New motilin agonists: a long and winding road

Abstract

The development of a prokinetic agent is a true challenge for the pharmaceutical industry. It is not sufficient to find a compound that causes contraction of gastrointestinal smooth muscle tissue. The end result should be the aboral transport of material present in the lumen and this requires the integration of motor events in space and time.1 In fact contraction by itself may prevent rather than accelerate transit, as is illustrated by the condition of pyloric stenosis. The pioneers of prokinetic therapy stressed that compounds accelerated gastric emptying by improving ‘gastropyloroduodenal coordination’ an expression which uses an adjective composed of Greek and Latin roots, with the complexity and dubious charm of a German compound noun. But it does deliver a clear message: gastric emptying involves coordination between different parts of the gastrointestinal tract.1, 2 Another example of integrated motor activity is provided by the migrating motor complex (MMC) the motor pattern of the fasted state. In particular the migration of phase 3 of the MMC from stomach to ileum acts as a housekeeper by clearing the lumen of residue and remnants of the previous meal.3 It may also function as a hunger signal.4 The discovery, first in dog5, 6 and then in man,7, 8 that exogenous administration of the peptide motilin induces phase 3 and that endogenous motilin plasma levels rise before the start of phase 3 activity in the stomach was met with great excitement. Clearly motilin was a natural prokinetic worthy of its name. Indeed, because John Brown used the contractile response of the gastric fundus as a bio-assay in the purification of the peptide,9 the term contractin could have been an option as well. In hindsight, motilin was obviously a better choice. It seemed logical to assume that motilin would also accelerate gastric emptying, but the first data were contradictory as acceleration, lack of effect and even retardation were reported.10-13 Meanwhile the limited role of the MMC in pathophysiology, and the difficulty of studying the mechanisms of a phenomenon with long periodicity (about 90 min) requiring multiple blood samples in addition to swallowing multiple catheters, led to a decline in interest in motilin. The situation changed dramatically when it was shown that erythromycin induced the MMC14 acting as a motilin agonist,15 and that erythromycin stimulated gastric emptying.16 The motilin receptor therefore became an interesting pharmacological target for the treatment of gastroparesis. While report after report confirmed the efficacy of erythromycin, more potent derivatives devoid of antibacterial properties were developed and a new class of prokinetics, the motilides, was born.17, 18 Several drug candidates emerged. The first one was EM-523,19 this together with its successor, EM-574,20 quietly left the scene at an early stage likely due to chemical instability and low bio-availability. Different strategies to remove or block a reactive hydroxyl group led to the synthesis of ABT-22921 and GM-611 (now known as mitemcinal).22 The motilide story received a serious drawback when development of ABT-229 was stopped because it failed to relieve symptoms in clinical trials of functional dyspepsia23 and diabetic gastroparesis.24 On the contrary, treatment seemed to have an adverse effect as symptoms worsened and the authors concluded that there was no future for the class of motilin agonists.23, 24 However it was pointed out that several factors associated with the drug (long half-life, tachyphylaxis, possible effect on gastric accommodation) and the study design (selection of patient population) may have contributed to the negative outcome.25 A comment on, and an elaboration of these arguments expressed the hope that these considerations ‘may encourage pharmaceutical companies to reconsider whether the medication or a derivative with improved pharmacokinetics should be given a second chance’.26 The message seems to have been understood as recent presentations and publications show that at least three new motilin agonists have been developed. In 2004 a report appeared describing the first synthetic non-peptide motilin agonist. BM-591348 (identified in the original paper as 11a) is the most potent non-peptide motilin agonist described to date.27 At the Digestive Disease Week in Chicago in May 2005 efforts to develop improved motilides were presented28 and at the World Congress in Montreal in September 2005 KOS-2187 was put forward as a candidate drug.29 Finally, in this issue of Neurogastroenterology & Motility, Park et al.30 present the first clinical study with atilmotin, a peptide analogue and novel motilin agonist. These three compounds represent the three possible strategies for targeting a peptide receptor: a synthetic non-peptide agonist (BM-591348) a non-peptide derived from a natural agonist (KOS-2187), and a peptide analogue (atilmotin). In drug development, synthetic non-peptide agonists are normally the first choice, as natural non-peptide agonists are usually not available and peptides are unpopular because their route of administration and pharmacokinetic properties severely limit their potential use. It may be noted that in the case of motilin the order was reversed. At a very early stage an application for the peptide was explored in postoperative ileus31 and the first effort of the pharmaceutical industry concerned the development of KW-5139, a Leu13 analogue of human motilin produced in large quantities by bio-engineering methods.32 The natural non-peptide agonist erythromycin and its semi-synthetic derivatives the motilides came next, and the first synthetic non-peptide agonist arrived on the scene only very recently. A careful reading of the literature suggests there may soon be other compounds. Atilmotin is a peptide analogue of the 1–14 fragment of human motilin. In atilmotin, the terminal amine of the first residue, Phe, is trimethylated, Arg12 is substituted by its d-isomer, Met13 by Leu and Gln14 by Lys. It was known that replacement of Met13 by Leu did not affect bioactivity33 and because Met13 may be easily oxidized, it has been common practice to replace Met13 in synthetic motilin by either leucine or nor-leucine. The methylation of Phe1 and the replacement of Arg12 by its d-isomer may be assumed to reduce enzymatic degradation, and the plasma half-life of atilmotin is indeed longer (around 10 min) than that of motilin (2–3 min).30 On the contrary, the methylation of Phe1 may also affect the potency, because it is known that this residue is a crucial element of the motilin pharmacophore.34 The replacements in residues 12 and 14 are outside the pharmacophore and probably without effect on potency. This seems to be confirmed by the potency of atilmotin to induce contractions in the rabbit duodenum (pEC50 = 7.72)30 which is not much different from the potency of the unaltered 1–14 fragment (pEC50 = 7.55) but lower than the potency of the full length molecule (pEC50 = 8.13).34 The authors administered 6, 30, 60 μg of atilmotin or vehicle (placebo) three times a day, namely 2 min after standardized breakfast, lunch and dinner. The meals were separated by 4-h intervals and the breakfast meal contained a tracer for the emptying of solids (99m Tc-eggs) and liquid (111 In-milk). Gastric, small intestinal and colonic transit was evaluated by scintigraphy using a method well validated by this group.35 It was found that gastric emptying of liquids and solids was accelerated at 10, 20 and 30 min but not at subsequent time points. For the first 20 min the effect was dose-dependent for both solids and liquids. Gastric half-emptying time was unaffected and there was also no effect on colonic filling or transit. The authors conclude that a further exploration of atilmotin in patients with motility abnormalities is warranted. This is the most careful and complete study ever on the effect of a motilin agonist on gastrointestinal transit. The authors only detected an effect on gastric emptying during the first 30 min, and suggest that during the next 60–90 min there is the opportunity for emptying under placebo to catch up with the initial effect of motilin. This may also explain why no effect was seen on small intestinal and colonic transit. As the authors point out, the short duration of action is most likely due to the short half-life of atilmotin. One may therefore conclude that if atilmotin was able to show a significant effect after a bolus injection, it may prove to be a much more potent prokinetic when given as an infusion over a longer period of time. In fact one may wonder why a bolus injection was selected, since in earlier studies on the effect of motilin on gastric emptying, infusions with durations between 75 and 120 min were used.12, 13, 36, 37 The authors do not explain the rationale for using a bolus injection, but two considerations may have played a role. On the one hand, a bolus of motilin given in the fasted state has a long lasting effect: phase 3 continues its migration in the small intestine long after plasma levels have returned to baseline. The same could have been true in the postprandial state, but their findings show that this is not the case. On the other hand, the authors may have wanted to avoid tachyphylaxis, a phenomenon that has been evoked to explain the failure of ABT-229.25 Still, the half-life of ABT-229 is several hours so that the duration of motilin receptor stimulation following an oral dose of ABT-229 far exceeds the short stimulation provoked by a bolus injection of atilmotin. As the authors note in their protocol tachyphylaxis was unlikely to occur. Whether tachyphylaxis played an important role in the failure of ABT-229 remains to be proven. However, three recent and independent in vitro studies all found that ABT-229 has unusually strong desensitizing properties.28, 38, 39 These studies also show that desensitization is not solely determined by potency as the order of potency to desensitize is ABT-229 > motilin > erythromycin, in contrast to the order of potency to activate the receptor in vitro (duodenum strips or cell lines expressing the motilin receptor) is motilin > ABT-229 > erythromycin. Apparently different structural elements determine receptor affinity and desensitization. For ABT-229 it has been suggested that the modification of the cladinose moiety is the culprit.38 Interestingly, motilin desensitization appears to be linked to the C-terminal end of the molecule.40 Therefore it is possible that atilmotin, which lacks eight C-terminal residues, is less potent than motilin in inducing desensitization. In vitro both KOS-2187 and erythromycin show minimal desensitization,29 while BM-591348's potency to desensitize is positioned between ABT-229 and motilin.39 Atilmotin may therefore behave in vitro as KOS-2187, but in vivo the situation could be even more favourable due to its short half-life. In any case, an exploration of the effect of infusions of atilmotin seems indicated. Compared with other prokinetics, motilin agonists have the advantage that the target receptor is mainly located in the gastric antrum and probably the most important factor contributing to the acceleration of gastric emptying by motilin agonists is the induction or amplification of antral contractions.41, 42 However the motilin receptor is also found in the lower oesophageal sphincter, the gastric fundus, the pylorus and the duodenum. While fundic contraction contributes to acceleration of gastric emptying, it may also reduce gastric accommodation, leading to the worsening of symptoms and this may have contributed to the failure of ABT-229.25 A recent study confirmed that motilin reduces gastric accommodation and increases satiety.43 Apparently the effect of motilin (and of erythromycin) on the gastric fundus is mediated via a smooth muscle receptor,43, 44 the effect on antral contractility via a neural mechanism.42 Therefore agonists specific for the neural receptor may provide a means to avoid the undesirable effect of reducing gastric accommodation. In the rabbit evidence for motilin receptor subtypes has been presented based on binding experiments after subcellular fractionation.45, 46 However there is no evidence that the human genome contains two structurally different receptors and at the mRNA level only a truncated inactive variant of the human receptor has been identified.47 In the rabbit, the potency of motilin to stimulate the neuronal motilin receptor is about one log unit higher than the potency to stimulate the smooth muscle receptor,48, 49 and this is most likely due to different coupling mechanisms rather than to the existence of two receptor subtypes.49 The same situation seems to exist in humans as the dose of erythromycin needed to stimulate neurally-mediated antral contractility is lower than the dose to stimulate atropine-insensitive contractions.42 Therefore selective stimulation of the neuronal pathway may be achievable by careful dosing of the motilin agonist. The observation that therapy with motilin agonists does not necessarily lead to adverse effects may be deduced from the recent clinical trial with mitemcinal, the only survivor of the ‘first generation motilides’. Indeed, while the outcome of this trial was somewhat disappointing, it was not because of the worsening of symptoms, but because symptomatic relief was limited to a small subset of patients.50 The trial under consideration with atilmotin was performed in volunteers and no adverse effects were observed. As the authors point out, patients with diabetic gastroparesis or functional dyspepsia may be more prone to the development of unfavourable symptoms.30 This should be kept in mind when planning clinical trials in patients, and as was suggested earlier by one of the authors, it may be indicated to select patients with only impaired gastric emptying and normal accommodation.26 Note in this respect that the subset of patients with a good response to mitemcinal had a body mass index below 35 kg m−2 and glycosylated haemoglobin levels (HbA1c) smaller than 10%, and that it is known that obesity and poor glycaemic control are risk factors for upper GI symptoms.51 One may also consider enhancing the chance for success by asking patient cooperation, such as voluntary reduction of the size of the meal and adjustment of the type of the meal. In a recent study treatment with low dose erythromycin and low-bulk diet resulted in a ‘dramatic short term improvement in the majority of patients’.52 Atilmotin is a peptide analogue and presently requires administration intravenously. This limits the therapeutic potential to patients for emergency surgery, as an aid in procedures such as endoscopy and duodenal intubation and in the acute treatment of hypomotility syndromes in critically ill patients or after surgery.53 In a recent review of prokinetics, these were basically the indications listed for erythromycin.54 Therefore it seems almost obligatory to develop an orally active formulation to make atilmotin successful, as the antibacterial properties of erythromycin are not a serious contra-indication for acute administration. The new non-peptide agonists and mitemcinal do not have this limitation but it remains to be seen whether they have an improved therapeutic profile compared with erythromycin on oral dosing. In our institution the distance between research laboratories and patient rooms is short, and a sometimes failing elevator, easily avoided by taking the stairs, is the only obstacle between them. For motilin agonists the road from bench to bedside has proven to be particularly tortuous. After so many years there is still no guarantee that the goal of clinical efficacy with motilin agonists can be achieved. However, in view of the possible patient benefit, the scientific community should support efforts to reach that goal by doing as much bench work as possible or, as in the study under consideration, by performing carefully planned clinical trials.