This study tests a new way to give CDK4/6 inhibitors, which are drugs for Metastatic Breast Cancer (MBC), in people aged 65 or older. Metastatic means cancer that has spread. CDK4/6 inhibitors like palbociclib and ribociclib help stop cancer cells from growing. Usually, these drugs are given at full dose, but they can cause side effects. This study compares starting with a lower dose and increasing it if the patient can handle it. This might help patients stay on the drug longer and get more benefits.

Participants will also take endocrine therapy, which are drugs that block hormones fueling cancer. They can choose which one they prefer with their doctor. Participants will be randomly assigned to one of two groups: one with the usual dosing and the other with the new dosing strategy. The main goal is to see which method helps patients stay on treatment longer.

• The study is for patients aged 65 or older with specific types of breast cancer.
• Participants must not have used CDK4/6 inhibitors for their cancer before.
• Participants will be closely monitored for side effects and treatment effectiveness.