Abstract 61P
Background
Triple-negative breast cancer (TNBC) is linked to poor prognosis and presents significant treatment challenges. Chemotherapy remains the backbone of therapy in this disease, and Carboplatin, a DNA crosslinking agent, is commonly used. It induces DNA damage, cell cycle arrest, and apoptosis. However, the effectiveness of carboplatin is frequently limited by resistance, emphasizing the need for novel therapies to target chemoresistant TNBC.
Methods
We developed chemoresistant patient-derived xenograft (PDX) cells (PDXC) and performed an shRNA high-throughput screen to identify genetic vulnerabilities that could resensitize them to carboplatin. To validate the synergy, we conducted cell viability assays using specific drugs and RNAi silencing. Western blotting, immunofluorescence, flow cytometry and RNA-sequencing were used to investigate the underlying molecular mechanisms. Our combination therapies were validated in vivo using PDXs.
Results
We identified ataxia telangiectasia and Rad3-related protein (ATR) as a promising target. We validated this finding using the ATR inhibitor elimusertib, which, when combined with carboplatin, significantly extended survival and reduced tumor growth in two PDX models. RNA sequencing of carboplatin-treated cells revealed 45 differentially regulated genes, all reversed upon addition of elimusertib. Notably, 38 of these genes were regulated by the transcription factor FOXM1, critical for mitotic transcription. Furthermore, carboplatin-exposed cells showed sensitivity to PKMYT1 depletion, essential for preventing unscheduled mitotic entry by phosphorylating CDK1. We found that the novel PKMYT1 inhibitor lunresertib synergized with carboplatin both in vitro and in vivo. Combining carboplatin with either ATR or PKMYT1 inhibition induced mitotic catastrophe in TNBC cells. As both elimusertib and lunresertib are in clinical development, PDXCs resistant to these drugs were generated to further explore the molecular biology in chemo-resistant TNBCs.
Conclusions
Our results highlight the susceptibility of TNBC to targeted cell cycle modulators when used with DNA damage agents like carboplatin and this vulnerability could enhance treatment efficacy in TNBC.
Clinical trial identification
Editorial acknowledgement
Legal entity responsible for the study
Juliet Guay and Dr. Mark Basik.
Funding
Cancer Research Society and Canadian Institutes of Health Research.
Disclosure
All authors have declared no conflicts of interest.