Your participation in the study starts with the double-blind placebo controlled period, which lasts for 24 weeks. During this time, neither you nor your doctor will know whether you are receiving the active medication or a placebo.

You will be assigned to one of three groups: high dose cladribine, low dose cladribine, or placebo. The assignment is done randomly, similar to flipping a coin.

Cladribine is a medication given as hard capsules that you take by mouth. If you receive placebo, you will take capsules that look identical but contain no active medication.

If you are taking corticosteroids (anti-inflammatory medications such as prednisone, prednisolone, or methylprednisolone), you will continue taking them at the same daily dose throughout the study. Your dose must remain stable and not exceed 20 milligrams per day for prednisone or prednisolone, or 16 milligrams per day for methylprednisolone.

If you are taking acetylcholinesterase inhibitors (medications that help improve muscle strength), you will continue taking them at the same daily dose throughout the study.

Your condition will be regularly evaluated using several assessment tools. These include the MG-ADL scale, which measures how your condition affects your daily activities; the QMG scale, which measures muscle strength through specific tests; and the MGC scale, which is a combined assessment of your symptoms.

Your quality of life will be assessed using the MG-QoL15r questionnaire, which evaluates how your condition affects your overall wellbeing.

Blood samples will be taken to monitor your lymphocyte count (a type of white blood cell) and to measure the levels of medication in your blood if you are receiving cladribine.

Your vital signs, such as blood pressure and heart rate, will be monitored regularly.

Any side effects or health changes you experience will be recorded and evaluated by your doctor.

You will receive the Shingrix vaccine, which protects against shingles (herpes zoster). This vaccine is given as an intramuscular injection, meaning it is injected into a muscle, typically in your upper arm.

The vaccine comes as a powder and liquid that are mixed together before injection.

At week 24, your response to treatment will be assessed. Your doctor will compare your current symptoms and test results to those recorded at the beginning of the study.

A responder is defined as someone who shows meaningful improvement in their assessment scores compared to the starting point.

After the initial 24-week period, the study continues with additional treatment periods.

If you initially received the full dose of cladribine and your symptoms improve, the time until you might need another course of treatment (called retreatment) will be monitored.

If your symptoms do not improve sufficiently or worsen, you may receive rescue treatment, which means additional or alternative medication to help manage your condition.

The total duration of your participation in the study extends beyond the initial 24 weeks, with the study estimated to continue until 2030.

Throughout the entire study, your safety will be closely monitored. Any adverse events (unwanted medical occurrences) will be recorded and classified by severity.

Special attention will be given to specific types of side effects that are of particular interest based on the known effects of cladribine.

Laboratory tests will continue to monitor your blood cell counts and other important health markers.

The study team will track all health changes to ensure your safety throughout your participation.