Topical Retinoids: Tretinoin, Adapalene and Tazarotene

Mechanism

Pharmacological Class and Group

Vitamin A derivatives for topical use of the first generation (Tretinoin/All-trans-retinoic acid), second generation (monomeric, not usually applied topically) and third generation (rigid polyaromatics: Adapalene and Tazarotene).

Mechanism of Action

Retinoids passively cross the cell membrane of keratinocytes thanks to their lipophilicity. In the cytosol, they are transported by intracellular retinoic acid binding proteins (CRABP-I and CRABP-II) to the cell nucleus, where they exert their direct epigenetic action.

Signaling of RAR and RXR Nuclear Receivers

1. Binding to Nuclear Receptors: Retinoids bind to two families of nuclear receptors belonging to the superfamily of steroid and thyroid hormone receptors: the Retinoic Acid Receptors (RAR, with subtypes α, β, γ) and the Retinoid X Receptors (RXR, with subtypes α, β, γ).

2. Heterodimerization and DNA Binding: Active receptors form stable heterodimers (RAR-RXR). This heterodimeric complex specifically binds to consensus regulatory sequences in the genome called Retinoic Acid Response Elements (RARE):

Retinoid-RAR-RXR Binding to RARE Transcriptional Activation/Repression

3. Effects on the Cell Cycle and Cohesion: They induce the transcription of terminal differentiation genes (filaggrin, loricrin) and repress the expression of hyperproliferation keratins (K6 and K16). Additionally, they reduce the cohesion of corneocytes by disorganizing the desmosomes and stimulating the natural desquamation of the stratum corneum.

4. Repression of Inflammation and Matrix Degradation: They indirectly repress the transcription factors AP-1 and NF-κB, decreasing the expression of matrix metalloproteinases (MMP-1, MMP-3) and limiting the synthesis of proinflammatory cytokines such as IL-1, TNF-α and IL-6.

Pharmacokinetics

High Resolution Pharmacokinetics

• Systemic Absorption: Minimal but variable. Absorption of topical tretinoin ranges between 1% and 5% of the total dose applied. Adapalene has negligible absorption (<0.01%), which significantly reduces its potential for systemic toxicity. Tazarotene, administered as a prodrug, is rapidly hydrolyzed in the skin to its active acid (tazarotenic acid), with a systemic absorption of approximately 1-6%.
• Protein Distribution and Binding: Circulating plasma Tazarotene is more than 99% bound to plasma proteins (albumin).
• Metabolism: The systemic remnant is metabolized in the liver by hydroxylation mediated by CYP450 isoenzymes (mainly CYP2C8 and CYP2C9), followed by glucuronidation.
• Elimination: It is excreted through the bile and fecal route, and to a lesser extent by renal excretion in the form of water-soluble metabolites.

Indicators and dose

Clinical Indications and Off-Label Uses

• Acne Vulgar (Severe, Moderate, Comedonic and Inflammatory): Cornerstone of topical treatment; They prevent the formation of microcomedones and reduce inflammatory lesions.
• Photoaging and Chronic Heliodermatitis: Reversal of epidermal atrophy, attenuation of fine wrinkles and mottled hyperpigmentation due to stimulation of collagenogenesis (type I and III collagen).
• Plaque Psoriasis (especially Tazarotene): Reduction of epidermal hyperplasia and scaling.
• Keratinization Disorders (off-label): X-linked ichthyosis, Darier disease, keratosis pilaris.

Dosage and Clinical Adjustment

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Known hypersensitivity to retinoids.
• Pregnancy in progress or immediate planning: Intrinsic teratogenic potential, although topical absorption is extremely low. The absolute precautionary principle applies.
• Concomitant use with highly erosive or peeling topical agents in the same anatomical area.

Relative Contraindications

• Active atopic dermatitis or erythematotelangiectatic rosacea (risk of severe exacerbation due to previous barrier dysfunction).
• Intense sun exposure or low phototypes prone to phototoxicity without adequate protection.

Adverse Effects (ADR) and Specific Toxicity

• Frequent (>10%): Retinoid Dermatitis (characterized by diffuse skin erythema, xeric peeling, pruritus, burning and sensation of skin tightness). It is associated with transient transepidermal water loss (TEWL).
• Frequent (1%-10%): Marked photosensitivity (reduction in the thickness of the stratum corneum that increases vulnerability to UV radiation), transient post-inflammatory hyperpigmentation or hypopigmentation.
• Rare (<0.1%): Severe eczematous eruptions requiring permanent suspension of therapy.

Drug Interactions of Clinical Relevance

• Benzyl Peroxide: First generation tretinoin is extremely chemically unstable and is immediately oxidized in the presence of oxidizing agents such as benzyl peroxide. Its application should be separated (tretinoin at night and benzyl peroxide in the morning). *Note:* Adapalene is oxidation stable and can be co-formulated or co-applied.
• Keratolytic Agents (Salicylic Acid, Glycolic Acid): Irritant synergistic effect. They drastically increase the risk of dermoepidermal erosion.

Safety in Pregnancy and Breastfeeding

• Pregnancy: Absolutely contraindicated. Although epidemiological data on topical absorption of tretinoin or adapalene do not show a significantly high rate of malformations compared to the general population, the severity of retinoid teratogenic syndrome (microtia, CNS anomalies, chronic cardiovascular defects) requires its total legal and clinical restriction throughout the entire gestational period.
• Breastfeeding: Not recommended. It is unknown whether it is excreted in clinically relevant quantities in breast milk, so it is not recommended, especially its direct application to the breast area.

Clinical

Clinical Management of Retinoid Dermatitis

Retinoid dermatitis is a dose-dependent adverse effect that usually appears during the first 2 to 4 weeks of treatment. It occurs due to the sudden stimulation of nuclear receptors and lipid disorganization of the stratum corneum:

Retinoid Application ↓ Corneocyte Cohesion ↑ TEWL Barrier Dysfunction and Local Inflammation

To mitigate this condition, titration of the application interval should be established: start with nighttime applications on alternate days for 2 weeks, use the amount equivalent to "a pea" for the entire facial surface, and use the "sandwich technique" (apply a layer of physiological moisturizing cream before and after the retinoid).