PET imaging of HER1-expressing xenografts in mice with 86Y-CHX-A''-DTPA-cetuximab

Abstract

Purpose: Cetuximab is a recombinant, human/mouse chimeric IgG(1) monoclonal antibody that binds to the epidermal growth factor receptor (EGFR/HER1). Cetuximab is approved for the treatment of patients with HER1-expressing metastatic colorectal cancer. Limitations in currently reported radiolabeled cetuximab for PET applications prompted the development of (86)Y-CHX-A''-DTPA-cetuximab as an alternative for imaging HER1-expressing cancer. (86)Y-CHX-A''-DTPA-cetuximab can also serve as a surrogate marker for (90)Y therapy.

Methods: Bifunctional chelate, CHX-A''-DTPA was conjugated to cetuximab and radiolabeled with (86)Y. In vitro immunoreactivity was assessed in HER1-expressing A431 cells. In vivo biodistribution, PET imaging and noncompartmental pharmacokinetics were performed in mice bearing HER1-expressing human colorectal (LS-174T and HT29), prostate (PC-3 and DU145), ovarian (SKOV3) and pancreatic (SHAW) tumor xenografts. Receptor blockage was demonstrated by coinjection of either 0.1 or 0.2 mg cetuximab.

Results: (86)Y-CHX-A''-DTPA-cetuximab was routinely prepared with a specific activity of 1.5-2 GBq/mg and in vitro cell-binding in the range 65-75%. Biodistribution and PET imaging studies demonstrated high HER1-specific tumor uptake of the radiotracer and clearance from nonspecific organs. In LS-174T tumor-bearing mice injected with (86)Y-CHX-A''-DTPA-cetuximab alone, (86)Y-CHX-A''-DTPA-cetuximab plus 0.1 mg cetuximab or 0.2 mg cetuximab, the tumor uptake values at 3 days were 29.3 +/- 4.2, 10.4 +/- 0.5 and 6.4 +/- 0.3%ID/g, respectively, demonstrating dose-dependent blockage of the target. Tumors were clearly visualized 1 day after injecting 3.8-4.0 MBq (86)Y-CHX-A''-DTPA-cetuximab. Quantitative PET revealed the highest tumor uptake in LS-174T (29.55 +/- 2.67%ID/cm(3)) and the lowest tumor uptake in PC-3 (15.92 +/- 1.55%ID/cm(3)) xenografts at 3 days after injection. Tumor uptake values quantified by PET were closely correlated (r (2) = 0.9, n = 18) with values determined by biodistribution studies.

Conclusion: This study demonstrated the feasibility of preparation of high specific activity (86)Y-CHX-A''-DTPA-cetuximab and its application for quantitative noninvasive PET imaging of HER1-expressing tumors. (86)Y-CHX-A''-DTPA-cetuximab offers an attractive alternative to previously labeled cetuximab for PET and further investigation for clinical translation is warranted.

Figure 1

(A) Biodistribution of ⁸⁶Y-CHX-A”-DTPA-cetuximab in selected organs of female athymic (NCr) nu/nu mice bearing human colorectal carcinoma LS-174T xenografts. Biodistribution data were obtained at 1, 2, 3 and 4 d after intravenous injection of ⁸⁶Y-CHX-A”-DTPA-cetuximab. (B). Dose-dependent receptor-meditated uptake of ⁸⁶Y-CHX-A”-DTPA-cetuximab in selected organs of female athymic (NCr) nu/nu mice bearing human colorectal carcinoma LS-174T 3 d after injection. All values are expressed as % ID/g. Data represent the mean value ± SEM from at least four determinations.

Figure 2

(A) Representative reconstructed and processed maximum intensity projections of female athymic (NCr) nu/nu mice bearing human colorectal carcinoma LS-174T xenografts injected i.v. with 3.8–4.0 MBq of ⁸⁶Y-CHX-A”-DTPA-cetuximab (B) 3.8–4.0 MBq of ⁸⁶Y-CHX-A”-DTPA-cetuximab co-injected with 0.1 mg cetuximab and (C) 3.8–4.0 MBq of ⁸⁶Y-CHX-A”-DTPA-cetuximab co-injected with 0.2 mg cetuximab. The tumors are indicated with a white arrow.

Figure 3

Representative reconstructed and processed maximum intensity projections of female athymic (NCr) nu/nu mice bearing (A) human pancreatic carcinoma SHAW, (B) human colorectal carcinoma HT29, (C) human prostate carcinoma DU145 and (D) human ovarian carcinoma SKOV3 tumor xenografts injected i.v. with 3.8–4.0 MBq of ⁸⁶Y-CHX-A”-DTPA-cetuximab. The tumors are indicated with a white arrow.

Figure 4

Time-activity curve and uptake values of ⁸⁶Y-CHX-A”-DTPA-cetuximab in female athymic (NCr) nu/nu mice bearing HER1-expressing human tumor xenografts assessed through quantitative small animal PET imaging. All uptake values derived from PET studies are expressed as % ID/cc. Data represent the mean value ± SEM from at least three determinations.