Abstract LB-B13: The application of high-dose low fraction image-guided irradiation (IGMI) in combination with anti-CTLA4 immunotherapy results in an additive inhibition of tumor growth in syngeneic tumor models
Authors: Mckenzie Andrew , Papadopoulou Nektaria , Yin Yinfei , Glenn Shonna , Garland Russell , Williams Neil , Shi Qian , Kumari Rajendra 
Published: May 2016

INTRODUCTION: Radiotherapy (RT) is used across multiple cancer types in a number of clinical settings. The primary mechanism of action is thought to be through irreversible damage to tumor cell or associated stromal DNA, but there is an increasing body of evidence that irradiation also mobilizes tumor-specific immunity.

Recent advances in RT with stereotactic and image-guided micro-irradiation (IGMI) have resulted in an increase in tumor specific targeting with a corresponding reduction in associated side-effects. The resultant use of high-dose, reduced fraction dosing regimens (hypofractionation) has resulted in improved clinical response. However, IGMI in the preclinical setting is less common and typically studies utilize whole body irradiation with lead shielding or simple single beam techniques to focus the radiation. Here we report the application of high-dose, low fraction IGMI to demonstrate an additive combination outcome of radiotherapy and immunotherapy and an abscopal effect in a preclinical model.

EXPERIMENTAL PROCEDURES: CT26 mouse colon and bioluminescent 4T1 mammary carcinoma cells were implanted subcutaneously into BALB/c mice. Subcutaneous tumor growth was monitored by caliper measurement and bioluminescent imaging (BLI) was carried out to confirm metastatic 4T1 lung tumors. Tumors (∼100-200mm3) were treated with hypofractionated IGMI, anti-CTLA4 therapy or a combination of both. For IGMI, mice were anaesthetized and CBCT images were acquired using the small animal research platform (SARRP; Xstrahl Ltd). Fractionated irradiation (5 × 6Gy) was administered to the isocenter of the tumor using a multi beam approach to spare the surrounding tissue. Bodyweight and clinical signs were monitored daily. At termination the primary and metastatic lung (4T1) tumors were collected and assessed for immune cell infiltration by FACs.

RESULTS: Response to the hypofractionated IGMI regimen and anti-CTLA4 either alone or in combination was well tolerated with no adverse effects or significant body weight-loss. Monotherapy with anti-CTLA4 exerted no statistically significant effect on primary or metastatic (4T1) tumor growth and treatment with RT resulted in a modest primary tumor growth inhibition (TGI; 40%). When the two treatments were combined, TGI increased (60%) which was statistically significant compared to RT monotherapy, indicating an additive effect. Additionally, there was a decrease in normalized lung cell counts following combination treatment (4T1) indicating an abscopal response on the lung metastases. This decrease in lung tumor burden corresponded to an increase in both the CD8+ T-cells and the ratio of Teff:Treg in the lung samples.

CONCLUSIONS: Hypofractionated IGMI when combined with anti-CTLA4 treatment had minimal side effects and showed statistically significant additive reduction in primary tumor growth. Furthermore, combination treatment resulted in a reduced metastatic lung burden indicating an abscopal effect that appears to be driven by an increase in CD8+ T-cells. The SARRP platform enables clinically relevant radiotherapy regimens to preclinical models facilitating improved evaluation of combinations strategies with immunotherapeutics and progression to the clinic.

Citation Format: Andrew Mckenzie, Nektaria Papadopoulou, Yinfei Yin, Shonna Glenn, Russell Garland, Neil Williams, Qian Shi, Rajendra Kumari. The application of high-dose low fraction image-guided irradiation (IGMI) in combination with anti-CTLA4 immunotherapy results in an additive inhibition of tumor growth in syngeneic tumor models. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr LB-B13.