Ointments - Summary Pharmaceutics PDF

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Ointments: Ointments are basically just semi-solid fatty bases that are applied to the skin. They are generally emulsions. They normally just consist of a fatty base that is applied to the skin. The fatty base may have an API added to it or not. In many cases, the fatty base is the only pharmacologically active substance. They are normally just applied to the skin to act as emollients meaning that they will not allow moisture evaporate from the skin and will make the surface of the skin more pliable. Ointments following preparation will be classes into three main groups depending on their physiochemical properties. The three groups are called the hydrophobic group, the water emulsifying ointments and the hydrophilic ointments. These preparations are generally opaque/translucent and they have a very viscous consistency. When they are applied to the skin, they will not be absorbed into the skin and they will not evaporate off. This means that the main reason that they are made and applied to the skin is so that they will stop the moisture loss from the skin itself. These are preparations that are mainly composed of a greasy thick oil (80%) that is called the base. The may also be used as a means of applying lipophilic drug molecules to the skin. They have very little risk of causing sensitization or irritation of the surface of body that they are applied to for the fact that they normally contain very few ingredients beyond the oil or fat that forms the base. They are often disliked by the patient due to their greasy consistency. The ointment bases are classed into four main groups called the hydrocarbon bases, the adsorption bases, the water-removable bases and the water soluble bases. 1. Oleaginous Bases: These bases are also called the hydrocarbon bases. When they are applied to the surface of the skin they are said to have an emollient effect. This means that they will prevent the escape of water from the surface of the skin. When ointments are said to be emollient, this means that they will hydrate the skin because of the fact that they will not allow the release of sweat droplets and so they will accumulate in the skin. They are often used as occlusive dressings because of the fact that they will remain on the surface for the skin for a long period of time before they dry out. Occlusive also means that they will not allow other materials and liquid penetrate the skin when they are present on the surface. These bases will incorporate very little water when they are applied to the surface of the skin and are extremely difficult to wash off for this reason after application. Water and other aqueous bases may be incorporated into the bases but only in very tiny amounts and with a large degree of difficulty. They may have powdered API’s incorporated into the bases. The most common hydrocarbon bases are yellow soft paraffin, white soft paraffin and liquid paraffin. All of these are isolated from crude petroleum. They may be straight chain or branched molecules.

When they have carbon chains in the length of C16 to C30, they are usually fluid and when they have carbon chains in the range of C30 to C50, they are usually solids. When the fluid hydrocarbons are mixed with the liquid hydrocarbons, the molecules of the fluid will be entrapped in the matrix of the solid hydrocarbons. These are said to be single phase bases as they only contain hydrocarbons although the hydrocarbon molecules may be of different lengths. Yellow soft paraffin is just yellow petroleum jelly and white soft paraffin is just purified yellow soft paraffin. Liquid paraffin is just a mixture of different shot chain hydrocarbon molecules. 2. Absorption Bases: There are two different types of absorption bases. The first group of bases are those that will permit the incorporation of water or other aqueous solutions which will result in the formation of an W/O emulsion. The second group pf bases are those that are already W/O emulsions and they will allow for the incorporation of even more water into the base. These bases are commonly used as emollients meaning that they hydrate the skin by means of not allowing sweat droplets move out of the skin. However they do not provide the same degree of occlusion as the hydrocarbon bases. Although they do contain some aqueous phase they are not easily removed from the surface of the skin. These bases are named for their water absorbing properties and depending on the amount of water that they absorb, they can make either O/W or W/O emulsions. They are able to absorb large amounts of water but still retain a semi-solid viscosity. These bases have a high index of compatibility with most drug molecules and are good for the incorporation of water soluble drug molecules. Lanolin is an example of an absorption base. This is a waxlike substance that is obtained from the wool of sheep. The substance is cleaned, deodorized and decolourized. It will not contain more than 0.25% water once it has been purified. Additional water may be incorporated into the base when it is mixed. 3. Water Removable Bases: These are bases that are in fact oil in water (O/W) emulsions and that look a lot like creams. Due to the fact that the external phase of the base is composed of water, it will be easily removed from the surface of the skin when it is washed. It is for this reason that they are commonly called water-washable bases. The above is an attribute that makes the ointment more acceptable for cosmetic reasons. These bases may be diluted with water and they will also absorb serous (blood) discharges on the surface of the skin. This is helpful for the treatment of many dermatological conditions. Hydrophilic ointment USP is an example of a water removable base. 4. Water Soluble Base:

These are bases that are also called greaseless bases because they do not contain any hydrocarbons. They are completely water washable. When water is added to these bases, they become very soft very quick, unlike the other bases that are able to incorporate large amounts of water and still retain a semi-solid structure. This means that it is very difficult to incorporate large amounts of water into the base. They are mostly commonly used for the incorporation of solid substances such as powdered drug molecules. PEG ointment is the most common and basic form water soluble base that can be used. PEG is a polymer of ethylene oxide. These polymers may have low and high molecular weights. The high molecular weight substances will have high viscosities and the low molecular weight substances will have very low viscosities. For example when PEG 3350 and PEG 350 are combined, this will result in the formation of a very pliable semisolid ointment. How to Choose the Correct Base: There are a number of factors that must be taken into consideration when it comes to the formulation of an ointment and the exact base that is chosen will depend on all of the following factors: -The desired release rate if the drug molecule. This will be affected by the solubility of the drug in the base, the viscosity of the base and a large number of other factors. -How the drug molecule is to be absorbed into the skin. The drug molecules may be absorbed topically or subcutaneously depending on how far down into the skin the base will penetrate. -How much occlusion you ant. An occlusive base will not allow water escape from the surface of the skin while a non-occlusive base will allow water escape from the surface of the skin. -The stability of the drug molecule in the base. This will be determined by the compatibility index of the base. -How easily you want to be able to remove the base from the surface of the skin once it has been applied. -The characteristics of the surface of the sin to which you are applying to the ointment....