Design, synthesis, and biological evaluation of a multifunctional neuropeptide-Y conjugate for selective nuclear delivery of radiolanthanides

Background Targeting G protein-coupled receptors on the surface of cancer cells with peptide ligands is a promising concept for the selective tumor delivery of therapeutically active cargos, including radiometals for targeted radionuclide therapy (TRT). Recently, the radiolanthanide terbium-161 ( 161 Tb) gained significant interest for TRT application, since it decays with medium-energy β-radiation but also emits a significant amount of conversion and Auger electrons with short tissue penetration range. The therapeutic efficiency of radiometals emitting Auger electrons, like 161 Tb, can therefore be highly boosted by an additional subcellular delivery into the nucleus, in order to facilitate maximum dose deposition to the DNA. In this study, we describe the design of a multifunctional, radiolabeled neuropeptide-Y (NPY) conjugate, to address radiolanthanides to the nucleus of cells naturally overexpressing the human Y 1 receptor (hY 1 R). By using solid-phase peptide synthesis, the hY 1 R-preferring [F 7 ,P 34 ]-NPY was modified with a fatty acid, a cathepsin B-cleavable linker, followed by a nuclear localization sequence (NLS), and a DOTA chelator (compound pb12). In this proof-of-concept study, labeling was performed with either native terbium-159 ( nat Tb), as surrogate for 161 Tb, or with indium-111 ( 111 In). Results [ nat Tb]Tb-pb12 showed a preserved high binding affinity to endogenous hY 1 R on MCF-7 cells and was able to induce receptor activation and internalization similar to the hY 1 R-preferring [F 7 ,P 34 ]-NPY. Specific internalization of the 111 In-labeled conjugate into MCF-7 cells was observed, and importantly, time-dependent nuclear uptake of 111 In was demonstrated. Study of metabolic stability showed that the peptide is insufficiently stable in human plasma. This was confirmed by injection of [ 111 In]In-pb12 in nude mice bearing MCF-7 xenograft which showed specific uptake only at very early time point. Conclusion The multifunctional NPY conjugate with a releasable DOTA-NLS unit represents a promising concept for enhanced TRT with Auger electron-emitting radiolanthanides. Our research is now focusing on improving the reported concept with respect to the poor plasmatic stability of this promising radiopeptide.