The purpose of the study is to understand how the study medicine PF-06823859 (dazukibart) works in people with idiopathic inflammatory myopathies (DM and PM). These disorders cause inflammation that weakens the muscles that are important for movement and may also cause skin rash in people with DM.
This study is seeking participants who:
* Are 18 years of age or older or minimum legal adult age as defined per local regulation, whichever is greater * Have active DM or active PM. * Are receiving a stable dose of 1 corticosteroid taken by mouth and/or 1 traditional immunosuppressant.
* Note: Corticosteroids and immunosuppressants are medicines that help reduce inflammation and may signal to the immune system not to attack the body.
Dermatomyositis (DM) is a rare disease that causes muscle inflammation that results in muscle weakness and low muscle stamina. Patients with DM have a characteristic skin rash. Polymyositis (PM) is a rare disease that involves mainly muscle inflammation resulting in muscle weakness, that can sometimes be painful. Patients with DM and PM may have trouble going up the steps, walking or getting to a standing position.
Some of the participants will receive the study medicine (dazukibart) and some will receive placebo (which is similar to study medicine but contains no medicine in it).
The study medicine or placebo will be given as an intravenous (IV) infusion (directly into the veins), which takes about 1 hour; every 4 weeks from Day 1 to Week 48 of the study. Both dazukibart and placebo and will be given at the study site.
The study will compare the experiences of people receiving study medication to those of the people who do not. This will help to see if dazukibart is safe and effective.
Participants will take part in this study for about 13 months. During this time, participants will have 15 study visits. These visits will be performed at the study site.
Male or female adults (≥18 years old or minimum legal adult age as defined per local regulation, whichever is greater)
Active dermatomyositis (DM) or polymyositis (PM) with age of onset
18 years old.
Must be receiving a stable dose of standard of care (SOC) background medications at the time of enrollment.
Myositis due to non-Idiopathic inflammatory myopathies (non-IIM)
Existing diagnosis of inclusion body myositis (IBM)
Presence of immune-mediated necrotizing myositis (IMNM)
Myositis with end-stage organ involvement
Parallel
PF-06823859
Druganti-interferon beta therapy
Placebo
DrugPlacebo for PF-06823859
Moderate change in Total Improvement Score (TIS)
Total Improvement Score 0 to 100 with higher scores indicating a better outcome.
Change from baseline in Manual Muscle Testing - 8 designated muscles (MMT-8)
Manual Muscle Testing (8 designated muscles) 0 to 150 with higher scores indicating a better outcome
Change from baseline in Cutaneous Dermatomyositis Disease Area and Severity Index Activity Score (CDASI-A) in participants with dermatomyositis (DM)
Cutaneous Dermatomyositis Disease Area and Severity Index Activity Score 0 to 100 with higher scores indicating a worse outcome. Only participants with baseline CDASI-A score ≥14 will be assessed.
Change from baseline in Investigator Global Assessment severity scale (IGA) in participants with dermatomyositis
Investigator Global Assessment severity scale 0 to 4 with higher scores indicating a worse outcome. Only participants with baseline IGA ≥2 will be assessed
Corticosteroid (CS) dose assessment
Normalized Area Under the Curve (AUC) of corticosteroid dose
