In vivo bodistribution, PET imaging, and tumor accumulation of ⁸⁶Y-and ¹¹¹In-antimindin/RG-1, engineered antibody fragments in LNCaP tumor-bearing nude mice

To optimize in vivo tissue uptake kinetics and clearance of engineered monoclonal antibody (mAb) fragments for radiother- apeutic and radiodiagnostic applications, we compared the bio-distribution and tumor localization of four ¹¹¹ In- and ⁸⁶Y-labeled antibody formats, derived from a single antimindin/RG-1 mAb, in a prostate tumor model. The IgG, diabody, single-chain variable domain (scFv), and novel miniantibody formats, composed of the human lgE-C _H4 and a modified lgG1 hinge linked to scFv domains, were compared. Methods; Antibodies were first derivatized with the bifunctional chelator CHX-A"-diethylenetriamine pentaacetic acid and then bound to the radiometal to create radiolabeled immunoconjugates. Human LNCaP xenografts were grown in nude mice, and ¹¹¹ln- or ⁸⁶Y-labeled antibodies were administered intravenously. Tissues were harvested at different times, and the level of antibody deposition was determined by measuring radioactivity. Whole-body small-animal PET of mice receiving ⁸⁶Y-labeled antibodies was performed at 6 time points and colocalized with simultaneous micro-CT imaging. Results; The biodistributions of ¹¹¹ In and ⁸⁶Y antibodies were quite similar. The blood, tumor, kidney, and liver tissues contained varying levels of radioactivity. The antibody accumulation in the tumor correlated with molecular size. The IgG steadily increased with time to 24.1 percentage injected dose per gram (%ID/g) at 48 h. The miniantibody accumulated at a similar rate to reach a lower level (14.2 %ID/g) at 48 h but with a higher tumor-to-blood ratio than the IgG. Tumor accumulation of the diabody peaked at 3 h, reaching a much lower level (3.7 %ID/g). A combination of rapid clearance and lower relative affinity of the scFv precluded deposition in the tumor. Small-animal PET results correlated well with the biodistribution results, with similar tumor localization patterns. Conclusion: The larger antibody formats (IgG and miniantibody) gave higher tumor uptake levels than did the smaller formats (diabody and scFv). These larger formats may be more suitable for radioimmunotherapy applications, evidenced by the preclinical efficacy previously shown by a report on the IgG format. The smaller formats were rapidly cleared from circulation, and the diabody, which accumulated in the tumor, may be more suitable for radiodiagnostic applications.