Ointments and Types of ointment bases

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Chapter: Pharmaceutical Drugs and Dosage: Semisolid dosage forms

Ointments are semisolid preparations that incorporate a lipid or hydrophobic excipient and are intended for external application to the skin or other muco-sal membranes.


Ointments

Ointments are semisolid preparations that incorporate a lipid or hydrophobic excipient and are intended for external application to the skin or other muco-sal membranes. An ointment usually contains <20% water and other volatile ingredients, such as ethanol, and >50% hydrocarbons, waxes, or polyols. Ointments are designed to soften or melt at body temperature, spread eas-ily, and have a smooth, nongritty feel. Ointments are typically used as (1) emollients to make the skin more pliable, (2) protective barriers to prevent harmful substances from coming in contact with the skin, and (3) vehicles for hydrophobic drugs.


Types of ointment bases

An ointment base forms the body of any ointment. Ointment bases are classified into four general groups: (1) hydrocarbon bases, (2) absorption bases, (3) emulsion or water-removable bases, and (4) water-soluble bases (Table 23.1).

Table 23.1 Various types of ointment bases


1. Hydrocarbon bases

Oily or oleaginous bases include hydrocarbons derived from petroleum, which are called hydrocarbon bases. These bases are anhydrous and insol-uble in water. These bases are used for their emollient effect (to hydrate the skin) and as an occlusive dressing. They cannot absorb or contain water. Thus, they can be protective to water labile drugs, such as bacitracin and tetracycline. However, they are greasy and not water washable. Thus, they can stain clothing and are generally not preferred. Oily- or fatty-base oint-ments may have hard, soft, or liquid paraffin bases, or mixtures of these, in such proportions as will render an ointment to be of suitable consistency.

Common hydrocarbon bases include the following:

·           Petrolatum: It is used as a base for water-insoluble ingredients. Yellow petrolatum or petrolatum jelly, for example, Vaseline®, melts at 38°C–60°C. Decolored petrolatum is known as white petrola-tum. Petrolatum forms an occlusive film on the skin and absorbs less than 5% water under normal conditions. Wax can be incorporated to stiffen the base. For example, yellow ointment contains 5% w/w yel-low wax and 95% w/w petrolatum.

·           Liquid petrolatum, also known as mineral oil, is a mixture of refined saturated hydrocarbons obtained from petroleum that are liquid at room temperature. It is used as a levigating agent to incorporate lipo-philic solids into ointments.

·           Synthetic esters are used as constituents of oleaginous bases. These esters include glycerol monostearate, isopropyl myristate, isopropyl palmitate, butyl stearate, and butyl palmitate.

·           Long-chain alcohols, such as cetyl alcohol and stearyl alcohol, are sometimes also incorporated in oleaginous bases. In addition, lanolin derivatives, such as lanolin oil and hydrogenated lanolin, are some-times used.

·           Plastibase® (ER Squibb & Co., Princeton, NJ) is a commercially available polyethylene-base gelled mineral oil. It is useful for the extem-poraneous preparation of ointments by cold incorporation of drugs, thus being suitable for heat-labile compounds.

2. Absorption bases

Absorption bases contain an oleaginous material and a water-in-oil (w/o) emulsifier so that they can absorb water to form or expand w/o emulsions. Absorption bases are useful as emollients, although they do not provide the degree of occlusion afforded by the oleaginous bases. Emollients are prepa-rations that soften and soothe the skin. These preparations may be used to reduce the dryness and scaling of skin. However, they are greasy because the external phase of the emulsion is oily. Absorption bases are not easily removed from the skin with water.

Absorption bases are of two types:

1. Anhydrous bases that permit the incorporation of aqueous solutions, resulting in the formation of w/o emulsions. These absorption bases are anhydrous vehicles composed of a hydrocarbon base and an addi-tive. The hydrocarbon base could be, for example, hydrophilic pet-rolatum and anhydrous lanolin. The additive is a miscible substance with polar groups (a surfactant), which functions as a w/o emulsifier. For example, cholesterol, lanosterol and other sterols, acetylated ste-rols, or the partial esters of polyhydric alcohols, such as monostearate or monooleate, can serve as additives.

2. Bases that are already w/o emulsions (emulsion bases) and permit the incorporation of small additional quantities of aqueous solutions. For example, lanolin and cold cream.

a. Lanolin is a w/o emulsion that can form an occlusive film on the skin and serve as an emollient, effectively preventing epidermal water loss. It retards but does not completely inhibit, transepi-dermal water loss. It can restore the water in the skin to a normal level of 10%–30%. Lanolin is a pale yellow substance obtained from sheep wool. It is chemically a wax, consisting of high molec-ular weight alcohols (e.g., sterols) and fatty acids. Lanolin can absorb twice its own weight of water. It is self-emulsifying and produces stable w/o emulsions. Lanolin is used to help prevent drying and chapping of the skin.

b. Cold cream is a semisolid white w/o emulsion prepared with cetyl ester wax, white wax, mineral oil, sodium borate, and purified water. Sodium borate combines with free fatty acids present in the waxes to form sodium salts of fatty acids (soaps) that act as emulsifiers. Cold cream is employed as an emollient and ointment base. For example, Eucerin cream is a w/o emulsion of petrolatum, mineral oil, mineral wax, wool wax, alcohol, and bronopol. It contains urea as the active ingredient and is used to help rehydrate dry, scaly skin.

3. Emulsion or water-removable bases and creams

Emulsion or water-removable bases are oil-in-water (o/w) emulsions. As these emulsion bases have an aqueous external phase, they are water wash-able or water removable. They are non/less greasy and occlusive than ole-aginous bases. They can be diluted with water and have a better cosmetic appearance. Highly viscous emulsion bases are commonly referred to as creams. These represent the most commonly used type of ointment base. The majority of dermatologic drug products are formulated in an emulsion or cream base.

An emulsion base has three component parts: (a) an internal oil phase, which is typically made of petrolatum and/or liquid petrolatum together with cetyl or stearyl alcohol; (b) an emulsifier; and (c) an aqueous phase. Drugs can be included in one of these phases before forming the emulsion or can be added to the formed emulsion.

Emulsion bases are of the following types:

·           Hydrophilic ointment is an o/w emulsion that uses sodium lauryl sul-fate as an emulsifying agent. It is readily miscible with water and is easily removed from the skin. A typical composition of hydrophilic ointment is listed in Table 23.2. In addition to these basic compo-nents, this base may also contain preservatives to control microbial growth. The preservative(s) could be methylparaben, propylparaben, benzyl alcohol, sorbic acid, or quaternary ammonium compounds. The aqueous phase contains the water-soluble components of the emulsion system, together with any additional stabilizers, antioxi-dants, and buffers that may be necessary for drug stability and pH control.

·           Vanishing cream is an o/w emulsion that contains a large percentage of water as well as a humectant (e.g., sorbitol, glycerin, or propylene glycol) that retards surface evaporation of water.

Table 23.2 A typical composition of hydrophilic ointment


Table 23.3 A typical composition of vanishing cream


 A typical composition of vanishing cream is listed in Table 23.3. It is a cosmetic product that is colorless when applied and is used as a foundation for powder or as a cleansing or moisturizing cream. The hydrophobic stearyl alcohol component in the formula helps to form a thin film when the water evaporates.

4. Water-soluble bases

Water-soluble bases absorb water to the point of solubility. They are water washable and may be anhydrous, or contain some water. Water-soluble bases are made of carbowax or polyethylene glycol (PEG) as the base. They are oil/lipid free and non/less occlusive. However, they may absorb water from the skin, thus dehydrating the skin, and may hinder percutaneous absorption.

PEGs are water soluble, nonvolatile, stable, and do not support the growth of mold. PEGs are polymers of oxyethylene units with different molecular weights. The number at the end of PEGs indicates their average molecular weight. Their melting point increases with increasing molecu-lar weight. Thus, PEGs with a molecular weight ≤400–600 are liquid at room temperature; PEGs with a molecular weight 800–2,000 are waxy or semisolid; and PEGs with a molecular weight >2,000 are solid at room temperature.

A typical composition of water-soluble base is listed in Table 23.4. The ointment is a blend of water-soluble PEG that forms a semisolid base. The base of PEGs alone is highly water soluble and does not allow incorpora-tion of more than 5% w/w water or aqueous solution to make an ointment. If greater quantities of water or aqueous component need to be added, a modified composition, such as with the addition of 5% w/w hydrophobic component may be used. 

Table 23.4 A typical composition of water-soluble base


A water-soluble base can solubilize water-soluble drugs and some water-insoluble drugs. The water-insoluble drugs are solubilized by the cosolvent action of the nonaqueous hydrophilic poly-mers present in the base. These bases are compatible with a wide variety of drugs.

Another water-soluble base is the ointment prepared with propylene gly-col and ethanol, which form a clear gel when mixed with 2% w/w hydroxy-propyl cellulose (HPC). This base is commonly used as a dermatologic vehicle.


Selection of ointment bases

An ointment base is chosen depending on

·           The solubility characteristics of the drug and the desired rate of drug release. For example, hydrophilic drug incorporated in an o/w base would be released immediately, whereas incorporation in a w/o emul-sion would lead to slower drug release.

·           Whether the final product is intended for drug absorption by the skin (percutaneous drug absorption) or not (topical application).

·           Typical properties of various ointment bases, such as water washabil-ity and tendency for skin occlusion.

·           Intended usage of the ointment, for example, a cosmetic use would require due attention to customer convenience factors such as water washability and nonstaining on the clothing. On the other hand, usage in a clinical setting, such as occlusive barrier on wounds that would be bandaged, might not require such considerations.


Methods of incorporation of drugs into ointment bases

In addition to the active drug, ingredients in ointment preparations can include oleaginous components, aqueous components, emulsifying agents, stiffeners, penetration enhancers, preservatives, and antioxidants. Oleaginous ointments may be prepared by levigation and fusion.

·           Levigation involves dispersing and/or grinding an insoluble drug into small particles while wet. Mixing of a base and other components over an ointment slab using a spatula can carry it out. Components such as liquid petrolatum serve as levigating agents by promoting the wetting of powders for incorporation into bases. Hydrophobic oint-ments and w/o emulsions and suspensions are typically prepared by levigation process to incorporate a powder and/or a small quantity of water or hydrophilic component into an oil base.

·           Fusion process involves melting components (such as paraffin, stearyl alcohol, white wax, yellow wax, and high molecular weight PEGs) together to form a homogeneous solution. Fusion method is used when the base contains solids that have higher melting points (e.g., waxes, cetyl alcohol, or glyceryl monostearate). This process is employed only when the components are stable at fusion tempera-tures. Hydrophilic o/w emulsions (such as water-removable ointments and creams) are typically prepared by the fusion process. The hydro-phobic components are melted together and added to the aqueous phase/water-soluble components containing an emulsifying agent with constant mixing until the mixture congeals.

Normally, drug substances are in fine powered forms before being dis-persed in the vehicle. Levigation of powders into a small portion of base may be facilitated by the use of a melted base or a small quantity of com-patible levigation aid, such as mineral oil or glycerin. Water-soluble salts of drugs are incorporated by dissolving them in a small volume of water and incorporating the aqueous solution into a compatible base.

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