Overview

This study looks at a study medicine called MK-1045 in people with lupus and rheumatoid arthritis (RA). The main goal of the study is to learn about the safety of MK-1045 and if people tolerate it when they receive it at different dose levels (amounts).

Eligibility

Inclusion Criteria:

• Has a body mass index between 18 and 32 kg/m\^2, inclusive
• Systemic lupus erythematosus (SLE): Has a diagnosis of SLE for at least 6 months and met the European Alliance of Associations for Rheumatology (EULAR)/ American College of Rheumatology (ACR) 2019 classification criteria
• SLE: Is taking at least one background therapy for SLE
• RA: Has a diagnosis of RA for at least 6 months and meets the 2010 ACR-EULAR classification criteria for RA

Exclusion Criteria:

• Has a known active infection (excluding fungal infection of nail beds), or any major episode of infection requiring hospitalization or treatment with anti-infectives within 8 weeks prior to the Day 1 dosing
• History of serious recurrent or chronic infection
• Is known to be infected with hepatitis B virus, hepatitis C virus, or human immunodeficiency virus
• Has evidence of active tuberculosis (TB), latent TB, or inadequately treated TB
• Has a significant or uncontrolled medical disease in any organ system not related to RA or SLE
• For RA participants, has a history of any arthritis with onset before age 17 years
• Has a current inflammatory condition other than SLE or RA that could interfere with disease activity assessments
• History of cancer (except fully treated nonmelanoma skin cancers or cervical carcinoma in situ after complete surgical removal) and is disease free for \<5 years before Day 1 dosing
• Has had a major surgery within 3 months prior to Screening or has a major surgery planned during the study.
• Has symptomatic heart failure (New York Heart Association class III or IV) or myocardial infarction or unstable angina pectoris within 6 months prior to Screening
• Has a severe chronic pulmonary disease requiring oxygen therapy
• Has current active lymphoproliferative disease, including lymphoma, or signs and symptoms suggestive of possible lymphoproliferative disease