An orally available, highly selective 5-hydroxytryptamine 2B (5-HT2B) receptor antagonist ameliorating pulmonary and dermal fibrosis

Background: 5-hydroxytryptamine (5-HT) is well known as a stimulator of tissue fibrosis and a significant role of peripheral 5-HT2B receptors in human fibrosis has been suggested. The pro-fibrotic effects of the 5-HT2B receptor are believed to be mediated through activation of the TGF-β/Smad signaling pathway. Aims: To investigate effects of a selective 5-HT2B receptor antagonist on fibrosis development and the TGF-β/Smad signaling pathway. Methods: The murine sclerodermatous chronic graft versus host disease (scGvHD) model was used to evaluate anti-fibrotic effects of the 5-HT2B receptor antagonist, AM1476. Oral, twice daily treatment with AM1476 was applied day 21 to 49. Pulmonary fibrosis was evaluated using hydroxyproline content, Sirius Red staining and Ashcroft score. Dermal thickness, myofibroblast counts and collagen production were used to evaluate dermal fibrosis. The number of phosphorylated Smad3 (pSmad3) positive cells was used to evaluate inhibition of TGF-β signaling. Plasma was collected for exposure analysis. Results: AM1476, significantly reduced all measured pulmonary and dermal readouts in the scGvHD model using a therapeutic treatment approach. Plasma exposure supported target engagement. The number of pSmad3 positive cells was significantly reduced in treated animals. Conclusion: Inhibition of 5-HT2B receptor activity resulted in anti-fibrotic effects in pulmonary and dermal fibrotic tissues. Reduction of pSmad3, reflecting TGF-β-inhibition, was observed. 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.