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

Triple arginines as molecular determinants for pentameric assembly of the intracellular domain of 5-HT3A receptors

Serotonin type 3A receptors are homopentameric ligand-gated ion channels that are thought to assemble via interactions involving the subunits’ extracellular and transmembrane domains. Pandhare et al. reveal that channel assembly is also determined by three arginine residues in the receptor’s intracellular domain.

Serotonin type 3 receptors (5-HT₃Rs) are cation-conducting pentameric ligand-gated ion channels and members of the Cys-loop superfamily in eukaryotes. 5-HT₃Rs are found in the peripheral and central nervous system, and they are targets for drugs used to treat anxiety, drug dependence, and schizophrenia, as well as chemotherapy-induced and postoperative nausea and emesis. Decades of research of Cys-loop receptors have identified motifs in both the extracellular and transmembrane domains that mediate pentameric assembly. Those efforts have largely ignored the most diverse domain of these channels, the intracellular domain (ICD). Here we identify molecular determinants within the ICD of serotonin type 3A (5-HT_3A) subunits for pentameric assembly by first identifying the segments contributing to pentamerization using deletion constructs of, and finally by making defined amino acid substitutions within, an isolated soluble ICD. Our work provides direct experimental evidence for the contribution of three intracellular arginines, previously implicated in governing the low conductance of 5-HT_3ARs, in structural features such as pentameric assembly.

Molecular Pharmacology of Phytocannabinoids

Cannabis sativa has been used for recreational, therapeutic and other uses for thousands of years. The plant contains more than 120 C₂₁ terpenophenolic constituents named phytocannabinoids. The Δ⁹-tetrahydrocannabinol type class of phytocannabinoids comprises the largest proportion of the phytocannabinoid content. Δ⁹-tetrahydrocannabinol was first discovered in 1971. This led to the discovery of the endocannabinoid system in mammals, including the cannabinoid receptors CB₁ and CB₂. Δ⁹-Tetrahydrocannabinol exerts its well-known psychotropic effects through the CB₁ receptor but this effect of Δ⁹-tetrahydrocannabinol has limited the use of cannabis medicinally, despite the therapeutic benefits of this phytocannabinoid. This has driven research into other targets outside the endocannabinoid system and has also driven research into the other non-psychotropic phytocannabinoids present in cannabis. This chapter presents an overview of the molecular pharmacology of the seven most thoroughly investigated phytocannabinoids, namely Δ⁹-tetrahydrocannabinol, Δ⁹-tetrahydrocannabivarin, cannabinol, cannabidiol, cannabidivarin, cannabigerol, and cannabichromene. The targets of these phytocannabinoids are defined both within the endocannabinoid system and beyond. The pharmacological effect of each individual phytocannabinoid is important in the overall therapeutic and recreational effect of cannabis and slight structural differences can elicit diverse and competing physiological effects. The proportion of each phytocannabinoid can be influenced by various factors such as growing conditions and extraction methods. It is therefore important to investigate the pharmacology of these seven phytocannabinoids further, and characterise the large number of other phytocannabinoids in order to better understand their contributions to the therapeutic and recreational effects claimed for the whole cannabis plant and its extracts.

The antidepressant bupropion is a negative allosteric modulator of serotonin type 3A receptors

The FDA-approved antidepressant and smoking cessation drug bupropion is known to inhibit dopamine and norepinephrine reuptake transporters, as well as nicotinic acetylcholine receptors (nAChRs) which are cation-conducting members of the Cys-loop superfamily of ion channels, and more broadly pentameric ligand-gated ion channels (pLGICs). In the present study, we examined the ability of bupropion and its primary metabolite hydroxybupropion to block the function of cation-selective serotonin type 3A receptors (5-HT_3ARs), and further characterized bupropion's pharmacological effects at these receptors. Mouse 5-HT_3ARs were heterologously expressed in HEK-293 cells or Xenopus laevis oocytes for equilibrium binding studies. In addition, the latter expression system was utilized for functional studies by employing two-electrode voltage-clamp recordings. Both bupropion and hydroxybupropion inhibited serotonin-gated currents from 5-HT_3ARs reversibly and dose-dependently with inhibitory potencies of 87 μM and 112 μM, respectively. Notably, the measured IC₅₀ value for hydroxybupropion is within its therapeutically-relevant concentrations. The blockade by bupropion was largely non-competitive and non-use-dependent. Unlike its modulation at cation-selective pLGICs, bupropion displayed no significant inhibition of the function of anion-selective pLGICs. In summary, our results demonstrate allosteric blockade by bupropion of the 5-HT_3AR. Importantly, given the possibility that bupropion's major active metabolite may achieve clinically relevant concentrations in the brain, our novel findings delineate a not yet identified pharmacological principle underlying its antidepressant effect.

Combinatorial Consensus Scoring for Ligand-Based Virtual Fragment Screening: A Comparative Case Study for Serotonin 5-HT(3)A, Histamine H(1), and Histamine H(4) Receptors

In the current study we have evaluated the applicability of ligand-based virtual screening (LBVS) methods for the identification of small fragment-like biologically active molecules using different similarity descriptors and different consensus scoring approaches. For this purpose, we have evaluated the performance of 14 chemical similarity descriptors in retrospective virtual screening studies to discriminate fragment-like ligands of three membrane-bound receptors from fragments that are experimentally determined to have no affinity for these proteins (true inactives). We used a complete fragment affinity data set of experimentally determined ligands and inactives for two G protein-coupled receptors (GPCRs), the histamine H1 receptor (H1R) and the histamine H4 receptor (H4R), and one ligand-gated ion channel (LGIC), the serotonin receptor (5-HT3AR), to validate our retrospective virtual screening studies. We have exhaustively tested consensus scoring strategies that combine the results of multiple actives (group fusion) or combine different similarity descriptors (similarity fusion), and for the first time systematically evaluated different combinations of group fusion and similarity fusion approaches. Our studies show that for these three case study protein targets both consensus scoring approaches can increase virtual screening enrichments compared to single chemical similarity search methods. Our cheminformatics analyses recommend to use a combination of both group fusion and similarity fusion for prospective ligand-based virtual fragment screening.

Metabolic kinetics of 5-hydroxytryptamine and the research targets of functional gastrointestinal disorders

5-Hydroxytryptamine (5-HT) is an important neurotransmitter in both the central and enteric nervous systems. It has diverse functions in regulating gastrointestinal motility and visceral sensitivity, emotion, appetite, pain and sensory perception, cognition, sexual activity and sleep. These functions are mainly associated with the metabolic kinetics of 5-HT in different tissues. Tryptophan hydroxylase is the rate-limiting enzyme and modulates serotonin synthesis. Vesicular monoamine transporter 1 plays a role in 5-HT storage and release. Degradation of 5-HT is mediated by monoamine oxidase-A. All these factors influence the action of 5-HT in vivo. Functional gastrointestinal disorders (FGIDs) are characterized by a series of symptoms including abdominal pain, diarrhea, constipation, anxiety and depression, in the absence of identifiable structural or biochemical abnormalities. They are frequently accompanied by changed gut motility or visceral sensitivity. An increasing body of research has found FGIDs to be closely associated with 5-HT, and drugs such as citalopram, paroxetine, venlafaxine, alosetron, tegaserod, prucalopride and mosapride have all been developed or discovered from the perspective of the metabolic kinetics of 5-HT. This review discusses the relationship between the metabolic kinetics of 5-HT and research targets in the field of FGIDs and suggests areas of future study that may be useful for understanding these disorders and identification of potential therapeutic targets.

Pharmacokinetic Evaluation and Safety Profile of a 15-Minute Versus 30-Second Infusion of Palonosetron in Healthy Subjects

Palonosetron is a potent, selective 5-HT(3) receptor antagonist effective in the prevention of acute and delayed chemotherapy-induced nausea and vomiting. In practice, 5-HT(3) receptor antagonists, including palonosetron, are often coadministered with dexamethasone over approximately 15 minutes, although the approval of palonosetron was based on administration as a 30-second infusion. This open-label, randomized, 2-way crossover trial compared the pharmacokinetics and safety of palonosetron 0.25 mg administered as a 15-minute 50-mL intravenous infusion with a 30-second 5-mL infusion. Aside from an anticipated 40% decrease in maximum plasma concentration after a 15-minute infusion, the pharmacokinetics of palonosetron (including area under the plasma concentration-time curve [AUC], plasma elimination half-life, total body clearance, and apparent volume of distribution at steady state) were similar for both treatments. Both treatments were well tolerated, with no significant changes in vital signs or electrocardiograms. Palonosetron infused over 15 minutes is well tolerated, with an AUC(0-infinity) equivalent to a 30-second infusion.

Pharmacokinetic and Safety Evaluation of Palonosetron, a 5-Hydroxytryptamine-3 Receptor Antagonist, in U.S. and Japanese Healthy Subjects

Palonosetron (Aloxi, Onicit) is a selective 5-HT(3) receptor antagonist recently approved by the Food and Drug Administration for the prevention of acute and delayed chemotherapy-induced nausea and vomiting. This study was performed to determine the pharmacokinetics and assess the safety and tolerability of intravenous (IV) palonosetron in healthy U.S. and Japanese subjects. Subjects were administered a single IV dose of palonosetron, ranging from 0.3 to 90 microg/kg in either of two randomized, double-blind, placebo-controlled, ascending-dose studies (n = 80 and n = 32, respectively). Serial blood samples were obtained in both studies to evaluate the pharmacokinetics of palonosetron and its N-oxide metabolite, M9. Intravenous palonosetron was well tolerated across a wide range of doses in both studies. The incidence and severity of adverse events (AEs) were similar between subjects receiving palonosetron and those receiving placebo, with no dose-dependent incidences. The most frequently reported AEs were headache, transient elevation of liver enzymes, and constipation. Systemic exposure (AUC and C(max)) for palonosetron generally increased with increasing dose. Mean total body clearance, elimination half-life, and apparent volume of distribution ranged from 1.11 to 3.90 mL/min/kg, 33.7 to 54.1 hours, and 3.85 to 12.6 L/kg, respectively, in U.S. subjects and from 2.58 to 3.50 mL/min/kg, 30.8 to 36.8 hours, and 6.96 to 9.85 L/kg, respectively, in Japanese subjects. The pharmacokinetics of palonosetron appeared to be independent of dose, with no dose adjustment required in Japanese subjects. The plasma concentration profile of palonosetron, as represented by a half-life of approximately 40 hours, may provide a clinical advantage over other 5-HT(3) antagonists.

5-HT(3) receptors

5-Hydroxytryptamine type 3 (5-HT(3)) receptors are cation-selective Cys loop receptors found in both the central and peripheral nervous systems. There are five 5-HT(3) receptor subunits (A-E), and all functional receptors require at least one A subunit. Regions from noncontiguous parts of the subunit sequence contribute to the agonist-binding site, and the roles of a range of amino acid residues that form the binding pocket have been identified. Drugs that selectively antagonize 5-HT(3) receptors (the "setrons") are the current gold standard for treatment of chemotherapy-induced and postoperative nausea and vomiting and have potential for the treatment of a range of other conditions.

Effects of grape seed proanthocyanidin on 5-hydroxytryptamine(3) receptors in NCB-20 neuroblastoma cells

Proanthocyanidin is a phenolic compound present in plants, that has antioxidant, antinociceptive, anti-emetic, and neuroprotective properties. We investigated the actions of proanthocyanidin from grape seeds on 5-hydroxytryptamine (5-HT)(3) receptors in NCB-20 neuroblastoma cells using a whole-cell voltage clamp technique. Co-treatment of proanthocyanidin (0.3-100 µg/ml) and 3 µM 5-HT (near EC(50)) produced a slight inhibition of 5-HT-induced inward peak current (I(5-HT)) in NCB-20 cells, but pretreatment with proanthocyanidin for 30 s before application of 5-HT induced a much larger inhibition of I(5-HT) in an irreversible, concentration- and time-dependent manner (IC(50)=6.5±0.4 µg/ml, Hill coefficient=2.5±0.1). Proanthocyanidin also produced a concentration-dependent inhibition of currents induced by 30 µM 5-HT, near-maximal concentration (IC(50)=22.1±0.4 µg/ml, Hill coefficient=2.4±0.1). High concentrations (≧30 µg/ml) of proanthocyanidin caused a concentration-dependent inhibition of the activation and desensitization of currents induced by 30 µM 5-HT. Further studies showed that pretreatment of 20 µg/ml proanthocyanidin caused not only a rightward shift of the dose-response curve for 5-HT (EC(50) shift from 2.7±0.4 to 6.2±0.5 µM), but also a decreased E(max) (inhibition by 37.5±1.3%). The proanthocyanidin-induced inhibition of 5-HT(3) receptors did not show a significant difference within the testing holding potential ranges (-50-+30 mV). These results suggest that proanthocyanidin inhibits 5-HT(3) receptor function in NCB-20 cells in a noncompetitive mode, and that this inhibitory effect of proanthocyanidin probably contributes to the pharmacological actions of proanthocyanidin.

Therapeutics of 5-HT3 receptor antagonists: current uses and future directions

The 5-Hydroxytryptamine3 (5-HT3) receptor is a member of the cys-loop family of ligand gated ion channels, of which the nicotinic acetylcholine receptor is the prototype. All other 5-HT receptors identified to date are metabotropic receptors. The 5-HT3 receptor is present in the central and peripheral nervous systems, as well as a number of non-nervous tissues. As an ion channel that is permeable to the cations, Na(+), K(+), and Ca(2+), the 5-HT3 receptor mediates fast depolarizing responses in pre- and post-synaptic neurons. As such, 5-HT3 receptor antagonists that are used clinically block afferent and efferent synaptic transmission. The most well established physiological roles of the 5-HT3 receptor are to coordinate emesis and regulate gastrointestinal motility. Currently marketed 5-HT3 receptor antagonists are indicated for the treatment of chemotherapy, radiation, and anesthesia-induced nausea and vomiting, as well as irritable bowel syndrome. Other therapeutic uses that have been explored include pain and drug addiction. The 5-HT3 receptor is one of a number of receptors that play a role in mediating nausea and vomiting, and as such, 5-HT3 receptor antagonists demonstrate the greatest anti-emetic efficacy when administered in combination with other drug classes.

Ramosetron versus ondansetron in the prevention of chemotherapy-induced gastrointestinal side effects: A prospective randomized controlled study

BACKGROUND

This study observed and compared the preventive effects of ramosetron and ondansetron on gastrointestinal side effects caused by cisplatin-containing chemotherapy.

METHODS

Fifty patients with malignant tumors undergoing their first chemotherapy were randomly divided into two groups, and each group received 0.3 mg of ramosetron and 16 mg of ondansetron in a prospective crossover comparison study.

RESULTS

Data were collected for analysis of the therapeutic effect in 47 cases and for adverse events in 50 cases. Both drugs showed similar results in regard to chemotherapy-induced gastrointestinal side effects, emesis and appetite loss on day 1, but by day 5, ramosetron was significantly better than ondansetron in terms of controlling appetite loss. From days 3-5, ramosetron tended to be more effective than ondansetron in its antiemetic action. The incidence of headache, fatigue and constipation was the same for both drugs.

CONCLUSIONS

Ramosetron is a long-lasting and safe antiemetic agent.

Interactions of metoclopramide and ergotamine with human 5-HT(3A) receptors and human 5-HT reuptake carriers

The actions of metoclopramide and ergotamine, drugs which are used as a combined migraine medication, on human (h)5-HT3A receptors and 5-HT reuptake carriers, stably expressed in HEK-293 cells, were studied with patch-clamp- and ([3H]5-HT)-uptake techniques. At clinical concentrations, metoclopramide inhibited peak and integrated currents through h5-HT3A receptors concentration-dependently (IC50 = 0.064 and 0.076 microM, respectively) when it was applied in equilibrium (60 s before and during 5-HT (30 microM) exposure). The onset and offset time constants of metoclopramide action were 1.3 and 2.1 s, respectively. The potency of metoclopramide when exclusively applied during the agonist pulse decreased more than 200-fold (IC50 = 19.0 microM, peak current suppression). Metoclopramide (0.10 microM) did not alter the EC50 of 5-HT-induced peak currents. In contrast to the lack of competitive interaction between metoclopramide and 5-HT in this functional assay, metoclopramide inhibited specific [3H]GR65630 binding to human h5-HT3A receptors in a surmountable manner. This seeming discrepancy between functional studies and radioligand binding experiments may be accounted for by (1) the slow kinetics of inhibition of peak currents by metoclopramide compared with the fast onset and offset kinetics of 5-HT-induced currents and (2) the low efficacy of metoclopramide in inhibiting radioligand binding (e.g. only 20% binding inhibition compared to 79% peak current suppression by 200 nM metoclopramide). At low concentrations (1-10 nM), ergotamine had no effect on 5-HT (30 microM)-induced peak currents. Above clinical concentrations, ergotamine (>3 microM) inhibited them. When both drugs were applied together (0.10 microM metoclopramide +0.001 to 0.01 microM ergotamine), an inhibition of both, peak and integrated current responses was observed. Neither metoclopramide (< or =30 microM) nor ergotamine (< or =30 microM) had an effect on the 5-HT reuptake carrier as they did not alter the citalopram-sensitive [3H]5-HT uptake.

Orally Active Benzoxazole Derivative as 5-HT3 Receptor Partial Agonist for Treatment of Diarrhea-Predominant Irritable Bowel Syndrome

During our search for therapeutic agents to treat diarrhea-predominant IBS, we found that 2-substituted benzoxazole derivatives have a characteristic 5-HT(3) receptor partial agonist activity with high affinity. Some of these compounds showed high in vitro metabolical stability, and 6g showed marked antidiarrhetic activity with little side effect of constipation in in vivo tests. Our results indicate that 5-HT(3) receptor partial agonists might be superior as therapeutic agents to the drugs currently used for IBS treatment.

The antiemetic effect of a novel tropisetron patch in anticancer agents-induced kaolin pica model using rats

The efficacy of a novel transdermal patch containing tropisetron, a 5-hydroxytryptamine 3 (5-HT(3)) receptor antagonist, against emesis induced by anticancer agents were evaluated, in comparison with the effect of traditional tropisetron injection, in rats. The antiemetic effects were assessed via the inhibitory activity on the anticancer agent-induced kaolin-consuming behavior, a pica model representing vomiting in emesis-resistant rodents. The tropisetron patch (10mg/patch, 3.5cm(2)) was attached on the shaved back area of rats. Eight h later, each anticancer agent, cisplatin (10mg/kg, i.v.), cyclophosphamide (200mg/kg, i.p.) or doxorubicin (8mg/kg, i.v.), was administered, and thereafter, daily kaolin consumption was measured for 3 days. In comparison, the effect of daily injection of tropisetron (2mg/kg, i.v.), given 10min, 24 and 48h after the anticancer agent administration, was also evaluated. Kaolin intake greatly increased to 21, 17 and 10 folds of control ingestion on the first day after administration with the anticancer agents, cisplatin, cyclophosphamide and doxorubicin, respectively, and then gradually decreased to near control level on day 3. Such anticancer agent-induced increases in the kaolin consumption were remarkably attenuated by the attachment of tropisetron patch, resulting in the reduction to half levels, which is comparable to the efficacy of daily tropisetron injection. In particular, the blood concentration of tropisetron following patch attachment reached a maximum level of 30-40ng/ml in 12h and exhibited a plateau until detachment of the patch, in contrast to a rapid elimination with a half-life of 2.21h after injection of the drug. Taken together, it is suggested that the novel tropisetron patch could be a promising regimen for the relief of emesis, based on the long-term antiemetic effects on the diverse anticancer agents and the convenience to use the transdermal delivery system for the cancer patients who have difficulty in taking drugs due to surgical operation or gastrointestinal dysfunction.

Pharmacokinetics, metabolism and excretion of intravenous [l4C]-palonosetron in healthy human volunteers

Palonosetron (Aloxi(R), Onicit(R)) is a potent, single stereoisomeric 5-HT(3) receptor antagonist developed to prevent chemotherapy-induced nausea and vomiting. The pharmacokinetics and metabolic disposition of a single intravenous [(14)C]-palonosetron (10 microg/kg, 0.8 microCi/kg) bolus dose were evaluated in six healthy volunteers (three males, three females) using serial blood, plasma, urine and fecal samples obtained over 10 days. The safety, tolerability and cardiac effects were assessed. Radiolabeled metabolic characterization revealed that unchanged palonosetron accounted for 71.9% of the total radioactivity in plasma over 96 h, with an extensive distribution volume (8.34 l/kg) and mean plasma elimination half-life of 37 h. Approximately 83% of the dose was recovered in urine ( approximately 40% as unchanged drug, with 50% metabolized; M9 and M4 were the major metabolites) and 3.4% in feces. Hydrolysis of urine samples suggests that the metabolites are not beta-glucuronide or sulfate conjugates of the parent drug or metabolites. The blood to plasma concentration ratio of the total radioactivity was 1.2, on average, indicating little selective partitioning in erythrocytes. Palonosetron was generally well tolerated; headache was the most frequently reported adverse event. Electrocardiograms and 72 h Holter monitoring revealed no clinically significant changes. Palonosetron circulates in plasma mainly as the parent drug. Renal elimination is the primary excretion route, with parent drug and metabolites M9 and M4 accounting for the majority of palonosetron disposition. These results indicate that both renal and hepatic routes are involved in the elimination of palonosetron from the body.

Influence of sodium substitutes on 5-HT-mediated effects at mouse 5-HT3 receptors

1 The influence of sodium ion substitutes on the 5-hydroxytryptamine (5-HT)-induced flux of the organic cation [14C]guanidinium through the ion channel of the mouse 5-HT3 receptor and on the competition of 5-HT with the selective 5-HT3 receptor antagonist [3H]GR 65630 was studied, unless stated otherwise, in mouse neuroblastoma N1E-115 cells. 2 Under physiological conditions (135 mm sodium), 5-HT induced a concentration-dependent [14C]guanidinium influx with an EC50 (1.3 microm) similar to that in electrophysiological studies. 3 The stepwise replacement of sodium by increasing concentrations of the organic cation hydroxyethyl trimethylammonium (choline) concentration dependently caused both a rightward shift of the 5-HT concentration-response curve and an increase in the maximum effect of 5-HT. Complete replacement of sodium resulted in a 34-fold lower potency of 5-HT and an almost two times higher maximal response. A low potency of 5-HT in choline buffer was also observed in other 5-HT3 receptor-expressing rodent cell lines (NG 108-15 or NCB 20). 4 Replacement of Na+ by Li+ left the potency and maximal effects of 5-HT almost unchanged. Replacement by tris (hydroxymethyl) methylamine (Tris), tetramethylammonium (TMA) or N-methyl-d-glucamine (NMDG) caused an increase in maximal response to 5-HT similar to that caused by choline. The potency of 5-HT was only slightly reduced by Tris, to a high degree decreased by TMA (comparable to the decrease by choline), but not influenced by NMDG. 5 The potency of 5-HT in inhibiting [3H]GR65630 binding to intact cells was 35-fold lower when sodium was completely replaced by choline, but remained unchanged after replacement by NMDG. 6 The results are compatible with the suggestion that choline competes with 5-HT for the 5-HT3 receptor; the increase in maximal response may be partly due to a choline-mediated delay of the 5-HT-induced desensitization. For studies of 5-HT-evoked [14C]guanidinium flux through 5-HT3 receptor channels, NMDG appears to be an 'ideal' sodium substituent since it increases the signal-to-noise ratio without interfering with 5-HT binding.

Amino acids in transmembrane domain two influence anesthetic enhancement of serotonin-3A receptor function

Alcohols and volatile anesthetics affect the function of members of the nicotinic acetylcholine (nACh) superfamily of receptors. Studies on glycine and GABA(A) receptors implicate amino acid residues within transmembrane (TM) regions two and three of these receptors as critical for alcohol and anesthetic enhancement of receptor function. The serotonin-3 (5-HT(3)) receptor is a member of the nicotinic acetylcholine receptor superfamily, sharing sequence and structural homology with the other members. We tested the hypothesis that amino acids of the 5-HT(3) receptor homologous to those shown to affect alcohol and anesthetic potentiation in GABA(A) and glycine receptors also determine the effects of these compounds on the 5-HT(3) receptor. Six 5-HT(3A) mutant cDNAs were generated by site-directed mutagenesis of two amino acids, phenylalanine-269 (14') and lecucine-270 (15') in transmembrane domain two (TM2). When assayed electrophysiologically in Xenopus oocytes, wild-type 5-HT(3) receptors exhibit enhancement of function by enflurane, halothane, isoflurane, chloroform and ethanol, but not by decanol and propofol. Mutations in transmembrane domain two markedly affected alcohol and anesthetic enhancement of 5-HT(3) receptor function. Some mutations had differential effects on the abilities of the isomers enflurane and isoflurane to potentiate 5-HT(3) receptor function.

A mutation in transmembrane domain II of the 5-hydroxytryptamine(3A) receptor stabilizes channel opening and alters alcohol modulatory actions

Mutant 5-hydroxytryptamine (5-HT)3A receptors, in which changes were made at Ile294, position 16', of the second transmembrane domain, were assessed for alterations in macroscopic response kinetics and modulation by alcohols. Function of heterologously expressed receptors was measured in Xenopus oocytes in the two-electrode voltage clamp configuration and in human embryonic kidney 293 cells using whole cell patch-clamp electrophysiological recordings with rapid drug application. Compared with the wild-type receptor, a decrease in the 5-HT EC50 value in the Ile294Thr mutant was observed, whereas an increase in the 5-HT EC50 value in the Ile294Leu mutant was measured. Ile294Thr receptors showed a marked reduction in the extent of desensitization. Ethanol and 2,2,2-trichloroethanol (TCEt) enhanced 5-HT-mediated currents in wild-type and Ile294Leu receptors, but inhibited or had little stimulatory effect in the Ile294Thr mutant. Kinetic analysis revealed that in the presence of TCEt, the slope of activation was unchanged in the Ile294Thr mutant and increased in the wild-type receptor. Alcohol cutoff was altered with wild-type = heptanol and Ile294Leu = hexanol. Kinetic changes in the Ile294Thr mutant that favor the open channel state, as well as reduction in the rate of channel activation in the presence of TCEt, likely underlie this mutant's altered response to n-chain alcohols.

Benefits and risks of newer treatments for chemotherapy-induced and postoperative nausea and vomiting

Nausea and vomiting are common adverse effects of chemotherapy, radiation therapy, anaesthesia and surgery. The incidence of chemotherapy-induced nausea and vomiting (CINV) is estimated to vary from 30 to 90%, depending on the type of chemotherapeutic agent used. Radiation-induced emesis varies with anatomical site radiated but is estimated to have an overall incidence of approximately 40%. The incidence of postoperative nausea and vomiting (PONV) depends on the type of anaesthesia and surgery, but overall is estimated to be 20-30%. Evidence-based medicine and meta-analysis have been used to direct medical therapy to help determine equivalence, optimal dose, timing, safety and efficacy of antiemetic medications. Concepts such as the number needed to treat and number needed to harm are helpful to guide the clinician regarding the benefits and risks of a particular treatment. The serotonin 5-HT(3) receptor antagonists ondansetron, granisetron, tropisetron and dolasetron have been important additions to the antiemetic armamentarium. The 5-HT(3) receptor antagonists are similar in chemical structure, efficacy and adverse effect profile. They appear to have no important differences among themselves in clinical outcomes for CINV and PONV. Headache, dizziness, constipation and diarrhoea are their most common adverse effects, and when they occur they are usually mild and easily managed. Haemodynamic changes and extrapyramidal adverse effects are uncommon. ECG changes such as prolonged corrected QT (QTc) interval are infrequent, dose-related and overall judged to be clinically insignificant. As most studies with the 5-HT(3) antagonists have been conducted on relatively healthy patients, caution should be exercised when these drugs are used in susceptible patients with co-morbidities. The clinician must weigh the benefit of administering an antiemetic for CINV or PONV against the risk of occurrence of an adverse event.

Inhaled drugs of abuse enhance serotonin-3 receptor function

Despite the prevalence of their use, little is currently known of the molecular mechanisms of action of inhaled drugs of abuse. Recent studies have shown effects on NMDA, GABA(A) and glycine receptors in vitro, suggesting that inhalants may exert at least some of their pharmacological effects on ligand-gated ion channels. Enhancement of serotonin-3 receptor function has been shown to play a role in the reinforcing properties of drugs of abuse. We tested the hypothesis that the commonly abused inhaled agents 1,1,1-trichloroethane, trichloroethylene, and toluene enhance serotonin-3 receptor function. All three inhalants significantly and reversibly potentiated, in a dose-dependent manner, serotonin-activated currents mediated by mouse serotonin-3A receptors expressed in Xenopus oocytes. Our findings add the serotonin-3 receptor to the growing list of molecular targets commonly affected by both inhalants and classic CNS depressants such as ethanol and the volatile anesthetics.

Direct inhibition by cannabinoids of human 5-HT3A receptors: probable involvement of an allosteric modulatory site

Excised outside-out patches from HEK293 cells stably transfected with the human (h) 5-HT3A receptor cDNA were used to determine the effects of cannabinoid receptor ligands on the 5-HT-induced current using the patch clamp technique. In addition, binding studies with radioligands for 5-HT3 as well as for cannabinoid CB1 and CB2 receptors were carried out. The 5-HT-induced current was inhibited by the following cannabinoid receptor agonists (at decreasing order of potency): 9-THC, WIN55,212-2, anandamide, JWH-015 and CP55940. The WIN55,212-2-induced inhibition was not altered by SR141716A, a CB1 receptor antagonist. WIN55,212-3, an enantiomer of WIN55,212-2, did not affect the 5-HT-induced current. WIN55,212-2 did not change the EC50 value of 5-HT in stimulating current, but reduced the maximum effect. The CB1 receptor ligand [3H]-SR141716A and the CB1/CB2 receptor ligand [3H]-CP55940 did not specifically bind to parental HEK293 cells. In competition experiments on membranes of HEK293 cells transfected with the h5-HT3A receptor cDNA, WIN55,212-2, CP55940, anandamide and SR141716A did not affect [3H]-GR65630 binding, but 5-HT caused a concentration dependent-inhibition. In conclusion, cannabinoids stereoselectively inhibit currents through recombinant h5-HT3A receptors independently of cannabinoid receptors. Probably the cannabinoids act allosterically at a modulatory site of the h5-HT3A receptor. Thus the functional state of the receptor can be controlled by the endogenous ligand anandamide. This site is a potential target for new analgesic and antiemetic drugs.

Randomized, double-blind, crossover, placebo-controlled trial of intravenous ondansetron for the prevention of intrathecal chemotherapy-induced vomiting in children

BACKGROUND

The purpose of this study was to determine the effectiveness of intravenous ondansetron in preventing vomiting after the administration of intrathecal chemotherapy in children.

PATIENTS AND METHODS

Twenty-six children (ages 18 mo to 15 y) receiving intrathecal chemotherapy with either methotrexate or the combination of methotrexate, hydrocortisone, and Ara-C for the prophylactic treatment of central nervous system leukemia were randomly assigned to receive an infusion of normal saline or ondansetron at one of two doses (0.15 or 0.45 mg/kg) 30 minutes before undergoing the procedure. One hundred forty-six infusions were administered (51 placebo, 47 at the lower ondansetron dose, and 48 at the higher dose). Each patient acted as his or her own control, and each patient was studied at least three times.

RESULTS

Twenty-three of 26 patients (88.5%) had postprocedural vomiting on at least one occasion. At least one episode of vomiting occurred during the 24 hours after the procedure in fifty-two of the procedures (35.6%). The incidence of vomiting was significantly greater after infusion of placebo than after either low-dose or high-dose ondansetron. The likelihood of severe vomiting was even more significantly reduced by the preadministration of ondansetron. Almost all of the intrathecal treatments associated with severe vomiting occurred after the infusion of placebo.

CONCLUSIONS

Vomiting induced by intrathecal chemotherapy can be greatly reduced by the intravenous administration of ondansetron before the procedure, and severe vomiting can be virtually eliminated.

The broad-spectrum anti-emetic activity of AS-8112, a novel dopamine D2, D3 and 5-HT3 receptors antagonist

The anti-emetic and pharmacological profile of AS-8112 ((R)-5-bromo-N-(1-ethyl-4-methylhexahydro-1H-1,4-diazepin-6-yl)-2-methoxy-6-methylamino-3-pyridinecarboxamide.2 fumarate), a novel and potent dopamine D2, D3 and 5-hydroxytryptamine-3 (5-HT3) receptors ligand, was investigated in the present study. In guinea-pig isolated colon, AS-8112 produced a rightward shift of the concentration-response curves of 2-methyl-5HT, a 5-HT3 receptor agonist (pA2 value of 7.04). Other 5-HT3 receptor antagonists also produced such a shift in the following antagonistic-potency order: granisetron> ondansetron=AS-8112>>metoclopramide. In mice, AS-8112 (1.0 - 3.0 mg kg(-1) s.c.) potently inhibited hypothermia induced by the dopamine D3 receptor agonist; R(+)-7-OH-DPAT (R(+)-7-hydroxy-2-(N,N-di-n-propylamino)tetraline) (0.3 mg kg(-1) s.c.). Domperidone and haloperidol, which have affinity for dopamine D3 receptor, also inhibited R(+)-7-OH-DPAT-induced hypothermia. In ferrets or dogs, AS-8112 dose-dependently inhibited emesis induced by R(+)-7-OH-DPAT, apomorphine, morphine or cisplatin with ID50 values of 2.22 microg kg(-1) s.c., 10.5 microg kg(-1) s.c., 14.2 microg kg(-1) i.v. and 17.6 microg kg(-1) i.v., respectively. Moreover, oral administration of AS-8112 significantly inhibited emesis induced by these emetogens. AS-8112 (0.3 mg kg(-1) i.v.) significantly inhibited emesis induced by cyclophosphamide and doxorubicin. In conclusion, AS-8112 is a potent dopamine D2, D3 and 5-HT3 receptors antagonist, and a novel anti-emetic agent with a broad-spectrum of anti-emetic activity. These results suggest that this compound is worthy of clinical investigation.

Association of two 3D QSAR analyses. application to the study of partial agonist serotonin-3 ligands

CATALYST and COMFA, two software packages for 3D QSAR studies, were associated to correlate the three-dimensional structures of 75 serotonin 5-HT3 ligands to their biological affinities. The conformational analysis and the influence of chemical function-based alignments (the basis of this association) on final results are discussed in this publication. These two analyses allow for precisely quantitating the weights of significant chemical groups or functions on the biological affinities.

Comparative review of 5-HT3 receptor antagonists in the treatment of acute chemotherapy-induced nausea and vomiting

Since their introduction, 5-HT3 receptor antagonists have become the agents of choice in the prevention of acute chemotherapy-induced nausea and vomiting and are generally superior to high-dose metoclopramide regimens. The availability of four different agents (ondansetron, granisetron, dolasetron, and tropisetron) within this class has prompted investigations into potential differences between the drugs, which appear to be few. More importantly, the results of recently conducted randomized comparative trials in patients receiving moderately or highly emetogenic chemotherapy have demonstrated similar efficacy. Although study designs and patient populations differed, seven large comparative trials in patients receiving highly emetogenic chemotherapy reported no significant differences in complete or complete plus major response rates among the agents. Similar results were generally reported in trials evaluating patients receiving moderately emetogenic chemotherapy. The safety and tolerability of these agents also appear to be similar. The most common adverse events include headache, gastrointestinal effects, lightheadedness, and sedation. All agents are available in both intravenous and oral dosage forms and may be administered as a single dose.

Bicuculline antagonizes 5-HT(3A) and alpha2 glycine receptors expressed in Xenopus oocytes

The present study examined the effects of bicuculline on the mouse 5-hydroxytryptamine(3A) receptor (5-HT(3A) receptor and the human alpha2 subunit of the glycine receptor. Bicuculline antagonized both the 5-HT(3A) receptor (IC(50)=20.12+/-0.39 microM) and the alpha2 glycine receptor (IC(50)=169.40+/-1.73 microM). A competitive form of antagonism by bicuculline was suggested by experiments in which the EC(50)s for 5-HT and glycine were increased in the 5-HT(3A) and alpha2 glycine receptors, respectively, as bicuculline concentrations were increased. A competitive nature of antagonism by bicuculline at the 5-HT(3A) receptor was also suggested by displacement of the competitive antagonist, [3H]GR65630 in SF21 insect cells expressing the 5-HT(3A) receptor (K(i)=19.01+/-0.71 microM). Our data and that of others reveal that bicuculline, a purported selective antagonist of the GABA(A) receptor, antagonizes at least one receptor subclass in every member of the superfamily of ligand-gated ion channels.

L-Type calcium channels in enterochromaffin cells from guinea pig and human duodenal crypts: an in situ study

BACKGROUND & AIMS

This study has investigated stimulus-secretion coupling of enterochromaffin cells by studying the cellular location and function of voltage-gated Ca(2+) channels within small intestinal crypts.

METHODS

Digital fluorescence imaging and electrochemical detection were used to measure intracellular Ca(2+) responses and serotonin (5-hydroxytryptamine [5-HT]) secretion in intact crypts isolated from guinea pig and human duodenum.

RESULTS

In fluo-3-loaded crypts, electrical depolarization with high K(+) solution increased cytosolic free [Ca(2+)] only in single cells subsequently identified by immunocytochemistry as enterochromaffin cells. In guinea pig enterochromaffin cells, the L-type Ca(2+) channel agonist FPL 64176 (3 micromol/L) did not change resting intracellular [Ca(2+)] but potentiated the depolarization-evoked increase in [Ca(2+)] (298 +/- 72 nmol/L) by 19 +/- 3-fold. In the majority of human enterochromaffin cells, FPL 64176 alone increased resting [Ca(2+)] by 423 +/- 171 nmol/L. Secretion studies in guinea pig crypts showed that high K(+) and FPL 64176 caused a 12-fold increase in 5-HT release. Noradrenaline caused increases in both enterochromaffin cell [Ca(2+)] and 5-HT release.

CONCLUSIONS

Using this approach, we have found that in duodenal crypts, enterochromaffin cells, but not other epithelial cells, contain L-type voltage-gated Ca(2+) channels involved in regulating 5-HT secretion. These data have implications for the pharmacological control of intestinal disorders involving enterochromaffin cell dysfunction.

Open-label comparison of the antiemetic efficacy of single intravenous doses of dolasetron mesylate in pediatric cancer patients receiving moderately to highly emetogenic chemotherapy

BACKGROUND

Nausea and vomiting are among the most unpleasant adverse side effects of cancer therapy.

PROCEDURE

An open-label dose-escalation study was conducted to assess the appropriate intravenous dose of dolasetron for pediatric patients undergoing chemotherapy. Patients received dolasetron in single intravenous doses of 0.6 (n = 10), 1.2 (n = 12), 1.8 (n = 12), or 2.4 (n = 12) mg/kg 30 min before receiving emetogenic chemotherapy. Pharmacokinetic parameters were evaluated at each dose level and efficacy was evaluated over the first 24 hr following the administration of dolasetron.

RESULTS

A complete response was achieved in 10% of patients given 0.6 mg/kg, 25% of patients given 1. 2 mg/kg, 67% of patients given 1.8 mg/kg, and 33% of patients given 2.4 mg/kg. Peak plasma concentrations (Cmax) were observed between 0. 33 and 0.75 hr following dolasetron infusion. Cmax and area under plasma concentration-time (AUC) increased with larger doses of dolasetron, while terminal disposition half-life (t1/2) and apparent clearance (Clapp) were not significantly changed with respect to dose. For 1.8-mg/kg dolasetron, the t1/2 was 4.98 hr and the maximum plasma concentration (tmax) 0.47 hr. Adverse events were mild to moderate. No serious events occurred. Conclusions. This study suggests that a single intravenous dose of 1.8 mg/kg is the optimum single intravenous dose for controlling chemotherapy-induced emesis in pediatric patients.

Safety, tolerability, antiemetic efficacy, and pharmacokinetics of oral dolasetron mesylate in pediatric cancer patients receiving moderately to highly emetogenic chemotherapy

PURPOSE

The safety, antiemetic efficacy, and pharmacokinetics of single oral doses of dolasetron, a new highly selective 5-HT3 receptor antagonist, were evaluated in children with cancer undergoing treatment with moderately to highly emetogenic chemotherapy.

PATIENTS AND METHODS

A total of 32 children, ages 3 to 18 years, were enrolled in a nonrandomized, multicenter, open-label, dose-escalation study. Three oral dose levels (0.6, 1.2, or 1.8 mg/kg) were studied. Safety, efficacy, and pharmacokinetic parameters were assessed over 24 hours at each dosage level.

RESULTS

The most effective dose was 1.8 mg/kg; 60% of the patients achieved a complete or major response (< or =2 emetic episodes in 24 hours). A complete response was achieved in 3 of 9 patients (33%) who received 0.6 mg/kg, 4 of 13 (31%) patients who received 1.2 mg/kg, and 5 of 10 (50%) patients who received 1.8 mg/kg of dolasetron. Overall, dolasetron was well tolerated. Adverse events were mild and similar to those reported in adults. Peak plasma concentrations (Cmax) of dolasetron's active reduced metabolite, MDL 74,156, were dose proportional and occurred, on the average, within 1 hour of oral administration. The half-life (t1/2) in plasma was approximately 6 hours for all dose levels, and the mean clearance (CLapp) was unrelated to dose.

CONCLUSIONS

Oral dolasetron is safe and effective in reducing chemotherapy-induced nausea and vomiting, particularly at the 1.8-mg/kg dose level. These results support further evaluation of oral dolasetron in larger randomized clinical trials in the pediatric cancer population.

Pharmacokinetics of granisetron in rat blood and brain by microdialysis

To characterize the pharmacokinetics of protein-free granisetron in blood and brain we implanted microdialysis probes into the jugular vein and cerebral frontal cortex of the rat. Granisetron (3 or 6 mg/kg, i.v., n=6) was then administered, and microdialysates from blood and brain were collected from both sites and assayed by a validated high-performance liquid chromatographic method. Pharmacokinetics parameters were calculated from the corrected dialysate concentrations of granisetron versus time data. The elimination half-lives of granisetron in blood and brain were 51.3+/-5.5 and 69.7+/-6.3 min for 6 mg/kg, and 50.7+/-4.3 and 74.3+/-12.5 min for 3 mg/kg, respectively. Granisetron rapidly entered the extracellular fluid of cerebral frontal cortex at Tmax of 24 min. The results suggest that simultaneous microdialysis in blood and brain can be usefully applied to study the pharmacokinetics of granisetron in the periphery and the central nervous system.

Definition of a pharmacophore for partial agonists of serotonin 5-HT3 receptors

A definition of a partial agonists serotonin 5-HT3 pharmacophore was carried out by considering a three-dimensional model which correlates the chemical structures of series of piperazinopyrrolothienopyrazines, piperazinopyridopyrrolopyrazines, piperazinopyrroloquinolaxines, piperazinopyridopyrroloquinoxalines, aminoalkyloximinopyrroloindoles, aminoalkyloximinothienopyrrolizines, and aminoalkyloximinopyrrolizines with the biological affinities. The model is formed by five features corresponding to two hydrogen bond acceptors, one aromatic ring, one hydrophobic group, and one positive ionizable site (quaternary ammonium ions). The nature of the features and the distances between them explain the partial agonist activities of these compounds.

Benzoxazole derivatives as novel 5-HT3 receptor partial agonists in the gut

A series of benzoxazoles with a nitrogen-containing heterocyclic substituent at the 2-position was prepared and evaluated for 5-HT3 partial agonist activity on isolated guinea pig ileum. The nature of the substituent at the 5-position of the benzoxazole ring affected the potency for the 5-HT3 receptor, and the 5-chloro derivatives showed increased potency and lowered intrinsic activity. 5-Chloro-7-methyl-2-(4-methyl-1-homopiperazinyl)benzoxazole (6v) exhibited a high binding affinity in the same range as that of the 5-HT3 antagonist granisetron, and its intrinsic activity was 12% of that of 5-HT. Compound 6v inhibited 5-HT-evoked diarrhea but did not prolong the transition time of glass beads in the normal distal colon even at a dose of 100 times the ED50 for diarrhea inhibition in mice. Compounds of this type are expected to be effective for the treatment of irritable bowel syndrome without the side effect of constipation.

Synthesis and biological investigations of [18F]MR18445, a 5-HT3 receptor partial agonist

18F Labelled MR18445 (4-[4-(4-[18F]fluorobenzyl)piperazino]-7-methoxypyrrolo++ +[1,2-alpha] quinoxaline), a selective 5-HT3 receptor partial agonist with nanomolar affinity, was synthesized and examined as a potential radioligand for PET imaging of brain 5HT3 receptors. Radiotracer was prepared by N-alkylation of the MR18491 precursor with 4-[18F]fluorobenzyl iodide. This latter was synthesized in a three-step procedure from 4-[18F]fluorobenzaldehyde obtained by 18F-nucleophilic displacement of 4-nitrobenzaldehyde, subsequently reduced to 4-[18F]fluorobenzyl alcohol and converted into reactive 4-[18F]fluorobenzyl iodide. The reduction step was performed on a column filled with NaBH4/Al2O3 and 4-[18F]fluorobenzyl alcohol was obtained with high reproducible yield (82-93% from 4-[18F]fluorobenzaldehyde) if there were traces of water in the system. The radiosynthesis of [18F]MR18445 required approximately 120 min. Semi-preparative HPLC purification followed by formulation gave injectable solutions of [18F]MR18445 with a radiochemical purity > 99%. The overall yield of the synthesis was mainly dependent upon the first step efficiency of aromatic incorporation of 18F- and varied from 12% to 29%. All the synthetic procedure was realized on a ZYMARK robotic system. Biological in vivo studies in rats showed that uptake of [18F]MR18445 in brain was rapid and high. No evidence of radiolabeled metabolites could be observed in the brain as late as 40 min after injection, despite the rapid appearance of metabolites in the plasma. However, neither phosphorimaging autoradiographic studies in rats nor PET experiments in baboons revealed specific binding of the radiotracer in brain, suggesting [18F]MR18445 is not suitable for 5-HT3 receptors PET studies.

A risk-benefit assessment of serotonin 5-HT3 receptor antagonists in antineoplastic therapy-induced emesis

Insight into the pathophysiology of antineoplastic therapy-induced nausea and vomiting led to the development of the serotonin 5-HT3 receptor antagonists as the most potent class of antiemetic agents. Among those which have been investigated are ondansetron, granisetron, tropisetron and dolasetron. A risk-benefit analysis of these drugs must not only account for the modest clinical differences in efficacy and tolerability, but also should include such issues as ease of use, route of administration, dosage considerations and patient preference. Pharmacokinetic and preclinical studies reveal distinctions among these antiemetics, but, overall, these distinctions do not translate in to clinically significant differences. In clinical trials, the most widely studied members of the 5-HT3 receptor antagonists are granisetron and ondansetron, which have been found to possess equivalent antiemetic efficacy. Dolasetron and tropisetron are also available, and some randomised trials have also documented their similar antiemetic activity, depending on the doses and schedules used. The equivalent efficacy of oral granisetron 2 mg versus intravenous ondansetron 32 mg has recently been demonstrated in prospective randomised clinical trials in patients receiving either highly emetogenic or moderately emetogenic antineoplastic therapy. The utilisation and efficacy of oral ondansetron and dolasetron in patients receiving moderately emetogenic antineoplastic therapy has also been documented. This review offers a brief overview of the pharmacokinetic and preclinical research on the 5-HT3 antagonists, a review of the comparative clinical trials of the major members of this class and a summary risk-benefit assessment that considers clinical applicability and cost, as well as efficacy and safety.

Phenylimidazolidin-2-one derivatives as selective 5-HT3 receptor antagonists and refinement of the pharmacophore model for 5-HT3 receptor binding

A possible bioisosterism between the benzamido and the phenylimidazolidin-2-one moieties has been suggested on the basis of the similarity between the molecular electrostatic potential (MEP) of metoclopramide, a D2 receptor antagonist with weak 5-HT3 receptor antagonist properties, and zetidoline, a D2 receptor antagonist. Starting from this premise, a series of phenylimidazolidin-2-one derivatives bearing a basic azabicycloalkyl or an imidazolylalkyl moiety were synthesized and evaluated for 5-HT3 receptor radioligand binding affinity ([3H]-GR 43,694). In vitro 5-HT3 receptor antagonist activity was tested in the guinea pig ileum assay (GPI). A number of high-affinity ligands were shown to be potent 5-HT3 receptor antagonists in vivo as determined by inhibition of the Bezold--Jarisch reflex in the anesthetized rat. In general, the imidazolylalkyl derivatives were found to be more active than azabicycloalkyls. 1-(3,5-Dichlorophenyl)-3-[(5-methyl-1H-imidazol-4-yl)methyl]imidazoli din-2-one (58), in particular, displayed very high affinity for the 5-HT3 receptor (Ki of 0.038 nM) with a Kb of 5.62 nM in the GPI assay, being more potent than the reference compounds (ondansetron, tropisetron, granisetron, and BRL 46,470) tested. 58 showed an ID50 comparable to that of ondansetron (2.2 micrograms/kg i.v.) in the Bezold--Jarisch reflex. A molecular modeling study based on this structurally novel series of compounds allowed the refinement of previously reported 5-HT3 receptor antagonist pharmacophore models.

Novel and selective partial agonists of 5-HT3 receptors. 2. Synthesis and biological evaluation of piperazinopyridopyrrolopyrazines, piperazinopyrroloquinoxalines, and piperazinopyridopyrroloquinoxalines

In continuation of our previous work on piperazinopyrrolothienopyrazine derivatives, three series of piperazinopyridopyrrolopyrazines, piperazinopyrroloquinoxalines, and piperazinopyridopyrroloquinoxalines were prepared and evaluated as 5-HT3 receptor ligands. The chemical modifications performed within these new series led to structure-activity relationships regarding both high affinity and selectivity for the 5-HT3 receptors that are in agreement with those established previously for the pyrrolothienopyrazine series. The best compound (8a) obtained in these new series is in the picomolar range of affinity for 5-HT3 receptors with a selectivity higher than 10(6). Four of the high-affinity 5-HT3 ligands (8a, 15a,b, and 16d) were selected in both the pyridopyrrolopyrazine and the pyrroloquinoxaline series and were characterized in vitro and in vivo as agonists or partial agonists. Compound 8a was also evaluated in the light/dark test where it showed potential anxiolytic-like activity at very low doses per os.

In vitro and in vivo predictors of the anti-emetic activity of tachykinin NK1 receptor antagonists

The ability of tachykinin NK1 receptor antagonists to inhibit GR73632 (D-Ala-[L-Pro9,Me-Leu8]substance P-(7-11))-induced foot tapping in gerbils was employed as an indirect measure of brain penetration and this was compared with their ability to prevent acute emesis induced by cisplatin in ferrets. (+)-GR203040 ((2S,3S and 2R,3R)-2-methoxy-5-tetrazol-1-yl-benzyl-(2-phenyl-piperidin- 3-yl)-amine), CP-99,994 ((2S,3S)-cis-3-(2-methoxybenzylamino)-2-phenyl piperidine) dihydrochloride), and L-742,694 (2-(S)-(3,5-bis(trifluoromethyl)benzyloxy)-3-(S)-phenyl-4-(5-(3-oxo-1,2, 4-triazolo)methylmorpholine) potently inhibited GR73632-induced foot tapping (ID50 < or = 0.85 mg/kg), and acute retching induced by cisplatin (ID50 < or = 0.18 mg/kg). RPR100893 ((3aS,4S,7aS)-7,7-diphenyl-4-(2-methoxyphenyl)-2-[(S)-2-(2-m ethoxyphenyl)proprionyl] perhydroisoindol-4-ol) was not a potent antagonist of retching (ID50 4.1 mg/kg) or foot tapping (ID50 > 10 mg/kg). High doses (3-10 mg/kg) of CGP49823 ((2R,4S)-2-benzyl-1-(3,5-dimethylbenzoyl)-N-[(4-quinolinyl)methyl] -4-piperineamine) dihydrochloride), FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-propyl]-N-methy l-N-phenylmethyl-L-3-(2-naphthyl)-alaninamide), and LY303870 ((R)-1-[N-(2-methoxybenzyl)acetylamino]-3-(1H-indol-3-yl)-2-[N-(2-(4-(pi peridinyl)piperidin-1-yl)acetyl)amino]propane) were required to inhibit foot tapping; these agents were not anti-emetic in this dose range. SR140333 ((S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl)piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane; 3-10 mg/kg) failed to inhibit foot tapping or emesis. Affinities for the human and ferret tachykinin NK1 receptor were highly correlated (r = 0.93, P = 0.0008). Inhibition of foot tapping in gerbils, but not NK1 receptor binding affinity, predicted anti-emetic activity in ferrets (r = 0.75, P < 0.01). These findings confirm that the anti-emetic activity of tachykinin NK1 receptor antagonists is dependent on brain penetration.

The tachykinin NK1 receptor antagonist PD 154075 blocks cisplatin-induced delayed emesis in the ferret

The activity of a selective tachykinin NK1 receptor antagonist, PD 154075 ([(2-benzofuran)-CH2OCO]-(R)-alpha-MeTrp-(S)-NHCH(CH3) Ph), was examined in radioligand binding studies, in a [Sar9,Met(O2)11]substance P-induced foot-tapping model in the gerbil, and in cisplatin-induced acute and delayed emesis in the ferret. In radioligand binding studies, PD 154075 showed nanomolar affinity for the human, guinea-pig, gerbil, dog and ferret NK1 receptors with an approximate 300 times lower affinity for the rodent NK1 receptor. Using NK2,NK3 receptors and a range of other receptor ligands, PD 154075 was shown to exhibit a high degree of selectivity and specificity for the human type NK1 receptor. Following subcutaneous administration PD 154075 dose dependently (1-100 mg/kg) antagonised the centrally mediated [Sar9,Met(O2)11] substance P-induced foot tapping in the gerbil with a minimum effective dose (MED) of 10 mg/kg. The ability of PD 154075 to readily penetrate into the brain following oral administration was confirmed by its extraction and high performance liquid chromatography assay from the rat brain. PD 154075 was shown to achieve a relatively fast and sustained brain concentration (brain/plasma ratios ranged from 0.27 to 0.41 during the time period of 0.25-12 h). Further pharmacokinetic studies revealed that the absolute oral bioavailability of PD 154075 in the rat was (mean +/- S.D.) 49 +/- 15%. PD 154075 (1-30 mg/kg, i.p.) dose dependently antagonised the acute vomiting and retching in the ferret measured for 4 h following administration of cisplatin (10 mg/kg, i.p.) with a MED of 3 mg/kg. The administration of a lower dose of cisplatin (5 mg/kg, i.p.) in the ferret induces both an acute (day 1) and delayed (days 2 and 3) phase of emesis. The i.p. administration of PD 154075, 10 mg/kg three times a day for 3 days, almost completely blocked both the acute and delayed emetic responses. In the same study, the 5-HT3 receptor antagonist ondansetron (1 mg/kg, i.p., t.i.d.) was also very effective against the acute emetic response observed during the first 4 h following cisplatin, but it was only weakly active against the delayed response. In conclusion, PD 154075 is a selective and specific high affinity NK1 receptor antagonist with good oral bioavailability which is effective against both acute and delayed emesis induced by cisplatin in the ferret.

New 2-piperazinylbenzimidazole derivatives as 5-HT3 antagonists. Synthesis and pharmacological evaluation

A series of 2-piperazinylbenzimidazole derivatives were prepared and evaluated as 5-HT3 receptor antagonists. Their 5-HT3 receptor affinities were evaluated by radioligand binding assays, and their abilities to inhibit the 5-HT-induced Bezold-Jarisch reflex in anesthetized rats were determined. Compound 7e (lerisetron, pKi = 9.2) exhibited higher affinity for the 5-HT3 receptor than did tropisetron and granisetron, while compound 7q (pKi = 7.5) had very low affinity for this receptor, showing that substitution on the N1 atom of the benzimidazole ring is essential for affinity and activity. The effect of substitution on the aromatic ring of benzimidazole by several substituents in different positions is also discussed. A strong correlation between the 5-HT3 antagonistic activity of the studied compounds and the position of substitution on the aromatic ring was established. Thus, while the 4-methoxy derivative 7m showed weak affinity for the 5-HT3 receptor (pKi = 6.7), the 7-methoxy derivative 7n exhibited the highest affinity (pKi = 9.4). Compounds 7e and 7n are now under further investigation as drugs for the treatment of nausea and emesis evoked by cancer chemotherapy and radiation.