PT - JOURNAL ARTICLE AU - Ganesan Vaidyanathan AU - Darryl McDougald AU - Jaeyeon Choi AU - Eftychia Koumarianou AU - Douglas Weitzel AU - Takuya Osada AU - H. Kim Lyerly AU - Michael R. Zalutsky TI - Preclinical Evaluation of <sup>18</sup>F-Labeled Anti-HER2 Nanobody Conjugates for Imaging HER2 Receptor Expression by Immuno-PET AID - 10.2967/jnumed.115.171306 DP - 2016 Jun 01 TA - Journal of Nuclear Medicine PG - 967--973 VI - 57 IP - 6 4099 - http://jnm.snmjournals.org/content/57/6/967.short 4100 - http://jnm.snmjournals.org/content/57/6/967.full SO - J Nucl Med2016 Jun 01; 57 AB - 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–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 ± 3.88 percentage injected dose per gram of tissue (%ID/g) at 1 h and 36.28 ± 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%–28% higher (P &lt; 0.05) than that of 18F-SFB-5F7. At 2 h, the tumor-to-blood ratio for 18F-RL-I-5F7 (47.4 ± 13.1) was significantly higher (P &lt; 0.05) than for 18F-SFB-5F7 (25.4 ± 10.3); however, kidney uptake was 28–36-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.