Topical effects of N-acetyl-L-hydroxyproline on ceramide synthesis and alleviation of pruritus

Erika Hashizume,¹ Tetsuo Nakano,² Ayako Kamimura,¹ Koji Morishita<sup>3

1</sup>Healthcare Products Development Center, Kyowa Hakko Bio, Tsukuba, Ibaraki, ²Technical Research Laboratories, Kyowa Hakko Bio, Hofu, Yamaguchi, ³Technology Development and Research Department, Kyowa Hakko Bio, Tokyo, Japan

Purpose: N-acetyl-l-hydroxyproline (AHYP) is an acetylated form of l-hydroxyproline that is used to treat skin ulcers and porphyria cutanea tarda. Its other biological and physiological effects on the skin have not been elucidated. We investigated the effects of AHYP on the skin-barrier function, focusing on ceramide synthesis and the effects of topical AHYP on atopic dermatitis.
Materials and methods: AHYP was applied to a three-dimensional cultured skin model. Ceramides were quantified by high-performance thin-layer chromatography. Serine palmitoyltransferase (SPT) is the rate-limiting enzyme in de novo ceramide synthesis, and the mRNA of its long-chain base subunit 1 (SPTLC1) was evaluated by quantitative reverse-transcription polymerase chain reaction. A clinical trial in the form of an intraindividual, comparative, double-blind, randomized, vehicle-controlled test involving 15 female subjects suffering from slight atopic dermatitis was performed. Subjects applied 1% (w/w) AHYP cream to one forearm and a control cream to the other forearm twice daily for 4 weeks. Skin condition was evaluated by measuring transepidermal water loss (TEWL). Dermatological observations were made by a dermatologist, and subjects evaluated their own pruritus intensity before beginning treatment and 4 weeks after the start of treatment.
Results: SPTLC1 expression and ceramide synthesis were significantly increased in an AHYP-treated skin model (P < 0.05). In the clinical trial, no adverse effects were observed in any subjects. TEWL was increased in the control-treated region of the forearm (P < 0.05) after 4 weeks' application, whereas there was no change in the AHYP-treated region of the forearm. Pruritus intensity declined in the AHYP-treated forearms between 0 and 4 weeks (P < 0.05), but there was no change in the control-treated forearms.
Conclusion: AHYP increased ceramide synthesis by upregulating SPTLC1 in a three-dimensional cultured skin model, and it prevented TEWL increase and alleviated pruritus in human subjects with slight atopic dermatitis.

Keywords: skin barrier, ceramide, pruritus, N-acetyl-l-hydroxyproline, amino acids