Aprea Therapeutics is pioneering the development of next-generation cancer therapeutics that regulate the DNA damage response (DDR).
DNA Damage Response
Cells are continuously exposed to endogenous and exogenous stress that can lead to DNA damage. To counter this lethal threat, cells have mechanisms to detect DNA damage, activate the appropriate repair pathway or, if irreparable, induce cell cycle arrest or apoptosis. These DDR processes are vital for cell survival.
Cancer cells rely on various alternative pathways to repair and resist DNA damage and replication stress. Many of these DDR-related genes are mutated across cancers, as loss of the DDR pathway allows cancer cells to rapidly evolve and grow out of control. Notably, functional loss of these pathways also creates a vulnerability in these cancers because mutation or loss of some DDR genes increases reliance on other DDR genes to support continued cancer cell growth. When mutation or loss of two DDR genes leads to cell death, the interplay between these genes is synthetic lethality. Importantly, selective targeting of specific members of the DDR pathway represents an attractive potential therapeutic approach for the treatment of cancer. Furthermore, because genes that are mutated in cancers continue to function normally in healthy tissues, this treatment approach can potentially reduce drug-induced toxicity while maintaining anti-cancer activity.
We have developed highly selective small molecule regulators of DDR proteins that play fundamental roles in these response pathways. Our regulators of ATR and WEE1 may capitalize on DDR-associated cancer cell vulnerabilities, as described below.
Aprea’s Approach to Synthetic Lethality and Modulators of DNA Damage Response
Leveraging synthetic lethality in therapeutic targeting of DDR represents an emerging strategy to treat a broad spectrum of cancers that currently lack effective treatments. Our team was the first to identify ATR as a drug target that synergistically kills cancer cells based on one of their most fundamental characteristics, oncogene expression. Aprea’s development pipeline is based on our discovery platforms and a rationally designed series of novel molecules.
Ataxia Telangiectasia and Rad3-related (ATR) and Checkpoint Kinase 1 (CHK1) are critical DNA damage response kinases that prevent the collapse of replication forks into DNA double strand breaks (DSBs). ATR is one of several key regulators of the response to defective DNA replication and DNA damage, which occurs more commonly in cancer cells than in normal cells.
In response to these cancer-associated genomic insults, ATR is activated to inhibit progression to cellular division and prevent the assembly of the SLX1-SLX4, MUS81-EME1 and XPF-ERCC1 (SMX) endonuclease (DNA cutting) complex. When ATR is inhibited, the SMX complex is inappropriately activated, promoting the cutting of replication forks into DSBs. In association with ATR’s fundamental roles in these replication responses, cells with increased oncogenic stress, p53 mutations and deficiencies in DDR pathways are predicted to have increased sensitivity to ATR inhibition. Accordingly, ATR inhibition is also predicted to sensitize cells to DNA-damaging chemotherapy, radiotherapy and PARP inhibitor treatments, making ATR inhibitors particularly attractive for the development of novel combination therapies.
We have developed a highly potent and selective macrocyclic inhibitor of ATR (ATRN-119) that has the potential to have less toxicity to normal tissues while continuing to capitalize on cancer vulnerabilities due to oncogenic stress and DDR pathway defects. ATRN-119 has received FDA IND approval (IND #141317) for a first-in-human clinical trial for cancer patients. This clinical trial is expected to begin in the third quarter of 2022.
WEE1 kinase is a key regulator of multiple phases of the cell cycle, most prominently in progression from G1 to S phase and from S/G2 to M phase through inhibitory phosphorylation of CDK2 and CDK1, respectively. Thus, when WEE1 is inhibited, both G1-S and G2-M checkpoints are abrogated, leading to premature S-phase and M-phase entry. Notably, the replication stress caused by cyclin E1 overexpression is transformed into toxic levels of DSBs and cancer cell death when WEE1 is inhibited. These findings suggest cyclin E overexpression as a cancer-associated vulnerability that may be capitalized on by WEE1 inhibitors.
We have developed a highly potent and selective WEE1 inhibitor, ATRN-W1051, that is distinct from other WEE1 inhibitors based on its potentially superior pharmacokinetic properties and selectivity regarding common off-targets (PLK1/2/3). ATRN-W1051 is expected to enter IND-enabling studies in 2022.