Halogenated Tryptophans Improve Integrin α _V β ₆ Affinity of Cyclic RGD Peptides and Provide Handles for Late‐Stage Derivatization

ABSTRACT Selective targeting of integrin α V β 6 is of high relevance for molecular imaging and therapeutic applications, yet ligands combining high affinity with structural and functional versatility remain limited. A structure–activity relationship (SAR) study of the nonapeptide cyclo ‐(‐Asp‐Leu‐Ala‐Phe‐ d ‐Pro‐NMeLys‐Phe‐Arg‐Gly‐) was performed through site‐specific substitution of phenylalanine residues by tryptophan and halogenated tryptophan derivatives, revealing a pronounced position‐dependent effect on integrin recognition. Substitution at position i+3 relative to the RGD sequence with halotryptophans is identified as an affinity hotspot, enabling picomolar binding to α V β 6 . In contrast, modification at position i+6 preserves high α V β 6 affinity while providing exceptional tolerance toward structural diversification and enabling late‐stage functionalization of unprotected peptides via palladium‐catalyzed cross‐coupling reactions. This establishes i+6 modification as a versatile platform for chemoselective derivatization without compromising affinity. Halotryptophans serve both as pharmacophores and synthetic handles. Notably, an anthracenyl‐functionalized derivative combines sub‐nanomolar α V β 6 affinity with water solubility and intrinsic fluorescence, demonstrating that even bulky π ‐systems can be accommodated at discrete positions without loss of potency. These integrin ligands are compatible with cellular applications and enable receptor‐specific detection by flow cytometry (FACS) as well as visualization by confocal fluorescence microscopy. This work defines a robust design for α V β 6 ‐targeting functional probes for molecular imaging and fluorescence‐guided tumor detection.