Abstract
Penetration, usually with finite dosing, provides data about the total active amount in the skin and permeation, being the most used methodology, usually with infinite dosing, leads to data about pharmacokinetic parameters. The main objective of this work is to assess if results from permeation, most of them at finite dose, may be equivalent to those from penetration usually at infinite dose. The transdermal behavior of four drugs with different physicochemical properties (diclofenac sodium, ibuprofen, lidocaine, and caffeine) was studied using penetration/finite and kinetic permeation/infinite dose systems using vertical Franz diffusion cells to determine the relationships between permeation and penetration profiles. Good correlation of these two in vitro assays is difficult to find; the influence of their dosage and the proportion of different ionized/unionized compounds due to the pH of the skin layers was demonstrated. Finite and infinite dose regimens have different applications in transdermal delivery. Each approach presents its own advantages and challenges. Pharmaceutical industries are not always clear about the method and the dose to use to determine transdermal drug delivery. Being aware that this study presents results for four actives with different physicochemical properties, it can be concluded that the permeation/infinite results could not be always extrapolated to those of penetration/finite. Differences in hydrophilicity and ionization of drugs can significantly influence the lack of equivalence between the two methodologies. Further investigations in this field are still needed to study the correlation of the two methodologies and the main properties of the drugs that should be taken into account.
Highlights
An enormous variety of experimental designs and evaluation procedures for in vitro experiments can be found in the literature
The results were compared with the Quantitative Structure Permeability Relationship (QSPRs) which are normally used to predict chemical absorption into and through the skin
Diclofenac sodium and ibuprofen present an acidic behavior while lidocaine and caffeine present a basic one, as ibuprofen and lidocaine partially unionized at the skin pH
Summary
An enormous variety of experimental designs and evaluation procedures for in vitro experiments can be found in the literature. The amount of active pharmaceutical ingredient (API) that has crossed the barrier is called “the amount permeated”. The amount of substance present in the skin is “the amount penetrated” [1]. In vitro permeation experiments are generally performed using diffusion cells, such as Franz diffusion cells. In vitro penetration experiments can use Franz diffusion cells; the data obtained are not much related to the flux through the skin and drug permeation but mainly related to the drug concentration in the different skin strata. A direct correlation between the penetration data and the permeation parameters has sometimes been found [3], since the diffusion through the stratum corneum (SC) is the rate limiting step
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