MK-1045 for Lupus or Rheumatoid Arthritis

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