Granisetron. A review of its pharmacological properties and therapeutic use as an antiemetic

Metabolism and disposition of 14C-granisetron in rat, dog and man after intravenous and oral dosing

1. The disposition and metabolic fate of 14C-granisetron, a novel 5-HT3 antagonist, was studied in rat, dog, and male human volunteers after intravenous and oral administration. 2. Complete absorption occurred from the gastrointestinal tract following oral dosing, but bioavailability was reduced by first-pass metabolism in all three species. 3. There were no sex-specific differences observed in radiometabolite patterns in rat or dog and there was no appreciable change in disposition with dose between 0.25 and 5 mg/kg in rat and 0.25 and 10 mg/kg in dog. Additionally, there were no large differences in disposition associated with route of administration in rat, dog and man. 4. In rat and dog, 35-41% of the dose was excreted in urine and 52-62% in faeces, via the bile. Metabolites were largely present as glucuronide and sulphate conjugates, together with numerous minor polar metabolites. In man, about 60% of dosed radioactivity was excreted in urine and 36% in faeces after both intravenous and oral dosing. Unchanged granisetron was only excreted in urine (5-25% of dose). 5. The major metabolites were isolated and identified by MS spectroscopy and nmr. In rat, the dominant routes of biotransformation after both intravenous and oral dosing were 5-hydroxylation and N1-demethylation, followed by the formation of conjugates which were the major metabolites in urine, bile and plasma. In dog and man the major metabolite was 7-hydroxy-granisetron, with lesser quantities of the 6,7-dihydrodiol and/or their conjugates.

Gastro-oesophageal afferent and serotonergic inputs to vagal efferent neurones

Peripheral 5-HT3 receptor mechanisms are involved in activation of gastrointestinal (GI) mucosal vagal afferent fibres. 5-HT3 receptor mechanisms in the central nervous system (CNS) may be involved in behavioural and reflex motility responses. This study investigates the processing of different sensory inputs in the CNS and the involvement of 5-HT3 receptors at these different levels. In Urethane (1.5 g/kg, i.p.) anaesthetized, splanchnectomized ferrets, the jugular vein was cannulated for intravenous (i.v.) drug injection, and the coeliac axis for intraarterial (i.a.) injection close to the upper GI tract. The carotid artery was intubated with a T-cannula for CNS-directed intracarotid (i.c.) injections. An intragastric cannula was used for fluid distension (40-50 ml), and an oesophageal catheter for balloon distension (2 ml). Efferent fibres were dissected from the right cervical vagus for single-unit recording. Nineteen single vagal efferent fibres were selected, with low frequency resting discharge (2.5 +/- 0.3 impulses/s), but no respiratory or cardiovascular phasic input. All responded rapidly (< 2.5 s) to gastric distension (532 +/- 230% change in firing rate) and oesophageal distension (300 +/- 170%). Gastric distension caused excitation in 14 fibres, inhibition in 4 fibres, and a biphasic response in 1. Oesophageal distension excited 16 and inhibited 3. Discharge was also influenced by i.a. injection of 5-HT or the 5-HT3 receptor agonist 2-methyl 5-HT (10-100 micrograms) in all fibres tested. These responses consisted of rapid (< 2.5 s) and powerful changes in firing rate, with excitation, inhibition or biphasic responses. 65% of responses to i.c. or i.v. injection were opposite in direction to those after close i.a. injection, indicating the activation of a different population of receptors. No differences were seen between effects of i.c. and i.v. injections. The 5-HT3 receptor antagonist granisetron (100 micrograms/kg, i.v.) blocked or reduced efferent responses to 5-HT receptor agonists, whereas responses to gastric and oesophageal distension were unchanged. Thus there is extensive convergence of inputs from gastric and oesophageal mechanoreceptors onto vagal motorneurones. These central effects of mechanical stimuli do not involve 5-HT3 receptor mechanisms. Other 5-HT3 receptor inputs are evident, probably peripherally from GI mucosal afferent fibres and from within the CNS.

Distribution and characterization of the [3H]granisetron-labelled 5-HT3 receptor in the human forebrain

The present study has demonstrated the distribution of [3H]granisetron-labelled 5-HT3 receptors in the human forebrain with relatively high levels of this receptor in homogenates of hippocampus, caudate nucleus, putamen, nucleus accumbens and amygdala. Lower levels of 5-HT3 receptors were found in other brain regions and the cervical vagus nerve. Pharmacological characterization of the labelled 5-HT3 receptor in human putamen homogenates identified a relatively low affinity for d-tubocurarine compared to the 5-HT3 receptor in NG108-15 neuroblastoma-glioma cell homogenates. In contrast, the affinities of 19 other 5-HT3 receptor ligands were not significantly different for the [3H]granisetron-labelled receptor in these two preparations. Such findings indicate that the human putamen 5-HT3 receptor displays a unique pharmacology which may have significance given the reported clinical potential of compounds active at this receptor when assessed in animal models of disease.

Labelling of 5-HT3 receptor recognition sites in the rat brain using the agonist radioligand [3H]meta-chlorophenylbiguanide

The binding of the tritiated derivative of the 5-HT3 receptor agonist meta-chlorophenylbiguanide ([3H]mCPBG) to rat cortical homogenates and whole rat brain sections was assessed in an attempt to further investigate the binding of agonists to the 5-HT3 receptor. In crude homogenates of rat cortex, no reproducible specific [3H]mCPBG (1.0 nM) binding (defined by either 10 microM granisetron, 100 microM 5-HT or 100 nM 'cold' mCPBG) was detected. Using autoradiographic techniques, in rat hindbrain sections, [3H]mCPBG (1.0 nM) labelled a differential distribution of specific binding sites (defined by the inclusion of granisetron, 1.0 microM). Specific binding was only detected within the dorsal vagal complex (nucleus tractus solitarius, area postrema and dorsal motor nucleus of the vagus nerve). An identical distribution of specific binding was detected in adjacent sections incubated with the selective 5-HT3 receptor radioligand, [3H](S)-zacopride (0.5 nM; non-specific binding defined by the inclusion of granisetron, 1.0 microM). No reproducible specific [3H]mCPBG (1.0 nM) binding (defined by the inclusion of granisetron, 1.0 microM) was detected within the rat forebrain. In contrast, [3H](S)-zacopride (0.5 nM) labelled specific sites (defined by the inclusion of granisetron, 1.0 microM) in some limbic brain structures (e.g. cerebral cortex, hippocampus, amygdala). These studies indicate that [3H]mCPBG labels the 5-HT3 receptor in rat brain tissue. However, the relatively high level of non-specific binding associated with this radioligand appears to mask the specific binding in regions which do not express relatively high densities of the 5-HT3 receptor.

Reduced levels of 5-HT3 receptor recognition sites in the putamen of patients with Huntington's disease

The present study assessed 5-HT3 receptor recognition site levels in homogenates of putamen derived from patients with clinically and neurochemically diagnosed Huntington's disease or Parkinson's disease and those from age-, sex- and post-mortem delay-matched neurologically and psychiatrically normal patients to investigate the cellular location of 5-HT3 receptors in the human putamen. Specific [3H]granisetron (0.91 nM) binding (defined by ondansetron, 10 microM) was significantly reduced in putamen homogenates from eight out of ten patients with Huntington's disease compared to similar homogenates from 'control' patients (72 +/- 6 and 39 +/- 8 fmol/g wet weight, mean +/- S.E.M., n = 10 and 8, tissue from 'control' and Huntington's disease patients, respectively, P = 0.004). In contrast, specific [3H]granisetron (1.04 nM) binding levels were similar in putamen homogenates from patients with Parkinson's disease when compared to homogenates from 'control' patients. The present results indicate that at least a proportion of the 5-HT3 receptor population in the human putamen is located on neurones that have their cell bodies within this brain region and that these receptors are not primarily located on dopamine neurone terminals in the human putamen.

Review article: 5-hydroxytryptamine agonists and antagonists in the modulation of gastrointestinal motility and sensation: clinical implications

Serotonin (5-hydroxytryptamine; 5-HT) is found in the enteric nervous system where it has been implicated in controlling gastrointestinal motor function. A number of receptor or recognition sites have been identified in the gut, but recently most attention has focused on the 5-HT3 and 5-HT4 receptors. The functional role of the 5-HT3 receptor remains incompletely understood, but it is probably involved in the modulation of colonic motility and visceral pain in the gut. A number of selective 5-HT3 antagonists have been developed including ondansetron, granisetron, tropisetron renzapride and zacopride. While the substituted benzamide prokinetics (for example, metoclopramide, cisapride) also block 5-HT3 receptors in high concentrations, their prokinetic action is believed to be on the basis of their agonist effects on the putative 5-HT4 receptor. Some 5-HT3 antagonists have 5-HT4 agonist activity (for example, renzapride, zacopride) and others do not (for example, ondansetron, granisetron), while tropisetron in high concentrations is a 5-HT4 antagonist. Based on the pharmacological data, it has been suggested that specific 5-HT antagonists and agonists may prove to be beneficial in a number of gastrointestinal disorders including the irritable bowel syndrome, functional dyspepsia, non-cardiac chest pain, gastrooesophageal reflux and refractory nausea. In this review, the rationale for the use of these compounds is discussed, and the available experimental evidence is summarized.

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.