Abstract P3-07-13: The next generation oral selective estrogen receptor degrader (SERD) camizestrant (AZD9833) is active against wild type and mutant estrogen receptor α

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.