1 The pharmacology of two novel 5-HT4 receptor agonists, RS 67333 (1-(4-amino-5-chloro-2-methoxy phenyl)-3-[1(n-butyl)-4-piperidinyl]-1-propanone HCl) and RS 67506 (1-(4-amino-5-chloro-2-methoxyphenyl)-3-[1-(2-methyl sulphonylamino)ethyl-4-piperidinyl]-1-propanone HCl) have been assessed in vitro and in vivo.2 RS 67333 and RS 67506 exhibited affinities (pKi=8.7 and 8.8, respectively) for the 5-HT4 binding sites, labelled with [3H]-GR 113808, in guinea-pig striatum. The Hill coefficients from these displacement curves were not significantly different from unity. The compounds exhibited lower affinities (< 6.0) at several other receptors including 5-HTA, 5-HTID, 5-HT2A, 5-HT2c, dopamine D,, D2 and muscarinic M,-M3 receptors. However, RS 67333 and RS 67506 did exhibit affinities for the a, (pKi=8.9 and 7.9, respectively) and Ca2 (pK,=8.0 and 7.3, respectively) binding sites. 3 At the 5-HT4 receptor mediating relaxation of the carbachol-precontracted oesophagus, RS 67333 and RS 67506 acted as potent (pECm 8.4 and 8.6, respectively), partial agonists (intrinsic activities, with respect to 5-HT were 0.5 and 0.6, respectively) with respect to 5-HT. Relaxant responses to RS 67333 or RS 67506 were surmountably antagonized by GR 113808 (10 nM), with apparent affinities (pKB) of 9.1 and 9.0, respectively. RS 67333 and RS 67506 induced dose-dependent increases in heart rate of the anaesthetized micropig (ED50 4.9 and 5.4 pg kg-',i.v.), with maximal increases of 35 and 47 beats min-', respectively. 4 RS 67333 and RS 67506, therefore, acted as potent, partial 5-HT4 receptor agonists in vitro and in vivo. These compounds, by virtue of their high potency and selectivity, may have some utility in elucidating the physiological role of 5-HT4 receptors.

3 In the rat isolated oesophagus, precontracted with carbachol, RS 39604 (30-300 nM) behaved as a competitive antagonist towards 5-HT-induced relaxation (pA2 =9.3; Schild slope= 1.0). We and others have shown previously that SB 204070 behaves as an unsurmountable antagonist in this preparation (pA2-10.5). In the guinea-pig isolated ileal mucosa, RS 39604 (30 nM) antagonized 5-MeOT-induced increase in short-circuit current (pA2=9.1). 4 In anaesthetized vagotomized micropigs, RS 39604, administered by the i.v. or intraduodenal (i.duod.) route, produced dose-dependent inhibition of 5-HT-induced tachycardia (ID50 = 4.7 gg kg-', i.v. and 254.5 gg kg-', i.duod). At maximal doses of 30 jg kg-', i.v. and 6 mg kg-', i.duod., the inhibitory effects of RS 39604 lasted for more than 6 h. In this preparation, SB 204070 was as potent as RS 39604 by the i.v. route but was inactive by the intraduodenal route at doses up to 3 mg kg-'. 5 In conscious mice, RS 39604, administered by the i.p. or p.o. route, produced dose-dependent inhibition of 5-hydroxytryptophan (5-HTP)-induced diarrhoea (ID50= 81.3 jg kg-', i.p. and 1.1 mg kg-', p.o.). In this assay, SB 204070 was inactive by the oral route at doses up to 30 mg kg-'. 6 In anaesthetized guinea-pigs, RS 39604 antagonized the contractile effect of 5-HT in the proximal colon by producing parallel, dextral displacement of the dose-response curve to 5-HT. The mean doseratios to 5-HT at 0.1 mg kg-', i.v., 1 mg kg', i.v. and 10 mg kg-', i.duod. were 4.6, 30.7 and 10.8, respectively. SB 204070 behaved as an unsurmountable antagonist in this assay. 7 In a model of visceral pain in conscious rats, RS 39604 (0.01-1 mg kg-', i.v.) did not affect colorectal distension-induced increases in arterial pressure whereas morphine (1 mg kg-', i.v.) produced significant inhibition of the response, implying that 5-HT4 receptors are not involved in nociception in this model. 8 The data suggest that RS 39604 is a high affinity and selective 5-HT4 receptor antagonist that is orally active and long-lasting in vivo. It is concluded that RS 39604 may be the preferable probe to use for investigating the physiological and pathophysiological role of 5-HT4 receptors in vivo.

The interaction of the novel antagonist, SDZ 205,557 (2-methoxy-4-amino-5-chloro benzoic acid 2-(diethylamino) ethyl ester), at 5-HT3 and 5-HT4 receptors has been assessed in vitro and in vivo. 2 In guinea-pig hippocampus and in the presence of 0.4 JLM 5-carboxamidotryptamine, 5-HT4-mediated stimulation of adenylyl cyclase was competitively antagonized by SDZ 205,557, with a pA2 value of 7.5, and a Schild slope of 0.81. In rat carbachol-contracted oesophagus, 5-HT4-receptor mediated relaxations were surmountably antagonized by SDZ 205,557 with a similar pA2 value (7.3). This value was agonist-independent with the exception of (R)-zacopride, against which a significantly lower value (6.4) was observed.

1 The pharmacological properties of RS 23597-190 (3-(piperdine-1-yl)-propyl-4-amino-5-chloro-2-methoxy benzoate hydrochloride) have been studied in vitro and in vivo.2 RS 23597-190 competitively antagonized 5-HT4 receptor-mediated relaxations of rat, carbachol precontracted oesophageal muscularis mucosae, (pA2 = 7.8 ± 0.1; Schild slope = 1.2 ± 0.2). Affinity estimates (-log KB) at 5-HT4 receptors using either renzapride or SC-53116 as agonists yielded a -log KB value of 8.0 ± 0.01. In contrast, RS 23597-190 failed to antagonize contractile responses to 5-HT of guinea-pig ileal 5-HT3 receptors, even at concentrations up to 10 tiM. 3 Increases in short-circuit current, induced by 5-HT, were studied in guinea-pig ileal mucosal sheets. Concentration-response curves to 5-HT were biphasic, with the high potency phase to 5-HT inhibited by RS 23597-190 and mimicked by 5-methoxytryptamine. The -log KB value for RS 23597-190 at the high potency phase was 7.3 confirming that 5-HT4 receptors mediated the high potency phase. min. Transient arrhythmic effects were noted after administration of the compound. 7 In conclusion, RS 23597-190 acts as a high affinity, selective competitive antagonist at 5-HT4 receptors. Thus, the compound appears to be a useful tool for 5-HT4 receptor identification in vitro. In vivo, the compound is rapidly metabolized in pigs such that 5-HT4 blockade is not maintained. However, in the rat, when given by infusion, RS 23597-190 antagonizes 5-HT3 mediated responses, at doses consistent with a low affinity 5-HT3 receptor. These data suggest that, under appropriate experimental conditions, RS 23597-190 may also be used in vivo to characterize further 5-HT4 receptor function.

5 At doses up to 1000 pg kg-', i.v., RS 25259-197 produced no meaningful haemodynamic changes in anaesthetized dogs. 6 In summary, RS 25259-197 is a novel, highly potent and orally active 5-HT3 receptor antagonist in vivo. With respect to its anti-emetic activity, RS 25259-197 appears to be a significant improvement over ondansetron in terms of potency and duration of action.