Topical Preparations

Introduction

In an overview, transdermal formulations aim to deliver the drug to therapeutically active levels at the target site (either local or systemic) through skin.

• It has been estimated that for topical products such as creams and gels, typically only between 1% and 3% of the applied dose is bioavailable.
• Bioavailability from patches is typically 30% to 70% for drugs such as buprenorphine and fentanyl.

NOTE: Topical treatments used for local effects have a decreased incidence of systemic side effects and generally provide faster relief. But, local reactions may occur as side-effects.

Formulation Options

Generally,

• Semisolid formulations are selected for increased residence on the skin;
• Transdermal patches are selected for extended drug delivery through the skin;
• Liquid formulations are selected for a rapid short-term input of permeant into the skin.

Cream or Ointment

At work (being at a hospital pharmacy or community pharmacy), we may find certain topical products come in both cream and ointment form, such as fusidic acid cream and ointment.

• Creams contain more than 20% water and up to 50% oil. They spread easily and are reasonably hydrating.
• Lotions contain the most water (compared to other topicals), are more fluid than creams and can be poured. They are easy to spread on the skin and they absorb quickly.
• Ointments contain the least water; 0-20% with the remainder composed of oil-based product(s). Ointments are good delivery vehicle to provide a barrier to exposure (e.g. organisms, sun) and they prevent moisture loss which helps burn and scar healing.

As a pharmacist, which product will you recommend? And more importantly, why? Factors affecting formulation type of topical therapy.

• Depending on the lesion type,
• For a wet, vesicular or weeping lesion, a 'wet' (usually aqueous-based) formulation is generally preferred, such as cream, lotion or gel.
• For a dry, thickened scaly lesion, a 'dry' (usually fatty) formulation is preferred, such as an ointment or paste.
• The skin site to be treated can affect the selection of the vehicle.
• For hairy areas, lotions, gels or sprays are usually preferable.
• For intertriginous areas (sites where skin may touch or rub such as the axilla of the arm), creams or lotions are usually preferred.

NOTE: Beyond the broad nature of the vehicle, specific formulation components may be required, dependent on the physicochemical properties of the drug (e.g. the drug pKa and the need for a buffer, the stability of the drug and the need for an antioxidant), clinical considerations (e.g. intact or broken skin, duration of use) and to improve patient adherence.

External Links

• Aulton's Pharmaceutics The Design and Manufacture of Medicines, 2022
  
• Topical Formulations, 2016