The 5-HT2B antagonist and 5-HT4 agonist activities of tegaserod in the anaesthetized rat
Introduction
5-Hydroxytryptamine (5-HT), which originates in enterochromaffin cells and intrinsic neurons of the alimentary tract, plays a pivotal physiological role in regulating gastrointestinal motility via an interaction with 5-HT1, 5-HT2, 5-HT3, and 5-HT4 receptors [1].
Activation of 5-HT4 receptors, located on motor neurons and interneurons in the alimentary tract results in facilitation of gastrointestinal motility by augmenting cholinergic and non-adrenergic, non-cholinergic neurotransmission [2]. Additionally, stimulation of 5-HT4 receptors on smooth muscle inhibits colonic circular muscle contractile activity in humans [3]. With respect to the role of the 5-HT2B receptor in gastrointestinal physiology, preclinical data indicate an involvement in the development and function of the enteric nervous system and alimentary tract [4], [5], [6]. 5-HT2B receptor mRNA and protein are expressed in the human and rodent gastrointestinal tract [5], [6], and their activation enhances neurogenically mediated contraction of colonic longitudinal smooth muscle [5].
Drug discovery efforts have identified several 5-HT ligands with clinical efficacy in gastrointestinal disorders. The 5-HT4 receptor agonists, cisapride (Propulsid®), prucalopride, renzapride, mosapride and tegaserod (Zelnorm®) have demonstrated efficacy in patients suffering disorders characterized by reduced gastrointestinal motility [7], [8], [9]. Notable amongst these agents is tegaserod [10], [11], [12], which is approved in several countries for the treatment of constipation-predominant irritable bowel syndrome in females and chronic constipation in males and females. Clinical studies have demonstrated that tegaserod increases gastric emptying and accelerates small intestinal and colonic transit in humans [13], [14]. Although the gastrointestinal actions of tegaserod are attributed to 5-HT4 receptor activation, the compound binds to 5-HT1 and 5-HT2 receptors, and is a potent 5-HT2B receptor antagonist [15]. The 5-HT2B receptor antagonist activity of tegaserod could have clinical significance.
The objective of this study was to investigate the 5-HT2B receptor antagonist activity of tegaserod in vivo, and to compare its potency in this respect to its 5-HT4 agonist activity. While the rat stomach fundus [16] and oesophagus [17] exist for assaying, respectively, 5-HT2B and 5-HT4 receptor agonist potency and efficacy in vitro, investigation of such activity in vivo, in rodents, is generally limited to intestinal transit or gastric emptying of test meals containing a marker [18], [19]. Implanted strain gauge force transducers have been used successfully in guinea pigs [20] to measure “real-time” gastrointestinal contractility, although due to the technical challenges, are more commonly used in larger species. In this study, digital sonomicrometry was used in anticipation that it would provide a sensitive means of recording real time changes in oesophageal and stomach fundus responsiveness in a less invasive manner than utilization of strain gauges or other forms of tissue tension measurement. Traditionally used in cardiovascular physiology and pharmacology studies, digital sonomicrometry utilizes miniature piezoelectric crystals to transmit and receive ultrasonic pulses, and the “time of flight” of the sound wave is measured with high sensitivity. As the speed of sound in biological tissue is fixed, a continuous record of inter-crystal distance is provided, and changes in muscle length are recorded. The in vivo sonomicrometry output (i.e. inter-crystal distance) is considered equivalent to that generated by traditional tissue bath methodology in which changes in muscle length are recorded isotonically. The application of this technique to the gastrointestinal field has been reported recently for measuring stomach motility in response to intravenous cholecystokinin [21]. Piboserod (SB-207266) and SB 206553, selective 5-HT4 and 5-HT2B/2C receptor antagonists, respectively [22], [23] and BW 723C86 and α-methyl 5-HT, selective 5-HT2B receptor agonists, were used to characterize definitively the pharmacological responses evoked by tegaserod. The oesophageal activity of tegaserod was compared to that produced by cisapride and mosapride, both of which are 5-HT4 receptor agonists [19], [24].
Section snippets
Materials and methods
All experiments were conducted in accordance with the principles of good laboratory animal care provided by the Animal Care and Usage Committee of Theravance Inc. Adult, male Sprague–Dawley rats (250–350 g, Harlan) were acclimated to the colony room (temperature controlled at 21 ± 1 °C and 12:12 h light-dark cycle commencing at 7:00 am) for at least 5 days following arrival. During this period, standard rat diet (Harlan Teklad) and drinking water were available ad libitum.
Rats were anaesthetized with
Stomach fundus
Following crystal placement, 30 min proved sufficient to establish a stable sonomicrometry recording of inter-crystal distance. No spontaneous changes in stomach fundus muscle length were observed after this stabilization period, and preparations remained viable for up to 2 h thereafter. Following i.v. administration, the selective 5-HT2B receptor agonist, BW 723C86 (0.01–1 mg kg−1), but not its vehicle, produced a dose-dependent reduction in fundus inter-crystal distance, consistent with
Discussion
Tegaserod (Zelnorm®) was introduced recently for the treatment of constipation-predominant irritable bowel syndrome and chronic constipation, and is currently under clinical evaluation in diabetic gastroparesis, gastro-oesophageal reflux disease and functional dyspepsia [7]. The clinical efficacy of tegaserod is attributed to its 5-HT4 receptor agonist activity, although it has affinity for other, non-5-HT4 receptors relevant to gastrointestinal function, such as the 5-HT2 receptor family [11].
Acknowledgements
We thank Drs Patrick Humphrey, Roger Thomas and Jan Smith for their comments on the manuscript during its preparation.
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