Physiology of chemotherapy-induced emesis and antiemetic therapy. Predictive models for evaluation of new compounds

The physiology of emesis has been studied for several hundred years, focusing on the different centres involved and the mechanics of expulsion. The vomiting centre receives inputs from various emetic detectors such as the gut, the vestibular labyrinths and the chemoreceptor trigger zone. Emesis is a common disabling effect in motion sickness, postoperative conditions and in radio- and chemotherapy. Our current understanding of the mechanisms has been provided mainly by the recent introduction of serotonin 5-HT3 receptor antagonists into therapeutic use. Nevertheless, despite the considerable advances made in the understanding of the different pathways involved in emesis, there are number of areas that still require experimental investigation. Different animal and human models are available to study the physiology of emesis and to evaluate the antiemetic activity of new compounds, but they need to be predictors of clinical situations.

[Postoperative nausea and vomiting]

Postoperative nausea and vomiting (PONV) is still a problem and is very discomforting for the patients. As surgery done on out-patient basis becomes more frequent, PONV will represent an important factor in the length of recovery room stay, and is in this way an economic problem. Based on the literature from 1985-1995 factors associated with increased risk, prevention and treatment of PONV are discussed in view of the physiological mechanisms. The conclusion is that PONV is a complex problem. It demands an individual evaluation with a careful preoperative visit including planning of both preanaesthetic and anaesthetic agents. Furthermore, a proper anaesthetic treatment of the per- and postoperative course is of high importance. Patients at high risk of PONV should be identified and given prophylactic antiemetics.

Innovative approaches in the treatment of emesis

Over the last 15 years, appreciation of the role of dopaminergic (D2) receptors and serotonergic (5-HT3) receptors has led to the development of a series of highly effective antiemetic agents. However, in spite of the suggestion of additional significant receptors (such as the NK-1 receptor and the opiate mu receptor), recent innovations in antiemetic treatment have concentrated on refinement of schedule, route, and dose. Single-dose regimens and oral formulations improve the convenience of antiemetic administration, while identification of the minimum effective dose has important economic implications. Involvement of experienced supportive care investigators in objective determination of utility scores for various supportive care modalities will be vital for rational inclusion of supportive care in pharmacoeconomic analysis, critical pathways, and clinical guidelines.

The effect of CP-99994 on the responses to provocative motion in the cat

1. The NK1 receptor antagonist CP-99994 has been shown to prevent vomiting elicited by both peripherally and centrally acting emetogens in ferrets and dogs. These results have now been extended to another stimulus, provocative motion, and another species, the cat. 2. CP-99994 displaced [3H]-substance P from cat cortex with IC50 of 0.52 +/- 0.08 nM. Following s.c. administration, peak plasma drug levels were achieved at 30 min. The plasma drug half life was 1.4 h. 3. Subcutaneous administration of CP-99994 inhibited motion-induced vomiting in the cat with an ED50 of 144 micrograms kg-1 but did not change the epiphenomena associated with provocative motion in the cat over the dose range of 30 to 300 micrograms kg-1. The antiemetic effect of CP-99994 can be attributed to antagonism of the NK1 receptor because its enantiomer, CP-100,263, which is 900 fold weaker as an NK1 antagonist, had no effects on any response to provocative motion. 4. The inhibitory effect of CP-99994 on motion-induced retching and vomiting is consistent with a central site of antiemetic action, potentially at the level of the motor nuclei responsible for these behaviours. 5. An investigation into whether the failure of CP-99994 to alter the epiphenomena will also predict a lack of anti-nausea effects in man will provide critical information on the neural organization of the emetic reflex.

[Role of serotonin in emesis]

There are now abundant evidence to confirm the role of serotonin (5-HT) and in particular, 5-HT3 receptors in the control of cisplatin-induced emesis. Emesis caused by cisplatin is associated with an increase in the concentration of 5-HT in the intestinal mucosa and in the area postrema. The intestinal mucosa contains enterochromaffin (EC) cells that synthesize and secrete approximately 80% of all 5-HT produced in the body. A selective 5-HT3 agonist, 2-methyl-5-HT, induced a dose-dependent increase in 5-HT level from the ileum. Furthermore, a selective 5-HT4-receptor agonist, 5-methoxytryptamine also induced a concentration-dependent increase of 5-HT level. Both 5-HT3 and 5-HT4 receptors may be involved in intestinal 5-HT release. It is proposed that anticancer drugs cause 5-HT release from the EC cells and that the released 5-HT stimulates the 5-HT3 receptors on the afferent vagal fibers, resulting in their deporalization. Electrical stimulation of the abdominal vagal afferents is capable of inducing emesis, and abdominal vagotomy suppresses cisplatin-induced emesis. These results indicate that the vagus is the major afferent pathway involved in the detection of emetic stimuli.

Modulation of vomiting by the medullary midline

Studies were conducted to investigate the possible role of the brainstem midline region as a source of facilitatory input for the vomiting process. Experiments were conducted using the "fictive vomiting' model in decerebrate, paralysed cats. Dysfunction of the medullary midline region produced by localized injections of the neurotoxin kainic acid abolished or greatly attenuated fictive vomiting. In addition, some respiratory-related midline neurons were found to fire in synchrony with co-active phrenic and abdominal nerve discharge during fictive vomiting. These experiments demonstrate the importance of the medullary midline for the normal occurrence of the vomiting process. An explanation for the post-lesion elimination of vomiting is that the lesions remove an important source of facilitatory input to spinal respiratory motoneurons and/or to the brainstem circuitry that mediates vomiting.

Postoperative nausea and vomiting. 1: Physiology and aetiology

There is a vast amount of research available relating to postoperative nausea and vomiting (PONV). This research has been analyzed and summarized in a series of two articles which draw out the main factors influencing the incidence and affecting the treatment of the condition. This, the first article, will describe the physiological processes that result in PONV. An analysis will then follow of the many factors that contribute to the complex multifactorial aetiology of PONV. These factors include personal, preoperative, intraoperative and postoperative variables which affect differing neurotransmitter centres in the chemoreceptor trigger zone (CTZ) which is situated in the brainstem. These variables will then be related to the appropriate neurotransmitter centre allowing a greater understanding of the most likely physiological pathways involved in specific cases of PONV. The second article will explore the management and treatment of PONV.

Autoradiographic distribution of [3H]sumatriptan-binding sites in post-mortem human brain

The anatomical distribution of [3H]sumatriptan-binding sites was analysed in brain tissue sections from 11 subjects. Relevant concentrations of [3H]sumatriptan-binding sites were seen in areas such as visual cortex > locus niger > globus pallidus > layers IV-V of the frontal cortex > subiculum > entorhinal cortex > nucleus tractus solitarius > nucleus trigeminalis caudalis. This distribution of [3H]sumatriptan-binding sites in the human brain shows some differences when compared with that of 5HT1D receptors, confirming that, besides 5HT1D, sumatriptan also binds to 5HT1F receptor subtype. Some species differences are evident between the distribution of [3H]sumatriptan-binding sites in the human brain and that reported for guinea-pig and rat brains, emphasizing that caution is needed in extrapolating experimental data from animals to humans. Furthermore, these data help to explain some of the therapeutic actions of sumatriptan. The remarkable levels of binding found in areas as nucleus tractus solitarius and nucleus trigeminalis caudalis suggest that in migraine attacks sumatriptan could exert its specific anti-emetic effects and, partly at least, induce analgesia by directly acting over these brain nuclei.

Extrinsic nervous control of retrograde giant contraction during vomiting in conscious dogs

Neural mechanisms controlling retrograde giant contraction during vomiting were studied in six conscious dogs with implanted strain gauge force transducers. The small intestine was divided into proximal (P), middle (M), and distal (D) segments. These segments were transplanted on intact mesenteric neurovascular pedicles. In three dogs, M and D segments were interchanged (group A). In three dogs, P and M segments were interchanged (group B). Before transplantation, apomorphine-induced vomiting caused retrograde giant contractions, starting from the M segment and rapidly migrating to the stomach. However, in group A, even after recovery of interdigestive migrating contractions migration, retrograde giant contractions during vomiting always originated in the distally interchanged M segment and jumped to the P segment without migration to the D segment. In group B, the retrograde giant contraction always originated in the proximally interchanged M segment and successively occurred in the distally interchanged P segment. We conclude that origination and migration of retrograde giant contractions are extrinsically controlled. These motor events during vomiting are thought to be a specific motor function that does not exist in the lower small intestine, and retrograde giant contraction during vomiting may originate in the mid-small intestine.

The pharmacology of the emetic response to upper gastrointestinal tract stimulation in Suncus murinus

This paper is the first to describe aspects of the mechanics of retching in the insectivore Suncus murinus (house musk shrew) and in an animal of such a small size (approximately 50 g). In anaesthetised animals using the novel stimulus of mechanical stimulation of the upper gastrointestinal tract as the provocative stimulus the frequency of retching was found to be about 4 retches/s, a much higher frequency than in other species (dog, cat, ferret). These studies show that quantification of retching in Suncus cannot be undertaken using direct observation. The temporal pattern of the emetic response was characterised in conscious Suncus using motion (1 Hz, 5 min) and nicotine (20 mg/kg s.c.). The ultrapotent capsaicin analogue resiniferatoxin (100 micrograms/kg s.c.) was discovered to be highly emetic and comparative studies showed that nicotine and resiniferatoxin induced the most intense responses with episodes (retches and a vomit) occurring every 10-15 s. The retching response to mechanical stimulation in the anaesthetised Suncus was not blocked by a 5-HT3 receptor antagonist (granisetron, 1-5 mg/kg s.c.), a tachykinin NK1 receptor antagonist (CP-99,994 20 mg/kg s.c. dihydrochloride salt (9+) -(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine) or morphine (2 mg/kg s.c.) but was blocked by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT 100 micrograms/kg s.c.). Suncus appears to be a suitable animal in which to study the pharmacology of the emetic response to mechanical stimulation of the gut. The results are discussed in the light of studies of the pharmacology of emesis in other species.

Serotonin mechanisms in chemotherapy-induced emesis in cancer patients

Emesis is a common side effect of chemotherapeutic drugs. Cisplatin, nitrogen mustard and dacarbazine induce increases in urinary 5-hydroxyindoleacetic acid (5-HIAA) in parallel with the development of the period of emesis which is sensitive to 5-HT3 receptor antagonists ('acute emesis'). It is suggested that these cytotoxics release serotonin from enterochromaffin cells, which then acts on 5-HT3 receptors to trigger the emetic response. Cyclophosphamide, on the other hand, induces a modest emetic response, partly sensitive to 5-HT3 receptor antagonists, but not associated with increases in urinary 5-HIAA. It is suggested that cyclophosphamide-induced emesis is not mediated by the release of serotonin from enterochromaffin cells. Although after high-dose cisplatin most emesis is sensitive to 5-HT3 receptor antagonists, patients often present a milder, although more prolonged form of emesis which is mostly resistant to 5-HT3 receptor antagonists (also known as 'delayed emesis'). This form of emesis is not associated with increases in urinary 5-HIAA (not due to serotonin released from the enterochromaffin cells). Treatment with p-chlorophenylalanine (a serotonin synthesis inhibitor) inhibited cisplatin-induced emesis and cisplatin-induced increases in urinary 5-HIAA excretion. In summary, these results indicate that in human patients, serotonin plays a fundamental role in chemotherapy-induced emesis. Serotonin released from enterochromaffin cells seems to mediate emesis sensitive to 5-HT3 receptor antagonists induced by cisplatin, dacarbazine and nitrogen mustard. Emesis sensitive to 5-HT3 receptor antagonists associated with cyclophosphamide treatment, is not mediated by the release of serotonin from enterochromaffin cells by the cytotoxic. Therefore, cyclophosphamide could induce serotonin release either from enteric serotonin nerves or from the CNS. Cisplatin-induced emesis resistant to 5-HT3 receptor antagonists ('delayed emesis') is not mediated by serotonin released from enterochromaffin cells.

Mechanisms of chemotherapy/radiotherapy-induced emesis in animal models

Animal models of chemotherapy/radiotherapy-induced emesis successfully predicted the clinical efficacy of the 5-HT3 receptor antagonists for the control of acute emesis. Further studies in animals have provided valuable information relating to the pathophysiology of emesis and the mechanism of action of 5-HT3 receptor antagonists. These agents inhibit emesis by blocking the action of 5-HT at 5-HT3 receptors on the vagus nerve in the gastrointestinal tract and in the hindbrain vomiting system. 5-HT is hypothesized to be released from enterochromaffin cells following cytotoxic therapy or radiation. The mechanism by which 5-HT is released from enterochromaffin cells is unknown and, although various mechanisms have been proposed, none of these have provided convincing supportive evidence. In collaboration with scientists at Glaxo we have pioneered two models of cisplatin-induced acute and delayed emesis [Rudd et al., 1994]. In the first model, ferrets are given a low dose of cisplatin (5 mg/kg i.p.) and observed for 3 days. A pattern of emesis similar to that seen in the clinic has been observed with two distinct phases of emesis. Ondansetron, and particularly ondansetron plus high-dose dexamethasone, are effective in reducing the emetic response over days 1-3. The second model uses a higher dose of cisplatin (10 mg/kg i.p.) and an observation period of 24 h. Part of the emetic response over this time is resistant to 5-HT3 receptor antagonism. Studies into the mechanism of the emesis induced in both models may give an insight into cisplatin-induced emesis in man that is not controlled with 5-HT3 receptor antagonists.

Pathophysiology, severity, pattern, and risk factors for carboplatin-induced emesis

Carboplatin has proven efficacy in the treatment of ovarian cancer and has been proven to be less toxic compared to the parent compound cisplatin. Nevertheless, emesis is still a major problem associated with carboplatin-containing chemotherapy. Several investigators have focussed on the understanding of the pathophysiology and pattern of cisplatin-induced emesis. Data describing both the pathomechanisms and pattern of carboplatin-induced emesis are still lacking. This paper combines data from the literature with our own experience with the pattern and control of carboplatin-induced emesis, and presents data contributing to the understanding of the underlying pathomechanisms. Carboplatin induces a significant increase in urinary 5-HIAA excretion, the main metabolite of serotonin. 5-HIAA excretion levels remain elevated over 3 days following chemotherapy. Carboplatin-induced emesis is observed in about 40% of patients despite anti-emetic prophylaxis with 5-HT3 antagonists. Vomiting after carboplatin extends over days 1-3 with an equal distribution regarding the severity on each day. Analysis of the pattern of emesis revealed that delayed emesis (> 24 h after chemotherapy) is a major problem associated with carboplatin therapy. Description of the pattern of emesis as 'prolonged emesis' seems to be appropriate. 5-HT3 receptor antagonists such as ondansetron seem to be efficacious both in the control of acute and prolonged emesis following carboplatin chemotherapy, but randomly controlled data comparing ondansetron with other anti-emetic regimens have not yet been published. Univariate analysis reveals gender and combination therapy containing carboplatin and cyclophosphamide and/or anthracyclines as risk factors for emesis.

Towards understanding the aetiology and pathophysiology of the emetic reflex: novel approaches to antiemetic drugs

The introduction of 5-HT3 antagonists, such as ondansetron, as antiemetic agents has transformed the management of patients receiving chemotherapy or radiation therapy. Studies in animal models with NK1 antagonists suggest that these represent a new class of antiemetic agents having a broader spectrum of activity than 5-HT3 antagonists. Compounds of this class may prove to be more effective in man against delayed emesis induced by cisplatin, post-operative nausea and vomiting and motion sickness. Thus, they have the potential to complement 5-HT3 antagonists and so provide a further advance in the management of nausea and vomiting.

An experimental model to study intracranial hypertension-induced vomiting in conscious dogs

An experimental model to study vomiting due to acute increases in intracranial pressure (ICP) was standardized in dogs. Chronic lateral cerebral ventricle cannulated animals were partially restrained in an observation chamber. The cannula was connected to a buffered saline pressure head which could be adjusted to the required height with the help of a pulley system to attain the desired intracranial pressure. In separate sets of experiments the ICP was raised and maintained at 20, 40, 60 and 80 mmHg for 30 min. The incidence of vomiting and/or retching was observed and the vomiting/retching score (V/R score) was calculated to quantify the severity. The V/R score increased with the level of ICP, maximum being at ICP 80 mmHg. At this level, the effect of pretreatment with propranolol, prazosin and ondansetron was studied. Prazosin and propranolol treatment reduced the vomiting/retching score, while ondansetron failed to do so. This model can be used for evaluating therapeutic interventions for elevated ICP-induced vomiting.

A hitherto unknown pattern of pathologic gastrointestinal motility--a cause of repeated vomiting?

We report the case of a 20-year-old woman with functional vomiting who presented with symptoms of anorexia nervosa. Antroduodenal and upper jejunal perfusion manometry was performed using an eight-lumen catheter. The investigation revealed a hitherto unknown motility pattern consisting of continuous simultaneous contractions at high frequency from the antrum down to the upper jejunum. The observation suggests that this disorder was related to the patients symptomatology.

Vomiting attack with ACTH and ADH discharge improves startle epilepsy

A 15-year-old boy with sequelae of perinatal asphyxia experienced intractable startle-induced epileptic seizures, which were transiently suppressed with episodic vomiting. His vomiting was associated with adrenocorticotropin and antidiuretic hormone discharge, and the alteration of urinary catecholamine excretion, which might modulate epileptic seizures. Because startle-induced epileptic seizures are resistant to conventional antiepileptic therapy, this case is informative for the treatment of startle epilepsy.

Involvement of dopamine D3 receptors in the area postrema in R(+)-7-OH-DPAT-induced emesis in the ferret

We investigated the possible involvement of dopamine D3 receptors in R(+)-7-hydroxy-2-(N,N-di-n-propylamino)tetraline (R(+)-7-OH-DPAT)-induced emesis in the ferret. The R(+)enantiomer of 7-OH-DPAT (0.03-1 mg/kg, s.c.) caused emesis in a dose-dependent manner, whereas the S(-)enantiomer, even at 1 mg/kg s.c. failed to induce emesis. Quinpirole (0.1-1.0 mg/kg) and apomorphine (0.3 mg/kg, s.c. only) also elicited an emetic response. S(-)-Eticlopride, which has a high affinity for the dopamine D3 receptor, antagonized R(+)-7-OH-DPAT (0.3 mg/kg, s.c.)-induced emesis (ID50 1.4 micrograms/kg, s.c.). R(+)-7-OH-DPAT (0.1-1.0 microgram) administered into the 4th cerebral ventricle dose dependently induced emesis within 1 min of dosing in ferrets. Intracerebroventricularly administered S(-)-eticlopride (0.01-1 microgram) also inhibited the emesis induced by s.c. administration of R(+)-7-OH-DPAT. The emetic effect of R(+)-7-OH-DPAT was unaffected by abdominal vagotomy but was markedly reduced by ablation of the area postrema. These results suggest that dopamine D3 receptors in the area postrema play an important role in R(+)-7-OH-DPAT-induced emesis in the ferret.

Changes in the volume of evacuated bile associated with emesis in dogs

Changes in the volume of evacuated bile as well as the extrahepatic biliary motilities associated with emesis were studied in decerebrated dogs. The motilities of both the gallbladder and sphincter of Oddi were enhanced with retching. The mean volume of evacuated bile significantly decreased during retching. These results suggested that the extrahepatic biliary motilities with emesis are not related to bile evacuation.

Marked depression of radiation-induced emesis by olfactory bulbectomy or pre-exposure using low doses

Acute emetic response to relatively low-doses of X-irradiation on suncus (Suncus murinus) was examined. The behaviors recorded for each subject, using a video-cassette recorder system, were (1) the number of emesis, during exposure to a dose of 3.0 Gy; (2) emesis threshold. Results showed that the emetic threshold was observed at 0.85 Gy, and the number of radioemesis during exposure was 29. However, the observed threshold dose became 2.22 Gy following olfactory bulbectomy. The emetic number decreased significantly (p <0.01), and reached a value one-fourth of the sham-control. The bulbectomized suncus showed a resistance to X-irradiation. Furthermore, I examined whether the animals could also acquire radio-resistance when they were subjected to a brief of dose X-rays (0.3 Gy) prior to a exposure to 3.0 Gy. Results showed that brief pre-exposure increased the observed threshold, a pattern that was exactly the same as shown in the bulbectomized animals. Increasing the pre-exposure dose further to 0.45-0.60 Gy, however, resulted in the complete disappearance of the effect. These results suggest that only the mice pre-irradiated with 0.30 Gy acquired resistance to radiation-induced emesis.