Anti-tumor effect of radioimmunotherapy using 177Lu-labeled anti-EGFR monoclonal antibody in head and neck squamous cell carcinoma xenograft model

Abstract

Background: Epidermal growth factor receptor (EGFR) has been one of the most comprehensively studied molecular targets in head and neck squamous cell carcinoma (HNSCC). However, as monotherapy, the use of monoclonal antibodies against EGFR have shown limited efficacy. In this study, we evaluated the feasibility of EGFR-targeted radioimmunotherapy using a 177Lu-labeled anti-EGFR monoclonal antibody in a SNU-1066 HNSCC mouse model.

Methods: 1. Radiolabelling of 177Lu and PCTA-cetuximab immunoconjugate Immunoconjugates were prepared by incubating cetuximab (20 mg) with the bifunctional chelator, 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-4-S-(4-isothiocyanatobenzyl)-3,6,9-triacetic acid (p-SCN-Bn-PCTA, Macrocyclics Inc., Dallas, TX, USA) (10 equivalents) in 100 mM sodium bicarbonate buffer (pH 8.5) at 4°C for overnight. Five hundred micrograms (250 μL) of immunoconjugate were reacted with 177Lu solution (37~370 MBq, 0.1 M sodium acetate buffer pH 6.5) for 60 min at room temperature or 37°C, with constant shaking.

2. Biodistribution study and Micro-SPECT/CT imaging SNU-1066 cells (1.2 × 107 cells) were subcutaneously injected into the right flank of 6-week-old female athymic mice. Biodistribution experiments were performed when tumor volume reached 100-300 mm3. Mice bearing SNU-1066 HNSCC tumors were intravenously injected with a 3.7 MBq dose of 177Lu-PCTA-cetuximab (n = 4) and mice were sacrificed at 14 days. Micro-SPECT/CT was performed on the 5th day after 177Lu-PCTA-cetuximab injection (12.95 MBq) in the SNU-1066 tumor model. At 30 min prior to 177Lu-PCTA-cetuximab injection, excess cold cetuximab (40 mg/kg mouse body weight) was intravenously injected in a blocking experiment. 3. Radioimmunotherapy using 177Lu-PCTA-cetuximab SNU-1066 cells (1.2 × 107 cells in PBS pH 7.4) were subcutaneously injected into the right flank of mice. SNU-1066 tumor-bearing mice were randomly divided into four groups when tumor volume reached 100-200 mm3 (n = 9 or 10 per group). HNSCC tumor mice model were intravenously administrated with saline, cetuximab (5 mg/kg, single dose or 10 mg/kg, three times a week for 2 weeks) or 177Lu-PCTA-cetuximab (12.95 MBq, single dose), respectively. Tumor volume was calculated by (long diameter × short diameter2) / 2 and body weight was measured three times a week.

Results: 1. Radiolabelling of 177Lu and PCTA-cetuximab immunoconjugate The average number of chelates per cetuximab was determined to be 4.0 ± 0.4 by MALDI mass spectrometry. 177Lu-PCTA-cetxuximab was prepared successfully with high radiolabeling yield and radiochemical purity (≥ 98%) which was confirmed by Instant Thin Layer Chromatography-silica gel (ITLC-sg).

2. Biodistribution study and Micro-SPECT/CT imaging The activity of 177Lu decreased over time in all organs except for the SNU-1066 tumor, which continued to accumulate radioactivity up to 7 days after administration. The maximum tumor uptake was 20.6 ± 5.2%ID/g on day 7 and then decreased gradually to 17.5 ± 4.9 %ID/g on day 14. The radioactivity in blood was 29.0 ± 3.6 %ID/g at 2 h, followed by relatively fast clearance by the end of 14 days (2.3 ± 0.4 %ID/g). The %ID/g of 177Lu-PCTA-cetuximab for all normal organs was always less than 10%, except for blood, liver and spleen.

3. Radioimmunotherapy using 177Lu-PCTA-cetuximab Treatment with a single-dose injection (12.95 MBq) of 177Lu-PCTA-cetuximab treatment showed marked regression of tumor volume. 177Lu-PCTA-cetuximab-treated SNU-1066 tumors on day 42 post-treatment showed a 70.3% reduction compared with that on day 2 after treatment. SNU-1066 tumor volume in the 177Lu-PCTA-cetuximab-treated group showed a statistically significant difference compared to the saline-treated groups by day 9 (P < 0.05), the single-dose cetuximab group by day 21 (P < 0.05) and the multiple-dose cetuximab group by day 35 (P < 0.05).

Conclusion: Our results suggest that cetuximab can function as an effective carrier for tumor-targeted delivery of radiation, and that radioimmunotherapy using 177Lu-PCTA-cetuximab is promising for the targeted therapy of EGFR-positive tumors compared to antibody-based immunotherapy.