Distinct In Vitro Binding Profile of the Somatostatin Receptor Subtype 2 Antagonist [Lu-177]Lu-OPS201 Compared to the Agonist [Lu-177]Lu-DOTA-TATE

Treatment of neuroendocrine tumours with the radiolabelled somatostatin receptor subtype 2 (SST2) peptide agonist [Lu-177]Lu-DOTA-TATE is effective and well-established. Recent studies suggest improved therapeutic efficacy using the SST2 peptide antagonist [Lu-177]Lu-OPS201. However, little is known about the cellular mechanisms that lead to the observed differences. In the present in vitro study, we compared kinetic binding, saturation binding, competition binding, cellular uptake and release of [Lu-177]Lu-OPS201 versus [Lu-177]Lu-DOTA-TATE using HEK cells stably transfected with the human SST2. While [Lu-177]Lu-OPS201 and [Lu-177]Lu-DOTA-TATE exhibited comparable affinity (K-D, 0.15 +/- 0.003 and 0.08 +/- 0.02 nM, respectively), [Lu-177]Lu-OPS201 recognized four times more binding sites than [Lu-177]Lu-DOTA-TATE. Competition assays demonstrated that a high concentration of the agonist displaced only 30% of [Lu-177]Lu-OPS201 bound to HEK-SST2 cell membranes; an indication that the antagonist binds to additional sites that are not recognized by the agonist. [Lu-177]Lu-OPS201 showed faster association and slower dissociation than [Lu-177]Lu-DOTA-TATE. Whereas most of [Lu-177]Lu-OPS201 remained at the cell surface, [Lu-177]Lu-DOTA-TATE was almost completely internalised inside the cell. The present data identified distinct differences between [Lu-177]Lu-OPS201 and [Lu-177]Lu-DOTA-TATE regarding the recognition of receptor binding sites (higher for [Lu-177]Lu-OPS201) and their kinetics (faster association and slower dissociation of [Lu-177]Lu-OPS201) that explain, to a great extent, the improved therapeutic efficacy of [Lu-177]Lu-OPS201 compared to [Lu-177]Lu-DOTA-TATE.