Background:Rheumatoid arthritis (RA) has been associated with increased cardiovascular (CV) risk and metabolic changes.Objectives:We wished to determine how the Janus kinase (JAK) inhibitor tofacitinib influences vascular pathophysiology and metabolites of the arginine and methionine-homocysteine pathways.Methods:Thirty RA patients with active disease were treated with either 5 mg bid or 10 mg bid tofacitinib and evaluated at baseline and after 6 and 12 months. We determined DAS28, CRP, IgM rheumatoid factor (RF) and anti-cyclic citrullinated peptide (aCCP) levels. We assessed brachial artery flow-mediated vasodilation (FMD), carotid intima-media thickness (IMT) and pulse-wave velocity (PWV) by ultrasound. We also determined plasma L-arginine, L-citrulline, L-ornithine, inducible nitric oxide synthase (iNOS), asymmetric (ADMA) and symmetric dimethylarginine (SDMA), L-N-monomethyl-arginine (L-NMMA), cysteine, homocysteine, and methionine levels.Results:Twenty-six patients completed the study. Tofacitinib treatment maintained FMD and PWV. Ten mg bid tofacitinib significantly increased L-arginine, L-ornithine, iNOS and methionine levels after 12 months. Tofacitinib transiently increased L-citrulline and L-NMMA and decreased homocysteine levels after 12 months. Based on L-citrulline, L-ornithine, ADMA and SDMA levels, L-arginine remained highly available for endothelial NO production. Multivariate analysis indicated variable correlations of L-arginine, L-citrulline, ADMA, L-NMMA, homocysteine and methionine with DAS28, CRP, ESR and RF but not with aCCP. Regarding vascular pathophysiology, only PWV and methionine correlated with each other after 12 months.Conclusion:Tofacitinib suppressed systemic inflammation in RA yielding stabilization of vascular function. It may exert CV protective effects in RA, at least in part, by shifting L-arginine metabolism to high arginine availability and decreasing homocysteine levels.Acknowledgements:This research was supported by the European Union and the State of Hungary and co-financed by the European Social Fund in the framework of TAMOP-4.2.4.A/2-11/1-2012-0001 ‘National Excellence Program ’(Z.S.); by the European Union grant GINOP-2.3.2-15-2016-00015 (Z.S.) and by the WI188341 investigator-initiated research (IIR) grant obtained from Pfizer US (Z.S.).Disclosure of Interests:Boglárka Soós: None declared, Attila Hamar: None declared, Anita Pusztai: None declared, Monika Czókolyová: None declared, Edit Végh: None declared, Szilvia Szamosi Speakers bureau: Roche, Zsófia Pethö: None declared, Katalin Gulyás: None declared, György Kerekes: None declared, Éva Szekanecz: None declared, Sándor Szántó Speakers bureau: Abbvie, MSD, Novartis, Consultant of: Abbvie, Novartis, Gabriella Szücs Speakers bureau: Actelion, Roche, Sager, Boehringer, Consultant of: Boehringer, Actelion, Sager, Uwe Christians: None declared, Jelena Klawitter: None declared, Tamas Seres: None declared, Zoltán Szekanecz Speakers bureau: Pfizer, Abbvie, Roche, Lilly, Novartis, Boehringer, Consultant of: Pfizer, Abbvie, Novartis, Grant/research support from: Pfizer