NIR Fluorescence Imaging of Colon Cancer with cRGD-ZW800-1—Letter
Authors: Susanne Kossatz, Johannes Notni
Journal: Clinical Cancer Research (2021)

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 comprehensive study of the selectivity profiles of RGD-binding peptides confirmed that the cyclo[RGDxK] motif is selective for αvβ3-integrin (3). It shows single-digit nanomolar αvβ3 activity, but its affinity to other RGD-binding integrins (most importantly, αvβ6, but also αvβ5, α5β1, αvβ8, and αIIbβ3) is definitely too low for in vivo targeting of these receptors. This widely accepted notion was further corroborated by a study of the cell uptake of different selective ligands for RGD-recognizing integrins conjugated to ferrocene, a cytotoxic agent (4). While even low concentrations of the respective cyclo-RGD pentapeptide conjugate reduced the viability of αvβ3-expressing cells, no effect was observed for αvβ6-positive cells, even if the conjugate was delivered in high concentrations, underscoring that cellular binding and uptake of cyclo-RGD pentapeptide conjugates is driven by αvβ3, but not αvβ6 (4).The lack of αvβ6 binding of cRGD pentapeptides strongly suggests that there is no causal relationship between tumor uptake of cRGD-ZW800–1 and presence of αvβ6 in tissue. Hence, we hold the view that the proposed tumor uptake mechanism is invalid. We are aware that this group previously determined the αvβ6-integrin–mediated cellular uptake of cRGD-ZW800–1 (5). In our view, these data neither support the authors' hypothesis. Uptake of cRGD-ZW800–1 by αvβ6-integrin–expressing HT29 cells could not be blocked by competition (no significance; ref. 5), which confirms that cellular uptake of the agent is not αvβ6-driven. We conclude that cRGD-ZW800–1 accumulation in the tumor tissue is certainly not caused by αvβ6-integrin expression, because the peptide in cRGD-ZW800–1 is not a ligand for αvβ6.See the Response, p. 4938S. Kossatz reports other support from Summit Biomedical Imaging outside the submitted work; in addition, S. Kossatz has a patent for "Methods of Cancer Detection using PARPi-FL" (WO2016164771) pending. J. Notni reports grants from Deutsche Forschungsgemeinschaft during the conduct of the study; in addition, J. Notni has a patent for avb6 integrin binding peptides pending and licensed; in addition, J. Notni is shareholder of TRIMT GmbH (Radeberg, Germany), a company active in the field of radiopharmaceutical research. No other disclosures were reported.