A comparison of EGF and MAb 528 labeled with In-111 for imaging human breast cancer

Our objective was to compare In-111-labeled human epidermal growth factor ( hEGF), a 53-amino acid peptide with anti-epidermal growth factor receptor ( EGFR) monoclonal antibody (MAb) 528 (IgG(2a)) for imaging EGFR-positive bre ast cancer. Methods: hEGF and MAb 528 were derivatized with diethylenetriam ine pentaacetic acid (DTPA) and labeled with In-111 acetate. Receptor bindi ng assays were conducted in vitro against MDA-MB-468 human breast cancer ce lls. Biodistribution and tumor imaging studies were conducted after intrave nous injection of the radiopharmaceuticals in athymic mice bearing subcutan eous MCF-7, MDA-MB-231, or MDA-MB-468 human breast cancer xenografts or in severe combined immunodeficiency mice implanted with a breast cancer metast asis (JW-97 cells). MCF-7, MDA-MB-231, JW-97, and MDA-MB-468 cells expresse d 1.5 x 10(4), 1.3 x 10(5), 2.7 x 10(5), and 1.3 x 10(6) EGFR/cell, respect ively in vitro. Results: In-111-DTPA-hEGF and In-111-DTPA-MAb 528 bound wit h high affinity to MDA-MB-468 cells (K-a of 7.5 x 10(8) and 1.2 x 10(8) L/m ol, respectively), In-111-DTPA-hEGF was eliminated rapidly from the blood w ith < 0.2% injected dose/g (%ID/g) circulating at 72 h after injection, whe reas In-111-DTPA-MAb 528 was cleared more slowly (3 %ID/g in the blood at 7 2 h). Maximum localization of In-111-DTPA-hEGF in MDA-MB-468 tumors (2.2 %I D/g) was 10-fold lower than with In-111-DTPA-MAb 528 (21.6 %ID/g). There wa s high uptake in the liver and kidneys for both radiopharmaceuticals. Tumor -to-blood ratios were greater for In-111-labeled hEGF than for MAb 528 (12: 1 versus 6:1), but all other tumor-to-normal tissue ratios were higher for MAb 528. MDA-MB-468 and JW-97 tumors were imaged successfully with both rad iopharmaceuticals, but tumors were more easily visualized using In-111-labe led MAb 528. There was no direct quantitative relationship between EGFR exp ression on breast cancer cell lines in vitro, and tumor uptake of the radio pharmaceuticals in vivo, but control studies showed that tumor uptake was r eceptor mediated. Conclusion: Our results suggest that the tumor uptake in vivo of receptor-binding radiopharmaceuticals is controlled to a greater ex tent by their elimination rate from the blood than by the level of receptor expression on the cancer cells. Radiolabeled anti-EGFR MAbs would be more effective for tumor imaging in cancer patients than peptide-based radiophar maceuticals such as hEGF, because they exhibit higher tumor uptake at only moderately lower tumor-to-blood ratios.