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stop elastin from cross-linking. The efficacy associated with using synthetic enzyme inhibitors varies with the product formulation and use conditions. More than one enzyme inhibitor can be incorporated into a multifunctional cosmeceuti-cal product, but as discussed earlier, the physical property requirements of each active must be met.

Some enzymes require other substances for them to function. If these substances are inorganic, such as a metal ion, they are called cofactors. Cofactors may be incorporated into cosmeceuticals; however, metal ions often adversely interact with other ingredients in the formulation and can create unstable products. If the substances are complex organic molecules, such as derivatives of a vitamin, they are called coenzymes. Multifunctional cosmeceuticals often incorporate coen-zymes in their formulations because they are fairly easy to formulate relative to enzymes, have better compatibility with other ingredients, and are usually less irritating to skin than many enzymes.

The proteolytic enzymes papain and bromelain have been used in cosmeceu-ticals to smooth or peel the skin. They must be used with extreme caution because their action is hard to stop, hence can cause severe skin irritation. Typically these actives are formulated alone or in combination with other enzymes into therapeutic cosmeceutical enzyme treatment products designed for professional use. If the enzyme formulations are too harsh to contain other cosmeceutical actives, they are often offered within a multifunctional line of cosmeceutical treatment products containing postenzyme treatments that soothe, calm, and condition the skin.

Superoxide Dismutase (SOD). A protective enzyme, SOD safeguards almost all living organisms from the damage caused by the free-radical oxygen species. Free-radical oxygen species damage cells by attacking unsaturated fatty acids in the cell membrane. In combination with the enzyme catalase, SOD completely converts these free-radical oxygen species into two water molecules plus oxygen. To further the interest in using SOD in cosmeceutical products, it is known that SOD tissue levels decrease with aging. Formulating products to contain stable SOD can be challenging, and the enzyme is irritating to skin. Modified SOD ingredients are available to aid formulation efforts.

Glycosaminoglycans [17]. Glycosaminoglycans (GAGs) are credited with maintaining the water content in skin and giving skin its elastic response. The most popular GAG used in cosmeceutical products is hyaluronic acid (HA), also called hyaluronan. A second GAG substance often used in cosmeceuticals is chondroitin. Present in both animal and human tissues at varying amounts, HA has a concentration of about 0.2 mg/mL in the human dermis. This very high ®

molecular weight linear polymer is anionically charged at physiological pH. When «

HA forms hydrogen bonds, it acquires a helix structure. This helps explain its characteristic viscoelastic properties when in solution. Owing to its molecular size, HA typically is found in the basal layer of the skin, and no higher toward the skin surface than the granular layer.

It has been documented that the skin content of HA declines with age, hence the interest in using it in cosmeceutical products. Being fairly easy to incorporate into formulations, it is found in many multifunctional cosmeceutical formulations. Research has shown that HA does not penetrate through intact skin. The therapeutic benefits to having HA in products most likely are related to effects perceived at the surface of the skin.

Herbal and Botanical Extracts, and Phytochemicals [18]. Plant byproducts are known to have therapeutic efficacy, hence are a major avenue for both drug discovery and cosmeceutical ingredients. A classic example of this from ancient times is the practice of chewing willow bark to alleviate pain. A major component in willow bark is salicylic acid. Salicylic acid is modified to make acetylsalicylic acid, which also is known as aspirin.

Many botanical ingredients have long histories of use from which their claims are drawn but lack documentation of their efficacy data. Because of consumer demand, the use of botanical extracts and phytochemicals in multifunctional cosmeceuticals has grown substantially. There often are several of these ingredients contained in one formula. Accordingly, it should be noted that botanical extracts are the fastest growing source of cosmeceutical allergens. A detailed discussion about specific ingredients in this category is beyond the scope of this chapter. With continued growth in documented therapeutic benefits and safety information about botanical extracts and phytochemicals, their use will also increase.

Hydroxy Acids [11,14,15,17,19-24]. A huge number of different groups of hydroxy acids exist. The groups of hydroxy acids commonly used in cosmeceuticals are a-hydroxy acids (AHAs). P-Hydroxy acids (BHAs) and a-keto acids (AKAs) also may be used. The most commonly used BHA is salicylic acid, which has been reported to yield effects similar to those of the AHAs. Enzymes may be used to effect conversion of in vivo AKAs to AHAs. The latter are also called fruit acids, reflecting their origin. For example, lactic acid is present in sour milk, honey, and tropical fruits and berries; malic acid is present in apples; citric acid is present in many fruits including oranges and lemons; glycolic and gluconic acids are present in sugarcane; and tartaric acid is present in grapes. Researchers do not completely agree on which AHA is most beneficial in cosmeceutical products; hence this discussion focuses on AHAs as a group of ingredients. Many multifunctional cosmeceuticals contain more than one type of AHA, or more than one form of an AHA.

The a-hydroxy acids have multifunctional effects on skin. They cause skin exfoliation by weakening the bonds between keratin cells and slowing cellular ker- |

atinization. They also increase the water content of skin. It has also been reported that AHAs may stimulate production of glycosaminoglycan, collagen, and elastin. ^

Blood flow may increase to the skin tissues as a result of the dilatation of surface blood vessels. All these effects are considered beneficial to the consumer. The shed- J

ding of skin cells helps reduce the appearance of dry skin, reduces the thickness of q t the stratum corneum so that it looks less thick or leathery, and serves to fade spots of hyperpigmentation. The maximum therapeutic effects obtained from using AHAs usually are noted after about 2 weeks of use. As usage continues, the rate of cell renewal declines. It has been reported that the stratum corneum will remain thinner up to 2 weeks after stopping AHA product application, but overall improvements in skin changes due to AHA use may last up to 6 months.

Many of the therapeutic benefits that result from using the AHAs depend upon how they are formulated into the product. Product effects will vary with the chemical and ionic form of the acid, and with the pH of the product. Un-ionized materials penetrate the layers of the skin more readily than ionized materials. Keeping the product pH below the ionization pH keeps the AHA in an un-ionized state and enhances its ability to penetrate into the skin. Note that ionization occurs at different pH values in different AHAs. The optimum product pH depends of the objectives for product performance and the AHAs being used. Adding a buffer to the product helps keep the pH of the product constant over time. Some researchers report that amino acid salts of the AHAs yield the same therapeutic effects as pure AHAs, but with reduced skin irritation. It has also been reported that natural sources for AHAs may contain other trace substances that help reduce skin irritation. There is much controversy over these reports. Regardless of the type of AHA used and the combination of AHAs in the product, the product goal should be to deliver the desired therapeutic benefit, with minimal skin irritation.

The amount of skin exfoliation resulting from AHA use usually depends upon the type and concentration of AHA in the product. The amount of time the product is in contact with the skin and frequency of use also will greatly impact therapeutic results. Products that cause a mild exfoliation usually contain 3-6% AHA. Products designed for facial peels in a professional salon environment contain as much as 30% AHA. Products designed for chemical facial peels in a medically supervised environment contain up to 50-70% AHA. As would be expected, the higher the AHA concentration, the more irritating the product will be to the skin, and the more closely contact time with the skin must be monitored. Citric acid has been reported to be the least irritating AHA, but also is reported to have less therapeutic activity that other AHAs, even at higher concentrations. AHA products should not be used around eye, nose, and lip tissues because they are extremely irritating to delicate tissues and mucous membranes. They are commonly incorporated into multifunctional cosmeceutical products along with moisturizers, vitamins, and herbal extracts.

Lipids [25-29]. The skin's barrier layer acts to regulate the permeability |

of substances to and from the deeper layers of the skin, and to maintain moisture in the stratum corneum. Many factors can disrupt the function or the barrier, resulting in dry, flaky skin. Many multifunctional cosmeceutical products are |

targeted at maintaining or restoring barrier function. Some of these products claim J

to do so by impacting the lipid content of the skin. Such products typically are multifunctional and contain other ingredients, such as vitamins, herbal extracts, moisturizers, and occasionally hydroxy acids.

Lipids are essential to maintain barrier layer function. They trap water in the upper layers of the skin, and prevent water loss. Ceramides, cholesterol, and fatty acids are three of the key lipids needed to maintain barrier function. These actives often are found in combination in cosmeceutical products claiming to enhance barrier function. Note that researchers have varied opinions regarding the ability of topically applied lipids to alter the skin barrier function in dry, yet healthy, skin.

Sphingolipids are a class of lipids found throughout the skin. Ceramides are a member of the sphingolipid class. Ceramides, along with other lipoid substances, create an organized lipid network that is necessary for normal barrier function. Cos-meceutical products containing ceramides are claimed to help restore impaired skin barrier function and protect skin. They are reported to act by increasing the waterholding capacity of the stratum corneum and replacing ceramides in deficient skin. Cholesterol and fatty acids have been shown to interact with the skin's lipid network to enhance its structure and water-holding ability. Many multifunctional cos-meceutical products contain cholesterol and/or fatty acids for this reason.

Urea [30,31]. Urea is a small, soluble molecule that exists in normal skin at a concentration of about 1%. It is a component of the natural moisturizing factor (NMF), with an NMF concentration up to 7%. It functions to increase the moisture content of skin by binding to both skin proteins and water, resulting in an increase in the water content of the stratum corneum. Research shows that water both alone and with cleaning solutions decreases the amount of urea in the skin. Applying a leave-on type of urea product after washing the skin, however, can significantly increase the urea content of the skin. Urea levels may stay high as long as 24 hours or more after product application, unless the skin is washed again. Increases in skin moisture were documented to accompany the increases in skin urea levels. Urea currently is used as a moisturizer in combination with a variety of other actives in multifunctional cosmeceuticals.

Urea is inherently multifunctional from a therapeutic standpoint. This attribute stems from varied activities based on its concentration in the product. In addition to moisturizing skin, it functions to degrade keratin and to aid in the relief of itching. Since it can degrade fibrin, it is used to remove surface crusts on skin wounds. These activities occur at concentration levels of 10-30%, which can be

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