Synthesis of Lu-177-Labeled, Somatostatin-2 Receptor-Targeted Metalla-Assemblies: Challenges in the Design of Supramolecular Radiotherapeutics

Self-assembled supramolecular coordination complexes(SCCs) holdpromise for biomedical applications in cancer therapy, although theirpotential in the field of nuclear medicine is still substantiallyunexplored. Therefore, in this study an exo-functionalizedcationic [Pd2L2]⁴⁺ metallacycle (L= 3,5-bis(3-ethynyl-pyridine)-phenyl), targeted to the somatostatin-2receptor (sst2R) and featuring the DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaceticacid) in order to bind the & beta;^-- and & gamma;-emitterlutetium-177, was synthesized by self-assembly following ligand synthesisvia standard solid-phase peptide synthesis (SPPS). This metallacyclewas then characterized by reverse-phase high-performance liquid chromatography(RP-HPLC), electrospray ionization mass spectrometry (ESI-MS), and ¹H and ¹H-DOSY NMR (DOSY = diffusion-ordered spectroscopy).A procedure for the radiolabeling of the metallacycle with ¹⁷⁷Lu was also optimized. The resulting [^nat/177Lu]Lu-DOTA-metallacycle,termed [^nat/177Lu]Lu-Cy, was evaluated concerningits stability and in vitro properties. The compoundwas more lipophilic compared to the reference [¹⁷⁷Lu]Lu-DOTA-TATE(logP Oct/H2O = -0.85 & PLUSMN; 0.10 versus -3.67 & PLUSMN; 0.04, respectively). While[^natLu]Lu-Cy revealed low stability in a DMEM/F12GlutaMax medium, it demonstrated good stability in other aqueous mediaas well as in DMSO. A high sst2R binding affinity (expressed as IC50) was determined in CHOsst2 cells (Chinese hamsterovary cells that were stably transfected with human sst2R). Moreover,the metallacycle exhibited high human serum albumin binding, as assessedby high-performance affinity chromatography (HPAC), and moderate stabilityin human serum compared to [¹⁷⁷Lu]Lu-DOTA-TATE (TATE =(Tyr³)-octreotate). In order to improve stability, a heterolepticapproach was used to develop a less sterically hindered cage-likeSCC that is potentially endowed with host-guest chemistry capability,which has been preliminarily characterized by RP-HPLC and ESI-MS.Overall, our initial results encourage future studies on sst2R-directedSCCs and have led to new insights into the chemistry of ss2R-directedSCCs for radiopharmaceutical applications. To broaden the scope of two-dimensionalmetalla-assembliesand to explore their application in nuclear medicine, a [Pd2L2](4+) metallacycle, functionalized with thetherapeutic nuclide lutetium-177 and targeted to the somatostatinreceptors of cancer cells, was designed and fully characterized. Thisstudy provides the first insight into the in vitro behavior and stability of this new family of radiolabeled supramolecules.