10015 Background: Patients with osteosarcoma recurrence have limited treatment options and poor outcomes. The observed response to cisplatin in osteosarcoma, in vitro susceptibility of osteosarcoma cell lines to ATR and PARP inhibitors, and the presence of mutations in genes involved in DDR served as the basis for the development of this trial. Methods: We conducted a phase II trial of olaparib and ceralasertib in patients aged 12-40 with recurrent osteosarcoma. The primary endpoint for patients with resectable disease limited to the lung (Cohort 2) was submission of paired pre- and post-treatment tumor samples. Secondary endpoints for Cohort 2 included event-free status at 12-months, event-free survival (EFS) and overall survival (OS). Patients received Olaparib 150mg twice a day on days 1-28 and ceralasertib 80mg twice a day on days 1-14 of a 28-day cycle. Patients with unilateral disease received 2 cycles of treatment prior to surgery. Patients with bilateral disease had one side resected, then received 2 cycles of treatment followed by surgery of the contralateral side. Tumor, blood and plasma samples were obtained from Cohort 1 (measurable unresectable disease, efficacy results previously reported) and Cohort 2 patients for correlative studies. Results: Between 3/2021-9/2024, 10 patients from 4 centers were enrolled in Cohort 2. Four patients had bilateral and six had unilateral lung metastases. Median age was 19.3 (range 15.7-24.6). Patients had received a median of three prior therapy regimens. Seven patients (70%, 95%CI: 35%-93%) underwent planned thoracic surgery and had paired pre- and post-treatment tumor samples submitted for correlative studies. Four patients (40%, 95%CI: 12%-74%) were event-free at 12 months and received >12 cycles of olaparib and ceralasertib. The 12-month EFS±SE and OS±SE were 40±15.5% and 90±9.5%, respectively. Initial review of clinical sequencing on a subset of Cohort 1 and 2 patients suggests therapy resistance may be associated with high-risk genomic features, like MYC amplification. Conclusions: Patients with recurrent resectable osteosarcoma limited to the lung parenchyma can be included as a separate cohort in phase II trials of novel agents. In this cohort, it is feasible to incorporate pre-surgical treatment with experimental agents followed by surgical resection of pulmonary nodules and collection of pre- and post-treatment tumor samples, enabling correlative biology. 12-month EFS was 40% but with a wide confidence interval crossing the confidence interval of historical 12-month EFS of 20% from prior COG studies. Sequencing and ctDNA analyses for all enrolled patients (both cohorts) with available samples (N=40 patients with tumor samples and N=48 patients with ctNDA samples) are underway to further assess potential biomarkers of treatment response. Clinical trial information: NCT04417062 .

1142 Background: PARP inhibitors (PARPi) olparib and talazoparib improve objective response rate (ORR) and disease-free survival (DFS) compared with chemotherapy in patients with metastatic breast cancer and germline BRCA 1/2 pathogenic variants (gBRCA 1/2 PVs). Although the ORR is approximately 60%, about 40% of patients do not respond, and nearly all responders eventually develop progressive disease. Therefore, new strategies to overcome PARPi resistance are needed. Preclinical studies suggest that ATR inhibition combined with olaparib may restore PARPi sensitivity. Phase I studies have shown that olaparib (Ola) plus the ATR inhibitor ceralasertib (Cerala) is safe and have established the recommended phase II dose. This prospective phase II study evaluated the ORR of Ola plus Cerala in patients with metastatic breast cancer with gBRCA 1/2 PVs who previously progressed on PARPi therapy. Methods: Eligible patients had metastatic breast cancer (HER2- negative), prior PARPi exposure, and no more than two prior lines of chemotherapy. After informed consent, all patients underwent the baseline research biopsy of a metastatic lesion and provided blood samples for exploratory analysis aimed at identifying mechanisms of PARPi resistance. Treatment consisted of olaparib 150 mg orally twice daily on days 1-28 plus ceralasertib 80 mg orally twice daily on days 1-14 of each 28-day cycle. The primary endpoint was ORR per RECIST 1.1. Secondary endpoints included safety, DFS, and duration of response. Exploratory endpoints focused on biomarker analysis of PARPi resistance. This abstract reports the primary end point. Results: Fifteen patients were enrolled. The median age was 39 years. Eight patients had hormone receptor-positive disease; 6 of these had received endocrine therapy before first-line PARPi. Two patients achieved a partial response (PR) and two had stable disease (SD), resulting and an ORR of 13.3% and a clinical benefit rate of 26.7%. No grade 4 toxicities were observed. Grade 3 adverse events included anemia (N=5), neutropenia (N=1), decreased white blood cell count (N=1), lymphopenia (N=1), and thrombocytopenia (N=1). Conclusions: The combination of olaparib plus ceralasertib is well tolerated and demonstrates clinical activity in patients with metastatic breast cancer with gBRCA 1/2 PVs who have progressed on prior PARPi therapy. A study of this combination in the first-line setting may help determine whether ATR inhibition can overcome or delay PARPi resistance. Biomarker analysis to identify mechanisms of resistance are ongoing and will be reported separately. Clinical trial information: NCT04090567 .

10005 Background: Osteosarcoma is the most common bone tumor of children, adolescents, and young adults and patients with recurrent osteosarcoma have very poor outcomes. The observed response to cisplatin in osteosarcoma, in vitro susceptibility of osteosarcoma cell lines to ATR and PARP inhibitors, and the presence of mutations in genes involved in DDR served as the basis for the development of this trial. Methods: In this single arm, open label phase II trial of olaparib and ceralasertib patients aged 12-40 weighing > 40 Kg with recurrent osteosarcoma and measurable unresectable disease were eligible for Cohort 1. The primary endpoint for Cohort 1 was event-free status at 4-months. Secondary endpoints included objective response rate (ORR) and event-free survival (EFS). After an early study amendment changing the dosing strategy based on clinical data generated from other trials, patients received Olaparib 150mg twice a day on days 1-28 and ceralasertib 80mg twice a day on days 1-14 of a 28-day cycle. Using a two-stage design and a planned sample size of 34 evaluable Cohort 1 patients receiving the amended dosing strategy there is 90% power to detect a 20% increase (39% vs 19%, selected based on a historical benchmark) in the proportion of patients who are event-free at 4-months. Interim analysis required ≥2 of 15 patients to be 4-months event-free to proceed to Stage 2; at Stage 2, ≥11 of 34 patients 4-months event-free for evidence of efficacy. Results: The study proceeded to full accrual based on the interim analysis. As of 1/21/2025 data-cut off, 38 patients from four centers were enrolled in Cohort 1 between November 2020 and November 2024; 37 were eligible and evaluable for safety, objective response, and survival analyses. Excluding four patients enrolled before the study amendment updating dosing, 33 patients were evaluable for the primary objective. Median age was 19.6 years (range 12.7-38.2). Patients received an average of 4.2 (range 1-9) prior therapy regimens, including 26 (70%) patients with prior multi-tyrosine kinase inhibitor treatment. Four of the 33 evaluable were event-free at 4-months (12%; 95% CI: 4%-29%). One of 37 patients had an objective response (ORR: 2.7%; 95% CI: 0.14%-16%). The 4-month EFS±SE was 13.5±5.6% (n = 37). The most common ≥ grade 3 adverse events were platelet count decreased and anemia (38% and 27%, respectively). Conclusions: The study did not meet the predetermined threshold for efficacy for Cohort 1; however, a subset of patients may be benefit from this combination treatment. Results of Cohort 2 (resectable osteosarcoma limited to the lung parenchyma) will be reported separately. Assessment of potential biomarkers of response is underway. Clinical trial information: NCT04417062 .

PURPOSE:Preclinical studies suggest the combination of the poly(adenosine diphosphate[ADP]-ribose) polymerase (PARP) inhibitor olaparib and the ataxia telangiectasia-mutated and Rad3-related (ATR) inhibitor ceralasertib has synergistic antitumor activity in homologous recombination proficient (HRP) and homologous recombination repair gene mutation (HRRm) patients. This study aims to determine the efficacy of olaparib plus ceralasertib in metastatic castration-resistant prostate cancer (mCRPC) with and without HRRm. PATIENTS AND METHODS:Thirty-five HRP and 12 HRRm PARP inhibitor-naïve patients with mCRPC progressing on ≥1 line of therapy or previous androgen receptor pathway inhibitor were enrolled. Patients received olaparib (300 mg twice daily) and ceralasertib (160 mg once daily, days 1-7) in 28-day cycles. HRRm was defined as biallelic inactivation of BRCA2 or BRCA1, monoallelic inactivation of ATM, or germline inactivation of any of these genes. The primary end point was disease response rate (dRR; confirmed prostate-specific antigen decline ≥50% and/or complete/partial radiographic response by RECIST v1.1) in HRP patients. Secondary end points included dRR in HRRm patients, progression-free survival (PFS), and safety/toxicity. Two-stage designs were used. RESULTS:Four of 35 (11%; 95% CI, 3.2 to 26.7) HRP and four of 12 HRRm patients responded. Median PFS was 8.2 months (95% CI, 5.3 to not reached) for HRP patients. Thirty-six percent of patients had ≥grade 3 toxicity, most commonly from anemia. Limitations include small, single-arm, nonrandomized trial design. CONCLUSION:Combining ceralasertib with olaparib had limited activity in patients with HRP mCRPC. HRRm response rate was not greater than previous single-agent PARP inhibitor clinical trials.

BACKGROUNDPhase 1 study of ATRinhibition alone or with radiation therapy (PATRIOT) was a first-in-human phase I study of the oral ATR (ataxia telangiectasia and Rad3-related) inhibitor ceralasertib (AZD6738) in advanced solid tumors.METHODSThe primary objective was safety. Secondary objectives included assessment of antitumor responses and pharmacokinetic (PK) and pharmacodynamic (PD) studies. Sixty-seven patients received 20-240 mg ceralasertib BD continuously or intermittently (14 of a 28-day cycle).RESULTSIntermittent dosing was better tolerated than continuous, which was associated with dose-limiting hematological toxicity. The recommended phase 2 dose of ceralasertib was 160 mg twice daily for 2 weeks in a 4-weekly cycle. Modulation of target and increased DNA damage were identified in tumor and surrogate PD. There were 5 (8%) confirmed partial responses (PRs) (40-240 mg BD), 34 (52%) stable disease (SD), including 1 unconfirmed PR, and 27 (41%) progressive disease. Durable responses were seen in tumors with loss of AT-rich interactive domain-containing protein 1A (ARID1A) and DNA damage-response defects. Treatment-modulated tumor and systemic immune markers and responding tumors were more immune inflamed than nonresponding.CONCLUSIONCeralasertib monotherapy was tolerated at 160 mg BD intermittently and associated with antitumor activity.TRIAL REGISTRATIONClinicaltrials.gov: NCT02223923, EudraCT: 2013-003994-84.FUNDINGCancer Research UK, AstraZeneca, UK Department of Health (National Institute for Health Research), Rosetrees Trust, Experimental Cancer Medicine Centre.

BACKGROUND:Based on a high incidence of genomic alteration in the cell cycle and DNA damage and response (DDR)-related pathways in small cell lung cancer (SCLC), the clinical efficacy of the DDR-targeting agent olaparib (PARP inhibitor) as monotherapy and in combination with ceralasertib (ATR inhibitor) in relapsed or refractory SCLC was evaluated.METHODS:As part of a phase 2 biomarker driven umbrella study, patients with SCLC and predefined DDR gene alterations who failed to benefit from prior platinum-based regimens were allocated to the olaparib monotherapy arm and nonbiomarker-selected patients were allocated to the olaparib and ceralasertib combination arm.RESULTS:In the olaparib monotherapy arm (n = 15), the objective response rate was 6.7% (one partial response), and the disease control rate was 33.3%, including three patients with stable disease. The median progression-free survival was 1.3 months (95% CI, 1.2-NA). In the combination arm (n = 26), the objective response rate and disease control rate were 3.8% and 42.3%, respectively, with one partial response and 10 patients with stable disease. The median progression-free survival was 2.8 months (95% CI, 1.8-5.4). Treatment was generally well tolerated except for one fatal case of neutropenic fever in the combination arm.CONCLUSIONS:Targeting DDR pathways with olaparib as a single agent or in combination with ceralasertib did not meet the predefined efficacy end point. However, disease stabilization was more evident in the combination arm. Further investigation of the combination of olaparib in SCLC should be performed with diverse combinations and patient selection strategies to maximize efficacy.

Background: Myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML) progressing after hypomethylating agent (HMA) treatment have very poor outcomes. MDS and CMML frequently harbor mutations in genes that regulate pre-mRNA splicing, including SF3B1, SRSF2, ZRSR2, and U2AF1. These mutations result in increased R-loops, which are resolved via ATR, suggesting ATR inhibition may be a therapeutic target in MDS, particularly in splicing factor mutant (SF mut) disease. We evaluated the oral ATR inhibitor ceralasertib in MDS and CMML and previously reported that monotherapy (RP2D 160mg PO BID d1-14 q28d) had an overall response rate (ORR) of 27% in patients with relapsed or refractory (R/R) MDS and CMML, while thrombocytopenia was the primary toxicity. We now describe response rates, toxicities, and patient outcomes with an alternate schedule of ceralasertib (NCT03770429). Methods: We enrolled adult patients with SF mut MDS or CMML, R/R after prior HMA exposure. Patients received ceralasertib 160mg BID on an alternative schedule of days 1-7 and 15-21 of a 28 day cycle (7on/7off). We evaluated this alternative schedule for dose limiting toxicities (DLTs) in the first 6 patients, and enrolled an expansion cohort of 14 patients in total. Responses were assessed according to IWG 2006 response criteria. This dose schedule was compared to the prior RP2D schedule (160mg BID 14on/14off). We assessed overall and progression-free survival for patients enrolled on both dose schedules. Results: A total of 44 patients were treated, including 30 patients on the 14on/14off schedule, and 14 patients on the 7on/7off alternate schedule of ceralasertib. The 7on/7off patients included 8 LR-MDS and 6 HR-MDS patients. The median age was 76 years (range 71-87) and 12/14 were male (86%). Reported grade 3 or higher adverse events (AEs) with 7on/7off dosing included febrile neutropenia (n=4), thrombocytopenia (n=2), and pneumonia (n=2), while 1 patient each reported anemia, GI bleeding, folliculitis, infection, sepsis, leukopenia, muscle weakness, dyspnea, and hypoxia. Fewer patients reported grade 4 thrombocytopenia in the 7on/7off schedule (1/14) than the 14on/14off dosing (11/30). No deaths occurred during the 7on/7off schedule, and there were no DLTs. Patients on the 7on/7off schedule were less likely to require a platelet transfusion compared to the 14on/14off schedule (p=0.038). Fewer patients with a baseline platelet count of 50k or higher required a platelet transfusion during the first cycle on the 7on/7off schedule (1 of 10) than the 14on/14off schedule (8 of 16). The ORR on the 7on/7off schedule was 29% (4 of 14 patients) and included one patient with marrow CR (improvement in BM blasts from 10% to 4%) without hematologic improvement (HI), one patient with RBC transfusion independence (TI) (LTB at baseline requiring 2U / 8 weeks, and achieving TI for 12 weeks), one patient with HI-E, and one patient with HI-N. 4/14 patients had progression to AML, all of whom had 10% or higher marrow blasts at study entry. The efficacy was similar to that reported on the 14on/14off schedule (ORR 27%, p=1.00). Overall survival (OS) was assessed for the entire study; the median OS was 12 months (95%CI 11, 27). There was no significant difference in OS between patients on the 14on/14off schedule (12mo, median f/u 40mo) vs 7on/7off schedule (13mo, median f/u 10mo; p=0.44). The median PFS for the entire cohort was 5.9mo (95%CI 4, 13) with no difference between dose schedules (p=0.98). We performed serial gene panel sequencing to assess clonal dynamics during treatment and relapse. Overall clonal structure was relatively stable during the treatment period, including splicing factor mutations. However, at the time of progression, RUNX1 mutation VAFs were enriched compared to enrollment (expanded n=6, new n=1), and generally were part of the dominant clone. Serial testing of serum inflammatory markers and cytokines is ongoing. Conclusion: Ceralasertib is an oral ATR inhibitor with clinical activity in patients with post-HMA MDS and CMML. The schedule of 160mg BID 7on/7off had similar response rates, OS, and PFS compared to a 14on/14off schedule, with a lower rate of severe thrombocytopenia, and this schedule merits further evaluation. At the time of progression, RUNX1 mutations were enriched; future efforts are ongoing to evaluate the mechanisms of response to ATRi in MDS and CMML.

The combination of radiotherapy/chemoradiotherapy and immune checkpoint blockade can result in poor outcomes in patients with locally advanced head and neck squamous cell carcinoma (HNSCC). Here, we show that combining ATR inhibition (ATRi) with radiotherapy (RT) increases the frequency of activated NKG2A+PD-1+ T cells in animal models of HNSCC. Compared with the ATRi/RT treatment regimen alone, the addition of simultaneous NKG2A and PD-L1 blockade to ATRi/RT, in the adjuvant, post-radiotherapy setting induces a robust antitumour response driven by higher infiltration and activation of cytotoxic T cells in the tumour microenvironment. The efficacy of this combination relies on CD40/CD40L costimulation and infiltration of activated, proliferating memory CD8+ and CD4+ T cells with persistent or new T cell receptor (TCR) signalling, respectively. We also observe increased richness in the TCR repertoire and emergence of numerous and large TCR clonotypes that cluster based on antigen specificity in response to NKG2A/PD-L1/ATRi/RT. Collectively, our data point towards potential combination approaches for the treatment of HNSCC. Immune checkpoint blockade can result in good outcomes in patients with metastatic head and neck squamous cell carcinoma (HNSCC), but not those with locally advanced disease. Here, the authors demonstrate that the addition of anti-NKG2A and anti-PD-L1 blockade to ATR inhibition and radiotherapy in the adjuvant setting can induce a robust antitumour immune response in mice.

8104 Background: In extensive stage small-cell lung cancer (SCLC), the standard first-line treatment is an immune checkpoint inhibitor (ICI) combined with chemotherapy. However, in the relapsed or refractory setting, the benefits of ICI-based combinations are modest, regardless of prior ICI exposure. Ceralasertib, a selective ATR inhibitor, not only enhances DNA damage, inducing apoptosis, but also affects immune and inflammatory pathways. This study assesses the safety and efficacy of combining durvalumab with ceralasertib in SCLC patients. Methods: This open-label, phase II, multi-cohort umbrella trial recruited relapsed and refractory SCLC patients independent of biomarker status. Initially, patients received 1500mg of durvalumab on day 1, followed by 240mg of ceralasertib twice daily from day 15 to 28 of a 28-day cycle (Dose A). An amendment aligned the dosing regimen to other studies to 240mg of ceralasertib twice daily from day 1 to 7, with 1500mg of durvalumab administered on day 8 (Dose B). The primary endpoint was the objective response rate (ORR), with a target enrollment of 40 evaluable patients. Results: Between June 2022 and May 2023, forty-two patients were recruited (Dose A: 27; Dose B: 15). The median age was 67 (range: 43-83), with a predominance of males (n = 36) and smokers (n = 40). ECOG PS scores were 0 (n = 1) or 1 (n = 41), and nearly half (n = 20, 47.6%) had prior ICI exposure. The ORR per RECIST was 9.5%, with a disease control rate of 26.2%, including 4 partial responses (PR) and 7 cases of stable disease (SD). Median progression-free survival (PFS) was 1.64 months (95% CI: 1.61-1.97), and overall survival was 7.16 months (95% CI: 6.07-13.9). The duration of responses was notable, showing PFS of 4.8, 17.0, and 18.3 months in ICI-naïve PR patients and 14.1 months in ICI-exposed patient currently still ongoing on treatment. A sustained clinical benefit of up to 10.7 months was observed in an ICI-naïve patient with SD. Hematologic adverse events (AEs) included thrombocytopenia as grade 4 (n = 9) and grade 3 (n = 3), anemia as grade 3 (n = 3), and neutropenia as grade 4 (n = 3) and grade 3 (n = 1). Other AEs were mostly grades 1 or 2 and manageable. Exploratory biomarker analysis is in progress and will be presented. Conclusions: Ceralasertib and durvalumab combination therapy have demonstrated durable responses in select patients with manageable hematologic toxicity. Biomarker research is ongoing to identify patients most likely to benefit from this treatment strategy. Clinical trial information: NCT04361825 .

Representativeness of Study Participants

Supplementary Figure 1. Single dose ceralasertib PK. PK from patients in cohorts 1 to 4 was in line with the PK model predictions (shadowed area) built with data from other studies. Supplementary Figure 2. ATM protein expression by IHC in patients with melanoma. Supplementary Figure 3. ctDNA clearance of mutant alleles from a responder to ceralasertib+paclitaxel; example 1. ctDNA was sequenced at baseline, day 61 and day 403. Re-staging scans were performed at day 63, day 117, intermediate days at approximate 8 week intervals (not shown) and day 426 (at the time of PD). The stream plot shows rapid clearance of the mutant alleles in line with RECIST v1.1 response. Supplementary Figure 4. CtDNA clearance of mutant alleles from a responder to ceralasertib+paclitaxel; example 2. ctDNA was sequenced at baseline, day 24, day 52 and day 241. Re-staging scans were performed at day 46, day 98, intermediate days at approximate 8 week intervals (not shown) and day 235. The stream plot shows rapid clearance of the mutant alleles prior to the day of the first re-staging scan which showed a RECIST v1.1 response.

Abstract Background: AcSé-ESMART is a European proof-of-concept, phase I/II, platform precision cancer medicine trial designed to explore targeted agents in a molecularly enriched population. Arm N explores the PARP inhibitor (PARPi) olaparib (ola) in combination with the Ataxia telangiectasia mutated rad3 related (ATR) inhibitor (ATRi) ceralasertib (cer). In pediatric cancer, the rationale for this combination is twofold: i) Many cancers show evidence of a constitutively active ATR pathway due to endogenous replication/transcription stress (RS) which can be targeted by ATRi. PARPi in this setting serves as additional replication stressor/ATRi sensitizer. ii) Classical BRCA alterations are rare and alterations conferring homologous recombination deficiency (HRD) likely require a combination approach including PARPi. This is a first-in-child study and we here report the results of the Phase I dose escalation part. Methods: Children and adolescents with relapsed or refractory cancer and advanced molecular profiling at relapse were eligible. All patients had to have molecular alterations consistent with HRD or RS in their tumor. Ola was administered orally twice daily (BID) continuously and cer orally BID on days 1-14 of a 28-day cycle. Dose level (DL) 1 was ola 150/100/50 mg BID and cer 80/40/30 mg BID in age group 12-18/6-<12/3-<6 years, respectively. Dose-escalation followed a Boin design, 12 patients treated at the recommended Phase II dose (RP2D) were required to conclude the Phase I part. Plasma for pharmacokinetics (PK) of both drugs was collected during Cycle 1. Results: Eighteen patients (8 sarcomas, 5 central nervous system tumors, 4 neuroblastomas, 1 carcinoma) with a median age of 16 years (range, 4;24) were enrolled over 3 dose levels between Feb 2021 and Sep 2022 and received a median of 3.5 cycles (range 1-12+; data cut-off Jan 2023). The combination was well tolerated overall, toxicity was mainly hematologic (thrombocytopenia, neutropenia, anemia) and gastrointestinal (nausea/vomiting). Five patients encountered dose-limiting toxicities (thrombocytopenia grade 3/4 requiring platelet transfusions > 7 days in all, and grade 4 neutropenia for > 7 days or with documented infection in 1 patient each; 0/3 DL-1, 2/12 in DL1 and 3/3 in DL2), suggesting DL1 as RP2D. One confirmed partial response (PR) was observed in a patient with pinealoblastoma (FANCA VUS), a prolonged stable disease converted to a PR after cycle 9 in a neuroblastoma (11q loss, ATRX VUS, in cycle 10). Four patients are ongoing in cycle 5/6/10/12, respectively. PK and retrospective biomarker analysis are ongoing to identify factors associated with clinical benefit and data will be presented. Conclusions: Ola and cer in combination are well tolerated. Recruitment in two expansion cohorts (HRD and RS) is ongoing at the RP2D, including confirmation of RP2D in younger children. Citation Format: Susanne A. Gatz, Alba Rubio San Simón, Baptiste Archambaud, Samuel Abbou, Morgane Cleirec, Amaury Leruste, Anne-Sophie Defachelles, Nicolas André, Jonathan Rubino, Souad Nebchi, Windy Rondof, Simon A. Smith, Peter G. Mortimer, Bienvenu Loembé, Gwenael Le Teuff, Gilles Vassal, Birgit Geoerger. Phase I/II study of the PARP inhibitor olaparib and ATR inhibitor ceralasertib in children with advanced malignancies: Arm N of the AcSé-ESMART trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT019.

Supplementary Table 1. Treatment Outcomes for individual trial participants and dose cohort (N=57)

Background Despite therapeutic gains from immune checkpoint inhibitors (ICI) in many tumor types, new strategies are needed to extend treatment benefits, especially in patients failing to mount effective antitumor T-cell responses. Radiation and drug therapies can profoundly affect the tumor immune microenvironment. Here, we aimed to identify immunotherapies to increase the antitumor response conferred by combined ataxia telangiectasia and Rad3-related kinase inhibition and radiotherapy. Methods Using the human papillomavirus (HPV)-negative murine oral squamous cell carcinoma model, MOC2, we assessed the nature of the antitumor response following ataxia telangiectasia and Rad3-related inhibitor (ATRi)/radiotherapy (RT) by performing RNA sequencing and detailed flow cytometry analyses in tumors. The benefit of immunotherapies based on T cell immunoreceptor with Ig and ITIM domains (TIGIT) and Programmed cell death protein 1 (PD-1) immune checkpoint blockade following ATRi/RT treatment was assessed in the MOC2 model and confirmed in another HPV-negative murine oral squamous cell carcinoma model called SCC7. Finally, immune profiling was performed by flow cytometry on blood samples in patients with head and neck squamous cell carcinoma enrolled in the PATRIOT clinical trial of combined ATRi/RT. Results ATRi enhances radiotherapy-induced inflammation in the tumor microenvironment, with natural killer (NK) cells playing a central role in maximizing treatment efficacy. We demonstrated that antitumor activity of NK cells can be further boosted with ICI targeting TIGIT and PD-1. Analyses of clinical samples from patients receiving ATRi (ceralasertib) confirm the translational potential of our preclinical studies. Conclusion This work delineates a previously unrecognized role for NK cells in the antitumor immune response to radiotherapy that can be augmented by small-molecule DNA damage–response inhibitors and immune checkpoint blockade.

Background Targeting the DNA damage repair (DDR) pathways is an attractive strategy for boosting cancer immunotherapy. Ceralasertib (AZD6738) is an oral kinase inhibitor of ataxia telangiectasia and Rad3 related protein, which is a master regulator of DDR. We conducted a phase II trial of ceralasertib plus durvalumab in patients with previously treated advanced gastric cancer (AGC) to demonstrate the safety, tolerability, and clinical activity of the combination.Methods This phase II, open-label, single-center, non-randomized study was designed to evaluate the efficacy and safety of ceralasertib in combination with durvalumab in patients with AGC. The study drug regimen was ceralasertib (240 mg two times a day) days 15–28 in a 28-day cycle in combination with durvalumab (1500 mg) at day 1 every 4 weeks. The primary end point was overall response rate (ORR) by Response Evaluation Criteria in Solid Tumors (V.1.1). Exploratory biomarker analysis was performed using fresh tumor biopsies in all enrolled patients.Results Among 31 patients, the ORR, disease control rate, median progression-free survival (PFS), and overall survival were 22.6% (95% CI 9.6% to 41.1%), 58.1% (95% CI 39.1% to 75.5%), 3.0 (95% CI 2.1 to 3.9) months, and 6.7 (95% CI 3.8 to 9.6) months, respectively. Common adverse events were manageable with dose modification. A subgroup of patients with a loss of ataxia telangiectasia mutated (ATM) expression and/or high proportion of mutational signature attributable to homologous repair deficiency (sig. HRD) demonstrated a significantly longer PFS than those with intact ATM and low sig. HRD (5.60 vs 1.65 months; HR 0.13, 95% CI 0.045 to 0.39; long-rank p<0.001). During the study treatment, upregulation of the innate immune response by cytosolic DNA, activation of intratumoral lymphocytes, and expansion of circulating tumor-reactive CD8 +T cell clones were identified in responders. Enrichment of the tumor vasculature signature was associated with treatment resistance.Conclusions Ceralasertib plus durvalumab has promising antitumor activity, with durable responses in patients with refractory AGC. Thus, a biomarker-driven trial is required.Trial registration NCT03780608.

88 Background: Men with metastatic, castration resistant prostate cancer (mCRPC) harboring DNA repair defects (̃20%) achieve a radiographic progression free survival of 7.4 months with PARP inhibitors (PARPi). Preclinical studies combining a PARPi (olaparib) and DNA damage checkpoint inhibitor (ATR inhibitor, ceralasertib) show synergy, providing the rationale to test this combination in men with mCRPC, including where single agent olaparib has been shown to be active. Methods: Two cohorts were accrued to a trial combining ceralasertib with olaparib in men a) with or b) without DNA repair defects. All patients progressed on ≥1 prior mCRPC therapy with no prior PARPi or platinum chemotherapy. The primary endpoint was disease response (confirmed PSA decline ≥50% and/or RECIST response), while disease progression was defined per Prostate Cancer Working Group 3 definition. Each cohort is analyzed independently for disease endpoints, while both groups were combined for toxicity assessments. Results: The 12 person DNA repair-deficient (DRDef) cohort allowed patients with germline BRCA2 loss (n = 4), somatic BRCA2 loss (n = 1) and ATM loss (n = 1 germline, n = 5 somatic and n = 1 somatic with unknown germline). 35 men without BRCA2/BRCA1 or ATM genomic loss were accrued to the DNA repair-proficient (DRPro) cohort. These men had next-generation sequencing (NGS) on contemporary biopsies (prior to enrolment without intervening therapy, 12), prior NGS on metastatic tissue (10), prior NGS on primary prostatic tissue (n = 3), or cell-free analyses (5). Five patients have incomplete cell-free analyses. At data cutoff (October 2021), in the DRDef cohort, the response rate by confirmed ≥50% PSA decline was 4/10 (40%) including 3 of 4 BRCA2 patients, and another is awaiting sufficient follow up; 1 of 6 ATM-deficient patients responded and another is awaiting sufficient follow up. All 4 DRDef responders remain on therapy (median of 8 months). For patients in the DRPro cohort who have completed therapy and response assessment (n = 21), 3 responded, one with a duration of 12 months, two with 6 months. An updated analysis will be presented. Conclusions: This analysis suggests potential activity of the doublet for DRDef (BRCA2 mainly) and DRPro mCRPC. Ongoing biomarker analysis (e.g. ATM IHC, contemporaneous cell-free DNA analysis rather than archived tissue) may help guide selection of patients most likely to benefit. Clinical trial information: NCT03787680.

Background: The majority of BRCA-mutant or HRR-deficient ovarian cancers are initially sensitive to PARP inhibitors (PARPis) but eventually develop resistance. The most commonly reported PARPi resistance mechanisms are (1) reversal of HRR deficiency through (a) reversion mutations in BRCA or other HRR genes (eg, RAD51C/D) or (b) DNA end resection rewiring with loss of 53BP1, REV7, or Shieldin complex components; or (2) stabilization of stalled replication forks through loss of PTIP or EZH2, preventing nuclease recruitment. The only confirmed PARPi resistance mechanism in the clinic to date is reversion mutations. Combined ATR and PARP inhibition is an attractive strategy that may overcome PARPi resistance, as has been shown in PARPi-resistant cell lines and patient-derived xenograft models (Yazinski et al Genes Dev 2017; Murai et al Oncotarget 2016; Kim et al Nat Commun 2020; data on file, AstraZeneca). The PARPi olaparib 300 mg twice daily (BID) continuously in combination with the ATR inhibitor ceralasertib 160 mg daily on days 1-7 of a 28-day cycle has demonstrated promising activity in the CAPRI study in patients with ovarian cancer who had progressed on PARPis (Wethington et al J Clin Oncol 2021). However, preclinical models of PARPi resistance suggest that a higher dose or duration of ceralasertib with a lower dose of olaparib may be more beneficial (data on file, AstraZeneca). Methods: D5330C00004 (NCT02264678) is a multicenter, modular, phase 1 study assessing ceralasertib in combination with other anti-cancer agents in adults with advanced solid malignancies. In Module 2, a lower dose of olaparib (150 mg BID continuous) and a longer duration of ceralasertib (80 mg BID, days 1-14 of a 28-day cycle) is now being explored in 2 expansion cohorts in patients with platinum-sensitive, high-grade, serous or endometrioid ovarian, fallopian tube, or primary peritoneal cancer with deleterious or suspected deleterious germline or somatic BRCA or RAD51C/D mutations, or HRR deficiency by Myriad MyChoice® or FoundationOne® CDx (F1CDx) assays. Patients will have progressed on prior PARPis after ≥6 months in the maintenance or treatment setting (≥12 months after first-line maintenance). Cohort 1 is enrolling ~30 patients immediately after progression on a PARPi; Cohort 2 is recruiting ~30 PARPi-treated patients after intervening platinum-based chemotherapy. Study endpoints include safety, tumor response by RECIST 1.1, and duration of response. Exploratory analyses include genomic and functional analysis of HRR restoration in tumor and circulating tumor DNA, and analysis of replication fork stability in organoid cultures established from mandatory fresh tumor biopsy samples. Recruitment began in July 2021 with the first patient dosed in August 2021. Citation Format: Geoffrey I. Shapiro, Bristi Basu, Anthony El-Khoueiry, Sophie Postel-Vinay, Seock-Ah Im, Sun Young Rha, Claire F. Friedman, Antoine Italiano, Yong Man Kim, Myong Cheol Lim, Rene Roux, Joyce F. Liu, Elhan Sanai, Simon A. Smith, Claire Smith, Sarah El Farhi, Alan Lau, Natalia Lukashchuk, Emma Dean, Matthew G. Krebs. Ceralasertib and olaparib in the treatment of homologous recombination repair (HRR)-deficient platinum-sensitive ovarian cancer after progression on PARP inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT201.

9514 Background: Alterations in DNA damage response (DDR) and repair are associated with genomic instability and increased somatic tumor mutational burden, and modulating DNA repair is a promising strategy to boost the efficacy of cancer immunotherapy. Ceralasertib is an oral inhibitor of the serine/threonine protein kinase Ataxia Telangiectasia and Rad3 Related (ATR), which is crucial to the cell’s response to replication stress. Methods: This phase 2 trial was designed to evaluate the efficacy and safety of ceralasertib in combination with durvalumab in patients with metastatic melanoma (MM) who had failed to anti-PD-1 therapy. The study drug schedule was: ceralasertib at 240 mg BD on days 15 to 28 in combination with durvalumab at 1500 mg on day 1 in a 28-day cycle. The primary end point was overall response rate (ORR) by RECIST (v1.1). To investigate markers predictive of clinical outcome, fresh tumor biopsies were obtained from all enrolled patients before treatment. Results: From August 2019 to May 2020, 30 MM patients (median # of lines, 2; range, 2 - 5) were enrolled. All enrolled patients were exposed to prior anti-PD-1 treatment (immediate failure, n = 23). The ORR was 30.0% (9 PRs, 10 SDs, 10 PDs), DCR 63.3%, median PFS 7.1 months (95% confidence interval (CI), 3.6-10.6), and median OS was 14.2 months (95% CI, 9.3-19.1). Common adverse events of any grade were anemia (n = 23, 76.7%), anorexia (n = 20, 66.7%) and thrombocytopenia (n = 19, 63.3%). Common adverse events of grade 3 or more included anemia (n = 10, 33.3%). One death occurred due to febrile neutropenia in a patient with a pre-existing wound infection. Conclusions: Ceralasertib in combination with durvalumab demonstrated a promising anti-tumor activity, particularly in melanoma patients who failed to standard of care including anti-PD1 treatment. Clinical trial information: NCT03780608.

TPS4166 Background: Ceralasertib (AZD6738) is a selective ATR inhibitor that causes stalled replication forks to collapse, leading to accumulation of double-strand DNA breaks, which is expected to have synergistic anti-tumor effects with immune checkpoint inhibitors (ICI) or PARP inhibitors. First, accumulation of DNA damage by ceralasertib induces tumor cell death, leads to the release of tumor-specific antigen, changing the tumor microenvironment to promote antigen presentation and enhances the anti-tumor effect of ICI. Second, by simultaneously inhibiting two DNA-damage response (DDR) pathways downstream of PARP and ATR, cancer cells are unable to repair damaged DNA, leading to cell death. Ceralasertib has demonstrated promising anti-tumor activity and manageable toxicity in combination with durvalumab or olaparib in solid tumors in a phase 1 study (NCT02264678). In preclinical studies, ceralasertib has shown potent anti-tumor effects in biliary tract cancer (BTC) as a monotherapy and in combination with chemotherapy (Nam, et al, 2019). Methods: This is an open-label, phase 2 umbrella study assessing the efficacy of ceralasertib in combination with durvalumab or olaparib in patients with advanced BTC. Eligible patients have histologically confirmed BTC (including intrahepatic or extrahepatic cholangiocarcinoma, gallbladder cancer, or ampullary cancer), have failed at least one chemotherapy and have ECOG performance status of 0-1. Patients who have received prior ICI, ATR or PARP inhibitor are excluded. Each cycle consists of 4 weeks. In ceralasertib /durvalumab cohort, 37 patients will receive durvalumab 1.5g on day 1 and ceralasertib 240mg twice daily on days 15-28. In ceralasertib /olaparib cohort, 37 patients receive ceralasertib 160mg once daily on days 1-7 and olaparib 300mg twice daily on days 1-28. The primary endpoint is disease control rate, with key secondary endpoints including overall response rate, progression-free survival, overall survival, and safety. Tissue and blood samples are being collected for translational biomarker studies. Clinical trial information: NCT04298021.