Comprehensive Structural Investigation of a Potent and Selective CXCR4 Antagonist via Crosslink Modification

Macrocyclization offers a valuable opportunity for improving the pharmacokinetic and pharmacodynamic characteristics of short bioactive peptides and exploring distinct features like cyclization tethers, size, and orientation is generally undertaken in the early development phase. Herein, starting from a potent and selective C-X-C chemokine receptor 4 (CXCR4) antagonist CXCL12-mimicking cyclic heptapeptide, we demonstrated that the disulfide bridge could be successfully replaced with a side chain-to-side chain lactam crosslink, which is commonly not enlisted among the conventional disulfide mimetics. An extensive investigation was carried out to explore the chemical space of the peptides, including macrocyclization width, stereochemical configuration, and lactam bond orientation, all of which were correlated with biochemical activity. We identified a new heptapeptide reproducing as whole the pharmacological profile of the parent peptide on CXCR4, including potency, selectivity, and antagonistic behavior. At this stage, computational studies were instructed to shed light on how the lactam cyclization features influenced the overall structure of 20 and, in turn, its ability to interact with the receptor. We envisage that these findings can give new momentum to the use of lactam cyclization as a disulfide bond mimetic and contribute to the enhancement of the repertoire for peptide-based drug development, thereby paving the way for novel avenues in therapeutic innovation.