Stroma-targeting therapies and local intervention in pancreatic ductal adenocarcinoma

Abstract

Pancreatic ductal adenocarcinoma (PDA) remains one of the deadliest forms of cancer, in part, because it is largely refractory to current therapies. The failure of most standard therapies, as well as promising immune therapies, may be largely ascribed to a highly unique and protective stromal microenvironment that presents significant biophysical barriers to effective drug delivery, as well as distribution and function of anti-tumor immune cells. This work supports a growing body of evidence implicating the critical nature of the stroma in tumor progression and resistance to therapy, and demonstrates how disruption of the stroma through focused stromal re-engineering approaches likely represents a key component of viable strategies to treat PDA with molecular and immune therapies. In addition to systemic therapies to combat PDA, local therapies are emerging as promising alternatives. Here, we investigate irreversible electroporation (IRE), a nonthermal focal therapy technique that induces widespread necrosis while sparing local blood vessels. These key characteristics make IRE an ideal treatment modality in the physiologically sensitive context of the pancreas, and may represent an unexplored approach to be used in concert with systemic therapies, such as stroma-targeting therapy (STT). In Chapter 1, we lay the groundwork to understand the unique pathology of PDA and the relatively recent—and at times, controversial—history of STT. We review the antifibrotic agent, halofuginone (HF), which is utilized in the majority of the preclinical intervention studies presented in this dissertation. Complementary to systemic STT approaches explored here, we describe electroporation, a nonthermal focal therapy technique which is widely used in the clinic in combination with chemotherapy, but has been proposed as a direct and monotherapeutic approach to ablate tissue in cancer within the last decade or so. Finally, we review models historically used to study novel therapeutics in PDA, and the predictive utility of each. In Chapter 2, the experimental methods used to conduct these studies are described in detail, including the genetically-engineered KPC mouse model our lab employs and the associated intervention studies that these mice were enrolled in. In Chapter 3, we present the results of our effort to disrupt the severe biophysical barriers characteristic of PDA through stromal re-engineering. This was largely done using the potent antifibrotic agent, HF. In an autochthonous, genetically engineered mouse model of PDA, we show that HF disrupts barriers to effective drug distribution and also results in increased number and distribution of cytotoxic T cells. Furthermore, we show that HF treatment directly targets carcinoma cells and leads to widespread intratumoral necrosis and reduced tumor volume. These data point to the multifunctional and critical role of the stroma in tumor protection and survival, and demonstrate how compromising tumor integrity through STT will likely be an instrumental component in treating PDA. In Chapter 4, we explored the therapeutic potential of local intervention in PDA through IRE. PDA is unable to be treated by a majority of focal therapies used in the clinic, such as heat-based radiofrequency, due to the sensitivity of the pancreas and proximity to the bile duct and major blood vessels. Here, we show that IRE is capable of imposing injury in subcutaneously engrafted tumors and inhibiting tumor volume growth. More importantly, we show that IRE induces widespread necrosis and an apparent immune response in a physiologically relevant model of PDA (i.e. the KPC mouse model). IRE presents a promising alternative—and possibly complementary—approach to systemic therapy. In Chapter 5, we summarize our findings and conclusions, discussing the broader implications of these results. Finally, we outline the future directions that these studies ought to go in. The experiments outlined are expected to flesh out the mechanisms at work in each of these therapeutic approaches, more comprehensively characterize the effects of STT and IRE in the context of long-term treatment, and build on the work done here by exploring the potential for added benefit with combination therapy.