Conjugation with (111)In-DTPA-poly(ethylene glycol) improves imaging of anti-EGF receptor antibody C225.

UNLABELLED: Significant liver uptake often limits the clinical application of radiolabeled antibodies in radioimmunodetection. The purpose of this study was to evaluate the gamma-imaging properties of an antiepidermal growth factor receptor (EGFR) antibody, C225, conjugated with heterofunctional poly(ethylene glycol) (PEG) with 1 terminus of the polymer attached to a radiometal chelator, diethylenetriaminepentaacetic acid (DTPA).

METHODS: Two preparations of PEG-modified C225, one with 20% and the other with 60% amine substitution, were labeled with (111)In. The conjugates, (111)In-DTPA-PEG-C225, were injected intravenously into nude mice with EGFR-positive A431 tumors. For comparison, C225 directly labeled with (111)In was also injected. In a competitive study, mice with A431 tumors were pretreated intravenously with 100-fold excess of native C225, followed by an injection of (111)In-DTPA-PEG-C225 30 min or 20 h later. In addition, (111)In-DTPA-PEG-C225 was injected into mice with EGFR-positive MDA-MB-468 tumors and EGFR-negative MDA-MB-435 tumors. Images were acquired at 5 min and at 2, 6, 24, and 48 h after injection of the radiotracers. Regions of interest (ROIs) were drawn on the computer images around the whole body, liver, muscle, and tumor. The counts per pixel in the tumor and normal tissues were calculated. At 48 h, the mice were killed and dissected. Blood, liver, muscle, and tumor samples were removed and the radioactivity of each sample was measured.

RESULTS: In A431 tumor xenografts, the tumor uptake of C225 modified with PEG was not significantly different than the uptake of unmodified (111)In-DTPA-C225. Uptake in the liver, however, was reduced by 38%-45%, and the reduction increased with increasing degree of PEG substitution. Tumors of A431 and MDA-MB-468 xenografts were clearly visualized with (111)In-DTPA-PEG-C225, whereas tumors of the MDA-MB-435 xenograft, which expresses low levels of EGFR, were not as readily visible. The tumor-to-blood ratios of (111)In-DTPA-PEG-C225 in A431 and MDA-MB-468 xenografts were about 3 fold higher than in MDA-MB-435 xenografts. Blocking EGFR by pretreatment with native C225 significantly reduced the uptake of (111)In-DTPA-PEG-C225 in the liver. The tumor-to-blood ratios in mice with A431 tumors were decreased 2.5-2.7 fold after pretreatment with a large excess of C225. Similar results were obtained with MDA-MB-468 tumor xenografts. In contrast, the tumor-to-blood ratios in mice with MDA-MB-435 tumor xenografts were not significantly different in C225-pretreated mice than in nonpretreated mice.

CONCLUSION: These findings indicate that (111)In-DTPA-PEG-C225 selectively localized to the tumors expressing high levels of EGFR. PEG-modification of C225 significantly reduced its liver uptake, resulting in improved visualization of EGFR-positive tumors. Using PEG as a linker between the monoclonal antibody and metal chelator is a useful strategy to optimize the imaging characteristics of antibody-based scintigraphic agents.