Diarrhoea-predominant irritable bowel syndrome (IBS-d) is associated with increased levels of 5-hydroxytryptamine (5-HT) that may contribute to the symptoms. Current treatment for IBS-d is limited. Whilst the 5-HT3 receptor antagonist alosetron delivers symptomatic relief, patients often experience constipation and more rarely potentially life-threatening ischemic colitis.  It is reasoned that a 5-HT3 receptor partial agonist would reduce some of the consequences of increased 5-HT levels, but by still retaining some 5-HT3 receptor activity, the side effect profile is predicted to be better than simple antagonists.  The efficacy of CSTI-300 is at least comparable to a current therapeutic, the 5-HT3 receptor antagonist alosetron, in an in vivo rat model replicating the viscera hypersensitivity patients with IBS-d experience (Roberts et al, 2018, this meeting).  Our studies have evaluated some of the 5-HT3 receptor partial agonist properties of CSTI-300, and the potential of the drug to treat IBS-d (Roberts et al, 2018, this meeting). Here we further investigate the in vitro pharmacology of CSTI-300.

Radioligand binding studies, functional intracellular calcium assays and whole-cell patch clamp electrophysiology were utilised to investigate the pharmacology of CSTI-300 at the human (h) 5-HT3A and 5-HT3AB receptors expressed in HEK293 cells. 

In radioligand binding assays, CSTI-300 displayed high affinity for the h5-HT3A and h5-HT3AB receptors, with a Ki of approximately 2.0 nM compared to around 400 nM for 5-HT.  5-HT and CSTI-300 allowed the cryptic orthosteric modulator 5-chloroindole to compete for h5-HT3A receptor binding sites (an action not evident with antagonists). In comparison to 5-HT, CSTI-300 displayed potent partial agonist activity (measured by an increase in [Ca2+]i) at the h5-HT3A and h5-HT3AB receptors (approximately 30-40% intrinsic efficacy).  Preincubation with the selective 5-HT3 receptor antagonist granisetron (500 nM) blocked either 5-HT or CSTI-300 induced responses at the h5-HT3A or h5-HT3AB receptors, demonstrating the selectivity of these ligands for the 5-HT3 receptors in this assay.  Electrophysiology experiments demonstrated that the CSTI-300 induced current had a slower rise time in comparison to 5-HT at the h5-HT3A receptor; with peak effect of approximately 20% relative to 5-HT.  

In summary, CSTI-300 displayed actions of a partial agonist at h5-HT3 receptor isoforms that support its therapeutic potential to reduce the symptoms experienced by patients with IBS-d, with the added benefit of likely reduced adverse effects mediated by high level inhibition of the 5-HT3 receptor.