Cytotoxicity of s-Receptor Ligands Is Associated with Major Changes of Cellular Metabolism and Complete Occupancy of the s-2 Subpopulation

Tumor cells can be selectively killed by application of s-ligands; high concentrations (20–100 mM) are often required, however, because either diffusion barriers must be passed to reach intracellular sites or the entire s-receptor population should be occupied to induce cell death. We measured receptor occupancies associated with the cytotoxic effect and dose-dependent changes of cellular metabolism in a tumor cell line. Methods: C6 cells (rat glioma) were grown in monolayers and exposed to (1)-pentazocine (s-1 agonist), AC915 (s-1 antagonist), rimcazole (s-1/s-2 antagonist), or haloperidol (s-1/s-2 antagonist). Occupancy of s-receptors by the test drugs was measured by studying the competition of the test drugs with cellular binding of the ligand 11C-SA4503. Metabolic changes were quantified by measuring cellular uptake of 18F-FDG, 18F-FLT, 11C-choline, or 11Cmethionine. Cytotoxicity was assessed by cellular morphology observation and cell counting after 24 h. Results: IC50 values (drug concentrations resulting in 50% occupancy of the available binding sites) of the test drugs for inhibition of cellular 11C-SA4503 binding were 6.5, 7.4, 0.36, and 0.27 mM for (1)-pentazocine, AC915, rimcazole, and haloperidol, respectively. EC50 values (dose required for a 50% reduction of cell number after 24 h) were 710, 819, 31, and 58mM, for pentazocine, AC915, rimcazole, and haloperidol, respectively. Cytotoxic doses of s-ligands were generally associated with increased uptake of 18F-FDG, decreased uptake of 18F-FLT and 11C-choline, and little change in 11C-methionine uptake per viable cell. Conclusion: IC50 values of the test drugs reflect their in vitro affinities to s-2 rather than tos-1 receptors. Because cytotoxicity occurred at concentrations 2 orders of magnitude higher than IC50 values for inhibition of cellular 11C-SA4503 binding, high (99%) occupancy of s-2 receptors is associated with loss of cell viability. 18F-FLT, 11C-choline, and 18F-FDG responded most strongly to drug treatment and showed changes corresponding to the cytotoxicity of the test compounds.