The antiemetic profile of zacopride

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Pathophysiological Mechanisms of Gastrointestinal Toxicity

Humans swallow a great variety and often large amounts of chemicals as nutrients, incidental food additives and contaminants, drugs, and inhaled particles and chemicals, thus exposing the gastrointestinal tract to many potentially toxic substances. It serves as a barrier in many cases to protect other components of the body from such substances and infections. Fortunately, the gastrointestinal tract is remarkably robust and generally is able to withstand multiple daily assaults by the chemicals to which it is exposed. Some chemicals, however, can affect one or more aspects of the gastrointestinal tract to produce abnormal events that reflect toxicity. It is the purpose of this chapter to evaluate the mechanisms by which toxic chemicals produce their deleterious effects and to determine the consequences of the toxicity on integrity of gastrointestinal structure and function. Probably because of the intrinsic ability of the gastrointestinal tract to resist toxic chemicals, there is a paucity of data regarding gastrointestinal toxicology. It is therefore necessary in many cases to extrapolate toxic mechanisms from infectious processes, inflammatory conditions, ischemia, and other insults in addition to more conventional chemical sources of toxicity.

On the risk concerns of zacopride, a moderate IK1 channel agonist with cardiac protective action

Zacopride, an IK1 agonist with moderate potency, could exert significant antiarrhythmic and cardiac protective effects. To date, there is no report to show that zacopride is proarrhythmic in both experimental studies and clinical trials. However, in certain cardiac pathological conditions, especially short QT syndrome and certain reentry tachycardia, zacopride is not suggested. Further studies are needed to precisely evaluate the potential arrhythmogenic risk of zacopride.

Looking beyond 5-HT(3) receptors: a review of the wider role of serotonin in the pharmacology of nausea and vomiting

The concept that 5-hydroxytryptamine (5-HT; serotonin) is involved in the emetic reflex was revealed using drugs that interfere with its synthesis, storage, release and metabolism ahead of the discovery of selective tools to modulate specific subtypes of receptors. This review comprehensively examines the fundamental role of serotonin in emesis control and highlights data indicating association of 5-HT1-4 receptors in the emetic reflex, whilst leaving open the possibility that 5-HT5-7 receptors may also be involved. The fact that each receptor subtype may mediate both emetic and anti-emetic effects is discussed in detail for the first time. These discussions are made in light of known species differences in emesis control, which has sometimes affected the perception of the translational value of data in regard to the development of novel anti-emetic for use in man.

Why can't rodents vomit? A comparative behavioral, anatomical, and physiological study

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity–key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.

Signals for nausea and emesis: Implications for models of upper gastrointestinal diseases

Nausea and vomiting are amongst the most common symptoms encountered in medicine as either symptoms of diseases or side effects of treatments. In a more biological setting they are also important components of an organism's defences against ingested toxins. Identification of treatments for nausea and vomiting and reduction of emetic liability of new therapies has largely relied on the use of animal models, and although such models have proven invaluable in identification of the anti-emetic effects of both 5-hydroxytryptamine(3) and neurokinin(1) receptor antagonists selection of appropriate models is still a matter of debate. The present paper focuses on a number of controversial issues and gaps in our knowledge in the study of the physiology of nausea and vomiting including: The choice of species for the study of emesis and the underlying behavioural (e.g. neophobia), anatomical (e.g. elongated, narrow abdominal oesophagus with reduced ability to shorten) and physiological (e.g. brainstem circuitry) mechanisms that explain the lack of a vomiting reflex in certain species (e.g. rats); The choice of response to measure (emesis[retching and vomiting], conditioned flavour avoidance or aversion, ingestion of clay[pica], plasma hormone levels[e.g. vasopressin], gastric dysrhythmias) and the relationship of these responses to those observed in humans and especially to the sensation of nausea; The stimulus coding of nausea and emesis by abdominal visceral afferents and especially the vagus-how do the afferents encode information for normal postprandial sensations, nausea and finally vomiting?; Understanding the central processing of signals for nausea and vomiting is particularly problematic in the light of observations that vomiting is more readily amenable to pharmacological treatment than is nausea, despite the assumption that nausea represents "low" intensity activation of pathways that can evoke vomiting when stimulated more intensely.

Inhibitory effect of zacopride on Cisplatin-induced delayed emesis in ferrets

We evaluated the antiemetic effect of zacopride, a potent 5-HT3-receptor antagonist with 5-HT4-receptor agonist properties, on delayed emesis caused by cisplatin (5 mg/kg, i.p.) in ferrets, compared with granisetron, a selective 5-HT3-receptor antagonist. Multiple intravenous injections of zacopride at 1 mg/kg, a dose that completely inhibited acute emesis caused bycisplatin (10 mg/kg, i.v.), significantly reduced delayed emesis. Granisetron (3.2 mg/kg) also reduced delayed emesis but this failed to reach statistical significance. The present study suggests that a combined 5-HT3-receptor antagonist/5-HT4-receptor agonist, like zacopride, may be useful against both acute and delayed emesis induced by cancer chemotherapy.

Roles of substance P and NK(1) receptor in the brainstem in the development of emesis

The emetic response is primarily a protective reflex occurring in a wide variety of vertebrates in response to the ingestion of toxic compounds. The role of the nuclei in the brainstem, including the area postrema, nucleus tractus solitarius, the dorsal motor nucleus of the vagus, and the central pattern generator for vomiting, as well as the involvement of the abdominal visceral innervation relevant to the emetic reflex, have all been discussed by many researchers. The introduction of serotonin 5-HT(3)-receptor antagonists into clinical practice allowed for a dramatic improvement in the management of vomiting. However, vomiting still remains a significant problem. The mechanism of the emetic response is even more complicated than was first thought. This review attempts to bring together some of the evidence suggesting the roles of substance P and its receptor, neurokinin NK(1) receptor, in the brainstem nuclei in the development of emesis. Accordingly, NK(1)-receptor antagonists might represent novel drugs for the management of major types of emesis.

Nitrogen-containing heteroalicycles with serotonin receptor binding affinity: development of gastroprokinetic and antiemetic agents

To obtain gastroprokinetic agents with more potent and selective activity than metoclopramide and cisapride, a series of N-(4-benzyl-2-morpholinylmethyl)benzamides were designed and prepared. Their synthesis and structure-activity relationships were described. As a result, mosapride was selected as a promising candidate for potent gastroprokinetic activity with selective 5-HT4 receptor agonistic activity. As an extension to this project, the novel benzamide and the carboxamide derivatives having 1-benzyl-4-methylhexahydro-1,4-diazepine ring in the amine moiety were prepared and evaluated for 5-HT3 receptor antagonistic activity. DAT-582 was identified as an antiemetic agent in cancer chemotherapy. The asymmetric synthesis of DAT-582 and the SAR studies were briefly reviewed. In further modifications of the N-(1-benzyl-4-methylhexahydro-1,4-diazepin-6-yl)benzamides, the novel nicotinamides with 1-ethyl-4-methylhexahydro-1,4-diazepin ring were found to have potent 5-HT3 and dopamine D2 and D3 receptor antagonistic activities and to show weak central nervous system depression and extrapyramidal syndrome. After extensive SARs, AS-8112 was selected as a broad antiemetic agent.

5-HT3 receptors are not involved in conditioned taste aversions induced by 5-hydroxytryptamine, ipecacuanha or cisplatin

We have used the rat to examine the involvement of the 5-HT3 receptor in the mechanism(s) of conditioned taste aversion induced by 5-hydroxytryptamine (5-HT) and selected emetic drugs. 5-HT, ipecacuanha and cisplatin all induced conditioned taste aversion at doses known to induce emesis in other species but the responses were resistant to treatment with the 5-HT3 receptor antagonists ondansetron and granisetron. Further, m-chlorophenylbiguanide, a selective and potent 5-HT3 receptor agonist, failed to induce a conditioned taste aversion. The data provide strong evidence that the 5-HT3 receptor is not involved in conditioned taste aversion mechanisms in the rat. Results are discussed in terms of the usefulness of the rat conditioned taste aversion paradigm to anti-emetic research.

Presynaptic 5-HT3 receptors evoke an excitatory response in dorsal vagal preganglionic neurones in anaesthetized rats

1. Recordings were made from a total of sixty-four vagal preganglionic neurones in the dorsal vagal motor nucleus (DVMN) of pentobarbitone sodium anaesthetized rats. The effects of ionophoretic administration of Mg2+ and Cd2+, inhibitors of neurotransmitter release, and the selective NMDA and non-NMDA receptor antagonists (+/-)-2-amino-5-phosphono-pentanoic acid (AP5) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) on the excitatory actions of the 5-HT3 receptor agonist 1-phenylbiguanide (PBG) were studied. 2. In extracellular recording experiments, PBG (0-40 nA) increased the firing rate of thirty-five of the thirty-nine neurones tested. The PBG-evoked excitation was attenuated by application of Mg2+ (1-10 nA) in sixteen of seventeen neurones or Cd2+ (2-10 nA) in seven of eight neurones tested. At these low ejection currents neither Mg2+ nor Cd2+ altered baseline firing rates and Mg2+ had no effect on the excitations evoked by DL-homocysteic acid (n = 4), NMDA (n = 4) or (AMPA; n = 2). 3. Ionophoresis of AP5 (2-10 nA), at currents which selectively inhibited NMDA-evoked excitations, attenuated PBG-evoked excitations in all eight neurones tested. DNQX (5-20 nA), at currents which selectively inhibited AMPA-evoked excitations, also attenuated PBG-evoked excitations (n = 3). 4. Intracellular activity was recorded in nine DVMN neurones. In six neurones ionophoretic application of PBG (10-200 nA) depolarized the membrane and increased firing rate whilst in the other three neurones, PBG had no effect on membrane potential though it increased synaptic noise (n = 3) and firing rate (n = 2). In all six neurones tested, ionophoresis of Mg2+ (10-120 nA) attenuated the PBG-evoked increases in synaptic noise and firing rate. 5. In conclusion, the data are consistent with the hypothesis that 5-HT3 receptor agonists activate DVMN neurones partly by acting on receptors located at sites presynaptic to the neurones. Activation of these receptors appears to facilitate release of glutamate, which, in turn, acts on postsynaptic NMDA and non-NMDA receptors to activate the neurones.

The anti-emetic activity of GK-128 in Suncus murinus

In Suncus murinus, various emetic responses and the anti-emetic activity of a new 5-hydroxytryptamine3 (5-HT3) receptor antagonist, GK-128 (2-[(2-methylimidazol-1-yl) methyl benzo[f]thiochromen-1-one monohydrochloride hemihydrate), were investigated. Cancer chemotherapeutic agents, cisplatin and cyclophosphamide, dose-dependently induced emesis of long-lasting duration. The 5-HT3 receptor agonist, 2-methyl-5-HT, and copper sulfate also induced emesis of short duration. However, another 5-HT3 receptor agonist, m-chlorophenylbiguanide, was not consistently emetic. GK-128 inhibited the emetic responses induced by chemotherapeutic agents and 2-methyl-5-HT with similar potency. The anti-emetic action of GK-128 was more potent than that of ondansetron, Y-25130, granisetron and metoclopramide. The order of potency of these drugs, except granisetron, was consistent with that of their 5-HT3 receptor binding affinity in rat cortex. GK-128 failed to inhibit copper sulfate-induced emesis. These data suggest that GK-128 has a potent inhibitory effect on emesis via the 5-HT3 receptor, and that the 5-HT3 receptor involved in emesis in Suncus murinus may be different from the classically defined 5-HT3 receptor in other animals such as rats, dogs and ferrets.

Possible involvement of peripheral 5-HT4 receptors in copper sulfate-induced vomiting in dogs

The involvement of visceral afferent fibers and 5-HT3 or 5-HT4 receptors in the vomiting induced by oral administration of copper sulfate was investigated in beagle dogs. Vomiting induced by copper sulfate (100 mg/kg) was inhibited markedly by bilateral abdominal vagotomy and bilateral greater splanchnic nerve section. The vomiting induced by copper sulfate was inhibited by blocking 5-HT4 receptors with high doses (1 and 3 mg/kg, i.v.) of ICS 205-930. On the other hand, blocking 5-HT3 receptors with MDL 72222 (0.5 and 5 mg/kg, i.v.) or low doses (0.01 mg/kg i.v.) of ICS 205-930 had no apparent effect on the vomiting induced by copper sulfate. Oral administration of a 5-HT4 receptor agonist, 5-methoxytryptamine (5-MT), caused vomiting at a dose of 100 mg/kg, and the vomiting was inhibited markedly by abdominal visceral nerve section or a high dose (1 mg/kg, i.v.), but not a low dose (0.01 mg/kg, i.v.), of ICS 205-930. Intravenous administration of 5-MT (10 mg/kg) failed to induce vomiting. These results suggest that the abdominal visceral afferent fibers and possibly peripheral 5-HT4 receptors play an important role in the vomiting induced by oral administration of copper sulfate in dogs.

Involvement of 5-HT3 receptors and vagal afferents in copper sulfate- and cisplatin-induced emesis in monkeys

The emetic effects of copper sulfate and cisplatin and the potential involvement of vagal afferent fibers and 5-HT3 receptors in the emesis were investigated in cynomolgus monkeys. Retching and vomiting induced by both oral (100 mg/kg) and intravenous (20 mg/kg) copper sulfate were inhibited markedly by abdominal vagotomy. Furthermore, the emetic response induced by oral copper sulfate was strongly inhibited by intravenous ICS 205-930 (0.1 mg/kg), a 5-HT3 receptor antagonist. Cisplatin (3 mg/kg, i.v.) caused severe retching and vomiting, and the number of emetic responses was much greater than that in other species. The emetic response induced by cisplatin was inhibited markedly by abdominal vagotomy or concurrent administration of ICS 205-930 (3 x 0.1 mg/kg, i.v.). These results suggest that the monkey is more sensitive to cisplatin than other species and that the vagal afferent terminals and 5-HT3 receptors play an important role in the emetic response induced by copper sulfate and cisplatin.

The effects of orally administered Y-25130, a selective serotonin3-receptor antagonist, on chemotherapeutic agent-induced emesis

The antiemetic effects of orally administered Y-25130, a potent and selective 5-HT3-receptor antagonist, were compared with those of ondansetron, granisetron, metoclopramide and domperidone. Y-25130 (0.1-1.0 mg/kg) dose-dependently prolonged the latency to the first vomiting and decreased the number of vomitings induced by cisplatin in dogs. The antiemetic effect of Y-25130 against cisplatin-induced vomiting was more potent than that of metoclopramide and ondansetron, but it showed little difference from that of granisetron. The emesis induced by the combined treatment of doxorubicin and cyclophosphamide was also inhibited by Y-25130 (0.1-1 mg/kg) in ferrets. The antiemetic effect of Y-25130 was more potent than that of metoclopramide, almost the same as that of granisetron and less potent than that of ondansetron. Because of a notable difference of potency ranking between Y-25130 and ondansetron in these two tests, a third test was performed to evaluate the inhibitory effect of Y-25130 in ferrets on cisplatin-induced emesis in comparison with that of ondansetron. The antiemetic effect of Y-25130 on cisplatin-induced emesis in ferrets was very similar to that of ondansetron. Domperidone did not inhibit these cytotoxic agents-induced emeses. These results suggest that Y-25130 is an orally active antiemetic compound against cisplatin and doxorubicin/cyclophosphamide-induced emeses; and its the antiemetic potency is similar to those of granisetron and ondansetron, but superior to those of metoclopramide and domperidone.

Pharmacological characterization of 5-hydroxytryptamine3 receptors in murine brain and ileum using the novel radioligand [3H]RS-42358-197: evidence for receptor heterogeneity

Previous studies have demonstrated species-specific differences in 5-hydroxytryptamine3 (5-HT3) receptors, but unequivocal evidence of 5-HT3 receptor subtypes, within a species, has not yet been obtained. The purpose of the current study was to test for heterogeneity in 5-HT3 receptors in murine tissues. 5-HT3 receptors in membranes derived from brain cerebral cortex of CD-1, C57Bl/6, and Swiss Webster mice and ileum of CD-1 mice were labeled with the 5-HT3 receptor antagonist [3H]RS-42358-197. Structurally diverse competing ligands were then used to characterize the binding site. [3H]RS-42358-197 bound with similar affinity in each of the cortical tissues (mean KD = 0.14 nM; range, 0.06-0.32 nM) but bound with lower affinity in ileal tissue (2.5 nM). The density of sites labeled with [3H]RS-42358-197 ranged from 10.4 fmol/mg of protein in Swiss Webster mouse cortex to 44.2 fmol/mg of protein in Sprague-Dawley rat cortex. Displacing ligands produced a pharmacologic profile of the [3H]RS-42358-197 binding site consistent with it being a 5-HT3 receptor: (R)-YM060 > (S)-zacopride > (R)-zacopride > MDL 72222 > 2-methyl-5-HT. However, > or = 10-fold differences in the affinity of certain ligands were found when comparing 5-HT3 binding sites in membranes from cerebral cortex of the different strains of mice and when comparing 5-HT3 binding sites in brain and ileal membranes prepared from the CD-1 mouse strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of cisplatin- and m-chlorophenylbiguanide-induced emesis in ferrets

We investigated the involvement of peripheral and central serotonin (5-HT)3 receptors in cisplatin- and 5-HT3 receptor agonist-induced emesis in ferrets. Cisplatin (10 mg/kg i.v.)-induced emesis was inhibited by intravenous YM060 (0.003-0.1 microgram/kg). A highly selective and potent 5-HT3 receptor agonist, m-chlorophenylbiguanide (1-10 mg/kg i.p.), dose dependently elicited emesis an effect which was inhibited by YM060 (0.003-0.3 microgram/kg i.v.). Vagotomy markedly reduced this emesis, and the combination of abdominal vagotomy and greater splanchnicectomy abolished emesis. Lesion of greater splanchnic nerves alone did not markedly inhibit emesis. Intracerebroventricularly (4th ventricle) administered YM060 inhibited cisplatin- and m-chlorophenylbiguanide-induced emesis only at higher doses (0.01-0.1 and 0.01-0.03 microgram, respectively). Intracerebroventricularly (4th ventricle) administered m-chlorophenylbiguanide (30-100 micrograms) produced only a weak retching response. These results indicate that stimulation of abdominal vagal afferent nerves via peripheral 5-HT3 receptors is important for triggering cisplatin- and m-chlorophenylbiguanide-induced emesis in ferrets.

Interaction of the antiemetic metopimazine and anticancer agents with brain dopamine D2, 5-hydroxytryptamine3, histamine H1, muscarine cholinergic and alpha 1-adrenergic receptors

The interactions of the antiemetic metopimazine (MPZ) and of the chemotherapeutic agents, cisplatin, carboplatin, doxorubicin, etoposide and vincristine were investigated at five neurotransmitter receptor binding sites. MPZ had nanomolar affinity for alpha 1, dopamine D2 and histamine H1 receptors, weak affinity for muscarinic cholinergic receptors, but no affinity for 5-hydroxytryptamine3 (5-HT3) receptors. Except for vincristine, which showed nanomolar affinity of muscarinic cholinergic receptors, none of the chemotherapeutic agents showed affinity for any of the receptors investigated at concentrations ranging between 10(-5) and 10(-7) M. Accordingly, chemotherapy-induced nausea and vomiting seems to be mediated by mechanisms other than the direct interaction of cytostatics with the neurotransmitter receptors investigated. Our finding that MPZ is without affinity for 5-HT3 receptors and therefore seems to mediate its antiemetic effect predominantly by dopamine D2 receptor blockade makes it an interesting drug for use in combinations with the new class of antiemetics, the 5-HT3 receptor antagonists. Data obtained in a recent clinical trial support this observation.

Pancopride, a potent and long-acting 5-HT3 receptor antagonist, is orally effective against anticancer drug-evoked emesis

Pancopride ((+-)N-(1-azabicyclo-[2,2,2]-oct-3-yl)-2-cyclopropylmethoxy-4-ami no-5-chlorobenzamide) is a new potent and selective 5-HT3 receptor antagonist, orally and parenterally effective against cytotoxic drug-induced emesis. In vitro, pancopride displayed high affinity (Ki = 0.40 nM) for [3H]GR65630-labelled 5-HT3 recognition sites in membranes from the cortex of rat brains. In vivo, pancopride antagonized 5-HT-induced bradycardia in anaesthetized rats when administered i.v. 5 min (ID50 = 0.56 microgram/kg) or p.o. 60 min (ID50 = 8.7 micrograms/kg) before 5-HT challenge. A single oral dose (10 micrograms/kg) of pancopride produced a significant inhibition of the bradycardic reflex over an 8-h period. Pancopride dose dependently inhibited the number of vomiting episodes and delayed the onset of vomiting induced by cisplatin in dogs (ID50 = 3.6 micrograms/kg i.v. and 7.1 micrograms/kg p.o.). Pancopride was also effective in blocking mechlorethamine- and dacarbazine-induced emesis. Unlike metoclopramide, pancopride was shown to lack any measurable antidopaminergic activity both in vitro and in vivo. These results support clinical data, indicating that pancopride will be a useful drug for treating cytostatic-induced emesis in humans.

Cisplatin-induced conditioned taste aversion: attenuation by dexamethasone but not zacopride or GR38032F

The 5-HT3 receptor antagonists zacopride and GR38032F are highly effective inhibitors of emesis induced by ionizing radiation and chemotherapeutic drugs such as cisplatin. The present study evaluated zacopride and GR38032F for efficacy in inhibiting the formation of the conditioned taste aversion (CTA) induced by cisplatin or lithium chloride in rats. The glucocorticoid dexamethasone, which has been reported to be effective against both the emetic and CTA-inducing effects of cisplatin, was included as a reference compound. When administered alone by i.p. injection, zacopride (0.1-10 mg/kg), GR38032F (10 mg/kg) and cisplatin (0.32-1.8 mg/kg) induced a CTA to an 0.1% saccharin solution; lower doses of each compound were ineffective. When administered as a pretreatment, neither zacopride (0.001-0.1 mg/kg) nor GR38032F (0.01-10 mg/kg) attenuated the CTA induced by cisplatin (0.32 and 0.56 mg/kg) or lithium chloride (10 mg/kg). In contrast, dexamethasone (0.32 and 1.0 mg/kg) attenuated the CTA induced by 0.32 but not 0.56 mg/kg of cisplatin. In an attempt to evaluate higher doses of zacopride against cisplatin without the potentially confounding factor that these doses by themselves induce a CTA, rats were injected with zacopride on three separate days prior to the aversion conditioning session. This pre-exposure treatment blocked the formation of the zacopride-induced CTA, but did not improve the efficacy of zacopride in attenuating the cisplatin-induced CTA. These results suggest that neither the cisplatin- nor the lithium-induced CTA in rats are due to effects that are sensitive to 5-HT3 receptor blockade.

Agonist effects at putative central 5-HT4 receptors in rat hippocampus by R(+)- and S(-)-zacopride; no evidence for stereo-selectivity

The EEG of halothane anaesthetized rats was recorded from an electrode implanted into the hippocampus. In the present study the effect of R(+)- and S(-)-zacopride, administered intra-cerebroventricularly, on different frequency bands of the EEG was investigated. Both enantiomers induced similar dose-dependent (5-20 micrograms) increases in all frequency bands. The effects of R(+)- and S(-)-zacopride were inhibited by pretreatment with a high dose of ICS 205-930 (1 micrograms i.c.v.), which suggests the involvement of 5-HT4 receptors. The lack of stereo-selectivity of the zacopride enantiomers is in contrast to observations made in in vitro studies.

Preliminary evidence for the involvement of the putative 5-HT4 receptor in zacopride- and copper sulphate-induced vomiting in the ferret

Previous studies of the mechanism of zacopride-induced emesis in ferrets have concluded that it is mediated predominantly by an antagonist effect on 5-HT3 receptors although the possibility of a contribution from an agonist effect at 5-HT4 receptors was not excluded. This study shows that zacopride (200 micrograms/kg p.o.)-induced emesis can be blocked by a 'high dose' (1000 micrograms/kg) of ICS205930 but not by a low dose (100 micrograms/kg) or by 'high doses' (1000 micrograms/kg) of another more selective 5-HT3 receptor antagonist granisetron. As ICS205930, at high doses, is reported to be a 5-HT4 receptor antagonist it appears likely that activation of 5HT4-receptors contributes to emesis induced by zacopride. 'High' doses of ICS205930, but not granisetron or ondansetron, can also block the vagally mediated emesis induced by oral CuSO4 suggesting that 5-HT4 receptors involved in emesis are closely associated with abdominal vagal afferents.

5-HT3 receptor antagonists. An overview of their present status and future potential in cancer therapy-induced emesis

The serotonin (5-hydroxytryptamine, 5-HT) antagonists, which bind at the type 3 receptor (5-HT3 receptor), have been evaluated in several preclinical models and found to be effective in alleviating cancer therapy-related emesis. The antiemetic efficacy of ondansetron (GRF-38032F, odanserin), granisetron (BRL-43694), tropisetron (ICS-205930), MDL-72222 and MDL-73147EF, batanopride (BMY-25801-01) and several others is at various stages of investigation. Ondansetron is currently marketed in several countries and the same will soon be true for granisetron. At this stage it is not yet possible to evaluate the comparative efficacy of each of these compounds, although recent preclinical data reveal some differences in the affinity of these compounds, for other receptors. Side effects related to these agents have been minor, consisting mainly of slight headaches; possible rises in liver enzymes related to some compounds need further evaluation. Future studies will need to determine the exact role of 5-HT3 antagonists, although their cost may confine their use to patients at high risk for side effects from metoclopramide.

5-HT3 receptor agonism may be responsible for the emetic effects of zacopride in the ferret

The racemic 5-HT3 receptor antagonist, zacopride (10-100 micrograms kg-1, i.m.) evoked an emetic response in ferrets. This property appeared to reside totally in the S-enantiomer which also produced emesis over the same dose range. This emesis could be prevented by pretreatment with ondansetron (1 mg kg-1, i.m.) or by R-zacopride (100 micrograms kg-1, i.m.). In urethane-anaesthetized ferrets, S-zacopride (0.3 micrograms kg-1, i.v.) evoked a profound Bezold-Jarisch reflex which was blocked by both ondansetron (30 micrograms kg-1, i.v.) and by R-zacopride (100 micrograms kg-1, i.v.). These results suggest that, in the ferret, S-zacopride possesses 5-HT3 receptor agonist properties which may be responsible for the emetic effect. In contrast R-zacopride does not appear to possess 5-HT3 receptor agonist properties in this species.

The antiemetic profile of Y-25130, a new selective 5-HT3 receptor antagonist

Y-25130( (+/-)N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro - 2H-1,4-benzoxazine-8-carboxamide hydrochloride) is a potent and selective 5-HT3 receptor antagonist free of dopamine receptor blocking activity. This compound was effective against emesis induced in animals by cytotoxic drugs or by total body X-radiation. When given prophylactically, the doses required to completely inhibit cisplatin-induced emesis in dogs and doxorubicin and cyclophosphamide-induced emesis in ferrets were 0.1 and 0.3 mg/kg i.v., respectively. Y-25130, at the dose of 0.3 mg/kg i.v., almost completely inhibited X-radiation-induced emesis in ferrets. When given during emesis, the doses required to completely inhibit cisplatin-induced emesis in dogs and doxorubicin- and cyclophosphamide-induced emesis in ferrets were 0.1 and 0.3 mg/kg i.v., respectively. The i.v. dose of 0.3 mg/kg of Y-25130 was enough to almost completely inhibit cisplatin-induced emesis in dogs for 24 h. From these results, it is suggested that Y-25130 may become an effective antiemetic drug against emesis induced by anticancer therapy.

Studies on the emetic and antiemetic properties of zacopride and its enantiomers

In ferrets, the oral emetic activity of zacopride was compared with its R- and S-enantiomers. Increasing doses of 0.01, 0.1, 1.0, 10.0 and 31.6 mg/kg of zacopride or its 2 enantiomers were each administered at hourly intervals to separate groups of animals until emesis occurred. The emetic (100%) dose for zacopride and its S-enantiomer was 0.11 mg/kg p.o. (cumulative dose). The R-enantiomer at a cumulative dose of 42.71 mg/kg p.o. produced emesis in 25% of the animals. By the i.p. route zacopride and its S-enantiomer were more potent than the R-enantiomer in blocking the emetic activity of 0.1 mg/kg p.o. of zacopride. The involvement of 5-HT3 mechanisms is indicated by a correlation between zacopride and its enantiomers to cause and prevent emesis and their affinity at 5-HT3 binding sites. Further, the putative 5-HT3 agonists, 2-methyserotonin and phenylbiguanide, at 10 mg/kg p.o., produced emesis that was blocked by zacopride (0.1 mg/kg i.p.) or ICS 205-930 (1 mg/kg i.p.). The results suggest that in the ferret the S-enantiomer is predominantly responsible for both the emetic and antiemetic properties of zacopride and that 5-HT3 agonism and antagonism are involved in these actions.

Antiemetic action of 5-HT3 receptor antagonists: review of preclinical and clinical results with ICS 205-930

These results thus indicate, that ICS 205-930 is a very effective antiemetic for cancer chemotherapy-induced emesis in man. Given as a single dose prior to the chemotherapeutic agent, ICS 205-930 inhibits emesis and nausea for at least 24 hr. Together with the lack of extrapyramidal side-effects, these properties of ICS 205-930 indicate a clear superiority to the current therapeutic standard metoclopramide.

Solubilization of a 5-HT3 binding site from rabbit small bowel muscularis membranes

A 5-HT3 binding site, with high affinity for (S-)[3H]zacopride, was solubilized from rabbit small bowel muscularis membranes utilizing 0.5% sodium cholate and 400 mM (NH4)2SO4. Approximately 72% of the (S-)[3H]zacopride binding activity was recovered in a form that retained the high affinity (Kd = 0.7 nM) and specificity for this radioligand that is characteristic of the membrane-bound receptor. ICS 205-930 and other 5-HT3 compounds were effective inhibitors and exhibited the same rank order of potency in the solubilized and membrane-bound preparations. The receptor-detergent complex did not sediment after centrifugation for 1 h at 150,000 x g and eluted between thyroglobulin (MW = 669,000) and apoferritin (MW = 443,000) when fractionated by high-performance liquid chromatography gel filtration. This is the first report of the solubilization of a 5-HT3 binding site.

Emetic activity of zacopride in ferrets and its antagonism by pharmacological agents

Zacopride administered orally was more emetic in fed than in fasted ferrets. The emetic activity of zacopride (0.1 mg/kg p.o.) was inhibited (100%) by 0.1 mg/kg i.p. of zacopride and 1 mg/kg i.p. of ICS 205-930. Haloperidol (3.16 mg/kg i.p.) and prochlorperazine (3.16 mg/kg i.p.) were weakly effective. N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide, a 5-HT1P antagonist, was inactive. Thus, the emetic activity of zacopride, like that of cisplatin, is blocked by 5-HT3 receptor antagonists.

Antagonism of [3H]zacopride binding to 5-HT3 recognition sites by its (R) and (S) enantiomers

[3H]Zacopride exhibits high affinity (Kd less than or equal to 1 nM) for 5-HT3 binding sites (inhibited by ICS 205-930) in rabbit intestinal muscularis and vagus nerve, human jejunum, rat intestinal muscularis and rat brain cortex. Its binding was inhibited by several 5-HT3 antagonists that displayed similar rank orders of potency in the tissues examined. Zacopride's (S) enantiomer was significantly more potent than its (R) enantiomer (21- to 42-fold in rabbit and human; 8- to 12-fold in rat) as an inhibitor of [3H]zacopride binding. These studies indicate that the utility of [3H]zacopride as a high affinity 5-HT3 ligand resides with the (S) enantiomer.

The actions of fenfluramine and interaction with 5-HT3 receptor-antagonists to inhibit emesis in the ferret

The racemate and (+)- and (-)-isomers of fenfluramine (5 mg kg-1 i.p., 1 h pretreatment) antagonized cisplatin-induced retching and vomiting in the ferret. The intravenous injection of (+/-)-fenfluramine administered on an established cisplatin-induced emesis antagonized the response within minutes of injection. The administration of a lower dose of (+/-)-fenfluramine (1.0 mg kg-1 i.p., 1 h pretreatment) failed to antagonize cisplatin-induced emesis when administered alone but enhanced the antiemetic effects of metoclopramide and ICS 205-930. This pretreatment with (+/-)-fenfluramine failed to enhance the antiemetic effects of zacopride. It is considered that an action of the racemate on presynaptic 5-HT/catecholaminergic systems to reduce neurotransmitter release may enhance the action of certain 5-HT3 receptor antagonists in controlling emesis induced by cisplatin.

Nerve growth factor induces 5-HT3 recognition sites in rat pheochromocytoma (PC12) cells

In rat pheochromocytoma (PC12) cells, nerve growth factor (7S NGF) induced the expression of recognition sites that bind the specific 5-HT3 antagonist (S-) [3H]zacopride. Culturing PC12 cells for 8-12 days in the presence of 50 ng/ml NGF increased the density (Bmax) of (S-) [3H]zacopride binding sites in cell membranes (0-100,000 x g fraction) from 0 to 105 fmoles/mg protein. This binding exhibited high affinity for (S-) [3H]zacopride (Kd = 0.8 nM), was specific (greater than 95%), and was inhibited by 5-HT3 compounds with a rank of potency (quipazine greater than ICS 205-930 greater than GR38032F greater than BRL24924 approximately MDL 72222 greater than phenylbiguanide greater than or equal to serotonin greater than 2-methyl-serotonin greater than metoclopramide) which was distinct from neuroblastoma cells. Thus, NGF-differentiated PC12 cells possess a 5-HT3 receptor and should be useful to investigate its regulation and biochemical mechanism of action.

Association of [3H]zacopride with 5-HT3 binding sites

An assay was developed for [3H]zacopride binding to 5-HT3 specific sites in membranes from rabbit ileum muscularis. The binding was rapid, saturable, reversible, salt-insensitive, unaffected by pH between 6.5 and 9.5, and of high affinity (apparent KD = 0.65 +/- 0.15 nM). ICS 205-930, a potent 5-HT3 antagonist that inhibited competitively, was utilized to define 5-HT3 specific binding. Other 5-HT3 antagonists and agonists, although exhibiting marked differences in potency, were also effective inhibitors; whereas, antagonists of other classes of serotonin receptors, guanyl nucleotides and numerous receptor-specific ligands, including peptide hormones, were inactive. Vagus nerve exhibited the greatest amount of 5-HT3 specific binding amongst rabbit tissues and virtually all of the [3H]zacopride was bound to 5-HT3 binding sites. In rabbit, rat and ferret a fairly uniform distribution of 5-HT3 binding sites was observed along the muscularis of the small bowel. [3H]Zacopride is a high-affinity ligand for detecting 5-HT3 binding sites and rabbit small bowel muscularis membranes are a sensitive system for evaluating the potency of 5-HT3 antagonists or agonists.