Key Inclusion Criteria:
1. Adult men or women 18 and ≤ 75 years of age at the time of signing the informed consent (ICF).
2. A diagnosis of definite or probable myositis according to American College of Rheumatology/European League Against Rheumatism 2017 (ACR/EULAR 2017) criteria:
1. Population 1: DM
* Diagnosis of DM with DM rash current or historical, or
2. Population 2: ASIM
* Anti-histidyl tRNA synthetase-(Anti-Jo-1) antibodies must be positive during screening by central laboratory testing, or
* One of following antibodies must be positive by historical testing: directed against anti-alanyl- (anti-PL-12), anti-threonyl-(anti PL-7), anti-asparaginyl-(anti-KS), anti-glycyl-(anti-EJ), anti-isoleucyl-(anti-OJ), anti-phenylalanyl-transfer RNA synthetase-(anti-ZO), anti-tyrosil-YRS(HA).
3. Currently active myositis with all the following (a, b, and c) during screening:
1. Manual Muscle Testing (MMT 8) score \< 142
2. At least 2 other abnormal core set measures (CSM) from the following list:
* Patient global disease activity (PtGDA) ≥ 2 cm in a 10 cm visual analog scale (VAS)
* Physician's Global Disease Activity (PhGDA) ≥ 2 cm in a 10 cm VAS
* Extramuscular activity ≥ 2cm in a 10 cm VAS
* At least one muscle enzyme 1.5 times upper limit of normal (ULN)
* Health assessment questionnaire-disability index (HAQ-DI) ≥ 0.5
3. Global muscle damage score ≤ 5 on a 10 cm VAS on the myositis damage index (MDI).
4. Participants should be on stable standard of care therapy if tolerated; if they are not able to tolerate it or have failed standard of care, medications should have a washed out period.
5. Participants should be willing to taper corticosteroid dose per protocol when stable or improving.
Exclusion Criteria:
1. Any condition that, in the opinion of the investigator or sponsor, would interfere with the evaluation of investigational product (IP) or interpretation of participant safety or study results.
2. Weight \> 160 kg (352 pounds) at screening.
3. Breastfeeding or pregnant women or women who intend to become pregnant anytime from signing the ICF through 6 months after receiving the last dose of IP.
4. History of clinically meaningful cardiac disease including unstable angina, myocardial infarction, congestive heart failure within 6 months prior to randomization; arrhythmia requiring active therapy, except for clinically insignificant extra systoles, or minor conduction abnormalities; or presence of clinically meaningful abnormality on electrocardiogram (ECG) if, in the opinion of the Investigator, it would increase the risk of study participation.
5. History of cancer within the past 5 years except cutaneous basal cell or squamous cell carcinoma treated with curative therapy.
6. Any underlying condition that in the opinion of the Investigator significantly predisposes the participant to infection (eg. hepatitis C).
7. Known history of a primary immunodeficiency or an underlying condition, such as known human immunodeficiency virus (HIV) infection, or a positive result for HIV infection per central laboratory.
8. All participants will undergo testing for hepatitis B virus serology as defined in the protocol.
9. Active tuberculosis (TB), or a positive interferon gamma (IFN-γ) release assay (IGRA) test at screening, unless documented history of appropriate treatment for active or latent TB according to local guidelines.
10. Any severe herpes virus family infection (including Epstein-Barr virus, cytomegalovirus \[CMV\]) at any time prior to randomization.
11. Opportunistic infection requiring hospitalization or parenteral antimicrobial treatment within 2 years prior to randomization.
12. Significant organ system involvement or myositis damage (global muscle damage score \> 5 on a 10 cm VAS scale on the MDI) that poses risks in the study or impedes assessments.
13. Diagnosis of immune-mediated necrotizing myopathy (IMNM) \[(positive 3-hydroxy-3-methylglutaryl-coenzyme A reductase (anti-HMGR), anti-signal recognition particle (anti- SRP), or antibody negative)\], inclusion body myositis (IBM) (including positive anti-cytosolic 5'-nucleotidase 1A (anti cN1A), or drug-induced myositis.
14. Current musculoskeletal, joint, or inflammatory disease, including significant joint contractures or calcinosis that in the opinion of the investigator, could interfere with the muscle strength assessments and confound the disease activity assessments.
15. Wheelchair bound participants.
16. Current inflammatory skin disease other than DM or ASIM that, in the opinion of the investigator, could interfere with the inflammatory skin assessments or confound the disease activity assessments.
17. Severe interstitial lung disease where respiratory symptoms limit participant function or progressive pulmonary fibrosis.
18. Myositis in overlap with another connective tissue disease that precludes the accurate assessment of a treatment response (for example, difficulty in assessing muscle strength in a scleroderma patient with associated myositis).
