Abstract C106: CBP501 potentiates the appearance of cisplatin-induced indicators of immunogenic cell death and promotes anti-tumor effects in an immuno-competent mouse model
Authors: Sakakibara Keiichi , Sato Takuji , Kufe Donald W. , Von Hoff Daniel D. , Kawabe Takumi 
Published: May 2016

Introduction: The CBP501 calmodulin-binding peptide is an anti-cancer drug candidate that has completed two Phase II clinical trials for patients with malignant pleural mesothelioma and non-small cell lung carcinoma (NSCLC). CBP501 was previously described as a unique G2 checkpoint-directed agent and as an enhancer of cisplatin (CDDP) uptake. In a post-trial analysis of patients with NSCLC, it was found that overall survival (OS) was prolonged in a subpopulation of patients with normal white blood cell counts (WBC) (WBC &lt; 8000). Here we show that modulation of immunogenic cell death might be a novel function of CBP501. Based on this novel activity of CBP501, combined treatment with PD-1 blocking antibodies in vivo was also explored.

Methods: CT26WT, a strain of CBP501 sensitive cells, were treated with clinically achievable dose levels of CDDP (10 or 20 uM) and 0.5 uM CBP501 for 0.75 h in vitro. After exchange with fresh drug-free medium, cells were incubated for 24, 48, or 72 h for IB analysis of phospho-eIF2-alpha, for calreticulin analysis by FACS, or for HMGB1 ELISA. The in vivo mouse study examined the effects of 3 dosing cycles for 3 anti-cancer agents, alone or in different combinations [CDDP: 5 mg/kg x 1/week, CBP501: 7.5 mg/kg x 3/week, anti-mPD1 antibody (RMP1-14): 200 ug x 1/week] using immuno-competent BALB/c mice bearing subcutaneous inoculated CT26 WT cells.

Results: In vitro, CDDP in combination with CBP501 elicited increased death in CT26WT cells, as well as an increase in different indicators of immunogenic cell death. These indicators include induction of phospho-eIF2-alpha, increase in cell surface-exposed calreticulin, and extracellular release of HMGB1. In an in vivo BALB/c mouse model, CDDP-treated mice showed a reduction of tumor growth by 52.7% as compared to vehicle-treated mice. CBP501 + CDDP showed an additional reduction of tumor growth by 63.1% as compared to vehicle-treated mice. Treatment with anti-mPD-1 antibody alone showed a slight reduction of tumor growth by 25.2% as compared to vehicle-treated mice. However, combined treatment with anti-mPD-1 + CDDP or anti-mPD-1 + CDDP + CBP501 showed significant reductions in the tumor growth in comparison to the vehicle-treated mice by 69.3% and 78.7%, respectively.

Conclusion: These results suggest that the anti-tumor activity of combined CBP501 + CDDP treatment is potentiated by inducing immunogenic cell death. This novel effect might contribute to the prolonged OS found in the phase II clinical trials. Combined treatments that include the anti-mPD-1 immune-checkpoint inhibitor were effective and shall be examined further. Further, immunohistochemical analyses of the effects of this combined treatment on tumor microenvironment are under way.

Citation Format: Keiichi Sakakibara, Takuji Sato, Donald W. Kufe, Daniel D. Von Hoff, Takumi Kawabe. CBP501 potentiates the appearance of cisplatin-induced indicators of immunogenic cell death and promotes anti-tumor effects in an immuno-competent mouse model. [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 C106.