Calcium Channel Blocker in Myotonic Dystrophy Type 1

Inclusion Criteria: * Male or Female between the ages 18 and 65, inclusive. * A genetically confirmed diagnosis of DM1, having a repeat expansion in the DMPK gene with at least 100 CTG repeats. * Grip strength less than 50% predicted based on age, height, and sex. * Video hand opening time is 4 sec or greater for at least one hand. * Ambulatory and able to walk 10 meters. * Willingness to discontinue anti-myotonia drugs at least 2 weeks prior to screening. * Highly effective method of contraception in women with childbearing potential. Exclusion Criteria: * Congenital DM1 as defined by symptom onset in the first 4 weeks of life. * Abnormal liver function tests (LFTs): alanine aminotransferase (ALT), or aspartate aminotransferase (AST) \>3 x upper limit of normal. Total bilirubin \> 1.5 mg/dL, or INR \> 1.3, or evidence of current active or chronic infection with hepatitis C, hepatitis B or other hepatobiliary conditions other than DM1 (or attributed to DM1) that cause abnormal liver laboratory parameters (e.g., hemochromatosis, Wilson's disease, autoimmune hepatitis) * Current or recent infection requiring antibiotic treatment within 2 weeks prior to screening. * Abnormal vital signs, including systolic blood pressure \< 90 mmHg and diastolic blood pressure \< 60 mmHg. * Treatment with concomitant medications with potential interactions with amlodipine, these may include but are not limited to sildenafil, cyclosporin, atorvastatin at high dosages (80 mg daily), simvastatin…