The 5-HT4 receptor agonist, tegaserod, is a potent 5-HT2B receptor antagonist in vitro and in vivo

Abstract

1 Tegaserod (Zelnorm) is a potent 5-hydroxytryptamine4 (5-HT4) receptor agonist with clinical efficacy in disorders associated with reduced gastrointestinal motility and transit. The present study investigated the interaction of tegaserod with 5-HT2 receptors, and compared its potency in this respect to its 5-HT4 receptor agonist activity. 2 Tegaserod had significant binding affinity for human recombinant 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi=7.5, 8.4 and 7.0, respectively). The 5-HT2B receptor-binding affinity of tegaserod was identical to that at human recombinant 5-HT4(c) receptors (mean pKi=8.4) in human embryonic kidney-293 (HEK-293) cells stably transfected with the human 5-HT4(c) receptor. 3 Tegaserod (0.1-3 microm) inhibited 5-HT-mediated contraction of the rat isolated stomach fundus potently (pA2=8.3), consistent with 5-HT(2B) receptor antagonist activity. Tegaserod produced, with similar potency, an elevation of adenosine 3',5' cyclic monophosphate in HEK-293 cells stably transfected with the human 5-HT4(c) receptor (mean pEC50=8.6), as well as 5-HT4) receptor-mediated relaxation of the rat isolated oesophagus (mean pEC50=8.2) and contraction of the guinea-pig isolated colon (mean pEC50=8.3). 4 Following subcutaneous administration, tegaserod (0.3 or 1 mg kg(-1)) inhibited contractions of the stomach fundus in anaesthetized rats in response to intravenous dosing of alpha-methyl 5-HT (0.03 mg kg(-1)) and BW 723C86 (0.3 mg kg(-1)), selective 5-HT2B receptor agonists. At similar doses, tegaserod (1 and 3 mg kg(-1) subcutaneously) evoked a 5-HT4 receptor-mediated increase in colonic transit in conscious guinea-pigs. 5 The data from this study indicate that tegaserod antagonizes 5-HT2B receptors at concentrations similar to those that activate 5-HT4 receptors. It remains to be determined whether this 5-HT2B receptor antagonist activity of tegaserod contributes to its clinical profile.

Figure 1

Effect of tegaserod and 5-HT on (a) [³H]GR113808-specific binding to membranes prepared from HEK-293 cells stably transfected with human recombinant 5-HT_4(c) receptors, and (b) [³H]LSD-specific binding to membranes prepared from CHO-K1 cells stably transfected with human recombinant 5-HT_2B receptors. Results are expressed as fitted curves to the mean (±s.d.) of 3–6 independent experiments.

Figure 2

Concentration–effect curves to tegaserod and 5-HT in a whole-cell cAMP accumulation assay using HEK-293 cells stably transfected with human recombinant 5-HT_4(c) receptors. Data are expressed as fitted curves to the mean (±s.d.) of three independent experiments, and as a percentage of the maximum cAMP accumulation response to 5-HT.

Figure 3

(a) Concentration–effect curves to tegaserod and 5-HT (each 0.001–10 μM; n=6 or 7, respectively) in the carbachol (3 μM)-precontracted rat isolated oesophageal tunica muscularis mucosa preparation. Values are expressed as the mean (±s.d.) percentage relaxation of that achieved by 5-HT. (b) Concentration–effect curves to tegaserod (0.0001–10 μM; n=6) and 5-HT (0.0001–3 μM; n=12) in the guinea-pig isolated colonic longitudinal muscle/myenteric plexus preparation. Values are expressed as the mean (±s.d.) percentage of the response to 5-HT.

Figure 4

Effects of tegaserod (0.03, 0.3 and 3 mg kg⁻¹) and its vehicle (10% SBE/CD, 2 ml kg⁻¹), each administered subcutaneously, on the colonic transit of dye in conscious guinea-pigs (n=10 for each group). The data (mean±s.d.) are expressed in terms of the time at which the first faecal pellet containing dye was excreted (^*P<0.05, ANOVA and Dunnett's post hoc test, compared to vehicle).

Figure 5

Influence of co-administration of subcutaneous piboserod (1 mg kg⁻¹) on the prokinetic activity of tegaserod (1 mg kg⁻¹ subcutaneously) in conscious guinea pigs (n=12–15 for each group). The effects of piboserod alone, its vehicle (D5W, 2 ml kg⁻¹) and tegaserod vehicle (10% SBE/CD, 2 ml kg⁻¹) are also shown. The data (mean±s.d.) are expressed with respect to the time at which the first faecal pellet containing dye was excreted (^*P<0.05, ANOVA and Dunnett's post hoc test), for comparison to vehicle.

Figure 6

(a) Concentration–effect curves to 5-HT (0.0001–1 μM; n=9), and 5-HT (0.0001–100 μM) in the presence of tegaserod (0.1, 1 and 3 μM; n=3, 4 and 2, respectively), in the rat isolated stomach fundus preparation. Values are expressed as a mean (±s.d.) percentage of the 5-HT maximum contraction in each tissue. (b) Schild regression analysis of the tegaserod-mediated antagonism of 5-HT-induced contraction in the rat isolated stomach fundus preparation.

Figure 7

(a) Influence of tegaserod (0.3 or 1 mg kg⁻¹; n=4–6) or its vehicle (10% SBE/CD, 1 ml kg⁻¹; n=5 or 6), each administered subcutaneously, on intravenous (a) α-methyl 5-HT (0.03 mg kg⁻¹)- or (b) BW 723C86 (0.3 mg kg⁻¹)-mediated contraction of the stomach fundus in isoflurane-anaesthetized rats. Also shown in (b) is the effect of subcutaneous pretreatment with SB 206553 (1 mg kg⁻¹; n=3) or its vehicle (0.9% saline, 1 ml kg⁻¹; n=3) on the BW 723C86-mediated responses. Data (mean±s.d.) are expressed in (a) with respect to the response (area and amplitude of the contraction) to the second α-methyl 5-HT challenge as a percentage of the first, and, in (b), as the amplitude (in mmHg) of the BW 723C86-evoked increase in stomach pressure (^*P<0.05 by (a) ANOVA and Dunnett's post hoc test, or (b) unpaired Student's t-test). Representative traces of responses to BW 723C86 (0.3 mg kg⁻¹ iv) are shown (inset in (b)) after pretreatment of rats with vehicle (10% SBE/CD; upper trace) or tegaserod (1 mg kg⁻¹; lower trace). The asterisk indicates the time of the BW 723C86 challenge.