Instructions for the genetic code
Epigenetics refers to a broad regulatory system that controls gene expression by modifying chromatin, which consists of DNA wrapped around an assembly of proteins called histones. The DNA included in chromatin is identical in each cell in the body, and the identity and function of each cell is determined by the specific set of genes that are expressed, or turned on or off, in a given cell. Whether a specific set of genes is turned on or off depends on the action of epigenetic regulators.
Epigenetic regulators change the architecture of chromatin, allowing it to adopt an open configuration to facilitate gene expression or, conversely, a closed configuration to suppress gene expression. An open chromatin configuration turns on a gene by allowing access and “readout” of the genetic information stored in DNA. A closed chromatin configuration turns off a gene by preventing access to DNA and results in silencing of gene expression.
We have focused our discovery and development efforts on three distinct classes of epigenetic regulators. First, epigenetic writers, which are enzymes that add chemical modifications on chromatin. Second, epigenetic readers, which are protein families that recognize chemical modifications on chromatin and bind to these modifications using specialized protein domains. Third, epigenetic erasers, which are enzymes that remove chemical modifications from chromatin. The action of each class of epigenetic regulators can alter chromatin configuration and control the expression of certain genes in different ways.
As illustrated in the graphic, epigenetic regulators within the writer, reader and eraser classes modify chromatin and affect gene expression by either adding, binding to or removing chemical tags, which are indicated by dots, on chromatin. In normal cells, these epigenetic mechanisms are tightly regulated so that genes are “turned on” or “turned off” as appropriate.
Abnormal cells, such as proliferating cancer cells, can usurp these epigenetic mechanisms, ultimately leading to disease. In certain contexts, the activity of an epigenetic regulator may be altered due to a genetic mutation, which may make certain cancer cells dependent on the activity of an individual epigenetic regulator for cancer cell growth. In other contexts, cancer cells may use the activity of an epigenetic regulator cooperatively with other cellular factors to exacerbate disease-promoting mechanisms and suppress the effectiveness of drug therapies, including chemotherapeutic agents, targeted agents (e.g., tyrosine kinase inhibitors) and immune-modulating agents (e.g., immune checkpoint inhibitors).
Epigenetics regulators also play a significant role in the differentiation of immune cell populations. The various immune cells in a human body arise from blood progenitor cells, or stem cells, through differentiation processes. Epigenetic regulators govern these differentiation processes by promoting and suppressing certain genes that are specific for each type of immune cell. Cancer cells can use epigenetic regulators to cause the differentiation process to produce or program immune cells that promote tumor immunity. The selective inhibition of these epigenetic regulators may alter this differentiation process and re-program immune cells that increase tumor immunity into immune cells that drive an anti-tumor response. This approach may allow immune cells to overcome resistance to immune checkpoint inhibitors.
Epigenetic inhibitor approaches
Epigenetic inhibition is particularly attractive as a therapeutic approach for several reasons:
- Small-molecule inhibitors can block the abnormal function of epigenetic regulators that cancer cells depend on for growth and potentially restore normal gene expression;
- Cells in the body utilize a large number of epigenetic regulators to control gene expression, which provides a large number of potential drug targets; and
- Biomarkers can be used to enrich for patients who are most likely to respond to epigenetic inhibition.
The early epigenetic inhibitors – such as histone deacetylase and DNA methyltransferase inhibitors – have not delivered on the full potential of the inhibition of epigenetic regulators as a class of cancer therapy. These drugs cause broad changes to gene expression across thousands of genes. This broad inhibition, as opposed to more selective inhibition, generally resulted in unintended effects accompanying the desired effect. Moreover, early epigenetic inhibitors were solely designed to alter gene expression in cancer cells to induce cancer cell death. This approach did not consider the importance of the cells surrounding the cancer cells, referred to as the tumor microenvironment, and the supporting role that epigenetic regulators play in sustaining the tumor microenvironment for cancer cell growth.
Since these early epigenetic drugs, biopharmaceutical development has focused on therapies targeting epigenetic regulators in genetically defined cancer contexts, and specifically in mutated epigenetic regulators with abnormal function that cancer cells depend on for growth. This approach has identified a small number of epigenetic targets and development opportunities for certain targets, including EZH2. Given that abnormal function of epigenetic regulators can arise for reasons other than genetic mutations, we believe that these genetically defined approaches, while valuable, underestimate the potential of identifying and specifically targeting epigenetic regulators in cancer.
We are a pioneer in the discovery and development of novel therapeutics that target the writer, reader and eraser classes of epigenetic regulators and modulate gene expression in a highly selective manner. Our efforts have demonstrated that these distinct classes of epigenetic regulators are broadly druggable and that selective reprogramming of gene expression is a promising therapeutic approach not only to induce cancer cell killing but also to enhance anti-tumor immunity.
Our integrated epigenetics platform includes a deep understanding of the biological context in which epigenetic regulators operate, the development of small-molecule product candidates that selectively modulate their activity and the design of clinical development programs supported by novel biomarker strategies.
Understanding the biological context. We have built a suite of tools that enables us to identify and validate epigenetic target biology. Specifically, we have built a chemical probe library comprising selective and potent small molecules that each inhibit the activity of a specific epigenetic regulator. In addition, we have also built libraries of genetic tools that help us understand the function of specific genes that encode epigenetic regulators. We routinely screen these small molecules and genetic tools across cellular and animal models of disease to identify and validate potential epigenetic targets. We have utilized these tools to identify epigenetic targets that, when inhibited, induce cancer cell death, sensitize tumor cells to an immune response, and reprogram immune-suppressive immune cells to enhance anti-tumor activity.
Development of small-molecule product candidates. Our small-molecule drug discovery engine is supported by our deep understanding of the writer, reader and eraser classes of epigenetic regulators. We have spent over ten years developing this understanding, and that expertise, combined with our capabilities in assay development, biochemistry, compound screening, medicinal chemistry and structural biology, provides a strong platform to continue to develop small-molecule epigenetic inhibitors. As a result, we have been able to develop multiple product candidates across our target classes and expect to continue to do so in the future.
Design of clinical development programs. We believe that our ability to link biological and clinical development to identify a path to registration for each of our product candidates is a crucial component of our platform. We prioritize indications based on the importance of a specific epigenetic target as a driver of a specific cancer. We also evaluate in vivo models, cancer cell line panels and human tumor samples to identify biomarkers that can be used to identify patient populations that may be most likely to respond to treatment with our product candidates. We create and use a range of assays in our preclinical and early clinical testing to validate hypotheses that we may use in later-stage testing to target specific patient populations.
Our approach to therapeutic agents is focused on epigenetic targets:
- whose inhibition modulates gene expression in a highly selective manner;
- with broad development opportunities, including biomarker-defined contexts, which we believe may expand the applicability of our product candidates to cancers with immune evasion or acquired drug resistance; or
- whose inhibition may reprogram immune-suppressive immune cells in the tumor microenvironment to enhance anti-tumor activity.
Our epigenetics platform allows us to intervene in diseases by targeting distinct classes of epigenetic regulators, including the following examples:
- EZH2—an epigenetic writer—is an enzyme that suppresses target gene expression by adding modifications to chromatin. Certain cancer and immune cells can use EZH2 to promote growth of cancer cells or suppress an anti-tumor response;
- BET proteins—a group of epigenetic readers—are proteins that bind to chromatin and enhance target gene expression. Certain cancer cells can use BET proteins to promote growth of cancer cells and inflammatory disorders; and
- LSD1—an epigenetic eraser—is an enzyme that suppresses target gene expression by removing modifications from chromatin. Certain cancer cells and immune cells can utilize lysine-specific demethylase 1A, or LSD1, to promote growth of cancer cells or suppress an anti-tumor immune response.
We believe that we can leverage our epigenetic platform to expand our discovery and development efforts into additional classes of epigenetic targets. We are also currently advancing several discovery programs against undisclosed epigenetic regulators focused on the tumor and immune microenvironment.
Therapeutic Applications of Epigenetics
Cancer is a heterogeneous group of diseases characterized by uncontrolled cell division and growth. Cancer can arise when the dysregulation of the cell’s gene expression program alters the identity of single normal cells.
This dysregulation can arise when there are changes in genes that control signaling pathways governing cell differentiation and function. Cancer cells can utilize this abnormal gene expression by using epigenetic regulators to activate pro-tumor genes or deactivate tumor suppressor genes. Furthermore, cancer cells can also use epigenetic regulators to activate resistance mechanisms against cancer treatments, including chemotherapy, targeted therapy and immunotherapy, and render the treatments less effective. We have designed our product candidates with the aim of treating various cancers by impacting these epigenetic regulators utilized by cancer cells.