A Phase IIa, Open-label Study to Evaluate the Safety, Pharmacokinetics and Preliminary Efficacy of Camizestrant in Combination With Atirmociclib in Participants With ER-positive, HER2-negative Advanced Breast Cancer (SERENA-1b)

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.