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

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 (¹⁶¹Tb) 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 ¹⁶¹Tb, 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 Y₁ receptor (hY₁R).

By using solid-phase peptide synthesis, the hY₁R-preferring [F⁷,P³⁴]-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 (^natTb), as surrogate for ¹⁶¹Tb, or with indium-111 (¹¹¹In).

[^natTb]Tb-pb12 showed a preserved high binding affinity to endogenous hY₁R on MCF-7 cells and was able to induce receptor activation and internalization similar to the hY₁R-preferring [F⁷,P³⁴]-NPY. Specific internalization of the ¹¹¹In-labeled conjugate into MCF-7 cells was observed, and importantly, time-dependent nuclear uptake of ¹¹¹In was demonstrated. Study of metabolic stability showed that the peptide is insufficiently stable in human plasma. This was confirmed by injection of [¹¹¹In]In-pb12 in nude mice bearing MCF-7 xenograft which showed specific uptake only at very early time point.

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.