Inclusion Criteria: - Fulfill one of the following criteria for dermatomyositis: 1. Bohan and Peter criteria (Bohan and Peter, 1975a; Bohan and Peter 1975b) 2. ACR/EULAR criteria (Lundberg et al, 2017) - Disease activity/severity fulfills one of the following three criteria: 1. MDGA ≥ 3 cm (0 - 10 cm Visual Analog Scale [VAS]) and MMT-8 score ≤ 142 (out of 150 total possible) 2. Sum of MDGA, PtGA and EMGA VAS scores is ≥ 10 cm (0-10 cm VAS for each) 3. MDGA ≥ 3 cm (0-10 cm VAS) and CDASI activity score of > 14 - Stable doses of immunosuppressive medications for DM as defined by: 1. Unchanged dose of oral corticosteroids ≤ 20 mg per day prednisone or equivalent for ≥ 4 weeks before Visit 1 2. Unchanged dose of immunosuppressive medications other than oral corticosteroids for ≥ 8 weeks before Screening Exclusion Criteria: - Unstable DM or DM with end-stage organ involvement at Screening or Visit 1 - Significant diseases or conditions other than DM that may influence response to the study drug or safety - Any of the following values for laboratory tests at Screening: 1. A positive pregnancy test (or at Visit 1) 2. Hemoglobin < 9 g/dL in males and < 8 g/dL in females 3. Neutrophils < 1.0 × 10^9/L 4. Platelets < 75 × 10^9/L 5. Creatinine clearance < 50 mL/min on screening blood test, per the Modification of Diet in Renal Disease Study or in 24 hour urine creatine clearance measurement