Investigating Resistance to Checkpoint Kinase 1 Inhibitors in Triple Negative Breast Cancer

Triple negative breast cancer (TNBC) is an aggressive and metastatic disease and is characterised by the absence of three therapeutically relevant receptors commonly found in breast cancers, namely oestrogen, progesterone, and human epidermal growth factor receptor 2. Loss of these targets makes hormone therapies and some targeted therapies ineffective, limiting treatment options available to patients with chemotherapy and surgery remaining the standard care of treatment. Loss of TP53 gene occurs at high frequency in TNBC making these cancers highly genomically unstable creating a dependency on remaining pathways to regulate the cell cycle and correct replication of DNA. Checkpoint Kinase 1 (CHK1) is a serine/threonine kinase activated in response to DNA damage to induce cell cycle arrest and/or replication fork stalling. DNA damage can be caused by exogenous agents such as genotoxic chemotherapy (e.g., gemcitabine), or innate in cancers with high levels of genomic instability and replication stress. Consequentially, tumours may become dependent on CHK1 for survival, making this kinase a potential therapeutic target. Preclinical data indicates that TNBC cells are sensitive to CHK1 inhibitors (CHK1i) SRA737 and Prexasertib, which are both in clinical trial. Unfortunately, resistance to anticancer drugs can emerge in tumours leading to patient relapse. It is therefore important to address this issue, even before a drug is approved for use. Consequently, this project aims to identify mechanisms, biomarkers, and strategies to overcome CHK1i resistance.

CHK1i resistant cell line models were generated from HCC38 and MDA-MB-468 TNBC cell lines to SRA737 and Prexasertib. HCC38 resistant sublines exhibit cross resistance to inhibitors of the CHK1 activator ATR (Ceralasertib) and cell cycle regulator WEE1 (Adavosertib). WEE1 phosphorylates CDK1/2 at Y15 inhibiting CDK activity and is present at higher levels in resistant sublines. Low doses of Adavosertib in combination with SRA737 resensitise resistant cells to CHK1i. Additionally, CDK2/1 inhibitor Roscovitine but not CDK1 inhibitor RO-3306 rescues the effect of SRA737 and Adavosertib drug combination in HCC38 parental and resistant sublines, suggesting resistant cells are dependent on maintenance of CDK2 inhibition for survival. Furthermore, HCC38 resistant sublines show resistance to the genotoxic agent Gemcitabine, suggesting further resistance mechanisms which may promote resilience to replication stress. However, low concentrations of Gemcitabine strongly resensitise HCC38 resistant sublines to CHK1i SRA737. Additionally, RNA sequencing of HCC38 parental and resistant sublines has identified a number of differentially expressed genes which may play a role in drug resistance.

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Project Aims

• Investigate and identify biomarkers and mechanisms of resistance to CHK1 inhibitors SRA737 and Prexasertib in Triple Negative Breast Cancer cell lines.

• Validate identified biomarkers/mechanisms of resistance to CHK1 inhibitors.

• Identify clinically relevant therapeutic strategies to overcome resistance.

Project Achievements

• Generated TNBC cell lines with acquired resistance to CHK1 inhibitors.

• Identified a CDK2 dependent mechanism of CHK1 inhibitor resistance.

• Showed WEE1 overexpression is a potential biomarker and driver of acquired resistance to CHK1 inhibitors.

• Demonstrated the use of either Adavosertib (WEE1i) or Gemcitabine (Anthracycline) can be used in combination with a CHK1 inhibitor to overcome acquired resistance to CHK1 inhibitors in TNBC.