Improved Tumor Responses with Sequential Targeted α-Particles Followed by Interleukin 2 Immunocytokine Therapies in Treatment of CEA-Positive Breast and Colon Tumors in CEA Transgenic Mice

Abstract

Targeted α-therapy (TAT) delivers high-linear-transfer-energy α-particles to tumors with the potential to generate tumor immune responses that may be augmented by antigen-targeted immunotherapy. Methods: This concept was evaluated in immunocompetent carcinoembryonic antigen (CEA) transgenic mice bearing CEA-positive mammary or colon tumors. Tumors were targeted with humanized anti-CEA antibody M5A labeled with ²²⁵Ac for its 10-d half-life and emission of 4 α-particles, as well as being targeted with the immunocytokine M5A-interleukin 2. Results: A dose response (3.7, 7.4, and 11.1 kBq) to TAT only, for orthotopic CEA-positive mammary tumors, was observed, with a tumor growth delay of 30 d and an increase in median survival from 20 to 36 d at the highest dose. Immunocytokine (4 times daily) monotherapy gave a tumor growth delay of 20 d that was not improved by addition of 7.4 kBq of TAT 5 d after the start of immunocytokine. However, TAT (7.4 kBq) followed by immunocytokine 10 d later led to a tumor growth delay of 38 d, with an increase in median survival to 45 d. Similar results were seen for TAT followed by immunocytokine at 5 versus 10 d. When a similar study was performed with subcutaneously implanted CEA-positive MC38 colon tumors, TAT (7.4 kBq) monotherapy gave an increase in median survival from 29 to 42 d. The addition of immunocytokine 10 d after 7.4 kBq of TAT increased median survival to 57 d. Immunophenotyping showed increased tumor-infiltrating interferon-γ-positive, CD8-positive T cells and an increased ratio of these cells to Foxp3-positive, CD4-positive regulatory T cells with sequential therapy. Immunohistochemistry confirmed there was an increase in tumor-infiltrating CD8-positive T cells in the sequential therapy group, strongly suggesting that immunocytokine augmented TAT can lead to an immune response that improves tumor therapy. Conclusion: Low-dose (7.4 kBq) TAT followed by a 4-dose immunocytokine regimen 5 or 10 d later gave superior tumor reductions and survival curves compared with either monotherapy in breast and colon cancer tumor models. Reversing the order of therapy to immunocytokine followed by TAT 5 d later was equivalent to either monotherapy in the breast cancer model.

Keywords: breast cancer; carcinoembryonic antigen; colon cancer; radionuclide therapy; targeted alpha therapy; targeted immunotherapy.

Graphical abstract

FIGURE 1.

(A) Treatment schema and color codes for TAT ²²⁵Ac-DOTA-M5A TAT doses in orthotopic breast cancer model. (B) Tumor growth curves. (C) Kaplan–Meier survival plot. (D) Weight loss. Groups contained 8 mice, with 2 mice removed at days 21 and 22 for blood analysis. **P < 0.01. ***P < 0.001.

FIGURE 2.

(A) Treatment schema for immunocytokine first followed by TAT (bottom) or TAT first followed by immunocytokine (top) in breast cancer model. Groups contained 8 mice, with 2 mice removed at days 21 and 22 for blood analysis. (B and C) Tumor growth curves (B) and Kaplan–Meier survival plot (C) for immunocytokine-first vs. TAT-first sequential therapy. P values are vs. untreated controls. **P < 0.01. ***P < 0.001. ICK = immunocytokine.

FIGURE 3.

(A) Treatment schema for TAT followed by immunocytokine 5 d later (top) or 10 d later (bottom) in breast cancer model. Groups contained 8 mice, with 2 mice removed at days 21 and 22 for blood analysis. (B and C) Tumor growth curves (B) and Kaplan–Meier survival plot (C). P values are vs. untreated controls. **P < 0.01. ***P < 0.001. ICK = immunocytokine.

FIGURE 4.

(A and B) Tumor growth curves (A) and Kaplan–Meier survival plot (B) for TAT dose response in colon cancer model (6 per group, with 2 removed at day 27). (C and D) Tumor growth curves (C) and Kaplan–Meier survival plot (D) for 2 × 7.4 kBq vs. 14.8 kBq of TAT (n = 5–6). P values are vs. untreated controls. **P < 0.01. ***P < 0.001.

FIGURE 5.

Treatment schema and TAT doses (A), tumor growth curves (B), and Kaplan–Meier survival plot (C) for TAT first followed by immunocytokine therapy in colon cancer model (7 per group). P values are vs. untreated controls. **P < 0.01. ***P < 0.001. ICK = immunocytokine.