Reactive Drug Metabolites Research Articles

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 targetsof reactive drug metabolites, and to elucidate the structure–activity relationships of drug's covalent binding to proteins and their clinical outcomes.

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 targetsof reactive drug metabolites, and to elucidate the structure–activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Time-dependent toxicity of sulfonamide reactive metabolites in cells of patients with sulfonamide hypersensitivity

Background Sulfonamide hypersensitivity is a major clinical problem for many patients and the mechanisms remain largely unknown. We have previously identified altered response to reactive drug metabolites as an important component of drug hypersensitivity. Methods Peripheral blood mononuclear cells from patients with a history of sulfonamide hypersensitivity (n=6) and normal controls (n=6) were incubated with increasing concentrations of sulfamethoxazole and sulfamethoxazole hydroxylamine over time with determinations of cell viability every 6 hours up to 36 hours. Cell viability was determined using an MTT method and the degree of apoptosis determined using FACS. Results A time-dependent decrease in viability was seen in the cells of both controls and patients, with significantly more cell death (p>0.05) seen in the cells of patients than controls after 12 hours of incubation (p>0.05). This was associated with a significant increase in cell death related to necrosis in the cells of patients (p>0.01) versus controls at time 24 hours. Conclusions Time-related delays in differential cellular sensitivity to reactive drug metabolites and differential mechanisms of cell death suggest that there are alterations in the ability of the cells of susceptible individiuals to respond that limits the ability of these cells to deal with reactive drug metabolites. Possible cellular mechanism(s) include may include altered ability to deal with reactive species. Clinical Pharmacology & Therapeutics (2004) 75, P26–P26; doi: 10.1016/j.clpt.2003.11.097

Increased reactive oxygen stress in cells of patients with drug hypersensitivity

Background Adverse drug reactions are important problems associated with therapy. The mechanisms of drug hypersensitivity remain largely unknown. We have previously identified altered response to reactive drug metabolites as an important component of drug hypersensitivity. Methods Peripheral blood mononuclear cells from patients with a history of sulfonamide hypersensitivity (n=6) and normal controls (n=6) were incubated with increasing concentrations of sulfamethoxazole and sulfamethoxazole hydroxylamine and the degree of reactive oxygen stress (ROS) determined using DH2TH after a 24 hour incubation. Cell viability was determined using an MTT method and the degree of apoptosis determined using FACS. Results A concentration-dependent increase in the amount of ROS was seen in the cells of both controls and patients, with significantly more ROS (p>0.05) seen in the cells of patients than controls. This was associated with a significant increase in cell death in the cells of patients (p>0.01) versus controls. The cells of controls showed significantly more apoptosis than did the cells of controls (p>0.05), with the cells of patients showing significantly more necrosis (p>0.05). No increases in ROS or cell death were seen in the presence of the parent drug. Conclusions The cells of patients who had reactions had significantly more ROS and necrosis than control cells, suggesting that altered response to ROS may be a critical factor in drug hypersensitivity. Clinical Pharmacology & Therapeutics (2004) 75, P26–P26; doi: 10.1016/j.clpt.2003.11.096

ORAL ADVERSE DRUG REACTIONS TO CARDIOVASCULAR DRUGS.

A great many cardiovascular drugs (CVDs) have the potential to induce adverse reactions in the mouth. The prevalence of such reactions is not known, however, since many are asymptomatic and therefore are believed to go unreported. As more drugs are marketed and the population includes an increasing number of elderly, the number of drug prescriptions is also expected to increase. Accordingly, it can be predicted that the occurrence of adverse drug reactions (ADRs), including the oral ones (ODRs), will continue to increase. ODRs affect the oral mucous membrane, saliva production, and taste. The pathogenesis of these reactions, especially the mucosal ones, is largely unknown and appears to involve complex interactions among the drug in question, other medications, the patient's underlying disease, genetics, and life-style factors. Along this line, there is a growing interest in the association between pharmacogenetic polymorphism and ADRs. Research focusing on polymorphism of the cytochrome P450 system (CYPs) has become increasingly important and has highlighted the intra- and inter-individual responses to drug exposure. This system has recently been suggested to be an underlying candidate regarding the pathogenesis of ADRs in the oral mucous membrane. This review focuses on those CVDs reported to induce ODRs. In addition, it will provide data on specific drugs or drug classes, and outline and discuss recent research on possible mechanisms linking ADRs to drug metabolism patterns. Abbreviations used will be as follows: ACEI, ACE inhibitor; ADR, adverse drug reaction; ANA, antinuclear antigen; ARB, angiotensin II receptor blocker; BAB, beta-adrenergic blocker; CCB, calcium-channel blocker; CDR, cutaneous drug reaction; CVD, cardiovascular drug; CYP, cytochrome P450 enzyme; EM, erythema multiforme; FDE, fixed drug eruption; I, inhibitor of CYP isoform activity; HMG-CoA, hydroxymethyl-glutaryl coenzyme A; NAT, N-acetyltransferase; ODR, oral drug reaction; RDM, reactive drug metabolite; S, substrate for CYP isoform; SJS, Stevens-Johnson syndrome; SLE, systemic lupus erythematosus; and TEN, toxic epidermal necrolysis.

Herbal bioactivation: The good, the bad and the ugly

It has been well established that the formation of reactive metabolites of drugs is associated with drug toxicity. Similarly, there are accumulating data suggesting the role of the formation of reactive metabolites/intermediates through bioactivation in herbal toxicity and carcinogenicity. It has been hypothesized that the resultant reactive metabolites following herbal bioactivation covalently bind to cellular proteins and DNA, leading to toxicity via multiple mechanisms such as direct cytotoxicity, oncogene activation, and hypersensitivity reactions. This is exemplified by aristolochic acids present in Aristolochia spp, undergoing reduction of the nitro group by hepatic cytochrome P450 (CYP1A1/2) or peroxidases in extrahepatic tissues to reactive cyclic nitrenium ion. The latter was capable of reacting with DNA and proteins, resulting in activation of H-ras oncogene, gene mutation and finally carcinogenesis. Other examples are pulegone present in essential oils from many mint species; and teucrin A, a diterpenoid found in germander ( Teuchrium chamaedrys) used as an adjuvant to slimming diets. Extensive pulegone metabolism generated p-cresol that was a glutathione depletory, and the furan ring of the diterpenoids in germander was oxidized by CYP3A4 to reactive epoxide which reacts with proteins such as CYP3A and epoxide hydrolase. On the other hand, some herbal/dietary constituents were shown to form reactive intermediates capable of irreversibly inhibiting various CYPs. The resultant metabolites lead to CYP inactivation by chemical modification of the heme, the apoprotein, or both as a result of covalent binding of modified heme to the apoprotein. Some examples include bergamottin, a furanocoumarin of grapefruit juice; capsaicin from chili peppers; glabridin, an isoflavan from licorice root; isothiocyanates found in all cruciferous vegetables; oleuropein rich in olive oil; dially sulfone found in garlic; and resveratrol, a constituent of red wine. CYPs have been known to metabolize more than 95% therapeutic drugs and activate a number of procarcinogens as well. Therefore, mechanism-based inhibition of CYPs may provide an explanation for some reported herb-drug interactions and chemopreventive activity of herbs. Due to the wide use and easy availability of herbal medicines, there is increasing concern about herbal toxicity. The safety and quality of herbal medicine should be ensured through greater research, pharmacovigilance, greater regulatory control and better communication between patients and health professionals.

Identification of coenzyme A-related tolmetin metabolites in rats: relationship with reactive drug metabolites

1. It has recently been proposed that acyl coenzyme A thioesters (acyl-CoAs) of xenobiotic carboxylic acids are electrophilic, reactive metabolites that may react with proteins. 2. The primary objective was to investigate the reactivity of the tolmetin acyl coenzyme A thioester (Tol-CoA). The second objective was to identify and quantify tolmetin (Tol) metabolites in vivo that were formed via Tol-CoA, e.g. the glycine (Tol-Gly) and taurine (Tol-Tau) conjugates. This finding would be indicative of Tol-CoA formation and thus of other acyl-CoA-related reactions that might occur, e.g. covalent binding to proteins. 3. In order to study the chemical reactivity, Tol-CoA (0.5 mM) was incubated with glutathione (5 mM) in a 0.1 M phosphate buffer (pH 7.4) at 37°C. Tol-CoA reacted rapidly with glutathione in vitro to form the S -acyl glutathione conjugate at a rate of 14.9 ± 0.7 µ M min − 1 (mean ± SD, n = 3) from 0 to 10 min. Compared with acyl-CoAs of other xenobiotic carboxylic acids, naproxen and clofibric acid, the rate by which Tol-CoA reacted with glutathione was high. 4. Following administration of 3 H-Tol (100 mg kg − 1, 200 µ Ci kg − 1, p.o.) to male Sprague-Dawley rats, Tol-Tau and Tol-Gly were identified in urine by electrospray ionization MS-MS in both positive- and negative-ion modes. The conjugates were only formed at trace levels (< 0.5%). However, the presence of Tol-Tau and Tol-Gly showed the reactive Tol-CoA was formed in vivo.

Reactive metabolites and adverse drug reactions: clinical considerations.

Idiosyncratic reactions can affect many different organ systems, either as -an isolated event (e.g., hepatitis) or as part of a syndrome (e.g., drug hypersensitivity syndrome). Formation of reactive metabolites of drugs in conjunction with a decreased ability for detoxification is believed to be the initiating step in many idiosyncratic reactions. The drug hypersensitivity syndrome, defined by the presence of fever, rash and internal organ involvement, is estimated to occur in approx 1 in 1000 to 1 in 10,000 exposures with drugs such as anticonvulsants sulfonamide antibiotics, allopurinol, and dapsone. Symptoms usually start within 2-8 wk of drug initiation. Serum sicknesslike reaction, most frequently found after 7-14 d of drug exposure, is distinguished by a fever, cutaneous eruption and arthralgias. Medications such as cefaclor, minocycline and bupropion are most frequently implicated in this reaction. In contrast, drug-induced lupus can occur l-2 yr after initiation of medication. Drug-induced lupus is characterized by musculoskeletal complaints and fever and weight loss. Drugs most commonly associated with drug-induced lupus include procainamide, hydralazine, chlorpromazine, isoniazid, and minocycline. Management of patients who develop idiosyncratic reactions includes discontinuation of the implicated drug, initiation of corticosteroids (when appropriate), and symptomatic relief as required. Internal organ involvement, which may initially be asymptomatic, should be monitored. Confirmatory or diagnostic tests are not readily available in most areas, except for research purposes.

Predicting drug-induced agranulocytosis: characterizing neutrophil-generated metabolites of a model compound, DMP 406, and assessing the relevance of an in vitro apoptosis assay for identifying drugs that may cause agranulocytosis

DMP 406 is a clozapine analogue developed by Dupont-Pharma for the treatment of schizophrenia. Unfortunately it caused agranulocytosis in dogs during preclinical studies. Clozapine also causes agranulocytosis and this is believed to be due to a reactive nitrenium ion metabolite produced by neutrophils. We studied the oxidation of DMP 406 by activated neutrophils and found that the major reactive species that is produced is not a nitrenium ion but rather an imine. This metabolite is similar to the reactive metabolite that has been proposed to be responsible for mianserin-induced agranulocytosis. Therefore we also studied the oxidation of mianserin by activated neutrophils and found that, although the major species is an iminium ion, it also bears a lactam moiety in the piperazine ring resulting from further oxidation. We usually find that HOCl is a good model system for the production of reactive metabolites of drugs that are formed by activated neutrophils, but in the case of both DMP 406 and mianserin, the products produced were significantly different than those formed by activated neutrophils. In contrast, the combination of horseradish peroxidase and hydrogen peroxide (HRP/H 2O 2) formed a very similar pattern of products, and this system was used to produce sufficient quantities of metabolites to allow for identification. The reactive metabolites of both DMP 406 and mianserin reacted with a range of nucleophiles, but in many cases the reaction was reversible. The best nucleophile for trapping these reactive metabolites was cyanide. It has been demonstrated that the products of clozapine oxidation by HRP/H 2O 2, presumably the nitrenium ion, induced apoptosis in neutrophils at therapeutic concentrations of clozapine. It has been suggested that this process is involved in the mechanism of clozapine-induced agranulocytosis. We tested DMP 406 and mianserin in this system to see if the ability of a reactive metabolite of a drug to cause apoptosis could predict the ability of that drug to cause agranulocytosis. We used clozapine as a positive control and we also tested olanzapine, a drug that forms a reactive metabolite similar to that of clozapine but is given at a lower dose and does not cause agranulocytosis. We found that DMP 406 did not increase apoptosis at concentrations below 50 μM, and although mianserin did increase apoptosis at 10 μM this is above the therapeutic concentration. Olanzapine caused an increase in apoptosis at the same concentration as clozapine (1 μM), but because its therapeutic concentration is lower, this concentration was above the pharmacological range. There was no increase in apoptosis with any drug in the absence of HRP/H 2O 2. These results indicate that this assay is unable to reliably predict the ability of different types of drugs to cause agranulocytosis. This is not a surprising result given that different drugs may induce agranulocytosis by different mechanisms.

Inhibition of tubulin assembly and covalent binding to microtubular protein by valproic acid glucuronide in vitro

Acyl glucuronides are reactive metabolites of carboxylate drugs, able to undergo a number of reactions in vitro and in vivo, including isomerization via intramolecular rearrangement and covalent adduct formation with proteins. The intrinsic reactivity of a particular acyl glucuronide depends upon the chemical makeup of the drug moiety. The least reactive acyl glucuronide yet reported is valproic acid acyl glucuronide (VPA-G), which is the major metabolite of the antiepileptic agent valproic acid (VPA). In this study, we showed that both VPA-G and its rearrangement isomers ( iso-VPA-G) interacted with bovine brain microtubular protein (MTP, comprised of 85% tubulin and 15% microtubule associated proteins {MAPs}). MTP was incubated with VPA, VPA-G and iso-VPA-G for 2 h at room temperature and pH 7.5 at various concentrations up to 4 mM. VPA-G and iso-VPA-G caused dose-dependent inhibition of assembly of MTP into microtubules, with 50% inhibition (IC 50) values of 1.0 and 0.2 mM respectively, suggesting that iso-VPA-G has five times more inhibitory potential than VPA-G. VPA itself did not inhibit microtubule formation except at very high concentrations (≥2 mM). Dialysis to remove unbound VPA-G and iso-VPA-G (prior to the assembly assay) diminished inhibition while not removing it. Comparison of covalent binding of VPA-G and iso-VPA-G (using [ 14C]-labelled species) showed that adduct formation was much greater for iso-VPA-G. When [ 14C]- iso-VPA-G was reacted with MTP in the presence of sodium cyanide (to stabilize glycation adducts), subsequent separation into tubulin and MAPs fractions by ion exchange chromatography revealed that 78 and 22% of the covalent binding occurred with the MAPs and tubulin fractions respectively. These experiments support the notion of both covalent and reversible binding playing parts in the inhibition of microtubule formation from MTP (though the acyl glucuronide of VPA is less important than its rearrangement isomers in this regard), and that both tubulin and (perhaps more importantly) MAPs form adducts with acyl glucuronides.

N-OXIDATION OF DRUGS ASSOCIATED WITH IDIOSYNCRATIC DRUG REACTIONS

Circumstantial evidence strongly suggests that most idiosyncratic drug reactions are due to reactive metabolites of drugs rather than due to the drugs themselves. Many of the drugs that are associated with idiosyncratic drug reactions contain nitrogen. There are many possible reasons for this association. One is simply that many drugs, especially CNS active drugs, contain nitrogen. In addition, nitrogen is relatively easy to oxidize because of its lone pair of electrons and many nitrogen-containing compounds readily undergo redox cycling, which can generate reactive oxygen species. There are several nitrogen-containing function groups that are especially associated with adverse reactions. These include aromatic amines, nitro compounds (nitro compounds are reduced to the same reactive intermediates as are formed by oxidation of the corresponding aromatic amine), hydrazines and compounds that can be oxidized to iminoquinone and related compounds. A greater attention to the issue of reactive metabolites during drug development would likely lead to safer drugs; however, not all drugs that form reactive metabolites are associated with a high incidence of idiosyncratic drug reactions. In addition to the presence of such a group, other factors, such as dose and electron density of the compound, appear to play a role in whether the drug containing such functional groups will be associated with a relatively high incidence of idiosyncratic drug reactions.

Anticonvulsant hypersensitivity syndrome in children: incidence, prevention and management.

Anticonvulsant hypersensitivity syndrome (AHS) is a rare, but potentially fatal, adverse reaction that occurs in patients, including children, who are treated with anticonvulsants. During metabolism of the anticonvulsant, toxic arene-oxide compounds are produced. AHS is associated with both cutaneous and systemic symptoms and is associated with multiorgan involvement. Liver damage, in particular, seems to be associated with fatal outcomes. The pathophysiology of AHS is still uncertain but it may be linked to a genetically determined inability to detoxify reactive drug metabolites. The prompt recognition of the first clinical signs of AHS, and the rapid withdrawal of the anticonvulsant, often avoids the progression of symptoms. Pharmacological treatment is essentially based on systemic corticosteroids in association with enteral nutrition, intravenous fluid augmentation, pain relief and ocular care. Intravenous immunoglobulins may also have a possible therapeutic role in some cases. Diagnostic tests, such as patch tests or in vitro assays, for AHS could help to identify patients at risk of developing the syndrome and could represent a first step of primary prevention when applied to relatives of patients.

Hypersensitivity adverse drug reactions in children: Pathophysiology and therapeutic implications

Background: Adverse drug reactions are common problems for children and adults. Among the most serious are hypersensitivity reactions. The pathogenesis of these reactions has not been understood, but recently there have been new insights into the mechanism(s) responsible for these complex adverse drug reactions. Objectives: The goals of this paper were to review potential mechanisms for hypersensitivity adverse drug reactions and to relate these mechanisms to the care of children with hypersensitivity reactions. Methods: A search for original research in the pathogenesis of hypersensitivity adverse drug reactions was done using MEDLINE from 1985 to 2001. Other databases were not consulted. The research reviewed was confined to that described in English. Results: Hypersensitivity adverse drug reactions appear to be initiated by metabolism of the parent drug to reactive intermediates. The ongoing propagation of these reactions appears to be mediated by immunity. The mechanism(s) involved in the immunopathogenesis of drug hypersensitivity reactions mediated by reactive drug metabolites remains unclear, but may include changes in T-cell responses mediated by changes in T-cell signaling. Conclusions: Hypersensitivity adverse drug reactions are important challenges for health care workers caring for children. Health care workers need to be vigilant for the development of hypersensitivity adverse drug reactions and be aware that these reactions may continue to evolve clinically despite discontinuation of the drug in question.

Phagocyte-mediated oxidation in idiosyncratic adverse drug reactions

The drug-metabolizing capacity of the liver is well known but cannot account for most idiosyncratic adverse drug reactions. Of the extrahepatic sources of reactive drug metabolites, the neutrophil has received the most attention because of its vast numbers and robust oxidizing machinery. Many drugs associated with autoimmunity are susceptible to oxidative transformation by the enzymatic action of myeloperoxidase, a protein released into the extracellular environment when neutrophils are activated. Production of the resulting drug metabolites within lymphoid organs maximizes their immune-perturbing effects. Mechanisms proposed for the initiation of drug-induced blood dyscrasias, hypersensitivity reactions, or lupus-like symptoms center around three views: (1) presentation of the implicated compound in the major histocompatibility complex of antigen-presenting cells via direct binding or after processing as a hapten bound to self-macromolecules, (2) direct cytotoxicity, or (3) interference in the development of T-cell tolerance in the thymus. How participation of reactive drug metabolites in these processes might lead to symptomatic disease is discussed.

Biological reactive intermediates in drug discovery and development: a perspective from the pharmaceutical industry.

The detection of chemically-reactive, electrophilic metabolites poses a particular problem in the discovery and development of drug candidates in pharmaceutical research, inasmuch as it is not possible to accurately predict the likely toxicological consequences of these intermediates in animal safety studies or in clinical trials. Advances in analytical instrumentation (notably liquid chromatography-tandem mass spectrometry [LC-MS/MS]) have facilitated the detection of reactive intermediates through the identification of the glutathione (GSH) adducts to which they normally give rise, while the increased use of radiolabeled tracers in drug development permits an early assessment to be made of the propensity of a drug candidate to undergo covalent binding to cellular macromolecules. Unfortunately, these advances in analytical methods for the detection and characterization of reactive drug metabolites have far outstripped our understanding of the mechanisms of foreign compound-mediated toxicities at the molecular level, and of the role of both covalent binding and oxidative stress in the cascade of events that lead ultimately to cellular injury or immune-mediated toxicities. In light of these uncertainties, it seems reasonable to argue that one should attempt to minimize, through structural modification, the extent to which a drug candidate undergoes metabolism to reactive intermediates. Therefore, it becomes imperative to have close collaboration between Drug Metabolism scientists and their counterparts in Medicinal Chemistry during both the discovery and early development phases.

Structure toxicity relationships--how useful are they in predicting toxicities of new drugs?

This chapter provides just a few newer examples of structural moieties found in drugs that have been associated with reactive metabolite formation and toxicities. For a discussion of several other structures in drugs that undergo metabolic activation to reactive intermediates, the reader is directed to previous volumes in this series and other chapters in this book, as well as a previous condensed review (Nelson, 1982). Since that review, some new knowledge allows us to better predict that some structural moieties are more likely than others to form drug reactive metabolites that may be involved in causing toxic effects in humans. For example, most aniline-, thiophene-, and nitroaromatic-containing drugs have had a relatively high incidence of adverse effects, and it would be prudent in the drug discovery process to avoid these substructures if possible. However, as illustrated by the case of olanzapine, these structures may be important for potent activity, and could therefore be beneficial in some cases. The glitazones represent a new class of drugs with a unique thiazolidinedione structure. This raises two important points. First, it demonstrates how limited our knowledge base is in regard to structure toxicity relationships when new structures are introduced. Our approaches must be very empirical and are far from quantitative for the reasons outlined in the introduction. Secondly, the glitazones point out the importance of benefit/risk considerations. This was a new structural class of drugs with a unique spectrum of action that is very beneficial in the treatment of a major disease. Despite some suspected risk of toxicity, based on early trials, troglitazone was approved for use with careful monitoring. This author believes that was the right decision, as was the decision to withdraw the drug when the risk became unacceptable, especially with the introduction of safer alternatives. If this were just another NSAID (e.g., bromfenac), there would be little reason for approval. In summary, as I pointed out previously (Nelson, 1982), with our limited knowledge of structure toxicity relationships, we can only make reasonable judgments as to risk assessment of a new drug in humans, and hope that we neither release a dangerous chemical entity nor, as importantly, abort an effective one.

Bile duct ligation promotes covalent drug-protein adduct formation in plasma but not in liver of rats given zomepirac

Acyl glucuronides are reactive electrophilic metabolites of carboxylate drugs, capable of undergoing hydrolysis, rearrangement and covalent binding reactions with proteins in vivo. Such covalent drug-protein adducts may be prerequisites for certain idiosyncratic immune and toxic responses in susceptible individuals. The present study examined the effect of experimental cholestasis on the extent and pattern of formation of protein adducts in plasma and liver of rats given the non-steroidal anti-inflammatory drug (NSAID) zomepirac (ZP). Groups of intact, bile-exteriorized and bile duct-ligated rats given a 50 mg/kg i.v. dose of ZP were studied for 24 hr. In intact rats, only 1.4% of the dose was recovered as the sum of ZP, ZP acyl glucuronide (ZAG) and its rearrangement isomers (iso-ZAG) in urine in 24 hr. In bile-exteriorized animals, 0.5% of the dose was recovered in urine in 24 hr, with 31.6% of the dose being recovered in bile (2.7% as ZP, 20.0% as ZAG and 8.9% as iso-ZAG). In the bile duct-ligated group, recovery of dose in 24 hr urine totalled 17.5% (1.7% as ZP, 6.7% as ZAG and 9.1% as iso-ZAG). ZAG and iso-ZAG were measurable in plasma only in the bile duct-ligated group, and covalent binding of ZP to plasma proteins was much higher (5–6 fold) than in intact or bile-exteriorized rats. Total adduct concentrations in liver were not significantly different among the three groups. Immunoblotting using a polyclonal ZP antiserum confirmed that serum albumin was a major target protein in plasma. The major ZP-modified bands in the livers of intact and bile-exteriorized rats were at about 110, 140 and 200 kDa. However, the bands at 110 and 140 kDa were much lower in the livers of bile duct-ligated rats. The results show that about 30% of ZP doses are normally excreted as ZAG and its isomers in bile, with only minor excretion in urine. Bile duct ligation shunts the glucuronide into blood (and urine), strongly promoting adduct formation with plasma proteins, and alters the pattern but not the total quantity of drug-modified proteins formed in the liver.

Is it Possible to More Accurately Predict which Drug Candidates will cause Idiosyncratic Drug Reactions

The unexpected occurrence of idiosyncratic drug reactions during late clinical trials or after a drug has been released can lead to a severe restriction in its use or failure to release/withdrawal. This leads to considerable uncertainty in drug development and has led to attempts to try to predict a drug's potential to cause such reactions. It appears that most idiosyncratic drug reactions are due to reactive metabolites; however, many drugs that form reactive metabolites are associated with a very low incidence of idiosyncratic drug reactions. Therefore. screening drug for their ability to generate reactive metabolites is likely to cause the rejection of many good drug candidates. There is evidence to suggest that an idiosyncratic drug reaction is more likely if there is some "danger signal'. Thus drugs that cause some degree of cell stress or damage may be more likely to lead to a high incidence of idiosyncratic drug reactions. The exact nature of the putative danger signals is unknown. However, a screen of the effects of drugs known to be associated with a high incidence of idiosyncatic reactions using expression genomics and proteomics may reveal a pattern or patterns of mRNA and protein expression that predict which drugs will cause a high incidence of idiosyncratic drug reactions. Although idiosyncratic drug reactions are not usually detected in animal tests because they are just as idiosyncratic in animals as they are in humans, it is likely that drug reactive metabolites would also cause similar cell stress in animals. It is more likely that in most cases it is differences in the immune response to the reactive metabolites that determine which individuals will develop an idiosyncratic reaction. If the expression of certain proteins in animals treated with a drug candidate could be used as a screening method to predict a drug's potential to cause a high incidence of idiosyncratic drug reactions, it would greatly facilitate the development of safer drugs.

Laboratory of Chemical Pharmacology, National Heart, Lung, and Blood Institute, NIH: a short history.

The Laboratory of Chemical Pharmacology (LCP) began in 1950 as the Section of Pharmacology within the National Heart Institute, the National Institutes of Health. Its first chief was Bernard B. Brodie, considered by many to be one of the fathers of modern pharmacology. Since its inception, LCP has made many significant contributions to the fields of pharmacology and toxicology. LCP was among the first to study (a) the effects of drugs on the turnover of serotonin and norepinephrine in brain and other tissues, (b) the absorption of drugs from the gastrointestinal tract and their passage across the blood-brain barrier, (c) the oxidation and reduction of drugs and other foreign compounds by liver microsomal enzymes (later known as the cytochrome P450 enzymes) and inhibitors and inducers of these enzymes, (d) the formation of toxic chemically reactive metabolites of drugs and other foreign compounds, and (e) mechanisms of immunological responses. Approximately 300 scientists worked in LCP during its existence, and they and their collaborators published more than 1,300 papers. This is a short history of the people who worked in it and of their contributions to biomedical sciences.

Initiation of autoimmunity by a reactive metabolite of a lupus-inducing drug in the thymus.

Drug-induced lupus is a side effect of deliberate ingestion of various medications, but its etiology, underlying mechanisms, and pathogenesis are puzzling. In vivo metabolic transformation of lupus-inducing drugs to reactive products explains how a heterogeneous set of drugs can mediate the same disease syndrome. Evidence has accumulated that drugs are transformed by extracellular oxidation from reactive oxygen species and myeloperoxidase produced when neutrophils are activated, maximizing the in situ accumulation of reactive drug metabolites within lymphoid compartments. The metabolite of procainamide, procainamide hydroxylamine, displays diverse biologic properties, but no apparent autoimmune effect has been observed. However, when procainamide hydroxylamine was introduced into the thymus of young adult normal mice, a delayed but robust autoimmune response developed. Disruption of central T-cell tolerance by intrathymic procainamide hydroxylamine resulted in the production of chromatin-reactive T cells that apparently drove the autoantibody response in the periphery. Drug-induced autoantibodies in this mouse model were remarkably similar to those in patients with procainamide-induced lupus. Therefore, this system has considerable promise to provide insight into the initiating events in drug-induced lupus and may provide a paradigm for how other xenobiotics might induce systemic autoimmunity.