Precision peptide theranostics: developing N- to C-terminus optimized theranostics targeting cholecystokinin-2...

Peptides are ideal for theranostic development as they afford rapid target accumulation, fast clearance from background tissue, and exhibit good tissue penetration. Previously, we developed a novel series of peptides that presented discreet folding propensity leading to an optimal candidate [⁶⁸Ga]Ga-DOTA-GA1 ([D-Glu]₆-Ala-Tyr-NMeGly-Trp-NMeNle-Asp-Nal-NH₂) with 50 pM binding affinity against cholecystokinin-2 receptors (CCK₂R). However, we were confronted with challenges of unfavorably high renal uptake. Methods: A structure activity relationship study was undertaken of the lead theranostic candidate. Prudent structural modifications were made to the peptide scaffold to evaluate the contributions of specific N-terminal residues to the overall biological activity. Optimal candidates were then evaluated in nude mice bearing transfected A431-CCK₂ tumors, and their biodistribution was quantitated ex vivo. Results: We identified and confirmed that D-Glu³ to D-Ala³ substitution produced 2 optimal candidates, [⁶⁸Ga]Ga-DOTA-GA12 and [⁶⁸Ga]Ga-DOTA-GA13. These radiopeptides presented with high target/background ratios, enhanced tumor retention, excellent metabolic stability in plasma and mice organ homogenates, and a 4-fold reduction in renal uptake, significantly outperforming their non-alanine counterparts. Conclusions: Our study identified novel radiopharmaceutical candidates that target the CCK₂R. Their high tumor uptake and reduced renal accumulation warrant clinical translation.