BIMU 8

BIMU 8 is an agonist of 5-HT4 with Ki values of 33.9 ± 8.0 nM and 12.6 ± 0.9 nM in guinea pig ileum and striatum, respectively [1, 2].

As a member of the seven transmembrane spanning G-protein-coupled family of receptors, the 5-HT4 receptor is positively coupled to adenylate cyclase. It exists in two isoforms (5-HT4S and 5-HT4L). These two isoforms differ in the sequence and length of their carboxy termini [3].

BIMU 8 significantly decreased the K+ current in colliculi neurons. This suggested a 5-HT4 receptor-mediated effect [4]. In neurons, BIMU 8 at concentrations ranging from 0.003-0.1 µM increased EPSP amplitude but did not change membrane potential. The EPSP potentiation induced by BIMU 8 was blocked by tropisetron (1 µM), a 5-HT3/5-HT4 receptor antagonist. But ondansetron (1 µM), a 5-HT3 receptor antagonist did not blocked the EPSP potentiation induced by BIMU 8 [5].

In the hot-plate test, BIMU 8 injected i.p. in the range of doses of 20-30 mg/kg significantly induced an increase in the pain threshold. 15 min after administration, the antinociceptive effect reached a maximum and hence diminished. This effect disappeared within 45 min. Choline uptake blocker HC-3 (1 µg per mouse i.c.v.), antimuscarinic drug atropine (5 mg/kg i.p.), 5-HT4 antagonists SDZ 205-557 (10 mg/kg i.p.) and GR 125487 (20 mg/kg i.p.) completely prevented the antinociception of BIMU 8 [1].

References:
[1].  Ghelardini C, Galeotti N, Casamenti F, et al. Central cholinergic antinociception induced by 5HT4 agonists: BIMU 1 and BIMU 8. Life sciences, 1996, 58(25): 2297-2309.
[2].  Yoshikawa T, Yoshida N, Mine Y, et al. Affinity of mosapride citrate, a new gastroprokinetic agent, for 5-HT4 receptors in guinea pig ileum. The Japanese Journal of Pharmacology, 1998, 77(1): 53-59.
[3].  Hegde SS, Eglen RM. Peripheral 5-HT4 receptors. The FASEB journal, 1996, 10(12): 1398-1407.
[4].  Fagni L, Dumuis A, Sebben M, et al. The 5-HT4 receptor subtype inhibits K+ current in colliculi neurones via activation of a cyclic AMP-dependent protein kinase. British journal of pharmacology, 1992, 105(4): 973-979.
[5].  Pan H, Galligan JJ. 5-HT1A and 5-HT4 receptors mediate inhibition and facilitation of fast synaptic transmission in enteric neurons. American Journal of Physiology-Gastrointestinal and Liver Physiology, 1994, 266(2): G230-G238.

Central cholinergic antinociception induced by 5HT4 agonists: BIMU 1 and BIMU 8.[Pubmed:8649219]

Life Sci. 1996;58(25):2297-309.

The antinociceptive effect of two 5-HT4 agonists, BIMU 1 and BIMU 8, were examined in mice and rats by using the hot-plate, abdominal constriction and paw-pressure tests. In both species, BIMU 1 (10-20 mg kg-1 s.c. and 40-60 mg kg-1 p.o. in mice; 20 mg kg-1 i.p. in rats) and BIMU 8 (20-30 mg kg-1 s.c. and 60 mg kg-1 p.o. in mice; 20 mg kg-1 i.p. in rats), produced significant antinociception which was prevented by atropine (5 mg kg-1 i.p.), hemicholinium-3 (1 microgram per mouse i.c.v.), SDZ 205-557 (10 mg kg-1 i.p.), GR 125487 (20 mg kg-1 i.p.) but not by naloxone (1 mg kg-1 i.p.), CGP 35348 (100 mg kg-1 i.p.) and reserpine (2 mg kg-1 i.p.). Moreover, BIMU 1 and BIMU 8 increase of pain threshold, is abolished by nucleus basalis magnocellularis (NBM) lesions in rats. SDZ 205-557 and GR 125487 which totally antagonized BIMU 1 and BIMU 8 antinociception did not modify morphine (7 mg kg-1 s.c.) or baclofen (4 mg kg-1 s.c.) antinociception. Intracerebroventricular injection in mice of BIMU 1 (3 micrograms per mouse) and BIMU 8 (10 micrograms per mouse), doses which were largely ineffective by parenteral routes, induces an antinociception whose intensity equaled that obtainable s.c., i.p. or p.o. In the antinociceptive dose-range, neither 5HT4 agonist impaired mice motor coordination evaluated by rota-rod test. On the basis of the above data, it can be postulated that BIMU 1 and BIMU 8 exerted an antinociceptive effect mediated by a central amplification of cholinergic transmission.

Conformational toggle switches implicated in basal constitutive and agonist-induced activated states of 5-hydroxytryptamine-4 receptors.[Pubmed:19168624]

Mol Pharmacol. 2009 Apr;75(4):982-90.

The extended classic ternary complex model predicts that a G protein-coupled receptor (GPCR) exists in only two interconvertible states: an inactive R, and an active R(*). However, different structural active R(*) complexes may exist in addition to a silent inactive R ground state (Rg). Here we demonstrate, in a cellular context, that several R(*) states of 5-hydroxytryptamine-4 (5-HT(4)) receptors involve different side-chain conformational toggle switches. Using site-directed mutagenesis and molecular modeling approaches, we show that the basal constitutive receptor (R(*)basal) results from stabilization of an obligatory double toggle switch (Thr3.36 from inactive g- to active g+ and Trp6.48 from inactive g+ to active t). Mutation of either threonine or tryptophan to alanine resulted in a lowering of the activity of the R(*)basal similar to the Rg. The T3.36A mutation shows that the Thr3.36 toggle switch plays a minor role in the stabilization of R(*) induced by 5-HT (R(*)-5-HT) and BIMU8 (R(*)-BIMU8) and is fully required in the stabilization of R(*) induced by (S)-zacopride, cisapride, and 1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-butyl-4-piperidinyl)-1-propanone (RS 67333) (R(*)-benzamides). Thus, benzamides stabilize R(*)-benzamides by forming a specific hydrogen bond with Thr3.36 in the active g+ conformation. Conversely, R(*)-BIMU8 was probably the result of a direct conformational transition of Trp6.48 from inactive g+ to active t by hydrogen bonding of this residue to a carboxyl group of BIMU8. We were surprised that the Trp6.48 toggle switch was not necessary for receptor activation by the natural agonist 5-HT. R(*)-5-HT is probably attained through other routes of activation. Thus, different conformational arrangements occur during stabilization of R(*)basal, R(*)-5-HT, R(*)-benzamides, and R(*)-BIMU8.

Synthesis and structure-activity relationships of potent and orally active 5-HT4 receptor antagonists: indazole and benzimidazolone derivatives.[Pubmed:9599243]

J Med Chem. 1998 May 21;41(11):1943-55.

A series of indole-3-carboxamides, indazole-3-carboxamides, and benzimidazolone-3-carboxamides was synthesized and evaluated for antagonist affinity at the 5-HT4 receptor in the rat esophagus. The endo-3-tropanamine derivatives in the indazole and benzimidazolone series possessed greater 5-HT4 receptor affinity than the corresponding indole analogues. 5-HT4 receptor antagonist affinity was further increased by alkylation at N-1 of the aromatic heterocycle. In a series of 1-isopropylindazole-3-carboxamides, replacement of the bicyclic tropane ring system with the monocyclic piperidine ring system or an acyclic aminoalkylene chain led to potent 5-HT4 receptor antagonists. In particular, those systems in which the basic amine was substituted with groups capable of forming hydrogen bonds showed increased 5-HT4 receptor antagonist activity. While some of these compounds displayed high affinity for other neurotransmitter receptors (in particular, 5-HT3, alpha1, and 5-HT2A receptors), as the conformational flexibility of the amine moiety increased, the selectivity for the 5-HT4 receptor also increased. From this series of compounds, we identified LY353433 (1-(1-methylethyl)-N-[2-[4-[(tricyclo[3.3.1.1(3, 7)]dec-1-ylcarbonyl)amino]-1-piperidinyl]ethyl]-1H-indazole-3- carboxamide) as a potent and selective 5-HT4 receptor antagonist with clinically suitable pharmacodynamics.