Radioiodinated Bicyclic RGD Peptide Derivatives for Enhanced Tumor Accumulation.

Background/Objectives: Integrin α(V)β(3) 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, [(125)I]bcRGD, exhibited high selectivity for α(V)β(3) 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 [(125)I]bcRGD derivatives were synthesized: [(125)I]bcRGD(pal) (with palmitic acid), [(125)I]bcRGD(iba) (with 4-(p-iodophenyl)butyric acid), and [(125)I]bcRGD(dimer) (a dimeric bicyclic RGD peptide). Their physicochemical properties, α(V)β(3)-selectivity, albumin-binding capacity, and biodistribution were assessed in vitro and in vivo using tumor-bearing mice. Tumor models included α(V)β(3)-high U-87 MG and α(V)β(3)-low A549 xenografts. Results: [(125)I]bcRGD(pal) and [(125)I]bcRGD(iba) exhibited prolonged blood retention (30-fold and 55-fold vs. [(125)I]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. [(125)I]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(V)β(3)-targeted imaging and therapy applications.