Background Microvascular injury is one of the first pathological events in systemic sclerosis, and precedes the fibrosis. A consequence of vascular damage is the exposure of subendothelial connective tissue that causes activation of platelets and local serotonin (5-hydroxytryptamine, 5-HT) release. Binding of 5-HT to 5-HT_2B receptors on e.g. fibroblasts results in increased myofibroblast differentiation and release of excessive amounts of matrix proteins subsequently leading to fibrosis. Thus, pharmacologic inhibition of 5-HT_2B receptor signalling may represent a new treatment opportunity for systemic sclerosis.

Objectives The objective of the present study was to evaluate a novel highly selective 5-HT_2B receptor antagonist for its ability to reduce the production of matrix proteins in human dermal fibroblasts and to ameliorate fibrosis in the tight-skin-1 model of systemic sclerosis.

Methods Dermal fibroblasts isolated from patients with systemic sclerosis were cultured with different concentrations of the 5-HT_2B receptor antagonist AM1125 with or without 1 μM 5-HT. Anti-fibrotic effects were evaluated by measuring matrix production, myofibroblast differentiation and TGF-β production. The tight-skin-1 model was used to evaluate anti-fibrotic effects in vivo using a therapeutic treatment approach. AM1125 was administered at 10 and 50 mg/kg orally, b.i.d. from week 5 to week 10. Hypodermal thickening, myofibroblast counts and collagen production (hydroxyproline) were evaluated at the end of the treatment period.

Results In vitro the 5-HT_2B receptor antagonist AM1125 dose-dependently reduced TGF-β, PAI, nuclear SMAD2/3 and collagens in human dermal fibroblasts. In addition, stress fiber formation and α-SMA mRNA were reduced indicating decreased myofibroblast differentiation. Therapeutic treatment with AM1125 at 50 mg/kg reduced hypodermal thickness (p<0.001), myofibroblast counts (p<0.05) and hydroxyproline content (p<0.01) in the tight-skin-1 model. In addition, the lower dose (10 mg/kg) reduced hypodermal thickness (p<0.001).

Conclusions The results demonstrate that the 5-HT_2B receptor antagonist AM1125 prevents pro-fibrotic events in human dermal fibroblasts and attenuates dermal fibrosis using a therapeutic treatment approach in the tight-skin-1 model. Thus, 5-HT_2B receptor antagonists, exemplified by the novel compound AM1125, could represent a new treatment opportunity for systemic sclerosis.

Disclosure of Interest C. Wenglén Employee of: AnaMar, L. Pettersson Employee of: AnaMar, H. Arozenius Employee of: AnaMar, G. Ekström Employee of: AnaMar