Systemic Retinoids: Isotretinoin and Acitretin

Mechanism

Pharmacological Class and Group

First generation systemic retinoids (Isotretinoin/13-cis-retinoic acid) and second generation (Acitretin/aromatic derivative of the active metabolite of etretinate).

Mechanism of Action

Isotretinoin and acitretin operate differentially in their target tissues:

• Isotretinoin: It is a prodrug with low intrinsic affinity for the RAR and RXR receptors. However, within the sebaceous cell, it isomerized to all-trans-retinoic acid and modulates follicular keratinocyte gene expression. Its most distinctive action is the direct induction of sebocyte apoptosis, drastically reducing glandular size and sebum production by more than 90%. This alters the follicular microenvironment and secondarily inhibits the proliferation of Cutibacterium acnes.
• Acitretin: It acts mainly on the nuclear receptors of the epidermal keratinocyte (particularly RAR-γ), normalizing differentiation and stopping the accelerated epidermal proliferation characteristic of psoriatic plaques.

Epithelial and Pilosebaceous Follicle Microenvironment Modulation

1. Sebocyte Apoptosis: Isotretinoin activates the transcription of the transporter protein p53 and pro-apoptotic genes such as FoxO3, inducing the effector caspase cascade in sebaceous gland cells.

2. Regulation of Infundibular Keratinization: They prevent retention hyperkeratosis of the follicular canal, facilitating the physiological drainage of cellular debris and preventing primary comedogenic occlusion:

Isotretinoin FoxO3 Activation Glandular Apoptosis ↓ Sebum by 90%

3. Induction of Differentiation Proteins by Acitretin: Acitretin suppresses the epidermal growth factor receptor (EGFR) signal transduction pathway, decreasing psoriasiform hyperplasia and reestablishing ordered cellular cohesiveness.

Pharmacokinetics

High Resolution Pharmacokinetics

• Bioavailability: Highly lipophilic. Absorption of isotretinoin is substantially doubled when administered with high-fat meals (bioavailability jumps from 30% to 60%). There are micronized formulations that reduce this dependency.
• Distribution: High apparent volume of distribution. Binding to plasma proteins greater than 99.9% (mainly albumin).
• Metabolism: Extensive hepatic first-pass metabolism. Isotretinoin is oxidized by CYP enzymes (CYP2C8, CYP2C9, CYP3A4, CYP2B6) to its main active metabolite: 4-oxo-isotretinoin. Acitretin is metabolized by isomerization and conjugation. *Pharmacokinetics Alert:* In the presence of ethanol, acitretin is metabolically transesterified in the liver to etretinate, a highly lipophilic retinoid with a terminal elimination half-life greater than 120 days.
• Half-life (t_1/2) and Excretion: The mean plasma elimination half-life of isotretinoin is 10-20 hours; Its metabolites are cleared through the kidneys and bile within 30 days. The plasma half-life of acitretin is approximately 50 hours, but due to the collateral synthesis of etretinate in patients who consume alcohol, it is retained for a long time in reserve adipose tissue.

Indicators and dose

Clinical Indications and Off-Label Uses

• Isotretinoin: Recalcitrant nodulocystic acne vulgaris, acne with severe risk of permanent physical or psychological scarring, moderate acne refractory to oral antibiotic regimens for 3 months.
• Acitretin: Generalized pustular psoriasis (Von Zumbusch), erythrodermic psoriasis, severe plaque psoriasis refractory to other systemic therapies.
• Severe rosacea (off-label): Especially recalcitrant papulopustular rosacea or initial stages of phymas (rhinophyma at low doses).
• Prevention of Squamous Cell Carcinomas (off-label): In solid organ transplant recipients with extreme skin tumor burden.

Dosage and Clinical Adjustment

Isotretinoin (Acne):

• Starting Daily Dose: 0.5 mg/kg/day orally, administered with the main meal to ensure optimal absorption.
• Usual Maintenance Range: 0.5 to 1.0 mg/kg/day.
• Traditional Cumulative Dose: The standard clinical goal used to be to achieve a total cumulative dose of 120 to 150 mg/kg at the end of the therapeutic cycle (to reduce relapse rates). Currently, the prolonged low-dose scheme (e.g., 10-20 mg/day) guided by complete clinical resolution plus a period of 2-3 months free of lesions is also validated.

Acitretin (Psoriasis):

• Starting Dose: 25 mg to 30 mg once daily orally.
• Maintenance: Adjust doses between 25 mg and 50 mg per day, evaluating tolerance and individual clinical efficacy.

Adjustment in Liver and Kidney Failure: Contraindicated in severe liver failure. In moderate renal failure, reduce the starting dose by half and monitor closely.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Pregnancy or suspected pregnancy: High penetrance major teratogen.
• Women of childbearing age who do not comply with the regulated pregnancy prevention program (iPLEDGE or EMA prevention plans).
• Severe liver failure or underlying cirrhosis.
• Severe kidney failure (for acitretin).
• Pre-existing severe hyperlipidemia (uncontrolled cholesterol or triglycerides).
• Concomitant use with tetracyclines or exogenous vitamin A.

Relative Contraindications

• Personal history of major depression or severe suicidal ideation (requires close psychiatric monitoring).
• Diabetes mellitus or prediabetes (potential induction of insulin resistance).
• Osteoporosis or history of metabolic bone disorders.

Adverse Effects (ADR) and Specific Toxicity

• Very common (>90%): Xerotic mucocutaneous effects (painful bilateral desquamative cheilitis in 100% of treated patients, conjunctival xerosis that prevents the use of contact lenses, generalized cutaneous xerosis and recurrent epistaxis due to dryness of the anterior nasal mucosa).
• Common (1%-10%): Reversible elevation of hepatic transaminases (AST/ALT), dyslipidemia (acute increase in serum triglycerides by 25% and total cholesterol), generalized myalgias and arthralgias after strenuous exercise.
• Rare (<0.1%): Reactive clinical depression, Benign intracranial hypertension (Pseudotumor cerebri).

Critical Toxicity: Teratogenicity and Retinoid Embryopathic Syndrome

Systemic retinoids are potent teratogens that directly alter embryonic neural crest development during early organogenesis. The rate of malformations in exposed newborns reaches 20-30%, coupled with a drastic increase in the risk of spontaneous abortion:

Systemic Retinoid Alteration of Hox Genes and Neural Crest Craniofacial, Cardiac and Thymus Malformations

Typical malformations include microtia or anotia, micrognathia, cleft palate, cardiac septal defects, transposition of the great vessels, hydrocephalus, thymus hypoplasia, and subsequent severe cognitive delay. The use of double contraceptive method is mandatory from one month before starting, during treatment and until one month after suspending isotretinoin. In the case of acitretin, the period of mandatory contraception is extended to 3 years post-treatment due to the risk of hepatic conversion to etretinate.

Drug Interactions of Clinical Relevance

• Antibiotics of the Tetracycline class (Doxycycline, Minocycline): Both drugs independently increase the pressure of the cerebrospinal fluid. Its synergistic coadministration is formally contraindicated due to the imminent risk of developing Pseudotumor cerebri.
• Vitamin A (Supplements): Additive synergism that can cause a severe clinical picture of acute hypervitaminosis A.
• Alcohol: Dramatically increases the transesterification of acitretin to etretinate, prolonging tissue persistence and the risk of teratogenicity from 2 months to 3 years.

Safety in Pregnancy and Breastfeeding

• Pregnancy: FDA Category Absolute contraindication of the first order.
• Breastfeeding: Absolutely contraindicated. Being lipophilic compounds, they are actively excreted in breast milk, posing a severe risk of systemic toxicity and arrest of skeletal growth in the infant.