This clinical trial is focused on studying treatments for HER2-negative early breast cancer, which is a type of breast cancer that does not have high levels of a protein called HER2. The study will compare two different treatment approaches. One group will receive a combination of two medications: elacestrant and ribociclib. Elacestrant is an anti-estrogen medication, which means it works by blocking the effects of estrogen, a hormone that can promote the growth of breast cancer cells. Ribociclib is a cyclin-dependent kinase (CDK) inhibitor, which helps to stop cancer cells from dividing and growing. The other group will receive a combination of an aromatase inhibitor (AI), which is another type of medication that lowers estrogen levels, and ribociclib. In some cases, a GnRH agonist, a type of medication that affects hormone levels, will also be used in premenopausal and perimenopausal women and men.

The purpose of the study is to evaluate which treatment combination is more effective as a neoadjuvant therapy, meaning it is given before the main treatment, usually surgery, to shrink the tumor. Participants in the study will be randomly assigned to one of the two treatment groups. The study will last for a period of six months, during which participants will take the medications orally. The effectiveness of the treatments will be assessed at the time of surgery, using a measure called the modified PEPI score, which helps to evaluate the response of the cancer to the treatment.

This trial aims to provide valuable information on the best treatment approach for patients with endocrine-responsive HER2-negative early breast cancer. By comparing the two treatment combinations, researchers hope to determine which is more effective in reducing the size of the tumor and improving outcomes for patients. The study will also look at other factors, such as changes in tumor size and the proportion of tumors that respond to the treatment, to gain a comprehensive understanding of the benefits of each treatment option.