Abstract 3813: Recruitment of macrophages, NK cells, and dendritic cells with metronomic cyclophosphamide-induced regression of gliosarcoma xenografts

Abstract

Abstract Metronomic chemotherapy displays improved therapeutic activity over conventional schedules for several malignancies in both preclinical and clinical studies. In particular, metronomic cyclophosphamide treatment (140 mg/kg BW every 6 days) induces dramatic, near-complete regression of established xenografts of human U251 and rat 9L gliosarcomas in s.c. scid mouse models, whereas maximum tolerated dose cyclophosphamide effects only moderate tumor growth delays. Furthermore, metronomic cyclophosphamide-induced regression of 9L tumors is accompanied by reduced microvessel density, induction of thrombospondin-1 (TSP1) and impaired drug uptake, all of which are consistent with the prevailing view that anti-angiogenesis is a major factor in the therapeutic effectiveness of metronomic chemotherapy. However, VEGF receptor inhibition induces substantially greater anti-angiogenesis in these tumor models but does not induce tumor regression. Therefore, we sought to investigate other non-anti-angiogenic mechanisms to explain the substantial regression and have found that it is strongly associated with recruitment of the host innate immune system. In regressing 9L and U251 tumors, host (mouse) PEDF (pigment epithelium-derived factor; Serpinf1) and PAI-1 (plasminogen activation inhibitor-1; Serpine1) expression is increased, with the levels of PEDF RNA being strongly correlated (r=0.89) with TSP1 RNA in large sets of 9L and U251 tumors isolated various times after initiation of metronomic cyclophosphamide therapy. Notably, PAI-1 and TSP1 are both expressed by tumor-associated macrophages. Metronomic cyclophosphamide also increased Fas, which stimulates macrophage activation, and the macrophage marker CD68, supporting the involvement of tumor-associated macrophages. Fas is also associated with cytotoxic lymphocytes, such as NK cells, and NK cell markers and cytotoxic effectors were dramatically increased in metronomic cyclophosphamide-treated tumors. Furthermore, dendritic cell markers important for cell-target interactions and antigen presentation were increased by metronomic cyclophosphamide treatment. Cytokines and chemokines were also up regulated in the metronomic cyclophosphamide-treated tumors, suggesting that chemokine and cytokine gradients help mobilize the host innate immune system in regressing 9L and U251 tumors. These findings highlight the ability of metronomic chemotherapy to induce macrophage, natural killer cell, and dendritic cell recruitment, thereby activating innate immunity leading to regression of established gliosarcomas and potentially other tumor types. Supported by NIH grant CA49248 (to DJW). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3813.