Solubilizing drugs for skin delivery, a decisive step
From the formulation to the skin
Three main steps govern drug diffusion:
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Solubility: the formulation must solubilize a sufficient amount of drug to reach the effective concentration at the target site
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Partition: the drug must partition out of the delivery vehicle into the upper layers of the stratum corneum
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Diffusion:
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the drug molecule diffuses through the stratum corneum mainly via the intercellular path
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the drug molecule diffuses through the stratum corneum mainly via the intercellular path
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Solubilizers for all types of drugs
The first step is therefore to solubilize the drug in the vehicle, and this can be achieved with a variety of components in the formulation: solvents, oils, surfactants and co-surfactants.
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Solvent |
Oils |
Co-surfactants |
Surfactants |
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Assessing solubility of drugs in lipid excipients
Obtaining solubility data in a sufficiently broad range of lipid excipients is the first step towards the development of a formulation, as solubilization of the drug is compulsory for skin delivery.
The solubility of a solute (API) in a solvent (lipid excipient) is measured as the saturation concentration, i.e. when adding more solute does not increase the concentration of the solution and the excess amount of solute begins to precipitate.
In liquid excipients, solubility data can be obtained by the equilibrium solubility method using UPLC/HPLC, whereas for solid and semi-solid lipid excipients techniques such as differential scanning calorimetry (DSC) and/or hot stage microscopy (HSM) are used.