89Zr-labeled ImmunoPET targeting the cancer stem cell antigen CD133 using fully-human antibody constructs.

Background: Cancer stem cells play an important role in driving tumor growth and treatment resistance, which makes them a promising therapeutic target to prevent cancer recurrence. Emerging cancer stem cell-targeted therapies would benefit from companion diagnostic imaging probes to aid in patient selection and monitoring response to therapy. To this end, zirconium-89-radiolabeled immunoPET probes that target the cancer stem cell-antigen CD133 were developed using fully human antibody and antibody scFv-Fc scaffolds.

Results: ImmunoPET probes [⁸⁹Zr]-DFO-RW03_IgG (CA = 0.7 ± 0.1), [⁸⁹Zr]-DFO-RW03_IgG (CA = 3.0 ± 0.3), and [⁸⁹Zr]-DFO-RW03_{scFv - Fc} (CA = 2.9 ± 0.3) were radiolabeled with zirconium-89 (radiochemical yield 42 ± 5%, 97 ± 2%, 86 ± 12%, respectively) and each was isolated in > 97% radiochemical purity with specific activities of 120 ± 30, 270 ± 90, and 200 ± 60 MBq/mg, respectively. In vitro binding assays showed a low-nanomolar binding affinity of 0.6 to 1.1 nM (95% CI) for DFO-RW03_IgG (CA = 0.7 ± 0.1), 0.3 to 1.9 nM (95% CI) for DFO-RW03_IgG (CA = 3.0 ± 0.3), and 1.5 to 3.3 nM (95% CI) for DFO-RW03_{scFv - Fc} (C/A = 0.3). Biodistribution studies found that [⁸⁹Zr]-DFO-RW03_{scFv - Fc} (CA = 2.9 ± 0.3) exhibited the highest tumor uptake (23 ± 4, 21 ± 2, and 23 ± 4%ID/g at 24, 48, and 72 h, respectively) and showed low uptake (< 6%ID/g) in all off-target organs at each timepoint (24, 48, and 72 h). Comparatively, [⁸⁹Zr]-DFO-RW03_IgG (CA = 0.7 ± 0.1) and [⁸⁹Zr]-DFO-RW03_IgG (CA = 3.0 ± 0.3) both reached maximum tumor uptake (16 ± 3%ID/g and 16 ± 2%ID/g, respectively) at 96 h p.i. and showed higher liver uptake (10.2 ± 3%ID/g and 15 ± 3%ID/g, respectively) at that timepoint. Region of interest analysis to assess PET images of mice administered [⁸⁹Zr]-DFO-RW03_{scFv - Fc} (CA = 2.9 ± 0.3) showed that this probe reached a maximum tumor uptake of 22 ± 1%ID/cc at 96 h, providing a tumor-to-liver ratio that exceeded 1:1 at 48 h p.i. Antibody-antigen mediated tumor uptake was demonstrated through biodistribution and PET imaging studies, where for each probe, co-injection of excess unlabeled RW03_IgG resulted in > 60% reduced tumor uptake.

Conclusions: Fully human CD133-targeted immunoPET probes [⁸⁹Zr]-DFO-RW03_IgG and [⁸⁹Zr]-DFO-RW03_{scFv - Fc} accumulate in CD133-expressing tumors to enable their delineation through PET imaging. Having identified [⁸⁹Zr]-DFO-RW03_{scFv - Fc} (CA = 2.9 ± 0.3) as the most attractive construct for CD133-expressing tumor delineation, the next step is to evaluate this probe using patient-derived tumor models to test its detection limit prior to clinical translation.