Radiolabelling of egg meals for gastric emptying studies: a comparison of 99mTc sulfur and 99mTc stannous colloids

OBJECTIVES: To establish whether 99mTc stannous (Tc-Sn) colloid is a suitable alternative to 99mTc sulfur (Tc-S) colloid for gastrointestinal studies, we compared the per cent binding to egg solids (%BS) and radiochemical purity (RCP) of both colloids in digesting media. METHODS: Egg white and yolk containing colloids were cooked separately and mashed. Samples of 4-5 g were digested over 2-3 h (37 degrees C) in excess simulated gastric juice (SGJ: 15 ml of 0.1 M HCl and 0.5 g.l-1 pepsin) or water, centrifuged, imaged with a gamma camera and the %BS computed. RCP was determined in aspirates taken from these preparations and from solutions of colloid added directly to SGJ. RESULTS: The %BS in egg white after 3 h in SGJ for both colloids were similar: Tc-Sn, 62+/-8 (n=12); Tc-S, 61+/-6 (n=8), but markedly lower than 95% (the often quoted literature value). Egg yolk was digested more rapidly than egg white: %BS after 2 h in SGJ for Tc-Sn colloid was 55+/-10 (n=5) in the yolk, compared to 77+/-4 (n=5) in the white (P<0.01). The RCP for Tc-S colloid alone in SGJ was >94% over 3 h but for Tc-Sn colloid was as low as 14%. For egg white, the RCP in SGJ was 91-96% for Tc-S and 80-91% for Tc-Sn. For egg yolk the RCPs in SGJ were similar for both colloids (>90%). The RCP in water digesting egg white or yolk containing either colloid was always lower than in the corresponding SGJ aspirates, indicating a leakage of small amounts of non-colloidal 99mTc. CONCLUSIONS: Although 99mTc-Sn almost completely dissociates in SGJ, once cooked in egg it is digested similarly to 99mTc-S. Variations in the size of digesting egg fragments and in SGJ composition can reconcile the lower %BS values obtained with previously reported higher results. 99mTc-Sn colloid cooked in egg appears suitable for gastrointestinal studies.