Manipulating the Tumor Microenvironment for Therapeutic Benefit

The physical tumor microenvironment contributes significantly to carcinogenesis, cancer progression and metastatic dissemination. Two main components of the tumor microenvironment, hypoxia and acidosis, are present in nearly every solid tumor and act as powerful selection forces against the tumor. Hypoxia and acidosis promote tumor heterogeneity and contribute to chemotherapy and radiotherapy resistance. This dissertation interrogates methods to target the tumor microenvironment including two novel studies describing mechanisms of buffer therapy resistance and targeting tumor hypoxia with vasodilators to enhance the efficacy of a hypoxia activated prodrug, TH-302. In the first study, mechanisms of buffer therapy resistance were identified and detailed. Many studies have shown that the acidity of solid tumors contributes to local invasion and metastasis. Oral pH buffers can specifically neutralize the acidic pH of tumors and reduce the incidence of local invasion and metastatic formation in multiple murine models. However, this effect is not universal as we have previously observed that metastasis is not inhibited by buffers in some tumor models, regardless of the buffer used. B16-F10 (murine melanoma), LL/2 (murine lung) and HCT116 (human colon) tumors are resistant to treatment with lysine buffer therapy, whereas metastasis is potently inhibited by lysine buffers in MDA-MB-231 (human breast) and PC3M (human prostate) tumors. In the current work, I confirmed that sensitive cells utilized a pH-dependent mechanism for successful metastasis supported by a highly glycolytic phenotype that acidifies the local tumor microenvironment resulting in morphological changes. In contrast, buffer-resistant cell lines exhibited a pH-independent metastatic mechanism involving constitutive secretion of matrix degrading proteases without elevated glycolysis. These results have identified two distinct mechanisms of experimental metastasis, one of which is pH-dependent (buffer therapy sensitive cells) and one which is pH-independent (buffer therapy resistant cells). Further characterization of these models has potential for therapeutic benefit. In the second study, improving the efficacy of hypoxia activated prodrug, TH-302, through induction of hypoxia was investigated. Pancreatic ductal adenocarcinomas are desmoplastic and hypoxic tumors, both of which are associated with poor prognosis. Hypoxia activated prodrugs, such as TH-302, are specifically activated in hypoxic environments and are now in a Phase III clinical trial in pancreatic cancer. Using animal models, we show that tumor hypoxia can be exacerbated using a vasodilator, hydralazine, improving TH-302 efficacy. Hydralazine reduces tumor blood flow through the Steal phenomenon, where atonal immature tumor vasculature fails to dilate in coordination with normal vasculature. The current study shows that MiaPaCa-2 tumors exhibit a Steal effect in response to hydralazine, resulting in decreased tumor blood flow and subsequent tumor pH reduction. The effect is not observed in SU.86.86 tumors with mature tumor vasculature, as measured by CD31 and smooth muscle actin (SMA) immunohistochemistry staining. Combination therapy of hydralazine and TH-302 resulted in a reduction in MiaPaCa-2 tumor volume growth after 18 days of treatment. Further optimization of hypoxia-inducing agents and dosing regimens may lead to increased TH-302 activity, potentially improving clinical outcome. The data presented here demonstrate methods to effectively target the tumor microenvironment for therapeutic benefit. Further investigation into mechanisms of action and biomarkers for therapy response may have important implications on clinical treatment regimens for cancer patients