Topical formulations are a vital part of pharmaceutical development. As it's applied directly to the skin, most of the patients prefer this dosage form.


Advantages of topical formulations

  • An increased dose of medication is applied where it is needed.
  • Avoid first pass effect.
  • There are reduced side effects and toxicity to other organs compared to systemic medications.
  • Suitable for self-medication.


Disadvantages of topical formulations

  • They can be time consuming to apply.
  • Sometimes, the regimen can be complicated, especially if several different formulations have been prescribed.
  • The applications may also be messy or uncomfortable.
  • Possibility of local skin irritation.


Topical formulations are made up in a vehicle, or base, which may be optimised for a particular site of the body or type of skin condition. The product may be designed to be moisturising or to maximise the penetration of an active ingredient, often a medicine, into or through the skin.


Topical formulation absorption / penetration through the skin depends on the following factors:

  • Thin skin absorbs more than thick skin — skin thickness varies with body site, age and the specific skin disorder.
  • Skin barrier function — this may be disrupted by dermatitis, ichthyosis and keratolytic agents (such as salicylic acid), so it may absorb more medication than intact, normal skin.
  • Physicochemical properties (partition coefficient, pH, solubility, particle size, molecular weight and polymorphism) of drug subsrances highly affecting the topical formulation.
  • The absorption of the active ingredient is greater where there is occlusion, such as in the skin folds, under dressings, or when a greasy, ointment formulation is used.
  • Small molecules are more easily absorbed through the skin than large molecules.
  • Lipophilic compounds are better absorbed than hydrophilic compounds.
  • Higher concentrations of the active ingredient may penetrate more than lower concentrations.
  • Other ingredients in the formulation may interact to increase or reduce potency or absorption rates.


Note: Minor differences in formulation may make surprising differences to the effectiveness of a topical medication.


Topical Formulation Excipients

Structure forming agents
  • Cetosteryl alcohol, Sorbitan, Mineral oils etc.

Preservatives
  • Benzyl alcohol, Sodium benzoate, Methyl paraben, Propyl paraben, Chlorocresol etc.

Antioxidants
  • Butyl hydroxyl toluene, Butyl hydroxyl anisole, Ascorbic acid etc.


Solubilizers
  • Lanolin, Cholesterol, Cholesterol esters etc.


Gelling agents
  • Carbomer, Carboxy methyl cellulose, Hydroxyl propyl cellulose, Xanthan gum etc.


Emollients
  • Glycerin, Mineral oil, Petrolatum, Isopropyl palmitate etc.

Suppository bases
  • Cocoa butter, Glycerin, Coconut oil, Gelatin, Hydrogenated vegetable oil, Polyethylene glycol etc.

Dermal Topical Formulation Types

  • Cream formulation: Thicker than a lotion, maintaining its shape, for example a 50/50 emulsion of oil and water. Requires preservative to extend shelf life. Often moisturizing.
  • Ointment formulation: Semi-solid, water-free or nearly water-free (80% oil). Greasy, sticky, emollient, protective, occlusive. No need for preservative so contact allergy is rare. May include hydrocarbon (paraffin), wool fat, beeswax, macrogols, emulsifying wax, cetrimide or vegetable oil (olive oil, arachis oil, coconut oil).
  • Solution formulation:  Water or alcoholic lotion containing a dissolved powder. 
  • Lotion formulation:  Usually considered thicker than a solution and more likely to contain oil as well as water or alcohol. A shake lotion separates into parts with time so needs to be shaken into suspension before use. 
  • Gel formulation: Aqueous or alcoholic monophasic semisolid emulsion, often based on cellulose and liquefies upon contact with skin. Often includes preservatives and fragrances.
  • Paste formulation: Concentrated suspension of oil, water and powder.
  • Spray formulation: 
    • Aerosol foam or spray: Solution with pressurized propellant. 
    • Powder formulation: Powder  Solid, for example talc (a mineral) or corn starch (vegetable). Powder formulation
    • Solid formulation: Antiperspirant or sunscreen stick. May melt on reaching body temperature (eg, a suppository).
    • Patch formulation: Drug delivery system allows precise dosing: includes an adhesive. 

The site of topical formulation

  • Palms and soles — an ointment or cream may be preferred.
  • Skin folds — use a cream or a lotion (ointments are too occlusive for these sites)
  • Hairy areas — a lotion, solution, gel, or foam is usually best.
  • Mucosal surfaces — take care to prescribe non-irritating formulations to avoid irritating eroded surfaces.

pH of topical formulation

The pH of topical formulation commonly suggest to adjust with skin. From this aspect the pH range fall between 5 to 7. But depending on the stability of topical formulation the pH range can slightly vary. 

Storage of topical formulation

Topical formulation should be stored in the indicated storage condition. Otherwise the efficacy of the formulation can be reduced or declined due through phase separation or denature of the product.



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