Species Differences in the Control of Gastric Motility
Species differences, pro-kinetic gastric activity of drugs and patient compliance: important considerations during drug development?
Human tissues, rather than animal models, are increasingly being used to predict drug effects on the gastrointestinal system, but is there evidence that species differences exist and are an important consideration during preclinical development?
As many readers of this article will be aware, the antibiotic erythromycin can have some extremely unpleasant gastrointestinal side-effects that may limit its use, in particular its propensity to cause debilitating stomach cramps. Thankfully, there are often alternative antibiotics that can be prescribed, but for companies developing new orally-delivered drugs (i.e. most drugs!), such side-effects are clearly something to be avoided. The Alzheimer’s drug galantamine is another oral medication that can cause such severe gastrointestinal side-effects that patients may need to stop taking their medication. Can such effects be reliably predicted in animal models or is there a need to understand how responses in animal models might translate to humans at an early (preclinical) stage of development?
Let’s first explore erythromycin’s mechanism of action. Erythromycin causes stomach cramps via activation of motilin receptors that are present in gastric smooth muscle; it acts as a pro-kinetic agent that stimulates gastric emptying1. The motilin receptor has been postulated to play an important role in gastrointestinal disorders2. Dass et al. hypothesised that low doses of motilin or erythromycin would facilitate neuronal function increasing gastric motility and exerting therapeutic benefits2 however at higher doses, when used as an antibiotic (typical dose of 500 or 1000 mg) many patients experience severe adverse effects.
We set out to explore the pharmacology of erythromycin in vitro using human isolated stomach tissues and to compare human responses with the functional effects of motilin receptor activation across species. Strips of fresh isolated human stomach were set up in tissue baths for measurements of force production. To more closely mimic the mechanism of smooth muscle contraction in vivo, which is primarily under the influence of the myenteric plexus, the contractions were evoked by trains of electrical field stimulation to activate nerves within this plexus. Such stimulation then leads to contractions of the stomach strips, apparent as a rhythmic series of contractions, as shown in figure 1. Erythromycin was then added in a cumulative manner (on top of the electrically-evoked contractions) to explore the concentration-response relationship between baseline tone (i.e. the baseline tension in the tissues between electrical field stimulation), the amplitude of the electrically-evoked contractions and any change in the duration of the responses (transient versus sustained changes in contractility).
The typical responses to erythromycin in human isolated stomach strips can be seen in figure 1.
Figure 1a. A typical oral dose of erythromycin is 500 or 1000 mg; the resultant plasma concentration (free erythromycin) is in the range of 0.1 to 1 mg/ml, which is roughly equivalent to the concentrations shown in the shaded area of the above data set. There is a clear effect on motility at a tissue bath concentration of 1 mM erythromycin.
Figure 1b. A typical oral dose of erythromycin is 500 or 1000 mg; concentration present in the stomach fluid (fasted volume of 25-40 ml; fed volume 250-400ml) is in the range of 1 to 10 mg/ml, which is approximately equivalent to the concentrations shown in the shaded area of the above data set. There is a pronounced effect on motility at a tissue bath concentration of 10 and 100 mM erythromycin.
In contrast to the prolonged increase in the amplitude of contractions observed in human stomach strips, rabbit stomach strips produced an increase in the amplitude of contractions that was transient rather than sustained (figure 2). This is a similar response to that seen by Jarvie et al. . In both cases the response had a half life of around 7 minutes and returned to baseline amplitude within 15 minutes; therefore, while a clear pro-kinetic effect could be observed, the duration of action differed markedly between rabbits and humans.
Figure 3. Raw data trace showing the addition of 1 mM erythromycin to a rabbit smooth muscle strip that was being electrically simulated by electrical field stimulation; a pronounced but transient increase in the amplitude of contractions was observed.
Erythromycin caused a dose-dependent increase in the amplitude of the electrically-evoked constriction of rabbit stomach circular smooth muscle at concentrations of 10 nM to 10 mM. At concentrations of erythromycin higher than 10 mM the electrically stimulated contractions were inhibited suggesting that erythromycin activates other pathways.
In guinea pig stomach, motilin agonists appeared to have an effect not only on the electrically-evoked contractions of the stomach smooth muscle but also a direct effect on baseline tone of the smooth muscle (figure 4).
Figure 4 Representative trace of the effects of motilin in guinea pig stomach circular smooth muscle.
The table below highlights the marked differences in response to erythromycin between various species. Rabbit stomach is perhaps the closest to human, but even here the nature of the response is quite different, with transient increases in the force of contraction and a second mechanisms. Clearly, using human stomach preparations is preferable and would avoid such concerns about species differences.
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