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13C-urea may be a suitable marker to assess the in vivo fate of colon-targeted dosage forms given by mouth. We postulated that release in the colon (urease-rich segment) of 13C-urea from colon-targeted capsules would lead to fermentation of 13C-urea by bacterial ureases into 13CO2. Subsequent absorption into the blood and circulation would lead to detectable 13C (as 13CO2) in breath. If, however, release of 13C-urea occurred in the small intestine (urease-poor segment), we expected detectable 13C (as 13C-urea) in blood but no breath 13C (as 13CO2). The differential kinetics of 13C-urea could thus potentially describe both release kinetics and indicate the gastrointestinal segment of release.The in vivo study consisted of three experiments, during which the same group of four volunteers participated.The kinetic model was internally valid. The appearance of 13C-in breath CO2 (Ffermented) and the appearance of 13C in blood as 13C-urea (Fnot fermented) show a high inverse correlation (Pearson's r=−0.981, P= 0.06). The total recovery of 13C (Ffermented+Fnot fermented) averaged 99%, indicating complete recovery of the administered 13C via breath and blood. 13CO2 exhalation was observed in all subjects. This indicates that 13C-urea was available in urease-rich segments, such as the caecum or colon.In this proof-of-concept study, 13C-urea was able to provide information on both the release kinetics of a colon-targeted oral dosage form and the gastrointestinal segment where it was released.