ABSTRACT
Overview 319
Historical Background 322
Cosmeceuticals 322
Retinol 322
Penetration, Absorption, and Cutaneous Metabolism of Topical Retinoids 323
Cellular Uptake of Retinol 324
Cutaneous Metabolism 325
Pharmacological Effects of Retinol In Vitro and In Vivo 326
Tretinoin Therapy in Photoaging 327
Toxicity 329
The Future 330
References 330
OVERVIEW
The retinoids are a diverse class of pharmacological compounds, consisting of
vitamin A (retinol) and its naturally occurring and synthetic derivatives, which
possess biological vitamin A activity (Tables 1 and 2). Vitamin A generically
encompasses retinol (vitamin A alcohol), retinal (vitamin A aldehyde), and
retinoic acid (vitamin A acid) (Fig. 1). In clinical use, retinoids have established
their effectiveness in treating acneiform eruptions (e.g., isotretinoin), disorders of
keratinization, such as psoriasis (e.g., acitretin), as well as some neoplastic pro-
cesses (e.g., tretinoin for leukemia, isotretinoin for squamous cell carcinomas).
Additional retinoids are currently being investigated, as novel uses of retinoids
already established in clinical practice. The main focus of retinoid usage in cos-
meceuticals has been its role as the mythical “fountain of youth” (i.e., reversal of
photoaging) (Table 3). Retinoids, like all drugs, have adverse effects, the most
infamous one being teratogenicity. Over 2000 derivatives have been developed
in the hope of finding retinoids with increased therapeutic efficacy coupled
with diminished local and systemic toxicity. The recent focus of retinoids has
been on topical delivery systems, as this route not only provides a safer
adverse effect profile, but also delivers a higher dose to a targeted area (i.e.,
the skin).
