Vehicles For Presenting Drugs To The Skin

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The formulations are described in order of decreasing water content. All water-based formulations must contain preservatives, e.g. chlorocresol, but these rarely cause allergic contact dermatitis.

Lotions or wet dressings

Water is the most important component. Wet dressings are generally used to cleanse, cool and relieve pruritus in acutely inflamed lesions, especially where there is much exudation, e.g. atopic eczema. The frequent reapplication and the cooling effect of evaporation of the water reduce the inflammatory response by inducing superficial vasoconstriction. Sodium chloride solution 0.9%, or solutions of astringent2 substances, e.g. aluminium acetate lotion, or potassium permanganate soaks or compresses of approx. 0.05%, can be used. The use of lotions or

2 Astringents are weak protein pr├ęcipitants, e.g. tannins, salts of aluminium and zinc.

wet dressings over very large areas can reduce body temperature dangerously in the old or the very ill.

Shake lotions, e.g. calamine lotion, are essentially a convenient way of applying a powder to the skin (see Dusting powders, p. 301) with additional cooling due to evaporation of the water. They are contraindicated when there is much exudate because crusts form. Lotions, after evaporation, sometimes produce excessive drying of the skin, but this can be reduced if oils are included, as in oily calamine lotion.

Creams

These are emulsions either of oil-in-water (washable; cosmetic 'Vanishing' creams) or water-in-oil. The water content allows the cream to rub in well. A cooling effect (cold creams) is obtained with both groups as the water evaporates.

Oil-in-water creams, e.g. aqueous cream (see emulsifying ointment, below), mix with serous discharges and are especially useful as vehicles for water-soluble active drugs. They may contain a wetting (surface tension reducing) agent (cetomacrogol). Aqueous cream is also used as an emollient (see below). Various other ingredients, e.g. calamine, zinc, may be added to it.

Water-in-oil creams, e.g. oily cream, zinc cream, behave like oils in that they do not mix with serous discharges, but their chief advantage over ointments (below) is that the water content makes them easier to spread and they give a better cosmetic effect. They act as lubricants and emollients, and can be used on hairy parts. Water-in-oil creams can be used as vehicles for lipid-soluble substances. A dry skin is mainly short of water, and oily substances are needed to provide a barrier that reduces evaporation of water, i.e. the presence of oils contributes to epidermal hydration.

Ointments

Ointments are greasy and are thicker than creams. Some are both lipophilic and hydrophilic, i.e. by occlusion they promote dermal hydration, but are also water miscible. Other ointment bases are composed largely of lipid; by preventing water loss they have a hydrating effect on skin and are used in chronic dry conditions. Ointments contain fewer preservatives and are less likely to sensitise. There are two main kinds:

Water-soluble ointments include mixtures of mac-rogols and polyethylene glycols; their consistency can be varied readily. They are easily washed off and are used in burn dressings, as lubricants and as vehicles that readily allow passage of drugs into the skin, e.g. hydrocortisone.

Emulsifying ointment is made from emulsifying wax (cetostearyl alcohol and sodium lauryl sulphate) and paraffins. Aqueous cream is an oil-in-water emulsion of emulsifying ointment.

Nonemulsifying ointments do not mix with water. They adhere to the skin to prevent evaporation and heat loss, i.e. they can be considered a form of occlusive dressing (with increased systemic absorption of active ingredients); skin maceration may occur. Nonemulsifying ointments are helpful in chronic dry and scaly conditions, such as atopic eczema, and as vehicles; they are not appropriate where there is significant exudation. They are difficult to remove except with oil or detergents and are messy and inconvenient, especially on hairy skin. Paraffin ointment contains beeswax, paraffins and cetostearyl alcohol.

Collodions

Collodions are preparations of cellulose nitrate (pyroxylin) dissolved in an organic solvent. The solvent evaporates rapidly and the resultant flexible film is used to hold a medicament, e.g. salicylic acid, in contact with the skin. They are irritant and inflammable and are used to treat only small areas of skin.

Pastes

Pastes, e.g. zinc compound paste, are stiff, semi-occlusive ointments containing insoluble powders. They are very adhesive and give good protection to circumscribed lesions, preventing spread of active ingredients to surrounding skin. Their powder content enables them to absorb a moderate amount of discharge. They can be used as vehicles, e.g. coal tar paste, which is zinc compound paste with 7.5% coal tar. Lassar's paste is used as a vehicle for dithranol in the treatment of plaque psoriasis.

EMOLLIENTS, BARRIER PREPARATIONS AND DUSTING POWDERS

Emollients hydrate the skin and soothe and smooth dry scaly conditions. They need to be applied frequently as their effects are short-lived. There is a variety of preparations but aqueous cream in addition to its use as a vehicle (above) is effective when used as a soap substitute. Various other ingredients may be added to emollients, e.g. menthol, camphor or phenol for its mild antipruritic effect and zinc and titanium dioxide as astringents.

Barrier preparations. Many different kinds have been devised for use in medicine, in industry and in the home to reduce dermatitis. They rely on water-repellent substances, e.g. silicones (dimethicone cream), and on soaps, as well as on substances that form an impermeable deposit (titanium, zinc, calamine). The barrier preparations are useful in protecting skin from discharges and secretions (colostomies, napkin rash) but they are ineffective when used under industrial working conditions. Indeed, the irritant properties of some barrier creams can enhance the percutaneous penetration of noxious substances. A simple after-work emollient is more effective.

Silicone sprays and occlusives, e.g. hydrocolloid dressings, may be effective in preventing and treating pressure sores.

Masking creams (camouflaging preparations) for obscuring unpleasant blemishes from view are greatly valued by the victims3. They may consist of titanium oxide in an ointment base with colouring appropriate to the site and the patient.

Dusting powders, e.g. zinc starch and talc,4 may

3 In the UK, the Red Cross offers a free cosmetic camouflage service through hospital dermatology departments.

4 Talc is magnesium silicate. It must not be used for dusting surgical gloves as it causes granulomas if it gets into wounds or body cavities.

cool by increasing the effective surface area of the skin and they reduce friction between skin surfaces by their lubricating action. Though usefully absorbent, they cause crusting if applied to exudative lesions. They may be used alone or as a vehicle for, e.g. fungicides.

Gels or jellies are semisolid colloidal solutions or suspensions used as lubricants and as vehicles for drugs. They are sometimes useful for treating the scalp.

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