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Background: Patients with a/mTNBC have limited treatment options and a poor prognosis (objective response rate [ORR] of 37%, median duration of response 6.5 months, median overall survival 15.5 months for 1L chemotherapy [Rugo, et al. Ann Oncol. 2021 LBA16]). Combining checkpoint inhibitors with 1L chemotherapy modestly improves outcomes but only in PD-L1–positive a/mTNBC, emphasizing a critical unmet need for patients with PD-L1–negative disease and for further improving outcomes in PD-L1–positive disease. BEGONIA (NCT03742102) is an ongoing 2-part, open-label platform study, evaluating safety and efficacy of D, an anti–PD-L1 antibody, combined with other novel therapies in 1L a/mTNBC, including Dato-DXd, an antibody-drug conjugate consisting of a humanized anti-TROP2 antibody covalently linked to a highly potent topoisomerase I inhibitor payload via a stable, tumor-selective, tetrapeptide-based cleavable linker. Early data from BEGONIA of D in combination with Dato-DXd showed promising responses. Here, we report updated results of Dato-DXd + D. Methods: Patients with unresectable a/mTNBC eligible for 1L treatment were enrolled, regardless of PD-L1 or TROP2 status, and received intravenous Dato-DXd 6 mg/kg + D 1120 mg every 3 weeks until progression or unacceptable toxicity. PD-L1, assessed using the VENTANA PD-L1 (SP263) Assay, was defined as high if ≥ 5% of the tumor area was populated by PD-L1–expressing tumor or immune cells. Primary endpoints were safety and tolerability. Secondary endpoints included investigator-assessed ORR (RECIST v1.1) and duration of response. Patients included in the efficacy analysis had ≥ 2 on-treatment disease assessments, progressed, died, or withdrew from the study. Results: As of April 8, 2022, 47 patients received Dato-DXd + D (39 ongoing) and 33 of those were included in the efficacy analysis. Median (range) follow-up was 7.5 (0–11) months. Patient age was a median of 51 years, 57% received prior treatment for early stage TNBC, and 60% had visceral metastases at baseline. Confirmed ORR was 26/33 (79%; 95% CI, 61–91); 2/33 patients (6%) had a complete response and 24/33 (73%) had a partial response. Confirmed response was irrespective of PD-L1 expression (PD-L1 high ORR, 4/5 [80%]; PD-L1 low, 16/21 [76%]; PD-L1 missing, 6/7 [86%] patients). Median duration of response was not reached; 100% of patients with a complete or partial response remained in response at 6 month follow-up, and 96% had an ongoing response at data cutoff. Adverse events (AEs) were manageable and consistent with the known safety profiles of each agent, with treatment-related AEs occurring in 41 patients (87%), any Grade 3/4 AEs in 17 patients (36%), and any serious AEs in 7 patients (15%). The most common all-Grade AEs were gastrointestinal (nausea in 26 patients [55%] and stomatitis in 24 patients [51%]). A low rate of diarrhea was reported (6 patients [13%], all Grade 1 or 2); 4 patients had anemia and 1 had neutropenia. There were no cases of interstitial lung disease/pneumonitis or thrombocytopenia. Nine patients (19%) and 11 patients (23%) underwent Dato-DXd dose reduction and delay, respectively; 14 (30%) had D dose delay. Treatment was discontinued due to an AE for 3 patients (6%). There were no deaths due to treatment-related AEs. Conclusions: In this updated analysis with additional patients and longer follow-up, the combination of Dato-DXd + D in 1L a/mTNBC demonstrated a manageable safety profile and compelling high response rates with promising durability. Although subgroups were small, responses occurred irrespective of PD-L1 expression. Further investigation of this treatment combination is warranted. Analysis of translational data is ongoing. Funding: AstraZeneca/Daiichi Sankyo Citation Format: Peter Schmid, Piotr Wysocki, Cynthia Ma, Yeon H. Park, Ricardo Fernandes, Simon Lord, Richard D. Baird, Catherine Prady, Kyung Hae Jung, Jamil Asselah, Robert Huisden, Ross Stewart, Petra Vuković, Ana T. Nunes, Zbigniew Nowecki. PD11-09 Datopotamab deruxtecan (Dato-DXd) + durvalumab (D) as first-line (1L) treatment for unresectable locally advanced/metastatic triple-negative breast cancer (a/mTNBC): updated results from BEGONIA, a phase 1b/2 study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD11-09.

Endocrine therapy forms the backbone treatment for patients with estrogen receptor (ER) positive tumors in both the adjuvant and metastatic setting. Aromatase inhibitors (AI) are the most common endocrine treatment option. Mutation of ESR1, the gene encoding ERα, is a common mechanism of resistance to AIs which leads to ligand independent activity of ERα. Camizestrant (AZD9833) is a next generation SERD and pure ER antagonist that is in Phase 3 trials (SERENA-4: NCT04711252; SERENA-6: NCT04964934). Here we report the preclinical and clinical activity of camizestrant in patients with ESR1 wild-type (ESR1wt) and mutant (ESR1m) tumors. The binding affinities of camizestrant, fulvestrant, and estradiol to wt ERα and ERα variants with mutations in the ligand binding domain were assessed. All three compounds exhibited reduced binding to mutant forms of ERα compared with wt ERα; the Y537S mutation had the greatest impact on binding. This was reflected in requirement for greater concentrations of camizestrant and fulvestrant to degrade and antagonize mutated ERα and to impact cellular proliferation in MCF-7 cells that expressed Y537S ESR1m compared to ESR1wt MCF-7 cells. Furthermore, while a 3 mg/kg dose of camizestrant achieved a maximal anti-tumor effect in a ESR1wt patient derived xenograft model, a 10 mg/kg was required for maximal effect in a D538G ESR1m model. Considering this difference between ESR1m and ESR1wt, pharmacokinetic/pharmacodynamic modelling of preclinical data predicted that a camizestrant dose of 75 mg would be maximally efficacious in patients with ESR1m tumors. Indeed, analysis of ESR1m circulating tumor DNA levels in patients from the SERENA-1 (NCT03616587) Phase 1 trial showed a clear effect of 14 days treatment with 75 mg camizestrant resulting in a >2-fold reduction in ESR1m variant allele frequency in 12/13 (92%) cases with complete clearance of ESR1m ctDNA in 7/13 (54%) cases. Interestingly, the clinical activity of camizestrant was higher in heavily pretreated patients with metastatic breast cancer with ESR1m tumors compared to those with no detectable mutation (ESR1m not detected). At a camizestrant dose of 75 mg, median progression-free survival was 8.3 months (maturity 12/15) in patients with ESR1m tumors compared to 5.6 months (8/9) in those with ESR1m not detected (data cut-off 6 October 2021). Camizestrant-induced ERα degradation was seen in both groups (mean reduction in H-score 42% in ESR1m tumors (n= 12 evaluable pairs) and 46% in tumors with ESR1m not detected (n=7)). Whole transcriptome analysis revealed a trend towards higher ERα activity at baseline in ESR1m tumors compared to ESR1m not detected; ERα activity reduced on treatment in both groups. Consistent with the clinical activity data, camizestrant induced more profound reductions in cell proliferation in ESR1m tumors compared to ESR1m not detected tumors (as seen by greater reductions in Ki67-positive tumor cells). These data demonstrate the activity of camizestrant in patients with ESR1m tumors. Clinical activity along with degradation and antagonism of the ERα is also seen in patients with tumors in which ESR1 mutations are not detected. In this heavily pre-treated Phase 1 patient population from SERENA-1, ESR1m may be a predictive biomarker to enrich for patients with maintained endocrine sensitivity. The SERENA-6 trial is investigating the efficacy and safety of camizestrant plus a CDK4/6 inhibitor in patients with metastatic breast cancer and detectable ESR1m. We acknowledge Helen Heffron, PhD, from InterComm International who provided medical writing support funded by AstraZeneca. Citation Format: Christopher Morrow, Larissa Carnevalli, Richard D. Baird, Tim Brier, Carmela Ciardullo, Natalie Cureton, Mandy Lawson, Robert McEwen, Myria Nikolaou, Anne Armstrong, Begoña Bermejo, Emiliano Calvo, Eva Ciruelos, Javier Garcia-Corbacho, Erika Hamilton, Jason Incorvati, Peter Kabos, Mafalda Oliveira, Manish R Patel, Manuel Ruiz-Borregó, Nicholas Turner, Chris Twelves, Christos Vaklavas, Danielle Carroll, Steven Ching, Nevena Cvetesic, Michelle DuPont, Lisa Gibbons, Alastair Mathewson, Rhiannon Maudsley, Pablo Morentin Gutierrez, Avinash Reddy, Jaime Rodriguez-Canales, Susana Ros, Dhivya Sudhan, Andy Sykes, David Whitson, Teresa Klinowska, Justin Lindemann. The next generation oral selective estrogen receptor degrader (SERD) camizestrant (AZD9833) is active against wild type and mutant estrogen receptor α [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-07-13.

Background AZD5305 is a selective PARP1 inhibitor (PARPi) generated for improved therapeutic index compared with non-selective PARPi. Measurement of PARylation inhibition has been used to determine the pharmacodynamics (PD) of PARPi in clinical trials, but the assays used were insensitive and non-specific. Here we report on a new, highly sensitive and specific PARylation assay as evaluated in the PETRA trial (NCT04644068). Methods PETRA is a Phase 1/2 clinical study investigating AZD5305 alone or in combination with other anticancer agents in patients (pts) with advanced solid tumors. Peripheral blood mononuclear cells (PBMC) collected at multiple timepoints during treatment (cycle [C] 0 day [D] 1 pre-dose, 1, 4, 24 and 48 hours [hrs] post-dose, and C1D1 pre-dose) were lysed, subjected to an ex-vivo PARylation reaction, and quantified with the novel MSD PARylation assay. Results PARylation data were available from 32 patients treated from 20 mg to 140 mg. AZD5305 significantly inhibited PARylation in PBMC with maximum PARylation inhibition occurring around time to plasma concentration maximum (Tmax, 1 to 4 hrs after dosing; Table). At longer timepoints, a majority of patients showed >90% PARylation inhibition up to 48 hrs, 72 hrs, and even 168 hrs. Inhibition >90% for up to 72 hrs was observed in 4/4 evaluable samples at 140 mg, while inhibition >90% for an extended period (up to 168 hrs) was observed in 3/4 samples at 20 mg and 1/2 samples at 60 mg. These data demonstrate that AZD5305 causes significant PARylation inhibition at doses ≥20 mg which is consistent with in vitro findings. Conclusions Significant and durable PARylation inhibition was demonstrated with the novel PARylation assay at all doses of AZD5305 in PBMC samples from the majority of pts tested. Table. Residual PARylation levels by dose in the PETRA study 20 mg 60 mg Time post-dose 1 hour 4 hours 1 hour 4 hours Patients with >90% PARylation inhibition, n (%) 9/15 (60.0) 10/15 (66.7) 11/13 (84.6) 10/13 (76.9) Median residual PARylation levels (residual PARylation), % 5.3 5.0 4.0 3.7 Citation Format: Benedetta Lombardi, Paola Marco-Casanova, Athanasios Karapetsas, Mya Hornett, Edit Lukacs, Ko Sugibayashi, Sabina Cosulich, Adam Dowson, Ganesh Moorthy, Jessica Brown, Timothy A. Yap, Alison M. Schram, Judith Balmana, Stephen J. Luen, Seock-Ah Im, Adam Sharp, Richard Baird, Elizabeth A. Harrington, Spiros Linardopoulos. A highly sensitive and specific PARylation assay confirms significant and durable target engagement by AZD5305 in patients [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 CT269.

Background: AZD5305 is a potent, highly selective PARP1 inhibitor and trapper with superior preclinical tolerability, target engagement and efficacy vs 1st generation dual PARP1/2 inhibitors (PARPi). This is the first report of the ongoing Phase 1/2a PETRA (NCT04644068) trial. Methods: Pts with advanced breast, ovarian, prostate or pancreatic cancer bearing germline or somatic BRCA1/2, PALB2 or RAD51C/D mutations received AZD5305 QD PO until disease progression. ECOG PS 0-2 and Hb ≥9.0 g/dL were required. Prior PARPi and platinum therapy were permitted. The primary objective was safety; secondary objectives included pharmacokinetics (PK) and pharmacodynamics in tumor and/or blood samples and response by RECIST v1.1, CA125 or PSA. Exploratory genomic analyses included zygosity evaluation and ctDNA response monitoring. Results: At data cutoff (Nov 17, 2021), 46 pts received AZD5305 10-90 mg QD (43.5% had prior PARPi; median 3.5 prior lines of therapy). AZD5305 was well tolerated across all doses without DLTs (Table). PK exposures were dose-proportional. Steady-state Ctrough was higher than 1st generation PARPi: specifically 6.3 and 31.9 fold above target effective concentration at 10 and 90 mg, respectively. PARylation inhibition was ≥90% at 10-40 mg QD (PBMCs) confirming target engagement. 7/25 (28%) pts had objective responses: 5 RECIST PRs (3 confirmed) and 2 additional pts with PSA50 responses (1 confirmed), including platinum- and PARPi-resistant pts. 13/22 (59%) RECIST-measurable pts had SD or PR up to 51+ weeks. ctDNA declined on treatment in 7/13 (54%) evaluable pts (3 complete, 4 >50% reductions) across doses. Conclusions: AZD5305 is a highly selective PARP1 inhibitor and trapper with excellent physiochemical properties and a wide therapeutic index. It led to maximal target engagement and showed promising clinical activity with favorable tolerability at exposures surpassing those of 1st generation PARPi. AZD5305 10 mg/d (n=8) AZD5305 20 mg/d (n=19) AZD5305 40 mg/d (n=13) AZD5305 60 mg/d (n=3) AZD5305 90 mg/d (n=3) Total (N=46) Most common (>10%) TRAEs, n (%) Any grade Any grade Any grade Any grade Any grade Grade ≥3 Any grade Nausea 3 (37.5) 5 (26.3) 1 (7.7) 1 (33.3) 0 0 10 (21.7) Anemia* 2 (25.0) 4 (21.1) 1 (7.7) 0 0 6 (13.0) 7 (15.2) Neutropenia* 3 (37.5) 2 (10.5) 1 (7.7) 0 1 (33.3) 2 (4.3) 7 (15.2) Thrombocytopenia* 1 (12.5) 2 (10.5) 2 (15.4) 0 0 1 (2.2) 5 (10.9) Fatigue and asthenia* 2 (25.0) 2 (10.5) 0 1 (33.3) 0 0 5 (10.9) Any TRAE leading to dose reduction 1 (12.5) 0 0 0 0 1 (2.2) Any TRAE leading to discontinuation 0 0 0 0 0 0 AE, adverse event; TRAE, treatment-related adverse event *Grouped Terms Citation Format: Timothy A. Yap, Seock-Ah Im, Alison M. Schram, Adam Sharp, Judith Balmana, Richard D. Baird, Jessica S. Brown, Maria Schwaederle, Elizabeth A. Pilling, Ganesh Moorthy, Spiros Linardopoulos, Adam Dowson, Carol Pound, Edit Lukacs, Sabina Cosulich, Stephen J. Luen. PETRA: First in class, first in human trial of the next generation PARP1-selective inhibitor AZD5305 in patients (pts) with BRCA1/2, PALB2 or RAD51C/D mutations [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 CT007.