Sponsor decision to prematurely stop the study, not linked to any safety concern.

Primary Objective:

To assess the incidence rate of dose-limiting toxicity and to confirm the recommended dose as well as the maximum tolerated dose of SAR439859 administered as monotherapy to Japanese postmenopausal women with estrogen receptor positive and human epidermal growth factor receptor 2-negative advanced breast cancer.

Secondary Objective:

• To characterize the overall safety profile of SAR439859 administered as monotherapy.
• To characterize the pharmacokinetic profile of SAR439859 administered as monotherapy.
• To evaluate the antitumor activity of SAR439859 administered as monotherapy and the clinical benefit rate (complete response, partial response and stable disease ≥ 24 weeks).

The duration of the study for an individual participant will include a period to assess eligibility (screening period) of up to 4 weeks (28 days), a treatment period of at least 1 cycle (28 days) of study treatment, and an End of Treatment (EOT) visit at least 30 days (or until the participant receives another anticancer therapy, whichever is earlier) following the last administration of study treatment. Study treatment may continue until precluded by unacceptable toxicity, disease progression, or upon participant's request.

Inclusion criteria :

• Participants must be postmenopausal women.
• Breast adenocarcinoma patients with locally advanced not amenable to radiation or surgery, inoperable and/or metastatic disease.
• Either the primary or any metastatic site must be positive for estrogen receptor (ER) (>1% staining by immunohistochemistry).
• Either the primary tumor or any metastatic site must be human epidermal growth factor receptor 2 non-overexpressing.
• Patients with at least 6 months of prior endocrine therapy.

Exclusion criteria:

• Eastern Cooperative Oncology Group Performance Status (ECOG) ≥2.
• Significant concomitant illness that would adversely affect participation in the study.
• Patients with a life expectancy less than 3 months.
• Patient not suitable for participation, whatever the reason.
• Major surgery within 4 weeks prior to first study treatment administration.
• Treatment with strong and moderate cytochrome P450 3A inhibitors/inducers.
• Patients with known endometrial disorders, uterine bleeding or ovarian cysts.
• Treatment with anticancer less than 2 weeks before first study treatment.
• Prior treatment with selective estrogen receptor down (SERD)-regulator (except fulvestrant for which a washout of at least 6 weeks is required).
• Inadequate hematological function.
• Inadequate renal function with serum creatinine ≥1.5 x upper limit of normal (ULN).
• Liver function: aspartate aminotransferase >3 x ULN, or alanine aminotransferase >3 x ULN. Total bilirubin >1.5 x ULN.
• Non-resolution of any prior treatment related toxicity to \

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org