The purpose of this study is to determine how safe and effective IMO-8400 is in adults with dermatomyositis.

This study will evaluate the safety and efficacy of IMO-8400 in adults with active dermatomyositis (DM).

Inclusion Criteria:

• Has definite or probable DM based on the criteria of Bohan and Peter
• Has a Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI)-Activity score ≥15
• Patients with muscle weakness are eligible; however having muscle weakness is not mandatory.
• Study participants must have a diagnostic evaluation for cancer if the diagnosis of DM was within 2 years prior to the Screening Visit

Exclusion Criteria:

• Has ongoing severe dysphagia (e.g., requires a feeding tube) for the 3 months prior to Screening

• Has known hypersensitivity to any oligodeoxynucleotide

• Has a history of drug or alcohol abuse within one year of screening, or evidence of drug abuse by urine drug screening

• Has body weight >140 kg

• Has a diagnosis of Juvenile DM, IBM, drug-induced toxic myopathy, metabolic myopathy, dystrophy, cancer-associated DM, or connective tissue disease-associated DM (e.g., overlap syndrome)

• Has received one or more of following prohibited treatments within the interval noted prior to Screening (Visit 1):

  1. Rituximab within 24 weeks (Note: patients who received rituximab are only eligible for inclusion if B-cell counts are confirmed to be within normal limits)
  2. Intravenous corticosteroids within 12 weeks
  3. Antimalarials (e.g., hydroxychloroquine) within 36 weeks
  4. Topical corticosteroids (excluding scalp) within 2 weeks

• Has evidence of or has required treatment for cancer (except for treated, non-invasive carcinoma of the skin or cured cervical carcinoma-in-situ) within 5 years

• Has interstitial lung disease requiring the use of supplemental oxygen