A Phase 2, Randomized, Open-Label Dose Optimization and Phase 3, Randomized, Double-Blind, Placebo-Controlled Study of LY4064809 Combined With a CDK4/6 Inhibitor and Endocrine Therapy in Adults With HR+, HER2-Advanced Breast Cancer With a PIK3CA Mutation Who Received No Prior Treatment for Advanced Breast Cancer (PIKALO-2)

Inclusion Criteria: * Are willing to follow contraception requirements. Contraceptive use by participants should be consistent with local regulations regarding the methods of contraception for those participating in clinical trials. * If assigned female at birth, pre-/peri- and postmenopausal status is allowed. Those with pre- or peri-menopausal status at study entry must agree to use ovarian function suppression with any locally approved gonadotropin-releasing hormone (GnRH) agonist. * If assigned male at birth with an estrogen receptor positive (ER+) breast cancer diagnosis, they must agree…