A new approach to labeling cells with technetium-99m, Part II. Evaluation of radiolabeled human-lymphocyte immune functions.

Technetium-99m-modified polylysine (MPL) is a mild and efficient method for cell labeling that is easily applicable to human lymphocytes. 99mTc-MPL uptake is maximal in 40 min at room temperature. Cell labeling efficiency (from 60 to 80%) increases with rising concentrations of cells and labeling agent. In vitro stability of 99mTc-MPL on lymphocytes incubated in serum at 37 degrees C is high. Microautoradiography indicates that 99mTc-MPL is equally distributed within the cells. The radiolabeling procedure does not alter expression of surface receptors involved in T lymphocyte effector functions, adhesion function, cytolytic activity or proliferative response to IL-2.

A new approach to labeling cells with technetium-99m, Part I. Preparation of modified polylysine and in vitro cell labeling.

A method for labeling cells with technetium-99m via hydrophilic, polycationic poly D-lysine modified by N-acetyl homocysteine has been developed. The modified polylysine (MPL) is labeled with 99mTc in > 95% yield and is stable for > 12 h. Maximum cell labeling is achieved by a 1-h incubation at room temperature with isolated leukocytes, granulocytes and peripheral blood mononuclear cells attaining 60-75% 99mTc incorporation, and red blood cells 35%. Ninety-two percent of the label is retained by leukocytes after a 1-h incubation at room temperature in 50% serum. The cell uptake of 99mTc-MPL is affected by the presence of negatively charged species in the medium; the inhibitory effects of 5% serum or serum albumin can be reversed by increasing the concentration of 99mTc-MPL, while those of heparin are not.

Uptake of the cation hexakis(2-methoxyisobutylisonitrile)-technetium-99m by human carcinoma cell lines in vitro.

Uptake of the cationic compound hexakis(2-methoxyisobutylisonitrile)-technetium-99m ([99mTc]MIBI) was examined in nine human tumor cell lines. The concentration of [99mTc]MIBI after a 1-h incubation with the compound varies from 5 to 28% of the activity in the external medium. In contrast, normal V79 cells (Chinese hamster lung fibroblasts) and human peripheral blood mononuclear cells exhibit a minimal uptake of less than 2% of the activity in the medium. Kinetic experiments with SW-13 cells indicate a rapid uptake over time (t1/2 of 10 min) until a steady state is approached whose concentration appears directly correlated with the extracellular concentration of [99mTc]MIBI with no evidence of saturation over the range tested (10(-12)-10(-9) M). [99mTc]MIBI is taken up by a temperature dependent process that is restricted to living cells. Microautoradiography demonstrates that [99mTc]MIBI is clustered in the cytoplasm around the nucleus. Given that depolarizing the plasma membrane potential in high K+ buffer results in lowering the uptake of [99mTc]-MIBI and that alteration of the mitochondrial membrane potential with valinomycin or nigericin induces, respectively, a significant decrease or increase of [99mTc]MIBI uptake, we propose that the plasma and mitochondrial membrane potentials play a major role in the uptake. These data suggest that the gamma emitter [99mTc]MIBI exhibits interesting tumor cell interaction characteristics with promise for in vivo tumor imaging.