The purpose of this study is to assess the safety and tolerability of CPTX2309 in healthy adult participants and adult participants with moderate to severe rheumatoid arthritis or systemic lupus erythematosus.

A first-in-human Phase 1, open label study to evaluate safety and tolerability of a single ascending dose (SAD) and multiple ascending dose (MAD) of CPTX2309 intravenously administered to healthy adult participants and adult participants with moderate to severe rheumatoid arthritis or systemic lupus erythematosus.

Inclusion Criteria:

• Male and female participants who are healthy as determined by the investigator based on review of medical history, physical examination, and clinical laboratory tests obtained during the screening period.
• Participant is willing and able to adhere to the study visit schedule and other protocol requirements.

RA Only:

• Clinical diagnosis of RA and fulfilling the 2010 ACR/EULAR classification criteria for RA
• Presence of rheumatoid factor or ACPA above the ULN
• Confirmation of at least moderate active disease at screening with the presence of at least 6 swollen and 6 tender joints at screening using the 68 (tender)/66 (swollen) joint count OR the presence of at least 3 joints with active synovitis via MRI assessment.

SLE Only:

• Clinical diagnosis of SLE and fulfilling the 2019 EULAR/ACR classification criteria for SLE
• Positive ANA>=1:80 and the presence of at least one of the following autoantibodies above the upper limit of normal (ULN): anti-double standard DNA (dsDNA) or anti-Smith (Sm).

Exclusion Criteria:

• Evidence of organ dysfunction or any clinically significant deviation from normal in physical examination, vital signs, or clinical laboratory tests beyond what is consistent with a healthy population in the region in which the study is conducted.
• Use of any investigational medical device or investigational drug within 30 days or 5 half-lives of the investigational drug (whichever is longer) prior to the first administration of CPTX2309.

RA Only:

- Participants diagnosed with Felty's syndrome

SLE Only:

• Active neuropsychiatric SLE, as defined by the CNS portion of SLEDAI at Screening, or signs of symptoms of neuropsychiatric SLE within 6 months prior to Screening (lupus headache permissible)
• Note: Other protocol-defined inclusion/exclusion criteria apply