Improved labelling of DTPA- and DOTA- conjugated peptides and antibodies with In in HEPES and MES buffer

Background: In single photon emission computed tomography [SPECT], high specific activity of In-labelled tracers will allow administration of low amounts of tracer to prevent receptor saturation and/or side effects. To increase the specific activity, we studied the effect of the buffer used during the labelling procedure: NaAc, NH4Ac, HEPES and MES buffer. The effect of the ageing of the InCl3 stock and cadmium contamination, the decay product of In, was also examined in these buffers. Methods: Escalating amounts of InCl3 were added to 1 μg of the diethylene triamine pentaacetic acid [DTPA]and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid [DOTA]-conjugated compounds (exendin-3, octreotide and anti-carbonic anhydrase IX [CAIX] antibody). Five volumes of 2-(N-morpholino)ethanesulfonic acid [MES], 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid [HEPES], NH4Ac or NaAc (0.1 M, pH 5.5) were added. After 20 min at 20°C (DTPA-conjugated compounds), at 95°C (DOTA-exendin-3 and DOTA-octreotide) or at 45°C (DOTA-anti-CAIX antibody), the labelling efficiency was determined by instant thin layer chromatography. The effect of the ageing of the InCl3 stock on the labelling efficiency of DTPA-exendin-3 as well as the effect of increasing concentrations of Cd (the decay product of In) were also examined. Results: Specific activities obtained for DTPA-octreotide and DOTA-anti-CAIX antibody were five times higher in MES and HEPES buffer. Radiolabelling of DTPA-exendin-3, DOTA-exendin-3 and DTPA-anti-CAIX antibody in MES and HEPES buffer resulted in twofold higher specific activities than that in NaAc and NH4Ac. Labelling of DTPA-exendin-3 decreased with 66% and 73% for NaAc and NH4Ac, respectively, at day 11 after the production date of InCl3, while for MES and HEPES, the maximal decrease in the specific activity was 10% and 4% at day 11, respectively. The presence of 1 pM Cd in the labelling mixture of DTPA-exendin-3 in NaAc and NH4Ac markedly reduced the labelling efficiency, whereas Cd concentrations up to 0.1 nM did not affect the labelling efficiency in MES and HEPES buffer. Conclusions: We showed improved labelling of DTPAand DOTA-conjugated compounds with In in HEPES and MES buffer. The enhanced labelling efficiency appears to be due to the reduced competitive chelation of cadmium. The enhanced labelling efficiency will allow more sensitive imaging of the biomarkers with SPECT.