Radioiodinated Bicyclic RGD Peptide Derivatives for Enhanced Tumor Accumulation

Background/Objectives: Integrin α_Vβ₃ plays a crucial role in tumor angiogenesis and cancer progression, making it a key target for radiolabeled probes used in imaging and therapy. A previously developed probe, [¹²⁵I]bcRGD, exhibited high selectivity for α_Vβ₃ but limited tumor accumulation due to rapid blood clearance. This study aimed to address this issue through two strategies: (1) conjugating albumin-binding molecules to enhance systemic circulation and (2) dimerizing RGD peptides to improve binding affinity via multivalency effects. Methods: Three [¹²⁵I]bcRGD derivatives were synthesized: [¹²⁵I]bcRGD_pal (with palmitic acid), [¹²⁵I]bcRGD_iba (with 4-(p-iodophenyl)butyric acid), and [¹²⁵I]bcRGD_dimer (a dimeric bicyclic RGD peptide). Their physicochemical properties, α_Vβ₃-selectivity, albumin-binding capacity, and biodistribution were assessed in vitro and in vivo using tumor-bearing mice. Tumor models included α_Vβ₃-high U-87 MG and α_Vβ₃-low A549 xenografts. Results: [¹²⁵I]bcRGD_pal and [¹²⁵I]bcRGD_iba exhibited prolonged blood retention (30-fold and 55-fold vs. [¹²⁵I]bcRGD, respectively) and increased tumor accumulation (3.9% ID/g and 3.6% ID/g at 2 h, respectively). Despite improved systemic circulation, tumor-to-blood ratios remained low (<1), indicating limited tumor retention. [¹²⁵I]bcRGD_dimer achieved significantly greater tumor accumulation (4.2% ID/g at 2 h) and favorable tumor-to-blood (22) and tumor-to-muscle (14) ratios, with a 5.4-fold higher uptake in U-87 MG tumors compared to A549 tumors. Conclusions: Dimerization was more effective than albumin binding in enhancing bcRGD's tumor-targeting potential. The dimeric probe demonstrated improved tumor accumulation, favorable pharmacokinetics, and preserved integrin selectivity. These findings provide a foundation for further structural optimization of bicyclic RGD peptides for integrin α_Vβ₃-targeted imaging and therapy applications.