RT Journal Article SR Electronic T1 Improved Tumor Targeting of Anti-HER2 Nanobody Through N-Succinimidyl 4-Guanidinomethyl-3-Iodobenzoate Radiolabeling JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 650 OP 656 DO 10.2967/jnumed.113.127100 VO 55 IS 4 A1 Pruszynski, Marek A1 Koumarianou, Eftychia A1 Vaidyanathan, Ganesan A1 Revets, Hilde A1 Devoogdt, Nick A1 Lahoutte, Tony A1 Lyerly, H. Kim A1 Zalutsky, Michael R. YR 2014 UL http://jnm.snmjournals.org/content/55/4/650.abstract AB Nanobodies are approximately 15-kDa proteins based on the smallest functional fragments of naturally occurring heavy chain–only antibodies and represent an attractive platform for the development of molecularly targeted agents for cancer diagnosis and therapy. Because the human epidermal growth factor receptor type 2 (HER2) is overexpressed in breast and ovarian carcinoma, as well as in other malignancies, HER2-specific Nanobodies may be valuable radiodiagnostics and therapeutics for these diseases. The aim of the present study was to evaluate the tumor-targeting potential of anti-HER2 5F7GGC Nanobody after radioiodination with the residualizing agent N-succinimidyl 4-guanidinomethyl 3-125/131I-iodobenzoate (*I-SGMIB). Methods: The 5F7GGC Nanobody was radiolabeled using *I-SGMIB and, for comparison, with Nε-(3-*I-iodobenzoyl)-Lys5-Nα-maleimido-Gly1-GEEEK (*I-IB-Mal-d-GEEEK), another residualizing agent, and by direct radioiodination using IODO-GEN (125I-Nanobody). The 3 labeled Nanobodies were evaluated in affinity measurements, and paired-label internalization assays were performed on HER2-expressing BT474M1 breast carcinoma cells and in paired-label tissue distribution measurements in mice bearing subcutaneous BT474M1 xenografts. Results: *I-SGMIB-Nanobody was produced in 50.4% ± 3.6% radiochemical yield and exhibited a dissociation constant of 1.5 ± 0.5 nM. Internalization assays demonstrated that intracellular retention of radioactivity was up to 1.5-fold higher for *I-SGMIB-Nanobody than for coincubated 125I-Nanobody or *I-IB-Mal-d-GEEEK-Nanobody. Peak tumor uptake for *I-SGMIB-Nanobody was 24.50% ± 9.89% injected dose/g at 2 h, 2- to 4-fold higher than observed with other labeling methods, and was reduced by 90% with trastuzumab blocking, confirming the HER2 specificity of localization. Moreover, normal-organ clearance was fastest for *I-SGMIB-Nanobody, such that tumor–to–normal-organ ratios greater than 50:1 were reached by 24 h in all tissues except lungs and kidneys, for which the values were 10.4 ± 4.5 and 5.2 ± 1.5, respectively. Conclusion: Labeling anti-HER2 Nanobody 5F7GGC with *I-SGMIB yields a promising new conjugate for targeting HER2-expressing malignancies. Further research is needed to determine the potential utility of *I-SGMIB-5F7GGC labeled with 124I, 123I, and 131I for PET and SPECT imaging and for targeted radiotherapy, respectively.