Benjamin Holmes, DVM

(CAR) T-cell therapy has revolutionized the treatment of hematologic malignancies since the initial approval of antiCD19 CAR T for acute lymphoblastic leukemia (ALL) in 2017.

 2021 proved an exceptionally busy year for approvals in CAR T therapeutics, with several ongoing trials showing that additional indications are not far behind.

Brexucabtagene autoleucel (Tecartus, brexu-cel), a CD19-directed CAR T product, was approved on October 1, 2021, by the FDA for the treatment of adults with relapsed or refractory (R/R) B-cell precursor ALL (B-ALL). Previously, the CAR T therapy was approved by the FDA in July 2020 for the treatment of adult patients with R/R mantle cell lymphoma.

The ALL indication was based on results from the single-arm phase 1/2 ZUMA-3 (NCT02614066) trial.

 Following completion of fl udarabine-cyclophosphamide lymphodepletion, brexu-cel was infused at 2 × 10

 cells/kg in over 50 adults. Phase 1 results indicated an overall complete remission rate of 69% in all patients and 83% in patients treated with 1 × 10

cells/kg. Minimal residual disease (MRD) was undetectable in all patients who responded. At the 1 × 10

 cells/kg dose level, the median duration of response (DOR) was 17.6 months with a median follow-up of 22.1 months, and the median overall survival (OS) was 16.1 months.

Phase 2 focused on the single infusion dose of 1 × 10

 cells/kg. Brexu-cel was successfully formulated for 65 of 71 enrolled patients and administered to 55 (77%). Complete remission or complete remission with incomplete hematologic recovery was observed in 39 patients (71%; 95% CI, 57%-82%; 

 < .0001) with 31 (56%) reaching complete remission. The median relapse-free survival was 11.6 months. Median OS was 18.2 months and OS was not reached among patients responding to therapy; MRD was negative in 38 patients (97%).

Cytokine release syndrome (CRS) occurred in 93% (grade ≥ 3, 13%) and neurological toxicities (NTs) in 78% of patients (grade ≥ 3, 38%) in the phase 1 analysis. Management for CRS and NTs was revised to include earlier steroid use for NTs and tocilizumab for CRS for the 1 × 10

 cells/kg group. CRS of grade 3 or higher occurred in 13 patients (24%) and NEs of grade 3 or higher occurred in 14 patients in phase 2 (25%).

Brexu-cel is a promising therapy, but further research is required to determine where it will fit in current treatment approaches or if it proves beneficial in other patients with ALL such as T-cell ALL.

Idecabtagene Vicleucel Idecabtagene vicleucel (Abecma, ide-cel) was approved by the FDA on March 26, 2021, for the treatment of adults with R/R multiple myeloma after 4 or more prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. This represented the first B-cell maturation antigen (BCMA)-directed CAR T agent approved.

Approval was based on results from the phase 2 KarMMa trial (NCT03361748). The efficacy evaluable population included 100 patients. The overall response rate was 72% (95% CI, 62%-81%) and 28% of patients achieved stringent complete response (sCR; 95% CI, 19%-38%). Of the patients who achieved an sCR, 65% (95% CI, 42%-81%) remained in remission for at least 12 months. The median DOR was 11 months (95% CI, 10.3-11.4) for all responders and 19 months (95% CI, 11.4–not evaluable) in patients who achieved sCR.

There is a boxed warning for CRS and NTs, but also for hemophagocytic lymphohistiocytosis (HLH)/macrophage activation syndrome (MAS) and prolonged cytopenias. CRS and NTs were mostly low grade with predictable early onset and early resolution. Grade 3 or higher CRS was observed in 12 (9%) patients. HLH/MAS occurred in 5 (4%) patients, including 1 who developed fatal multi-organ HLH/ MAS and another with HLH/MAS which contributed to the fatal outcome of bronchopulmonary aspergillosis. Prolonged grade 3 or 4 neutropenia was noted in 52 (41%) patients and grade 3 or 4 thrombocytopenia in 62 (49%).

 “Patients can become triple refractory to the main classes of drugs against myeloma,” said Michael Jain, MD, PhD, “and CAR T is very beneficial for these difficult to treat patients.” The recommended dose of idecabtagene vicleucel is 300 to 460 × 106 CAR-positive T cells.

Axicabtagene ciloleucel (Yescarta, axi-cel), another CD19-directed CAR T therapy, was approved by the FDA on March 5, 2021, for the treatment of adults with R/R follicular lymphoma (FL) after 2 or more lines of therapy. Approval was based on data from the single-arm, open-label phase 2 ZUMA-5 trial (NCT03105336). The efficacy analysis included 81 patients. The objective response rate (ORR) was 91% (95% CI, 83%-96%) with a complete remission rate of 60%. The median DOR was not reached with a 1-year continued remission rate of 76.2% (95% CI, 63.9%-84.7%). Patients who underwent leukapheresis (n = 123) experienced an ORR of 89% (95% CI, 83%-94%) with a complete remission rate of 62%. CRS occurred in 88% (grade ≥ 3, 10%) and NTs in 51% (grade ≥ 3, 26%) of all patients with non-Hodgkin lymphoma treated with auto-cel.

Recent data published from ZUMA-5 evaluated patients with progression within 24 months from initiation of the first anti-CD20–containing chemoimmunotherapy (POD24). The overall response rate was 92% among 81 patients with POD24 and 92% among 41 patients without POD24, with a median of 23.3 months follow-up time. Estimated progression-free survival (PFS) rates were 55% for patients with POD24 and 84% for patients without POD24. Grade 3 or greater CRS occurred in 9% of patients with POD24 and in 2% of patients without, whereas grade 3 or greater NTs occurred in 17% of both groups. ZUMA-5 is ongoing with more data to come.

Jain, an assistant member in the Blood and Marrow Transplant and Cellular Immunotherapy Department at Moffi tt Cancer Center, commented on the benefits of CAR T therapies for patients with FL: “Patients don’t have to be on chronic daily therapy. They receive their CAR T-cell infusion, may have to remain in the hospital a week, and then go quite a long time without the need for continued oral medications.”

A third CD19-directed CAR T therapy, lisocabtagene maraleucel (Breyanzi, liso-cel), was approved by the FDA on February 5, 2021, for the treatment of adults with R/R large B-cell lymphoma (LBCL) after 2 or more lines of systemic therapy, including diffuse large B-cell lymphoma (DLBCL) not otherwise specified (including DLBCL arising from indolent lymphoma), high-grade BCL, primary mediastinal LBCL, and FL grade 3B. Approval was based on results from the single-arm, open-label phase 1 TRANSCEND-NHL-001 (NCT02631044) trial.

The ORR of the 256 efficacy-evaluable patients was 73% (95% CI, 66.8%-78.0%) with a complete response rate of 53% (95% CI, 46.8%-59.4%). At a follow-up of 12 months, the median DOR was 16.7 months in all patients and had not yet been reached in patients who achieved a complete response. Of patients who responded, 60.4% remained in response at 6 months and 54.7% continued to respond by 12 months.

CRS occurred in 42% of patients (grade ≥ 3, 2%) and NTs in 30% (grade ≥ 3, 10%). Fatal NT occurred in 3 patients. Approval of liso-cel contains a risk evaluation and mitigation strategy due to the risk of fatal or life-threatening CRS and NTs. The recommended IV infusion dose contains 50 to 110 × 10

CAR-positive viable T cells with a 1:1 ratio of CD4 and CD8 components, preceded by fludarabine and cyclophosphamide.

Cytokine Release Syndrome and Neurologic Toxicities for CAR T 

All approved CAR T therapies come with a boxed warning for CRS and NTs. Jain had this to say concerning CRS, “The real-world data rates of severe CRS are a lot lower than many of the trials.” Initially, there was hesitancy to treat CRS earlier in treatment for fear of decreasing CAR T efficacy and expansion. Jain continued, “We are much less afraid of affecting CAR T-cell efficacy and are intervening much earlier. If a patient develops a fever within 72 hours of treatment, they are immediately given tocilizumab, even with no other symptoms.”

Regarding NTs, death is rare, but longer hospital stays and the need for further immunosuppression with steroids is common. However, steroids increase risk of infection and fatigue in already frail patients. NTs are manageable, and Jain has hope for future management strategies: “How much the NT is due to CAR T vs CD19 specifically is unknown. It’s interesting that rates of NT in myeloma BCMA CAR are far lower than [with] the CD19 CARs. Maybe that’s something that can be solved in the future.”

Top-line results from the ZUMA-7 trial (NCT03391466) indicate that axi-cel is superior to second-line standard-of-care treatment for R/R LBCL. ZUMA-7 is a randomized, global, multicenter phase 3 study evaluating one-time infusion of axi-cel compared with standard-of-care second-line treatment for patients with R/R LBCL. According to a press release, after a median follow-up of 2 years, the study met the primary end point of event-free survival (HR, 0.398; 

< .0001) and the key secondary end point of ORR. Not enough data are available yet to determine analysis of OS, although a preliminary trend was seen in favor of axi-cel. Safety results were consistent with previous safety data, with grade 3 or higher CRS observed in 6% of patients and 21% experiencing grade 3 or higher NEs.

Top-line results present the potential for a paradigm shift in the treatment of LBCL.

 “I think that CAR T cells have the potential to move to the frontline for a subset of patients, for those that are not going to be responsive to our standard treatments. I think that’s an approach that has merit, to try and use a powerful weapon at an earlier time point,” Jason Westin, MD, leader of the DLBCL research team at The University of Texas MD Anderson Cancer Center, said in an interview with Targeted Therapies in Oncology™.

Liso-cel demonstrates highly statistically significant improvement in event-free survival, complete response rate, and PFS compared with standard-of-care second-line treatment for patients with LBCL according to top-line results from the randomized, multicenter, phase 3 TRANSFORM trial (NCT03575351). Liso-cel was compared with salvage therapy followed by high-dose chemotherapy and hematopoietic stem cell transplant in patients with R/R LBCL.

 An interim analysis conducted by an independent review committee revealed the superiority of liso-cel in the primary and key secondary end points. It was too soon to determine OS data and safety results were consistent with the TRANSCEND-NHL-001 trial (NCT02631044).

Tisagenlecleucel (Kymriah, tisa-cel) is a CAR T therapy that was approved by the FDA for the treatment of pediatric and young adult patients up to 25 years of age with R/R B-ALL in 2017 as well as for adult patients with R/R DLBCL in 2018.

Stephen J. Schuster, MD, director of the Lymphoma Program, director of Lymphoma Translational Research, and Robert and Margarita Louis-Dreyfus Professor in Chronic Lymphocytic Leukemia and Lymphoma Clinical Care and Research at the University of Pennsylvania, told Targeted Therapies in Oncology™ that tisa-cel “is a biologically based therapy, meaning that [patients with] prior response to chemotherapy, or [those with] diseases [that] have resistance mechanisms to chemotherapy, will still respond to this.”

ELARA (NCT03568461) is a phase 2, single-arm, open-label trial evaluating the efficacy and safety of tisa-cel in patients with R/R FL. The primary end point is complete response rate over a period of 2 years after treatment.

Safety was evaluated in 97 patients who received tisa-cel and 94 of these patients were evaluated for efficacy. The complete response rate was 66% (95% CI, 56%-75%) and ORR was 86% (95% CI, 78%-92%). The DOR for patients who achieved complete response was estimated to be 94% (95% CI, 82%-98%) at 6 months, with an estimated PFS rate of 76% (95% CI, 65%-84%). CRS occurred in 49% of patients (grade ≥ 3, 0%) and 9% of patients experienced NTs (0% grade 3, 1 grade 4 patient who recovered). These data suggest tisa-cel may be an effective therapy for patients with R/R FL.

Further studies are required to determine patient selection and incorporation into existing treatment protocols.

A comparison of clinical outcomes from ZUMA-5 and SCHOLAR-5 indicates substantial and statistically significant improvement for axi-cel in comparison with alternative treatments for patients with R/R FL. SCHOLAR-5 was an international retrospective study to evaluate treatment patterns and clinical outcomes of real-world late lines of therapy for FL. Data regarding patients with R/R FL who initiated a third or higher line of therapy were extracted, and data from the pivotal DELTA trial (NCT01282424) of delalisib (Zydelig) were included in the SCHOLAR-5 cohort. SCHOLAR-5 and ZUMA-5 cohorts were balanced for patient characteristics through propensity scoring with 85 SCHOLAR-5 patients and 86 ZUMA-5 patients selected for comparison; however, more SCHOLAR-5 patients than ZUMA-5 patients had an ECOG score of 1 at baseline despite propensity scoring.

ORRs in those who received at least 2 prior lines of therapy were 49.9% in SCHOLAR-5 patients and 94.2% in ZUMA5 patients (odds ratio [OR], 16.2; 95% CI, 5.6-46.9). The complete response rate was also significantly improved in the ZUMA-5 cohort (79.1%) vs SCHOLAR-5 (29.9%) (OR, 8.86; 95% CI, 4.3-18.225). Median PFS and OS rates were not reached for ZUMA-5, but were 12.7 and 59.8 months, respectively, for SCHOLAR-5. These data indicate axi-cel has the potential for significant improvement in treatment options in R/R FL.

A single infusion of ciltacabtagene autoleucel (cilta-cel), another BCMA-directed CAR T product, produced early, deep, and durable responses in heavily pretreated patients with multiple myeloma per recent study results from the phase 1b/2 CARTITUDE-1 trial (NCT03548207). Patients in the study had multiple myeloma and had received 3 or more prior lines of therapy or were not responsive to a proteasome inhibitor, an immunomodulatory drug, or an anti-CD38 antibody.

Cilta-cel was administered to 97 patients with a median of 6 prior lines of therapy as of September 1, 2020. The overall response rate was 97% (95% CI, 91%-99%) with 67% achieving sCR. The median time to complete response or better was 2 months with a median time to first response of 1 month. MRD was evaluable in 57 patients and 93% were MRD negative at 10-5. The PFS rate was 77% (95% CI, 66%-84%) and the OS rate was 89% (95% CI, 80%-94%) at 12 months, with median PFS and DOR not reached.

 CRS was observed in 95% of patients and resolved in all but 1 patient who experienced grade 5 CRS/hemophagocytic lymphohistiocytosis. Per this data, cilta-cel appears to provide highly effective treatment for patients with heavily pretreated multiple myeloma, who are traditionally difficult to treat.

Jain commented on the trial results: “I hesitate to compare trials, but the results from the CARTITUDE-1 trial, from a raw-data perspective, appear even better than [those of] ide-cel.”

The FDA has granted a priority review to cilta-cel with an extended Prescription Drug User Fee Act target date of February 28, 2022.

One of the problems facing CAR T therapy is a greater demand than supply. As Jain explained: “Cellular therapies are very complex compared to making chemicals or even an antibody. This is an autologous process and the determination of how to scale is as big a problem as efficacy or CAR design. More companies involved and more experience will help.” As CAR T achieves more indications and more companies join in production, there is potential for providing this therapy to many patients in need.

If oncologists are concerned about adverse effects or patients tolerating therapy, Jain hopes to alleviate those concerns. “Not every patient is going to have that difficult a time with CAR T. The comorbidities you may think would exclude them from this type of therapy (decreased heart, lung, or kidney function) are manageable. Don’t hesitate to refer patients to a center for the diseases they have. They may benefit despite being more frail,” he said.

With the rapid advancement of CAR T, there is no doubt that many patients in the future will reap the benefits. Questions remain as to how much benefit CAR T will provide, how to select patients who will benefit the most, and how to scale production to reach more patients. Ongoing research hopes to answer those questions.

1. Ahmad A. CAR-T Cell Therapy. Int J Mol Sci.2020;21(12):4303.doi:10.3390/ijms21124303

2. U.S. FDA Approves Kite’s Tecartus, the First and Only CAR T Treatment for Relapsed or Refractory Mantle Cell Lymphoma. News release. Kite. July 24, 2020. Accessed November 17, 2021. 

3. FDA approves brexucabtagene autoleucel for relapsed or refractory B-cell precursor acute lymphoblastic leukemia. FDA. Updated October 21, 2021. Accessed November 17, 2021, 2021. 

4. Shah BD, Bishop MR, Oluwole OO, et al. KTE-X19 anti-CD19 CAR T-cell therapy in adult relapsed/refractory acute lymphoblastic leukemia: ZUMA-3 phase 1 results. 

.2021;138(1):11-22. doi:10.1182/blood.2020009098

5. Shah BD, Ghobadi A, Oluwole OO, et al. KTE-X19 for relapsed or refractory adult B-cell acute lymphoblastic leukaemia: phase 2 results of the single-arm, open-label, multicentre ZUMA-3 study. 

.2021;398(10299):491-502. doi:10.1016/S0140-6736(21)01222-8

6. FDA approves idecabtagene vicleucel for multiple myeloma. FDA. Accessed November 17, 2021. 

7. U.S. Food and Drug Administration Approves Bristol Myers Squibb’s and bluebird bio’s Abecma (idecabtagene vicleucel), the First Anti-BCMA CAR T Cell Therapy for Relapsed or Refractory Multiple Myeloma. News release. Bristol Myers Squibb; March 26, 2021. Accessed November 17, 2021. 

8. FDA grants accelerated approval to axicabtagene ciloleucel for relapsed or refractory follicular lymphoma. FDA. Updated March 08, 2021. Accessed November 17, 2021. 

9. Jacobson CA, Chavez JC, Sehgal A, et al. Outcomes in ZUMA-5 with axicabtagene ciloleucel (axi-cel) in patients (pts) with relapsed/refractory (R/R) indolent non-Hodgkin lymphoma (iNHL) who had the high-risk feature of progression within 24 months from initiation of first anti-CD20–containing chemoimmunotherapy (POD24). 

. 2021;39(suppl15):7515. doi:10.1200/JCO.2021.39.15_suppl.7515

10. FDA approves lisocabtagene maraleucel for relapsed or refractory large B-cell lymphoma. FDA. Updated February 5, 2021. Accessed November 18, 2021. 

11. Abramson JS, Palomba ML, Gordon LI, et al. Lisocabtagene maraleucel for patients with relapsed or refractory large B-cell lymphomas (TRANSCEND NHL 001): a multicentre seamless design study. 

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12. Oluwole OO, Bishop MR, Gisselbrecht C, et al. ZUMA-7: A phase 3 randomized trial of axicabtagene ciloleucel (Axi-Cel) versus standard-of-care (SOC) therapy in patients with relapsed/refractory diffuse large B cell lymphoma (R/R DLBCL). 

. 2018;36(suppl15):TPS7585. doi:10.1200/JCO.2018.36.15_suppl.TPS7585

13. Kite Announces Yescarta® CAR T-cell Therapy Improved Event-Free Survival by 60% Over Chemotherapy Plus Stem Cell Transplant in Second-Line Relapsed or Refractory Large B-cell Lymphoma. News release. Gilead Sciences Inc.; June 28, 2021. Accessed November 17, 2021. 

14. Bristol Myers Squibb Announces Positive Topline Results from Phase 3 TRANSFORM Trial Evaluating Breyanzi (lisocabtagene maraleucel) Versus Chemotherapy Followed by Stem Cell Transplant in Second-line Relapsed or Refractory Large B-cell Lymphoma. News release. Bristol-Myers Squibb Company; June 10, 2021. November 17, 2021. 

15. Schuster SJ, Dickinson MJ, Dreyling MH, et al. Efficacy and safety of tisagenlecleucel (Tisa-cel) in adult patients (Pts) with relapsed/refractory follicular lymphoma (r/r FL): Primary analysis of the phase 2 Elara trial. 

.2021;39(suppl15):7508. doi:10.1200/JCO.2021.39.15_suppl.7508

16. Dickinson M, Popplewell L, Kolstad A, et al. ELARA: A phase II, single-arm, multicenter, open-label trial investigating the efficacy and safety of tisagenlecleucel in adult patients with refractory/relapsed follicular lymphoma (r/r FL). 

. 2019;37(suppl15):TPS7573. doi:10.1200/JCO.2019.37.15_suppl.TPS7573

17. Ghione P, Patel A, BobilloS, et al. A Comparison of Clinical Outcomes from ZUMA-5 (Axicabtagene ciloleucel) and the International SCHOLAR-5 External Control Cohort in Relapsed/Refractory Follicular Lymphoma (R/R FL). Presented at: 2021 European Hematology AssociationCongress; June 2021; virtual. Abstract LB1904. 

18. Berdeja JG, Madduri D, Usmani SZ, et al. Ciltacabtagene autoleucel, a B-cell maturation antigen-directed chimeric antigen receptor T-cell therapy in patients with relapsed or refractory multiple myeloma (CARTITUDE-1): a phase 1b/2 open-label study. 

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19. Legend Biotech Announces Extension of PDUFA Date for Cilta-Cel. News release. Legend Biotech Corporation. November 1, 2021. Accessed November 18, 2021. 

Articles

With the rapid advancement of CAR T, there is no doubt that many patients in the future will reap the benefits.

CAR T-Cell Therapy Indications Grow Significantly in 2021

Dysreguation of dysfunction of the DNA damage response (DDR) system can result in genomic instability, a well-known hallmark of many cancers.

 DDR deficiency can lead to greater potential for cancer development and malignant transformation (

), but DDR can also be a mechanism of resistance to chemotherapy and radiotherapy.

 Targeted anti-cancer therapy approaches aim to exploit genetic or functional defects in DDR via synthetic lethal mechanisms with limited effects on healthy tissues.

PARP is the best-understood component of DDR, and mutations in genes within the Fanconi anemia/BRCA pathway (such as 

, BRCA2) that are common in breast, ovarian, prostate, and pancreatic cancers are most sensitive to PARP inhibitors.

 Currently approved PARP inhibitors include olaparib (Lynparza), rucaparib (Rubraca), niraparib (Zejula), and talazoparib (Talzenna).

 However, acquired PARP inhibitor resistance is common.

More targeted therapies aimed at inhibiting DNA damage kinases, classical nonhomologous end joining, and alternative end joining have come from continued DDR research.

Two regulators of DDR of interest are the DNA damage kinases ataxia telangiectasia mutated Rad3-related kinase (ATR) and its downstream effector checkpoint kinase 1 (CHK1).

The ATR-CHK1 pathway regulates and coordinates processes such as inhibition of replication origin firing, protection of stressed replication forks, cell cycle arrest, and DNA repair.

 ATR’s contributions to early delay in the M phase entry, regulation of late-phase DDR, and prevention of chromosome breaks during S phase are well described.

ATR’s integral role in DDR has made it 

an appealing target for cancer treatment, 

and several ATR inhibitors are being evaluated 

tumors. Haider S. Mahdi, MD, a gynecologic 

oncologist at UPMC Hillman Cancer Center 

assistant professor in the Department of 

Obstetrics, Gynecology, and Reproductive 

Sciences at the University of Pittsburgh 

School of Medicine in Pennsylvania, commented 

inhibitor therapy: “There are trials looking 

at patients with solid tumors with deficiency 

in the DDR pathway; most of them are early-phase 

BAY 1895344: A Potent and Synergistic ATR Inhibitor

BAY 1895344, a novel selective ATR inhibitor, demonstrated high selectivity and potency in a variety of tumor types with DDR defects in initial in vivo and in vitro studies.8 The synthetic lethality of tumor-intrinsic DNA-repair deficiencies and the ATR blockade was confirmed with pronounced in vivo antitumor efficacy of the agent in xenograft models.

Superior antitumor activity as monotherapy when compared with ceralasertib (AZD6738) and berzosertib (M6620; VX-970) was observed; however, BAY 1895344 was most effective in combination with DNA damage-inducing or repair-compromising therapies.

Synergistic antitumor activity was observed with external beam radiation therapy in colorectal cancer, the PARP inhibitor olaparib in breast and prostate cancers, and with the androgen receptor antagonist darolutamide (Nubeqa) in prostate cancer. However, dose-dependent toxicity was observed in combination with carboplatin, and the combination was less effective than that of BAY 1895344 alone. BAY 1895344 shows promise as a combination therapy, especially with PARP inhibitors and is being evaluated in a phase 1 study in patients with solid tumors and lymphomas (NCT03188965).

Another study compared castration-resistant prostate cancer (CRPC) cell lines treated with BAY 1895344, olaparib, or BAY 1895344 plus anti–PD-L1. BAY 1895344 destabilized the PD-L1 protein by directly repressing ATR-CHK1 signaling. This resulted in an autocrine, apoptotic response in CRPC cells. Combination of BAY 1895344 with anti–PD-L1 therapy resulted in robust immune activation in the mouse model with a synergistic, T-cell−dependent therapeutic response. Combining ATR-targeted agents with immune checkpoint blockade for patients with CRPC is supported by these mechanisms.

Ceralasertib in Combination Regimens Ceralasertib + PARP Inhibition

The addition of an ATR inhibitor to a PARP inhibitor is theorized to combat PARP inhibitor resistance. Olaparib was combined with the ATR inhibitor ceralasertib on days 1 to 7 of a 28-day cycle continuously in 25 patients with relapsed or refractory cancers harboring DDR alterations as part of the Olaparib Combinations trial (NCT02576444). Primary end points were confirmed complete response (CR) or partial response (PR) rates and clinical benefit rate (CBR; CR + PR + stable disease [SD] at 16 weeks).

The overall response rate was 8% and the 

response was 22 months (range, 18-26+) and 

the median duration of clinical benefit was 

ATM (ataxia-telangiectasia mutated) mutations 

were found in 5 patients, and 2 of these 

achieved CR or SD ongoing at 24 months or 

more. Out of 7 patients with BRCA-mutated 

PARP inhibitor–resistant high-grade serous 

5 had SD ranging from 16 to 72 weeks (CBR, 

86%). Safety of ceralasertib combined with

full-dose olaparib was confirmed, though 

dose reduction due to myelosuppression was 

required in 5 patients. Signs of clinical benefi

t were observed specifi cally in patients with 

PARP inhibitor–resistant ovarian cancer, 

pre-treated with a median of 4 prior therapies 

(range, 0-10) and patients with highgrade 

inhibitor–resistant but still saw clinical 

benefit. “Low response rate could be due to 

the fact we didn’t limit the previous number 

of lines of therapy,” commented Mahdi. He 

noted this study used higher doses of PARP 

inhibition in comparison to ATR inhibition. 

“PARP inhibitor–resistant models show ATR 

is the driver. In this study, we administered 

a higher dose of PARP with lower dose of 

ATR inhibition, so maybe we need to focus 

cancer and was tested in combination with 

radiation and ceralasertib in tumor models. 

radiation survival relative to monotherapy 

of either agent. Therapeutic concentrations 

of olaparib plus radiation and ceralasertib 

high concentrations of olaparib resulted in 

ATR-mediated replication stress response 

antagonized by ceralasertib. Tumor growth 

was significantly delayed in HRR-proficient 

tumor xenografts with the regimen. Success 

development of this combination for tumors 

A phase 1 study (NCT02264678) evaluated carboplatin 

of ceralasertib in 21-day cycles in patients with 

phase 2 dose was 40 mg once daily on days 

1 and 2 for ceralasertib with carboplatin at 

Common treatment-emergent adverse events (AEs) observed with the combination were anemia (39%), thrombocytopenia (36%), and neutropenia (25%). Dose-limiting toxicities of grade 4 thrombocytopenia, including a combination of grade 4 thrombocytopenia and grade 3 neutropenia, occurred in 3 of the 36 patients.12 Four patients achieved PR, 2 of which were confirmed and 2 were unconfirmed. Additionally, SD was observed in 18 of 34 (53%) response-evaluable patients. Absent or low ATM or SLFN11 protein expression was found in the 2 patients with confirmed PRs.

Mahdi commented that platinum-based chemotherapy agents have significant toxicity overlap with ATR inhibition. He stressed that the ATR inhibition dose and/or the chemotherapy dose should be limited because it could be potentially harmful due to increased toxicity. “The potentially more successful approach is to combine it with another agent that is not DNA damaging, such as gemcitabine alone, or [with] the topoisomerase inhibitors, such as topotecan,” he said.

In a preclinical study, the novel ATR inhibitor M4344 was evaluated as monotherapy and in combination with antitumor agents in multiple cancer cell lines, patient-derived tumor organoids, and mouse xenograft models. As a single agent, the potency of M4344 was compared with other investigational ATR inhibitors: BAY 1895344, berzosertib, ceralasertib, and VE-821. M4344 was found to be highly potent; by inducing mitotic catastrophe and DNA damage, its ability to kill cancer cells surpassed that of all the other ATR inhibitors. Additionally, the cytotoxic effects of M4344 were augmented when it was combined with DNA-damaging agents such as topoisomerase inhibitors (eg, topotecan and irinotecan), gemcitabine, cisplatin, and talazoparib; significant synergy was observed. The expression of 2 sets of genes—1 related to replication stress and 1 to neuroendocrine differentiation—was associated with enhanced response to M4344, as was inactivation of the chromatin remodeling genes 

 and BRG1. These findings suggest that gene expression signatures and specific genetic tests can be used to predict M4344 activity. Tumors with neuroendocrine characteristics, such as small cell lung cancer and prostate cancer, were significantly correlated with cytotoxic response to M4344. Evaluating these factors may help optimize patient selection for M4344 therapy.

M4344 demonstrated 100-fold selectivity for ATR over 308 of 312 kinases tested. M4344 suppressed prostate cancer cell proliferation at similar concentrations and similarly to BAY 1895344, but was more potent than berzosertib, ceralasertib, and VE-821. Additionally, M4344 is chemically distinct from other ATR inhibitors. This warrants further studies to determine binding sites and possible interactions between M4344 and ATR.

Berzosertib: Raising Questions in ATR Inhibition Applications

Berzosertib is an intravenous, highly potent, selective ATR inhibitor. A first-in-human, open-label phase 1 trial (NCT02157792) evaluated the safety and activity of ascending doses of berzosertib in combination with gemcitabine, with or without cisplatin, in patients with advanced solid tumors. Out of 52 patients receiving berzosertib and gemcitabine alone, 4 had a total of 7 dose-limiting toxicities. In the 8 patients receiving additional cisplatin, 3 patients had a total of 3 dose-limiting toxicities.

Across both cohorts, 5 patients achieved PRs with all 5 having previously received cisplatin or carboplatin. SD was achieved in 29 patients in the berzosertib and gemcitabine cohort plus 4 patients in the triplet cohort.

Another phase 1 study (NCT02157792) evaluated berzosertib in combination with cisplatin in patients with advanced solid tumors refractory or resistant to standard of care; patients received ascending doses of cisplatin and berzosertib every 3 weeks. A total of 31 patients received berzosertib (90-210 mg/m

) across 7 dose levels. PR was observed in 4 patients, all of whom had disease progression following platinum-based chemotherapy, and SD was reached in 15 patients. Neutropenia (20.0%) and anemia (16.7%) were the most common grade 3 or higher treatment-emergent AEs. Two dose-limiting toxicities were observed (grade 3 hypersensitivity and grade 3 increase in alanine aminotransferase).

A phase 2 study (NCT02567409) evaluated berzosertib in combination with cisplatin and gemcitabine in patients with advanced urothelial carcinoma. A total of 87 patients were randomized to receive cisplatin with gemcitabine plus berzosertib (n = 46) or cisplatin with gemcitabine alone (n = 41).

Median progression-free survival was 8.0 months for both treatment groups. Median overall survival was 14.4 months in the berzosertib group but 19.8 months in patients receiving cisplatin with gemcitabine alone (adjusted HR, 1.42; 95% CI, 0.76-2.68). Higher rates of grade 3 or higher thrombocytopenia (59% vs 39%) and neutropenia (37% vs 27%) were observed with the berzosertib combination vs cisplatin plus gemcitabine alone. AEs within the treatment arm led to necessary dose reductions below the median control arm dose for cisplatin (

“We need to look at specific subsets of cancers and loss-of-function status to determine the functional implications of alteration in the DDR pathway,” Mahdi commented. “It could also be that there is a mutational gene, but the machinery is still intact, so we aren’t seeing expected results.”

A 2018 pivotal study revealed a mitosisspecific, R loop–driven ATR pathway responsible for suppressing chromosome instability.

 This finding indicates that ATR regulate genomic stability through 2 separate pathways. During S phase, ATR suppresses R loop– associated genomic instability; paradoxically, ATR also mediates the centromeric function of R loops during mitosis.

Replication stress during S phase and chromosome instability during mitosis are 2 mechanisms of overall genomic instability observed in cancer cells.

Lee Zou, PhD, scientific co-director, Massachusetts General Hospital Cancer Center, and the James & Patricia Poitras Endowed Chair for Cancer Research, Harvard Medical School, Boston, stated, “ATR inhibition is primarily used to target DNA replication stress at this time. There may be opportunities to target mitotic defects with ATR inhibition. Biomarkers predictive of these defects will be important.”

Questions remain regarding ATR inhibition potency, molecular signatures to best predict activity, and most effective therapeutic combinations. ATR kinase and its ATR-CHK1 pathway are essential to DNA integrity, and the clinical success of ATR inhibition, particularly in combination with other DNA-damaging therapies, will require strategies that limit the occurrence of dose-limiting toxicity.

Mahdi indicated that important considerations in ATR inhibition research include, “focusing on higher doses of ATR inhibition and lower doses of PARP inhibition. Either we reverse PARP inhibitor resistance or have another targeted therapy after PARP inhibitor progression. We should also look beyond PARP inhibition or DNA-damaging therapy, combining ATR inhibitors with other agents without overlapping toxicities.”

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ATR’s integral role in DNA damage repair has made it an appealing target for cancer treatment, and several ATR inhibitors are being evaluated for efficacy and safety across solid tumors.

Benjamin Holmes, DVM | Authors

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