbamate (RWJ-333369) and Lamotrigine and Valproic Acid

PURPOSE

To characterize possible pharmacokinetic interactions between the new antiepileptic drug carisbamate (RWJ-333369) and valproic acid (VPA) or lamotrigine (LTG) following multiple dosing in healthy subjects.

METHODS

Two open-label, sequential-design studies were conducted in 24 healthy adults. In Study 1, subjects received carisbamate alone (5 days 250 mg q12h; 5 days 500 mg q12h), then VPA alone (7 days 300 mg q12h; 7 days 500 mg q12h), and then a combination of VPA (500 mg q12h) and carisbamate (5 days 250 mg q12h; 5 days 500 mg q12h). In Study 2, subjects received carisbamate alone as in Study 1, then LTG alone (14 days 25 mg q12h; 14 days 50 mg q12h), and then combination of LTG (50 mg q12h) and carisbamate (3 days 250 mg q12h; 14 days 500 mg q12h).

RESULTS

Coadministration of VPA or LTG had minimal effect on carisbamate mean C(max) and AUC(ss) values. Mean VPA-C(max) and AUC(ss) values were approximately 15% lower when given concomitantly with carisbamate. However, the 90% confidence intervals (CIs) for the C(max) and AUC(ss) ratio with/without carisbamate were within the 80-125% equivalence range, C(max) 82-89%; AUC(ss) 81-88%. Mean LTG C(max) and AUC(ss) values were approximately 20% lower when given concomitantly with carisbamate. The 90% CIs with and without carisbamate for LTG C(max) and AUC(ss) were 79-86% and 75-81%, respectively. This modest change is not considered clinically significant.

CONCLUSIONS

There were no clinically significant interactions between carisbamate and VPA or LTG. Concomitant administration of carisbamate with VPA or LTG was generally safe and well tolerated.

Idiosyncratic adverse reactions to antiepileptic drugs

Idiosyncratic drug reactions may be defined as adverse effects that cannot be explained by the known mechanisms of action of the offending agent, do not occur at any dose in most patients, and develop mostly unpredictably in susceptible individuals only. These reactions are generally thought to account for up to 10% of all adverse drug reactions, but their frequency may be higher depending on the definition adopted. Idiosyncratic reactions are a major source of concern because they encompass most life-threatening effects of antiepileptic drugs (AEDs), as well as many other reactions requiring discontinuation of treatment. Based on the underlying mechanisms, idiosyncratic reactions can be differentiated into (1) immune-mediated hypersensitivity reactions, which may range from benign skin rashes to serious conditions such as drug-related rash with eosinophilia and systemic symptoms; (2) reactions involving unusual nonimmune-mediated individual susceptibility, often related to abnormal production or defective detoxification of reactive cytotoxic metabolites (as in valproate-induced liver toxicity); and (3) off-target pharmacology, whereby a drug interacts directly with a system other than that for which it is intended, an example being some types of AED-induced dyskinesias. Although no AED is free from the potential of inducing idiosyncratic reactions, the magnitude of risk and the most common manifestations vary from one drug to another, a consideration that impacts on treatment choices. Serious consequences of idiosyncratic reactions can be minimized by knowledge of risk factors, avoidance of specific AEDs in subpopulations at risk, cautious dose titration, and careful monitoring of clinical response.

Chemical toxicology: reactive intermediates and their role in pharmacology and toxicology

Reactive intermediates formed during the metabolism of drugs have been investigated extensively over the past decades. Today, interest in reactive intermediates in drug discovery is focused on minimising bioactivation in hopes of reducing the risk of causing so-called idiosyncratic toxicity. These efforts are justified based on the 'hapten hypothesis', namely, that on binding to protein, reactive intermediates may elicit an immune response to the modified protein, leading to a cascade of events that ultimately manifests as a toxic outcome. However, the pharmacological action of certain drugs depends on reactive intermediates that modify critical amino acid residues of proteins, typically enzymes, thereby altering their activity. Thus, the notion that reactive intermediates are inherently dangerous is unjustified. When a reactive intermediate is necessary for the desired pharmacological effect of a drug, the selectivity it displays towards the target protein is crucial, as off-target binding may produce unwanted toxicities. On the other hand, reactive intermediates may play no role in toxicity. This review provides a balanced perspective, primarily focusing on the proposed role of reactive intermediates in drug toxicity, while also highlighting examples in which they are involved in causing the desired pharmacology. It is hoped that this knowledge can help scientists involved in drug discovery and development in their challenging task of producing safe and effective drugs.

Involvement of enzymes other than CYPs in the oxidative metabolism of xenobiotics

Although the majority of oxidative metabolic reactions are mediated by the CYP superfamily of enzymes, non-CYP-mediated oxidative reactions can play an important role in the metabolism of xenobiotics. The (major) oxidative enzymes, other than CYPs, involved in the metabolism of drugs and other xenobiotics are: the flavin-containing monooxygenases, the molybdenum hydroxylases (aldehyde oxidase and xanthine oxidase), the prostaglandin H synthase, the lipoxygenases, the amine oxidases (monoamine, polyamine, diamine and semicarbazide-sensitive amine oxidases) and the alcohol and aldehyde dehydrogenases. In a similar manner to CYPs, these oxidative enzymes can also produce therapeutically active metabolites and reactive/toxic metabolites, modulate the efficacy of therapeutically active drugs or contribute to detoxification. Many of them have been shown to be important in endobiotic metabolism, and, consequently, interactions between drugs and endogenous compounds might occur when they are involved in drug metabolism. In general, most non-CYP oxidative enzymes appear to be noninducible or much less inducible than the CYP system, although some of them may be as inducible as some CYPs. Some of these oxidative enzymes exhibit polymorphic expression, as do some CYPs. It is possible that the contribution of non-CYP oxidative enzymes to the overall metabolism of xenobiotics is underestimated, as most investigations of drug metabolism in discovery and lead optimisation are performed using in vitro test systems optimised for CYP activity.

Fluorofelbamate

The incidence of refractory seizures has remained at 30-40%, even with the approval of nine new anticonvulsants over the past 12 years. In attempts to reduce seizure frequency and severity, physicians routinely resort to combining two or more anticonvulsants, ideally with different mechanisms of action. These combinatorial therapies are difficult to administer for both patient and caregiver and often result in tolerability issues. Hence, a broad spectrum anticonvulsant, with multiple mechanisms of action, that is well tolerated, would provide physicians with an important option in their armamentarium to control seizures. Felbamate initially fit this profile and was demonstrated to effectively control both partial and generalized seizures in clinical studies supporting registration. Unfortunately, unanticipated idiosyncratic toxicity was observed after approval and the drug is now relegated to second or third line therapy, depending on patient history and seizure type. Epileptologists still prescribe this drug for refractory seizures, and a recent communication indicates that 35,000 to 46,000 new patients have tried Felbatol (MedPointe Pharmaceuticals, Somerset, NJ) since 1995. The continued utilization of Felbatol, in light of its risk:benefit issues, highlights the need for new efficacious therapeutic options. Fluorofelbamate (MedPointe Pharmaceuticals), a phase I drug candidate, was designed to retain the broad spectrum multimechanistic activity of felbamate, with a modified metabolism that has demonstrated, in vitro, to avoid the production of the reactive metabolite believed to cause the idiosyncratic toxicity. This drug candidate is one of several carbamates either in development or currently on the market for treatment of seizures and other CNS disorders.

Progress report on new antiepileptic drugs: a summary of the Eigth Eilat Conference (EILAT VIII)

The Eigth Eilat Conference on New Antiepileptic Drugs (AEDs)-EILAT VII, took place in Sitges, Barcelona from the 10th to 14th September, 2006. Basic scientists, clinical pharmacologists and neurologists from 24 countries attended the conference, whose main themes included a focus on status epilepticus (epidemiology, current and future treatments), evidence-based treatment guidelines and the potential of neurostimulation in refractory epilepsy. Consistent with previous formats of this conference, the central part of the conference was devoted to a review of AEDs in development, as well as updates on marketed AEDs introduced since 1989. This article summarizes the information presented on drugs in development, including brivaracetam, eslicarbazepine acetate (BIA-2-093), fluorofelbamate, ganaxolone, huperzine, lacosamide, retigabine, rufinamide, seletracetam, stiripentol, talampanel, valrocemide, JZP-4, NS1209, PID and RWJ-333369. Updates on felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine and new extended release oxcarbazepine formulations, pregabalin, tiagabine, topiramate, vigabatrin, zonisamide and new extended release valproic acid formulations, and the antiepileptic vagal stimulator device are also presented.

Pharmacokinetic Interaction Study between the New Antiepileptic and CNS Drug RWJ-333369 and Carbamazepine in Healthy Adults

PURPOSE

To characterize the possible pharmacokinetic interaction between the new antiepileptic and CNS drug RWJ-333369 and carbamazepine (CBZ) following multiple dosing in healthy subjects.

METHODS

In an 8-week, open-label, sequential design study, 24 healthy adults received multiple-dose RWJ-333369 alone (5 days 250 mg q12h; 5 days 500 mg q12h), then after a 4-day washout, multiple-dose CBZ alone (3 days 100 mg q12h; 3 days 200 mg q12h; 22 days 300 mg q12h), and then combination of CBZ (300 mg q12h), and RWJ-333369 (5 days 250 mg q12h; 5 days 500 mg q12h).

RESULTS

At steady-state following multiple dosing, RWJ-333369 peak plasma concentration (C(max)) and area under the concentration-time-curve within the dosing interval (AUCss) increased in proportion to dose. The C(max) and AUCss of CBZ were similar when given alone or concomitantly with RWJ-333369. The 90% confidence intervals for the ratio of CBZ C(max) and AUCss with/without RWJ-333369 were: 94-104% and 95-104%, respectively (well within the equivalence range of 80-125%). When RWJ-333369 was administered with CBZ, its mean (SD) oral clearance increased from 3.2 L/h to 4.9 L/h and consequently its mean half-life was shortened from 10.4 (1.9) h to 7.4 (1.2) h, and mean AUCss and C(max) were reduced by 37% and 30%, respectively.

CONCLUSIONS

There was no effect of multiple-dose RWJ-333369 on CBZ pharmacokinetics. CBZ induced RWJ-333369 clearance, resulting in shortened half-life and decreased exposure (AUCss) and C(max). Concomitant administration of RWJ-333369 with CBZ was generally safe and tolerated.

Pharmacokinetics of the New Antiepileptic and CNS Drug RWJ-333369 Following Single and Multiple Dosing to Humans

PURPOSE

To characterize the pharmacokinetics of the new antiepileptic and CNS drug RWJ-333369 following single and multiple oral doses to healthy subjects, including the effect of food on bioavailability.

METHOD

Two studies were conducted. The first study had a randomized, double-blind, placebo-controlled, sequential, ascending-dose crossover design. Subjects were divided into four dose groups (100, 250, 500, and 750 mg) of 10 to 11 subjects each. RWJ-333369 or placebo was administered for two 7-day periods, separated by a 14-day washout. In the second study RWJ-333369 (750 mg) was administered to 12 healthy subjects under fasted and fed conditions. Plasma and urine samples were analyzed for RWJ-333369 by liquid chromatography-mass spectroscopy. Safety was assessed throughout the studies.

RESULTS

Mean (range) pharmacokinetic parameters in the above studies were: oral clearance (CL/F) 3.4-4.2 L/h, half-life (t(1/2)) 10.6-12.8 h, and renal clearance (CLr) 0.042-0.094 L/h, indicating that RWJ-333369 is eliminated primarily by metabolism. These parameters were not significantly different (p > 0.05) for the four dose groups and for single and multiple dosing. C(max) and AUC increased proportionally with dose and decreased with food by 11% and 5%, respectively.

CONCLUSIONS

Following single and repetitive (q12h) doses of 100-750 mg, RWJ-333369 had linear pharmacokinetics; food did not alter pharmacokinetics to a clinically relevant extent. RWJ-333369 is extensively metabolized and has a low CL/F that equals < 5% of the liver blood flow. Thus, orally administered RWJ-333369 has no hepatic first-pass effect. The 12-h half-life will enable bid dosing with an immediate-release oral formulation.

The absorption, metabolism, and excretion of the novel neuromodulator RWJ-333369 (1,2-ethanediol, [1-2-chlorophenyl]-, 2-carbamate, [S]-) in humans

RWJ-333369 (1,2-ethanediol, [1-2-chlorophenyl]-, 2-carbamate, [S]-; CAS Registry Number 194085-75-1) is a novel neuromodulator in clinical development for the treatment of epilepsy. To study the disposition of RWJ-333369, eight healthy male subjects received a single oral dose of 500 mg of (14)C-RWJ-333369. Urine, feces, and plasma were collected for analysis for up to 1 week after dosing. Radioactivity was mainly excreted in urine (93.8 +/- 6.6%) and much less in feces (2.5 +/- 1.6%). RWJ-333369 was extensively metabolized in humans, since only low amounts of parent drug were excreted in urine (1.7% on average) and feces (trace amounts). The major biotransformation pathways were direct O-glucuronidation (44% of the dose), and hydrolysis of the carbamate ester followed by oxidation to 2-chloromandelic acid, which was subsequently metabolized in parallel to 2-chlorophenyl glycine and 2-chlorobenzoic acid (mean percentage of the dose for the three acids together was 36%). Other routes were chiral inversion followed by O-glucuronidation (11%), and aromatic hydroxylation in combination with sulfate conjugation (5%). In plasma, unchanged drug accounted for 76.5% of the total radioactivity, with the R-enantiomer and the O-glucuronide of the parent drug as the only measurable plasma metabolites. With the use of very sensitive liquid chromatography-tandem mass spectrometry techniques, only traces of aromatic (pre)mercapturic acid conjugates were detected in urine (each <0.3% of the dose), suggesting a low potential for reactive metabolite formation. In conclusion, the disposition of RWJ-333369 in humans is characterized by virtually complete absorption, extensive metabolism, and unchanged drug as the only significant circulating species.

New antiepileptic drugs that are second generation to existing antiepileptic drugs

In the last decade, 10 new antiepileptic drugs (AEDs) have been introduced that offer appreciable advantages in terms of their favourable pharmacokinetics, improved tolerability and lower potential for drug interactions. However, despite the large therapeutic range of old and new AEDs, approximately 30% of the patients with epilepsy are still not seizure free and, consequently, there is a substantial need to develop new AEDs. The new AEDs currently in development can be divided into two categories: drugs with completely new chemical structures such as lacosamide (formally harkoseride), retigabine, rufinamide and talampanel; and drugs that are derivatives or analogues of existing AEDs that can be regarded as second-generation or follow-up compounds of established AEDs. This article focuses on the second category and thus critically reviews the following second-generation compounds: eslicarbazepine acetate or BIA-2-093 and 10-hydroxy carbazepine (carbamazepine derivatives); valrocemide and NPS 1776 (isovaleramide; valproic acid derivatives); pregabalin and XP13512 (gabapentin derivatives); brivaracetam (ucb 34714) and seletracetam (ucb 44212; levetiracetam derivatives); and fluorofelbamate (a felbamate derivative). In addition, a series of valproic acid derivatives that are currently in preclinical stage has also been evaluated because some lead compounds of this series have a promising potential to become new antiepileptics and CNS drugs. For any of these follow-up compounds to become a successful second generation to an existing AED, it has to be more potent, safer and possess favourable pharmacokinetics, including low potential for pharmacokinetic and pharmacodynamic drug interactions.

In vitro and animal models of drug-induced blood dyscrasias

Drug-induced blood dyscrasias can be either acute and predictable or delayed and unpredictable (idiosyncratic). The predictable toxicity is relatively easy to reproduce with in vitro models, although they may not work for drugs that require bioactivation. It is very unlikely that idiosyncratic blood dyscrasias can be modeled in vitro, although some drugs (or their reactive metabolites) that cause idiosyncratic reaction are toxic to bone marrow cells in vitro. Although the mechanisms of idiosyncratic reactions are poorly understood, there is evidence that most are due to reactive metabolites and some are immune-mediated. Therefore screening drugs for their bioactivation by myeloperoxidase, the major oxidative enzyme in bone marrow, may provide some measure of the risk that a drug will cause blood dyscrasias. Several examples of drug-induced idiosyncratic agranulocytosis, aplastic anemia and thrombocytopenia are presented, but better in vivo models are clearly needed to gain a clearer understanding of these adverse reactions.

Pharmacokinetic Variability of Newer Antiepileptic Drugs

A new generation of antiepileptic drugs (AEDs) has reached the market in recent years with ten new compounds: felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide. The newer AEDs in general have more predictable pharmacokinetics than older AEDs such as phenytoin, carbamazepine and valproic acid (valproate sodium), which have a pronounced inter-individual variability in their pharmacokinetics and a narrow therapeutic range. For these older drugs it has been common practice to adjust the dosage to achieve a serum drug concentration within a predefined 'therapeutic range', representing an interval where most patients are expected to show an optimal response. However, such ranges must be interpreted with caution, since many patients are optimally treated when they have serum concentrations below or above the suggested range. It is often said that there is less need for therapeutic drug monitoring (TDM) with the newer AEDs, although this is partially based on the lack of documented correlation between serum concentration and drug effects. Nevertheless, TDM may be useful despite the shortcomings of existing therapeutic ranges, by utilisation of the concept of 'individual reference concentrations' based on intra-individual comparisons of drug serum concentrations. With this concept, TDM may be indicated regardless of the existence or lack of a well-defined therapeutic range. The ten newer AEDs all have different pharmacological properties, and therefore, the usefulness of TDM for these drugs has to be assessed individually. For vigabatrin, a clear relationship between drug concentration and clinical effect cannot be expected because of its unique mode of action. Therefore, TDM of vigabatrin is mainly to check compliance. The mode of action of the other new AEDs would not preclude the applicability of TDM. For the prodrug oxcarbazepine, TDM is also useful, since the active metabolite licarbazepine is measured. For drugs that are eliminated renally completely unchanged (gabapentin, pregabalin and vigabatrin) or mainly unchanged (levetiracetam and topiramate), the pharmacokinetic variability is less pronounced and more predictable. However, the dose-dependent absorption of gabapentin increases its pharmacokinetic variability. Drug interactions can affect topiramate concentrations markedly, and individual factors such as age, pregnancy and renal function will contribute to the pharmacokinetic variability of all renally eliminated AEDs. For those of the newer AEDs that are metabolised (felbamate, lamotrigine, oxcarbazepine, tiagabine and zonisamide), pharmacokinetic variability is just as relevant as for many of the older AEDs. Therefore, TDM is likely to be useful in many clinical settings for the newer AEDs. The purpose of the present review is to discuss individually the potential value of TDM of these newer AEDs, with emphasis on pharmacokinetic variability.

Improving the decision-making process in structural modification of drug candidates: reducing toxicity

The rule of three, relating to activity-exposure-toxicity, presents the single most difficult challenge in the design and advancement of drug candidates to the development stage. Absorption, distribution, metabolism and excretion (ADME) studies are widely used in drug discovery to optimize this balance of properties necessary to convert lead compounds into drugs that are both safe and effective for human patients. Idiosyncratic drug reactions (IDRs; referred to as type B reactions, which are mainly caused by reactive metabolites) are one type of adverse drug reaction that is important to human health and safety. This review highlights the strategies for the decision-making process involving substructures that, when found in drugs, can form reactive metabolites and are involved in toxicities in humans; the tools used to reduce IDRs are also discussed. Several examples are included to show how toxicity studies have influenced and guided drug design. Investigations of reactive intermediate formation in subcellular fractions with the use of radiolabeled reagents are also discussed.

Progress report on new antiepileptic drugs: a summary of the Seventh Eilat Conference (EILAT VII)

The Seventh Eilat Conference on New Antiepileptic Drugs (AEDs) (EILAT VII) took place in Villasimius, Sardinia, Italy from the 9th to 13th May 2004. Basic scientists, clinical pharmacologists and neurologists from 24 countries attended the conference,whose main themes included advances in pathophysiology of drug resistance, new AEDs in pediatric epilepsy syndromes, modes of AED action and spectrum of adverse effects and a re-appraisal of comparative responses to AED combinations. Consistent with previous formats of this conference, the central part of the conference was devoted to a review of AEDs in development, as well as updates on second-generation AEDs. This article summarizes the information presented on drugs in development, including atipamezole, BIA-2-093, fluorofelbamate, NPS 1776, pregabalin, retigabine, safinamide, SPM 927, stiripentol, talampanel,ucb 34714 and valrocemide (TV 1901). Updates on felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine,topiramate, vigabatrin, zonisamide, new oral and parenteral formulations of valproic acid and SPM 927 and the antiepileptic vagal stimulator device are also presented.

Therapeutic drug monitoring of the newer antiepileptic drugs

The aim of the present review is to discuss the potential value of therapeutic drug monitoring (TDM) of the newer antiepileptic drugs (AEDs) felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, and zonisamide. Studies of the relationship between serum concentrations and clinical efficacy of these drugs are reviewed, and the potential value of TDM of the drugs is discussed based on their pharmacokinetic properties and mode of action. Analytical methods for the determination of the serum concentrations of these drugs are also briefly described. There are only some prospective data on the serum concentration-effect relationships, and few studies have been designed primarily to study these relationships. As TDM is not widely practiced for the newer AEDs, there are no generally accepted target ranges for any of these drugs, and for most a wide range in serum concentration is associated with clinical efficacy. Furthermore, a considerable overlap in drug concentrations related to toxicity and nonresponse is reported. Nevertheless, the current tentative target ranges for felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine (10-hydroxy-carbazepine metabolite), tiagabine, topiramate, vigabatrin, and zonisamide are 125 to 250 micromol/L, 70 to 120 micromol/L, 10 to 60 micromol/L, 35 to 120 micromol/L, 50 to 140 micomol/L, 50 to 250 nmol/L, 15 to 60 micromol/L, 6 to 278 micromol/L, and 45 to 180 micromol/L, respectively. Further systematic studies designed specifically to evaluate concentration-effect relationships of the new AEDs are urgently needed. Although routine monitoring in general cannot be recommended at present, measurements of some of the drugs is undoubtedly of help with individualization of treatment in selected cases in a particular clinical setting.

Mechanisms of idiosyncratic drug reactions: the case of felbamate

Idiosyncratic drug reactions (IDR) are a specific type of drug toxicity characterized by their delayed onset, low incidence and reactive metabolite formation with little, if any, correlation between pharmacokinetics or pharmacodynamics and the toxicological outcome. As the name implies, IDR are unpredictable and often result in the post marketing failure of otherwise useful therapies. Examples of drugs, which have failed as a result of IDR in recent years, include trovafloxacin, zileuton, troglitazone, tolcapone and felbamate. To date there exists no pre-clinical model to predict these adverse drug reactions and a mechanistic understanding of these toxicities remains limited. In an attempt to better understand this class of drug toxicities and gain mechanistic insight, we have studied the IDR associated with a model compound, felbamate. Our studies with felbamate are consistent with the theory that compounds which cause IDR undergo bioactivation to a highly reactive electrophilic metabolite that is capable of forming covalent protein adducts in vivo. In additon, our data suggest that under normal physiological conditions glutathione plays a protective role in preventing IDR during felbamate therapy, further emphasizing a correlation between reactive metabolite formation and a toxic outcome. Clinical studies with felbamate have been able to demonstrate an association between reactive metabolite formation and a clinically relevant toxicity; however, additional research is required to more fully understand the link between reactive metabolite formation and the events which elicit toxicity. Going forward, it seems reasonable that screening for reactive metabolite formation in early drug discovery may be an important tool in eliminating the post-marketing failure of otherwise useful therapies.

Reactivity of atropaldehyde, a felbamate metabolite in human liver tissue in vitro

Antiepileptic therapy with a broad spectrum drug felbamate (FBM) has been limited due to reports of hepatotoxicity and aplastic anemia associated with its use. It was proposed that a bioactivation of FBM leading to formation of alpha,beta-unsaturated aldehyde, atropaldehyde (ATPAL) could be responsible for toxicities associated with the parent drug. Other members of this class of compounds, acrolein and 4-hydroxynonenal (HNE), are known for their reactivity and toxicity. It has been proposed that the bioactivation of FBM to ATPAL proceeds though a more stable cyclized product, 4-hydroxy-5-phenyltetrahydro-1,3-oxazin-2-one (CCMF) whose formation has been shown recently. Aldehyde dehydrogenase (ALDH) and glutathione transferase (GST) are detoxifying enzymes and targets for reactive aldehydes. This study examined effects of ATPAL and its precursor, CCMF on ALDH, GST and cell viability in liver, the target tissue for its metabolism and toxicity. A known toxin, HNE, which is also a substrate for ALDH and GST, was used for comparison. Interspecies difference in metabolism of FBM is well documented, therefore, human tissue was deemed most relevant and used for these studies. ATPAL inhibited ALDH and GST activities and led to a loss of hepatocyte viability. Several fold greater concentrations of CCMF were necessary to demonstrate a similar degree of ALDH inhibition or cytotoxicity as observed with ATPAL. This is consistent with CCMF requiring prior conversion to the more proximate toxin, ATPAL. GSH was shown to protect against ALDH inhibition by ATPAL. In this context, ALDH and GST are detoxifying pathways and their inhibition would lead to an accumulation of reactive species from FBM metabolism and/or metabolism of other endogenous or exogenous compounds and predisposing to or causing toxicity. Therefore, mechanisms of reactive aldehydes toxicity could include direct interaction with critical cellular macromolecules or indirect interference with cellular detoxification mechanisms.

Anticonvulsant and antiepileptogenic effects of fluorofelbamate in experimental status epilepticus

PURPOSE

To examine the seizure-protective properties of fluorofelbamate, a felbamate analog, on acute and chronic seizures in an experimental model of self-sustaining status epilepticus (SSSE).

METHODS

SSSE was induced by stimulation of the perforant path for 30 min (PPS) through chronically implanted electrodes in free-running adult male Wistar rats. Fluorofelbamate was injected intravenously (i.v.) either 10 min, or 40 min after SSSE induction. Seizure and spike profiles were analyzed off-line.

RESULTS

Fluorofelbamate injected during the early stages of SSSE (10 min after the end of PPS), shortened the duration of seizures in a dose-dependent manner. While a dose of 50 mg kg(-1) was ineffective, 100 and 200 mg kg(-1) reduced cumulative seizure time from 393 +/- 10 min to 15 +/- 8 min and 2.4 +/- 0.5 min respectively. Administration of fluorofelbamate (200 and 300 mg kg (-1)) at a late stage of SSSE, which is refractory to treatment with conventional anticonvulsants, also significantly attenuated seizures. Acute fluorofelbamate treatment (200 mg kg(-1) 10 min after PPS) significantly decreased the frequency of spontaneous seizures which follow SSSE after a 'latent' interval. Moreover, in contrast to control animals, fluorofelbamate-treated rats showed regression of spontaneous seizures, and an apparent remission of epilepsy within 2 months after SSSE.

CONCLUSIONS

Acute treatment of SSSE with fluorofelbamate showed strong anticonvulsant effects even during the late stages of SSSE. In this model, it also displayed antiepileptogenic properties: it reduced the severity of chronic epilepsy after SSSE and lead to apparent remissions of that epilepsy.

Progress report on new antiepileptic drugs: a summary of the Fifth Eilat Conference (EILAT V)

The Fifth Eilat Conference on New Antiepileptic Drugs (AEDs) took place at the Dan Hotel, Eilat, Israel, 25-29 June 2000. Basic scientists, clinical pharmacologists and neurologists from 20 countries attended the conference, whose main themes included recognition of unexpected adverse effects, new indications of AEDs, and patient-tailored AED therapy. According to tradition, the central part of the conference was devoted to a review of AEDs in development, as well to updates on AEDs that have been marketed in recent years. This article summarizes the information presented on drugs in preclinical and clinical development, including AWD 131-138, DP-valproate, harkoseride, LY300164, NPS 1776, NW 1015, pregabalin, remacemide, retigabine, rufinamide and valrocemide. The potential value of an innovative strategy, porcine embryonic GABAergic cell transplants, is also discussed. Finally, updates on felbamate, fosphenytoin, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide, and the antiepileptic vagal stimulator device are presented.

New antiepileptic drugs in children: which ones for which seizures?

Until 1993, carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PB), and valproate (VPA) accounted for the great majority of the prescriptions written for the treatment of epilepsy. Since 1993, five antiepileptic drugs (AEDs) have been released in the United States, and at least three additional drugs are expected to be released by the end of the year 2000. As a group, these newer drugs differ from the established drugs in terms of their pharmacokinetics, interaction potential, and adverse effects. In addition, any one of the newer drugs may achieve seizure control in situations in which an established drug had not. The newer drugs certainly represent a welcome addition to the existing options for the treatment of epilepsy in children. However, the availability of several new AEDs represents a therapeutic dilemma for the clinician because optimal use of these drugs has not yet been established. This is particularly true in children because (i) newer drugs are often studied less frequently in children, (ii) pharmacokinetics in children differ from those in adults, (iii) children may have different adverse effects, and (iv) children have a broader spectrum of various seizure types and epilepsy syndromes. In the first part of this review, the clinical pharmacology of the currently available newer AEDs is discussed individually, with special emphasis on data in children. In particular, pharmacokinetics, interactions, dosage and titration, efficacy spectrum, and adverse effect profile is discussed for each drug. In the second part, an attempt is made to determine the place for the newer drugs in the treatment of the different pediatric seizures and epilepsy syndromes.

Quantification in patient urine samples of felbamate and three metabolites: acid carbamate and two mercapturic acids

PURPOSE

Previously we proposed and provided evidence for the metabolic pathway of felbamate (FBM), which leads to the reactive metabolite, 3-carbamoyl-2-phenylpropion-aldehyde. This aldehyde carbamate was suggested to be the reactive intermediate in the oxidation of 2-phenyl-1,3-propanediol monocarbamate to the major human metabolite 3-carbamoyl-2-phenylpropionic acid. In addition, the aldehyde carbamate was found to undergo spontaneous elimination to 2-phenylpropenal, commonly known as atropaldehyde. Moreover, atropaldehyde was proposed to play a role in the development of toxicity during FBM therapy. Evidence for atropaldehyde formation in vivo was reported with the identification of modified N-acetyl-cysteine conjugates of atropaldehyde in both human and rat urine after FBM administration. Identification of the atropaldehyde-derived mercapturic acids in urine after FBM administration is consistent with the hypothesis that atropaldehyde is formed in vivo and that it reacts with thiol nucleophiles. Based on the hypothesis that the potential for toxicity will correlate to the amount of atropaldehyde formed, we sought to develop an analytic method that would quantify the amount of relevant metabolites excreted in patient urine.

METHODS

We summarize the results of an LC/MS method used to quantify FBM, 3-carbamoyl-2-phenylpropionic acid and two atropaldehyde-derived mercapturic acids in the patient population.

RESULTS

Analysis was performed on 31 patients undergoing FBM therapy. The absolute quantities of FBM and three metabolites were measured.

CONCLUSIONS

This method demonstrated sufficient precision for the identification of patients exhibiting "abnormal" levels of atropaldehyde conjugates and may hold potential for patient monitoring.

Metabolism of the lipid peroxidation product, 4-hydroxy-trans-2-nonenal, in isolated perfused rat heart

The metabolism of 4-hydroxy-trans-2-nonenal (HNE), an alpha, beta-unsaturated aldehyde generated during lipid peroxidation, was studied in isolated perfused rat hearts. High performance liquid chromatography separation of radioactive metabolites recovered from [3H]HNE-treated hearts revealed four major peaks. Based on the retention times of synthesized standards, peak I, which accounted for 20% radioactivity administered to the heart, was identified to be due to glutathione conjugates of HNE. Peaks II and III, containing 2 and 37% radioactivity, were assigned to 1, 4-dihydroxy-2-nonene (DHN) and 4-hydroxy-2-nonenoic acid, respectively. Peak IV was due to unmetabolized HNE. The electrospray ionization mass spectrum of peak I revealed two prominent metabolites with m/z values corresponding to [M + H]+ of HNE and DHN conjugates with glutathione. The presence of 4-hydroxy-2-nonenoic acid in peak III was substantiated using gas chromatography-chemical ionization mass spectroscopy. When exposed to sorbinil, an inhibitor of aldose reductase, no GS-DHN was recovered in the coronary effluent, and treatment with cyanamide, an inhibitor of aldehyde dehydrogenase, attenuated 4-hydroxy-2-nonenoic acid formation. These results show that the major metabolic transformations of HNE in rat heart involve conjugation with glutathione and oxidation to 4-hydroxy-2-nonenoic acid. Further metabolism of the GS-HNE conjugate involves aldose reductase-mediated reduction, a reaction catalyzed in vitro by homogenous cardiac aldose reductase.