Inclusion Criteria:
* Male and female patients ≥18 years of age at the time of consent.
* A body mass index (BMI) within the range of 15 - 38 kg/m2. BMI = Body weight (kg) / \[Height (m)\]2.
* Subjects diagnosed with a probable or definite DM according to 2017 European League Against Rheumatism/American College of Rheumatology (2017 EULAR/ACR).
* Subjects must have proof of vaccination against Neisseria meningitidis (MenACWY and MenB), Streptococcus pneumoniae (PCV13 or PCV15 and PPSV23), and Haemophilus influenzae type b (Hib) taken at least 2 weeks prior to NM8074 administration as per national and local guidelines. If the window of vaccination is short, then patients will be prophylactically treated with appropriate antibiotics.
* Female partners of child-bearing potential (WOCBP), defined as all women physiologically capable of becoming pregnant, must have a negative pregnancy test at screening and must agree to use highly effective methods of contraception during dosing and for at least 8 weeks after stopping the investigational drug.
* Male patients and partners of child-bearing potential must agree to use contraceptives, and male patients must agree to refrain from donating sperm for the duration of the study and for at least 8 weeks after stopping the investigational drug.
Exclusion Criteria:
* Subjects with drug-induced myositis
* Subjects who have interstitial lung disease requiring the use of supplemental oxygen.
* Use of other investigational drugs at the time of enrollment, or within 5 half- lives of enrollment or within 3 months to study day 1, whichever is longer.
* History of currently active or suspicion of active bacterial, viral, or fungal infection within 2 weeks prior to first dose, or history of unexplained, recurrent bacterial infections.
* Subjects currently or previously diagnosed with cancer or who finished their cancer treatment within 2 years of the start of the clinical trial.
* Subjects with the history of bone marrow, hematopoietic stem cells, or solid organ transplantation.
* Has a currently active or known history of meningococcal disease or N. meningitidis infection.
* Evidence of active malignant disease or malignancies diagnosed within the previous 5 year
* Clinically significant medical or psychological conditions or risk factors that, as per the Investigator's judgment, could hinder the patient's participation in the study, introduce additional risks for the patient, or complicate the evaluation of the patient or study outcomes.
* Pregnant, planning to become pregnant, or nursing female subjects.
* Females with a positive pregnancy test result at Screening or on Day 1.
