Study to Evaluate the Safety and Tolerability of Camizestrant in Combination With Atirmociclib in Women With Advanced Breast Cancer

Main Inclusion Criteria:

* Participants with advanced adenocarcinoma of the breast and must have received prior adequate therapy in accordance with local practice for their tumor type and stage of disease.
* Metastatic or locoregionally recurrent disease and radiological or objective evidence of progression on or after the last systemic therapy prior to starting investigational medicinal products.
* Eastern cooperative oncology group (ECOG)/World Health Organization (WHO) performance status 0 to 1, and a minimum life expectancy of 12 weeks.
* At least one lesion that is measurable and/or non-measurable, as per RECIST 1.1 and that can be accurately assessed at baseline and is suitable for repeated assessment by computed tomography (CT), magnetic resonance imaging (MRI), or plain X-ray, or clinical examination.
* Menopausal status

  * Pre-menopausal women must start GnRH agonist therapy at least 4 weeks before study treatment and continue throughout the study.
  * Post-menopausal women must meet one of these criteria: bilateral oophorectomy, age ≥60 years, age ≥50 years with ≥12 months amenorrhea and intact uterus without hormonal therapy, or age \<60 years with ≥12 months amenorrhea and post-menopausal hormone levels.
* Histological or cytological confirmation of adenocarcinoma of the breast.
* Participants of childbearing potential must agree to use one highly effective contraceptive measure.
* Documentation of ER-positive tumor irrespective of progesterone receptor status.

Main Exclusion Criteria:

* A participant who has received 2 or more lines of CDK4/6 inhibitors in the advanced disease setting.
* A participant who has received prior camizestrant or atirmociclib treatment in the advanced disease setting.
* Patients previously treated with other next generation selective estrogen receptor degrader (SERDs) or other experimental ETs in the advanced disease setting.
* Patients previously treated with other experimental cyclin-dependent kinase (CDK) inhibitors are not eligible.
* Inability to swallow oral medications.
* Any unresolved toxicities of Grade ≥ 2 from prior anti-cancer therapy (with the exception of alopecia).
* Presence of life-threatening metastatic visceral disease.
* Any evidence of severe or uncontrolled systemic diseases.
* Contraindication to or known intolerance/hypersensitivity of/to camizestrant or atirmociclib.