Ceralasertib (AZD6738) is a selective oral inhibitor of the ATR kinase, a central regulator of the cellular response to DNA replication stress. Tumors characterized by high replication stress or impaired DNA repair rely heavily on ATR signaling for survival, making ATR inhibition an attractive therapeutic strategy. This condensed article summarizes the biological activity, mechanistic insights, and optimized dosing strategies for ceralasertib in preclinical models. The focus is placed on how ceralasertib functions as a standalone therapy and how it enhances the activity of chemotherapies and PARP inhibition.
Introduction
ATR is a serine/threonine kinase activated in response to stalled replication forks and single-stranded DNA regions. When replication is impeded, ATR regulates checkpoint activation, stabilizes replication structures, and delays mitotic entry. By ensuring that damaged or under-replicated DNA is not passed into mitosis, ATR preserves genome stability. Tumors with chronic replication stress or with deficiencies in DNA repair pathways, such as ATM loss or homologous recombination defects, show heightened reliance on ATR-mediated repair.
Ceralasertib (AZD6738) was developed as a potent, orally bioavailable small-molecule ATR inhibitor. Because ATR function is essential in managing replication stress, ceralasertib has potential to selectively impair cancer cells while sparing normal tissues, especially when used at optimized schedules. Preclinical studies have demonstrated both intrinsic antitumor activity of ceralasertib and a strong rationale for combining it with replication stress–inducing agents such as carboplatin, irinotecan, and PARP inhibitors.
Materials and Methods Summary
A broad collection of cancer cell lines was examined to evaluate sensitivity to ceralasertib. Growth inhibition assays quantified GI50 values after 72-hour drug exposure. Synergy between ceralasertib and other agents was assessed using the Loewe additivity model. Mechanistic studies employed reporters of homologous recombination repair and break-induced replication, while pharmacodynamic readouts included pRAD50, γH2AX, and pCHK1. Xenograft and patient-derived tumor models were used to assess in vivo antitumor efficacy and to identify dosing schedules that balance tolerability with therapeutic effect. Drug exposures were correlated with both tumor control and observed biomarker modulation.
Results
Differential Sensitivity to Ceralasertib Monotherapy
Across nearly 300 cancer cell lines, sensitivity to ceralasertib varied widely. Only a subset showed high responsiveness at concentrations consistent with on-target ATR inhibition. Hematologic cancers demonstrated greater sensitivity than most solid tumors. No single genomic alteration fully predicted response, but several features enriched among responsive lines included CCNE1 amplification, ARID1A mutations, SETD2 loss, ATRX alterations, and functional deficits in ATM signaling. These features align with known contributors to replication stress or impaired repair coordination, providing potential biomarkers for patient selection.
In responsive models, ceralasertib induced hallmark ATR inhibition effects such as reduced CHK1 phosphorylation and increased γH2AX and pRAD50. These changes indicate accumulation of DNA damage and activation of DNA double-strand break signaling, consistent with reduced fork protection and impaired homologous recombination.
Mechanistic Evidence of Repair Inhibition
Reporter assays showed that ceralasertib inhibited both homologous recombination repair and break-induced replication. This suggests that ATR inhibition not only destabilizes replication forks but also compromises downstream repair processes needed to resolve stalled replication structures. Together, these effects sensitize tumor cells to agents that generate single-stranded DNA or impede replication progression.
Dosing Considerations for Monotherapy
Tumor control by ceralasertib monotherapy required sustained exposure. Continuous or near-continuous dosing was more effective than intermittent low-density schedules. Pharmacodynamic analysis confirmed that free plasma concentrations correlated with induction of γH2AX, pCHK1, and pRAD50, supporting the need for maintained pathway suppression to achieve tumor growth control.
Although monotherapy responses were generally limited to stasis rather than regression, ceralasertib created a biologically primed environment for combinations, especially with therapies that further elevate replication stress.
Combination Strategies
Chemotherapy Combinations
Ceralasertib showed strong synergy with carboplatin and irinotecan. Both agents generate replication-blocking lesions—carboplatin through crosslinks and irinotecan through topoisomerase I inhibition—which increase replication stress and thereby augment the dependency on ATR signaling.
Carboplatin Combination
In vivo evaluation demonstrated that adding ceralasertib improved responses but required careful schedule optimization to minimize toxicity. A maximum of three consecutive days of ceralasertib dosing was generally tolerated in combination. The order of administration was critical: dosing ceralasertib after carboplatin produced superior tumor regressions compared with pre-treatment or non-overlapping regimens. Continued ATR inhibition for several days following carboplatin was necessary to collapse damaged forks and drive tumor regression.
Irinotecan Combination
The optimal schedule for irinotecan required ceralasertib coverage during the first 24 hours following irinotecan exposure. Twice-daily dosing of ceralasertib was necessary to maintain adequate pathway inhibition and achieve combination benefit. Coadministration or delayed administration without sustained exposure produced little enhancement. These findings underscore that replication stress–inducing therapies require ATR inhibition at precise temporal windows to convert DNA lesions into lethal damage.
Combination with the PARP Inhibitor Olaparib
The synergy between ceralasertib and PARP inhibitors arises from convergent mechanisms: PARP inhibition increases single-strand breaks and traps PARP on DNA, causing replication fork stalling, while ATR inhibition prevents the rescue and repair of these collapsed forks. In BRCA1-mutant cells, low doses of ceralasertib dramatically shifted olaparib sensitivity, producing large increases in DNA damage and chromosomal instability.
In BRCA2-mutant patient-derived xenografts, three to five days per week of ceralasertib combined with olaparib was sufficient to achieve complete tumor regression. Notably, increasing the intensity of either agent—higher olaparib dosing or twice-daily ceralasertib—enabled complete regressions even in BRCA-proficient tumors, reflecting a strong dependency on ATR in the context of combined fork stalling and defective repair.
Dose-Schedule Optimization and General Principles
Across all combination studies, several consistent principles emerged:
- Ceralasertib is the primary driver of combination potency, with its scheduling more influential than that of partner drugs.
- Higher doses with shorter durations can match lower doses with extended duration, providing flexibility for clinical tolerability.
- Sustained ATR inhibition during peak replication stress is essential—typically within the first 24–72 hours after chemotherapy or PARP inhibition.
- Genetic context modulates sensitivity, with BRCA-mutant and ATM-deficient tumors requiring fewer doses for robust response.
These observations emphasize that optimal therapeutic performance arises from synchronizing ATR inhibition with DNA damage induction.
Discussion
Ceralasertib exhibits significant promise as both monotherapy and as a combination partner. Although monotherapy primarily induces tumor stasis, its biological effects generate vulnerabilities that can be exploited through rational combinations. The drug’s ability to impair multiple aspects of replication recovery—including homologous recombination and fork restart—makes it especially potent when paired with agents that impose replication stress.
The findings support clinical evaluation strategies that prioritize biomarker-enriched patient groups, optimized dosing windows, and treatment schedules tailored to each partner drug. By aligning ATR inhibition with the timing of replication interference, it is possible to achieve profound tumor regressions at doses that remain tolerable.
Conclusion
Ceralasertib (AZD6738) is a powerful ATR inhibitor with demonstrated ability to control tumor growth and enhance the effects of DNA-damaging therapies. Its activity across multiple cancer models underscores the therapeutic potential of ATR blockade. The success of ceralasertib hinges not only on its biological mechanism but also on the careful optimization of dose intensity, frequency, and temporal sequencing. These preclinical insights provide a strong foundation for clinical development and for designing regimens that maximize antitumor efficacy while maintaining patient tolerability.