A Phase 1 clinical trial combining two DNA-damage drugs has produced an objective response rate of 37.8 percent in women with BRCA1/2-mutated, HER2-negative breast cancer who had never taken a PARP inhibitor before. The study, published in the British Journal of Cancer, paired ceralasertib, an ATR kinase inhibitor, with olaparib, a widely prescribed PARP inhibitor, and enrolled 142 patients with advanced solid tumors. The trial found the combination delivered manageable safety alongside encouraging early activity in a carefully selected patient group.
Among 37 patients in the BRCA-mutant expansion cohort, 14 achieved confirmed responses—one complete and 13 partial—with a median duration of response lasting 9.8 months. When restricted to the 30 patients whose BRCA mutations were centrally verified by next-generation sequencing, the response rate climbed to 40.0 percent, median response duration stretched to 11.0 months, and median progression-free survival reached 9.0 months. Median overall survival in this confirmed group hit 26.2 months. By contrast, the parallel cohort of 25 patients with BRCA wild-type triple-negative breast cancer—lacking homologous recombination repair mutations—produced zero objective responses, with median progression-free survival of only 3.1 months. Safety events were nearly universal, affecting 97.3 percent of patients in the BRCA-mutant group and 96.0 percent in the triple-negative cohort, but severe toxicity was less common, at 43.2 percent and 36.0 percent respectively. The most frequent serious side effects were blood-related: anemia and neutropenia in the BRCA-mutant cohort, and anemia, thrombocytopenia, and neutropenia in the triple-negative group. Adverse events forced discontinuation in just two of 37 patients in the BRCA-mutant arm and one of 25 in the triple-negative arm, with no deaths from side effects in the BRCA-mutant cohort.
The investigators concluded that no new safety signals emerged beyond those already associated with each drug class. They set the recommended Phase 2 dose at ceralasertib 160 mg once daily on days 1 to 7 of each 28-day cycle, plus olaparib 300 mg twice daily given continuously, a schedule shaped by dose-limiting blood toxicity that appeared when researchers tried to push ceralasertib higher or extend dosing days. Paired tumor biopsies showed rises in phosphorylated RAD50, gamma-H2AX, and phosphorylated CHK1—confirming target engagement—though the small sample size limited interpretation. The report notes that exploratory biomarker analyses found responders showed a higher proportion of ATM-low tumors and lower Schlafen-11 expression, with 85.7 percent of responders classified as SLFN11-low, but neither marker was strongly linked to clinical outcome.
The biological logic behind the pairing hinges on how cancer cells handle DNA damage. ATR stabilizes stalled DNA replication forks and activates checkpoints, buying cells time to repair damage before dividing; blocking ATR lets double-strand breaks pile up, driving genomic chaos and cell death. PARP inhibitors like olaparib trap PARP1 on DNA, causing stalled forks to collapse into double-strand breaks that are normally fixed by the homologous recombination pathway, where BRCA1 and BRCA2 play central roles. In tumors carrying BRCA mutations, homologous recombination is broken, so PARP inhibitors kill cells through synthetic lethality—two separate weaknesses that are individually survivable but lethal together. PARP inhibition also ramps up replication stress, making tumor cells increasingly reliant on ATR for survival, and preclinical work had shown selective synergy between ceralasertib and olaparib in BRCA-deficient models and in most models of acquired PARP inhibitor resistance.
The authors place their findings alongside landmark PARP inhibitor monotherapy trials in BRCA-mutated metastatic breast cancer, where median progression-free survival has ranged from 4.0 to 8.6 months and response rates have varied widely depending on prior platinum exposure. Without a monotherapy comparison arm, the report acknowledges, the specific contribution of ceralasertib to the observed activity can't be definitively determined, and a parallel randomized trial called VIOLETTE found no significant progression-free survival benefit from adding ceralasertib to olaparib in triple-negative breast cancer, possibly because blood toxicity forced a relatively low ceralasertib dose. Still, the authors argue that further work should identify which patient groups—defined by prior PARP exposure, specific mutations, or biomarkers—are most likely to benefit from adding ATR inhibition to PARP therapy, since predictive biomarkers for ceralasertib remain elusive and replication stress still can't be measured accurately in clinical tissue samples.