The purpose of this clinical trial is to evaluate the efficacy and safety of mexiletine hydrochloride in patients with spinal and bulbar muscular atrophy.

The main questions it aims to answer are:

Does mexiletine hydrochloride improve the ALSFRS-R score in spinal and bulbar muscular atrophy patients?

Participants will:

Take mexiletine hydrochloride or a placebo every day for 3 months Visit the hospital once every 4 weeks for evaluations.

Inclusion Criteria:

1. Male patients with a CAG repeat count of 38 or more for the androgen receptor gene in genetic testing and a confirmed diagnosis of SBMA
2. Patients with muscle weakness (limb weakness and atrophy, or bulbar palsy) due to lower motor neuron lesion
3. Patients with a total ALSFRS-R score of ≥ 24 and ≤ 42 at screening
4. Patients who are at least 18 years old and less than 80 years old at the time of consent
5. Patients who give their voluntary written consent after having received adequate information on this study (However, if the patient is unable to sign the consent form due to the condition of the disease, a person equivalent to a regal representative must be present to provide written explanation, the prospective candidate must verbally consent to participate in the study, and a person equivalent to a regal representative must sign the consent form on behalf of the patient. The person who is to be the regal representative may sign the document on his/her behalf, noting the circumstances and his/her relationship to the subject.)

Exclusion Criteria:

1. Patients who have participated or are participating in a clinical trial within 12 weeks prior to enrollment
2. Patients with a history of hypersensitivity to any component of this drug product
3. Patients with a conduction disturbance (such as second- or third-degree atrioventricular block without a pacemaker, or left bundle branch block)
4. Patients with Brugada-type ECG
5. Patients with severe heart failure or heart disease (myocardial infarction, valvular disease, cardiomyopathy, etc.)
6. Patients with sinus bradycardia (<50 beats/minute)
7. Patients with systolic blood pressure of 90 mmHg or less
8. Patients with serum potassium level less than 3.5 mmol/L
9. Patients on antiarrhythmic drugs
10. Patients on antiepileptic drugs that affect to sodium channels
11. Patients on theophylline
12. Patients on narcotics
13. Patients who used Mexiletine within 1 month prior to enrollment or used Mexiletine for expectations of improvement in symptoms of SBMA
14. Patients with serious complications
15. Patients who cannot agree to use contraception during the study period
16. Other Patients deemed inappropriate by the investigator or subinvestigator

If the principal investigator, clinical trial office, main stakeholder conclude that secondary use of individual data obtained in this clinical trial is beneficial for additional analysis, the secondary use of data excluding personal information will be acceptable after publication of results.