Activated charcoal as a potential radioactive marker for gastrointestinal studies

The scintigraphic measurement of colonic transit is currently performed using ¹¹¹ In ion exchange resin pellets delivered to the colon in a capsule coated with a pH sensitive polymer, methacrylate, which dissolves in the distal ileum. However, in the USA, this requires an investigational drug permit. Our aim was to evaluate the in vitro binding characteristics of activated charcoal in milieus that mimicked gastric and small intestinal content. The in vitro incubation of activated charcoal was performed with Na ⁹⁹Tc^mO₄, ⁹⁹Tc^m-DTPA, ¹¹¹ InCl₃, ¹¹¹ In-DTPA, ²⁰¹TlCl and ⁶⁷Ga-citrate in the pH range 2–4 and pH 7.2 at 37$$C. We estimated theassociation of radiopharmaceuticals with the activated charcoal over a 3 h in vitro incubation. With the exception of ⁶⁷Ga-citrate, the association of activated charcoal with the other radiopharmaceuticals was approximately 100% throughout the 3 h incubation. In conclusion, activated charcoal appears to adsorb avidly with common radioisotopes, and appears promising as an alternative to resin ion exchange pellets used for the measurement of gastrointestinal transit by scintigraphy.