2. Skin Delivery - Creams, ointments and other topical preparations

What does topical mean in pharmacy?

• Applied to external body surfaces for localized effect

• Can include skin, scalp, eye, nasal, buccal, rectal & vaginal areas

• In this context, refers only to skin & scalp (non-mucosal)

Why is treatment given locally to the skin?

• Enhance skin’s barrier function

• Target drug to specific skin layers (e.g. outer layer for fungal infections, deeper for eczema)

• Fewer side effects vs oral drug delivery

What are the 3 main layers of the skin & key features?

• Epidermis: outer layer; stratum corneum (outermost), stratum basale (where skin cells form); no blood vessels

• Dermis: contains blood vessels, sweat glands, hair follicles, sebaceous glands

• Subcutaneous: fatty tissue layer that cushions the skin

How does target location affect topical treatment?

• Above/on SC: easier – e.g. sunscreens, insect repellents

• Targeting SC: e.g. fungal infections

• Below SC (epidermis/dermis): harder – requires drug to cross SC = percutaneous absorption

What types of topical dosage forms are available?

• Semisolids (halfway between solid & liquid): ointments, pastes, creams, lotions, jellies, gels, liniments, collodions, plasters

• Topical solutions, soaps/shampoos, tinctures, powders, aerosols/foams, topical patches

What is the difference between occlusive & non-occlusive topical formulations?

• Occlusive: prevents water loss, ↑ skin hydration

• Good for dry skin, not for infected skin

• Forms a barrier/shield on skin

• Enhances drug penetration through skin

What are ointments & what are they made of?

• Semisolid preparations for external use on skin/mucosa

• Made from hydrocarbons (not water-soluble)

• Common ingredients:

  • Soft paraffin (main base)

  • Liquid paraffin (to thin)

  • Hard paraffin (to thicken)

What are the properties of hydrocarbon ointments?

• Occlusive & emollient

• ↑ skin hydration by trapping moisture

• Keeps skin supple

• Greasy & hard to wash off with water

• Useful if drug is unstable in water

What are water-soluble ointments?

• Washable & water-soluble

• Example: macrogol (PEG) ointment

• Longer chain length = higher melting point

• Used in:

  • Bactroban (mupirocin) for skin infections

  • Iodosorb (iodine) for wound care

What are the properties of water-soluble ointments?

• Water soluble & washable

• Non-greasy

• Non-occlusive or less occlusive than hydrocarbon ointments

• Do not lock in moisture as effectively as hydrocarbon bases

What are creams & why are they popular for topical use?

• Two phases: oil & water (O/W or W/O)

• O/W more common for drug delivery

• Need emulsifiers to keep stable & preservatives due to water content

More popular because:

• Softer, less sticky than ointments

• Spread easily, good patient acceptability

• O/W creams leave a thin film after water evaporates

• Less occlusive than ointments (don’t lock in moisture as well)

What are examples of W/O emulsifiers and creams?

• Wool alcohols (lanolin alcohols): Key W/O emulsifier, rich in cholesterol & lanesterol

• Hydrous wool fat (hydrous lanolin): 7 parts wool fat, 3 parts water, softer than wool fat

• Beeswax: Traditional W/O emulsifier

• Examples of W/O creams:

  • Drapolene Cream (for nappy rash)

  • Boots Chilblain Cream

What are examples of O/W emulsifiers and creams?

• Emulsifying Waxes (O/W):

  • Emulsifying wax BP: Sodium lauryl sulphate & cetostearyl alcohol (1:9)

  • Cetrimide emulsifying wax BPC: Cetrimide & cetostearyl alcohol (1:9)

  • Cetomacrogol emulsifying wax BPC: Cetomacrogol 1000 & cetostearyl alcohol (8:2)

• Examples of O/W creams:

  • Aqueous Cream (O/W): Emulsifying ointment (O), water (W), preservative

  • Hydrocortisone cream (O/W): Mineral oil (O), water (W), emulsifiers, co-solvents, preservative

What are the properties & an example of topical lotions?

• Properties:

  • Liquid with insoluble solids that need suspension

  • Low viscosity → spreads easily

  • Dries quickly on skin after application

• Example: Calamine lotion

  • Contains calamine, zinc oxide (solids)

  • Glycerol (moisturises), phenol (antibacterial)

  • Bentonite (keeps solids suspended), water

  • Pink colour from iron oxide in calamine

What are gels & how do gelling agents work?

• Gels:

  • Solid or semisolid preparations

  • Usually transparent or translucent

  • Require gelling agents (natural: tragacanth, pectin, alginate; synthetic: carbomer, cellulose)

  • E.g. Ibugel, isotrexin gel

• Gelling agents:

  • Form a cross-linked network in liquid

  • Gels = liquid dispersed in solid (solid is continuous phase)

  • Hydrogels form when hydrophilic polymers swell in water

What are carbomer gels & how are they formed?

• Carboxy vinyl polymers

• Form low viscosity dispersions in acidic water

• Gel forms when neutralized with alkali

How does the target site affect the choice of topical formulation?

• Sunscreens → stay on surface → use gels, sprays, creams, lotions (not ointments)

• Fungal infections → affect stratum corneum → target outer skin layers (not ointments)

• Inflamed skin → barrier issues → ointments useful (occlusive & protective)

How is transdermal delivery different from topical delivery?

• Topical → local effect, no bloodstream entry

• Transdermal → drug enters bloodstream

• Avoids breakdown in stomach/gut

• ↑ patient compliance