The purpose of the study is to evaluate the efficacy and safety of subcutaneously administered cladribine versus placebo to stop inflammation and treat disease progression of non-active secondary progressive multiple sclerosis. Multiple sclerosis is an inflammatory disease of the central nervous system. In most patients, it starts with a relapsing course (RMS) which is caused by acute inflammatory lesions in the brain and spinal cord. RMS transforms at later stages into progressive disease (secondary progressive MS). Currently approved disease-modifying treatments are effective in reducing clinical relapses and brain and spinal lesions visible in MR, but they perform poorly in preventing disease progression and overall disability accumulation. The growing evidence shows that disease progression partially depends on chronic inflammation present in the CNS. Drugs, which may cross the blood-brain barrier and reach inflammatory cells residing in the CNS might be effective in this stage of the disease. Cladribine is one of the DMT approved for RMS. It is a synthetic purine analog with selective lymphocyte toxicity, which enter the CNS and is found in cerebrospinal fluid. In patients treated with cladribine, the oligoclonal bands tend to disappear proving that neuroinflammation is diminished. The participants of this clinical trial with the later non-active stage of MS are enrolled to be treated with cladribine subcutaneously or a non-active comparator (placebo) for 6 months and followed for the next 2 years, with an MRI scan and clinical evaluation every 6 months. The main questions it aims to answer are if in the non-active stage of MS cladribine is potent to lessen brain volume loss and if it is potent to attenuate inflammation in the CNS.