External preparations

36


External preparations




Study Points



image Skin structure and sites of action of drugs


image The types and functions of solid, liquid and semi-solid skin preparations


image The ingredients used in skin preparations


image Dispensing preparations for use on the skin


image Transdermal drug delivery for systemic activity



Introduction


Skin is the largest organ in the body and has three distinct regions. The hypodermis is the innermost and is often called subcutaneous fat. The dermis is the bulk of the thickness of the skin and contains blood vessels, nerve fibres, sweat glands and hair follicles. The outermost region is the epidermis, which is made up of several layers. One of these layers is the stratum basale, in which cells divide and as they move towards the surface, they change appearance and function. The outermost layer, the stratum corneum, acts as the skin barrier. It is made up of about 20 layers of dead keratinized cells. The hair follicles and sweat ducts pass through the stratum corneum to reach the surface. A simplified diagram showing the main skin structures is given in Figure 36.1.



There are a large number of diseases which may affect different regions of the skin. Any drug used will need to reach the site of the disease in order to act. Unless it is for a surface effect only, the drug must either pass through the stratum corneum or go through hair follicles or sweat ducts. Examples of drugs applied to the skin and their sites of action are shown in Figure 36.1. Once in the skin, a lipid-soluble drug will tend to accumulate in lipid regions, while more water-soluble drugs will tend to enter the blood capillaries and be removed from the skin. There are also many metabolic enzymes in the skin which can deactivate drugs.


Effective formulation makes it possible to achieve adequate and reproducible percutaneous absorption, which is close to zero order kinetics. As a consequence, absorption through the skin from toxic materials is possible and so gloves should always be worn when preparing external preparations.


There are an increasing number of drugs that are effective against skin diseases, but drugs are not the only way of treating skin conditions. Creating physiological changes in the skin can also be beneficial. The main change is to control the moisture content of the skin. Normal skin has 10–25% moisture in the stratum corneum. This level may be reduced in, for example, eczema, or increased, as in skin maceration between the toes. By using an occlusive product (that is, an oily product), water leaving the body through the skin will be trapped and the moisture content will increase. These products are called emollients. An excess of moisture may be removed using an astringent, a hygroscopic material or, to a lesser extent, a dusting powder. Where an oily vehicle is needed, but moisture must not increase, adding solid particles to the vehicle will allow water to escape. Lubrication of sensitive skin is achieved by using finely divided solids, applied either as a powder or, more efficiently, as a suspension. Cooling the skin relieves inflammation and eases discomfort. It is achieved by evaporating a solvent, usually water or a water and alcohol mixture. Volatile solvents sprayed on the skin give intense cooling.



Types of skin preparation


There are a large number of different types of external medicine, ranging from dry powders through semi-solids to liquids. The names are often traditional making classification difficult. Figure 36.2 illustrates the formulation of the main types of preparation used on the skin.





Liquids


Soaks have an active ingredient dissolved in an aqueous solvent and are often used as astringents, for cooling or to leave a film of solid on the skin. Oily vehicles can be used in bath additives to leave an emollient film on the skin surface.


Lotions are aqueous solutions, suspensions (see Example 36.1) or emulsions that cool inflamed skin and deposit a protective layer of solid.



Example 36.1


Prepare 100 mL Compound Sulfur Lotion BPC.




























  Master formula For 100 mL
Precipitated sulfur 40 g 4 g
Quillaia tincture 5 mL 0.5 mL
Glycerol 20 mL 2 mL
Industrial denatured alcohol 60 mL 6 mL
Calcium hydroxide solution to 1000 mL to 100 mL

Liniments are alcoholic or oily solutions or emulsions (see Example 36.2) designed to be rubbed into the skin. The medicament is usually a rubefacient.



Example 36.2


Prepare 100 mL Methyl Salicylate Liniment BP.
















  Master formula For 100 mL
Methyl salicylate 250 mL 25 mL
Arachis oil to 1000 mL to 100 mL

Applications are solutions or emulsions that frequently contain parasiticides (see Example 36.3).



Example 36.3


Prepare 100 mL of Benzyl Benzoate Application BP.




















  Master formula For 100 mL
Benzyl benzoate 250 g 25 g
Emulsifying wax 20 g 2 g
Purified water, freshly boiled and cooled to 1000 mL to 100 mL

Paints and tinctures are concentrated aqueous or alcoholic antimicrobial solutions.


Collodions are organic solvents containing a polymer and drug.


There are also many other liquid products including shampoos and foot washes.



Semi-solids


Ointments are usually oily vehicles that may contain a surfactant to allow them to be washed off easily (barrier creams). They are used as emollients, or for drug delivery either to the surface or for deeper penetration.


Creams are traditionally oil-in-water (o/w) emulsions while oily creams are water-in-oil (w/o) emulsions. However, there are also ‘creams’ that are not emulsions. Emulsified creams usually give cooling, are less greasy than ointments and can be used for drug delivery onto or into the skin. They require antimicrobial preservatives.


Pastes are vehicles (aqueous or oily) with a high concentration of added solid. This makes them thick so they do not spread and so localizes drug delivery (e.g. Dithranol in Lassar’s Paste, see Example 36.9). They can also be used for sun blocks.


Gels are usually aqueous gels used for lubrication or applying a drug to the skin. Oily gels are also available where occlusion is required.



Ingredients used in skin preparations




Oily vehicles


Oils used in external preparations come from one of three sources.


Mineral oils (paraffins) are the most widely used. They are complex mixtures of mainly saturated hydrocarbons, which are available in different fractions. Different names are used in different pharmacopoeias (Table 36.1).



Light liquid paraffin is not normally used in external medicines. Soft paraffin is the main ingredient in many products, with liquid or hard paraffin being used to thin or thicken them respectively. There are two forms of soft paraffin: yellow and white. The latter has been bleached, residues of which may remain. As a general rule, white is used with white or pale coloured ingredients, while yellow is used for darker ingredients. The paraffins are occlusive and chemically inert, but do not give good skin penetration.


Vegetable oils come from plant sources such as castor, olive, peanut and coconut. The latter two would be rarely used because of severe reactions in patients with nut allergies.


Synthetic oils, such as the silicone oils (Dimethicone BP), are used as water repellents and occlusives because they are very hydrophobic. The semi-synthetic isopropylmyristate is similar to vegetable oil in its properties and use.



Emulsifying agents


Liquid and semi-solid emulsions, both o/w and w/o, are used externally and require the addition of emulsifying agents. The latter may also be added to an oil without water as in Emulsifying Ointment BP. The presence of a surfactant usually increases the skin penetration of any drug. A wide range of materials can be used as surfactants, either alone or in combinations. Selection is made in view of the type of emulsion required (o/w or w/o) and the charge on the other ingredients (anionic, cationic or non-ionic).




Emulsifiers – o/w


Emulsifying waxes each contain two ingredients: cetostearyl alcohol and a surface-active agent, as shown in Table 36.2. All three bases are waxy solids that mix with oily materials. Addition of water produces an o/w emulsion – a cream. Both the non-aqueous blends and the creams are easily washed off the skin. Varying the amount of bodying agent, usually cetostearyl alcohol, can control consistency. The ratio of oil to water will also alter the consistency of a cream.


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Related posts:

  1. Oral unit dosage forms
  2. Solutions
  3. Public health
  4. Emulsions

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Jun 24, 2016 | Posted by in PHARMACY | Comments Off on External preparations

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