A Clinical Study of MK-1045 in People With Lupus or Rheumatoid Arthritis (MK-1045-004)

Who May Qualify: - 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 Who Should NOT Join This Trial: - 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 weakened immune system 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 Always talk to your doctor about whether this trial is right for you.