This is a multicountry, multicenter, observational study in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer receiving first-line endocrine-based therapy with an aromatase inhibitor in combination with a CDK4/6 inhibitor. The study aims to describe the prevalence of ESR1 mutations at baseline and the emergence of ESR1 mutations over time using circulating tumor DNA testing in routine clinical practice.

Patients receiving first-line treatment for at least 6 months and no more than 18 months, without evidence of disease progression at study entry, may undergo baseline ESR1 mutation testing. Patients with a negative baseline result may undergo longitudinal monitoring approximately every 3 months, for up to 18 months or 6 testing timepoints, to assess emergence of ESR1 mutations. The study will also describe mutation subtypes, testing methods used in routine practice, selected clinical characteristics, and treatment patterns across participating countries.

This is a multicountry, multicenter, observational study designed to determine the prevalence of ESR1 mutations at baseline and to assess the emergence of ESR1 mutations during longitudinal surveillance in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer (HR+/HER2- aBC) receiving first-line treatment with an aromatase inhibitor (AI) in combination with a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor.

Eligible patients are adults with histologically or cytologically confirmed HR+/HER2- advanced breast cancer who have been receiving first-line AI plus CDK4/6 inhibitor therapy for at least 6 months and no more than 18 months, without evidence of disease progression according to investigator assessment, and who are willing and able to provide blood samples for circulating tumor DNA (ctDNA) testing at predefined intervals.

Following informed consent, baseline data will be collected in electronic case report forms and will include sociodemographic characteristics, clinical and tumor history, testing methods, and treatment patterns. A baseline blood sample will be collected for ctDNA-based ESR1 mutation testing using testing methods applied in routine clinical practice, including but not limited to quantitative polymerase chain reaction (qPCR), digital polymerase chain reaction (dPCR), and next-generation sequencing (NGS), according to local availability and site capability. Testing will be performed in validated laboratories in accordance with local standard operating procedures.

Patients who are negative for ESR1 mutation at baseline will undergo longitudinal ctDNA monitoring approximately every 12 weeks (+/-4 weeks), for up to 18 months or 6 testing time points from initial testing, whichever occurs first. Patients who test positive for ESR1 mutation at baseline or during follow-up will discontinue further study surveillance and will continue to receive routine clinical care as determined by the treating physician. The date of first ESR1 mutation detection will be recorded.

The primary objectives are to determine the prevalence of ESR1 mutations at baseline and the emergence rate and time to emergence of ESR1 mutations during the surveillance period among patients who are ESR1 negative at initial testing. Secondary objectives include assessment of ESR1 mutation frequency by duration of first-line therapy and by testing method, distribution of specific ESR1 mutation subtypes, co-mutations with other clinically relevant biomarkers when available, patient clinical and sociodemographic characteristics, testing and treatment patterns, and associations between ESR1 mutation status and relevant patient or treatment factors.

Approximately 3,000 patients are planned to be enrolled at about 30 sites in 15 countries across Asia, Latin America, and the Middle East and Africa. The estimated recruitment period is 12 months.

Inclusion Criteria:

• Age 18 years or older at the time of informed consent and willing and able to provide informed consent before any study-related procedures.
• Histologically- or cytologically-confirmed hormone receptor-positive (ER- and/or progesterone receptor-positive), human epidermal growth factor receptor 2-negative (HER2-negative) advanced breast cancer.
• Receiving first-line therapy with an aromatase inhibitor in combination with a CDK4/6 inhibitor for at least 6 months and no more than 18 months, with no evidence of disease progression at study entry, as assessed by the investigator.
• Able and willing to provide a blood sample for circulating tumour DNA testing for ESR1 mutation assessment at approximately quarterly intervals.

Exclusion Criteria:

• Evidence of disease progression during first-line aromatase inhibitor plus CDK4/6 inhibitor therapy, based on investigator assessment.
• Known ESR1 mutation status at study entry.

Adults with HR-positive/HER2-negative advanced breast cancer (aBC), receiving first-line aromatase inhibitor (AI) plus CDK4/6 inhibitor therapy for 6-18 months, without disease progression, and with unknown ESR1 status at baseline.

Qualified researchers can request access to anonymized individual patient-level data from AstraZeneca group of companies sponsored clinical trials via the request portal Vivli.org. All requests will be evaluated as per the AZ disclosure commitment: https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

Yes, indicates that AZ are accepting requests for IPD, but this does not mean all requests will be shared

AstraZeneca will meet or exceed data availability as per the commitments made to the EFPIA PhRMA Data Sharing Principles. For details of our timelines, please rerefer to our disclosure commitment at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

When a request has been approved AstraZeneca will provide access to the anonymized individual patient-level data via secure research environment Vivli.org.

Signed Data Usage Agreement (non-negotiable contract for data accessors) must be in place before accessing requested information.