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Research Techniques Made Simple: Drug Delivery Techniques, Part 1: Concepts in Transepidermal Penetration and Absorption

      Introduction

      In dermatology, topical therapies are usually the first line and mainstay of treatment for the majority of skin conditions. Most topical preparations are available in a variety of potencies and delivery systems. Practitioners must carefully choose from this vast array based on the potency required, location of intended use, product elegance, and likelihood of patient compliance. Unfortunately, information concerning which preparation is truly best, regarding actual penetration and delivery to the site of action, is not readily available. In general, many practitioners believe that ointments and foams enhance penetration when compared to creams, gels, and powders. However, this is not always the case. Aside from vehicles, there are a variety of chemical and physical enhancement techniques that influence topical penetration. As physician-scientists, dermatologists should be aware of the basic mechanisms involved in topical absorption and should be able to assess whether a preparation is likely to exert its desired effect. In this article, we explore the inherent properties of the epidermis and the physiology of passive diffusion and aim to clarify the definition of the terms “absorption” and “penetration.”

      Accumulation Versus Penetration

      The terms “absorption,” which is the accumulation of drug in the skin, and “penetration,” which is a measure of flux/transport across the skin, are often incorrectly used interchangeably. Penetration is quantifiable as the amount of substance that crosses the skin per unit area per unit time. By contrast, “absorption,” or “accumulation,” refers to the amount of a substance that builds up in the skin over a certain time period. The accumulated substance may remain in the skin or leave the skin to enter the systemic circulation. Whether the substance exerts a biological effect, is inert, is metabolized (and at what rate), and/or is soluble can impact the substance’s concentration in the skin at any given time, making the results difficult to compare and reproduce.
      Figure thumbnail fx1
      One approach to solving the penetration problem is the use of triggered release mechanisms. This allows the drug to be released independently after it has been delivered deep into the hair follicle. The drug can then pass through the hair follicle and into the surrounding skin. Currently, a number of these release triggers are under investigation and include radiofrequency, ultrasound, light, enzymatic reactions, and pH manipulation (
      • Shah V.P.
      • Maibach H.I.
      • Jenner J.
      Topical Drug Bioavailability, Bioequivalence, and Penetration.
      ).

      Barrier Properties of the Skin

      The stratum corneum of the stratified epidermis functions as the single greatest barrier against drug penetration (
      • Bouwstra J.A.
      • Gooris G.S.
      • Dubbelaar F.E.
      • et al.
      Phase behavior of lipid mixtures based on human ceramides: coexistence of crystalline and liquid phases.
      ;
      • Jain K.K.
      ). A major component of this barrier is extracellular lipids, which are extruded into the extracellular space as cells transition from the granular layer to the stratum corneum. In conjunction with this lipid-rich extracellular space, limited desquamation of corneocytes provides a homeostatic layer of cells that protects the underlying epidermis. Together, this arrangement of corneocytes in the stratum corneum and the formation of a lipid-rich extracellular space has been coined the “bricks-and-mortar” model (Figure 1) (
      • Bolognia J.
      • Jorizzo J.L.
      • Schaffer J.V.
      ;
      • Bouwstra J.A.
      • Ponec M.
      The skin barrier in healthy and diseased state.
      ;
      • Lampe M.A.
      • Burlingame A.L.
      • Whitney J.
      • et al.
      Human stratum corneum lipids: characterization and regional variations.
      ;
      • Hachem J.P.
      • Crumrine D.
      • Fluhr J.
      • et al.
      pH directly regulates epidermal permeability barrier homeostasis, and stratum corneum integrity/cohesion.
      ;
      • Elias P.
      • Feingold K.
      • Fartasch M.
      Epidermal lamellar body as a multifunctional secretory organelle.
      ;
      • Shah V.P.
      • Maibach H.I.
      • Jenner J.
      Topical Drug Bioavailability, Bioequivalence, and Penetration.
      ;
      • Jain K.K.
      ).
      Figure thumbnail gr1
      Figure 1Basic structure of the stratum corneum. (a) “Bricks-and-mortar” model of the epidermis. The extracellular space of the stratum granulosum and stratum corneum exhibits hydrophobic properties due to its lipid-rich composition. This is in contrast to the layers below the stratum granulosum, which exhibit a hydrophilic environment due to the lipid-poor, desmosome-rich composition. pH = ~7.3 in stratum granulosum; pH = ~5.0 in the mid–stratum corneum. (b) Formation of lipid-rich extracellular space. Expulsion of lamellar bodies and conversion of lipids into final end products.
      The lipid-rich, hydrophobic, extracellular space of the stratum corneum is predominated by ceramides, cholesterol, and free fatty acids (
      • Bouwstra J.A.
      • Ponec M.
      The skin barrier in healthy and diseased state.
      ;
      • Shah V.P.
      • Maibach H.I.
      • Jenner J.
      Topical Drug Bioavailability, Bioequivalence, and Penetration.
      ). This extracellular space is formed by the conversion of lipids extruded from lamellar bodies in the stratum granulosum (i.e., glucosylceramides, sphingomyelin, cholesterol, and phospholipids). The conversion of these precursor lipids into their final end products occurs mainly by the following enzymes, which are also extruded from lamellar bodies: β-glucocerebrosidase, acid sphingomyelinase, secretory phospholipase A2, and proteases. In contrast to the extracellular space of the stratum corneum, the extracellular space in the layers including and between the stratum basale and the stratum granulosum consists of a hydrophilic environment predominated by proteinaceous molecules such as desmosomes (
      • Elias P.M.
      • Goerke J.
      • Friend D.S.
      Mammalian epidermal barrier layer lipids: composition and influence on structure.
      ). Because of the importance of the stratum corneum lipids in barrier function, it is likely that there are many enhancers/excipients that affect these lipids in some way. Some substances such as DMSO and azone are thought to distort the packing geometry of these intercellular lipids. Oleic acid, azone, and terpenes have been shown to induce discreet domains in the stratum corneum lipids where the excipient is concentrated, resulting in barrier defects. Surfactants such as sodium lauryl sulfate may work by solubilizing stratum corneum lipids (
      • Williams A.C.
      • Barry B.W.
      Penetration enhancers.
      ).
      It has been reported that the conversion of lipids into their final end products is facilitated by acidification of the extracellular space of the stratum granulosum and the stratum corneum. This “acid mantle” is created primarily via two mechanisms (
      • Hachem J.P.
      • Crumrine D.
      • Fluhr J.
      • et al.
      pH directly regulates epidermal permeability barrier homeostasis, and stratum corneum integrity/cohesion.
      ). The first mechanism involves a non–energy dependent sodium–proton exchanger on the surface of upper epidermal keratinocytes, which pumps protons extracellularly. The second mechanism involves conversion of phospholipids into free fatty acids via phospholipase A2, which provides an acidic environment through the inherently acidic moieties on the free fatty acids. Together, the sodium–proton exchanger and free fatty acids create the necessary pH for optimal activity of β-glucocerebrosidase and acid sphingomyelinase, which convert precursor sphingolipids into ceramides, a very important constituent of the stratum corneum (
      • Hachem J.P.
      • Crumrine D.
      • Fluhr J.
      • et al.
      pH directly regulates epidermal permeability barrier homeostasis, and stratum corneum integrity/cohesion.
      ).
      Just as the “acid mantle” facilitates the formation of a lipid-rich extracellular space to limit drug penetration, so too does it indirectly limit desquamation, further fortifying the barrier function of the stratum corneum (
      • Hachem J.P.
      • Crumrine D.
      • Fluhr J.
      • et al.
      pH directly regulates epidermal permeability barrier homeostasis, and stratum corneum integrity/cohesion.
      ). Particularly, the low pH of the extracellular space in the upper epidermis limits the degradation of corneodesmosomes, mostly desmoglein 1, by proteases. In contrast, if the pH were higher, protease activity in the extracellular space would increase and desquamation would occur at a more rapid rate. This further illustrates the importance of pH on the barrier function of the skin.

      Regional Variation

      Epidermal permeability changes depending on body site. This knowledge is important when choosing the correct potency and vehicle of a topical preparation, both for efficacy and for avoiding adverse effects. Although most clinicians believe that skin penetration correlates with skin thickness, differences in drug penetration can actually be explained by variations in the number of lamellar membranes (lipid weight %), membrane structure, and/or lipid composition (i.e., sphingomyelin:ceramide ratio) (
      • Bolognia J.
      • Jorizzo J.L.
      • Schaffer J.V.
      ).

      Physiology of Passive Transport and Enhancers

      Transport across the stratum corneum is passive. Fick’s law (
      • Jain K.K.
      ), a passive diffusion model, can therefore be used to calculate skin permeation as follows
      J=KDh(c0-c1)


      Figure thumbnail fx2
      Although the inherent properties of the stratum corneum and drug are important factors in penetration, the hydration of the epidermis is an additional factor to consider. Although it is not entirely clear by which mechanism water affects skin permeability, it is understood that water typically enhances drug penetration. One hypothesis is that water increases topical drug delivery by exploiting a network of “aqueous pores” (Figure 2). Normally, this network, which consists of lacunae at sites of corneodesmosome degradation, is interrupted (
      • Williams A.C.
      • Barry B.W.
      Penetration enhancers.
      ). However, under certain conditions, such as extensive hydration, occlusion, and sonophoresis, the lacunae form connections that create a pathway through the stratum corneum (
      • Williams A.C.
      • Barry B.W.
      Penetration enhancers.
      ;
      • Bolognia J.
      • Jorizzo J.L.
      • Schaffer J.V.
      ). The existence of these pores not only adds to the already numerous transepidermal pathways but also provides future opportunities for drug delivery manipulation (
      • Bolognia J.
      • Jorizzo J.L.
      • Schaffer J.V.
      ). Of particular interest, vehicle characteristics (reviewed in Table 1) can be influential in increasing hydration (
      • Barry B.W.
      Novel mechanisms and devices to enable successful transdermal drug delivery.
      ).
      Figure thumbnail gr2
      Figure 2“Pore” pathway within the stratum corneum. Aqueous pores represent discontinuous lacunar domains formed by the degradation of corneodesmosomes. Under certain conditions, such as extensive hydration, occlusion, and sonophoresis, these pores enlarge, extend, and connect, creating a continuous pathway through the stratum corneum.
      Adapted from
      • Bolognia J.
      • Jorizzo J.L.
      • Schaffer J.V.
      with permission from Elsevier.
      Table 1The effect of various vehicles on the permeability of the stratum corneum
      Table thumbnail gr3
      Furthermore, the addition of excipients, nonactive additives, can promote penetration. Some of the most studied excipients are water, DMSO, azone, N-methyl-2-pyrrolidone, 2-pyrrolidone, fatty acids, ethanol, propylene glycol, urea, menthol, and essential oils. The advantages and disadvantages of these excipients are outlined in Table 2 (
      • Williams A.C.
      • Barry B.W.
      Penetration enhancers.
      ).
      Table 2The advantages and disadvantages of excipients
      Table thumbnail gr4
      Finally, another mode of enhancement to consider is the use of nanoparticles, especially given that their use is the most active direction of research in transepidermal drug delivery. In particular, ultrafine nanoparticles ranging from 1 to 100 nm can be utilized to encapsulate drug molecules and enhance penetration. Some common nanoparticles include carbon nanotubes, fullerenes, quantum dots, metals (Ag, Au), metal oxides (TiO2, ZnO, Fe2O3, SiO2), and lipophilic nanoparticles (
      • DeLouise L.A.
      Applications of nanotechnology in dermatology.
      ). The size, shape, rigidity, hydrophobicity, and charge of these nanoparticles can be exploited to optimize drug delivery (i.e., to protect labile drugs, control release of drugs, and target drug delivery). However, adverse effects may exist with the use of nanoparticles because they tend to eventually accumulate in peripheral tissue, causing inflammation and damage, no matter their inherent size or chemical properties (
      • Minchin R.
      Nanomedicine: sizing up targets with nanoparticles.
      ).

      Conclusion

      As high-volume prescribers of topical drugs, dermatologists should be aware of the factors that influence topical drug efficacy and safety. Specifically, it is important to know what effects varying individual parameters (i.e., excipients, vehicles, etc.) can have on enhancing drug penetration into the epidermis. When presented with a topical drug, a dermatologist can then appropriately decide a drug’s true capabilities versus the claims made about the drug. In Part 2 of this Research Techniques Made Simple, we will discuss the techniques used to objectively assess topical bioavailability, giving the reader an even better appreciation of transepidermal drug delivery.

      Conflict of Interest

      The authors state no conflict of interest.

      CME Accreditation

      This activity has been planned and implemented in accordance with the Essential Areas and Policies of the Accreditation Council for Continuing Medical Education through the joint sponsorship of the Duke University School of Medicine and Society for Investigative Dermatology. The Duke University School of Medicine is accredited by the ACCME to provide continuing medical education for physicians. To participate in the CME activity, follow the link provided. Physicians should only claim credit commensurate with the extent of their participation in the activity.
      To take the online quiz, follow the link below:

      Acknowledgments

      The authors recognize Joke Bouwstra, PhD for her expertise, contribution, and oversight during the preparation of this manuscript.

      SUPPLEMENTARY MATERIAL

      A PowerPoint slide presentation appropriate for journal club or other teaching exercises is available at http://dx.doi.org/10.1038/jid.2015.343.

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