A Study to Investigate the Efficacy and Safety of Letrozole SIE Compared With Femara® (Both Combined With the CDK4/6 Inhibitor Ribociclib) in Postmenopausal Women With HR-Positive, HER2-Negative, Inoperable Locally Advanced or Metastatic Breast Cancer

This is a multicenter, randomized, double-blind, double-dummy, active-controlled Phase III study comparing Letrozole SIE with Femara® (both in combination with the CDK4/6 inhibitor ribociclib) in postmenopausal women with HR-positive, HER2-negative, inoperable locally advanced or metastatic breast cancer. The study employs double-blinding with matching placebos (double-dummy technique) to minimize bias on the part of participants, investigators, and analysts. Participants will be randomized in a 1:1 ratio to receive either Letrozole SIE (experimental use) or Femara® (active comparator), both in combination with the CDK4/6 inhibitor ribociclib. Randomization occurs after Screening and confirmation of eligibility and will be stratified by visceral metastasis (Yes/No) and prior adjuvant endocrine therapy (Yes/No).

The Treatment Period will last from Cycle 1 Day 1 (C1D1) until study intervention discontinuation due to disease progression, symptomatic deterioration, unacceptable toxicity, withdrawal of consent, loss to follow-up, participant's death, or end of study, whichever occurs first. After study intervention discontinuation, a post-discontinuation visit will be performed. Then, participants will be followed until withdrawal of consent, loss to follow-up, the participant's death, or end of study, whichever occurs first.