Gastric water emptying as a critical parameter for oral drug absorption can be investigated by several imaging techniques or by the interpretation of pharmacokinetics of appropriate substances. Recently introduced salivary caffeine kinetics is a valuable tool, but the required caffeine abstinence limits its applicability. To avoid the caffeine abstinence, stable isotope-labeled caffeine might be used, but the representability and transferability of kinetics for evaluation of gastric emptying must be demonstrated. Thus, salivary caffeine pharmacokinetics were compared for naturally occurring 12C-caffeine and 13C3-caffeine after the administration of water under fasting conditions in six healthy young subjects. For this purpose, an ice capsule containing the two caffeine species was administered with 50 mL tap water. Gastric water emptying was simultaneously quantified using magnetic resonance imaging (MRI). Gastric emptying of 50 mL of water could be successfully evaluated. The salivary caffeine kinetics of 13C3- and 12C-caffeine were nearly congruent and showed good linear correlations in all subjects, with a mean correlation coefficient of 0.96 in pooled data. Thus, the substitution of natural 12C caffeine with stable isotope-labeled 13C3-caffeine offers the opportunity for broader application of the salivary caffeine gastric emptying technique and increases the robustness of the method against environmental contamination with caffeine.
Keywords: UNGAP; caffeine; gastric emptying; magnetic resonance imaging (MRI); saliva tracers; stable isotopes.