Although the pathogenesis of scleroderma is not fully understood, activation of connective‐tissue‐type mast cells (CTMCs) has been implicated in various fibrotic diseases.

Our previous study showed that the number of CTMCs was markedly increased during fibrous proliferation in the skin of a scleroderma model, namely tight‐skin (Tsk) mice. Because mast cells express numerous bioactive factors, such as cytokines, growth factors, proteases, and others, it is crucial to identify the primary factors that may be involved in the pathogenesis of scleroderma. Our previous study also showed that a CTMC‐specific protease, chymase‐4, was selectively upregulated in accordance with the development of skin fibrosis in Tsk mice.

To further elucidate the role of chymase secreted from CTMCs, we evaluated the therapeutic effects of a synthetic chymase‐specific inhibitor, SUN‐C8257, on the development of skin fibrosis in Tsk mice. SUN‐C8257 (50 mg kg⁻¹ day⁻¹) was administered via intraperitoneal injection in 13‐week‐old Tsk mice for a period of 2 weeks.

Treatment with SUN‐C8257 significantly reduced chymase activity by 43% and the chymase‐4 mRNA level by 47%, and also decreased the thickness of the subcutaneous fibrous layer of Tsk mice by 42% compared with that of Tsk mice injected with vehicle.

Furthermore, immunohistochemical analysis revealed that transforming growth factor (TGF)‐beta1 staining in the fibrous layer of Tsk skin was markedly reduced by the treatment with SUN‐C8257. This chymase inhibitor may prevent the chymase‐dependent pathway that activates the latent TGF‐beta1 in fibrous tissue, and may exhibit beneficial effects that inhibit the development of fibrosis.

In conclusion, our results strongly support the assumption that CTMC‐derived chymase may play a key role in the pathogenesis of scleroderma.