Percutaneous absorption

Solubility of Foreign Molecules in Stratum Corneum Brick and Mortar Structure

The barrier function of the skin is mainly assured by its outermost layer, stratum corneum (SC). One key aspect in predicting dermal drug delivery and in safety assessment of skin exposure to chemicals is the need to determine the amount of chemical that is taken up into the SC. We here present a strategy that allows for direct measures of the amount of various solid chemicals that can be dissolved in the SC in any environmental relative humidity (RH). A main advantage of the presented method is that it distinguishes between molecules that are dissolved within the SC and molecules that are not dissolved but might be present at, for example, the skin surface. In addition, the method allows for studies of uptake of hydrophobic chemicals without the need to use organic solvents. The strategy relies on the differences in the molecular properties of the added molecules in the dissolved and the excess states, employing detection methods that act as a dynamic filter to spot only one of the fractions, either the dissolved molecules or the excess solid molecules. By measuring the solubility in SC and delipidized SC at the same RHs, the same method can be used to estimate the distribution of the added chemical between the extracellular lipids and corneocytes at different hydration conditions. The solubility in porcine SC is shown to vary with hydration, which has implications for the molecular uptake and transport across the skin. The findings highlight the importance of assessing the chemical uptake at hydration conditions relevant to the specific applications. The methodology presented in this study can also be generalized to study the solubility and partitioning of chemicals in other heterogeneous materials with complex composition and structure.

Dissolving microneedles: Applications and growing therapeutic potential

Microneedles are a rapidly developing method for the transdermal delivery of therapeutic compounds. All types of microneedles, whether solid, hollow, coated, or dissolving function by penetrating the stratum corneum layer of the skin producing a microchannel through which therapeutic agents may be delivered. To date, coated and hollow microneedles have been the most successful, despite suffering from issues such as poor drug loading capabilities and blocked pores. Dissolving microneedles, on the other hand, have superior drug loading as well as other positive attributes that make it an ideal delivery system, including simple methods of fabrication and disposal, and abundantly available materials. Indeed, dissolvable microneedles can even be fabricated entirely from the therapeutic agent itself thus eliminating the requirement for additional excipients. This focused review presents the recent developments and trends of dissolving microneedles as well as potential future directions. The advantages, and disadvantages of dissolving microneedles as well as fabrication materials and methods are discussed. The potential applications of dissolving microneedles as a drug delivery system in different therapeutic areas in both research literature and clinical trials is highlighted. Applications including the delivery of cosmetics, vaccine delivery, diagnosis and monitoring, cancer, pain and inflammation, diabetes, hair and scalp disorders and inflammatory skin diseases are presented. The current trends observed in the microneedle landscape with particular emphasis on contemporary clinical trials and commercial successes as well as barriers impeding microneedle development and commercialisation are also discussed.

Physicochemical and biopharmaceutical aspects influencing skin permeation and role of SLN and NLC for skin drug delivery

The skin is a complex and multifunctional organ, in which the static versus dynamic balance is responsible for its constant adaptation to variations in the external environment that is continuously exposed. One of the most important functions of the skin is its ability to act as a protective barrier, against the entry of foreign substances and against the excessive loss of endogenous material. Human skin imposes physical, chemical and biological limitations on all types of permeating agents that can cross the epithelial barrier. For a molecule to be passively permeated through the skin, it must have properties, such as dimensions, molecular weight, pKa and hydrophilic-lipophilic gradient, appropriate to the anatomy and physiology of the skin. These requirements have limited the number of commercially available products for dermal and transdermal administration of drugs. To understand the mechanisms involved in the drug permeation process through the skin, the approach should be multidisciplinary in order to overcome biological and pharmacotechnical barriers. The study of the mechanisms involved in the permeation process, and the ways to control it, can make this route of drug administration cease to be a constant promise and become a reality. In this work, we address the physicochemical and biopharmaceutical aspects encountered in the pathway of drugs through the skin, and the potential added value of using solid lipid nanoparticles (SLN) and nanostructured lipid vectors (NLC) to drug permeation/penetration through this route. The technology and architecture for obtaining lipid nanoparticles are described in detail, namely the composition, production methods and the ability to release pharmacologically active substances, as well as the application of these systems in the vectorization of various pharmacologically active substances for dermal and transdermal applications. The characteristics of these systems in terms of dermal application are addressed, such as biocompatibility, occlusion, hydration, emollience and the penetration of pharmacologically active substances. The advantages of using these systems over conventional formulations are described and explored from a pharmaceutical point of view.

Injectable Capsaicin for the Management of Pain Due to Osteoarthritis

Capsaicin is a potent agonist of the TRPV1 channel, a transduction channel that is highly expressed in nociceptive fibers (pain fibers) throughout the peripheral nervous system. Given the importance of TRPV1 as one of several transduction channels in nociceptive fibers, much research has been focused on the potential therapeutic benefits of using TRPV1 antagonists for the management of pain. However, an antagonist has two limitations. First, an antagonist in principle generally only affects one receptor. Secondly, most antagonists must have an ongoing presence on the receptor to have an effect. Capsaicin overcomes both liabilities by disrupting peripheral terminals of nociceptive fibers that express TRPV1, and thereby affects all of the potential means of activating that pain fiber (not just TRPV1 function). This disruptive effect is dependent on the dose and can occur within minutes. Thus, unlike a typical receptor antagonist, continued bioavailability at the level of the receptor is not necessary. By disrupting the entire terminal of the TRPV1-expressing nociceptive fiber, capsaicin blocks all the activation mechanisms within that fiber, and not just TRPV1 function. Topical capsaicin, an FDA approved treatment for neuropathic pain, addresses pain from abnormal nociceptor activity in the superficial layers of the skin. Effects after a single administration are evident over a period of weeks to months, but in time are fully reversible. This review focuses on the rationale for using capsaicin by injection for painful conditions such as osteoarthritis (OA) and provides an update on studies completed to date.

Skin penetration and tissue permeation after topical administration of diclofenac

OBJECTIVE

Topical delivery of drugs is an alternative to oral administration, often with similar efficacy but potentially a more favorable tolerability profile. However, topical formulations need to be able to penetrate the skin and permeate to the target areas in quantities sufficient to exert a therapeutic effect. Many factors can affect this process, including the physicochemical properties of the drug, the formulation used, and the site and mode of application. It is believed that measurement of drug concentrations at the sites of action may be an indicator of their likely efficacy. This review addresses these issues, with reference to topically administered diclofenac in osteoarthritis.

METHODS

Articles relevant to this review were identified after a systematic search of Medline and Embase, using the key words "diclofenac", "topical administration" and "osteoarthritis" in the search strategy.

RESULTS

The sparse data available indicate that topical diclofenac can penetrate and permeate to deeper tissues, with a lower plasma to tissue ratio than oral diclofenac. The tissue diclofenac levels after topical delivery are sustained over time (at least several hours). However, there is not enough data to establish how diclofenac levels in the joint compare with IC₅₀ levels (50% of the maximum inhibition of prostaglandin synthesis) established following oral administration.

CONCLUSIONS

After topical application, diclofenac can penetrate the skin and permeate to deeper tissues, where it reaches a concentration that appears to be sufficient to exert a therapeutic effect. More robust methods are required for in vivo characterization to better estimate the clinical efficacy of topically applied drugs.

Skin permeation: The years of enlightenment

Considerable advances in our understanding of the mechanisms by which drugs permeate the skin barrier have been made over the past 60 years. The key publications, which have influenced the field of skin permeation research are highlighted in the present review. The methodologies commonly employed for estimation of skin permeability are discussed as are the mechanisms proposed for skin permeation. The principal findings from the commonly employed in vitro and in vivo models are considered as well as the applications of enhancers and surfactants for optimization of skin delivery. As these studies presaged the emergence of transdermal drug delivery research in the 1970s, early approaches to model and predict dermal and transdermal absorption are also outlined. The published work on skin permeability in this period embodies the fundamental literature sources for consultation by scientists new to and currently engaged in transdermal drug delivery.

Skin deep

Over the past 30 or so years there has been a considerable advance in our knowledge of the mechanisms of skin permeation. This has largely been brought about by the development of sophisticated biophysical techniques and increased computing powers. The advanced technology has clearly provided indications, at a molecular level, about routes of permeation and how the barrier function can be modulated by excipients with which actives are formulated. This publication reviews some of the advances that have been made and mathematical models that have been constructed to predict percutaneous penetration and transdermal delivery. The models also indicate the various enhancement strategies that can be used in dermal penetration. In the past, it has been difficult to identify precise mechanisms of action of the different classes of enhancer but a combination of appropriate biophysical techniques, mathematical modelling and chemometric analysis can help identify the contributing processes. The models can also be used to indicate rate control in transdermal delivery, whether it is in the applied delivery device or in the skin.

Recent developments in topical and transdermal delivery

Over the past decade there has been considerable interest shown in an understanding of the mechanisms of skin penetration and its enhancement. Sophisticated biophysical techniques have been used to probe, at a molecular level, the ways in which chemical and physical enhancement can be achieved. Two types of chemical penetration enhancers can be identified and their co-administration can result in synergistic effects. Another attractive method for enhancing skin penetration is the use of supersaturation but problems associated with stability must be addressed. Biotech drugs are inherently difficult to administer and recent progress using iontophoresis and sonophoresis has identified that medicines of this type can be delivered percutaneously. It is also possible to use the percutaneous route to monitor plasma levels non-invasively. Coupling non invasive monitoring with controlled delivery allows the possibility of creating transdermal systems which act with bio feedback.

Drug delivery systems. 6. Transdermal drug delivery

Transdermal drug delivery system has been in existence for a long time. In the past, the most commonly applied systems were topically applied creams and ointments for dermatological disorders. The occurrence of systemic side-effects with some of these formulations is indicative of absorption through the skin. A number of drugs have been applied to the skin for systemic treatment. In a broad sense, the term transdermal delivery system includes all topically administered drug formulations intended to deliver the active ingredient into the general circulation. Transdermal therapeutic systems have been designed to provide controlled continuous delivery of drugs via the skin to the systemic circulation. The relative impermeability of skin is well known, and this is associated with its functions as a dual protective barrier against invasion by micro-organisms and the prevention of the loss of physiologically essential substances such as water. Elucidation of factors that contribute to this impermeability has made the use of skin as a route for controlled systemic drug delivery possible. Basically, four systems are available that allow for effective absorption of drugs across the skin. The microsealed system is a partition-controlled delivery system that contains a drug reservoir with a saturated suspension of drug in a water-miscible solvent homogeneously dispersed in a silicone elastomer matrix. A second system is the matrix-diffusion controlled system. The third and most widely used system for transdermal drug delivery is the membrane-permeation controlled system. A fourth system, recently made available, is the gradient-charged system. Additionally, advanced transdermal carriers include systems such as iontophoretic and sonophoretic systems, thermosetting gels, prodrugs, and liposomes. Many drugs have been formulated in transdermal systems, and others are being examined for the feasibility of their delivery in this manner (e.g., nicotine antihistamines, beta-blockers, calcium channel blockers, non-steroidal anti-inflammatory drugs, contraceptives, anti-arrhythmic drugs, insulin, antivirals, hormones, alpha-interferon, and cancer chemotherapeutic agents). Research also continues on various chemical penetration enhancers that may allow delivery of therapeutic substances. For example, penetration enhancers such as Azone may allow delivery of larger-sized molecules such as proteins and polypeptides.

Some observations on the pharmacology of 'deep-heat', a topical rubifacient

A topically applied rubifacient delivered by aerosol (Deep-Heat) was studied. After spray application to the forearms of volunteers, without massage, the erythema produced was measured by thermography and correlated with the concentration of 2 salicylate components of the mixture found in local and systemic venous blood. Maximum erythema occurred at about 30 minutes, while blood salicylate levels were maximal between 20 and 30 minutes after application. Methyl salicylate was absorbed before ethyl salicylate. Over the time period of the erythematous response oxygen levels in local venous blood were raised. Finally, platelets collected from venous blood draining from the sprayed site, when induced to clump by the addition of arachidonic acid in an aggregometer, showed increased resistance to clumping when compared with control cells. The mechanism of these observed phenomena and the mode of action of the constituents of Deep-Heat are discussed.