Using topical therapy: how, why, when? (Proceedings)

The key to success when utilizing topical therapy involves an accurate assessment of the lesion or problem as well as an understanding of the basic principles of topical preparations. In addition, knowledge of the various formulations available as well as of the drugs and their limitations when applied topically, along with choosing an appropriate physical mode of therapy are all critical factors to consider when utilizing topical therapy.

Pharmacokinetics: the skin

The skin serves as a protective organ and maintains an effective barrier, thus preventing escape of water and essential electrolytes as well as keeping harmful substances out. This is accomplished via a two-compartment system comprised of hydrophobic lipids and hydrophilic proteins. Vehicles utilized in topical therapy serve not only to increase penetration, especially through the hydrophobic lipid portion, but also to decrease permeation and target the skin appendages, in some situations. Concentration and solubility of drug in the vehicle agent also play a role in the effectiveness of a topical product. Most topically applied drugs are absorbed systemically to some degree, influenced by dermal blood flow, increased body temperature and vasoconstriction.

Types of topicals

Emulsifying agents

Emulsions are a dispersion of two or more immiscible liquid phases. Alone they are unstable and will separate, however when combined with an emulsifying agent, such as an ionic (sodium lauryl sulfate) or nonionic (polysorbate, sorbitan ester) surfactant they can be used effectively in topical form.

Liquid preparations

These come in the form of wet dressings, baths or soaks, lotions and aerosols or sprays. Wet dressings are used to hydrate the skin and may be useful when the goal is to debride necrotic or devitalized tissue. Bathing therapy serves to cleanse the skin and hair coat, deliver an active ingredient to the affected skin and is very helpful in decreasing exposure to environmental allergens. This is especially of importance when managing patients with atopic dermatitis. Hydrotherapy can either rehydrate or even dehydrate the skin when an occlusive type rinse is not part of the regimen. Novasomes and spherulites are types of sustained-release microvesicle technology that improves the ability to maintain skin hydration, in particular. Rinses or dips are medications in a water-soluble form, typically associated with antiparasitic agents and antifungal agents. Lotions are suspensions of powders in water that dry leaving a fine powder of medication behind. The residual, or leave-on products are available in this form.

Solid preparations

Powders are uncommonly used in veterinary medicine since they are often messy and difficult to use in abundantly haired patients. They do promote drying and commonly contain talk or some type of starch. Some formulations contain potent topical corticosteroids that can cause significant suppression of the hypothalamic-pituitary-adrenal axis if used frequently.

Semisolid preparations

Most of the localized topical agents utilized in veterinary medicine come in the form of creams, gels, ointments, pastes and fixed dressings. Creams are emulsions of oil-in-water that are miscible with the skin and absorb water, thus they are non-occlusive and often desirable treatment delivery forms for our dermatologic patients. Gels are colloidal dispersions that liquefy upon contact with the skin and are also non-occlusive in nature. Ointments can be water soluble, containing polyethylene glycol, or emulsifiable, containing water-in-oil mixtures. Water repellant ointments contain white petroleum or mineral oil. Most are occlusive, however these provide better penetration than creams or lotions. Incorporating powders into an ointment creates a paste. They are well tolerated with residual activity, however because they are messy they are rarely used in veterinary patients.

Indications for topical therapy

A few clinical situations exist that can be managed solely with topical therapy, however most topical agents are utilized as adjunctive treatments. Bacterial and fungal skin infections are typically treated both systemically and topically, however adjunctive therapies are a large part of the treatment regimen. Allergic and/or pruritic conditions also lend themselves well to topical therapeutics.

Limitations to topical therapy

Veterinary patients are hairy, thus the challenge when prescribing topical therapy. Patients often do not tolerate topical medications either, thus another challenge when contact time with the affected area is important to successful therapy. Many diseases are more generalized or multifocal which can also prove difficult, and client compliance is also of concern when product application is messy and / or the patient resists application of the prescribed medication. Additional limitations to topical therapy involve the lack of availability of single ingredient preparations. Most topical agents are polypharmaceuticals that contain an antibacterial agent, an antifungal agent as well as a glucocorticoid. Most indications for topical therapy do not require all 3 types of ingredients; frequently the glucocorticoid is not indicated or even contraindicated in the treatment of infectious dermatologic diseases.

Other types of topical treatments

Astringents are used to "dry" the skin; humectants are used to "wet" the skin. Astringents act by precipitating proteins via the use of tannins (witch hazel), aluminum acetate (Burrow's solution), or 0.9% saline. Humectants utilize hygroscopic substances that attract water, such as colloidal oatmeal, propylene glycol, urea and lactic acid, at lower concentrations. Emollients are occlusive agents that increase hydration by preventing water loss via the use of oils or bandages.

Functions of topical therapy

Topical therapies can be recommended for a multitude of reasons, however most commonly they are used to treat cutaneous infections, control pruritus, remove allergens, eradicate cutaneous parasites and for management of seborrheic conditions.

Antibacterial and Antifungal Therapy

This is the group of topical therapies utilized most frequently in our practice. Multiple options are available and these are typically utilized as adjunct therapies. Typically chlorhexidine, benzoyl peroxide and ethyl lactate are the most frequently chosen ingredients; iodine is not frequently used in the small animal section of our practice due the presence of organic material in association with secondary bacterial skin infections in dogs, primarily.

A large number of our canine patients have both bacterial and fungal (Malassezia spp.) infections, thus combination products are heavily utilized; shampoos, medicated wipes, sprays and flushes are often prescribed for our patients. Lime sulfur dip/spray is another product utilized for it's antiparastic activity (Demodex gatoi in cats, Sarcoptes scabiei in dogs), antipruritic activity as well as a topical agent necessary for the successful treatment of dermatophytosis in cats (in combination with oral antifungal therapy).

Antipruritic Therapy

Bathing therapy is of paramount importance for management of the allergic patient, particularly the environmentally allergic patient. Decreasing exposure to allergens via manual removal and providing hydrotherapy to the skin plays an important role in management of pruritus in these patients. These agents often provide some steroid-sparing benefits as well and many serve as adjunct treatments, often used in combination. Colloidal oatmeal, topical anesthetics, topical hydrocortisone and menthol are all often helpful in reducing the severity of pruritus while other therapeutic strategies are utilized, such as allergen specific immunotherapy, cyclosporine therapy or oral corticosteroid therapy.

Antiseborrheic Therapy

Shampoos are the mainstay of therapy frequently contain both keratolytic and keratoplastic agents. These include crude coal tar, sulfur, salicylic acid, selenium sulfide and benzoyl peroxide. Crude coal tar is an excellent keratoplastic agent and carries the benefit of being antipruritic in addition to being a great degreasing agent as well. Unfortunately, it can also be irritating, drying and phototoxic and can stain lightly colored hair coats and fabrics. Sulfur is generally found in combination with a tar product or with salicylic acid. It is mildly keratolytic and keratoplastic, but the activity is increased when combined with salicylic acid. Sulfur at higher concentrations also has antiparasitic and antifungal properties. Salicylic acid is also keratolytic and keratoplastic and the activity is synergistic when combined with sulfur. Selenium sulfide is a potent degreasing agent, thus the anti-Malassezia spp. effect is enhanced. It is also keratolytic and available over the counter. Benzoyl peroxide is a very commonly used ingredient and provides degreasing and follicular flushing action in addition to the keratolytic and antibacterial benefits.

When treating severely affected seborrheic animals, often a potent degreasing shampoo is applied first, then rinsed, and a humectant or emollient product may follow. This type of regimen is particularly useful in the treatment of dogs with sebaceous adenitis. With the advent of the new topical agents, such as those that contain phytosphingosine, additional options are available for the patient with a primary seborrheic condition.

The availability of topical products now marketed for veterinary patients can be overwhelming at times since it is impossible to stock every product from every manufacturer. Knowing what to expect from the active ingredients in each product and finding the right characteristics associated with that product will aid in the successful utilization of topical therapy in the dermatologic patient.