Proof-of-concept study on the suitability of 13C-urea as a marker substance for assessment of in vivo behaviour of oral colon-targeted dosage forms.

BACKGROUND AND PURPOSE (13)C-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 (13)C-urea from colon-targeted capsules would lead to fermentation of (13)C-urea by bacterial ureases into (13)CO(2). Subsequent absorption into the blood and circulation would lead to detectable (13)C (as (13)CO(2)) in breath. If, however, release of (13)C-urea occurred in the small intestine (urease-poor segment), we expected detectable (13)C (as (13)C-urea) in blood but no breath (13)C (as (13)CO(2)). The differential kinetics of (13)C-urea could thus potentially describe both release kinetics and indicate the gastrointestinal segment of release. EXPERIMENTAL APPROACH The in vivo study consisted of three experiments, during which the same group of four volunteers participated. KEY RESULTS The kinetic model was internally valid. The appearance of (13)C-in breath CO(2) (F(fermented)) and the appearance of (13)C in blood as (13)C-urea (F(not fermented)) show a high inverse correlation (Pearson's r=-0.981, P= 0.06). The total recovery of (13)C (F(fermented)+F(not fermented)) averaged 99%, indicating complete recovery of the administered (13)C via breath and blood. (13)CO(2) exhalation was observed in all subjects. This indicates that (13)C-urea was available in urease-rich segments, such as the caecum or colon. CONCLUSIONS AND IMPLICATIONS In this proof-of-concept study, (13)C-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.