ATR Inhibitors Offer New Line of Attack on DNA Repair Network

Cancer cells that manipulate the DNA damage response (DDR) to foster the genomic instability that underlies many of their hallmark processes become heavily reliant on intact pathways for their survival, creating a targetable Achilles heel that can be exploited therapeutically.

The DDR is a network of signaling pathways that senses and repairs damaged DNA. The central kinases of the DDR, including the ataxia telangiectasia mutated and Rad3-related kinase (ATR) protein, are attractive targets. ATR coordinates the response to replication stress (RS), a driving force of cancer, as well as the result of treatment with certain types of cancer therapy.

Small molecule ATR inhibitors have been in clinical development for several years, and although various novel drugs have come and gone, 2 clear leaders have emerged from the pack. Merck’s M6620 and AstraZeneca’s AZD6738 are both being evaluated in multiple clinical trials across different tumor types.

Overcoming DNA Damage

Following the paradigm of synthetic lethality established by PARP inhibitors, ATR inhibitors are being evaluated mostly in rational combinations designed to push the cancer cell to the breaking point, but new insights are offering potential inroads into targeting ATR with monotherapy.Cells are constantly exposed to a host of DNA-damaging assaults, from external threats such as ultraviolet radiation and chemical toxicity to the hazards of replicating the genome for cell division. To maintain genomic integrity, cells evolved the DDR, a highly coordinated network of signaling pathways that sense DNA damage, interact with checkpoints to control the cell cycle, and perform repair.

Several kinases of the phosphatidylinositol kinase-related kinase family of proteins are central to the DDR. Among them, the ATR protein is responsible primarily for the repair of 1 of the major types of DNA damage: single-stranded breaks. In particular, ATR plays a vital role in responding to RS, which occurs if DNA damage accumulates and impedes the fundamental process of DNA replication.

During DNA replication, the double helix is unwound and partially unzipped, creating a fork at the area where replication is taking place. Under RS, damage to the DNA strands can cause replication to slow down or stall, and if the damage goes unrepaired, the fork may collapse, leading to cell death.^1-3

Replication protein A binds to single-stranded DNA at sites of damage and the resulting complex is a trigger for ATR recruitment through its binding partner ATR-interacting protein. Full activation of ATR requires a conformational change triggered by the subsequent binding of ATR activator proteins, with evidence suggesting different activator proteins are responsible for initiating different pathways of ATR activation in response to distinct types of DNA damage.

Figure. Mechanisms of DNA Damage Response Provide Therapeutic Targets³

A Cancer Paradox

Upon activation, ATR orchestrates a multi-faceted response through activation of numerous downstream effector proteins. Best characterized is the CHK1 kinase; through CHK1, ATR is able to arrest the cell cycle at the S and G2/M phase checkpoints, to reduce RS by blocking global origin “firing” (needed to duplicate a genome’s worth of DNA in a timely and efficient manner) and to trigger the appropriate DNA repair pathways^3-5 (Figure³).Cancer cells have characteristically unstable genomes, which fosters the development of many hallmark processes.⁶ Given DDR’s seminal role in maintaining genome integrity, defects can promote tumorigenesis. Most common is loss of the G1 cell cycle checkpoint, allowing cancer cells to enter the cell cycle unchecked.

Paradoxically, the same oncogenic DDR defects cause cancer cells to become highly reliant on intact repair processes—analogous to oncogene addiction—to avoid catastrophic levels of genomic instability that provoke apoptosis.

When combined with other oncogenic alterations that promote a high proliferation rate, cancer cells have to deal with dividing more often and in the presence of greater levels of DNA damage than normal cells, contributing to high levels of RS observed from the earliest stages of tumorigenesis.

Drugging Addicted Cancers

As a key player in the RS response and in regulating the S and G2/M cell cycle check-points, ATR is vital to a cancer cell’s ability to continue to thrive under these conditions. It is also instrumental to resistance to cancer therapies, such as DNA damage—inducing chemotherapies (eg, topoisomerase inhibitors) and ionizing radiation, which induce high levels of RS.^5,7-9The realization that DDR perturbations can render cancer cells highly dependent on compensatory pathways led to the pursuit of small molecule inhibitors targeting this Achilles heel. This paradigm of synthetic lethality, whereby combined assaults on DDR processes can be uniquely fatal to cancer cells, has proved most clinically successful in the development of PARP inhibitors in patients with with homologous recombination repair defects.¹⁰

As our understanding of the DDR has expanded, so has the list of therapeutic targets. Several pharmaceutical companies have pursued ATR inhibitors, and 4 agents continue to be evaluated in clinical trials (Table). AstraZeneca and Vertex Pharmaceuticals emerged as clear leaders of the pack, the latter having now sold its development program to Merck.

M6620 (formerly VX-970) was the first ATR inhibitor to enter clinical development. Initial phase I clinical trial data emerged in 2016, from a study conducted in the United Kingdom. Findings were presented for 17 patients who received M6620 as mono-therapy and 17 who were treated with the ATR inhibitor plus carboplatin. In the monotherapy cohort, 1 patient with colorectal cancer (CRC) who displayed loss of the ATM protein (another integral DDR kinase) had a complete response (CR) lasting for more than 20 months, and 5 patients had stable disease (SD). Among those treated with the combination, there was a notable partial response (PR) lasting 6 months in a patient with BRCA1-mutant, platinum-refractory, PARP inhibitor—resistant ovarian cancer with a TP53 mutation and 5 patients had SD.¹¹

An ongoing trial is assessing the combination of M6620 and chemotherapy in patients with advanced solid tumors, including CRC, non—small cell lung cancer (NSCLC), breast cancer, pancreatic cancer, and other tumor types. Among the first 50 patients, the best response was PR for 4 patients with breast cancer, neuroendocrine pancreatic cancer, NSCLC, and cancer of unknown primary.¹²

In a separate presentation, investigators reported that among 28 patients who received a combination of M6620 and cisplatin, 4 patients had a PR, including 3 with platinum-resistant or -refractory mesothelioma, ovarian cancer, and triple-negative breast cancer (TNBC).¹³

More recently, results from a dose-expansion cohort in patients with metastatic TNBC were presented. The 35 patients enrolled to date, 18 of whom were BRCA1/2 wild type, received intravenous cisplatin at a dose of 75mg/m² on day 1, in combination with M6620 at a dose of 140 mg/m² on days 2 and 9 of each 21-day cycle. The preliminary unconfirmed objective response rate was 38.9%, with all PRs as best overall response and the longest duration of response at 183 days. Grade 3 or higher treatment-related adverse events (TRAEs) experienced by more than 1 patient included neutropenia, anemia, vomiting, and nausea.¹⁴

In a separate phase I study, M6620 was combined with topotecan in 3-week cycles. Among the 21 patients enrolled to date, there were 2 with PRs (including 2 in 5 patients with small cell lung cancer [SCLC]) and 7 with SD (including a patient with SCLC). The most common TRAEs included anemia, leukopenia, neutropenia, lymphopenia, and thrombocytopenia.¹⁵

AstraZeneca is developing ceralasertib (AZD6738), the first orally administered ATR inhibitor. Early clinical trial data have demonstrated antitumor activity in a variety of cancer types in combination with chemotherapy and other DNA-damaging drugs.

Among ongoing clinical trials is a modular phase I study in which AZD6738 is being combined with carboplatin, the PARP inhibitor olaparib (Lynparza), or the immune checkpoint inhibitor durvalumab (Imfinzi) in patients with advanced cancers. Patients (N = 36) were treated with doses ranging from 20 to 60 mg given either once or twice daily, using sequential or concurrent schedules ranging from 2 to 17 days in combination with carboplatin area under the curve 5 given every 21 days. There were 3 PRs in patients with cervical cancer, ATM-loss CRC, and ATM-mutant ovarian cancer. The most common TRAEs included thrombocytopenia, neutropenia, and anemia.¹⁶

In more recent data, 45 patients have been treated with the combination of AZD6738 and olaparib in 10 cohorts ranging from 60 mg once daily to 240 mg twice daily for 5 to 14 days, in combination with olaparib ranging from 100 to 300 mg twice daily in a continuous infusion. There was 1 CR, 5 PRs, and 1 unconfirmed PR among 3 patients with breast cancer and 1 each with ovarian, prostate, pancreatic, and ampullary cancer. Thrombocytopenia and neutropenia were dose-limiting toxicities (DLTs) and grade 3 or higher TRAEs included anemia, fatigue, and reduced appetite.

Twenty-five patients have been treated with the combination of AZD6738 and durvalumab in 5 cohorts, with 1 to 2 of weeks monotherapy run-in followed by durvalumab at a dose of 1500 mg on day 1, in combination with AZD6738 at 80 to 240 mg once or twice daily for 1 week (on days 22-28) or 2 weeks (on days 15-28). There was 1 CR, 2 PRs, and 1 unconfirmed PR in 3 patients with advanced NSCLC and 1 with head and neck squamous cell carcinoma. Responses occurred independently of PD-L1 expression status. TRAEs included 1 instance of grade 3 anemia, and there was 1 DLT of thrombocytopenia.¹⁷

Table. Ongoing Clinical Trials of ATR Inhibitors

Although the majority of clinical trials are focused on combination therapy, ongoing research is attempting to offer potential roads to ATR inhibitor monotherapy by identifying biomarkers of response and fostering better understanding of the ATR pathway within the context of the DDR, as well as the varied functions of the ATR protein. Recently, investigators at Massachusetts General Hospital identified a role for ATR in regulating the spindle apparatus, the cellular machinery to which the duplicate chromosomes attach and are pulled apart into new cells during cell divison. The hypothesis is that ATR facilitates the attachment of the centromeres to the spindle; ATR inhibition, therefore, leads to whole chromosome missegregation. The investigators postulate that ATR inhibitors could prove particularly effective in cancers that have both high levels of RS and chromosomal instability.¹⁸

References

1. Abdel-Magid AF. ATR inhibitors as potential treatment for cancers. ACS Med Chem Lett. 2018;9(4):292-293. doi: 10.1021/acsmedchemlett.8b00107.
2. Lecona E, Fernandez-Capetillo O. Targeting ATR in cancer. Nat Rev Cancer. 2018;18(9):586-595. doi: 10.1038/s41568-018-0034-3.
3. Minchom A, Aversa C, Lopez J. Dancing with the DNA damage response: next-generation anti-cancer therapeutic strategies. Ther Adv Med Oncol. 2018;10:1758835918786658. doi: 10.1177/1758835918786658.
4. Yekezare M, Gómez-González B, Diffley JFX. Controlling DNA replication origins in response to DNA damage — inhibit globally, activate locally. J Cell Sci. 2013;126(pt 6):1297-1306. doi: 10.1242/jcs.096701.
5. Rundle S, Bradbury A, Drew Y, Curtin NJ. Targeting the ATR-CHK1 axis in cancer therapy. Cancers (Basel). 2017;9(5). doi: 10.3390/cancers9050041.
6. Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell. 2011;144(5):646-674. doi: 10.1016/j.cell.2011.02.013.
7. Gaillard H, García-Muse T, Aguilera A. Replication stress and cancer. Nat Rev Cancer. 2015;15(5):276-289. doi: 10.1038/nrc3916.
8. Halazonetis TD, Gorgoulis VG, Bartek J. An oncogene-induced DNA damage model for cancer development. Science. 2008;319(5868):1352-1355. doi: 10.1126/science.1140735.
9. Macheret M, Halazonetis TD. DNA replication stress as a hallmark of cancer. Annu Rev Pathol. 2015;10:425-448. doi: 10.1146/annurev-pathol-012414-040424.
10. Lord CJ, Ashworth A. PARP inhibitors: synthetic lethality in the clinic. Science. 2017;355(6330):1152-1158. doi: 10.1126/science.aam7344.
11. O'Carrigan B, Luken MJdM, Papadatos-Pastos D, et al. Phase I trial of a first-in-class ATR inhibitor VX-970 as monotherapy (mono) or in combination (combo) with carboplatin (CP) incorporating pharmacodynamics (PD) studies. J Clin Oncol. 2016;34(suppl 15):2504. doi: 10.1200/JCO.2016.34.15_suppl.2504.
12. Plummer ER, Dean EJ, Evans TRJ, et al. Phase I trial of first-in-class ATR inhibitor VX-970 in combination with gemcitabine (Gem) in advanced solid tumors (NCT02157792). J Clin Oncol. 2016;34(suppl 15):2513. doi: 10.1200/JCO.2016.34.15_suppl.2513.
13. Shapiro G, Wesolowski R, Middleton M, et al. Abstract CT012: phase 1 trial of first-in-class ATR inhibitor VX-970 in combination with cisplatin (Cis) in patients (pts) with advanced solid tumors (NCT02157792). Cancer Res. 2016;76(14)(suppl):CT012. doi: 10.1158/1538-7445.Am2016-ct012.
14. Telli M, Lord S, Dean E, et al. Abstract OT2-07-07: ATR inhibitor M6620 (formerly VX-970) with cisplatin in metastatic triple-negative breast cancer: preliminary results from a phase 1 dose expansion cohort (NCT02157792). Cancer Res. 2018;78(4)(suppl):OT2-07-07-OT02-07-07. doi: 10.1158/1538-7445.Sabcs17-ot2-07-07.
15. Thomas A, Redon CE, Sciuto L, et al. Phase I study of ATR inhibitor M6620 in combination with topotecan in patients with advanced solid tumors. J Clin Oncol. 2018;36(16):1594-1602. doi: 10.1200/jco.2017.76.6915.
16. Yap TA, Krebs MG, Postel-Vinay S, et al. Phase I modular study of AZD6738, a novel oral, potent and selective ataxia telangiectasia Rad3-related (ATR) inhibitor in combination (combo) with carboplatin, olaparib or durvalumab in patients (pts) with advanced cancers. Eur J Cancer. 2016;69(suppl 1):S2. doi: 10.1016/S0959-8049(16)32607-7.
17. Krebs MG, Lopez J, El-Khoueiry A, et al. Abstract CT026: phase I study of AZD6738, an inhibitor of ataxia telangiectasia Rad3-related (ATR), in combination with olaparib or durvalumab in patients (pts) with advanced solid cancers. Cancer Res. 2018;78(13)(suppl):CT026. doi: 10.1158/1538-7445.Am2018-ct026.
18. Kabeche L, Nguyen HD, Buisson R, Zou L. A mitosis-specific and R loop—driven ATR pathway promotes faithful chromosome segregation. Science. 2018;359(6371):108-114. doi: 10.1126/science.aan6490.