Neuroleptic blockade of the effect of various neurotransmitter substances

Focus on Clozapine Withdrawal- and Misuse-Related Cases as Reported to the European Medicines Agency (EMA) Pharmacovigilance Database

BACKGROUND

Clozapine is of high clinical relevance for the management of both treatment-resistant schizophrenia and psychotic disturbances with concurrent drug misuse. Although the molecule presents with a range of well-known side-effects, its discontinuation/withdrawal syndrome has been only anecdotally described.

AIMS

the 2005-2018 European Medicines Agency (EMA) dataset of Adverse Drug Reactions (ADRs) was analyzed to identify and describe possible clozapine withdrawal- and misuse-/abuse-/dependence-related issues.

METHOD

A descriptive analysis of clozapine-related ADRs was performed when available, data on ADRs' outcome, dosage, and possible concomitant drug(s) were considered.

RESULTS

Out of 11,847 clozapine-related ADRs, some 599 (5.05%) were related to misuse/abuse/dependence/withdrawal issues, including 258 withdrawal-related (43.1%); 241 abuse-related (40.2%); and 80 intentional product misuse-related (13.3%) ADRs. A small number of overdose- and suicide-related ADRs were reported as well. Clozapine was typically (69.2%) identified alone, and most (84.7%) fatalities/high-dosage intake instances were reported in association with a history of substance abuse.

CONCLUSIONS

Previous suggestions about the possibility of a clozapine discontinuation/withdrawal occurrence are here supported, but further studies are needed. However, the misuse/abuse cases here identified might be difficult to interpret, given the lack of studies highlighting the possible recreational use of clozapine. The high-dosage intake, fatal outcomes and clozapine/polydrug abuse issues reported here may, however, be a reason for concern.

Acepromazine and Chlorpromazine as Pharmaceutical-grade Alternatives to Chlorprothixene for Pupillary Light Reflex Imaging in Mice

Studies of visual responses in isoflurane-anesthetized mice often use the sedative chlorprothixene to decrease the amount of isoflurane used because excessive isoflurane could adversely affect light-evoked responses. However, data are not available to justify the use of this nonpharmaceutical-grade chemical. The current study tested whether pharmaceutical-grade sedatives would be appropriate alternatives for imaging pupillary light reflexes. Male 15-wk-old mice were injected intraperitoneally with 1 mg/kg chlorprothixene, 5 mg/kg acepromazine, 10 mg/kg chlorpromazine, or saline. After anesthetic induction, anesthesia maintenance used 0.5% and 1% isoflurane for sedative- and saline-injected mice, respectively. A photostimulus (16.0 log photons cm^-2 s^-1; 470 nm) was presented to the right eye for 20 min, during which the left eye was imaged for consensual pupillary constriction and involuntary pupil drift. Time to immobilization, loss of righting reflex, physiologic parameters, gain of righting reflex, and degree of recovery were assessed also. The sedative groups were statistically indistinguishable for all measures. By contrast, pupillary drift occurred far more often in saline-treated mice than in the sedative groups. Furthermore, saline-treated mice took longer to reach maximal pupil constriction than all sedative groups and had lower heart rates compared with chlorpromazine- and chlorprothixene-sedated mice. Full recovery (as defined by purposeful movement, response to tactile stimuli, and full alertness) was not regularly achieved in any sedative group. In conclusion, at the doses tested, acepromazine and chlorpromazine are suitable pharmaceutical-grade alternatives to chlorprothixene for pupil imaging and conceivably other in vivo photoresponse measurements; however, given the lack of full recovery, lower dosages should be investigated further for use in survival procedures.

PTS: a pharmaceutical target seeker

Identifying protein targets for a bioactive compound is critical in drug discovery. Molecular similarity is a main approach to fish drug targets, and is based upon an axiom that similar compounds may have the same targets. The molecular structural similarity of a compound and the ligand of a known target can be gauged in topological (2D), steric (3D) or static (pharmacophoric) metric. The topologic metric is fast, but unable to represent steric and static profile of a bioactive compound. Steric and static metrics reflect the shape properties of a compound if its structure were experimentally obtained, and could be unreliable if they were based upon the putative conformation data. In this paper, we report a pharmaceutical target seeker (PTS), which searches protein targets for a bioactive compound based upon the static and steric shape comparison by comparing a compound structure against the experimental ligand structure. Especially, the crystal structures of active compounds were taken into similarity calculation and the predicted targets can be filtered according to multi activity thresholds. PTS has a pharmaceutical target database that contains approximately 250 000 ligands annotated with about 2300 protein targets. A visualization tool is provided for a user to examine the result.

Database URL: http://www.rcdd.org.cn/PTS

Shape-based reprofiling of FDA-approved drugs for the H₁ histamine receptor

Reprofiling of existing drugs to treat conditions not originally targeted is an attractive means of addressing the problem of a decreasing stream of approved drugs. To determine if 3D shape similarity can be used to rationalize an otherwise serendipitous process, we employed 3D shape-based virtual screening to reprofile existing FDA-approved drugs. The study was conducted in two phases. First, multiple histamine H(1) receptor antagonists were identified to be used as query molecules, and these were compared to a database of approved drugs. Second, the hits were ranked according to 3D similarity and the top drugs evaluated in a cell-based assay. The virtual screening methodology proved highly successful, as 13 of 23 top drugs tested selectively inhibited histamine-induced calcium release with the best being chlorprothixene (IC(50) 1 nM). Finally, we confirmed that the drugs identified using the cell-based assay were all acting at the receptor level by conducting a radioligand-binding assay using rat membrane.

Ondansetron results in improved auditory gating in DBA/2 mice through a cholinergic mechanism

The 5-HT(3) receptor antagonist, ondansetron, has been shown to correct the auditory gating deficit in medicated schizophrenia patients. Inhibition of 5-HT(3) receptors releases acetylcholine, the endogenous ligand for nicotinic acetylcholine receptors. The schizophrenia-related auditory gating deficit is modulated, in part, by nicotinic acetylcholine receptors, as is the mouse (DBA/2) model of the deficit. The present study assessed the effects of both acute and chronically administered ondansetron on auditory gating in DBA/2 mice. Auditory gating is defined as a decrease in amplitude of response to the second of a paired identical auditory stimulus presented 0.5 s following an initial auditory stimulus. Acute ondansetron administration at the lowest dose (0.1 mg/kg, IP) tested had no effect, while other doses (0.33 and 1 mg/kg, IP) produced improvements in auditory gating. The improvements were produced through both an increase in response to the first auditory stimulus and a decrease in the response to the second auditory stimulus. Co-administration of an alpha7 nicotinic acetylcholine receptor antagonist, alpha-bungarotoxin, or the alpha4beta2 nicotinic acetylcholine receptor antagonist dihydro-beta-erythroidine, with the 0.33 mg/kg dose of ondansetron blocked the improvement in auditory gating produced by ondansetron alone. There was no difference in response between the chronically injected mice and naive mice. Both showed improved auditory gating, thus, demonstrating no "carry over" effect of daily injections. These data demonstrate that indirect stimulation of nicotinic acetylcholine receptors by ondansetron can improve auditory gating parameters in DBA/2 mice.

Has dopamine a physiological role in the control of sexual behavior? A critical review of the evidence

The role of dopaminergic systems in the control of sexual behavior has been a subject of study for at least 40 years. Not surprisingly, reviews of the area have been published at variable intervals. However, the earlier reviews have been summaries of published research rather than a critical analysis of it. They have focused upon the conclusions presented in the original research papers rather than on evaluating the reliability and functional significance of the data reported to support these conclusions. During the last few years, important new knowledge concerning dopaminergic systems and their behavioral functions as well as the possible role of these systems in sexual behavior has been obtained. For the first time, it is now possible to integrate the data obtained in studies of sexual behavior into the wider context of general dopaminergic functions. To make this possible, we first present an analysis of the nature and organization of sexual behavior followed by a summary of current knowledge about the brain structures of crucial importance for this behavior. We then proceed with a description of the dopaminergic systems within or projecting to these structures. Whenever possible, we also try to include data on the electrophysiological actions of dopamine. Thereafter, we proceed with analyses of pharmacological data and release studies, both in males and in females. Consistently throughout this discussion, we make an effort to distinguish pharmacological effects on sexual behavior from a possible physiological role of dopamine. By pharmacological effects, we mean here drug-induced alterations in behavior that are not the result of the normal actions of synaptically released dopamine in the untreated animal. The conclusion of this endeavor is that pharmacological effects of dopaminergic drugs are variable in both males and females, independently of whether the drugs are administered systemically or intracerebrally. We conclude that the pharmacological data basically reinforce the notion that dopamine is important for motor functions and general arousal. These actions could, in fact, explain most of the effects seen on sexual behavior. Studies of dopamine release, in both males and females, have focused on the nucleus accumbens, a structure with at most a marginal importance for sexual behavior. Since accumbens dopamine release is associated with all kinds of events, aversive as well as appetitive, it can have no specific effect on sexual behavior but promotes arousal and activation of non-specific motor patterns. Preoptic and paraventricular nucleus release of dopamine may have some relationship to mechanisms of ejaculation or to the neuroendocrine consequences of sexual activity or they can be related to other autonomic processes associated with copulation. There is no compelling indication in existing experimental data that dopamine is of any particular importance for sexual motivation. There is experimental evidence showing that it is of no importance for sexual reward.

SCH 23390--a selective dopamine D-1 receptor antagonist with putative 5-HT1 receptor agonistic activity

The selective dopamine D-1 receptor antagonist SCH 23390 has been tested in vitro in the rat fundus model and in vivo in the electrically stimulated flexor reflex model. In the fundus model, SCH 23390 showed a potent agonistic activity compared to that of different 5-HT receptor agonists. Pindolol, 1-propranolol and pirenperone showed no or only weak inhibition of the SCH 23390-induced contractions in the fundus strip whereas methysergide was a potent inhibitor. The 5-HT3 receptor antagonist ICS 205-930 did not induce an inhibitory effect. In the electrically stimulated flexor reflex model in pithed rats, SCH 23390 induced a marked increase of the reflex. This increase was slightly inhibited by a mixed dopamine (DA) D-1/D-2 antagonist cis(Z)-flupentixol and by a specific DA D-2 antagonist YM 09151-2. Different reference antagonists: bicuculline (GABAergic), propranolol (beta-adrenergic), scopolamine (muscarinic), yohimbine (alpha 2-adrenergic), prazosin (alpha 1-adrenergic) were all without an antagonist effect on the SCH 23390-induced increase of the flexor reflex. Ketanserin, a selective 5-HT2 receptor antagonist, showed a weak and short-lasting inhibition of the SCH 23390 effect in high doses, whereas ritanserin showed only 35% inhibition of the SCH 23390-induced flexor reflex at a dose of 1.3 mumol/kg i.v. The mixed 5-HT1/5-HT2 antagonists methiothepin and metergoline showed a marked inhibitory effect at 2.6 mumol/kg i.v. and 3.1 mumol/kg i.v., respectively (1.3 mg/kg i.v.). These findings suggest that SCH 23390 might possess 5-HT1 receptor agonist activity.

Pharmacological effects of a specific dopamine D-1 antagonist SCH 23390 in comparison with neuroleptics

Neuroleptics such as thioxanthenes (cis(Z)-flupentixol and cis(Z)-clopenthixol) and phenothiazines (fluphenazine and perphenazine), which block both dopamine (DA) D-1 and D-2 receptors and the butyrophenones (haloperidol and spiroperidol), which block D-2 receptors only, are equipotent both behaviorally and clinically. A new compound SCH 23390 which selectively blocks DA D-1 receptors, resembles many neuroleptics in its pharmacological profile: antistereotypic effects in mice, rats and dogs, cataleptogenic effect and inhibitory effect on amphetamine circling. In contrast SCH 23390 has no effect on apomorphine-induced vomiting in dogs and little effects on 6-OHDA-denervated supersensitive DA receptors, stimulated by the DA agonist 3-PPP. In a series of experiments where methylphenidate-induced stereotyped gnawing in mice was inhibited by neuroleptics, it was shown that concomitant treatment with scopolamine or diazepam attenuated the effect of butyrophenones (D-2 antagonists). The same treatment attenuated the effect of phenothiazines, to a lesser extent, and hardly attenuated the effect of thioxanthenes and SCH 23390 at all. It is concluded that DA D-1 receptors are as important as D-2 receptors for the expression of neuroleptic activity in most animal models believed to be predictive of antipsychotic and extrapyramidal side-effect potential. However, the D-1 antagonist is less sensitive than D-2 antagonists to antimuscarinic compounds and benzodiazepines.

Clozapine--a serotonin antagonist?

The effect of clozapine on the central serotonergic transmission system was studied by investigation of open-field motility of rats after microinjection of drugs into nucleus accumbens and median raphe nucleus. Previous work has shown that LSD in low doses potentiates apomorphine-induced hypermotility and that this LSD effect is induced by a serotonin agonist action in median raphe nucleus. Clozapine, injected into median raphe nucleus (0.05 micrograms), suppressed the LSD effect in the same manner as serotonin antagonists did. Since alpha-adrenergic drugs, injected into median raphe nucleus, caused locomotor stimulant effects, an alpha- adrenalytic action of clozapine was excluded. Clozapine, injected into nucleus accumbens (0.2 micrograms), increased apomorphine-induced hypermotility, whereas the dopamine antagonist haloperidol suppressed it. Our results suggest a serotonin antagonist action of clozapine.

Influence of dixyrazine on intestinal and renal vasoconstrictor responses during fentanyl-nitrous oxide anaesthesia

In cats (n = 24) anaesthetized with fentanyl-nitrous oxide and diazepam, stimulation of the hypothalamic defence-alarm area (DA) or afferent activation of somatic pain fibres (SA), elicited a pronounced increase in intestinal (DA 297%, SA 107%) and renal (DA 214%, SA 90%) vascular resistance as well as a decrease in diuresis. These stress-related responses were markedly counteracted by dixyrazine (0.15-0.5 mg X kg-1 b.w.i.v.), especially in the kidney where the subsequent increase in vascular resistance to DA and SA stimulations amounted to only 25% and 13%, respectively, while diuresis increased. Corresponding data for stimulation-induced increases in intestinal vascular resistance after dixyrazine were DA 156% and SA 28%. Dixyrazine is suggested to act both through interaction with peripheral alpha-adrenergic mechanisms in control of vascular tone and through central nervous cardiovascular reflex depression. In man (n = 7), during a similar form of anaesthesia, portal vein blood flow (1137 +/- 177 ml) was measured by the continuous thermodilution method. Preportal tissue vascular resistance during surgery decreased significantly (11.3 vs 8.7 kPa X min X ml-1 X 10(-3] after i.v. dixyrazine (0.15 mg X kg-1 b.w.). A concomitant increase in oxygen uptake in preportal tissues occurred (19.9 ml min-1 vs 24.5 ml X min-1).

Neuroleptic inhibition of 6,7-ADTN-induced hyperactivity after injection into the nucleus accumbens. Specificity and comparison with other models

The dopamine agonist 6,7-ADTN induced locomotor stimulation after injection into the nucleus accumbens of rats. This hyperactivity was stereospecifically antagonized by peripheral injection of typical and atypical neuroleptics, except sulpiride, regardless of structural class. The potency of 14 neuroleptics correlated significantly to that in other neuroleptic in vivo models in rats, i.e. cataleptogenic and amphetamine-antagonistic (stereotypy, circling) activity. Sulpiride was inactive after peripheral injection, whereas intraacumbens sulpiride antagonized 6,7-ADTN-induced hyperactivity with a higher potency than that found for locally injected cis-(Z)-flupentixol, haloperidol and clebopride. Blockade of alpha 1-adrenergic receptors by systemic prazosin or locally injected phentolamine also antagonized 6,7-ADTN, whereas antagonists of alpha 2-adrenergic, beta-adrenergic, muscarinic and serotoninergic receptors were without effect. Furthermore, the cis-(Z)-flupentixol and haloperidol-induced 6,7-ADTN blockade was only slightly or not reversed by additional antimuscarinic (scopolamine) or antiserotonergic (methysergide) treatment. It is concluded that antagonism of 6,7-ADTN-induced hyperactivity is a relevant model for detecting neuroleptic activity with minimal influence of anticholinergic and antiserotonergic activity, but that the dopamine receptor within the nucleus accumbens responsible for the locomotor stimulation is similar to that involved in other behavioural effects of neuroleptics. It is suggested that an apparent preferential limbic action of some neuroleptics (e.g. clozapine and chlorprothixene) is more likely to be explainable by their simultaneous blocking effect on dopaminergic, alpha-adrenergic and muscarinic receptors.

Pharmacological specificity of conditioned avoidance response inhibition in rats: inhibition by neuroleptics and correlation to dopamine receptor blockade

The inhibitory effect of 36 neuroleptic compounds on conditioned avoidance response (CAR) and unconditioned escape response (UER) has been studied in rats. All neuroleptics antagonized CAR in doses below those inhibiting UER and below those inducing catalepsy. Stereospecificity was shown in two cases. Significant correlation was found between CAR inhibitory and cataleptogenic potency. Also inhibition of amphetamine induced stereotypy, affinity to 3H-haloperidol binding sites in vitro and clinical potency was significantly correlated to CAR inhibition. CAR and UER inhibition induced by cis(Z)-flupentixol and haloperidol was attenuated by scopolamine, but was only weakly influenced by methysergide and prazosin. Among a wide range of other CNS active compounds tested, CAR was inhibited by alpha 1-adrenergic antagonists, benzodiazepines, a barbiturate, GABA agonists, morphine and a serotonin agonist, but in doses inducing other motor disturbances. It is concluded that CAR inhibition is a sensitive test for dopamine receptor antagonists. However, additional alpha-adrenergic activity found for some neuroleptics (e.g. clozapine, chlorprothixene) may contribute to the CAR inhibitory potency. Additional antimuscarinic activity of neuroleptics may moderately attenuate CAR inhibition whereas serotonin receptor blockade is of minor importance.

A comparison of the effects of neuroleptics on phencyclidine-induced behaviors in the rat

The dose-response effects of neuroleptic pretreatment on phencyclidine (PCP; 3 or 5 mg/kg)-induced locomotor activity, stereotyped behaviors and ataxia were quantified in groups of male rats using rating scales recently developed in this laboratory. Three butyrophenone neuroleptics consistently produced dose-dependent antagonism of the behavioral effects of PCP administration. Fluphenazine antagonized the behavioral effects produced by 3 mg/kg PCP but not those produced by 5 mg/kg PCP. Each of the other neuroleptics examined (chlorpromazine, thioridazine, mesoridazine, triflupromazine, cis-flupenthixol) had no consistent antagonistic effect or actually enhanced one or more of the behavioral effects of PCP. Some neuroleptics slightly reduced PCP locomotion or stereotypies at high doses, but these effects were probably a non-specific consequence of the synergistic ataxia-producing properties of these drugs. In a second set of experiments, atropine sulfate pretreatment increased PCP-induced locomotor activity and stereotyped behaviors but had no effect on ataxia; pretreatment with physostigmine produced opposite effects. Combined pretreatment with haloperidol and atropine sulfate significantly reduced only haloperidol antagonism of PCP-induced ataxia, thus suggesting that non-dopoaminergic effects of neuroleptics may interfere with their ability to antagonize PCP.

Chronic treatments with zotepine, thioridazine, and haloperidol affect apomorphine-elicited stereotypic behavior and striatal 3H-spiroperidol binding sites in the rat

Apomorphine (AP)-elicited sterotypic behavior and striatal 3H-spiroperidol binding sites were studied in rats given 3 weeks of chronic treatment with one of the following neuroleptic drugs: zotepine (10 or 20 mg/kg/day IP); thioridazine (10 or 20 mg/kg/day IP); haloperidol (2 or 5 mg/kg/day IP). On days 10-12 after the chronic neuroleptic treatment, enhancement of AP-elicited stereotypy was seen in the high- and low-dose haloperidol-treated, as well as in the high-dose thioridazine and zotepine-treated rats when compared to that of saline-injected controls. No significant change in the response to AP was found in the low-dose thioridazine and zotepine-treated animals. Significant increases in the concentration of striatal 3H-spiroperidol binding sites were seen after treatment with all three neuroleptics, both high and low doses. A positive correlation was found between AP-elicited stereotypy and the concentration of striatal 3H-spiroperidol binding sites in the haloperidol-treated and control rats. However, no such correlation was seen after chronic thioridazine and zotepine treatments.

Neuroleptic influence on hyperthermia induced by 5-hydroxytryptophan and p-methoxy-amphetamine in MAOI-pretreated rabbits

5-Hydroxytryptophan (5-HTP) and p-methoxyamphetamine (p-MA) induce dose-dependent, lethal hyperthermia when applied intravenously to monoamine oxidase inhibitor (MAOI) pretreated rabbits. The time course of hyperthermia and the doses required to induce hyperthermia varies between the two substances. Results with alpha-MT and PCPA suggest that 5-HTP hyperthermia depends on 5-HT formation, release of endogenous 5-HT, and the presence of catecholamines, whereas p-MA-induced hyperthermia most likely is a result of indirect 5-HT release. Some neuroleptics (piflutixol, spiroperidol and methiotepine) are extremely potent inhibitors of the induced hyperthermia, Also the 5-HT receptor blocking agent methergoline antagonizes hyperthermia induced by the two substances in rather low doses. On the other hand cis (Z)-flupenthixol is a very weak antagonist of 5-HTP but a more potent inhibitor of p-MA hyperthermia. It is concluded that both 5-HT and catecholamine (dopamine) receptor blockade is required to antagonize 5-HTP hyperthermia and that antagonism of p-MA induced hyperthermia is primarily a result of influence on the 5-HT system.