RA-FLS express KCa1.1 (BK, Slo1, MaxiK, KCNMA1) as their major plasma membrane potassium channel. synoviocytes (FLS) in rheumatoid arthritis (RA-FLS) contribute to joint inflammation and damage characteristic of the disease. 8 Center for Drug Discovery, Baylor College of Medicine, Houston, TX, 77030, USA. 7 Biology of Inflammation Center, Baylor College of Medicine, Houston, TX, 77030, USA. 6 Department of Molecular Physiology and Biophysics, Mail Stop BCM335, Room S409A, Baylor College of Medicine, Houston, TX, 77030, USA.5 Division of Rheumatology, Icahn School of Medicine at Mount Sinai, New York, NY, 10029, USA.4 Graduate Program in Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, TX, 77030, USA.3 Interdepartmental Graduate Program in Translational Biology and Molecular Medicine, Baylor College of Medicine, Houston, TX, 77030, USA.2 Department of Biophysics and Cell Biology, Faculty of Medicine, University of Debrecen, Debrecen, 4032, Hungary.1 Department of Molecular Physiology and Biophysics, Mail Stop BCM335, Room S409A, Baylor College of Medicine, Houston, TX, 77030, USA.Overall, these studies emphasize the efficacy of targeting FLS in reducing disease severity and suggest selective KCa1.1 inhibitors as potential therapeutics in the treatment of RA. The use of the selective peptide KCa1.1 inhibitor IbTX provides for a novel targeted pharmacological approach to inhibit KCa1.1 expressed on FLS while minimizing side effects. Ex vivo analysis of RA-FLS demonstrated that KCa1.1 inhibition alters integrin expression and activation through modulation of calcium homeostasis and Akt phosphorylation, resulting in decreased invasiveness.Ĭonclusion: KCa1.1 is an attractive therapeutic target for the treatment of RA by inhibiting the invasive phenotype of FLS through modulating integrin expression. Side effects, including tremors and incontinence, were significantly reduced in IbTX-treated rats compared to those treated with paxilline. X-rays and histological analysis of joints from each treatment group indicated that the KCa1.1 blocker-treated rats had less bone and cartilage damage and reduced synovial hyperplasia, fibrosis, and immune infiltrates compared to vehicle-treated animals. Results: Both paxilline and IbTX significantly reduced clinical signs of disease in PIA by approximately 55% (p<0.001) and 65% (p<0.001), respectively, as determined by a standard scoring system of paw inflammation. Flow cytometry and ex vivo functional assays were used to assess the effects of KCa1.1 inhibition on the expression and activation of signaling molecules involved with FLS invasion. Side effects, including incontinence and tremors, were determined in healthy rats given a single treatment of either IbTX, paxilline, or vehicle. After three weeks of treatment, X-rays and histology were completed on paws of rats from each treatment group. Disease severity was measured daily using a standard scoring system. Methods: Starting at disease onset, rats with the pristane-induced arthritis (PIA) model of RA were given either vehicle, paxilline, or IbTX. Here, we investigated the efficacy of the peptide KCa1.1 inhibitor iberiotoxin (IbTX), which has a limited biodistribution, in reducing disease severity in an animal model of RA, assessed IbTX’s side effects, and determined the mechanism by which KCa1.1 regulates FLS invasiveness. However, KCa1.1 is expressed in a variety of tissues and systemic KCa1.1 block induces side effects that preclude paxilline’s use as a potential therapeutic in humans. Selectively inhibiting KCa1.1 with the small molecule paxilline reduces the in vitro invasiveness of FLS and reduces disease severity in multiple rat models of RA. We have previously found that FLS from RA patients and from rats with a model of RA express higher levels of the KCa1.1 potassium channel at their plasma membrane than FLS from patients with osteoarthritis or from healthy rats. There are currently no therapeutics that specifically target the pathogenic phenotype of FLS. Background/Purpose: Fibroblast-like synoviocytes (FLS) develop a high degree of invasiveness during rheumatoid arthritis (RA), leading to joint degradation.