Transport, binding, and uptake kinetics of tin and technetium in the in-vitro Tc-99m labeling of red blood cells

These studies were undertaken to define the mechanisms involved in the BNL kit methods for labelling red blood cells (RBC) in vitro with Tc-99m. The studied systems included the widely used method requiring plasma separation prior to incubation of ''tinned'' cells with pertechnetate, and a newer method that enables specific labeling of RBC in whole blood and uses chemical inactivation of excess tin in the plasma thus eliminating the need for cell separation prior to the addition of pertechnetate. The following were investigated in depth: 1) kinetics of Sn(II) uptake by RBC using above (stannous citrate) its; 2) kinetics of Tc-99m uptake by ''tinned'' RBC; 3) role of oxidation and chelation in removal of extracellular Sn(II); 4) effect of plasma and other suspending media; 5) sites of binding of Sn(II) and Tc-99m within the RBC. Only Sn(II), and not Sn(IV), was taken up by RBC. The uptake was initially rapid, then asimtotic at <40% after 30 min. The uptake was irreversible and dependent on tin concentration, citrate concentration, plasma, and temperature. Technetium uptake by tinned RBC in blood exhibited similar kinetics but the asimtote was at >90%. Evidence was found for a plasma-bound Sn(II) species that resists oxidation but is slowly dissociable. Chelants such as EDTA or citrate compete successfully for plasma-bound tin and render it oxidizable. Both Sn(II) and Tc-99m bind predominantly to hemoglobin within the cell. The membrane did not appear to be the limiting factor in uptake rates