Efficacy of a topically applied drug depends on three things: whether the drug is applied, the delivery of the active molecule by its vehicle, and the inherent potency of the active molecule.
Adherence to topical treatment is often poor and decreases over time.
Factors that affect penetration of applied drug include soluble medication concentration, regional variations in skin barrier properties, application frequency, and effects of the vehicle on barrier function.
The primary barrier that limits the percutaneous absorption of compounds is the stratum corneum.
The delivery of a topical agent is highly dependent on the physicochemical properties of the vehicle; this influences the kinetics of release and absorption and the onset, duration, and extent of a biologic response.
Either the vehicle or its active ingredient(s) may cause local toxicity, systemic toxicity, or both.
Topical therapy involves three key steps: topical application, percutaneous absorption, and binding of the active molecule to its target site. Sensible topical drug therapy involves selection of an appropriate agent that patients are able and willing to procure and apply (which may depend on the areas of the body affected), a vehicle that will effectively deliver the active molecule without irritation, and an active drug of the appropriate potency, along with a defined duration of use that maximizes efficacy and minimizes adverse side effects. Behind each of these considerations are basic principles that help guide the practitioner toward a rational plan of therapy.
The first principle of topical treatment is the ubiquity of poor adherence. Adherence is an often-ignored aspect of medication efficacy. Nonadherence to prescribed medicines in chronic conditions is between 30% and 50%.1,2 Poor adherence to topical treatment is a common cause of poor response to drugs and is linked with poor outcomes in diseases such as psoriasis, atopic dermatitis, and acne.3
Poor adherence may be intentional or unintentional. Intentional nonadherence is driven by the patient’s perception of the need for treatment weighed against their concern for toxicities and other costs. Unintentional nonadherence results from forgetfulness or lack of knowledge on prescribed regimens. Depression, socioeconomic status, single marital status, complicated treatment regimens, and high prescription costs are associated with nonadherence.1,4-6
There are varying patterns of poor adherence. Primary nonadherence refers to when patients do not fill their prescription or initiate treatment. Secondary nonadherence occurs when patients initiate treatment but use the medication poorly. Secondary nonadherence includes both poor execution (using the medication intermittently) and early discontinuation of treatment.
A variety of approaches have been promoted and tested as means to promote better adherence to topical treatment. Written instructions, discussing treatment expectations, and early follow-up visits after receiving a prescription can improve adherence and decrease the overall required visits. Simplifying treatment regimens, prescribing patient preferred vehicles, treatment reminders, and support groups also improve adherence.1...