Survey on the pharmacodynamics of the new antipsychotic risperidone.

This review reports on the pharmacodynamics of the new antipsychotic risperidone. The primary action of risperidone is serotonin 5-HT2 receptor blockade as shown by displacement of radioligand binding (Ki: 0.16 nM), activity on isolated tissues (EC50: 0.5 nM), and antagonism of peripherally (ED50: 0.0011 mg/kg) and centrally (ED50: 0.014 mg/kg) acting 5-HT2 receptor agonists in rats. Risperidone is at least as potent as the specific 5-HT2 receptor antagonist ritanserin in these tests. Risperidone is also a potent dopamine D2 receptor antagonist as indicated by displacement of radioligand binding (Ki: 1.4 nM), activity in isolated striatal slices (IC50: 0.89 nM), and antagonism of peripherally (ED50: 0.0057 mg/kg in dogs) and centrally acting D2 receptor agonists (ED50: 0.056-0.15 mg/kg in rats). Risperidone shows all effects common to D2 antagonists, including enhancement of prolactin release. However, some central effects such as catalepsy and blockade of motor activity occur at high doses only. Risperidone is 4-10 times less potent than haloperidol as a central D2 antagonist in rats and it differs from haloperidol by the following characteristics: predominant 5-HT2 antagonism; LSD antagonism; effects on sleep; smooth dose-response curves for D2 antagonism; synergism of combined 5-HT2/D2 antagonism; pronounced effects on amphetamine-induced oxygen consumption; increased social interaction; and pronounced effects on dopamine (DA) turnover. Risperidone displays similar activity at pre- and postsynaptic D2 receptors and at D2 receptors from various rat brain regions. The binding affinity for D4 and D3 receptors is 5 and 9 times weaker, respectively, than for D2 receptors; interaction with D1 receptors occurs only at very high concentrations. The pharmacological profile of risperidone includes interaction with histamine H1 and alpha-adrenergic receptors but the compound is devoid of significant interaction with cholinergic and a variety of other types of receptors. Risperidone has excellent oral activity, a rapid onset, and a 24-h duration of action. Its major metabolite, 9-hydroxyrisperidone, closely mimics risperidone in pharmacodynamics. Risperidone can be characterized as a potent D2 antagonist with predominant 5HT2 antagonistic activity and optimal pharmacokinetic properties.

Pharmacological profile of the new potent neuroleptic ocaperidone (R 79,598).

Ocaperidone, a new benzisoxazolyl piperidine neuroleptic, was compared with haloperidol, risperidone and ritanserin in a large series of pharmacological tests. Ocaperidone inhibited dopamine agonist (apomorphine, amphetamine or cocaine)-induced behavioral effects at low doses (0.014-0.042 mg/kg) and was, thereby, equipotent with haloperidol (0.016-0.024 mg/kg) and 2.0 to 8.3 times more potent than risperidone. Ocaperidone completely blocked the dopamine agonist behavior at slightly higher doses (0.064 mg/kg) and was, thereby, more potent and efficacious than haloperidol (0.097-0.13 mg/kg) and risperidone (0.59-1.17 mg/kg). The dissociation between inhibition of apomorphine behavior and induction of catalepsy was as high for ocaperidone (22) as for risperidone (20) and higher than for haloperidol (8), suggesting risperidone-like low extrapyramidal side effect liability. Ocaperidone also antagonized serotonin agonist (tryptamine, mescaline or 5-hydroxytryptophan)-induced behavioral effects (0.011-0.064 mg/kg) and was, thereby, equipotent with risperidone (0.014-0.056 mg/kg) and at least as potent as ritanserin (0.037-0.13 mg/kg). Ocaperidone displayed its serotonin and dopamine antagonism at the same dose levels, in contrast to risperidone, which was a predominant serotonin antagonist. Apart from protection from compound 48/80 lethality (0.042 mg/kg) and norepinephrine lethality (0.097 mg/kg), which were not considered to hinder its clinical application, no additional secondary effects were observed at low doses of ocaperidone. In the apomorphine test in dogs, ocaperidone was very potent (i.v., s.c. and p.o. ED50 values: less than 1.0 micrograms/kg) and showed a rapid onset (less than 0.5 h) and long duration of action (24 h) after p.o. administration. Ocaperidone is concluded to be a highly potent and efficacious dopamine-D2 antagonist with concomitant, equivalent serotonin 5-HT2 antagonism. Ocaperidone is expected to exert pronounced haloperidol-like effects on the positive symptoms of schizophrenic patients but with risperidone-like low extrapyramidal side effect liability and improved patient compliance.

Behavioral disinhibition and depression in amphetaminized rats: a comparison of risperidone, ocaperidone and haloperidol.

The mixed serotonin-2/dopamine-D2 antagonists risperidone and ocaperidone were compared with the specific D2 antagonist haloperidol for their ability to antagonize amphetamine (10 mg/kg, s.c.)-induced stereotypy in rats. Four successive stages of amphetamine antagonism were differentiated: 1) disinhibition: reversal of stationary stereotypy into the hyperactivity normally observed with lower doses of amphetamine; 2) inhibition: the first significant reduction of activity; 3) normalization: reduction of activity to the level of nonamphetaminized rats; and 4) suppression: reduction of activity to 50% of the level of nonamphetaminized rats. Ocaperidone and risperidone were equipotent with haloperidol for disinhibition (0.0062-0.011 mg/kg). However, the disinhibition was maintained over a wider dose range with risperidone (factor 84) than with haloperidol (9.0) and ocaperidone (4.1) and was also more pronounced in magnitude with risperidone. Ocaperidone was equipotent with haloperidol for inhibition (0.013-0.025 mg/kg) and normalization (0.074-0.080 mg/kg) but 4.4 times less potent for suppression of activity (0.71 vs. 0.16 mg/kg). Risperidone became progressively less potent than haloperidol: 4.4 times for inhibition, 9.6 times for normalization and 22 times for suppression of activity. The present data are consistent with the hypothesis that serotonin-2 antagonism compensates for the functional consequences of D2 receptor blockade. The implications for the clinical application of the compounds are discussed.

Levocabastine: pharmacological profile of a highly effective inhibitor of allergic reactions.

Levocabastine, selected from a series of cyclohexylpiperidine derivatives protects rats from compound 48/80-induced anaphylactic shock for at least 16 h at the oral dose of 0.0015 mg/kg. At the same dose histamine skin reactions and at slightly higher doses passive cutaneous anaphylactic reactions are inhibited. Blockade of passive cutaneous anaphylactic reactions is obtained with levocabastine, despite absence of peripheral serotonin antagonism and any other known non-specific action that may facilitate inhibition of passive anaphylaxis. In dogs allergic reactions are inhibited at oral doses 40 times lower than ketotifen. In guinea-pigs orally and topically administered levocabastine are remarkably effective against allergic conjunctivitis.

Ritanserin reduces abuse of alcohol, cocaine, and fentanyl in rats.

Rats that had received 3% alcohol, 0.01% cocaine, or 0.002% fentanyl as the only beverage over 10 days showed marked preference for the drug solution when water was made available as a second fluid in a separate bottle. Treatment with low doses of ritanserin, a specific central serotonin 5-HT2 antagonist, rapidly reversed drug preference without changing total fluid intake. Quantitatively, the reduction in drug consumption was greater for alcohol than for cocaine and greater for cocaine than for fentanyl. This is probably related to differences in the reinforcing potential of the three drugs.

General pharmacology of the four gastrointestinal motility stimulants bethanechol, metoclopramide, trimebutine, and cisapride.

The pharmacological profile of bethanechol (CAS 674-38-4), metoclopramide (CAS 364-62-5), trimebutine (CAS 39133-31-8) and cisapride (CAS 81098-60-4) was studied in a series of simple pharmacological tests in rats and dogs. Bethanechol stimulated both gastric emptying and intestinal propulsion but displayed also the well-known behavioral effects of a direct muscarinic acetylcholine receptor agonist. Metoclopramide showed the profile of a centrally active dopamine D2 antagonist. In addition, metoclopramide displayed a stimulant effect on spontaneous gastric emptying in rats, an effect that could not be related to dopamine D2 antagonism. The only effect observed with trimebutine was protection from castor oil diarrhea, probably due to its reported interaction with peripheral opiate receptors. Cisapride was a potent stimulant of gastric emptying in rats, 7 times more potent than metoclopramide. Cisapride was also a very specific gastrokinetic, over a large dose range (specificity ratio: greater than or equal to 20) devoid of effects indicative for direct interaction with dopamine or acetylcholine receptors. The relationship between the differential activity profiles of the compounds in the present study and differences in their mechanism of action and side-effect liability is discussed.

Functional interaction between serotonin-S2 and dopamine-D2 neurotransmission as revealed by selective antagonism of hyper-reactivity to tryptamine and apomorphine.

The functional significance of the interaction between serotonergic and dopaminergic neurotransmission is still uncertain. To document this interaction further, specific behavioral responses of rats to tryptamine and apomorphine were studied. The sequential injection of these agonists, at time intervals with minimal direct behavioral interference, was used to observe response changes with respect to a single challenge. The antagonists haloperidol, ritanserin and risperidone, with known actions on serotonin-S2 (5-HT2) and dopamine-D2 (D2) receptors were used to evaluate effective antagonism of single and sequential challenges. When tryptamine was preceded by an apomorphine challenge the effective doses of the 5-HT2 antagonists ritanserin and risperidone for 50% inhibition of the seizures increased by a factor of 2.5. The dose-response curve of haloperidol remained virtually unchanged, apparently because of the potent dopamine-D2 antagonism associated with these doses which may block the potentiating effect of apomorphine. When apomorphine was preceded by a tryptamine challenge, the total agitation score of the control animals increased by 59% on the average. Haloperidol was equally effective against the enhanced as against the unenhanced apomorphine response. Ritanserin reduced agitation only by the part corresponding to the tryptamine enhancement. Risperidone's activity against the enhanced agitation started at very low doses and was complete at a dose still about 2.5 times lower than that required against the single apomorphine challenge. Mutual enhancement of tryptamine and apomorphine appears to occur even at a time when the behavioral effects of the first agonist are no longer manifest. The enhanced agitation remains largely dopamine-D2-specific and the enhanced seizures serotonin 5-HT2-specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Normalization of small intestinal propulsion with loperamide-like antidiarrheals in rats.

Gastrointestinal propulsion and the presence of diarrhea were assessed in rats pretreated with various opioids and challenged orally with either castor or paraffin oil, which both contained phenol red as a marker of gastrointestinal propulsion. In solvent-pretreated rats, diarrhea was always observed within 90 min after castor oil, reflecting a state of hyperpropulsive activity of the gut, but never (up to 8 h) after paraffin oil, reflecting normal intestinal propulsion (which amounted to an average distance of 91% of the total length of the small intestine in 90 min). Paraffin oil propulsion was blocked (to values less than 60%) by all opioids tested with the exception of the gut-selective compounds loperamide, loperamide oxide and fluperamide oxide (ED50s: greater than or equal to 160 mg/kg). Castor oil diarrhea was antagonized by all opioids tested and, at comparable but slightly (1.3-2.6 times) higher doses, propulsion was normalized to values (less than 100%) comparable to those measured in paraffin oil-challenged control rats. Castor oil propulsion was further reduced to subnormal values (less than 60%) by still higher doses of the opioids, comparable to those that blocked propulsion after paraffin oil. However, the required dose increment varied consistently among the opioids tested and ranged, depending on gut selectivity, from a factor 2.3 times the antidiarrheal dose for narcotic analgesics such as pethidine and dextromoramide to greater than 300 for antidiarrheals such as loperamide, loperamide oxide and fluperamide oxide. Protection from diarrhea and normalization of propulsion showed a close correlation; both failed to correlate with central analgesic activity and are thought to be mediated via peripheral opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

[Serotonin antagonism involved in the antipsychotic effect. Confirmation with ritanserine and risperidone]. / Antagonisme de la sérotonine impliqué dans l'effet antipsychotique. Confirmations par la ritansérine et le rispéridone.

Shortly after the introduction of the first neuroleptics a serotonin hypothesis of schizophrenia has been proposed. But neuroleptics in animals and in man were found to produce effects more consistently related to inhibition of the dopaminergic than of any other type of neurotransmission. However, two early neuroleptics, pipamperone and clozapine, act pharmacologically more on 5-HT2 than on D2 receptors. Both have a distinct clinical profile and low EPS liability. The development of selective 5-HT2-antagonists, devoid of LSD-like properties, resulted in a first compound, ritanserin. Clinically, the highly specific 5-HT2-antagonism of ritanserin improves dysthymia, increases slow wave sleep and supports classical neuroleptic treatment by decreasing negative symptoms and EPS. These properties, being valuable by themselves, have been associated to dopamine D2-antagonism in the new antipsychotic risperidone, which is an extremely potent 5-HT2-antagonist. At doses of 5 mg daily risperidone acts on both negative and positive symptoms of schizophrenia in the virtual absence of EPS.

'Tardive' dyskinesia: etiological and therapeutic aspects.

Common hypotheses may dominate medical opinion: tardive dyskinesia is semi-officially defined as an iatrogenic extrapyramidal disorder caused by long-term administration of antipsychotic drugs. Its prevalence is still increasing, reaching an average of 30% in recent studies. However, the development of these abnormal involuntary movements may be related to neurologic processes inherent in the treated disease and high prevalence figures may be the result of artificial calculations and extremely sensitive criteria. This hypothesis that tardive dyskinesia may be in some part idiopathic is supported by historical descriptions of dyskinesia in schizophrenic patients long before neuroleptics became available and has also emerged in several recent studies, despite concurrent treatment of the patients with neuroleptics. The available data indicate that "mental disease" is by far the most important risk factor for dyskinesia, followed by age, female sex and, ultimately, antipsychotic treatment. Not surprisingly, the disorder often does not respond to neuroleptic withdrawal and may improve slowly despite ongoing treatment to prevent psychotic relapse. In the absence of a generally applicable therapy for the disease processes leading to dyskinesia, the factors that can be controlled (overestimated need for antipsychotics, routine association with anticholinergics, suboptimal psychosocial support) must remain the basis of prevention of "tardive" dyskinesia.

Rapid desensitization and down-regulation of 5-HT2 receptors by DOM treatment.

The regulation of the 5-HT2 receptor-mediated head twitch response and of 5-HT2 receptor binding in the frontal cortex was studied in rats treated repeatedly with the 5-HT2 agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) (2.5 mg/kg, s.c.). Four injections in 24 h produced a near maximal reduction in the behaviour (-70%) and in the Bmax for [3H]ketanserin binding (-41%). The KD values tended to increase slightly. 5-HT2 receptors reappeared, with half-lives of 5.5 to 3 days. In view of the reported anomalous 5-HT2 receptor regulation by antagonists and the regular regulation by agonists, we propose a refinement in the receptor regulation theory.

Is in vivo dissociation between the antipropulsive and antidiarrheal properties of opioids in rats related to gut selectivity?

The antipropulsive activity of a series of opioids in the charcoal test was compared with their antidiarrheal activity in the castor oil test and their analgesic activity in the tail withdrawal test. The obtained antipropulsive/antidiarrheal potency ratios varied from 0.71 to greater than 552 [pethidine (oral ED50's in mg/kg: 21.5/30.2), fentanyl (0.77/0.49), dextromoramide (5.39/2.90), methadone (14.2/6.38), codeine (98.4/10.8), morphine (56.6/5.21), diphenoxylate (8.15/0.54), nufenoxole (74.7/1.72), difenoxin (7.10/0.16), loperamide oxide (greater than 160/0.34) and loperamide (greater than 160/0.29)]. The above ratios correlated with the gut selectivity of the compounds as defined by their analgesic/antidiarrheal potency ratios (r = 0.92, P less than 0.001). Furthermore, inhibition of propulsion was found to correlate with central analgesic activity (r = 0.93, P less than 0.001) but not with protection from diarrhea (r = 0.023, P greater than 0.05). Indeed, gut-selective opioids such as loperamide and loperamide oxide failed to affect propulsion up to doses more than 450 times their antidiarrheal doses. In contrast, alpha 2-adrenoceptor agonists delayed propulsion at doses comparable to their antidiarrheal doses [clonidine (0.085 vs 0.021), lidamidine (2.35 vs 1.66)] and anticholinergics inhibited propulsion even at doses many times below their antidiarrheal doses [atropine (0.26 vs 9.30), dexetimide (0.13 vs 5.03) and isopropamide (0.78 vs 74.6)]. The present results indicate that the in vivo inhibition of gastrointestinal propulsion by opioids in rats is mediated by a central action. Effects on intestinal fluid transport or, alternatively, on motility events distal to the ileocecal junction rather than effects on propulsion through the small intestine, seem to be the primary mechanism of antidiarrheal action of gut-selective opioids such as loperamide and loperamide oxide.

Pharmacology of risperidone (R 64 766), a new antipsychotic with serotonin-S2 and dopamine-D2 antagonistic properties.

Comparative studies of the benzisoxazole derivative risperidone (R 64 766) were made with ritanserin, a selective centrally acting serotonin-S2 antagonist and with haloperidol, a selective centrally acting dopamine-D2 antagonist. Risperidone like ritanserin shows activity in all tests related to serotonin-S2 antagonism, but at even lower doses (peripheral S2-antagonism at 0.0011 mg/kg, central S2-antagonism at 0.014 mg/kg). Like haloperidol, risperidone shows activity in all tests related to dopamine-D2 antagonism; activity in rats for both compounds starts at 0.016 mg/kg, but some central nervous system controlled functions, including the induction of catalepsy, are relatively much less affected by risperidone. Qualitatively, risperidone is a mixed serotonin-dopamine antagonist. Quantitatively, its study in dogs reveals potent dopamine-D2 antagonistic activity with excellent p.o. bioavailability and a relatively long duration of action. From the obtained pharmacological data, risperidone could be expected to possess the complementary clinical effects of a ritanserin-like serotonin-S2 and an haloperidol-like dopamine-D2 antagonist. Serotonin-S2 antagonism may improve the quality of sleep, reduce negative and affective symptoms in schizophrenic patients and decrease extrapyramidal symptoms induced by classical neuroleptics. Because risperidone is a dopamine-D2 antagonist, antidelusional, antihallucinatory and antimanic actions are expected. The first clinical studies indicate that two additional therapeutic targets, which are not reached with classical neuroleptics, may be obtained with risperidone in the monotherapy of schizophrenia and related disorders: very important contact and mood-elevating properties and extrapyramidal symptoms-free maintenance therapy.

Differential effects of the new antipsychotic risperidone on large and small motor movements in rats: a comparison with haloperidol.

Risperidone, a new antipsychotic agent, was studied for its effects on spontaneous motor activity in rats in comparison with haloperidol. Motor activity was recorded via the optical scanning technique (horizontal and vertical activity) and via a recently developed technique based on the piezo-electric principle which, in contrast to optical scanning, is very sensitive to small, stationary movements (piezo activity). Risperidone and haloperidol at low doses depressed both vertical activity (ED50s: 0.062 and 0.038 mg/kg, respectively) and horizontal activity (0.18 and 0.060 mg/kg, respectively). With increase of dose, motor activity decline was significantly faster with haloperidol than with risperidone. Moreover, haloperidol also rapidly depressed piezo activity (ED50: 0.085 mg/kg) whereas risperidone depressed this component of motor behaviour at much higher doses only (ED50: 2.80 mg/kg). Visual inspection did not reveal abnormal behavioural movements following the test compounds. Risperidone, therefore, preserves normal small movements over a much larger dose interval than haloperidol; this effect may be related to its relatively low cataleptogenic activity and potentially also to a reduced EPS liability. The present results further confirm that the piezo technique may complement the optical scanning method, and thereby enhance the information on the extent that test compounds modify behaviour.

Electrophysiologic, antiarrhythmic and hemodynamic effects of transcainide.

Transcainide was selected as an antiarrhythmic drug with potential clinical application. In isolated dog, sheep and rabbit Purkinje fibres, in dog and guinea-pig trabecular preparations and in the guinea-pig right auricle, transcainide decreases the rate of rise of the transmembrane action potential, with no effect on normal spontaneous activity and calcium-mediated action potentials; it inhibits early after-depolarizations. The effect on the rate of rise is very slow in onset. In vivo a prolongation of QRS duration is observed. In dogs, the drug is effective against post-infarction and ouabain-induced ventricular arrhythmias, and abolishes acetylcholine and aconitine-induced atrial fibrillation; it elevates the threshold of electrically induced ventricular fibrillation. Hemodynamic studies in anaesthetized and unaesthetized dogs indicate that transcainide moderately decreases contractility, while slightly increasing frequency. No major side effects are seen. Preliminary data on the pharmacokinetics suggest that in the dog the observed effects after i.v. infusion are related to the parent drug. Transcainide is an antiarrhythmic of the local anaesthetic type, with very slow kinetics. It is characterized by a good oral absorption and a long duration of action.