When identifying selective G protein-coupled receptor agonists as potential drugs, it is essential to confirm that their intrinsic activity, measured using the recombinant receptor, correctly translates to the receptor expressed naturally in the therapeutically relevant tissue. Thus, it cannot be assumed that the way a receptor couples to its effector mechanisms in a host cell is the same in native tissue. Different densities of receptor expression can also change the efficacy of an agonist in both recombinant and native-expressed receptors. These basic principles of agonist drug discovery are nicely illustrated by 5-HT4 receptor agonists.
This month sees the publication of an important study which confirms the clinical efficacy and safety of the selective 5-HT4 receptor agonist prucalopride (2 mg), given once daily for 4 weeks to elderly, nursing home patients suffering from constipation.1 As anticipated with a gastrointestinal (GI) prokinetic agent, the most frequent adverse events were diarrhoea and abdominal pain. Critically, there were no differences in a range of cardiovascular measures on Holter monitoring, including ECG QTc, ECG morphology parameters or incidence of supraventricular and ventricular dysrhythmias. Such studies are an important and normal step in the development of any new drug, especially one in which the target receptor is found in both the gut and the heart. This study also finds consistency with a number of observations which indicate that selective 5-HT4 receptor agonists can demonstrate high intrinsic activity (relative to 5-HT) in isolated GI tissues where the agonist is required to facilitate electrically evoked, cholinergically mediated contractions (thought to be representative of an ability to promote GI motility), but low intrinsic activity when the same compound is tested in other tissues expressing the receptor, including cardiac muscle. This concept has been highlighted by analysing the effects of prucalopride and other 5-HT4 receptor agonists on porcine gastric and cardiac tissues, using the operational model of agonism.2 It was concluded that 5-HT4 receptors on enteric neurones may be more efficiently coupled to mechanisms which facilitate their transmission, relative to the receptors on cardiac muscle. This pattern of activity means that when partial 5-HT4 receptor agonists are given as drugs, they selectively increase GI motility without necessarily influencing functions in other tissues. The explanation is unclear but one possibility is that variations in distributions of COOH-terminal splice variants of the 5-HT4 receptor endow different coupling efficiencies and/or desensitization liabilities.3 Most tissues with 5-HT4 receptors express the 5-HT4(b) and 5-HT4(a) isoforms, for example, but only the human intestine has so far been found to express the 5-HT4(d) isoform; in recombinant systems, the 5-HT4 receptor agonist, 5-HT3 receptor antagonist and GI prokinetic agent renzapride behaved as a full agonist at the 5-HT4(d) isoform but as a partial agonist at the 5-HT4(g) isoform.4 These data provide an attractive hypothesis, although considerable uncertainty remains.5
The cardiac safety study with prucalopride represents another step in the long process of translating 5-HT4 receptor pharmacology to the clinic. It began with metoclopramide in 1964, synthesized to improve on the anti-dysrhythmic properties of procainamide but shown to possess anti-emetic activity (via antagonism at dopamine D2 receptors and at a later date and in higher doses, at 5-HT3 receptors) and an ability to stimulate gastric motility. This discovery led to several attempts to identify analogues which did not antagonize D2 receptors in the brain (thereby removing the side effect of akathisia) but retained the anti-emetic and/or gastric stimulant properties. After the existence of the 5-HT4 receptor was suggested in 1988 (in mouse embryo colliculi neurones) it was proposed that agents such as metoclopramide exerted GI prokinetic activity via 5-HT4 receptor activation.6 This created for the first time, a defined target for drug discovery but not before new ‘GI prokinetic agents’ had been moved into the clinic. The most successful were mosapride, a compound with low intrinsic activity at the native 5-HT4 receptor and marketed in Japan for upper GI disorders,7 and cisapride, marketed worldwide in 1989 for gastro-oesophageal reflux disease, non-ulcer dyspepsia and gastroparesis. In 1995, the rank-order of affinity of cisapride for different receptors was reported as 5-HT2A > 5-HT4 = α1 adrenoceptor > 5-HT3 = D2,8 indicating a lack of selectivity for the 5-HT4 receptor; additional affinity for 5-HT2B receptors (pKi 8.4)7 was later discovered. Complications arising from drug–drug interactions (cisapride is metabolized by the cytochrome P450 3A4 isoenzyme) and more especially, reports of QT interval prolongation, cardiac dysrhythmias and the discovery of potent and direct activity at the human Ether-a-go-go Related Gene (hERG) encoded K+ channel led to withdrawal of cisapride from general use in 2000.5,9 Another ‘gastric prokinetic’, renzapride, potently antagonized 5-HT3 receptors and was subsequently shown to activate 5-HT4 receptors. This mixed pharmacology and the initial success of the 5-HT3 receptor antagonist alosetron for treatment of female diarrhoea-predominant irritable bowel syndrome (IBS) patients, prompted an evaluation of renzapride in patients with constipation-predominant IBS. However, whilst well tolerated, with ‘no propensity to cause disturbances in heart rhythm at the therapeutic dose and at a dose five times this level’, renzapride was abandoned because of insufficient clinical benefit (http://www.drugs.com/clinical_trials/results-renzapride-phase-iii-clinical-trial-4052.html). The emerging and possible link between rare cases of ischaemic colitis and the use of 5-HT3 receptor antagonists in IBS patients, and the contrasting abilities of 5-HT4 receptor activation and 5-HT3 receptor antagonism to, respectively, increase and reduce colonic motility,5 may each have contributed to a poor ‘risk-benefit’ outcome. With respect to the 5-HT4 receptor however, the mixed pharmacology of both cisapride and renzapride meant that the clinical promise of this receptor remained untranslated.
Tegaserod was approved by the FDA in 2002 for female constipation-predominant patients with IBS and then for patients with chronic idiopathic constipation under the age of 65. The compound was described in 1995 in terms of its ability to facilitate electrically evoked, cholinergically mediated contractions in guinea-pig isolated ileum; selectivity was determined against 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C and 5-HT3 receptors. However, as a 5-HT4 receptor agonist, its intrinsic activity was not optimal (in the electrically stimulated guinea-pig ileum the intrinsic activity relative to 5-HT was originally reported as 0.2),10 a characteristic compounded by poor solubility and/or tissue penetration properties.5,9 European Union (EU) registration was not achieved and the end came after a cross study analysis found 13 of 11 600 patients treated with tegaserod had ischaemic cardiovascular events, compared to 1 on placebo.5 Several of the treated patients had pre-existing cardiovascular problems, so the real association of tegaserod with these adverse events has since been questioned;11 links to 5-HT4 receptor or hERG activity have also been dismissed.5 However, high regulatory risk-aversion in patients with functional bowel disorders, combined with poor efficacy, contributed to the eventual withdrawal of tegaserod.11 Significantly, following its approval tegaserod was found to antagonize at human 5-HT2B receptors (this receptor was only cloned in 1992 and named in 1994) with a binding affinity equal to that for the h5-HT4(c) receptor (pKi 8.4 each).7 It has recently been suggested that 5-HT2B receptor antagonism may be antinociceptive12 and reduce colonic motility.13 The latter would oppose the prokinetic benefits of 5-HT4 receptor activation, adding to the poor prokinetic efficacy of tegaserod. Thus, the low intrinsic activity of tegaserod and its affinity for 5-HT2B receptors once again meant that the promise of the 5-HT4 receptor remained untested.
Thirty-five years after the description of metoclopramide and 13 years after the first characterization of the 5-HT4 receptor, the pharmacology of prucalopride was published (following abstracts in 1997).14 This molecule shows high selectivity for human 5-HT4(a) and 5-HT4(b) receptor isoforms, compared with a range of other receptors, including 5-HT2B and 5-HT3; the intrinsic activity in models of enteric nerve function (relative to 5-HT) was not assessed. However, in human isolated colon, prucalopride was subsequently shown to increase cholinergic and nitrergic motor nerve function, with greater efficacy than tegaserod.15 Following demonstration of clinical benefit in phase III trials, the committee for Medicinal Products for Human Use (European Medicines Agency) recommended in July 2009 that marketing authorization should be granted for prucalopride ‘for the symptomatic treatment of chronic constipation in women in whom laxatives fail to provide adequate relief’ (http://www.emea.europa.eu/). If the recommendation is accepted, prucalopride will be the first oral compound for the treatment of severe chronic constipation to be approved in the 27 member states of the EU.
The brief history of 5-HT4 receptor agonists highlights the fact that the development of prucalopride took place when other 5-HT4 receptor agonists (and 5-HT3 receptor antagonists) were failing. Such a poor track record in the translation of GI 5-HT research has certainly not encouraged the pharmaceutical industry to remain in this area of research (prucalopride was not finally developed by the company which discovered this molecule). Progress has been achieved because prucalopride (i) has high intrinsic activity at 5-HT4 receptors expressed by human GI nerves, (ii) is selective in its action, (iii) was progressed for the right clinical indication and (iv) because of persistence and dedication during development. Other compounds are following, including ATI-7505 and TD-51085. Their progress will be watched with great interest, as each will now be working towards ways of differentiating themselves from prucalopride. Perhaps at last, 5-HT4 receptor agonists are being given the chance to show what they can do.
Finally, it should be noted that translation of 5-HT4 receptor agonist pharmacology to difficult-to-treat patients with chronic constipation within the EU, follows that of lubiprostone, a locally acting ClC-2 chloride channel activator, approved by the FDA in 2006 for adults with chronic idiopathic constipation (lubiprostone is not yet available within the EU). For both prucalopride and lubiprostone, it is significant that the first indication is for the treatment of chronic constipation, rather than constipation-predominant IBS. This trend towards first evaluating the effects of new drugs which change GI motility in disorders where disrupted GI motility is known to exist (rather than in functional bowel disorders where changes in motility are uncertain) seems likely to increase the success rate for GI drug development.