Semisolid Dosage Forms

• Semisolid dosage forms such as ointments, creams, and gels are for topical application.
• These forms can be applied to the skin, surface of the eye, or used nasally, vaginally, and rectally.
• Most semisolid preparations deliver therapeutic agents for their intended effects.
• Unmedicated preparations serve as protectants or lubricants due to their physical properties.
• Topical preparations can be used for both local and systemic drug delivery.
• Topical drugs are designed for delivery into the skin to treat skin disorders, with the skin as the target organ.
• Transdermal products deliver drugs through the skin for systemic effects, where the skin is not the target organ.
• Systemic drug absorption from topical products is a concern during pregnancy or nursing due to potential transfer to the fetus or infant.
• Penetration enhancers are sometimes added to topical formulations to increase systemic drug delivery.
• The rate of drug movement across the skin depends on the drug's concentration in the vehicle.
• Physicochemical properties of the drug substance, such as solubility, partition coefficient, pKa, and molecular weight, are factors.
• Vehicle characteristics, including hydrophilic-lipophilic balance and viscosity, influence drug movement.
• The condition of the skin, whether broken, hydrated, or otherwise affected, impacts drug absorption.
• Ointments are semisolid preparations for external application to the skin or mucous membranes.
• Ointments can be medicated or unmedicated, with unmedicated ones used for protection, emollience, or lubrication.
• Ointment bases can be used for their physical effects or as vehicles for medicated ointments.
• The USP classifies ointment bases into four groups: oleaginous, absorption, water-removable, and water-soluble.

Ideal Properties of Ointment Bases

• Stable.
• Neutral in reaction.
• Non-greasy.
• Not degreasing in action.
• Nonirritating.
• Non-dehydrating.
• Non-hygroscopic.
• Water-removable.
• Compatible with medications.
• Free from objectionable odor.
• Non-staining.
• Capable of serving as medium for drugs that are water and lipid soluble.
• Efficient on dry, oily, or moist skin.
• Composed of readily available components of known chemical composition.
• Easily compounded by the pharmacist.
• Can melt or soften at body temperature.

Oleaginous Bases (Hydrocarbon Bases)

• Oleaginous bases are also known as hydrocarbon bases.
• They have an emollient effect upon application to the skin.
• Protect against moisture loss.
• Act as effective occlusive dressings.
• Remain on the skin for extended periods without drying out.
• Oleaginous bases are difficult to wash off due to their immiscibility with water.
• Water and aqueous preparations can be incorporated in small amounts with difficulty.
• Liquid petrolatum (mineral oil) can be used as a levigating agent for incorporating powdered substances.
• Examples of oleaginous bases include petrolatum, white petrolatum, yellow ointment, and white ointment.
• Petrolatum is a purified mixture of semisolid hydrocarbons from petroleum, with a yellow to light amber color. Also known as yellow petrolatum or petroleum jelly.
• White petrolatum is a purified, wholly or nearly decolorized mixture of semisolid hydrocarbons from petroleum, also known as white petroleum jelly.
• Yellow ointment contains yellow wax and petrolatum. Yellow wax is purified wax from bee honeycomb.
• White ointment differs from yellow ointment by using white wax (bleached and purified yellow wax) and white petrolatum.

Absorption Bases

• Two types exist: those that permit the incorporation of solutions, forming w/o emulsions, and those that are w/o emulsions and allow further incorporation of aqueous solutions.
• Used as emollients.
• Not easily removed from the skin by water washing due to the oleaginous external phase.
• Do not provide as much occlusion as oleaginous bases.
• Examples include hydrophilic petrolatum, lanolin, and modified lanolin.
• Hydrophilic petrolatum contains cholesterol, stearyl alcohol, white wax, and white petrolatum.
• Commercial products like Aquaphor and Aquabase can absorb several times their weight in water.
• Useful for incorporating water-soluble drugs into oleaginous ointment bases.
• Lanolin (anhydrous lanolin) is a purified wax-like substance from sheep wool that is cleaned, deodorized, and decolorized, containing no more than 0.25% water.

Water-Removable Bases (Water-Washable)

• Oil-in-water emulsions (o/w) resembling creams.
• Easily washed from the skin due to the aqueous external phase.
• Can be diluted with water or aqueous solutions.
• Able to absorb serous discharges.
• Hydrophilic Ointment, USP, exemplifies this type of base, containing methylparaben, propylparaben, sodium lauryl sulfate, propylene glycol, stearyl alcohol, white petrolatum, and purified water.

Water-Soluble Bases (Greaseless)

• Do not contain oleaginous components.
• Completely water washable.
• Soften greatly with water addition, limiting the incorporation of large amounts of aqueous solutions.
• Example: Polyethylene Glycol Ointment NF, made with PEG 3350 and PEG 4000.

Ointment Base Selection

• The selection of the best ointment base depends on several factors. Release rate of the drug substance from the base is a factor to consider, in addition to the desirability of topical or systemic action.
• Occlusion of moisture from the skin should also be considered
• The stability of the drug in the ointment base must be considered.
• Consider the effect of the drug on the consistency of the base.
• Water washability of the base should be considered.
• Characteristics of the surface to which it is applied must also be considered
• Ointments are generally applied to dry, scaly skin, while creams are applied to weeping or oozing surfaces.

Preparation of Ointments

• Ointments are prepared by either incorporation or fusion, based on the ingredients involved.

Incorporation Method

• Components are mixed until a uniform preparation is achieved.
• On a small scale a pharmacist can mix components using a mortar and pestle, or a spatula on an ointment slab.
• Hard rubber or silicone spatulas should be used if components react with metal, such as iodine.
• The ointment is prepared by thoroughly rubbing and working the components together on a hard surface until smooth.
• Pulverization by intervention can incorporate a gummy material, such as camphor. It is dissolved in a solvent and spread out on the pill tile.
• The solvent evaporates, leaving a thin film onto which the other ingredients are spread.
• Liquid substances, or solutions of drugs, are added, considering the base's capacity to accept the required volume.
• Hydrophilic ointment bases readily accept aqueous solutions, unlike oleaginous ointments

Levigation in Ointment Preparation

• Particle size of a powder or crystalline material should reduced before incorporating it into the ointment base to avoid grittiness.
• Particle size reduction is achieved by levigating, or mixing the solid with a vehicle in which it is insoluble to create a smooth dispersion ("wet grinding").
• The levigating agent, like mineral oil or glycerin, should be compatible with the drug and base.
• The levigating agent is used in an equal volume to the solid material.
• A mortar and pestle are used in levigation.
• Levigation reduces particle size and disperses the substance in the vehicle.
• Solids soluble in a solvent that does not affect drug stability or efficacy may be dissolved and added to the ointment base.
• On a large scale, ointment or roller mills can be used to force coarsely formed ointments through stainless steel or ceramic rollers for uniformity.