I. INTRODUCTION.
Drugs applied to the surface of the skin have four possible outcomes. (1) They may remain on the surface, where they may exert their medicinal qualities. (2) They may have local effects on the stratum corneum. (3) They may be absorbed by the skin and exert deeper effects on the epidermis and dermis. (4) They may be absorbed and exert systemic effects.
A. Structure and function of the skin
1. Functions of the skin. The skin is considered the largest organ of the body. It has several key functions in normal and diseased states, including maintenance of hydration; protection from chemical/mechanical damage, radiation, and infectious agents; sensation; aiding in motion and shape; metabolism; and communication. These functions are accomplished in part by the anatomical structure of the skin and in part by the physiological actions of the skin, especially the epidermis. These same physiological processes are responsible for the behavior of drugs applied to or on the skin. For example, desquamation is an important part of the physiologic renewal of the epidermis. Any substances, including drugs, placed on the skin will naturally be removed by this normal turnover of epithelial cells, thus limiting the residual effects of topical medication.
2. Structure of the skin. The skin consists of three basic layers, from outside to inside: the epidermis, the dermis, and the hypodermis.
a. The epidermis. The epidermis is the most crucial layer affecting drug absorption and pharmacokinetics. There are four anatomic layers in the skin. From the deepest layer to the most superficial, these are the stratum basale (basal cell layer), the stratum spinosum (prickle cell layer), the stratum granulosum (granular cell layer), and the stratum corneum (horny layer). The germinal layer is the stratum granulosum and keratinocytes move outward through these layers and undergo several important physiologic alterations, in a process called cornification. This process includes production of various proteins that contribute to the development of a thick cell envelope and production of intercellular lipids that contribute to the lipid layer found between keratinocytes. The epidermis is constantly regenerating in the basal cell layer and sloughing from the stratum corneum. The epidermal turnover time in the dog is approximately 21–25 days
(1) Keratinocytes. Keratinocytes undergo several physiologic changes as they move from the stratum basale to the stratum corneum. The cell envelope becomes thick and insoluble; however, the keratinocyte is very hygroscopic and may absorb large amounts of water or hydrophilic substances. Keratinocytes may act as reservoirs of hydrophilic substances, which then diffuse long after the topical agent has been applied.
(2) Epidermal lipids. The keratinocytes are surrounded by lipids produced in lamellar granules of keratinocytes and sebaceous glands. These lipids include ceramides, fatty acids, cholesterol, squalene, sterol/wax esters, and sphingolipids. These lipids form a bipolar layer between the keratinocytes and play a key role in the barrier functions of the skin and in keratinocyte cohesion within the epidermis. Obviously, hydrophobic, the lipids retard passage of hydrophilic substances, such as water through the layer.
B. Factors affecting cutaneous pharmacokinetics
1. Routes of drug passage. Drugs may move through the skin by movement through the epidermis, through the hair follicles, or through apocrine or sebaceous glands. The vast majority of drug movement occurs through the epidermis; however, the movement through other structures could alter the pharmacokinetics required for good drug absorption.
2. Two-compartment system (Figure 13-1). The structure of the epidermis consisting of keratinocytes surrounded by a lipid layer has been compared to a brick and mortar wall, where the keratinocytes are the “bricks” and the lipids produced by the keratinocytes and sebaceous glands surround the keratinocytes like mortar in a brick wall. This results in a lipophobic (or hydrophilic) structure, the keratinocytes, that are surrounded by a lipohilic (or hydrophobic) layer, the bipolar lipids. In order for drugs to pass through the epidermis, they must pass through the lipid layer. Logically, lipid soluble drugs are generally able to accomplish this more readily than water-soluble drugs.
3. Reservoir actions of the epidermis. Many drugs, especially hydrophilic drugs, may be concentrated within the keratinocytes or bound to epidermal sites providing a high concentration and an osmotic gradient for drugs to move passively into the deeper layers of the epidermis and dermis, even after the drug has been removed from the surface of the skin. This principle is used in topical application of systemically active medications, such as patches for pain or motion sickness.
FIGURE 13-1. Two-compartment model of the skin. Intercellular lipids allow lipophilic substances to pass through the skin more readily than hydrophilic substances. (Modified from Fig. 53.4. of Adams’s Veterinary Pharmacology and Therapeutics, 8th ed. 2001).
C. Factors affecting drug disposition
1. Properties of the topical agent
a. Properties of the active agent. Because of the lipid layer between the keratinocytes, lipid soluble drugs penetrate the epidermis more efficiently. Water-soluble drugs are more likely to be absorbed by keratinocytes, but have a difficult time penetrating the epidermis.
(1) Solubility of the drug. The solubility of the drug has a major effect on the absorption of the drug into the skin. The drug must be soluble enough to be carried in high concentrations in the vehicle, but it must not be too soluble, which would tend to hold the drug in the vehicle and not allow it to diffuse into the keratinocytes or lipid layer of the epidermis.
(a) Partition coefficient. The partition coefficient is the term that describes the affinity of a drug for the lipid phase of the skin. A higher partition coefficient suggests better movement of a drug into the lipid components of the skin, and thus better topical absorption.
(2) Concentration of the drug. Since drug movement into and through the stratum corneum is a passive process (i.e., diffusion), higher concentrations of the drug within the vehicle will promote greater movement of the drug into the keratinocytes or lipid layer.
(3) Molecular weight. Smaller molecular weight compounds will pass through the lipid barrier more readily than larger molecular weight compounds. This remains the case even with hydrophilic compounds, which in general, do not pass through the lipid layer readily.
b. Vehicle. The vehicle carries the active drug, in which it is suspended or dissolved, and provides a delivery mechanism for active agents. An oil-based vehicle allows for better movement of the drug into the lipid barrier. Vehicles may, in some cases, have therapeutic effects by themselves, for example, by functioning as occlusive dressings which increase cutaneous hydration.
c. Occlusive dressings. Occlusive dressings may include a physical dressing or a layer of the vehicle that prevents loss of the drug from the surface of the skin. Bandages, even band-aids, act as occlusive dressing by preventing removal of the drug by mechanical means or evaporation. An oil preparation may also act as an occlusive dressing when applied after other medications. Bath oil applied after bathing is an example of the use of an occlusive dressing. The oil slows down water loss through the skin and from keratinocytes that have absorbed water in the bathing process.
2. Cutaneous factors affecting drug disposition
a. Thickness of the stratum corneum. Movement of a drug through the skin involves movement through the lipid layer surrounding the stratum corneum, and in some cases, movement through the keratinocytes. Drug movement is more difficult as the stratum corneum becomes thicker and is enhanced by application on areas of the skin, such as the axillary, inguinal, and abdominal regions, where the epidermis is thinner.
b. Integrity of the stratum corneum. Damage to the stratum corneum, from excoriations or abrasions, will reduce the effectiveness of the barrier function and increase the drug absorption.
c. Alterations in physiological behavior. Desquamation is an important factor in epithelial turnover. Inflammatory diseases, metabolic disorders, and familial conditions, such as familial seborrhea, may alter the desquamation of keratinocytes by increasing or decreasing the turnover of keratinocytes. Loss of keratinocytes would definitely change disposition of any drugs using the stratum corneum as a reservoir and would increase the loss of any drug remaining on the surface of the skin.
d. Temperature. Increased temperature of the skin will allow for enhance absorption of most drugs by increasing solubility of the drug and increasing vascular flow to the area.
e. Hydration status of the epidermis. As the epidermis becomes dehydrated, keratinocytes become shrunken and the cell envelope tougher, resulting in less drug absorption.
D. Indications for topical therapy. Topical therapy using any of the agents or formulations described in this chapter may be used for a wide range of dermatological problems. The most common uses in veterinary dermatology are (1) for control of ectoparasites, (2) for control of pruritus, (3) for treatment of cutaneous infections by bacteria, yeast, and dermatophytes, and (4) as an aid for the management of seborrhea and other scaling disorders.
II. PHARMACOLOGICAL AGENTS
A. Antimicrobial agents
1. Topical antibacterial agents
a. Preparations and chemistry. Topical antibacterial agents fall into one of two main categories, antibiotics or antiseptic agents. However, keratolytic agents and other compounds may also exhibit antibacterial properties. Glycotechnology is the use of simple mono- or polysaccharide sugar moieties to competitively inhibit binding of microbes to the skin surface.
(1) Various antibiotics have been used topically, including neomycin, gentamicin, chloramphenicol, bacitracin, polymyxin B sulfates, nitrofurazone, and others. Mupirocin is a highly effective topical antibiotic in an ointment formulation. It is highly effective against staphylococci and many gram-negative bacteria.
(2) Antiseptics used on the skin include alcohol, acetic acid, propylene glycol, chlorhexidine, iodophors, phenols (e.g., resorcinol, hexylresorcinol), potassium permanganate, triclosan, and sodium hypochlorite.
(3) Many of the topical keratolytic agents found in shampoos used for antiseborrheic therapy also demonstrate potent antimicrobial properties. These include sulfur, salicylic acid, selenium disulfide, ethyl lactate, and benzoyl peroxide. Benzoyl peroxide is highly effective as a topical antibacterial agent used in a shampoo formulation.
(4) Glycotechnology. It is recognized that some monosaccharides or polysaccharides will bind to lectins found on microbes. These lectins, or binding sites, are used by the bacteria to attach to keratinocytes which contain other sugars on their surface. This competitive inhibition is incorporated in some shampoos available commercially, such as Keratolux® (containing salicylic acid) and KetoChlor® (containing chlorhexidine and ketoconazole), and will likely be found in other topical agents in the future.
b. Mechanism of action. The mechanism of action of antibiotic varies with the class of drug. Antiseptics work through various mechanisms, including denaturing proteins that damage cell walls or membranes, lowering surface tension, inhibition of essential enzymes, or by acting as reactive oxidating or alkylating agents. Mono- and polysaccharides bind to lectins on microbes, preventing their adherence to keratinocytes.
c. Therapeutic uses. Topical antibacterial therapy in the form of ointments, creams, and sprays is indicated for focal bacterial infections, such as acute moist dermatitis and wounds. Generalized bacterial dermatitis, such as folliculitis, is best managed with shampoos or rinses containing antibacterial agents such as benzoyl peroxide.
d. Administration. Topical antibacterial agents are used once or twice daily on affected areas. Shampoos containing residual agents, such as benzoyl peroxide, are administered once or twice weekly in most cases.
e. Adverse effects. Any topical antibacterial agent may potentially induce irritation or result in a contact allergic reaction. Such problems are infrequently reported.
2. Topical antifungal therapy
a. Topical therapy for yeast (Malassezia pachydermatis)
(1) Preparations and chemistry. Several antibiotics and antiseptics have efficacy against yeast including azoles (miconazole, clotrimazole, enilconazole, and ketoconazole), nystatin, iodine, chlorhexidine, selenium sulfide, and lime sulfur.
(2) Mechanism of action. Antiseptics, such as chlorhexidine and iodine, work through various mechanisms. The azole compounds inhibit ergosterol synthesis, which disrupts fungal cell membranes.
(3) Therapeutic uses. Anti-yeast medications are indicated to treat localized or generalized yeast infections. Their primary use is to manage infections caused by M. pachydermatis.
(4) Administration
(a) Localized infections. Malassezia are found in areas of higher humidity and temperature, such as lip folds, interdigital areas (both dorsally and ventrally), axillary and inguinal regions, or any intertriginous area.
i. The goal is to clean and dry the affected area. This may be done using detergents or commercial products designed with astringents included in the formulation.
ii. Excellent products for “spot” treatment of focal lesions include towelettes containing miconazole and chlorhexidine (Malaseb Toweletts®) or acetic acid plus boric acid (Malcetic Wipes®). The latter product is safe around the eyes, making it ideal for treating periocular infections.
(b) Generalized Malassezia infections. Generalized or multifocal yeast infections require wider distribution of medication. Shampoos or rinses are best suited for this purpose.
i. Active ingredients found in shampoo or rinse formulations that are effective in controlling Malassezia infections include selenium sulfide, miconazole, ketoconazole, enilconazole, lime sulfur, acetic acid-boric acid, and chlorhexidine.
ii. Shampoos must be applied 2–3 times weekly to effectively control yeast infections of the skin. Contact time of 10 minutes is important to achieve the best results from these agents.
(5) Adverse effects. Patients may rarely develop irritation or hypersensitivity to the active agent or other ingredients of the products. Clinical signs may include redness or pruritus following topical application. A patient may rarely develop urticarial reactions to topical medications.
b. Topical therapy for dermatophytosis
(1) Preparations and chemistry. Agents found in commercial veterinary formulations with activity against dermatophytes include (in decreasing order of effectiveness) lime sulfur, enilconazole, chlorhexidine, povidone iodine, and ketoconazole. The combination of miconazole and chlorhexidine appears to have superior activity over the use of either agent alone. Antiseptics, including sodium hypochlorite, also have some activity against dermatophytes.
(2) Mechanism of action. The agents work in various manners. The azoles inhibit ergosterol synthesis in fungal cell walls (see Chapter 15 for detailed information).
(3) Therapeutic uses. Topical antifungal agents are used as adjunctive therapy for dermatophytes infections in companion animals. They are often used as the sole treatment in large animals.
(4) Administration.
(a) Formulations. Antifungal agents may be administered in lotions (e.g., Conofite®), sprays or rinses (e.g., Malaseb Rinse®), and shampoos (e.g., Malaseb® shampoo, Miconazole® shampoo, KetoChlor®).
(b) Application. Products are generally applied once or twice weekly as adjunctive therapy to systemic antifungal therapy.
(5) Adverse effects. Possible adverse effects include irritation or hypersensitivity reactions to ingredients of these formulations. Products should be used with caution around eyes to avoid irritation.
B. Antipruritic agents
1. Preparation and chemistry. Substances present for topical application that reduce pruritus include glucocorticoids (e.g., hydrocortisone), antihistamines (e.g., diphenhydramine), hydrating agents, sulfur, tar, topical anesthetics (e.g., lidocaine, pramoxine), and cooling agents (e.g., methol, camphor). Any topical therapy that moisturizes the skin tends to reduce pruritus, since dry or dehydrated skin exacerbates pruritus. This may include bathing and/or the use of occlusive dressings.
2. Mechanism of action. There are many mechanisms for induction of pruritus, which is thought primarily in dermatology conditions to arise from irritation of unmyelinated nerves by various cytokines (e.g., leukotriene B4, kallikreins, prostaglandins), external enzymes (e.g., proteases), and other inflammatory mediators (e.g., kinins, opioids, acetylcholine). As might be expected, there are many mechanisms of action to disrupt that sensation. Glucocorticoids work by reducing the inflammation and the production of various cytokines that induce pruritus. Topical anesthetics are presumed to desensitize the peripheral nerves or receptors that trigger pruritus. The cooling agents will substitute a cool sensation that functionally may displace the sensation of itch. Cool soaks or dressings work to reduce the pruritic threshold thorough several mechanisms including desensitizing nerve receptors.
3. Therapeutic uses and administration. Topical antipruritic agents may reduce pruritus in allergic, parasitic, seborrheic, neoplastic, and other skin conditions where inflammation or dry skin is present. Topical therapy is rarely effective as the sole treatment for moderate or severe pruritus. It is best used as adjunctive therapy or for spot treatment of focally pruritic lesions. Hydrating agents works best in allergy skin diseases and conditions where the skin is dehydrated (i.e., dry). Spot treatment of focal lesions may be applied once or more daily to affected areas in lotions, creams, ointments, or sprays. Generalized pruritus is best managed with agents formulation in shampoos.
4. Adverse effects. In general, these topical antipruritic agents are safe. Rarely, a patient may show focal cutaneous irritation at the site of application. Repeated applications of topical anesthetics may result in a contact allergic reaction, although is not considered a problem for products containing pramoxine.
C. Anti-inflammatory and immunomodulating agents
1. Preparations. Various substances may have anti-inflammatory effects, including cool water applications, glucocorticoids, dimethyl sulfoxide (DMSO), aloe vera, and others.
a. Glucocorticoids vary greatly in their topical potency. The more common topical glucocorticoids for skin and ear use are, in order of increasing potency, hydrocortisone, prednisone and prednisolone, triamcinolone, dexamethasone, betamethasone, mometasone, and flucinolone. The salt used to formulate the glucocorticoid will affect the pharmacokinetics, especially t½. For example, triamcinolone generally has a stronger antiinflammatory effect on the skin and ears than dexamethasone, despite their opposite potencies in vivo.
b. Tacrolimus. Tacrolimus is a macrolide produced by Streptomyces tsukubaensis. It is available for topical administration as a 0.03% and 0.1% ointment (Protopic®), although the higher concentration is primarily used in veterinary dermatology. Pimecrolimus (Elidel Cream®) is a similar drug that is also available commercially but is not used extensively in veterinary medicine.
c. Imiquimod. Imiquimod is an immune response modifier sold as a 5% cream (Aldara®).
2. Mechanism of action. The mechanisms of action vary in this group.
a. Glucocorticoids, the most commonly used anti-inflammatory drugs, exert their anti-inflammatory properties by decreasing migration of and phagocytosis by inflammatory cells, decreased elaboration of C-reactive protein, suppression of cytokine production and release from inflammatory cells, and prevention of the release of lysosomal enzymes that cause tissue damage and generation of leukotactic substances.
b. Tacrolimus has antiinflammatory and immunomodulating effects through calcineurin inhibition, which results in inhibition of various cytokines (interleukin-2, 3, and 4; TNF-a, and interferon) and down-regulation of cytokine expression in inflammatory cells. Tacrolimus has similar properties as cyclosporine but is reportedly 10–100 times more potent when applied topically.
c. Imiquimod actives immune cells by ligating toll-like receptors (TLR7, TLR8), resulting in cytokine production and release (interferon-a), interleukin-6, and tumor necrosis factor-α. In addition, imiqimod may activate Langerhans cells to activate additional immunoregulatory responses.
3. Therapeutic uses and administration.
a. Topical anti-inflammatory agents are used to reduce the inflammation caused by various skin disorders. They are used most often for their antipruritic activities.
b. Tacrolimus has been shown to be effective in the management of perianal fistula, effective in reducing the severity of clinical lesions in atopic dermatitis, and for management of various autoimmune skin conditions, such as discoid lupus erythematosus.
c. Imiquimod has been used in human medicine to manage various cutaneous neoplasms (e.g., basal cell carcinoma), precancerous lesions (e.g., Bowen’s disease), genital warts, and actinic (i.e., solar) keratosis. In veterinary medicine, the drug has been shown in limited studies to be effective in managing sarcoids in horses. Other potential uses, that need additional investigation, include Bowen’s disease in cats and cutaneous herpes virus infections.
4. Adverse effects
a. All topical glucocorticoids are absorbed and may exert systemic effects, though the degree to which they are absorbed varies from agent to agent with the formulation and salt used. For example, mometasone has significantly more potency than betamethasone, but is absorbed much less and therefore is safer for prolonged topical application. Specific adverse effects may be iatrogenic hyperadrenocorticism, suppression of the hypothalamic-pituitary-adrenal axis, and cutaneous manifestations of hyperadrenocorticism (e.g., comedone formation, cutaneous atrophy, alopecia).
b. Tacrolimus is very safe when applied topically, though gloves should be worn by humans applying the medication. Rarely, mild erythema may be seen at the site of application.
c. Imiquimod does induce inflammation, so cutaneous erythema, edema, and pruritus may be seen following topical application. Gloves should be worn by humans applying this compound.
D. Astringents
1. Preparations and chemistry. The principal astringents are salts of aluminum, zinc, iron, and bismuth or salts that contain these metals (e.g., permanganates). Tannins and other polyphenolic compounds also act as astringents. Commonly used astringents in veterinary medicine include zinc sulfate and aluminum acetate (Domeboro® solution).
2. Mechanism of action. Astringents are substances that precipitate protein, reduce permeability of the cell membrane, and reduce transcapillary movement of plasma proteins, and thus inflammation, edema, and exudation.
3. Therapeutic uses. Astringents are used to dry the skin, toughen the skin, and promote healing. They are also used to coagulate blood (i.e., styptic actions).
4. Administration. These agents are applied topically to dry moist areas. They are helpful in the topical management of acute moist dermatitis and other moist pyoderma, for irritant dermatitis (e.g., urine scald), and for drying the ear canal in suppurative otitis externa and yeast infections of the ears.
5. Adverse effects. Astringents may cause mild irritation following topical application. Overuse may cause excessive drying of the skin, which can result in secondary infections.
E. Demulcents
1. Preparation and chemistry. Demulcents may be applied as sticky lotions that adhere to the skin or as powders that mix with fluids excreted from the affected area. Substances that have demulcent properties include alginates, mucilages, starches, dextrins, gums, sugars, and polymeric polyhydric glycols. The most common demulcents used for skin and ear application in veterinary dermatologic formulations are propylene glycol, polyethylene glycol, and glycerin.
2. Mechanisms of action. Demulcents adhere to the skin to provide physical protection and to allow prolonged topical exposure of any drug that might be suspended in the preparation used. Some demulcents, such as propylene glycol, have bacteriostatic and fungistatic properties. Most demulcents have hydrophilic colloidal properties that allow them to serve as emulsifiers and suspending agents for water-soluble agents.
3. Therapeutic uses and administration. Demulcents are protective agents that are used to relieve irritation and irritation, especially of abraded tissues or mucous membranes. They are applied topically to irritated areas. Propylene glycol is commonly used because of its antimicrobial actions and physical properties: nongreasy, nonstaining, and the ability to spread easily over the surface.
4. Adverse effects. Topical irritation may be seen in some animals following repeated application of propylene glycol on the skin or in the ear canal.
F. Adsorbents
1. Preparations and chemistry. Adsorbents are inert and insoluble substances, generally fine powders, that may consist of starch, calcium carbonate, boric acid, zinc oxide, titanium dioxide, or talc.
2. Mechanism of action. Adsorbents generally absorb moisture on the skin and act as dessicants.
3. Therapeutic uses and administration.
a. Adsorbents may be used as a vehicle for parasiticides or antimicrobial agents.
b. Powders may be used to absorb moisture on the skin as long as they are not allowed to form adherent crusts.
4. Adverse effects. If applied to wet surfaces, adsorbents may form crusts of powder and liquid. Powders may promote granulation tissue, especially if used within body cavities.
G. Protectives
1. Preparations and chemistry. Mechanical protectives may form an adherent film that may be flexible or inflexible. Ingredients considered as protectives include lanolin, mineral oil, olive oil, zinc stearate, petrolatum, silicones, and various polymers. Several products recently introduced into veterinary dermatology contain sugars that may serve as protectives against microbes.
2. Mechanism of action. A protective may provide a physical and/or occlusive layer on the skin. In addition, these materials may also serve as a vehicle for medications. These agents may protect from physical (e.g., radiation) or chemical insult to the skin. The most recent agents released in veterinary medicine in this class are shampoos containing monosaccharides.
3. Therapeutic uses and administration.
a. Protectives may be applied to wounds or nonhealing ulcers.
b. Petrolatum or other protectives may be applied daily around inguinal region to protect the skin from irritation from urinary or fecal incontinence.
4. Adverse effects. Protectives may be occlusive and may therefore allow infections to spread. They should not be used without the appropriate antimicrobial therapy in infected skin.
H. Humectants and moisturizing agents
1. Preparations. Many substances have hygroscopic qualities and will attract water, including α-hydroxy acids (lactic acid, malic acid, glycolic acid, and others), urea, colloidal oatmeal, propylene glycol, glycerin, and DMSO.
2. Mechanism of action. These agents generally attract water molecules, which are chemically bound to the agent. They are often used in conjunction with an emollient to retain water that has been attracted or bound to the skin.
3. Therapeutic uses and administration. Humectants are indicated whenever the skin is dry or dehydrated.
4. Adverse effects. Adverse effects are uncommon. Overuse could potentially lead to excessive maceration of the skin.
I. Emollients
1. Preparations and chemistry. Emollients are bland fatty substances that are applied locally to the skin. Numerous animal fats and oils may act as emollients, including lanolin and lard. Sources found in vegetable oils include olive oil, castor oil, cottonseed oil, corn oil, coconut oil, peanut oil, persic oil, sesame oil, and cocoa butter. Waxes and other hydrocarbons, such as glycerin, isopropyl myrisate, beeswax, paraffin, and petrolatum also serve as emollients. Products containing fatty acids and phytosphingosines will also act as emollients when applied topically.
2. Mechanism of action. Emollients act to increase the tissue moisture content by (1) preventing moisture loss by acting as an occlusive dressing or protective, (2) increasing the water holding capabilities through the use of humectants, and (3) altering the desquamation of the stratum corneum.
3. Therapeutic uses and administration. Emollients are indicated in patients with dry and scaly skin to increase cutaneous hydration and soften the skin. Their chief use is as a vehicle for many lipid-soluble topical medications. Emollients used after shampoo therapy will increase or reduce evaporative water loss from the skin by forming an occlusive dressing.
4. Adverse effects. Emollients may result in retention of moisture that may trap bacteria and possibly favor growth of some organisms, including anaerobic bacteria. Some emollients, such as lanolin, may promote formation of comedones.
J. Keratolytic and keratoplastic agents (antiseborrheic agents)
1. Preparations. Benzoyl peroxide, urea, salicylic acid, sulfur, selenium disulfide, resorcinol, coal tar, various phenols, and sulfur all have keratolytic actions. Tar (from coal or pine distillation), sulfur, salicylic acid, and sulfur have keratoplastic properties.
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