Two Goals That Sound the Same but Are Not
Topical and transdermal are often used interchangeably. They should not be.
A topical formulation delivers drug to the skin or the tissue beneath it. The goal is local effect: treat a skin condition, reduce inflammation in a joint just below the surface, or act on target tissue within the accessible layers of the dermis. Systemic exposure is often undesirable and should be minimised.
A transdermal formulation uses the skin as a route of administration to the systemic circulation. The goal is to bypass the gastrointestinal tract, avoid first-pass metabolism, and deliver drug at a controlled rate into the bloodstream. Systemic exposure is exactly what you want.
The formulation strategies, the regulatory requirements, and the development challenges are substantially different for each. Getting them confused at the start of a programme creates problems that are expensive to fix later.
The Skin Barrier: The Common Challenge
Whether you want to keep the drug local or get it into the blood, you have to deal with the stratum corneum. This outermost layer of skin, roughly 10 to 20 micrometres of densely packed dead keratinocytes embedded in a lipid matrix, is one of the most effective biological barriers in the body. It evolved to keep the outside world out. That function works against topical and transdermal drug delivery alike.
Flux through the stratum corneum follows Fick’s law of diffusion: it is proportional to the concentration gradient, the diffusivity of the drug in the membrane, and the membrane area, and inversely proportional to the membrane thickness. Drugs with low molecular weight (below approximately 500 Daltons), intermediate lipophilicity (log P between 1 and 4), and low melting point tend to permeate best. Most pharmaceutically active molecules do not meet all three criteria, which is why formulation science is required to make topical and transdermal delivery work for most drugs.
Formulation Types and Their Clinical Applications
| Formulation Type | Typical Clinical Use | Key Formulation Consideration |
| Creams (O/W or W/O emulsion) | Topical dermatology; anti-inflammatory; antifungal | Emulsion stability; preservative system; skin tolerability; drug partitioning into aqueous or oil phase |
| Ointments (oil-based or hydrocarbon) | Chronic skin conditions; wound healing; occlusive therapy | High oil content increases skin hydration and drug absorption; patient acceptability vs efficacy trade-off |
| Gels (hydrophilic polymer-based) | Topical NSAIDs; acne; hormonal gel formulations | Drug solubility in aqueous gel base; alcohol content and skin tolerability; spreading and drying on application |
| Patches (transdermal) | Systemic delivery of analgesics, hormones, antiemetics, smoking cessation agents | Membrane-controlled or matrix-controlled release; adhesive compatibility with drug; skin irritation from prolonged contact |
| Lotions (low-viscosity emulsion) | Large-body-surface-area application; hair loss; mild skin conditions | Ease of application; evaporation of vehicle after application; residue on skin or clothing |
| Foams (pressurised aerosol) | Scalp conditions; dermatology; rectal applications | Propellant selection; drug stability in pressurised container; foaming behaviour on skin |
Permeation Enhancers: Helping the Drug Cross the Barrier
Chemical penetration enhancers work by temporarily disrupting the stratum corneum lipid structure, increasing the fluidity of the lipid bilayers and creating pathways for drug diffusion. Common classes include alcohols, fatty acids, terpenes, and surfactants. The challenge is selectivity: an enhancer powerful enough to meaningfully increase drug flux often causes skin irritation, limiting the concentration and contact time that patients will tolerate.
Formulation development for topical products with a permeation enhancer must balance in vitro flux data against skin tolerability assessment. An enhancer that doubles flux but causes erythema in 20% of patients is not a development success.
Regulatory and Bioequivalence Considerations
For generic topical products, demonstrating bioequivalence is more complex than for oral dosage forms. A simple pharmacokinetic study is often not appropriate because systemic exposure is low and local bioavailability in the skin is what matters clinically. The FDA’s product-specific guidance documents for generic topical products describe the expected bioequivalence study designs, which may include in vitro release testing (IVRT), tape stripping studies to measure drug concentration in the stratum corneum, or clinical endpoint studies.
Ardena’s Topical Formulation Capabilities
Ardena’s formulation team has experience developing semi-solid and topical dosage forms for both branded and generic programmes. Development work includes formulation screening, in vitro permeation studies using Franz diffusion cells, and stability testing of semi-solid systems to ICH guidelines. For programmes requiring regulatory bioequivalence strategies, the team can advise on the appropriate study design based on the FDA and EMA product-specific expectations.
