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Introduction: Transforming growth factor beta (TGFβ) cytokines (TGFβ1, TGFβ2, and TGFβ3) play critical roles in tissue fibrosis. However, treatment with systemic pan-TGFβ inhibitors have demonstrated unacceptable toxicities. In this study, we evaluated the safety, tolerability, pharmacokinetics, and pharmacodynamics of RO7303509, a high-affinity, TGFβ3-specific, humanized immunoglobulin G1 monoclonal antibody, in healthy adult volunteers (HVs). Methods: This phase 1a, randomized, double-blind trial included six cohorts for evaluation, with each cohort receiving single doses of placebo or RO7303509, administered intravenously (IV; 50 mg, 150 mg, 240 mg) or subcutaneously (SC; 240 mg, 675 mg, 1200 mg). The frequency and severity of adverse events (AEs) and RO7303509 serum concentrations were monitored throughout the study. We also measured serum periostin and cartilage oligomeric matrix protein (COMP) by immunoassay and developed a population pharmacokinetics model to characterize RO7303509 serum concentrations. Results: The study enrolled 49 HVs, with a median age of 39 (range 18–73) years. Ten (27.8%) RO7303509-treated subjects reported 24 AEs, and six (30.8%) placebo-treated subjects reported six AEs. The most frequent AEs related to the study drug were injection site reactions and infusion-related reactions. Maximum serum concentrations (Cmax) and area under the concentration–time curve from time 0 to infinity (AUC0–inf) values for RO7303509 appeared to increase dose-proportionally across all doses tested. Serum concentrations across cohorts were best characterized by a two-compartment model plus a depot compartment with first-order SC absorption kinetics. No subjects tested positive for anti-drug antibodies (ADAs) at baseline; one subject (2.8%; 50 mg IV) tested positive for ADAs at a single time point (day 15). No clear pharmacodynamic effects were observed for periostin or COMP upon TGFβ3 inhibition. Conclusion: RO7303509 was well tolerated at single SC doses up to 1200 mg in HVs with favorable pharmacokinetic data that appeared to increase dose-proportionally. TGFβ3-specific inhibition may be suitable for development as a chronic antifibrotic therapy. Trial Registration: ISRCTN13175485. [ABSTRACT FROM AUTHOR]