Combination Strategies for the Treatment of KRAS Mutant Colorectal and Pancreatic Cancer

Abstract

A large percentage of human cancers show mutations in RAS, a critical activator of the mitogen activated protein kinase (MAPK) signaling cascade that has been well documented for its role in driving tumorigenesis. While there have been numerous attempts to inhibit RAS signaling, efforts have largely been unsuccessful. The major reason leading to the lack of success is the small size of the RAS protein, which prevents the discovery of adequate binding sites for small molecules. While directly inhibiting RAS has not been achieved except in isolated cases, there has been much better success at inhibiting downstream effectors of RAS. Inhibition of downstream effectors have focused on MAPK pathway kinases RAF, MEK and ERK. Additionally, inhibition of PI3K as well as the human growth factor receptors (HERs) have been moderately successful. However, resistance and adaptive signaling in response to therapy is prevalent. A new trend of combination therapies is emerging within the field, allowing effective inhibition of multiple proteins in ways that reduce the ability of tumor cells to escape inhibition. The goal of this dissertation is to delineate downstream effectors that are effective in blocking RAS signaling when combined with another kinase inhibitor and evaluate efficacy of these combination strategies. Focus was placed on developing novel strategies for the treatment of patients diagnosed with KRAS-activated pancreatic or colorectal cancer, an isoform of the RAS oncogene. These cancers show high mutation rates in KRAS: up to 90% in pancreatic cancer and 30-50% in colorectal cancer. The high mutation rate and lack of effective therapies for patients diagnosed KRAS-mutant disease is a critical unmet need. These two chapters approach the problem in different ways. In Chapter 2, which explores co-inhibition of MEK and CDK4/6, a screen was employed to find the most responsive cancer cell line models of a panel. This study identified two pancreatic cancer models, L3.6pl and UM59, that exhibited the highest response to treatment. In these models, it was found that COX-2 expression was higher than in those that did not respond as well. These findings identify a potential biomarker that could have implications for current and future clinical trials evaluating this treatment strategy. Chapter 3 evaluates the activity of a rationally designed molecule that inhibits PI3K and EGFR. These are two prominent proteins central to growth signaling in the cell that participate with RAS for malignant growth. The activity of MTX-211 was screened for activity on the NCI-60 panel, a curated collection of cancer cell line models from different tissues. From this screen, it emerged that MTX-211 was most effective in colorectal cancer models and in models with a PIK3CA mutation. Further evaluation of MTX-211 identified MEK, a downstream signaling effector of RAS, as an ideal combination partner. Inhibition of cancer cells with a MEK inhibitor activates EGFR and PI3K signaling, which MTX-211 can block. This is a novel combination strategy that is effective in targeting adaptive signaling that emerges from single agent MEK inhibition and causes apoptosis in treated cells. It was further found that MTX-211 was effective as a single agent and in combination with a MEK inhibitor in several animal studies of tumor-bearing mice.