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

IntroductionIncorporation of fluorine-18 (18F) into radiotracers by capturing ionic [18F]-species can greatly accelerate and simplify radiolabeling for this important positron emission tomography (PET) radioisotope. Among the different strategies, the incorporation of aluminum [18F]fluoride (Al[18F]2 +) into NOTA chelators has recently emerged as a robust approach to peptide radiolabeling. This study presents Al[18F]2 +-radiolabeling of an αvβ6 integrin-targeted peptide (NOTA-PEG28-A20FMDV2) and its in vitro and in vivo evaluation.MethodsAluminum [18F]fluoride was prepared at r.t. from [18F]fluoride (40 MBq–11 GBq) and introduced into NOTA-PEG28-A20FMDV2 (1) in sodium acetate (pH 4.1; 100°C, 15 min). The radiotracer Al[18F] NOTA-PEG28-A20FMDV2 (2) was purified by HPLC, formulated in PBS and evaluated in vitro (stability; binding and internalization in αvβ6(+) and αvβ6(−) cells) and in vivo (paired αvβ6(+) and αvβ6(−) xenograft mice: PET/CT, biodistribution, tumor autoradiography and metabolites).ResultsThe radiotracer 2 was prepared in 90 ± 6 min (incl. formulation; n = 3) in 19.3 ± 5.4% decay corrected radiochemical yield (radiochemical purity: > 99%; specific activity: 158 ± 36 GBq/μmol) and was stable in PBS and serum (2 h). During in vitro cell binding studies, 2 showed high, αvβ6-targeted binding (αvβ6(+): 42.4 ± 1.2% of total radioactivity, ratio (+)/(−) = 8.4/1) and internalization (αvβ6(+): 28.3 ± 0.5% of total radioactivity, (+)/(−) = 11.7/1). In vivo, 2 maintained αvβ6-targeted binding (biodistribution; 1 h: αvβ6(+): 1.74 ± 0.38% ID/g, (+)/(−) = 2.72/1; 4 h: αvβ6(+): 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).ConclusionThe Al[18F]/NOTA-based radiolabeling was rapid and efficient, and the radiotracer 2 showed good αvβ6-selectivity in vitro and in vivo. However, in contrast to A20FMDV2 labeled with covalently bound [18F]-prosthetic groups (e.g., [18F]fluorobenzoic acid), 2 demonstrated significant trapping in kidneys, similar to radiometal-labeled chelator-analogs of 2.