This is a Phase 3 multicenter, double-blind, randomized, placebo-controlled study assessing the efficacy and safety of lenabasum for the treatment of dermatomyositis. Approximately 150 subjects will be enrolled in this study at about 60 sites in North America, Europe, and Asia. The planned duration of double-blind treatment with study drug is up to 52 weeks.

Subjects will be randomized to receive lenabasum 20 mg twice per day, lenabasum 5 mg twice per day, or placebo twice per day in a 2:1:2 ratio. The primary efficacy outcome at Week 28 will compare lenabasum 20 mg BID to placebo the Total Improvement Score (TIS), which is a weighted composite measure of improvement from baseline in six endpoints: Physician Global Assessment of Disease Activity, Physician Assessment of Extramuscular Disease Activity, Patient Global Assessment of Disease Activity, Health Assessment Questionnaire (patient-reported disability), Manual Muscle Testing (MMT), and muscle enzymes.

Inclusion Criteria:

• Fulfill at least 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 at least 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