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■a stored in the liver, then circulated via specific cellular binding proteins to the tissues, where it is converted to retinoic acid. Excessive vitamin A intake results in toxicity. A toxic dose of vitamin A varies with the individual, but is reported to be about 12,000 IU per kilogram of body weight. Symptoms of vitamin A toxicity can include some or all of the following: dry, peeling skin, headache, altered mental status, blurred vision, hair loss, and jaundice.

Vitamin A increases skin cell turnover. The epidermis produces more protein to become thicker, and collagen production may increase. Topically applied pure vitamin A is predominantly absorbed into the skin epidermis with only a small amount penetrating into the dermis. It is unstable in the presence of light and oxygen. When used in cosmeceutical products, vitamin A stability is poor, and it typically is inactivated within days. Since it is an oil, it often is used and classified as a skin emollient in multifunctional cosmeceutical products. Note, however, that consumers may be unaware that is serves these functions, and with a casual review of the ingredient list may think it has antiphotoaging benefits.

To enhance its stability in products, vitamin A has been chemically modified to make a variety of vitamin A esters. These esters are converted to retinol, then retinoic acid, upon absorption into the skin. The most common vitamin A ester used in multifunctional cosmeceuticals is retinyl palmitate, which is reported to be more stable than pure vitamin A. Formulation studies have been ongoing for years to understand how to keep retinyl palmitate stable for the life of a product. Its stability is affected by light, oxygen, heat, trace metals, and a product pH outside 5-6. To maintain stability, retinyl palmitate must be added to a formulation below a temperature of 40°C. Stability results have been more acceptable for products that have a stable pH between 5 and 6, contain antioxidant ingredients, and have no trace metals present. Product color stability is best for retinyl palmitate that is protected by multilamellar vesicle liposomes. Some studies indicate that retinyl palmitate penetration into the skin is enhanced when retinyl palmitate is formulated into multifunctional cosmeceutical products that also contain the AHA gly-colic acid. It also has been shown that the type of product vehicle will impact the stability of retinyl palmitate, as well as its ability to penetrate into the skin.

Retinoic acid, also called tretinoin or all-trans retinoic acid, is available at concentrations of 0.02% to 0.1% in a emulsion, gel, or liquid vehicles via a physician's prescription. The 0.02% and 0.05% emulsion products are considered to be multifunctional cosmeceuticals because they reverse the signs of photoaging while moisturizing the skin in a fairly aesthetic cream form. They are drugs that ^

were approved by the FDA upon completion of the NDA process. They are the |

only products officially indicated by the FDA for use to diminish fine wrinkles, ®

varied skin hyperpigmentation, and skin roughness. They do not alleviate deep wrinkles, sun-damaged skin associated with cancer, coarse skin, or skin yellowing. Improvements in the indicated areas most often are perceivable after 4-6 | months of use. Improvements after 6 months of use are marginal. The benefits of retinoic acid are maintained with continued product usage. Once discontinued, the skin may return to its original state. The amount of time for skin to return to its original state varies with the individual, the person's response to the product, and the amount of time that it was used. All clinical studies for 0.05% retinoic acid incorporated a complete multifunctional skin care regimen that included sunscreens and protective clothing.

Retinoic acid is a known skin irritant and causes photosensitivity. The degree of side effects associated with retinoic acid use vary, but most often include skin redness, itching, burning, stinging, peeling, and dryness. Symptoms typically subside within 6 months of continued use. Patients may opt to reduce the frequency of product application to manage skin reactions. About 4% of patients discontinue retinoic acid use because of their inability to tolerate adverse skin reactions. Retinoic acid use is not recommended during pregnancy.

Vitamin B [32,38]. There are many different types of vitamin B, and their functions are not the same. The two types of vitamin B used most often in multifunctional cosmeceuticals are P-carotene and panthenol. Different types of vitamin B commonly are combined a multifunctional cosmeceutical product.

P-Carotene, found in carrot oil, acts predominantly as an antioxidant. When consumed in the diet, it is transformed by the body into vitamin A. Product formulation with P-carotene is challenging because it stains the skin yellow. Decolorizing P-carotene so that it will not stain the skin often results in the loss of its therapeutic activity.

Panthenol is active in the form called d-panthenol. It is found naturally in liver, queen bee jelly, rice bran, and molasses. It is absorbed after topical application, and converted in the skin to d-panthenoic acid. d-Panthenoic acid works with skin enzymes to enhance skin cell growth and promote skin healing. d-Panthenol also is classified as a moisturizer in cosmeceutical products.

Vitamin C [11,15,23,32,38-40]. Vitamin C also is known as l-ascorbic acid. It is found in citrus fruits, hip berries, fresh tea leaves, and paprika. Vitamin C is therapeutically multifunctional in that it is recognized as an antioxidant and functions as a cofactor that is necessary for many in vivo reactions to occur. It facilitates collagen production in tissue cells, reduces the likelihood of infection in wounds, and converts reacted vitamin E back into its active form, restoring the capacity of vitamin E to function as a free-radical scavenger. The l-ascorbic acid isomer of vitamin C can be absorbed after topical application and has been shown to reduce the amount of ultraviolet photodamage to skin cells. It is documented to act in concert with other skin antioxidants to yield this benefit. Vitamin C also has been reported to stimulate skin collagen production under certain conditions. Research «

continues to examine the extent of these effects on healthy, intact human skin. ^

Since vitamin C is such a good antioxidant, it is difficult to keep stable in a formulation. Stable formulations that contain 10% l-ascorbic acid in an acidic §

t aqueous form are available, but significant clinical data demonstrating their antiphotoaging benefits are lacking. Research also is investigating the usefulness of vitamin C derivatives. Ascorbyl palmitate and phosphate are contained in many multifunctional cosmeceutical products. These esters are easier to formulate into products than l-ascorbic acid, but more research in needed to document their ability to penetrate the human stratum corneum to produce a therapeutic effect.

Vitamin E [11,15,32,41,42]. Vitamin E is a fat-soluble vitamin, which in its most potent form is known as a-tocopherol. It has multifunctional benefits. It most commonly is used in multifunctional cosmeceutical products as a moisturizer, and as a stabilizer for other ingredients against oxidation. Research shows that vitamin E has additional benefits for the skin: it is necessary to stabilize cell membranes; it is thought to control parameters impacting the structure and function of various lipoproteins; and its antioxidant effects are thought to be protective against substances that may have damaging effects on the skin. Its free-radical scavenging activity makes for its photoprotective effects against ultraviolet radiation. Research indicates that the application of topical vitamin E can result in increased vitamin E content in the epidermis for at least 24 hours. It enhances the ability of other antioxidants within the skin to protect skin tissues, and is noted to stop ultraviolet-induced immunosupression and tumorigenesis. Vitamin E has been noted to reduce the incidence of various skin irritations. Caution must be exercised, however, because some topical products containing 10-20% vitamin E have been reported to irritate the skin, resulting in delayed eczema or rash-type reactions.

Vitamin E stability is compromised by heat, light, and the presence of metal ions. Its oxidation is relatively slow, however, thus allowing its widespread use in many multifunctional cosmeceutical products. Vitamin E can be modified into an ester form, and the most popular ester used in cosmeceuticals is vitamin E acetate, a compound that is very stable against oxidation. When orally ingested, vitamin E acetate is transformed into vitamin E. The same conversion must be made in the skin after topical application to yield vitamin E benefits, but studies have not definitively confirmed this conversion. Understanding more about the penetration of vitamin E acetate into the skin, and the rate at which it is converted into vitamin E, will shed more light on the potential value of using this ester instead of vitamin E. Vitamin E acetate is not thought to be photoprotective.

Pure a-tocopherol rarely exists in nature, but it is commercially produced and available. Natural vitamin E, a mixture of different forms of vitamin E, is present in wheat germ, corn, sunflower seeds, grape seeds, soybean oils, alfalfa, and lettuce.

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