In Vivo PET Imaging of the Cancer Integrin αvβ6 Using 68Ga-Labeled Cyclic RGD Nonapeptides

Expression of the cellular transmembrane receptor alpha v beta 6 integrin is essentially restricted to malignant epithelial cells in carcinomas of a broad variety of lineages, whereas it is virtually absent in normal adult tissues. Thus, it is a highly attractive target for tumor imaging and therapy. Furthermore, alpha v beta 6 integrin plays an important role for the epithelial-mesenchymal interaction and the development of fibrosis. Methods: On the basis of the Ga-68 chelators TRAP (triazacyclononane-triphosphinate) and NODAGA, we synthesized mono-, di-, and trimeric conjugates of the alpha v beta 6 integrin-selective peptide cyclo(FRGDLAFp(NMe)K) via click chemistry. These were labeled with 68Ga and screened regarding their suitability for in vivo imaging of alpha v beta 6 integrin expression by PET and ex vivo biodistribution in severe combined immunodeficiency mice bearing H2009 tumor (human lung adenocarcinoma) xenografts. For these, alpha v beta 6 integrin expression in tumor and other tissues was determined by 136 immunohistochemistry. Results: Despite the multimers showing higher alpha v beta 6 integrin affinities (23-120 pM) than the monomers (260 pM), the best results that is, low background uptake and excellent tumor delineation were obtained with the TRAP-based monomer Ga-68-avebehexin. This compound showed the most favorable pharmacokinetics because of its high polarity (log D = -3.7) and presence of additional negative charges (carboxylates) on the chelator, promoting renal clearance. Although tumor uptake was low (0.65% 0.04% injected dose per gram tissue [%1D/g]), it was still higher than in all other organs except the kidneys, ranging from a maximum for the stomach (0.52 +/- 0.04 %ID/g) to almost negligible for the pancreas (0.07 +/- 0,01 %ID/g). A low but significant target expression in tumor, lung, and stomach was confirmed by immunohistochemistry. Conclusion: Because of highly sensitive PET imaging even of tissues with low alpha v beta 6 integrin expression density, we anticipate clinical applicability of 68Ga-avebehexin for imaging of alpha v beta 6 tumors and fibrosis by PET.