A recent article by de Valk and colleagues reported the use of the fluorescent agent cRGD-ZW800–1 for visualization of tumor tissue in humans during surgery (1). The authors hypothesize that their compound, which “consists of a cyclic pentapeptide (cRGD) conjugated to the 800 nm zwitterionic NIR fluorophore ZW800–1” [sic], is able to recognize αvβ6-integrin. The authors confirmed presence of αvβ6-integrin in excised tumor tissue by IHC and concluded that the observed accumulation of cRGD-ZW800–1 is driven by αvβ6-integrin expression.

Figure 6 of this article reveals that the “(cRGD)” [sic] peptide in cRGD-ZW800–1 is cyclo[RGDyK]. Such cyclic pentapeptides are known for decades, however, not as ligands for αvβ6-, but for αvβ3-integrin (2). Peptides of the cyclo[RGDxK] (x = y, f) type were extensively used for addressing angiogenic processes in vivo, because it was shown in 1991 that αvβ3-integrin plays an important role for angiogenesis. A recent...

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