Expression of the cellular transmembrane receptor αvβ6 integrin is essentially restricted to malignant epithelial cells in carcinomas of a broad variety of lineages, while it is virtually absent in normal adult tissues. Thus, it is a highly attractive target for tumor imaging and therapy. Furthermore, αvβ6 integrin plays an important role for the epithelial-mesenchymal interaction and the development of fibrosis.
Methods: On the basis of the Gallium-68 chelators TRAP (triazacyclononane-triphosphinate) and NODAGA, we synthesized mono- di- and trimeric conjugates of the αvβ6 integrinselective peptide cyclo(FRGDLAFp(NMe)K) via click chemistry (CuAAC). These were labeled with gallium-68 and screened regarding their suitability for in-vivo imaging of αvβ6 integrin expression by positron emission tomography (PET) and ex-vivo biodistribution in SCID mice bearing H2009 tumor (human lung adenocarcinoma) xenografts. For these, αvβ6 integrin expression in tumor and other tissues was determined by β6 immunohistochemistry (IHC).
Results: Despite the multimers showed higher αvβ6 integrin affinities (23–120 pM) than the monomers (260 pM), best results, that is, low background uptake and excellent tumor delineation, were obtained with the TRAP-based monomer ⁶⁸Ga-Avebehexin. This compound showed the most favourable pharmacokinetics due to 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, %ID/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 IHC.
Conclusion: Because of highly sensitive PET imaging even of tissues with low αvβ6 integrin expression density, we anticipate clinical applicability of ⁶⁸Ga-Avebehexin for imaging of αvβ6 tumors and fibrosis by PET.