THU0353 AN ORALLY AVAILABLE HIGHLY SELECTIVE 5-HYDROXYTRYPTAMINE 2B (5-HT2B) RECEPTOR ANTAGONIST AMELIORATING PULMONARY AND DERMAL FIBROSIS IN PRECLINICAL MODELS OF SYSTEMIC SCLEROSIS

Background: Serotonin or 5-hydroxytryptamine (5-HT) is well known as a stimulator of tissue fibrosis and a significant role of peripheral 5-HT2B receptors in fibrosis has been suggested with the receptor being upregulated in fibrotic tissues. In addition, agonism of the 5-HT2B receptor has been implicated in human tissue fibrosis caused by drugs known to activate the receptor. Pharmacologic inhibition of 5-HT2B receptor signalling consequently represents a promising treatment strategy for fibrotic disorders including systemic sclerosis. 5-HT is released from platelets activated upon vascular damage, one of the first pathological events in systemic sclerosis. The local 5-HT concentration is increased and leads to activation of 5-HT2B receptors on e.g. fibroblasts. The pro-fibrotic effects of 5-HT and the 5-HT2B receptor are believed to be mediated through activation of the TGF-β/Smad signaling pathway. Objectives: The objective of the present study was to evaluate a novel highly selective orally available 5-HT2B receptor antagonist, AM1476, for its ability to reduce pulmonary and dermal fibrosis in the sclerodermatous chronic graft-versus-host disease model and dermal fibrosis in the tight-skin-1 model of systemic sclerosis. Effects on the TGF-β/Smad signaling pathway were investigated in vivo. Exposure of AM1476 was measured to ensure proper target engagement. Methods: The murine sclerodermatous chronic graft-versus-host disease (cGvHD) model was used to evaluate anti-fibrotic effects after therapeutic dosing of the 5-HT2B receptor antagonist, AM1476. The compound was orally administered at 1, 10 and 30 mg/kg b.i.d. from day 21, several days after the first clinical signs of cGvHD, to day 49. Dermal thickness, myofibroblast counts and collagen production were used to evaluate dermal fibrosis. Effects on pulmonary fibrosis were measured using hydroxyproline content, Sirius Red staining and Ashcroft score. Plasma was collected for PK analysis at different timepoints in each treatment group using sparse sampling, on the last day of the experiment. The tight-skin-1 model was used to evaluate anti-fibrotic effects after therapeutic treatment. AM1476 was orally administered at 10 mg/kg, b.i.d. from week 5 to week 10. Hypodermal thickening, myofibroblast counts and hydroxyproline content in skin biopsies were evaluated at the end of the treatment period. The number of phosphorylated Smad3 (pSmad3) positive cells was used to evaluate inhibition of TGF-β signaling. Results: The 5-HT2B receptor antagonist AM1476, significantly reduced all measured dermal and pulmonary fibrosis readouts in the cGvHD model using an oral therapeutic treatment approach. Pharmacokinetic analysis of plasma samples supported 5-HT2B receptor engagement. Therapeutic treatment of dermal fibrosis in the tight-skin model effectively and significantly reduced hypodermal thickening, number of myofibroblast and hydroxyproline content. The number of pSmad3 positive cells was significantly reduced in skin samples isolated from treated animals. Conclusion: Inhibition of 5-HT2B receptor activity resulted in pronounced anti-fibrotic effects in pulmonary and dermal fibrotic tissues. AM1476 was well tolerated without obvious signs of toxicity. Effects on pSmad3, reflecting TGF-β-inhibition, will be evaluated as a potential biomarker in upcoming clinical trials. The highly selective 5-HT2B receptor antagonist AM1476 represents a promising drug candidate for treatment of fibrotic conditions and is currently in development for systemic sclerosis. Disclosure of Interests: Christina Wenglen Employee of: Employed by AnaMar AB, Lund, Sweden, Helena Arozenius Employee of: AnaMar AB, Lund, Sweden, Lars Pettersson Employee of: AnaMar AB, Lund, Sweden, Gunilla Ekstrom Employee of: AnaMar AB, Lund, Sweden DOI: 10.1136/annrheumdis-2019-eular.6808