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

Time to reach TIS ≥ 20 (first "minimal clinical improvement")

The Total improvement score (TIS) assesses minimal, moderate, and major clinical response, and is assessed using the ACR/EULAR criteria. It is measured on a 0 to 100 scale. Higher scores represent improvement; zero indicates no improvement or worsening (from baseline).

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Percentage of participants with TIS ≥ 20

The Total improvement score (TIS) assesses minimal, moderate, and major clinical response, and is assessed using the ACR/EULAR criteria. It is measured on a 0 to 100 scale. Higher scores represent improvement; zero indicates no improvement or worsening (from baseline).

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Time to reach TIS ≥ 40 (first "moderate clinical improvement")

The Total improvement score (TIS) assesses minimal, moderate, and major clinical response, and is assessed using the ACR/EULAR criteria. It is measured on a 0 to 100 scale. Higher scores represent improvement; zero indicates no improvement or worsening (from baseline).

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Percentage of participants with TIS ≥ 40

The Total improvement score (TIS) assesses minimal, moderate, and major clinical response, and is assessed using the ACR/EULAR criteria. It is measured on a 0 to 100 scale. Higher scores represent improvement; zero indicates no improvement or worsening (from baseline).

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Change in MMT8 score

The manual muscle testing-8 (MMT8) is a physician assessment of muscle strength in a set of 8 designated muscles tested bilaterally and axially. The highest total potential MMT8 score is 150

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Change in Patient Global Assessment of Disease Activity (PGA)

The Patient Global Assessment of Disease Activity (PGA) is a tool that measures a patient's global evaluation of their overall disease activity at the time of assessment using a 10-cm VAS. The participant rates their overall disease activity by drawing a vertical mark on a 10-cm VAS from the left end of the line (no evidence of disease activity) to the right end of the line (extremely active or severe disease activity).

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Change in Physician Global Assessment of Disease Activity (MDGA)

The Physician Global Assessment of Disease Activity (MDGA) is a tool that measures the physician's global evaluation of the participant's overall disease activity, defined as potentially reversible pathology or physiology resulting from IIM. The physician rates disease activity on the MDGA using a 10-cm VAS. Overall disease activity is rated by drawing a vertical mark on a 10-cm VAS from the left end of the line (no evidence of disease activity), midpoint of the line (moderate disease activity), and the right end of the line (extremely active or severe disease activity).

Time frame: phase 2: up to 24 weeks; phase 3: up to 52 weeks

Proportion of participants who have at least moderate improvement (≥40) in TIS at week 52 and adhere to an oral prednisone dosage of ≤5 mg/day (or equivalent) from week 44 onward

The Total improvement score (TIS) assesses minimal, moderate, and major clinical response, and is assessed using the ACR/EULAR criteria. It is measured on a 0 to 100 scale. Higher scores represent improvement; zero indicates no improvement or worsening (from baseline).

Time frame: Phase 3: up to 52 weeks

Change in CK abnormality grades at week 52

CK: creatine kinase

Time frame: Phase 3: up to 52 weeks

Change in CDASI activity score at week 52

The Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI) is a skin-specific outcome measure used to assess disease in patients with Dermatomyositis (DM). Disease in 15 different anatomical locations is rated using 3 activity measures (erythema, scale, erosion/ulceration) and 2 damage measures (poikiloderma, calcinosis). The resulting activity and damage scores range from 0 to 100 and 0 to 32, respectively. Higher scores indicate greater disease severity.

Time frame: Phase 3: up to 52 weeks