Panitumumab-DOTA-111In: An Epidermal Growth Factor Receptor Targeted Theranostic for SPECT/CT Imaging and Meitner-Auger Electron Radioimmunotherapy of Triple-Negative Breast Cancer

Epidermal growth factor receptors (EGFR) are overexpressed in triple-negative breast cancer (TNBC) and are an attractive target for the development of theranostic radiopharmaceuticals. We studied anti-EGFR panitumumab labeled with 111In (panitumumab-DOTA-In-111) for SPECT/CT imaging and Meitner-Auger electron (MAE) radioimmunotherapy (RIT) of TNBC. Panitumumab-DOTA-In-111 was bound, internalized, and routed to the nucleus in MCF7, MDA-MB-231/Luc, and MDAMB-468 human breast cancer (BC) cells dependent on the EGFR expression level (1.5 x 10(4), 1.7 x 10(5), or 1.3 x 10(6) EGFR/cell, respectively). The absorbed dose in the nuclei of MCF7, MDA-MB-231/Luc, and MDA-MB-468 cells incubated with 4.4 MBq of panitumumab-DOTA-In-111 (20 nM) was 1.20 +/- 0.02, 2.2 +/- 0.1, and 25 +/- 2 Gy, respectively. The surviving fraction (SF) of MDAMB-231/Luc cells treated with panitumumab-DOTA-In-111 (10-300 nM; 1.5 MBq/mu g) was reduced as the absorbed dose in the cell increased, with clonogenic survival reduced to an SF = 0.12 +/- 0.05 at 300 nM corresponding to 12.7 Gy. The SFs of MDA-MB-468, MDA-MB-231/Luc, and MCF7 cells treated with panitumumab-DOTA-111In (20 nM; 1.7 MBq/mu g) were < 0.01, 0.56 +/- 0.05, and 0.67 +/- 0.04, respectively. Unlabeled panitumumab had no effect on SF, and irrelevant IgG-DOTA-In-111 only modestly reduced the SF of MDA-MB-231/Luc cells but not MCF7 or MDA-MB-468 cells. The cytotoxicity of panitumumab-DOTA-In-111 was mediated by increased DNA double-strand breaks (DSB), cell cycle arrest at G2/M-phase and apoptosis measured by immunofluorescence detection by flow cytometry. MDA-MB-231/Luc tumors in the mammary fat pad (MFP) of NRG mice were clearly imaged with panitumumab-DOTA-In-111 by microSPECT/CT at 4 days postinjection (p.i.), and biodistribution studies revealed high tumor uptake [18 +/- 2% injected dose/g (% ID/g] and lower normal tissue uptake (< 10% ID/g). Administration of up to 24 MBq (15 mu g) of panitumumab-DOTA-In-111 to healthy NRG mice caused no major hematological, renal, or hepatic toxicity with no decrease in body weight. Treatment of NOD SCID mice with MDA-MB-231 tumors with panitumumab-DOTA-In-111 (22 MBq; 15 mu g) slowed tumor growth. The mean time for tumors to reach a volume of >= 500 mm(3) was 61 +/- 5 days for RIT with panitumumab-DOTA-In-111 compared to 42 +/- 6 days for mice treated with irrelevant IgG(2)-DOTA-In-111 (P < 0.0001) and 35 +/- 3 days for mice receiving 0.9% NaCl (P < 0.0001). However, tumors regrew at later time points. The median survival of mice treated with panitumumabDOTA-In-111 was 70 days versus 46 days for IgG2-DOTA-In-111 (P < 0.0001) or 40 days for 0.9% NaCl (P < 0.0001). We conclude that panitumumab-DOTA-In-111 is a promising theranostic agent for TNBC. Increasing the administered amount of panitumumabDOTA-In-111 and/or combination with radiosensitizing PARP inhibitors used for treatment of patients with TNBC may provide a more durable response to RIT.

Automated summary

The study investigated the use of anti-EGFR panitumumab labeled with 111In for SPECT/CT imaging and radioimmunotherapy of triple-negative breast cancer (TNBC). The results showed that panitumumab-DOTA-In-111 is a promising theranostic agent for TNBC, and combining it with radiosensitizing PARP inhibitors may provide a more durable response to therapy.