Abstract 3261: Impact of CTLA-4 blockade in conjunction with metronomic chemotherapy on preclinical breast cancer growth

Abstract

Abstract Although we previously reported that metronomic cyclophosphamide can augment the anti-tumor impact of anti-CTLA-4 immunotherapy, the effectiveness of this strategy to treat drug resistant tumors remains to be evaluated. To that end, the parental, drug sensitive, EMT-6/P murine breast cancer, or its cisplatin (DDP) or cyclophosphamide (CTX) resistant variants, were implanted into Balb/c mice (n = 5-8 mice per group), either subcutaneously or orthotopically. Established tumors where monitored for relative growth following treatment with anti-CTLA-4 antibody (mouse anti-CD152, clone 9H10, given at 100ug on day 1 and 35ug on day 6, i.p.), alone or in combination with; a) metronomic CTX (ldCTX; 20mg/kg/day), b) Bolus (150mg/kg) plus ldCTX, or c) sequential treatment with gemcitabine (160mg/kg every 3 days). EMT-6/P tumors responded to anti-CTLA-4 therapy, but this response was less effective when combined with Bolus plus ldCTX. Anti-CTLA-4 could be effectively combined with either ldCTX (without a bolus), or with a regimen of sequential gemcitabine. The EMT-6/P tumor showed consistent results in subcutaneous tumors and in those that were orthotopically implanted into the mammary fat pad. Tumor responses were less pronounced in drug resistant EMT-6/CTX or EMT-6/DDP tumor models than in the EMT-6/P tumor, but nonetheless even in the drug resistant models CTLA-4 targeting could be effectively combined with metronomic CTX or sequential gemcitabine (p<0.05). A number mice implanted with the parent or with the drug resistant tumors developed spontaneous metastases under continuous therapy, including mice bearing subcutaneous tumors, and about 20% of the mice on the combination therapies showed complete tumor responses. Of these mice, over 90% rejected subsequent EMT/6 tumor grafts, indicating that the anti-CTLA-4 plus metronomic chemotherapy regimens did not impair immunological memory. Furthermore, the efficacy of a therapy of anti-CTLA-4 with sequential continuous gemcitabine was confirmed in an independent syngeneic CT-26 colon cancer model. We conclude that metronomic chemotherapy can combined with anti-CTLA-4 therapy, both in syngeneic breast and colon cancer models, and in breast tumors selected in vivo for resistance to alkylating agents. Sequential therapy of anti-CTLA-4 followed by gemcitabine is effective in both chemotherapy naïve tumors and in tumors selected in vivo for resistance to CTX or to DDP, although tumor relapses can occur, in some cases accompanied by the development spontaneous metastases. Our results suggest a strategy for improving the anti-tumor efficacy of anti-CTLA-4 based therapies. Citation Format: Karla Parra, Paloma Valenzuela, Alejandra E. Gallegos, Natzidielly Lerma, Luis Reza, Georgialina Rodriguez, Urban Emmenegger, Marian Manciu, Teresa Di Desidero, Guido Bocci, Mitchell S. Felder, Robert A. Kirken, Giulio Francia. Impact of CTLA-4 blockade in conjunction with metronomic chemotherapy on preclinical breast cancer growth. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3261.