PET imaging of EGFR expression in nude mice bearing MDA-MB-468, a human breast adenocarcinoma.

BACKGROUND AND OBJECTIVE

Cetuximab is a monoclonal antibody that binds to and inhibits the epidermal growth factor receptor (EGFR). EGFR overexpression has been observed in a subset of breast cancers. The purpose of this study was to evaluate 64Cu-labeled cetuximab as an imaging agent using MDA-MB-468 breast cancer cells.

METHODS

Cetuximab was coupled with an N-sulfosuccinimide ester of DOTA, purified, and labeled with the positron-emitting nuclide, 64Cu. Receptor-binding specificity and affinity of 64Cu-DOTA-cetuximab were studied using human MDA-MB-468 breast cancer cells, which express high levels of EGFR. Micropositron emission tomography and biodistribution studies were performed in athymic nude mice bearing MDA-MB-468 cell xenografts. Blocking studies with cold cetuximab were also performed to determine the specific binding of cetuximab.

RESULTS

The radiochemical yield was 97.1 ± 1.1%. The specific activity was 1.5 Ci/μm cetuximab and the affinity to EGFR-positive MDA-MB-468 cells was high (KD=0.4 nmol/l). Both biodistribution and micropositron emission tomographic imaging studies with 64Cu-DOTA-cetuximab showed higher tumor uptake at 24 h (20.91 ± 2.49% ID/g, standardized uptake values of 9.6) than at 4 h (11.65 ± 3.89% ID/g, standardized uptake values of 4.9). Tumor uptake was significantly reduced from 20.91 ± 2.49% ID/g at 24 h to 14.42 ± 0.85% ID/g in a 1-h blocking study (P=0.00).

CONCLUSION

Cetuximab can be labeled with 64Cu without compromising its biological activity. The tumor uptake was excellent with high tumor/muscle (7.97 ± 1.78 at 4 h, 15.91 ± 6.04 at 24 h) and reasonable tumor/blood (0.5 ± 0.18 at 4 h, 2.12 ± 0.86 at 24 h) ratios. Blocking studies showed the specific binding of the labeled antibody to tumor tissue.