A Phase 1/2 Study of PRO-203 in Healthy Volunteers and Participants With Systemic Sclerosis.

A two-part study of PRO-203 administered subcutaneously in healthy adult volunteers and participants with Systemic Sclerosis (SSc).

Key Inclusion Criteria (All Participants) * Is male or female, age 18 to 65 years, inclusive, at Screening. * Able to provide Informed Consent. * Absolute B cell count \> 25 cells/uL. Additional Inclusion for Part 1 (Closed to Enrollment) * In good general health, determined by no clinically significant findings in the of the investigator from medical history, physical examination, 12-lead ECG, clinical laboratory findings, and vital signs at Screening and Day -1 (participants with Gilbert's disease with associated abnormalities of liver function tests are eligible for enrollment). * Up to date vaccination status per local guidelines (including but not limited to influenza vaccine, COVID booster and hepatitis B vaccine). Additional Inclusion for Part 2 * Fulfill 2013 ACR/ EULAR criteria for classification of SSc with a total score of ≥ 9. * Active disease defined as at least two of the following at screening: * Disease duration ≤ 2 years (since onset of first-non-Raynaud-symptom), or * Elevated acute phase reactant levels (CRP ≥ 6 mg/L, erythrocytes sedimentation rate \[ESR\] ≥ 28 mm/1h, or platelet count ≥ 330,000/µL), or * Baseline mRSS ≥10 with evidence of progression, defined as mRSS increase at least 3 units, or involvement of 1 new body area and mRSS increase at least 2 units, or involvement of 2 new body areas (each within the previous 6 months), or * ≥ 1 tendon friction rub, or * Elevation of CK or aldolase \> 2 × the upper limit of normal (ULN) consistent with SSc-related myopathy, or * Progressive fibrosing interstitial lung disease (ILD) as defined by at least one of the following criteria at any time within the prior 2 years: 1. relative decline in forced vital capacity (FVC) % predicted ≥ 10%, or 2. relative decline in FVC % predicted ≥ 5% to \