Targeting the Internal Epitope of Prostate-Specific Membrane Antigen with ⁸⁹Zr-7E11 Immuno-PET

Abstract

The potential of the positron-emitting ⁸⁹Zr has been recently investigated for the design of radioimmunoconjugates for immuno-PET. In this study, we report the preparation and in vivo evaluation of ⁸⁹Zr-desferrioxamine B (DFO)-7E11, a novel ⁸⁹Zr-labeled monoclonal antibody (mAb) construct for targeted imaging of prostate-specific membrane antigen (PSMA), a prototypical cell surface marker highly overexpressed in prostate cancer. The ability of ⁸⁹Zr-DFO-7E11 to delineate tumor response to therapy was also investigated, because it binds to the intracellular epitope of PSMA, which becomes available only on membrane disruption in dead or dying cells. Methods: 7E11 as a marker of dying cells was studied by flow cytometry and microscopy of cells after antiandrogen-, radio-, and chemotherapy in LNCaP and PC3 PSMA–positive cells. The in vivo behavior of ⁸⁹Zr-DFO-7E11 was characterized in mice bearing subcutaneous LNCaP (PSMA-positive) tumors by biodistribution studies and immuno-PET. The potential of assessing tumor response was evaluated in vivo after radiotherapy. Results: In vitro studies correlated 7E11 binding with markers of apoptosis (7-amino-actinomycin-D and caspase-3). In vivo biodistribution experiments revealed high, target-specific uptake of ⁸⁹Zr-DFO-7E11 in LNCaP tumors after 24 h (20.35 ± 7.50 percentage injected dose per gram [%ID/g]), 48 h (22.82 ± 3.58 %ID/g), 96 h (36.94 ± 6.27 %ID/g), and 120 h (25.23 ± 4.82 %ID/g). Excellent image contrast was observed with immuno-PET. 7E11 uptake was statistically increased in irradiated versus control tumor as measured by immuno-PET and biodistribution studies. Binding specificity was assessed by effective blocking studies at 48 h. Conclusion: These findings suggest that ⁸⁹Zr-DFO-7E11 displays high tumor–to–background tissue contrast in immuno-PET and can be used as a tool to monitor and quantify, with high specificity, tumor response in PSMA-positive prostate cancer.