CD47 as a potential target for molecular imaging

Objectives The goal of this study was to evaluate CD47, a protein upregulated on cancer cells where it protects them against attack by the host immune system, as a PET imaging target for cancer diagnosis and prognosis.

Methods OV10, a CD47-deficient cell line, was transfected with human CD47. Anti-CD47 antibody B6H12 was conjugated to the Zr chelator, desferroxamine, and the resulting complex labeled with Zr-89 at 37°C for 1 hour. CD47-sufficient and -deficient cells were incubated with 2 µCi (0.5 µg) of radiolabeled antibody for 35 minutes at 37°C in humidified atmosphere supplemented with 5% CO2. Unbound antibody was removed and the cells were solubilized in PBS containing 0.1% SDS, the lysate was transferred to microfuge tubes and cell-associated activity detected with a gamma-counter. Protein content was measured with the bicinchoninic acid assay and the data expressed as counts per milligram of protein. These cell lines were used to generate subcutaneous xenograft tumors in athymic NCR nu/nu mice. Imaging data was acquired with either Inveon PET/CT or Focus 220 (Siemens Medical Solutions) and reconstructed with the maximum aposteriory probability (MAP) algorithm. The data was analyzed with Inveon Research Workstation software.

Results The antibody-chelate complex was efficiently labeled with Zr-89 at 4 µCi/µg specific activity. The complex was bound by the CD47-sufficient and not CD47-deficient cells. Image analysis of tumor-bearing animals demonstrated 4.5 fold higher tracer uptake in the CD47-expressing tumors. Analysis of post-imaging biodistribution showed that the tracer accumulated in CD47 negative tumors at levels similar to those of the blood, 2.1 %ID/g vs. 2.7 %ID/g, respectively; while tracer uptake in the CD47 positive tumors was 2.5 fold higher, 4.9 %ID/g.

Conclusions These results indicate that the anti-CD47 antibody can be effectively labeled with Zr-89 and used as an imaging probe for tumors over-expressing this marker