Exploring macrocyclization strategies to design novel octreotate-based radioconjugates

Peptides derived from the cyclic tetradecapeptide somastostatin exhibit a strong affinity primarily towards the G-protein coupled somatostatin receptor subtype 2 (SSTR2), which is overexpressed in neuroendocrine tumors. These somatostatin analogs, such as octreotide (TOC) or octreotate (TATE), are typically cyclized through a disulfide bridge. To address the potential fragility of this linkage in vivo, four distinct stapling strategies were explored to develop novel TATE derivatives with improved stability. Each approach induced a different distance between the two sulfhydryl groups involved into the macrocyclization. Additionally, the stapling linkers were designed to present a third functional group required for the regioselective insertion of a metal chelate. Ultimately, six stapled octreotate derivatives (stTATE-01/06), possessing 3 to 6 chemical bonds between the two cysteine residues, were synthesized and radiolabeled with indium-111. Evaluation of their affinity to SSTR2, conducted through a competitive binding assay, aimed to identify the most effective stapling strategy. However, a significant loss of affinity was observed for all stapled peptides compared to the gold standard DOTA-TATE, confirming that these macrocyclization approaches were detrimental to the biological activity of the new SSTR2 ligands.