@article {Vaidyanathan967, author = {Ganesan Vaidyanathan and Darryl McDougald and Jaeyeon Choi and Eftychia Koumarianou and Douglas Weitzel and Takuya Osada and H. Kim Lyerly and Michael R. Zalutsky}, title = {Preclinical Evaluation of 18F-Labeled Anti-HER2 Nanobody Conjugates for Imaging HER2 Receptor Expression by Immuno-PET}, volume = {57}, number = {6}, pages = {967--973}, year = {2016}, doi = {10.2967/jnumed.115.171306}, publisher = {Society of Nuclear Medicine}, abstract = {The human growth factor receptor type 2 (HER2) is overexpressed in breast as well as other types of cancer. Immuno-PET, a noninvasive imaging procedure that could assess HER2 status in both primary and metastatic lesions simultaneously, could be a valuable tool for optimizing application of HER2-targeted therapies in individual patients. Herein, we have evaluated the tumor-targeting potential of the 5F7 anti-HER2 Nanobody (single-domain antibody fragment; \~{}13 kDa) after 18F labeling by 2 methods. Methods: The 5F7 Nanobody was labeled with 18F using the novel residualizing label N-succinimidyl 3-((4-(4-18F-fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate (18F-SFBTMGMB; 18F-RL-I) and also via the most commonly used 18F protein{\textendash}labeling prosthetic agent N-succinimidyl 3-18F-fluorobenzoate (18F-SFB). For comparison, 5F7 Nanobody was also labeled using the residualizing radioiodination agent N-succinimidyl 4-guanidinomethyl-3-125I-iodobenzoate (125I-SGMIB). Paired-label (18F/125I) internalization assays and biodistribution studies were performed on HER2-expressing BT474M1 breast carcinoma cells and in mice with BT474M1 subcutaneous xenografts, respectively. Small-animal PET/CT imaging of 5F7 Nanobody labeled using 18F-RL-I also was performed. Results: Internalization assays indicated that intracellularly retained radioactivity for 18F-RL-I-5F7 was similar to that for coincubated 125I-SGMIB-5F7, whereas that for 18F-SFB-5F7 was lower than coincubated 125I-SGMIB-5F7 and decreased with time. BT474M1 tumor uptake of 18F-RL-I-5F7 was 28.97 {\textpm} 3.88 percentage injected dose per gram of tissue (\%ID/g) at 1 h and 36.28 {\textpm} 14.10 \%ID/g at 2 h, reduced by more than 90\% on blocking with trastuzumab, indicating HER2 specificity of uptake, and was also 26\%{\textendash}28\% higher (P \< 0.05) than that of 18F-SFB-5F7. At 2 h, the tumor-to-blood ratio for 18F-RL-I-5F7 (47.4 {\textpm} 13.1) was significantly higher (P \< 0.05) than for 18F-SFB-5F7 (25.4 {\textpm} 10.3); however, kidney uptake was 28{\textendash}36-fold higher for 18F-RL-I-5F7. Conclusion: 18F-RL-I-5F7 is a promising tracer for evaluating HER2 status by immuno-PET; however, in settings in which renal background is problematic, strategies for reducing its kidney uptake may be needed.}, issn = {0161-5505}, URL = {https://jnm.snmjournals.org/content/57/6/967}, eprint = {https://jnm.snmjournals.org/content/57/6/967.full.pdf}, journal = {Journal of Nuclear Medicine} }