Synthesis of <sup>177</sup>Lu-Labeled, Somatostatin-2 Receptor-Targeted Metalla-Assemblies: Challenges in the Design of Supramolecular Radiotherapeutics
Sandra Deiser, Marike Drexler, Guillermo Moreno‐Alcántar, Maximilian Irl, Claudia Schmidt, Thomas Günther, Angela Casini
Abstract
Self-assembled supramolecular coordination complexes (SCCs) hold promise for biomedical applications in cancer therapy, although their potential in the field of nuclear medicine is still substantially unexplored. Therefore, in this study an exo -functionalized cationic [Pd 2 L 2 ] 4+ metallacycle (L = 3,5-bis(3-ethynylpyridine)phenyl), targeted to the somatostatin-2 receptor (sst2R) and featuring the DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) in order to bind the β – - and γ-emitter lutetium-177, was synthesized by self-assembly following ligand synthesis via standard solid-phase peptide synthesis (SPPS). This metallacycle was then characterized by reverse-phase high-performance liquid chromatography (RP-HPLC), electrospray ionization mass spectrometry (ESI-MS), and 1 H and 1 H-DOSY NMR (DOSY = diffusion-ordered spectroscopy). A procedure for the radiolabeling of the metallacycle with 177 Lu was also optimized. The resulting [ nat/177 Lu]Lu-DOTA-metallacycle, termed [ nat/177 Lu]Lu- Cy, was evaluated concerning its stability and in vitro properties. The compound was more lipophilic compared to the reference [ 177 Lu]Lu-DOTA-TATE (log P Oct/H 2 O = −0.85 ± 0.10 versus −3.67 ± 0.04, respectively). While [ nat Lu]Lu- Cy revealed low stability in a DMEM/F12 GlutaMax medium, it demonstrated good stability in other aqueous media as well as in DMSO. A high sst2R binding affinity (expressed as IC 50 ) was determined in CHO sst2 cells (Chinese hamster ovary cells that were stably transfected with human sst2R). Moreover, the metallacycle exhibited high human serum albumin binding, as assessed by high-performance affinity chromatography (HPAC), and moderate stability in human serum compared to [ 177 Lu]Lu-DOTA-TATE (TATE = (Tyr 3 )-octreotate). In order to improve stability, a heteroleptic approach was used to develop a less sterically hindered cage-like SCC 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-directed SCCs and have led to new insights into the chemistry of ss2R-directed SCCs for radiopharmaceutical applications.