Janus kinase (JAK)–mediated cytokine signaling has emerged as an important therapeutic target for the treatment of inflammatory diseases such as rheumatoid arthritis (RA). Accordingly, JAK inhibitors compose a new class of drugs, among which tofacitinib and baricitinib have been approved for the treatment of RA. Periarticular bone erosions contribute considerably to the pathogenesis of RA. However, although the immunomodulatory aspect of JAK inhibition (JAKi) is well defined, the current knowledge of how JAKi influences bone homeostasis is limited. Here, we assessed the effects of the JAK inhibitors tofacitinib and baricitinib on bone phenotype (i) in mice during steady-state conditions or in mice with bone loss induced by (ii) estrogen-deficiency (ovariectomy) or (iii) inflammation (arthritis) to evaluate whether effects of JAKi on bone metabolism require noninflammatory/inflammatory challenge. In all three models, JAKi increased bone mass, consistent with reducing the ratio of receptor activator of NF-κB ligand/osteoprotegerin in serum. In vitro, effects of tofacitinib and baricitinib on osteoclast and osteoblast differentiation were analyzed. JAKi significantly increased osteoblast function (P < 0.05) but showed no direct effects on osteoclasts. Additionally, mRNA sequencing and ingenuity pathway analyses were performed in osteoblasts exposed to JAKi and revealed robust up-regulation of markers for osteoblast function, such as osteocalcin and Wnt signaling. The anabolic effect of JAKi was illustrated by the stabilization of β-catenin. In humans with RA, JAKi induced bone-anabolic effects as evidenced by repair of arthritic bone erosions. Results support that JAKi is a potent therapeutic tool for increasing osteoblast function and bone formation.

Several bone conditions, eg, bone cancer, osteoporosis, and rheumatoid arthritis (RA), are associated with a risk of developing persistent pain. Increased osteoclast activity is often the hallmark of these bony pathologies and not only leads to bone remodeling but is also a source of pronociceptive factors that sensitize the bone-innervating nociceptors. Although historically bone loss in RA has been believed to be a consequence of inflammation, both bone erosion and pain can occur years before the symptom onset. Here, we have addressed the disconnection between inflammation, pain, and bone erosion by using a combination of 2 monoclonal antibodies isolated from B cells of patients with RA. We have found that mice injected with B02/B09 monoclonal antibodies (mAbs) developed a long-lasting mechanical hypersensitivity that was accompanied by bone erosion in the absence of joint edema or synovitis. Intriguingly, we have noted a lack of analgesic effect of naproxen and a moderate elevation of few inflammatory factors in the ankle joints suggesting that B02/B09-induced pain-like behavior does not depend on inflammatory processes. By contrast, we found that inhibiting osteoclast activity and acid-sensing ion channel 3 signaling prevented the development of B02/B09-mediated mechanical hypersensitivity. Moreover, we have identified secretory phospholipase A2 and lysophosphatidylcholine 16:0 as critical components of B02/B09-induced pain-like behavior and shown that treatment with a secretory phospholipase A2 inhibitor reversed B02/B09-induced mechanical hypersensitivity and bone erosion. Taken together, our study suggests a potential link between bone erosion and pain in a state of subclinical inflammation and offers a step forward in understanding the mechanisms of bone pain in diseases such as RA.

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Career situation of first and presenting authorStudent for a master or a PhD.IntroductionMany cytokines relevant to rheumatoid arthritis (RA) rely on the janus kinase – signal transducer and activator of transcription (JAK-STAT) signaling pathway. The JAK-inhibitors tofacitinib and baricitinib, targeting JAK3/JAK1 and JAK1/JAK2 respectively, have been approved for treatment of RA1. While currently available therapies reduce inflammation, erosive damage in involved joints is still irreversible. However, preliminary data suggests an influence of JAK inhibition on local bone healing.ObjectivesTo study the role of JAK-inhibition in osteoblast and osteoclast-mediated bone homeostasis and its capacity to alleviate structural bone damage in vivo.MethodsFor steady state analysis C57BL/6 (WT) mice received tofacitinib QD by oral gavage for 6 weeks. WT mice of the ovariectomy-induced osteoporosis model (OVX) obtained tofacitinib BID by oral gavage for 6 weeks. For the serum-induced arthritis model (SIA) WT mice received tofacitinib or baricitinib BID by oral gavage for 14 days. Readout covered serum cytokine levels (ELISA), µCT and mRNA analysis of bone (qPCR) and clinical scoring. Murine osteoclasts (OC) were assessed for differentiation (TRAP staining) and resorption (Von Kossa staining). Murine mesenchymal stem cell (MSC)-derived and primary osteoblasts (OB) were analyzed for differentiation (qPCR) and function (Alizarin red staining).ResultsIn unchallenged WT mice, treatment with tofacitinib increased trabecular bone density in tibia and reduced RANKL/OPG ratio in serum. These results, along with elevated trabeculae numbers, also applied to spinal bone in tofacitinib-treated OVX mice. In SIA, baricitinib and tofacitinib ameliorated disease manifestation and inhibited trabecular/cortical bone loss. In vitro JAK-inhibition exhibited no effect on differentiation and function of OCs. However, it enhanced OCN expression in MSC-derived OBs at day 1 after osteoblastic induction and led to decreased Igf1 and increased Dkk1 expression at day 7. Accordingly, primary OBs showed reduced RANKL expression during JAK-inhibition. Moreover, both MSC- and primary OBs responded to JAK-inhibition with increased mineralization.ConclusionsOur findings indicate that JAK-inhibition by tofacitinib and baricitinib increases bone formation in vivo, in both steady-state and pathological conditions, presumably as a result of increased mineralization capacity by osteoblasts.ReferenceBaker KF, Isaacs JD. ARD 2018.AcknowledgementsWork funded by Pfizer and Eli Lilly.Disclosure of InterestS. Adam Grant/research support from: Pfizer, Eli Lilly, N. Simon: None declared, U. Steffen: None declared, F. Andes: None declared, D. Müller: None declared, S. Culemann: None declared, D. Andreev: None declared, M. Hahn: None declared, C. Scholtysek: None declared, G. Schett: None declared, G. Krönke: None declared, S. Frey: None declared, A. Hueber: None declared.