Abstract: Berzosertib (also known as VE-822, VX-970, or M6620) is a first-in-class, intravenously administered small-molecule inhibitor of the Ataxia telangiectasia and Rad3-related (ATR) kinase. It has emerged as a promising targeted therapy in precision oncology, particularly for tumors characterized by high replication stress and genomic instability. Small cell lung cancer (SCLC), an aggressive malignancy marked by frequent p53 and Rb1 loss, high mutation burden, and rapid development of platinum resistance, represents a critical unmet medical need. By inhibiting ATR, berzosertib impairs the DNA damage response and cell cycle checkpoint activation, leading to synthetic lethality in SCLC cells. Clinical evaluations have demonstrated that berzosertib, particularly in combination with the topoisomerase inhibitor topotecan, yields durable regressions and significant objective response rates in patients with relapsed, platinum-resistant SCLC. This review synthesizes the pharmacological activity, molecular mechanisms, current limitations, and future perspectives of berzosertib in the treatment of SCLC based on recent literature.
1. Introduction
Small cell lung cancer (SCLC) is a highly aggressive neuroendocrine tumor characterized by a high frequency of p53 and Rb1 loss, profound genomic instability, and a high mutation burden [2]. While SCLC initially responds to platinum-based chemotherapy, the disease frequently relapses and becomes refractory or resistant to standard treatments, making the management of relapsed SCLC a major clinical challenge [2]. The underlying mechanisms of this resistance are complex, but the reliance of these tumors on specific DNA repair pathways provides a unique therapeutic vulnerability.
Berzosertib (formerly known as VE-822, VX-970, or M6620) is a first-in-class, intravenously administered inhibitor of the Ataxia telangiectasia and Rad3-related (ATR) kinase [1][2]. ATR is a critical component of the DNA damage response (DDR) network. By targeting ATR, berzosertib exploits the high endogenous replication stress (RepStress) inherent in SCLC, driving cancer cells toward synthetic lethality [3][4]. Consequently, berzosertib is currently under extensive clinical evaluation as both a monotherapy and in combination with DNA-damaging agents to overcome chemoresistance in SCLC and other solid tumors [1].
2. Pharmacological Activity
Berzosertib exhibits a well-characterized pharmacokinetic (PK) profile. It demonstrates moderate to high clearance (approximately 60 L/h), a high volume of distribution (~1250 L), and an elimination half-life (t1/2) of 17 hours [1]. The pharmacokinetics of berzosertib are dose-linear across a wide range (18–480 mg/m2) and remain unchanged upon coadministration with combination drugs [1]. As a monotherapy, it is generally well tolerated with no dose-limiting toxicities (DLTs) observed at doses up to 480 mg/m2, and the recommended phase 2 dose (RP2D) for monotherapy was established at 240 mg/m2 given once weekly via intravenous infusion [1][2].
In the context of SCLC, berzosertib has shown remarkable pharmacological activity when combined with topotecan. In an investigator-initiated phase I study (NCT02487095) evaluating patients with advanced solid tumors, the RP2D for the combination was determined to be topotecan 1.25 mg/m2 (Days 1–5) and berzosertib 210 mg/m2 (Days 2 and 5) in 21-day cycles [1][2]. This combination yielded an objective response rate (ORR) of 36% [1]. Notably, 3 out of 5 patients (60%) with platinum-refractory SCLC achieved a partial response (PR) or prolonged stable disease (SD) [2]. The therapeutic targeting of ATR with this combination has yielded durable regressions specifically in small cell neuroendocrine cancers exhibiting high replication stress [3].
3. Molecular Mechanism of Action
The molecular mechanism of berzosertib centers on the inhibition of the ATR-CHK1 signaling pathway. During the cell cycle, replication stress or DNA-damaging agents cause the stalling of replication forks and the generation of single-stranded DNA (ssDNA). ATR is activated by recognizing replication protein A (RPA)-coated ssDNA at these stalled forks or at resected DNA double-strand breaks [2]. Once activated, ATR phosphorylates its primary downstream effector, checkpoint kinase 1 (CHK1), which subsequently prevents genomic instability by stabilizing the replication fork, ensuring sufficient deoxynucleotide supply, and suppressing the G2-M cell cycle transition to allow time for DNA repair [2].
Berzosertib potently inhibits ATR kinase activity, thereby blocking CHK1 phosphorylation and abrogating the cell cycle checkpoints [2][3]. In SCLC, which is characterized by high endogenous replication stress and frequent loss of the G1 checkpoint (due to p53 and Rb1 mutations), cells are overly reliant on the ATR-mediated G2-M checkpoint for survival [2][3]. By inhibiting ATR, berzosertib forces these cancer cells into premature mitosis with unrepaired DNA damage, leading to replication fork collapse and cell death (mitotic catastrophe) [2]. Furthermore, tumors responding to the berzosertib and topotecan combination display marked enrichment for pathways associated with cell-cycle progression and DNA repair, including E2F target genes, the G2-M checkpoint, and the Fanconi DNA repair pathways, consistent with a replication stress phenotype [3].
4. Structure-Activity Relationship (SAR)
While the provided literature focuses predominantly on the clinical and molecular pharmacology of berzosertib rather than its detailed chemical structure-activity relationship, it is established as a highly potent and selective first-in-class ATR inhibitor [2][4]. Unlike other advanced ATR inhibitors such as ceralasertib (AZD6738) and elimusertib (BAY1895344), which are formulated for oral administration, berzosertib is administered intravenously [3]. Preclinical studies have also demonstrated that berzosertib possesses good blood-brain barrier (BBB) penetration, which has provided a rationale for its evaluation as a radiosensitizer in models of non-small cell lung cancer (NSCLC) brain metastases [4].
5. Current Limitations
Despite its promising efficacy, the clinical application of berzosertib faces several limitations. First, its intravenous route of administration requires clinical infusions, which may be less convenient for patients compared to the oral route utilized by other ATR inhibitors in development [3]. Second, toxicity remains a significant challenge when berzosertib is combined with DNA-damaging cytotoxic chemotherapy. In the phase I trial combining berzosertib with topotecan, grade 3/4 toxicities were predominantly related to myelosuppression, including anemia (19%), leukopenia (19%), neutropenia (19%), lymphopenia (14%), and thrombocytopenia (10%) [2]. This necessitates careful dose and schedule adjustments to limit toxicity to normal replicating cells [3].
Additionally, intrinsic and acquired resistance mechanisms pose a hurdle. For example, the silencing of the Schlafen 11 (SLFN11) gene is considered a potential mechanism of chemoresistance in SCLC [2]. Finally, there is an unmet clinical need for robust, easily accessible predictive biomarkers. While ATM deficiency and transcriptomic replication stress signatures can predict response to ATR inhibitors, routine clinical assessment of these markers (e.g., via immunohistochemistry for ATM) is complicated by technical challenges and the difficulty of interpreting variants of unknown significance [3].
6. Future Perspectives
The future of berzosertib in SCLC and other malignancies is highly promising, particularly in combination regimens. The combination of berzosertib with topoisomerase inhibitors is viewed as a potential breakthrough for platinum-refractory SCLC [2]. To this end, a global phase II pivotal trial (NCT04768296) evaluating the berzosertib-topotecan combination in patients with relapsed, platinum-resistant SCLC is currently underway. This trial notably utilizes an Asia-inclusive multiregional design, incorporating safety run-in cohorts in Japan and China to enable seamless globalization of the drug's development [1].
Beyond topotecan, berzosertib is being actively evaluated in combination with other novel agents, including lurbinectedin and sacituzumab govitecan [1]. There is also significant interest in exploring trimodality therapies that combine ATR inhibitors with immune checkpoint inhibitors and ionizing radiation, leveraging the ability of ATR inhibitors to modulate the tumor immune microenvironment [2]. Moving forward, the integration of multivariate omics analyses and artificial intelligence to evaluate predictive biomarkers—such as SLFN11 expression and transcriptomic replication stress signatures—will be critical for patient selection and the successful implementation of berzosertib in precision oncology [3].
