Peptide Use in Cosmetics: Mechanisms of Action and Clinical Findings

Abstract
Peptides are biologically active molecules composed of short amino acid chains that play a critical role in cellular signaling processes. Their use in cosmetic and dermocosmetic products has increased significantly in recent years, particularly in the prevention and treatment of skin aging. Peptides exert effects through various mechanisms including stimulating collagen synthesis, maintaining extracellular matrix (ECM) integrity, modulating neuromuscular transmission, and inhibiting enzymatic degradation. Clinical studies demonstrate that formulations containing peptides produce positive results such as reducing wrinkle depth, increasing skin elasticity, and improving hydration levels. This review comprehensively addresses the classification, mechanisms of action, and clinical efficacy of peptides used in cosmetics in light of current literature.

Introduction
Skin aging is a complex biological process resulting from the combination of intrinsic (genetic and chronological) and extrinsic (UV radiation, environmental factors) processes. In this process, the decrease of structural proteins such as collagen and elastin, the increase in matrix metalloproteinase (MMP) activity, and the slowing of cellular renewal play important roles.

Peptides have gained an important place in cosmetic science as functional molecules capable of targeting these biological processes. The development of cosmetic peptides began with the demonstration that the Gly-His-Lys (GHK) peptide stimulates collagen synthesis, and this discovery accelerated research into the effects of peptides on skin biology.

Today, cosmetic peptides are classified into four main groups:
Signal peptides
Carrier peptides
Neurotransmitter inhibitor peptides
Enzyme inhibitor peptides

Mechanisms of Action of Peptides
Signal peptides stimulate fibroblasts to increase synthesis of type I collagen, type III collagen, and elastin. In particular, palmitoyl pentapeptide (Matrixyl) mimics collagen fragments to create a "damage signal" and activates cellular repair mechanisms. This contributes to the restructuring of the extracellular matrix. Carrier peptides play a role in the transport of metal ions. One of the best-known examples, the GHK-Cu complex, accelerates wound healing, enhances antioxidant defense, and supports collagen production.

Neurotransmitter inhibitor peptides modulate synaptic transmission to reduce muscle contractions. For example, acetyl hexapeptide-8 (Argireline) targets the SNAP-25 protein to reduce neurotransmitter release, thereby improving the appearance of expression-induced wrinkles. Enzyme inhibitor peptides suppress the activity of MMPs to reduce collagen and elastin degradation. This mechanism is particularly important in slowing the photoaging process.

Clinical Findings and Efficacy
The efficacy of peptides in cosmetics is supported by numerous in vitro and in vivo studies. In randomized controlled clinical trials, formulations containing palmitoyl pentapeptide have been shown to provide significant reduction in wrinkle depth after 12 weeks of use. Similarly, oral or topical application of collagen peptides has been reported to increase skin elasticity and improve dermal density. Additionally, peptide-based formulations are reported to strengthen skin barrier function by increasing stratum corneum hydration. Studies involving GHK-Cu have demonstrated that this complex not only increases collagen synthesis but also exhibits anti-inflammatory effects and accelerates wound healing.

However, the efficacy of peptides depends on factors such as molecular size, stability, formulation type, and carrier systems. In particular, crossing the skin barrier is an important factor limiting the bioavailability of peptides. For this reason, combinations with nanotechnological carrier systems are increasingly being researched.

Formulation and Future Perspectives
Stability and penetration are of critical importance for peptides to be effective in cosmetic formulations. Since peptides are generally susceptible to enzymatic degradation, they must be protected with appropriate carrier systems. Liposomes, nanoemulsions, and solid lipid nanoparticles are the main systems used for this purpose. In the future, peptides are expected to be used in combination with personalized cosmetics, biomimetic design, and smart carrier systems. Furthermore, genomic and proteomic approaches will enable target-oriented peptide design, bringing a new dimension to cosmetic science.

Conclusion
Peptides are important active ingredients in cosmetic science with proven biological efficacy and multifaceted mechanisms of action. By increasing collagen synthesis, reducing enzymatic degradation, and modulating neuromuscular transmission, they target the fundamental mechanisms of skin aging. Clinical studies support the positive effects of peptides in reducing wrinkles, increasing skin elasticity, and improving hydration levels. However, for peptides to achieve maximum efficacy, they must be supported with appropriate formulation strategies and carrier systems. In the future, the integration of peptide-based cosmetics with nanotechnology and personalized treatment approaches will further increase the efficacy of these compounds. Therefore, multidisciplinary research and advanced clinical studies will continue to play a critical role in the development of cosmetic peptides.

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