Insights into synergistic interactions in binary mixtures of chemical permeation enhancers for transdermal drug delivery

Efficiency of fatty acids as chemical penetration enhancers: mechanisms and structure enhancement relationship

PURPOSE

The present study evaluated the effects of fatty acids commonly present in cosmetic and topical formulations on permeation enhancement across human epidermal membrane (HEM) lipoidal pathway when the fatty acids saturated the SC lipid domain without cosolvents (Emax).

METHODS

HEM was treated with neat fatty acids or fatty acid suspensions to determine Emax. A volatile solvent system was used to deposit fatty acids on HEM surface to compare fatty acid enhancer efficiency in topical volatile formulations with Emax. To elucidate permeation enhancement mechanism(s), estradiol (E2beta) uptake into fatty acid-treated SC lipid domain was determined.

RESULTS

Emax of fatty acids was shown to increase with their octanol solubilities and decrease with their lipophilicities, similar to our previous findings with other enhancers. Emax of solid fatty acids was shown to depend on their melting points, an important parameter to the effectiveness of the enhancers. The E2beta uptake results suggest that enhancer-induced permeation enhancement across HEM is related to enhanced permeant partitioning into the SC lipid domain.

CONCLUSIONS

The results suggest Emax as a model for studying the permeation enhancement effect of the fatty acids and their structure enhancement relationship.

Enhancement of transdermal drug delivery via synergistic action of chemicals

Transdermal drug delivery is an attractive alternative to conventional techniques for administration of systemic therapeutics. One challenge in designing transdermal drug delivery systems is to overcome the natural transport barrier of the skin. Chemicals offer tremendous potential in overcoming the skin barrier to enhance transport of drug molecules. Individual chemicals are however limited in their efficacy in disrupting the skin barrier at low concentrations and usually cause skin irritation at high concentrations. Multicomponent mixtures of chemicals, however, have been shown to provide high skin permeabilization potency as compared to individual chemicals without necessarily causing irritation. Here we review systems employing synergistic mixtures of chemicals that offer superior skin permeation enhancement. These synergistic systems include solvent mixtures, microemulsions, eutectic mixtures, complex self-assembled vesicles and inclusion complexes. Methods for design and discovery of such synergistic systems are also discussed.

Effects of chemical enhancers on human epidermal membrane: Structure-enhancement relationship based on maximum enhancement (E(max))

Chemical penetration enhancers are widely used in transdermal pharmaceuticals as well as cosmetic products. Selection of suitable enhancers in topical formulations requires an understanding of the mechanism of action of these enhancers. The objective of the present study was to evaluate the enhancement effects of a number of commonly known enhancers and cosmetic ingredients on permeation across human epidermal membrane (HEM). The potencies of these chemical enhancers-maximum enhancement, E(max)-were compared at their highest thermodynamic activity in equilibrium with HEM (i.e., solubility equilibrium). This was achieved by the treatment of HEM with the enhancer or phosphate buffered saline (PBS) saturated with the enhancer. Passive transport experiments were then conducted with a model permeant corticosterone to determine the effects of these enhancers on the lipoidal pathway of HEM. The results suggest that E(max) of an enhancer is related to its octanol/water partition coefficient and its solubility in the HEM lipid domain. A relationship between enhancer E(max) and its solubility in silicone elastomer was also observed, suggesting the use of silicone solubility to predict enhancer potency. Based on the E(max) results, some common topical ingredients were found to be more potent enhancers than a number of well-known chemical enhancers.

Microneedles permit transdermal delivery of a skin-impermeant medication to humans

Drugs with poor oral bioavailability usually are administered by hypodermic injection, which causes pain, poor patient compliance, the need for trained personnel, and risk of infectious disease transmission. Transdermal (TD) delivery provides an excellent alternative, but the barrier of skin's outer stratum corneum (SC) prevents delivery of most drugs. Micrometer-scale microneedles (MNs) have been used to pierce animal and human cadaver skin and thereby enable TD delivery of small molecules, proteins, DNA, and vaccines for systemic action. Here, we present a clinical study of MN-enhanced delivery of a medication to humans. Naltrexone (NTX) is a potent mu-opioid receptor antagonist used to treat opiate and alcohol dependence. This hydrophilic and skin-impermeant molecule was delivered from a TD patch to healthy human subjects with and without pretreatment of the skin with MNs. Whereas delivery from a standard NTX TD patch over a 72-h period yielded undetectable drug plasma levels, pretreatment of skin with MNs achieved steady-state plasma concentrations within 2 h of patch application and were maintained for at least 48 h. The MNs and NTX patch were well tolerated with mild systemic and application site side effects. The MN arrays were painless upon administration and not damaged during skin insertion, and no MNs were broken off into the skin. This human proof-of-concept study demonstrates systemic administration of a hydrophilic medication by MN-enhanced TD delivery. These findings set the stage for future human studies of skin-impermeant medications and biopharmaceuticals for clinical applications.

Piezoelectric control of needle-free transdermal drug delivery

Transdermal drug delivery occurs primarily through hypodermic needle injections, which cause pain, require a trained administrator, and may contribute to the spread of disease. With the growing number of pharmaceutical therapies requiring transdermal delivery, an effective, safe, and simple needle-free alternative is needed. We present and characterize a needle-free jet injector that employs a piezoelectric actuator to accelerate a micron-scale stream of fluid (40-130 microm diameter) to velocities sufficient for skin penetration and drug delivery (50-160 m/s). Existing jet injectors, powered by compressed springs and gases, are not widely used due to painful injections and poor reliability in skin penetration depth and dose. In contrast, our device offers electronic control of the actuator expansion rate, resulting in direct control of jet velocity and thus the potential for more precise injections. We apply a simple fluid-dynamic model to predict the device response to actuator expansion. Further, we demonstrate that injection parameters including expelled volume, jet pressure, and penetration depth in soft materials vary with actuator expansion rate, but are highly coupled. Finally, we discuss how electronically-controlled jet injectors may enable the decoupling of injection parameters such as penetration depth and dose, improving the reliability of needle-free transdermal drug delivery.

Controlled release in transdermal pressure sensitive adhesives using organosilicate nanocomposites

Polydimethyl siloxane (PDMS) based pressure sensitive adhesives (PSA) incorporating organo-clays at different loadings were fabricated via solution casting. Partially exfoliated nanocomposites were obtained for the hydroxyl terminated PDMS in ethyl acetate solvent as determined by X-ray diffraction and atomic force microscopy. Drug release studies showed that the initial burst release was substantially reduced and the drug release could be controlled by the addition of organo-clay. Shear strength and shear adhesion failure temperature (SAFT) measurements indicated substantial improvement in adhesive properties of the PSA nanocomposite adhesives. Shear strength showed more than 200% improvement at the lower clay loadings and the SAFT increased by about 21% due to the reinforcement provided by the nano-dispersed clay platelets. It was found that by optimizing the level of the organosilicate additive to the polymer matrix, superior control over drug release kinetics and simultaneous improvements in adhesive properties could be attained for a transdermal PSA formulation.

Probing the skin permeation of fish oil/EPA and ketoprofen

The complexation of EPA with ketoprofen was probed in order to rationalise co-operative skin permeation enhancement behaviour observed previously. The modulation of aromatic protons of ketoprofen was determined using (1)H NMR spectra from different formulations containing varying concentrations of fish oil and a control saturated triglyceride. Molecular modelling of possible complexes of ketoprofen with constituents of fish oil was performed. NMR data revealed a dose-dependent change in chemical shift in the aromatic protons of ketoprofen on addition of fish oil and/or EPA. Similar patterns were observed in both cases, although the free fatty acid induced changes in more protons. Molecular modelling results indicate quite large binding energies of all complexes considered, varying between ca. 90 and 160 kJ mol(-1). The geometries of these complexes shows strong O-H...O hydrogen bonds in all cases, and in the case of the complex of ketoprofen with free EPA there is also some evidence of C-H... pi and/or pi-pi interactions, giving rise to regiospecifically solvated complexes. If strongly bound ketoprofen:EPA complexes can form, then the permeation enhancement of EPA by ketoprofen could be attributed to such a complex. Once the complex is formed, the triglyceride/free fatty acid could aid permeation of associated ketoprofen into the lipophilic stratum corneum via the pull effect. Once permeated, the more hydrophilic ketoprofen could aid the permeation of the triglyceride/free fatty acid through the epidermis, again via the pull effect. This could explain the synergistic permeation enhancement seen with these compounds.