Comparison of the metabolism of iron labeled transferrin (Fe TF) and indium labeled transferrin (In TF) by the erythropoietic marrow

This study compared the metabolism of In TF and Fe TF by the erythropoietic marrow. There were marked differences observed between the biologic behavior of these two metals bound to transferrin in all parameters studied. These include: slower plasma clearance of In TF; significantly greater percentage of ⁵⁹Fe incorporation in the circulating red cells with only minimal labeling of peripheral red cells with indium and rapid disappearance of the indium labeled cells from the circulation; significantly greater accumulation of ⁵⁹Fe in the skeleton; irradiated rabbits' legs showed the expected bone and bone marrow decrease in the uptake of ⁵⁹Fe transferrin at 48 hr following 500 rads to the right tibia fibula unit but no change in ¹¹¹In TF or ⁹⁹Tc(m) sulfur colloid uptake; in the control nonirradiated legs, 60% of the ⁵⁹Fe but only 20% of ¹¹¹In was in the extracted marrow, indicating that the bone concentration of indium was greater than that of ⁵⁹Fe (P < 0.001); a more rapid uptake of ⁵⁹Fe TF by reticulocyte rich blood which at 60 min was nine times that of In TF. These results suggest that the metabolism of indium is significantly different from that of iron with respect to erythropoiesis. The use of ¹¹¹In TF as a bone marrow scanning agent must be approached with caution in view of the high indium to iron ratio in normal bone and the unchanged accumulation of indium by bone marrow in which the erythroid marrow has been damaged by irradiation.