GISTS: Gastrointestinal Stromal Tumors

Abstract

1664 Objectives A high specific activity (SA) of 111In-labeled tracers will allow administration of low amounts of tracer to prevent receptor saturation and/or side-effects. To increase the SA, we studied the effect of the buffer used during the labeling procedure: NaAc, NH4Ac, HEPES and MES-buffer. The effect of ageing of the 111InCl3 stock and cadmium contamination, the decay product of 111In, was also examined in these buffers. Methods Escalating amounts of 111InCl3 were added to 1 µg of the DTPA- and DOTA-compound (Exendin-3, Octreotide and anti-CAIX antibody, cG250). Five volumes of MES, HEPES, NH4Ac or NaAc (0.1 M, pH 5.5) were added. After 20 min at 20 °C (DTPA-conjugated peptides), at 95 °C (DOTA-Exendin-3 and DOTA-Octreotide) or 45 °C (DOTA-cG250), the labeling efficiency was determined by ITLC. The effect of ageing of the 111InCl3 stock was examined on the labeling efficiency of DTPA-Exendin-3 The effect of escalating concentrations of Cd2+, the decay product of 111In, on labelling efficiency of DTPA-Exendin-3 was determined. Results Specific activities obtained for DTPA-Octreotide and DOTA-cG250 were 5 times higher in MES and HEPES buffer. Radiolabeling of DTPA-Exendin-3, DOTA-Exendin-3 and DTPA-cG250 in MES and HEPES buffer resulted in two times higher specific activities than in NaAc and NH4Ac. Labeling of DTPA-Exendin-3 decreased with 66% and 73% for NaAc and NH4Ac, respectively at day 12 after the production date of 111InCl3, while for MES and HEPES the maximal decrease in specific activity was 10% and 4% at day 12, respectively. The presence of 0.1 nmol Cd2+ in the labeling mixture of DTPA-Exendin-3 in NaAc and NH4Ac markedly reduced labeling efficiency, whereas Cd2+ concentrations up to 100 nmol did not affect the labeling efficiency in MES and HEPES buffer. Conclusions In conclusion, we showed improved labeling of DTPA- and DOTA-conjugated compounds with 111In in HEPES and MES buffer. The enhanced labeling efficiency could be due to the reduced competitive chelation of cadmium