We synthesized disulfide-based cyclic RGD pentapeptides bearing a near-infrared fluorescent dye (cypate), represented by cypate-c(CRGDC) (1) for integrin-targeted optical imaging. These compounds were compared with the traditional lactam-based cyclic RGD counterpart, cypate-c(RGDfK) (2). Molecular modeling suggests that the binding affinity of 2 to integrin αvβ3 is an order of magnitude higher than that of 1. This was confirmed experimentally, which further showed that substitution of Gly with Pro, Val and Tyr in 1 remarkably hampered the α vβ3 binding. Interestingly, cell microscopy with A549 cells showed that 1 exhibited higher cellular staining than 2. These results indicate that factors other than receptor binding affinity to αvβ3 dimeric proteins mediate cellular uptake. Consequently, 1 and its analogs may serve as valuable molecular probes for investigating the selectivity and specificity of integrin targeting by optical imaging.
- Disulfide-based cyclization
- Integrin αβ binding
- Near-infrared fluorescent probe
- Optical imaging
- RGD peptide