In vitro and in vivo evaluation of the effects of aluminum [18F]fluoride radiolabeling on an integrin αvβ6-specific peptide

Introduction: Incorporation of fluorine-18 (F-18) into radiotracers by capturing ionic [F-18]-species can greatly accelerate and simplify radiolabeling for this important positron emission tomography (PET) radioisotope. Among the different strategies, the incorporation of aluminum [F-18]fluoride (Al[F-18](2+)) into NOTA chelators has recently emerged as a robust approach to peptide radiolabeling. This study presents Al[F-18](2+) -radiolabeling of an alpha(v)beta(6) integrin-targeted peptide (NOTA-PEG(28)-A20FMDV2) and its in vitro and in vivo evaluation. Methods: Aluminum [F-18]fluoride was prepared at r.t. from [F-18]fluoride (40 MBq-11 GBq) and introduced into NOTA-PEG(28)-A20FMDV2 (1) in sodium acetate (pH 4.1; 100 degrees C, 15 min). The radiotracer Al[F-18] NOTA-PEG(28)-A20FMDV2 (2) was purified by HPLC, formulated in PBS and evaluated in vitro (stability; binding and internalization in alpha(v)beta(6)(+) and alpha(v)beta(6)(-) cells) and in vivo (paired alpha(v)beta(6)(+) and alpha(v)beta(6)(xenograft mice: PET/CT, biodistribution, tumor autoradiography and metabolites). Results: The radiotracer 2 was prepared in 90 +/- 6 mm (incl. formulation; n = 3) in 19.3 +/- 5.4% decay corrected radiochemical yield (radiochemical purity: >99%; specific activity: 158 +/- 36 GBq/mu mol) and was stable in PBS and serum (2 h). During in vitro cell binding studies, 2 showed high, alpha(v)beta(6)-targeted binding (alpha(v)beta(6)(+): 42.4 +/- 1.2% of total radioactivity, ratio (+)/(-) = 8.4/1) and internalization (alpha(v)beta(6)(+): 28.3 +/- 0.5% of total radioactivity, (+)/(-) = 11.7/1). In vivo, 2 maintained alpha(v)beta(6)-targeted binding (biodistribution; 1 h: 04,138(+): 1.74 +/- 038% ID/g, (+)/(-) = 2.72/1; 4 h: alpha(v)beta(6)(+): 1.21 0.56% ID/g, (+)/(-) = 4.0/1; 11% intact 2 in tumor at 1 h), with highest uptake around the tumor edge (autoradiography). Most of the radioactivity cleared rapidly in the urine within one hour, but a significant fraction remained trapped in the kidneys (4 h: 229 44% ID/g). Conclusion: The Al[F-18]/NOTA-based radiolabeling was rapid and efficient, and the radiotracer 2 showed good alpha(v)beta(6)-selectivity in vitro and in vivo. However, in contrast to A20FMDV2 labeled with covalently bound [F-18]-prosthetic groups (e.g., [F-18]fluorobenzoic acid), 2 demonstrated significant trapping in kidneys, similar to radiometal-labeled chelator-analogs of 2. (C) 2014 Elsevier Inc. All rights reserved.