A Study to Investigate the Efficacy and Safety of Efgartigimod PH20 SC in Adult Participants With Active Idiopathic Inflammatory Myopathy.

Inclusion Criteria:

• Ability to consent in the jurisdiction in which the study is taking place andcapable of giving signed informed consent.

• A definite or probable clinical diagnosis of idiopathic inflammatory myopathy (IIM)

• One of the following medical histories: Diagnosis of dermatomyositis (DM) orjuvenile dermatomyositis (JDM), Diagnosis of polymyositis (PM) (includingantisynthetase syndrome (ASyS)), Diagnosis of immune-mediated necrotizing myopathy (IMNM)

• Diagnosed with active disease as defined by the presence of at least 1 of thefollowing criteria: Abnormal levels of at least 1 of the following enzymes: creatinekinase (CK), aldolase, lactate dehydrogenase, aspartate aminotransaminase (AST),alanine aminotransferase (ALT), based on central laboratory results;Electromyography demonstrating active disease within the past 3 months; Activedermatomyositis (DM) skin rash; Muscle biopsy indicative of active idiopathicinflammatory myopathy (IIM) in the past 3 months; Magnetic resonance imaging withinthe past 3 months indicative of active inflammation

• Muscle weakness

• Receiving a permitted background treatment for idiopathic inflammatory myopathy.

• Contraceptive use consistent with local regulations, where available, forindividuals participating in clinical studies. Women of childbearing potential musthave a negative serum pregnancy test during screening and a negative urine pregnancytest at baseline before receiving investigational medicinal product (IMP).

The full list of inclusion criteria can be found in the protocol.

Exclusion Criteria:

• A clinically significant active infection at screening

• A COVID-19 polymerase chain reaction (PCR)-positive test before enrollment

• Any other known autoimmune disease that, in the investigator's opinion, wouldinterfere with an accurate assessment of clinical symptoms of idiopathicinflammatory myopathy (IIM) or put the patient at undue risk

• A history of malignancy unless considered cured by adequate treatment, with noevidence of recurrence for ≥ 3 years before the first administration of theinvestigational medicinal product (IMP). Adequately treated participants with thefollowing cancers can be included at any time: Basal cell or squamous cell skincancer ; Carcinoma in situ of the cervix; Carcinoma in situ of the breast;Incidental histological finding of prostate cancer

• Severe muscle damage

• Glucocorticoid-induced myopathy that the investigator considers the primary cause ofmuscle weakness or permanent weakness linked to a non-idiopathic inflammatorymyopathy (IIM) cause

• Juvenile myositis (JDM) diagnosed > 5 years from screening or juvenile myositis withextensive calcinosis or severe calcinosis.

• Uncontrolled interstitial lung disease or any other uncontrolled idiopathicinflammatory myopathy (IIM) manifestation that, in the opinion of the investigator,would be likely to require treatment with prohibited medication during the study

• Other inflammatory and noninflammatory myopathies: inclusion body myositis, overlapmyositis), metabolic myopathies, muscle dystrophies or a family history of muscledystrophy, drug-induced or endocrine induced myositis, and juvenile myositis (otherthan juvenile dermatomyositis (JDM))

• Clinically significant disease, recent major surgery or intends to have surgeryduring the study, or has any other condition in the opinion of the investigator thatcould confound the results of the trial or put the patient at undue risk

• Known hypersensitivity reaction to investigational medicinal product (IMP) or 1 ofits excipients

• Received a live or live-attenuated vaccine less than 4 weeks before screening.

• Positive serum test at screening for active viral infection with any of thefollowing conditions: Hepatitis B virus (HBV); Hepatitis C virus (HCV); HIV

• Participant has previously participated in an efgartigimod clinical trial andreceived at least 1 dose of investigational medicinal product (IMP).

• Participant is concurrently participating in any other clinical study, including anoninterventional study.

• Participant has a current or history (ie, within 12 months of screening) of alcohol,drug, or medication abuse.

• Participant is pregnant or lactating or intends to become pregnant during the study.

• Participant has severe renal impairment .

• Participant is institutionalized by a court or other governmental order or is in adependent relationship with the sponsor or investigator.

The full list of exclusion criteria can be found in the protocol.