Chapter 217. Topical Retinoids

In topical preparations, retinoids are widely used as prescription drugs as well as cosmeceuticals. In some of these products, retinoids are naturally occurring compounds while others are synthetic molecules. All retinoids, however, share predictable pharmacology in eliciting human skin responses due to the fact that retinoid effects are primarily mediated through their intranuclear retinoid receptors acting as transcription factors. Indeed, the discovery and characterization of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) have been pivotal to our understanding of the retinoid action mechanism and gave rise to the idea that retinoids act similarly to hormones.1,2 In 1976, Michael Sporn and his colleagues originally defined retinoids as both the naturally occurring compounds with vitamin A activity and the synthetic analogs of retinol. This concept is no longer adequate. Now, retinoid is defined as any molecule that, by itself or through metabolic conversion, binds to and activates the RARs, thereby eliciting transcriptional activation of retinoic acid-responsive genes that results in specific biologic responses.

Retinoid Receptors

The discovery and characterization of RARs as having molecular features that are similar to steroid/thyroid hormone receptors were landmark findings.1,2 A characteristic common to these receptors is that they bind to regulatory regions in DNA called hormone response elements, or target sequences, and activate gene transcription in a ligand-dependent manner. Therefore, they are referred to as ligand (hormone)-dependent transcription factors. These receptors bind to specific DNA sequences as dimers. Unlike receptors for the steroid hormones, which form homodimers (two identical monomers); RAR, vitamin D receptor (VDR), and thyroid hormone receptors (TR) bind to their elements with greater affinity as heterodimers (two different monomers). The critical partner for heterodimerization and ultimate functioning of RAR, VDR, and TR is the RXR,3 whose physiologic ligand is 9-cis-retinoic acid.4 The ability of RXRs to interact with many receptors suggests their importance as regulatory proteins. There are three different members of RAR (α, β, and γ) and RXR (α, β, and γ), each encoded by different genes. Furthermore, each of the receptors has isoforms, adding to the diversity of retinoid receptors.

Receptor-Selective Retinoids

The identification and functional characterization of retinoid receptor families cleared the way for drug discovery. Many synthetic compounds have no structural similarities to all-trans-retinol or retinoic acid, yet are still considered retinoids by virtue of their ability to activate the receptor(s), therefore mediating the retinoid effect. Adapalene, tazarotene, and bexarotene are three synthetic retinoids that fit this description and have been used topically in humans (Fig. 217-1). Adapalene has restricted receptor specificity, possessing poor affinity for RAR-α, higher affinity for RAR-β and -γ, and no interaction with RXR-α.5 Tazarotene cannot directly bind to RARs or RXRs. However, its metabolite, tazarotenic acid, has receptor selectivity for RARs (RAR-β > RAR-γ > RAR-α),6 and is felt to be the principal active form. Bexarotene is an RXR-selective retinoid (rexinoid), which is 100-fold more potent at binding to RXR than to RAR receptor.7

Retinoid Receptors in Human Skin

Systematic analysis of relative levels of each RAR and RXR has been made in human skin and shows that the human epidermis expresses RAR-α, RAR-γ, RXR-α, and RXR-β messenger RNAs (mRNAs).8,9 The relative levels of retinoid receptor mRNAs mirror their relative protein levels. In nuclei from human epidermal cells, there are five times more total RXRs than RARs.9 For RAR proteins, 87% are RAR-γ and 13% are RAR-α with no detectable RAR-β. Of the RXR proteins, 90% are RXR-α. Furthermore, only RXR-α and RAR (mostly γ) heterodimers bind to retinoic acid-responsive elements (RAREs).9,10 At their physiologic levels, RARs and RXRs in human skin bind RAREs as heterodimers and transactivate these response elements in response to RAR, but not to RXR ligands. However, the RXR is key, as the heterodimer will not bind to the RARE without the presence of the RXR protein. Ultraviolet (UV) irradiation of human skin causes a rapid and significant reduction of retinoid receptors, both RAR-γ and RXR-α.11 Although this decrease in the receptor levels is transient and returns to pre-UV treatment levels within 2 days, UV irradiation results in functional impairment of receptor-dependent retinoid responsiveness in human skin. Interestingly, pretreatment of skin with all-trans-retinoic acid before UV irradiation reduces receptor loss and accelerates receptor recovery.11

Retinoid Target Genes and Their Activation Cascade in Human Skin

Retinoid actions are mediated by retinoid receptors. Given that the receptors are transcription factors, the ultimate skin effects of retinoids (phenotypic changes) must be accomplished through regulated gene expression. The best-established mechanism by which retinoid receptors modulate gene expression is activation of retinoid target genes through direct binding to RARE in the gene promoter, thereby stimulating the basal transcriptional machinery. In skin, cellular retinoic acid binding protein (CRABP) II, cellular retinol binding protein (CRBP), retinoic acid 4-hydroxylase (CYP26), and keratin 6—all of which contain RAREs—are regulated by all-trans-retinoic acid.12–15 However, the products of these genes are unlikely to be the effectors of the retinoid response and thus cannot account for the pleiotropic effects of retinoids in skin. Undoubtedly, additional RARE-regulated genes will be identified that encode for proteins that function to modulate cutaneous growth and differentiation either directly or indirectly. A likely scenario is that these protein products, in turn, activate other non-RARE-containing genes in a cascading reaction to produce the clinical features of retinoid action in skin (Fig. 217-2).

Molecular description responsible for the most striking histologic feature of all-trans-retinoic acid-treated skin, the marked thickening of the epidermis, illustrates this point.12,16 The epidermal hyperplasia is caused by the increased proliferation of basal keratinocytes. There is also an increase in differentiation markers involucrin, loricrin, filaggrin, and epidermal transglutaminase.15 These epidermal changes collectively translate to clinical desquamation and peeling. This hyperproliferative response of epidermis to retinoic acid is mediated by its nuclear receptor.17,18 Among the genes regulated by retinoid receptors involved in the cell growth control (keratinocyte proliferation), ligands for epidermal growth factor (EGF) receptor appear to be critically important. In human skin in vivo, topical retinoic acid induces heparin-binding (HB)-EGF and amphiregulin (AR), which stimulate basal cell growth via the cell surface EGF receptor activation.19,20 Interestingly, neither HB-EGF nor AR possesses any putative RARE. Therefore, a direct modulation of HB-EGF or AR by RARE promoters seems unlikely. Rather, RAR/RXR is likely regulating other transcription factor(s) that would, in turn, regulate AR and HB-EGF gene expression (Fig. 217-3). The identity of such transcription factor(s) remains to be determined.

Nomenclature

All-trans-retinoic acid (tretinoin), which binds to and activates RARs, is derived from sequential oxidation of all-trans-retinol (or vitamin A) and all-trans-retinaldehyde. It is a 20-carbon molecule that consists of a cyclohexenyl ring, a side chain with four double bonds (all arranged in trans-configuration), and a carboxylic-acid end group (Fig. 217-4A). The numbering of the carbon atoms is as shown in Figure 217-4B. The terms 9-cis- (alitretinoin) and 13-cis-retinoic acid refer to stereoisomers of all-trans-retinoic acid in which the double bond that begins with the ninth and thirteenth carbon atoms, respectively, is in the cis- rather than trans-configuration. The 9-cis-retinoic acid binds to the RXR-α receptor with high affinity, being 40 times more potent than all-trans-retinoic acid on RXR-α. 9-cis-retinoic acid can also bind RAR receptors thus at times referred to as a panagonist of retinoid receptors.4,21 The fourth carbon atom is located in the cyclohexenyl ring of retinoic acid and is involved in a hydroxylation reaction to generate 4-hydroxy-retinoic acid (see Fig. 217-4C). The addition of a hydroxyl group to the cyclohexenyl ring renders the molecule more polar, making it more amenable to excretion/elimination from cells and organisms. It is also much less active biologically. 4-Hydroxy-all-trans-retinoic acid can be further oxidized to 4-oxo-retinoic acid.

A group of compounds referred to as retinyl esters functions as the molecular storage form of retinol. The compounds are formed by esterification of retinol with fatty acids (see Fig. 217-4D), which specify the ester. For example, retinyl palmitate is generated when retinol is esterified with palmitic acid. Hydrolysis of retinyl esters regenerates retinol. Finally, β-carotene is a symmetric 40-carbon molecule (twice the number of carbons in retinol or retinoic acid) present in green and yellow–orange fruits and vegetables. As suggested by stoichiometry, one molecule of this carotenoid can potentially generate two molecules of retinol by cleavage of the central double bond (see Fig. 217-4E).

Esterification and Oxidation of Retinol

Topical treatment of human skin with all-trans-retinol increases retinyl ester levels in the epidermal layer by more than tenfold.12 This reaction is catalyzed by two enzymes: (1) lecithin/retinol acyltransferase and (2) acyl-coenzyme A/retinol acyltransferase. In human keratinocytes, lecithin/retinol acyltransferase has the predominant retinol-esterifying activity.22

Sequential oxidation of all-trans-retinol forms all-trans-retinoic acid, with all-trans-retinaldehyde as the intermediate metabolite. The first step (oxidation of all-trans-retinol to all-trans-retinaldehyde) is rate limiting for all-trans-retinoic acid formation. Topical application of all-trans-retinol to human skin results in histologic and molecular alterations that mimic those following all-trans-retinoic acid treatment.12 These include epidermal hyperplasia, epidermal spongiosis, compaction of the stratum corneum, and induction of CRBP, CRABP-II, and CYP26.22,23 In retinol-treated skin, all-trans-retinoic acid is minimally detectable and is no different from levels found in untreated normal skin. The lack of significant accumulation of all-trans-retinoic acid in retinol-treated skin is a result of the tightly regulated conversion of retinol to retinaldehyde and the effective hydroxylation of all-trans-retinoic acid that is formed by CYP26. The observation that all-trans-retinol and retinoic acid elicit similar responses in human skin, but that retinol does so without detectable increases in retinoic acid levels, indicates that endogenously synthesized retinoic acid is a much more efficient activator of retinoid pathways than exogenously supplied retinoic acid.

Hydroxylation of All-trans-Retinoic Acid

In human skin all-trans-retinoic acid is catabolized primarily to the more polar 4-hydroxy-all-trans-retinoic acid, which is further metabolized to 4-oxo-retinoic acid. 4-Hydroxy-retinoic acid is tenfold less potent in inducing retinoid responses in human keratinocytes and mouse skin than all-trans-retinoic acid.24 In untreated normal human skin, CYP26 activity is minimally detectable. Topical administration of all-trans-retinoic acid or all-trans-retinol to human skin, however, increases its activity several fold.23,25 This hydroxylase activity is also induced in skin treated with 9-cis- or 13-cis-retinoic acid, but remarkably, it has substrate specificity for the all-trans-retinoic acid isomer only and does not hydroxylate 9-cis- or 13-cis-retinoic acid, retinol, or retinaldehydes.23 With all-trans-retinoic acid, CYP26 activity is maximally induced after 24 hours of occlusive treatment. The CYP26 activity can be effectively inhibited by ketoconazole and a related azole, liarozole.16,25 By blocking this major inactivation pathway of all-trans-retinoic acid, topical liarozole can serve as a retinoic acid mimetic or amplify human skin responses to all-trans-retinol and all-trans-retinoic acid.16 This approach of targeting CYP26 has given rise to a novel class of new chemical entities referred to as retinoic acid metabolism blocking agents (RAMBAs). A study involving another RAMBA, talarozole, demonstrated that topical application led to increased mRNA expression of CRABP II and CYP26.26 The drug development progress for RAMBA has been slow. A challenge for RAMBAs of the future is to selectively target CYP26 without affecting other enzymes such as steroid metabolizing aromatase or 17,20-lysase.

Until recently, clinical use of topical retinoids has been limited to all-trans-retinoic acid, which is approved in the United States for the treatment of acne, photoaged skin, and melasma. Topical adapalene and tazarotene have also received approval for acne; tazarotene has received approval for psoriasis and photoaging. More recently, bexarotene was approved for management of cutaneous T-cell lymphoma and alitretinoin was approved for patients with Kaposi sarcoma.

It is widely accepted that topical retinoids are extremely effective for acne therapy, especially for comedonal lesions. Of the different classes of antiacne medications, retinoids are thought to be the best, if not the only, agents to normalize the abnormal follicular epithelial differentiation/desquamation important in the pathogenesis of acne lesions. Therefore, the use of retinoids can also provide protection against the development of new lesions. This prophylactic property is the basis for including topical retinoid in almost all antiacne regimens. A potential for aggravating inflammation exists in treating inflammatory acne (i.e., papules and pustules) with topical retinoids, but when properly administered, this type of acne also responds well to retinoids (see Section “Dosing Regimen”).

Fine wrinkles and dyspigmentation are two features of photoaged skin that are improved by topical tretinoin or tazarotene. Several weeks of treatment are required before clinical improvement is appreciated.27–29 For the effacement of fine wrinkles by topical tretinoin, partial restoration of markedly reduced levels of collagen in sun-exposed skin toward those seen in sun-protected skin appears to be responsible.28,30 Tretinoin's ability to improve photoaging is specific and does not result from the irritation or retinoid dermatitis frequently produced by this compound.29 Topical adapalene is not approved for the treatment of photoaging; however, a recent study indicates that it also holds promise in ameliorating the clinical features of photodamage.31

Primary cutaneous T-cell lymphomas are characterized by clonal proliferation of skin-homing malignant T lymphocytes. As an RXR-selective retinoid, bexarotene inhibits tumor cell growth, encourages terminal differentiation, and induces apoptosis.7 It may also play a role in chemoprophylaxis.32,33

Alitretinoin gel was approved in 1999 for the management of cutaneous Kaposi sarcoma (KS), which is caused by human herpes virus 8 (HHV-8). Alitretinoin's mechanism of action in KS is not entirely clear, but presumably relates to inhibition of cellular proliferation as well as induction of apoptosis seen with other retinoids. Other topical retinoids such as all-trans-retinoic acid and 13-cis-retinoic acid have been shown to inhibit KS cellular proliferation via inhibition of autocrine growth factors such as oncostatin M, tumor necrosis factor-α, and IL-6.34–36 Furthermore, altretinoin and tretinoin have been reported to inhibit herpes simplex virus replication thus alitretinoin may have an antiviral role against HHV-8.37 Alitretinoin has also been shown to have anti-inflammatory properties.38 In clinical trials, most patients saw improvement after 4–8 weeks of treatment with the most significant response occurring after 14 weeks of therapy.39

Besides the approved indications, topical retinoids have been demonstrated to be effective in the treatment of several other conditions. Most studies have used tretinoin because it is the first topical retinoid to be developed and has been available the longest. Studied conditions include, but are not limited to, postinflammatory hyperpigmentation in blacks,40 actinic dyspigmentation in Chinese and Japanese individuals,41 and early stretch marks.42 The controlled studies show more than therapeutic efficacies. They also provide valuable information to dispel some of the myths about retinoid use in humans. For example, African-Americans and Asians tolerate topical tretinoin as well as, if not better than,40,41,43 Caucasians.28,44 Furthermore, the often observed retinoid dermatitis does not usually lead to postinflammatory hyperpigmentation in those with greater constitutive pigmentation.

Many other skin disorders have been reported to be improved by topical retinoids, but most of these have not been rigorously studied; thus, their therapeutic claims should be interpreted with caution. Molluscum contagiosum, warts, and various forms of ichthyosis may be improved by topical retinoids to a variable degree. In psoriasis, especially, irritation of treated skin has limited the use. Topical tazarotene, which is approved for psoriasis, does not appear to have fully overcome the irritation problem45; thus, it is typically used in combination with topical steroids.

With such a wide variety of skin conditions treatable by topical retinoids, their use has included all age groups, perhaps with the exception of neonates. The use of topical retinoids in pregnancy is an emotional issue. As discussed in Section “Risks and Precautions,” teratogenicity is not caused by topical retinoids. However, because none of the dermatologic conditions seen in pregnancy that may respond to topical retinoids (i.e., acne, melasma, stretch marks) is life-threatening to the mother or the fetus, it seems prudent to delay the treatment until after delivery. In a study that demonstrated that early, inflammatory stretch marks were improved by topical tretinoin, all pregnancy-related stretch marks were treated postpartum.42 Therefore, even in this pregnancy-associated condition, the therapeutic benefit could be achieved by instituting the treatment after delivery.

For decades, tretinoin was the only topical retinoid available for clinical use sold under the trade name Retin-A. Now, tretinoin is also available in other formulations (eTable 217-0.1). Adapalene is also available in varying formulations and more recently, combination medications have been introduced (eTable 217-0.1). For acne and psoriasis, topical tazarotene is available and for photoaging treatment, tretinoin and tazarotene are approved for use. Tretinoin 0.05%/hydroquinone 4%/fluocinolone 0.1% is a topical combination approved for the treatment of melasma. Different formulations allow some flexibility in terms of tailoring the therapy to an individual's skin dryness or oiliness. Finally, more recently approved topical retinoids include bexarotene and alitretinoin, sold in gel formulations.

Regardless of the retinoid preparation or the patient's age, the most important element in topical therapy is patient/guardian education. It must be clearly explained to each patient that, as part of the treatment, local skin irritation, characterized by redness and peeling, can be expected. The concept that clinical improvement correlates with the degree of irritation has been erased through a large, controlled clinical study in which 0.025% and 0.1% tretinoin were shown to be equally efficacious, but the former was significantly less irritating than the latter.29 Therefore, unlike most medications for which the dosing schedule may be set as once or twice daily, administration of a topical retinoid should be titrated depending on the skin reaction. For some individuals, it may be applied only twice a week, for others four times a week, and this can be increased, as tolerated, to a once-daily regimen. This method of individualizing topical therapy minimizes unwanted acute retinoid dermatitis.

Under the nonprescription category, there are countless “natural retinoid” preparations with various claims (mostly antiaging) being sold throughout the world. Most of these contain retinyl esters, especially retinyl palmitate, retinaldehyde, or retinol. Whether any of these products can deliver retinoid activity to human skin is subject to question. Percutaneous absorption (especially for retinyl esters), adequate concentrations, and stable formulations (especially for retinol) are some of the important unknowns for this group of products. Based on human in-vivo work, all-trans-retinol and all-trans-retinaldehyde hold the most promise of these natural retinoids in being biologically active,12,46 provided the stability of the compound can be maintained with a proper formulation. When the sun-protected skin of individuals older than 80 years of age was treated for 7 days with 1% retinol, fibroblast growth and dermal collagen were increased significantly.47 Concomitantly, retinol markedly reduced the levels of matrix-degrading metalloproteinases that are elevated in aged skin. These findings suggest that retinol may reverse and partially prevent skin atrophy that accompanies intrinsic aging. A recent clinical study has confirmed that topical retinol improves the fine wrinkles associated with atrophic, naturally aged skin of the elderly.48

Adverse Effects

By far the most common adverse effect associated with topical retinoid use is local skin irritation characterized by erythema, peeling, dryness, tightness, and burning sensation. This predictable skin response is temporary, but troubling, for many patients. It tends to peak within the first month of treatment and diminishes thereafter. It responds to a temporary reduction in the frequency or amount of retinoid application and to liberal use of emollients. The requirement for RAR/RXR receptors in retinoid stimulation of basal keratinocyte proliferation was presented in Section “Retinoid Target Genes and Their Activation Cascade in Human Skin.” Retinoid activation of the receptors is followed by subsequent induction of HB-EGF and AR in human skin in vivo19,20 (see Fig. 217-3). The importance of this signaling pathway in the hyperplastic response of the epidermis to topical retinoids was recently demonstrated when pharmacologic antagonists of EGF receptor tyrosine kinase activity blocked retinoid-induced epidermal thickening in human skin organ culture and in vivo.19,20 This observation suggests that it may be possible to reduce the clinically undesirable effects of retinoids on epidermal keratinocytes. However, the erythema response following topical retinoic acid may not be retinoid-receptor mediated because all-trans-retinol, which induces epidermal hyperplasia and CRABP-II mRNA-like retinoic acid (two indicators of receptor activation), is minimally associated with clinical erythema.12

For the more recently approved bexarotene and alitretinoin, local irritation is also the most common side effect. With alitretinoin, the local erythema can increase to edema and vesiculation with continued use. However, most reactions are mild-to-moderate with only 7% of patients requiring treatment withdrawal in clinical trials.39 Finally, the central hypothyroidism seen with systemic bexarotene is not observed in the gel formulation.49,50

Systemic retinoid exposure has been well documented and established as a cause of embryonic death and congenital malformation, and understandably, there is concern about potential teratogenicity from long-term topical retinoid use. Systemic absorption of retinoids from topical application is negligible, and the levels of endogenous retinoic acid in the blood are not increased by twice daily application of 0.025% tretinoin to more than 40% of body area over 1 month. Furthermore, controlled topical administration of tretinoin at doses used for acne therapy (2 g of 0.025% gel applied daily to the face, neck, and upper part of the chest for 14 days) has less influence on plasma levels of endogenous retinoids than diurnal and nutritional factors.51 Indeed, a large, population-based study demonstrated no excess risk of birth defects in offspring born to mothers who were exposed to topical tretinoin during pregnancy.52 Therefore, no evidence exists for teratogenicity of topical tretinoin in humans.

“Sun sensitivity” is a frequently discussed subject with topical retinoid use and requires clarification. When formally tested in humans, topical tretinoin does not lower the minimal erythema dose of UVB light.53 Because its presence in human skin does not increase the likelihood of sunburn reaction, tretinoin is not a phototoxic agent. The patients who complain of such sun sensitivity, describe an uncomfortable skin sensation that is felt within minutes of being in the sun rather than hours later. This timeline is not consistent with a typical sunburn reaction, which takes a few hours to be noticed. Furthermore, this sensation is often said to be accentuated in warmer temperatures, which suggests participation of infrared irradiation (heat).

Related to sun sensitivity is the issue of photocarcinogenesis. In an animal model of photocarcinogenesis, topical tretinoin has caused skin cancer. However, when human skin was grafted onto mice with severe combined immunodeficiency disease, gross inadequacy of the commonly used rodent model of photocarcinogenesis was demonstrated.54 Specifically, the traditional rodent models significantly overestimate the human carcinogenic potential of tested agents. Topical retinoids, on the contrary, appear to have a protective effect against UV-induced premalignant and malignant lesions. In those predisposed by nevoid basal cell carcinoma syndrome or xeroderma pigmentosum to the development of nonmelanoma skin cancer, systemic retinoids have provided effective protection. In this regard, topical tretinoin's ability to prevent UV induction of c-Jun is relevant.55 c-Jun is a proto-oncoprotein that is minimally detectable in normal human skin in vivo. Its partner c-Fos; however, is constitutively expressed. UV irradiation to human skin does not affect the level of c-Fos expression, but it markedly induces c-Jun protein, which can then heterodimerize with c-Fos, forming a complete active AP-1 transcription factor.55 The critical importance of AP-1 in mediating carcinogenic transformation of papillomas has been demonstrated.56 In human squamous cell carcinomas, c-Jun expression is elevated, and systemic retinoids prevent carcinomas of the head and neck. Mechanisms involved in this chemopreventive effect of the retinoid likely include c-Jun suppression. These clinically observed anticarcinogenic activities of retinoids are also supported by in vitro data, demonstrating that tretinoin treatment of human skin upregulates the antigen-presenting activity of Langerhans cells without concomitant increase in autoreactivity.57 Such a retinoid effect would improve cutaneous immune responsiveness to tumor antigens. Therefore, topical retinoic acid is not a carcinogen in humans.

Drug Interaction and Compatibilities

Retinoids, in general, are photolabile and therefore can be photoinactivated. Based on this chemistry, it is prudent to apply the agents in the evening rather than before the start of the day. Because the major avenue of tretinoin inactivation is through CYP26, drugs that modulate the activity of this enzyme can potentially cause drug interactions. Ketoconazole and liarozole are effective inhibitors of CYP26 and, therefore, are RAMBAs.16,25 Concurrent use of these azoles and topical tretinoin can increase the amount and prolong the half-life of tretinoin locally in the skin, thereby aggravating local side effect. Other than retinoids, no other compounds have been shown to induce CYP26.

The use of vitamin D3 and its analogs has increased in dermatology, and, in particular, for psoriasis. Vitamin D effects in human skin are largely mediated via its nuclear receptor VDR. As previously discussed in Section “Retinoid Receptors,” VDR is a member of the steroid/retinoic acid/thyroid hormone receptor superfamily. Like RAR, VDR functions in human skin mainly as a heterodimer with RXR (VDR-RXR). In contrast with retinoid signaling via RAR-RXR, in which the presence of RXR-ligand confers no additional effect, RXR-ligand provides a synergistic effect with VDR-ligand in vitamin D signaling.58 Therefore, topical retinoids that possess, or through metabolic conversion, acquire RXR selectivity can positively influence vitamin D pharmacology in human skin.