PPFA 18 - Topical formulations

Describe the different types of semi-solid preparations ,common excipients and their purposes , stability and packahing of formulations , advantages and disadvantages and making semi-solids extemporaneously

What is the structure of the skin ?

Skin is the largest organ, with 3 main layers:

Hypodermis (subcutaneous fat):
deepest, fatty layer.
Dermis: contains blood vessels, nerves,
sweat glands, hair follicles.
Epidermis: outermost layer, includes the
stratum corneum (barrier made of ~20
layers of dead keratinised cells).

Hair follicles and sweat ducts pass through the
epidermis to the surface.

What are the uses of semi-soids for the skin and the types ?

Local uses:
• Inflammation
• Pain
• Itching
• Protection
• Lubricant
• Emollient

Systemic action too (transdermal)

Types- ointments,creams,pastes,gels

Describe the properties ointments and their uses ?

Greasy semi-solid preparations
Use an oily (usually anhydrous) base.
May include a surfactant to help with washing off if needed.

Uses:
• As emollients to soften and protect the skin.
• For local drug delivery to the skin.
• For systemic drug delivery via the skin.
• For application to the eyes.
• Highly occlusive, making them very effective for treating dry skin.

Describe the properties and uses of creams ?

Type: Semi-solid emulsions
Can be oil-in-water (o/w) or water-in-oil (w/o)
Effect: Cooling sensation on the skin
Appearance: Less greasy, so patients like them more
Occlusion: Less occlusive (better for moist or weeping skin)
Water content: Contain water → need preservatives

Uses:
As emollients (moisturisers)
For local drug delivery

What are some examples of creams

Examples:
1.o/w creams (e.g., aqueous cream)
Less greasy
Cooling
Need preservatives

2.w/o creams (e.g., oily cream)
Greasy and more occlusive
Form a stronger barrier on the skin

What creams should be used - clinical case

Patients with eczema often complain of dry, cracked, itchy skin.
Which type of cream—o/w or w/o—would you choose, and why?

For eczema, the most suitable cream type is generally:

Water-in-oil (w/o) creams (also called oily creams)

These are preferred because they are:
❑More occlusive → they form a protective barrier over the skin
❑Better at reducing transepidermal water loss
❑More hydrating, which helps repair the damaged skin barrier
seen in eczema
❑Less irritating for very dry, inflamed skin compared to lighter
o/w creams

Why do eczema patients not use o/w creams ?

Why not o/w creams?
-o/w creams are less greasy and more cosmetically appealing
but they are less occlusive and therefore less effective for very dry,
barrier-impaired skin like eczema.
-They also contain more water → often need more preservatives, which can occasionally irritate sensitive skin.

What are the properties and uses of pastes ?

Texture: Thick, stiff preparations
Composition: Ointments containing up to
50% solid powder
Effect: Create a thick, protective, and
impermeable layer on the skin
Purpose: Provide precise, localised treatment

Uses:
❑ For specific, well-defined areas — e.g., for
psoriasis
❑ Skin protectants — e.g., for nappy rash
❑ Sunblocks

What are the properties and uses of gels?

Structure: Two-component semi-solids
Base: A polymer forms a gel matrix (e.g., carbomer)

•Water content: High — helps drug move through the gel to the action site

Uses:
•Lubrication — e.g., US gel
•Topical drug delivery — e.g., ibuprofen gel
•Oral use — e.g., Bonjela

What are the excipients used for topical formulations ?

Oily vehicles:
Mineral oils: occlusive but limited penetration
• Liquid paraffin
• Soft paraffin – white and yellow
• Paraffin wax
Vegetable oils: occlusive and good penetration but can turn rancid
• Arachis
• Castor
• Olive
• Coconut
Synthetic oils: water repellent and occlusive
• Dimethicone
• isopropyl myristate

What are the excipients used for topical formulations 2 ?

Water miscible vehicles
• Water, alcohol and macrogols (polyethylene glycols).
• Alcohol is used for cooling via evaporation

Gelling agents
• Variety of viscosities
• Aqueous gels:
• Tragacanth, alginates, pectin, gelatin, methylcellulose,
carbomer, polyvinyl alcohol

Emulsifying agents (surfactants)
• Help oil and water to form a stable uniform mix
• Will also increase the penetration of the drug

For water in oil:
• Lanolin (Wool fat), wool alcohol, hydrous wool fat, Bees wax

For oil in water:
• Emulsifying waxes: mostly used for extemp, based on cetostearyl alcohol
• Polyethylene glycols
• Macrogols/carbowaxes
• Non-ionic surfactants (Sorbitan laurate)

What are the excipients used for topical formulations 3 ?

Humectants
• Reduce the loss of water from creams and gels
• E.g. glycerol, propylene glycol
Solids
• Usually added to occlusive bases (pastes)
• E.g. zinc oxide, talc
Preservatives
• E.g. parabens (methyl hydroxybenzoate or methyl
parabens, propylhydroxy benzoate) and EDTA
• Can cause allergy/skin sensitisation
Anti-oxidants
• Prevent decomposition

What affects the stability of semi-solid dosage forms and what happens ?

Most semi-solid dosage forms will be affected by:
• Heat
• Light
• Microorganism growth

• Emulsions can crack or cream.
• Oils can turn rancid
• Products low in water are usually the most stable
• Ointments and pastes
• Some may require special storage instructions
• E.g. keep in the fridge

What is the packaging used for topical treatments ?

• Glass/plastic jars
  • Aluminium/plastic tubes
  • Pump dispensers used for emollients to promote
  use of large quantities
  • Nozzles for application
  • Sachets for measured dose applications
  • E.g. Aldara, Testogel

What are the advantages and disadvantages of ointments and pastes ?

Advantages
• More stable than creams and gels
• Occlusive and protective
• Good for dry skin
• Less need for preservatives
• Some ointments can be used as washes

Disadvantages
• Greasy and sticky so less acceptable to patients
• Slow absorption into skin
• Not to be used on macerated skin
• High oil content preps can be flammable

What are the advantages and disadvantages of creams and gels ?

Advantages
• Non greasy and more acceptable to patients
• Rapid absorption into the skin
• Have a cooling effect on evaporation
• Can be used on macerated skin
• Gels have less additives

Disadvantages
• Susceptible to microbial contamination
• Require preservatives which can cause sensitisation
• Less stable and creams can crack (short expiry)
• Short duration of action due to rapid absorption

How do you make semi-solids extemporaneously ?

Always make an excess! (usually 5g)
✓ Involve mixing semi-solids, liquids and solids together
✓ May need to consider equivalent weights for liquids
✓ May need to grind and sieve powder increase surface area and reduce clumping
✓ Often adding ingredients to a ready-made base
✓ May be diluting the strength of a readily made semi-solid with another base
✓ Packaging – amber jar of appropriate size and lined lid

What is trituration ?

Trituration:
• Method of incorporation of liquids or powders into a semi-solid base using a ceramic tile
• Powders – doubling up and mixed using levigation
• Liquids – making a small well and then mixed in gradually

What is levigation ?

Levigation:
• Applying pressure to incorporate a powder into a semisolid
• Avoids a gritty product

What do you look for in extemporaneous final product ?

Final check:
• Uniformity – spread out on tile thinly to look for grains and unmixed liquids, colour distribution if applicable
• Correct final weight
• Appearance: sides of jar covered, no bubbles visible, smooth surface
• Clean jar with lid lined

Quality Control
• Expiry max 1 month – or shortest date of the ingredients
• Content uniformity – not possible in extemp hence the need for good manufacturing procedure

Summary

Topical drugs mainly act locally, but some can produce systemic effects.

▪ Skin formulations come as solids, liquids, or semi-solids.

▪ Vehicles (bases) used in liquid and semi-solid preparations include:
Water-based
Water-miscible
Oily
Emulsified (o/w or w/o)