Study Evaluating Efficacy and Safety of Froniglutide (PF1801) in Patients With Idiopathic Inflammatory Myopathy

Key Inclusion Criteria: * Diagnosis of IIM (DM or polymyositis \[PM\]) as per Bohan and Peter classification criteria * MMT-8 ≤125 units and two out of the following CSM items, or MMT-8 \>125 units and three out of the following CSM items, together with verifiable muscular weakness A. PGA VAS ≥2 cm B. SGA VAS ≥2 cm C. HAQ-DI ≥0.25 D. Extramuscular activity (MDAAT) ≥2 cm E. Increase in one or more muscle enzyme (CK, LDH, AST, ALT, aldolase) values (must be ≥1.3 × ULN) * On treatment with standard of care (immunosuppressants and/or corticosteroids) for \>12 weeks and on stable therapy for at least 4 weeks Key Exclusion Criteria: * Inclusion body myositis (IBM) or amyopathic DM * Severe muscle damage (myositis damage index \[MDI\] \>7/10 cm). Permanent deterioration caused by reasons other than PM/DM, or myositis with cardiac involvement * Clinically significant renal/hepatic impairment * Severe interstitial lung disease requiring supportive oxygen therapy