Application of a combined effect compartment and binding model for gastric acid inhibition of AR-HO47108: a potassium competitive acid blocker, and its active metabolite AR-HO47116 in the dog.

The effect of AR-HO47108, a potassium competitive acid blocker, and its active metabolite AR-HO47116 was studied in Heidenhain pouch dogs following administration of single oral and intravenous doses of the two compounds. The histamine-stimulated acid secretion was measured in different periods after dose up to 24h. All data obtained in the different studies was pooled and analyzed by non-linear mixed effects modelling. It was found that there is a delay between the plasma concentration-time peak and the maximum inhibitory effect and that the effect persisted longer than anticipated from the plasma concentration half-lives of the compounds. In addition, it was found that the peak effect was reached earlier at higher doses. The effect data was well described by a combined effect compartment and binding model and both distribution to the biophase i.e. the canaliculus of the parietal cell and a rate limiting binding interaction between drug and enzyme appear to contribute to the observed delay. In addition, a secretion rate dependent washout from the biophase may contribute. Furthermore, because the parent compound and metabolite bind to the same enzyme, the effect is determined by competition between the two for the same enzyme. The metabolite was found to be less potent than the parent compound, with Kd values of the combined model of 125 and 11.2 nM for the metabolite and parent compound, respectively. However, the metabolite is generated in high concentrations that rapidly exceed the concentration of parent compound after oral administration of parent compound, and this together with its longer plasma half-life will make its contribution to the overall effect increase with time and slightly prolong the duration of the effect.