A Study to Investigate Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of AZD5492 in Adult Participants With Systemic Lupus Erythematosus or Idiopathic Inflammatory Myopathies or Rheumatoid Arthritis.

To be considered for this study, you must be an adult between 18 and 70 years old.

If you have Systemic Lupus Erythematosus (lupus), you need to have a confirmed diagnosis and your lupus should be active and moderately to severely affecting you. Also, your current treatments shouldn't be working well enough or you might have struggled with side effects from at least three different past treatments.

If you have Idiopathic Inflammatory Myopathies (IIM), you must have a 'probable' or 'definite' diagnosis of certain types of IIM, but not others like inclusion body myositis. Your muscle inflammation needs to be active and you should have tried standard treatments, including at least one biologic medicine, immunoglobulin, or cyclophosphamide, without good results or with side effects. Similarly, if you have Rheumatoid Arthritis, you'll need to meet specific criteria for diagnosis and active disease.

If you decide to take part, you'll first have a screening period that could last up to 42 days. During this time, doctors will check if the study is right for you. Depending on which part of the study you're in, you will receive the study medication, AZD5492, either once or twice (with the two doses given 7 days apart).

Your participation will last at least 180 days (about 6 months) after you receive the dose. You will have a number of study visits to the clinic to monitor your health. These visits will be more frequent at the beginning (for example, daily for a few days, then weekly), and then they will become less frequent. For example, in Part 1, visits are on Days 1 to 4, then Day 8, 15, 22, 30, 60, 90, 120, 150, and 180. Some participants might need to have follow-up visits for up to 12 months after the study starts. You'll have regular checks, blood tests, and other assessments to see how you're doing and how the medicine is affecting you.