TRANSPORT OF GA-67 AND IN-111 ACROSS A MEMBRANE - ROLE OF PLASMA-BINDING AND CONCENTRATION GRADIENTS

If i.v. injected radiogallium does not firmly bind to circulating transferrin in plasma, the transvascular transport of the radionuclide may be affected by diffusion of unbound radionuclide and interstitial transferrin, and may therefore not be specific for transferrin transport. Using equilibrium dialysis, the avidity of Ga-67 and In-111 to bind to plasma transferrin was compared by evaluating transport of unbound radionuclide across a semipermeable membrane, as a function of the plasma (transferrin) concentration gradient. One chamber of a dialysis cell was filled with human plasma and Ga-67 or In-111, and the other, separated by the membrane, with increasing concentrations of plasma in normal saline. The half-time (mean +/- S.D.) for reaching equilibrium between the chambers decreased (P < 0.05) for Ga-67 from 13.1 +/- 3.3 to 4.9 +/- 1.6, 3.7 +/- 1.2 and 3.0 +/- 0.5 h, at plasma concentrations of 0, 25, 50 and 100%, respectively. The equilibrium half-time for In-111 did not fall at increasing plasma concentrations and was higher (P < 0.005) than for Ga-67 at 25 to 100% plasma. Hence, Ga-67 is not as avidly bound to plasma transferrin as In-111, since, in contrast to In-111, dissociation of the Ga-67-transferrin complex and transmembrane diffusion of unbound Ga-67 increases when the plasma transferrin concentration gradient decreases. In vivo, the transvascular transport of Ga-67 may be influenced by diffusion of unbound radionuclide, depending on the interstitial transferrin concentration, and may not be a specific indicator of transvascular protein transport.