Peroxidase-catalysed oxidation of different dibenzazepine derivatives

Interaction of clozapine and its nitrenium ion with rat D2 dopamine receptors: in vitro binding and computational study

The interaction of diazepine analogues like clozapine or olanzapine with D2 receptor was greatly affected by a mixture of HRP/H(2)O(2) known to induce the formation of nitrenium ion. Unlike diazepine derivatives, the oxidative mixture had low impact on the affinity of oxa- and thiazepine derivatives such as loxapine, clothiapine or JL13 for the D2 receptor. Molecular docking simulations revealed a huge difference between the mode of interaction of clozapine nitrenium ion and the parent drug. Electronic and geometric changes of the tricyclic ring system caused by the oxidation appeared to prevent the compound finding the correct binding mode and could therefore explain the difference observed in binding affinities.

Minimising the potential for metabolic activation in drug discovery

Investigations into the role of bioactivation in the pathogenesis of xenobiotic-induced toxicity have been a major area of research since the link between reactive metabolites and carcinogenesis was first reported in the 1930s. Circumstantial evidence suggests that bioactivation of relatively inert functional groups to reactive metabolites may contribute towards certain drug-induced adverse reactions. Reactive metabolites, if not detoxified, can covalently modify essential cellular targets. The identity of the susceptible biomacromolecule(s), and the physiological consequence of its covalent modification, will dictate the resulting toxicological response (e.g., covalent modification of DNA by reactive intermediates derived from procarcinogens that potentially leads to carcinogenesis). The formation of drug-protein adducts often carries a potential risk of clinical toxicities that may not be predicted from preclinical safety studies. Animal models used to reliably predict idiosyncratic drug toxicity are unavailable at present. Furthermore, considering that the frequency of occurrence of idiosyncratic adverse drug reactions (IADRs) is fairly rare (1 in 1000 to 1 in 10,000), it is impossible to detect such phenomena in early clinical trials. Thus, the occurrence of IADRs during late clinical trials or after a drug has been released can lead to an unanticipated restriction in its use and even in its withdrawal. Major themes explored in this review include a comprehensive cataloguing of bioactivation pathways of functional groups commonly utilised in drug design efforts with appropriate strategies towards detection of corresponding reactive intermediates. Several instances wherein replacement of putative structural alerts in drugs associated with IADRs with a latent functionality eliminates the underlying liability are also presented. Examples of where bioactivation phenomenon in drug candidates can be successfully abrogated via iterative chemical interventions are also discussed. Finally, appropriate strategies that aid in potentially mitigating the risk of IADRs are explored, especially in circumstances in which the structural alert is also responsible for the primary pharmacology of the drug candidate and cannot be replaced.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

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

JL 13, an atypical antipsychotic: a preclinical review

The extensive pharmacological evaluation of JL 13 as an atypical antipsychotic drug has revealed a close similarity to clozapine, however with some major advantages. JL 13 was characterized as a weak D(2) antagonist, both in vitro and in vivo, with a strong affinity for the D(4) and the 5-HT(2A) receptors. It has no affinity for the 5-HT(2C) receptor. In vivo microdialysis experiments in rat showed that JL 13, like clozapine, preferentially increased extracellular dopamine concentrations in the prefrontal cortex compared to nucleus accumbens or striatum. Behavioral studies showed that JL 13, like clozapine, has the profile of an atypical antipsychotic. Thus, JL 13 did not antagonize apomorphine-induced stereotypy nor did it produce catalepsy, but it antagonized apomorphine-induced climbing in rodents. It was inactive against d-amphetamine-induced stereotypy but antagonized d-amphetamine-induced hyperactivity in the mouse. Likewise, in the paw test, it was more effective in prolonging hindlimb retraction time than prolonging forelimb retraction time. Like other antipsychotic drugs, JL 13 reversed the apomorphine- and amphetamine-induced disruption of prepulse inhibition. In a complex temporal regulation schedule in the dog, JL 13 showed a high resemblance with clozapine without inducing sialorrhea, palpebral ptosis or any significant motor side effects. In rats and squirrel monkeys JL 13 induced a high degree of generalization (70%) to clozapine. Regarding behavioral toxicology, JL 13 did not produce dystonia or Parkinsonian symptoms in haloperidol-sensitized monkeys. After acute administration, again like clozapine, JL 13 induced only a transient increase in circulating prolactin. Last but not the least, regarding a possible hematological toxicity, unlike clozapine, JL 13 did not present sensitivity to peroxidase-induced oxidation. Moreover, its electrooxidation potential was close to that of loxapine and far from that of clozapine. Taking all these preclinical data into account, it appears that JL 13 is a promising atypical antipsychotic drug.

Y-931, a novel atypical antipsychotic drug, is less sensitive to oxidative phenomena

The oxidation behavior of Y-931, a potent atypical antipsychotic drug, was compared with that of clozapine and olanzapine. In two enzymatic systems (horseradish peroxidase (HRP)/glutathione (GSH) and HRP/H(2)O(2)/GSH) which generate thiyl radicals, clozapine markedly strengthened the electron paramagnetic resonance (EPR) signal for the radical. Olanzapine, Y-931 and the major metabolites (compounds 1-3) had no or minimal effect on the intensity of this signal. In addition, the redox potential values for the three derivatives were in accord with the EPR spin trapping results. In toxicological experiments in human leukocytes, a concentration-dependent toxicity was observed when neutrophils were incubated with clozapine (1-10 micromol/l) and H(2)O(2) (1 mmol/l). However, Y-931 and olanzapine did not show remarkable toxicity under the conditions.

Oxidation sensitivity may be a useful tool for the detection of the hematotoxic potential of newly developed molecules: application to antipsychotic drugs

Some antipsychotic agents have been found to produce agranulocytosis and aplastic anemia. The oxidation phenomena and/or the formation of free radicals has been suggested to be causally related to various hematological disorders, e.g., agranulocytosis. Using five experimental conditions, we tested the oxidative potential of compounds with and without a history of hematological side effects, e.g., agranulocytosis and aplastic anemia. A statistical analysis was undertaken for each experimental condition and a multivariate analysis combining all results was performed. Two peroxidase-induced free radical models did not successfully discriminate between drugs with and without a history of causing hematologic problems (<70%). The lipid peroxidation system provided even less satisfactory discrimination, with only 56.25% correct classification. However, an 87.5% correct classification was obtained when using the oxidation potentials of these drugs determined at pH 4.7 and at pH 7.4. A multivariate analysis taking into account the five variables provided 87.5% success in classification. The two clusters were better discriminated in terms of a "distance coefficient." In a second analysis, the putative antipsychotic pyridobenzodiazepine analogues (JL5, JL8, JL18, and JL25) were classified in the cluster of toxic compounds, while the oxa- and thiazepine analogues (JL2, JL3, and JL13) were classified as nontoxic compounds. On the other hand, a few metabolites of clozapine and fluperlapine were classified in the toxic compound group. The procedure described herein is, to our knowledge, the first which classifies molecules of different structures as well as different pharmacological profiles according to their hematotoxic potential. Such a procedure could be used to predict drug-induced hematological side effects.

Comparative oxidation of loxapine and clozapine by human neutrophils

The clozapine-induced agranulocytosis could be due to the formation of a reactive intermediate formed in polymorphonuclear neutrophils and granulocyte precursors with the myeloperoxidase-hydrogen peroxide system. On the contrary, no case of agranulocytosis has been described for loxapine, an other neuroleptic drug with a very close structural analogy. We have compared the clozapine and loxapine interaction with the oxidative burst and particularly with this enzymatic complex. On the one hand, the assay of the oxidative species demonstrated a different impact for the two neuroleptics. The 50% inhibitory concentration was 92 microM for hydrogen peroxide and 40 microM for hypochlorous acid for loxapine. The loxapine target is located before the myeloperoxidase-hydrogen peroxide system in the oxidative stream, whereas clozapine diverts the chlorination pathway of the enzyme. On the other hand, the in vitro metabolism of drugs by the myeloperoxidase-hydrogen peroxide system has been investigated by mass spectrometry. Loxapine remains inert but clozapine undergoes the oxidation. The glutathione or ascorbate addition in the medium leads to a removal of the oxidation. Glutathione is able to trap the toxic intermediate and could avoid its formation.

JL 13, a potential successor to clozapine, is less sensitive to oxidative phenomena

The oxidation behaviour of JL 13, a promising antipsychotic, was investigated in comparison with clozapine and loxapine, by measuring their direct "radical scavenging" abilities and their efficacies in inhibiting the lipid peroxidation. In the lipid peroxidation system, the reactivity of these compounds with free radicals produced by gamma-irradiation of linoleic acid may be presented as follows: JL 13 = loxapine < clozapine. In two enzymatic systems (HRP/GSH and HRP/H2O2/ GSH) which generate the thiyl free radicals, clozapine produces a strong enhancement of the thiyl-radical EPR signal intensity while JL 13 and loxapine exhibit no or minimal effect on this signal. The redox potential values for the three derivatives confirm the spectro-photometric and EPR results. Following this study, we show that JL 13, although presenting a preclinical clozapine-like profile, appears less sensitive to oxidation than clozapine.

Metabolism of clozapine by human neutrophils: evidence for a specific oxidation of clozapine by the myeloperoxidase system with inhibition of enzymatic chlorination cycle

The use of clozapine, an unique antipsychotic drug, raises the real problem of drug-induced polymorphonuclear neutrophil cytotoxicity. Clozapine prescription has been restricted due to a 1-2% incidence of drug-induced agranulocytosis. The exact mechanism of this adverse effect is not yet known. The myeloperoxidase-hydrogen peroxide system could play a key role in the initiation of agranulocytosis. Therefore, we have investigated the clozapine effects on hydrogen peroxide and hypochlorous acid, evaluated the peroxidase-mediated metabolism of clozapine by mass spectrometry analysis because myeloperoxidase uses hydrogen peroxide and chloride producing hypochlorous acid in its chlorination cycle, and thus could oxidise clozapine in its peroxidation cycle. First, evidence for inhibition of hypochlorous acid production and scavenging of hydrogen peroxide by clozapine were demonstrated in vitro, in different cell-free and cellular systems. Results are consistent with an inhibition of the myeloperoxidase chlorination cycle when clozapine is oxidised in the peroxidation cycle. Secondly, ion-spray mass spectrometry analysis allowed us to confirm clozapine oxidation by the myeloperoxidase system. Actually, clozapine N-oxide with a m/z at 343 was formed. It could be the final step of the metabolisation of clozapine via two successive univalent oxidations mediated by peroxidase. We suggest that generation of a free cation radical, CLZ(o+), was the initial step. CLZ(o+) is a very reactive species and may play an important role in the onset of agranulocytosis either by direct toxicity or via an immunological mechanism. However, this assumption does not exclude the possible role of other metabolic ways involving, in particular, N-desmethylclozapine.