Targeting melanocortin receptor type 1 with small peptides

Neuroendocrine signaling in the skin with a special focus on the epidermal neuropeptides

The skin, which is comprised of the epidermis, dermis, and subcutaneous tissue, is the largest organ in the human body and it plays a crucial role in the regulation of the body's homeostasis. These functions are regulated by local neuroendocrine and immune systems with a plethora of signaling molecules produced by resident and immune cells. In addition, neurotransmitters, endocrine factors, neuropeptides, and cytokines released from nerve endings play a central role in the skin's responses to stress. These molecules act on the corresponding receptors in an intra-, juxta-, para-, or autocrine fashion. The epidermis as the outer most component of skin forms a barrier directly protecting against environmental stressors. This protection is assured by an intrinsic keratinocyte differentiation program, pigmentary system, and local nervous, immune, endocrine, and microbiome elements. These constituents communicate cross-functionally among themselves and with corresponding systems in the dermis and hypodermis to secure the basic epidermal functions to maintain local (skin) and global (systemic) homeostasis. The neurohormonal mediators and cytokines used in these communications regulate physiological skin functions separately or in concert. Disturbances in the functions in these systems lead to cutaneous pathology that includes inflammatory (i.e., psoriasis, allergic, or atopic dermatitis, etc.) and keratinocytic hyperproliferative disorders (i.e., seborrheic and solar keratoses), dysfunction of adnexal structure (i.e., hair follicles, eccrine, and sebaceous glands), hypersensitivity reactions, pigmentary disorders (vitiligo, melasma, and hypo- or hyperpigmentary responses), premature aging, and malignancies (melanoma and nonmelanoma skin cancers). These cellular, molecular, and neural components preserve skin integrity and protect against skin pathologies and can act as "messengers of the skin" to the central organs, all to preserve organismal survival.

Up- or Downregulation of Melanin Synthesis Using Amino Acids, Peptides, and Their Analogs

Harmonious synthesis and distribution of melanin in the skin contribute to the expression of beauty and the maintenance of health. When skin pigmentary disorders occur because of internal or external factors or, when there is a need to artificially increase or reduce the pigmentation level of the skin for aesthetic or therapeutic purposes, various pharmacological therapies are applied but the results are not always satisfactory. Studies have been conducted to improve the efficacy and safety of these treatment strategies. In this review, we present the latest studies regarding peptides and related compounds that may be useful in artificially increasing or reducing skin melanin levels. Certain analogs of α-melanocyte stimulating hormone (MSH) and oligopeptides with the sequences derived from the hormone were shown to promote melanin synthesis in cells and in vivo models. Various amino acids, peptides, their analogs, and their hybrid compounds with other chemical moieties were shown to inhibit tyrosinase (TYR) catalytic activity or downregulate TYR gene expression. Certain peptides were shown to inhibit melanosome biogenesis or induce autophagy, leading to decreased pigmentation. In vivo and clinical evidence are available for some compounds, including [Nle⁴-_D-Phe⁷]-α-MSH, glutathione disulfide, and glycinamide hydrochloride. For many other compounds, additional studies are required to verify their efficacy and safety in vivo and in clinical trials. The accumulating information regarding pro- and antimelanogenic activity of peptides and related compounds will lead to the development of novel drugs for the treatment of skin pigmentary disorders.

The First Human Clinical Trial on the Skin Depigmentation Efficacy of Glycinamide Hydrochloride

A previous study identified certain low molecular anti-melanogenic peptides that share a common sequence with α-melanocyte stimulating hormone (MSH) and end with a glycinamide moiety. Glycinamide itself also showed anti-melanogenic activity in cell-based assays, but neither glycine nor acetyl glycinamide were active, which indicated a special structure and activity relationship. The aim of this study was to examine the skin depigmentation efficacy of glycinamide hydrochloride in human subjects. The primary skin irritation potential of glycinamide hydrochloride was evaluated by patch testing in 30 human subjects. The skin depigmentation efficacy of glycinamide hydrochloride was evaluated in a double-blinded clinical test in 21 human subjects. The test product and a control product were applied to designated sites on the right or left side of the face twice daily for eight weeks. Skin color parameters, i.e., the melanin index, the L* value (representing skin lightness), a* value (redness), and b* value (yellowness) were measured using instruments. The individual topology angle (ITA^o, representing skin color) was calculated from L* and b values. The degree of skin pigmentation was visually assessed by two testers. The primary skin irritation test showed that a solution containing glycinamide hydrochloride up to 10% did not induce any adverse skin responses. In the efficacy test, the test product significantly reduced the melanin index, and increased L* value and ITA^o after two weeks of application relative to the baseline value at the start of the test. It also significantly lowered the degree of pigmentation after 6 weeks of application, relative to the baseline value. Differences in the melanin index, L* value, ITA^o and the degree of pigmentation between the test and control groups became statistically significant after six weeks or eight weeks of application. No signs of skin irritation were observed during the efficacy test. The present study suggests that glycinamide hydrochloride has great potential to be used in the control of skin hyperpigmentation.