Evaluation of an integrin αvβ6-specific peptide labeled with [18F]fluorine by copper-free, strain-promoted click chemistry

Introduction: Click chemistry, particularly the Huisgen 1,3-dipolar cycloaddition of an alkyne with an azide, has quickly become popular for site-specific radiolabeling. Recently, strain-promoted click chemistries have been developed, eliminating the need for potentially toxic copper catalysts. This study presents radiolabeling of an alpha(v)beta(6) integrin targeting peptide (A20FMDV2) via strain-promoted click using a fluorine-18 prosthetic group, and in vitro and in vivo evaluation. Methods: The radiotracer [F-18]FBA-C-6-ADIBON(3)-PEG(7)-A20FMDV2 (1) was prepared from [F-18]FBA-C-6-ADIBO (2) and N-3-PEG(7)-A20FMDV2 (ethanol; 10 min; 35-45 degrees C). HPLC-purified and formulated radiotracer 1 was evaluated in vitro by cell binding (DX3puro beta 6, alpha(v)beta(6)-positive; and DX3puro, alpha(v)beta(6)-negative control) and serum stability, and in vivo using PET/CT imaging and biodistribution studies in mice. Results: The radiotracer 1 was readily prepared and purified (from 2: 40+/-4 min including HPLC, 11.9+/-3.2% decay corrected isolated radiochemical yield, >99% radiochemical purity, n = 4) and displayed good stability (1 h: >99%, saline; 94.6%, serum). Strong alpha(v)beta(6)-targeted binding was observed in vitro (DX3puro beta 6 cells, 15 min: 43.2% binding, >6:1 for DX3puro beta 6:DX3puro). In the mouse model DX3puro beta 6-tumor binding was low (1 h: 0.47+/-0.28% ID/g, 4 h: 0.14+/-0.09% ID/g) and clearing from the bloodstream was via the renal and hepatobiliary routes (urine: 167+/-84% ID/g at 1 h, 10.3+/-4.8% ID/g at 4 h; gall bladder: 95+/-33% ID/g at 1 h, 63+/-11% ID/g at 4 h). Conclusion: Copper-free, strain-promoted click chemistry is an attractive, straightforward approach to radiolabeling. Although the [F-18]FBA-C-6-ADBIO-based prosthetic group did not interfere with alpha(v)beta(6)-targeted binding in vitro, it did influence the pharmacokinetics, possibly due to its size and lipophilic nature. (C) 2013 Elsevier Inc. All rights reserved.