Principles of Topical Therapy in Dermatology
Topical therapy is a cornerstone of clinical dermatology. It delivers the drug directly to the skin lesion, maximizing its biological effect while keeping systemic side effects to a minimum. How well a topical treatment works, however, depends not only on the active pharmaceutical ingredient (API) but also on the vehicle it is formulated in, the condition of the epidermal barrier (stratum corneum) and how the product is actually applied.
At Haute-Derma, topical treatment follows five core pharmacologic principles, with the vehicle tailored to each patient, to achieve the best possible absorption through the skin and fully restore the skin surface.
Penetration Kinetics Across the Epidermal Barrier
Stratum Corneum Penetration
The stratum corneum is the main physical barrier against outside substances, organized like "bricks and mortar." A topically applied drug can cross it by three main routes:
- Intercellular pathway: The main route for most lipophilic (fat-loving) drugs, which travel through the lipid layers between corneocytes.
- Transcellular pathway: Hydrophilic (water-loving) drugs pass directly through the corneocytes and their cytokeratin network.
- Transappendageal pathway: Rapid absorption through hair follicle openings and sweat glands. These cover less than 1% of the skin surface but matter for large molecules.
Factors That Affect the Rate of Absorption
- Hydration of the stratum corneum: A well-hydrated stratum corneum lets drugs penetrate 4–5 times better. Occlusion, or applying medication right after a warm bath, locks moisture into the tissue and widens the spaces between corneocytes.
- Stratum corneum thickness by body site: Absorption varies enormously: mucosa/scrotal skin (100%) > eyelids/face (30%) > scalp (20%) > trunk (10%) > palms/soles (1%).
- Inflamed skin: Skin that has lost its epidermal barrier (weeping, eroded or scaling) absorbs drugs into the systemic circulation much faster.
Vehicle Principles
A guiding rule in dermatology: "If it's wet, dry it with wet dressings or solutions; if it's dry or thickened, moisturize it with ointments or creams."
- Solutions and soaks (wet dressings): Cool the skin through evaporation, relieve itch, loosen crusts and cause mild vasoconstriction. Used for acute, weeping lesions (oozing contact dermatitis, acute shingles).
- Gels and lotions: Water- or light alcohol-based vehicles that leave no greasy residue. Well suited to hairy areas (scalp), skin folds and oily, acne-prone skin.
- Creams (water-in-oil or oil-in-water emulsions): Easy to spread and cosmetically pleasant. Suitable for subacute lesions, redness and mild dryness.
- Ointments (water-free lipid/hydrocarbon base): The most occlusive vehicle. Ointments prevent transepidermal water loss (TEWL) and maximize drug penetration. Used for chronic, dry, thickened or lichenified lesions (lichen simplex chronicus, psoriasis).
Comparison of Topical Vehicle Properties
| Vehicle | Oil / Water Composition | Main Biological Effect |
|---|---|---|
| Solution / lotion | Water / alcohol > oil | Cooling, evaporative, drying |
| Gel | Polymer dispersed in water | Dries quickly, non-greasy |
| Cream | Oil-and-water emulsion | Light moisturizing, cosmetically pleasant |
| Ointment | Oil / petrolatum (100% lipid) | Occlusive, seals in moisture, deep penetration |
| Paste | Fine powder (zinc oxide) + ointment | Protective, reduces friction, absorbs fluid |
Fingertip Unit - FTU
One of the most common reasons for treatment failure or side effects is applying too little or too much medication. Dermatologists use the fingertip unit (FTU) to standardize dosing.
- What is 1 FTU? The amount of ointment or cream squeezed from a tube with a 5 mm nozzle along an adult's index finger, from the tip to the crease of the last joint.
- Weight and area equivalents: 1 FTU ≈ 0.5 g of medication, enough to cover an area equal to two adult palms (about 2% of body surface area, BSA).
FTU guide for adult body areas:
- Face and neck: 2.5 FTU (~1.25 g)
- One hand (both sides): 1 FTU (~0.5 g)
- One arm and forearm: 4 FTU (~2.0 g)
- One foot: 2 FTU (~1.0 g)
- One thigh and lower leg: 8 FTU (~4.0 g)
- Back, or chest and abdomen: 7 FTU each (~3.5 g)
How to Apply Topical Medication Correctly at Haute-Derma
Cleanse and Prepare the Skin: Gently wash the affected skin with warm water or a mild soap-free cleanser. Pat dry with a soft towel, leaving the skin slightly damp.
Optimal Application Technique
- Squeeze out the right number of FTUs for the area to be treated.
- Dot the medication evenly over several points in the affected area with a fingertip, then spread it gently in the direction of hair growth. Avoid rubbing hard, which can irritate small blood vessels.
Combining Medication and Moisturizer
- Apply the treatment (corticosteroid, TCI, retinoid) first, wait 15–20 minutes for it to absorb fully, then apply a barrier-repair moisturizer on top.
- If the skin is very dry, a moisturizer can be applied first to soothe it, with the treatment layered on top.
Preventing Tachyphylaxis and Side Effects: Potent topical corticosteroids are used as intermittent (pulse) therapy: applied 2 days a week (Saturday and Sunday) or alternated with a TCI (tacrolimus) to avoid tolerance and skin atrophy.
