This clinical trial is focused on studying the effects of a medication called MAVENCLAD, which contains the active substance cladribine, on a condition known as Multiple Sclerosis (MS). MS is a disease that affects the central nervous system, which includes the brain and spinal cord. The study aims to understand how cladribine treatment influences the activity of certain cells in the brain called microglial cells. These cells are part of the immune system in the brain and can become activated in MS, potentially contributing to the disease process.

The purpose of the study is to observe changes in microglial cell activity in different areas of the brain before and after 18 months of treatment with cladribine. Participants will undergo imaging tests, such as PET (Positron Emission Tomography) and MRI (Magnetic Resonance Imaging), to evaluate these changes. These imaging techniques help visualize the brain and assess the effects of the treatment on microglial cells and other brain structures.

Throughout the study, researchers will also monitor changes in the overall brain structure, including white and gray matter, and the presence of MS-related lesions. The study will compare these findings with those from healthy individuals and untreated MS patients to better understand the impact of cladribine on the disease. The study is expected to last for 18 months, providing valuable insights into the potential benefits of cladribine for people with Multiple Sclerosis.