Comparative evaluation of red cell-labeling parameters of three lipid-soluble- 111In-chelates: effect of lipid solubility on membrane incorporation and stability constant on transchelation.

A rabbit red cell model was used to determine the cell labeling properties of three lipid-soluble 111In-complexes: 111In-oxine, 111In-acetylacetone, and 111In-tropolone. Partition coefficients (olive oil/buffer) were measured to determine the lipid solubility and were 3.54, 7.93, and 18.18 for 111In-oxine, 111In-acetylacetone, and 111In-tropolone respectively. The effect of the concentration of these three chelating agents on labeling efficiencies was studied. The factors influencing the labeling efficiencies of these complexes such as cell density, time of incubation, influence of temperature, pH, effect of plasma proteins, and citrate ion concentration in the cell-labeling medium were studied. Labeling yields as high as 95.15 +/- 4.15% were achieved with 111In-tropolone after a 10-min incubation at 37 degrees C. The optimum pH for cell labeling was 6.5. Excess citrate ion (greater than 3.02 mg/ml) and small amounts of plasma proteins (greater than 10 microliter/ml) decreased the labeling efficiencies in all three cases. Distribution of these 111In-complexes in membrane, membrane fragments, and hemoglobin was studied after hemolysis. In spite of the higher lipid solubility of 111In-tropolone, the transchelation capacity appears to be similar to that of 111In-oxine. 111In-acetylacetone had the highest transchelation capacity.