Retinoids and Carotenoids in Dermatology

Vitamin A has been shown to play a distinct role in the development of normal epithelial cells ever since seminal work in 1925 demonstrated that lack of vitamin A in laboratory rats would result in cutaneous hyperkeratosis as well as mucosal hyperplasia and metaplasia. Vitamin A is now recognized to be essential for maintaining epithelial and mucosal surfaces, good vision, adequate hematopoiesis, and immunocompetence.

The activity or lack of vitamin A has been known from ancient Egyptian times (3500 BCE), when “roasted ox liver, pressed, applied (to the eye)” was shown to be effective in eliminating “sharew,” which was probably night blindness. Seventh-century Chinese physicians recommended the use of pig's liver to alleviate this condition. Fast-forward to the 19th century when an Austrian naval physician, Eduard Schwarz, proved that boiled ox liver was effective in treating hemeralopia, better known as night blindness.¹

Vitamin A refers to chemicals derived from C₂₀-β-ionones that demonstrate biological activity shown by all-trans-retinol. From here, definitions become murky. For example, retinoids comprise retinol analogs and compounds that may have vitamin A or retinoid activity. They have a chemical structure of cyclic and polar end groups, plus a polyene side chain. Carotenoids have the formula C₄₀H₅₆ and include carotene, which gives color to carrots. Carotenoids also act as provitamins.

This symposium is a complete exposition of current knowledge of the retinoids and carotenoids. The editors have meticulously edited the 19 chapters, which have been executed by distinguished basic and clinical scientists in the field. The amount of detail is exhaustive and provides a synopsis of current knowledge.

After the introductory sections, the book presents clinically relevant discussions of acne therapy and the antiaging uses of the retinoids. This includes the topical agents available by prescription (tretinoin, adapalene, and tazarotene) and the cosmeceutical retinaldehyde.

From the first use of tretinoin for acne in the 1960s and the oral use of isotretinoin in the 1980s, the treatment of this sebaceous gland disorder has been revolutionized. The retinoids can also be helpful in treating psoriasis; diseases of cornification, such as ichthyosis and cutaneous T-cell lymphomas; and skin cancers. The substantial adverse effects of oral retinoids are delineated, with the teratogenic effects being the most prominent. Other findings include skin dryness, cheilitis, bone pain, night blindness, glare sensitivity, and alteration of color perception. Topical retinoids may cause burning and stinging, while the cancer scare—laboratory animals developed cancer when irradiated—has been long forgotten.

There are other retinoids in the pipeline, such as liarozole and rambazole. The US Food and Drug Administration has not approved oral tazarotene, although its topical form is the only retinoid approved for the treatment of psoriasis. Acitretin remains the only oral retinoid available for the treatment of psoriasis and is used as first-line therapy by many psoriasis experts in the treatment of palmar-plantar psoriasis, generalized pustular psoriasis of von Zumbusch, and psoriatic erythroderma.

Carotenoids—there are 600 recognized types found in fruits and vegetables—are labile compounds that can create a number of breakdown products whose significance remains to be determined. The carotenoids most commonly found in the skin are lycopene, beta-carotene, and oxidized carotenoids (xanthophylls), including zeaxanthin and lutein. They are not essential for humans and hence cannot be called vitamins. Beta-carotene can be used in the treatment of erythropoietic protoporphyria and possibly in the prevention of UV radiation–induced oxidative stress. This suggests that they play a role in mediating photodamage and the aging process, as well as acting as antioxidants. There seems to be no significant adverse event associated with ingestion of carotenoids save for carotenoderma, the harmless yellow skin discoloration that results from eating too many carrots.

When placed in its proper perspective, this book will prove a useful source for sorting out and keeping up to date on the burgeoning field of retinoids and carotenoids.