Two lymphoscintigraphic agents compared in vitro and in vivo in rabbit knees

Two 99mTc-labelled colloidal radiopharmaceuticals [99mTc-albumin colloid (A) and 99mTc-rhenium sulfide colloid (B)] were compared in vitro in saline, plasma, and synovial fluid with regard to particle size, radiochemical purity, protein binding level, and stability at varying acidity. The purpose was to compare the two compounds in studies of lymphatic drainage from the knee joint of rabbits. Particle size for A was uniform at about 10 nm, but for B it varied between 40 and 80 nm. The radiochemical purity was lowest for B (90%-95%), with a small fraction of minor particles. B was protein bound, which was only the case for A in joint fluid from synovitic knees. A faster disappearance of A than B was observed from the knee joint, with lesser retention in the regional lymph nodes. A negligible uptake of colloid was seen in the liver. The major fraction of radioactivity absorbed from the knee was excreted in the urine. The differences between in vitro and in vivo stability are probably due to phagocytosis in the synovial membrane.