Dimethylamino Acid Esters as Biodegradable and Reversible Transdermal Permeation Enhancers: Effects of Linking Chain Length, Chirality and Polyfluorination

Proline, hydroxyproline, and pyrrolidone carboxylic acid derivatives as highly efficient but reversible transdermal permeation enhancers

Overcoming the skin barrier properties efficiently, temporarily, and safely for successful transdermal drug delivery remains a challenge. We synthesized three series of potential skin permeation enhancers derived from natural amino acid derivatives proline, 4-hydroxyproline, and pyrrolidone carboxylic acid, which is a component of natural moisturizing factor. Permeation studies using in vitro human skin identified dodecyl prolinates with N-acetyl, propionyl, and butyryl chains (Pro2, Pro3, and Pro4, respectively) as potent enhancers for model drugs theophylline and diclofenac. The proline derivatives were generally more active than 4-hydroxyprolines and pyrrolidone carboxylic acid derivatives. Pro2-4 had acceptable in vitro toxicities on 3T3 fibroblast and HaCaT cell lines with IC₅₀ values in tens of µM. Infrared spectroscopy using the human stratum corneum revealed that these enhancers preferentially interacted with the skin barrier lipids and decreased the overall chain order without causing lipid extraction, while their effects on the stratum corneum protein structures were negligible. The impacts of Pro3 and Pro4 on an in vitro transepidermal water loss and skin electrical impedance were fully reversible. Thus, proline derivatives Pro3 and Pro4 have an advantageous combination of high enhancing potency, low cellular toxicity, and reversible action, which is important for their potential in vivo use as the skin barrier would quickly recover after the drug/enhancer administration is terminated.

Transdermal Drug Delivery Systems and Their Use in Obesity Treatment

Transdermal drug delivery (TDD) has recently emerged as an effective alternative to oral and injection administration because of its less invasiveness, low rejection rate, and excellent ease of administration. TDD has made an important contribution to medical practice such as diabetes, hemorrhoids, arthritis, migraine, and schizophrenia treatment, but has yet to fully achieve its potential in the treatment of obesity. Obesity has reached epidemic proportions globally and posed a significant threat to human health. Various approaches, including oral and injection administration have widely been used in clinical setting for obesity treatment. However, these traditional options remain ineffective and inconvenient, and carry risks of adverse effects. Therefore, alternative and advanced drug delivery strategies with higher efficacy and less toxicity such as TDD are urgently required for obesity treatment. This review summarizes current TDD technology, and the main anti-obesity drug delivery system. This review also provides insights into various anti-obesity drugs under study with a focus on the recent developments of TDD system for enhanced anti-obesity drug delivery. Although most of presented studies stay in animal stage, the application of TDD in anti-obesity drugs would have a significant impact on bringing safe and effective therapies to obese patients in the future.

Current Status of Amino Acid-Based Permeation Enhancers in Transdermal Drug Delivery

Transdermal drug delivery (TDD) presents many advantages compared to other conventional routes of drug administration, yet its full potential has not been achieved. The administration of drugs through the skin is hampered by the natural barrier properties of the skin, which results in poor permeation of most drugs. Several methods have been developed to overcome this limitation. One of the approaches to increase drug permeation and thus to enable TDD for a wider range of drugs consists in the use of chemical permeation enhancers (CPEs), compounds that interact with skin to ultimately increase drug flux. Amino acid derivatives show great potential as permeation enhancers, as they exhibit high biodegradability and low toxicity. Here we present an overview of amino acid derivatives investigated so far as CPEs for the delivery of hydrophilic and lipophilic drugs across the skin, focusing on the structural features which promote their enhancement capacity.

Permeation enhancers in transdermal drug delivery: benefits and limitations

Introduction: Transdermal drug delivery has several clinical benefits over conventional routes of drug administration. To open the transdermal route for a wider range of drugs, including macromolecules, numerous physical and chemical techniques to overcome the natural low skin permeability have been developed.Areas covered: This review focuses on permeation enhancers (penetration enhancers, percutaneous absorption promoters or accelerants), which are chemicals that increase drug flux through the skin barrier. First, skin components, drug permeation pathways, and drug properties are introduced. Next, we discuss properties of enhancers, their various classifications, structure-activity relationships, mechanisms of action, reversibility and toxicity, biodegradable enhancers, and synergistic enhancer combinations.Expert opinion: Overcoming the remarkable skin barrier properties in an efficient, temporary and safe manner remains a challenge. High permeation-enhancing potency has long been perceived to be associated with toxicity and irritation potential of such compounds, which has limited their further development. In addition, the complexity of enhancer interactions with skin, formulation and drug, along with their vast chemical diversity hampered understanding of their mechanisms of action. The recent development in the field revealed highly potent yet safe enhancers or enhancer combinations, which suggest that enhancer-aided transdermal drug delivery has yet to reach its full potential.

Esters of terpene alcohols as highly potent, reversible, and low toxic skin penetration enhancers

Skin penetration/permeation enhancers are compounds that improve (trans)dermal drug delivery. We designed hybrid terpene-amino acid enhancers by conjugating natural terpenes (citronellol, geraniol, nerol, farnesol, linalool, perillyl alcohol, menthol, borneol, carveol) or cinnamyl alcohol with 6-(dimethylamino)hexanoic acid through a biodegradable ester linker. The compounds were screened for their ability to increase the delivery of theophylline and hydrocortisone through and into human skin ex vivo. The citronellyl, bornyl and cinnamyl esters showed exceptional permeation-enhancing properties (enhancement ratios up to 82) while having low cellular toxicities. The barrier function of enhancer-treated skin (assessed by transepidermal water loss and electrical impedance) recovered within 24 h. Infrared spectroscopy suggested that these esters fluidized the stratum corneum lipids. Furthermore, the citronellyl ester increased the epidermal concentration of topically applied cidofovir, which is a potent antiviral and anticancer drug, by 15-fold. In conclusion, citronellyl 6-(dimethylamino)hexanoate is an outstanding enhancer with an advantageous combination of properties, which may improve the delivery of drugs that have a limited ability to cross biological barriers.

Galactosyl Pentadecene Reversibly Enhances Transdermal and Topical Drug Delivery

PURPOSE

To study new skin penetration/permeation enhancers based on amphiphilic galactose derivatives.

METHODS

Two series of alkyl and alkenyl galactosides were synthesized and evaluated for their enhancing effect on transdermal/topical delivery of theophylline (TH), hydrocortisone (HC) and cidofovir (CDV), reversibility of their effects on transepidermal water loss (TEWL) and skin impedance, interaction with the stratum corneum using infrared spectroscopy, and cytotoxicity on keratinocytes and fibroblasts.

RESULTS

Initial evaluation identified 1-(α-D-galactopyranosyl)-(2E)-pentadec-2-ene A15 as a highly potent enhancer - it increased TH and HC flux through human skin 8.5 and 5 times, respectively. Compound A15 increased the epidermal concentration of a potent antiviral CDV 7 times over that reached by control and Span 20 (an established sugar-based enhancer). Infrared spectroscopy of human stratum corneum indicated interaction of A15 with skin barrier lipids but not proteins. These effects of A15 on the skin barrier were reversible (both TEWL and skin impedance returned to baseline values within 24 h after A15 had been removed from skin). In vitro toxicity of A15 on HaCaT keratinocytes and 3T3 fibroblasts was acceptable, with IC₅₀ values over 60 μM.

CONCLUSIONS

Galactosyl pentadecene A15 is a potent enhancer with low toxicity and reversible action.

Decylglucoside-based microemulsions for cutaneous localization of lycopene and ascorbic acid

Cutaneous delivery of combinations of antioxidants offers the possibility of enhanced protection against UV-radiation. In this study, we investigated the potential of sugar-based microemulsions containing monoglycerides to promote simultaneous cutaneous delivery of lycopene and ascorbic acid, and increase tissue antioxidant activity. Lycopene and ascorbic acid were incorporated (0.04% and 0.2% (w/w), respectively) in decylglucoside-based microemulsions containing isopropyl myristate mixed with monocaprylin (ME-MC), monolaurin (ME-ML) or monoolein (ME-MO) as oil phase. The microemulsions increased lycopene delivery into porcine ear skin by 3.3- to 8-fold compared to a drug solution. The effect of microemulsions on ascorbic acid cutaneous delivery was more modest (1.5-3-fold), and associated with an approximately 2-fold increase in transdermal delivery. According to their penetration-enhancing ability, the microemulsions were ranked ME-MC>ME-MO>ME-ML. This superiority of ME-MC coincided with a stronger effect in decreasing skin electrical resistance. After 18 h of treatment, the viability of bioengineered skin treated with ME-MC was 2.2-times higher compared to Triton-X100 (moderate irritant), demonstrating that ME-MC is less cytotoxic. Skin treatment with ME-MC containing both antioxidants increased the tissue antioxidant activity by 10.2-fold, but no synergism between the antioxidants was observed.

Esters and amides of hexanoic acid substituted with tertiary amino group in terminal position and their activity as transdermal permeation enhancers

Series of alkyl esters of 6-diethylamino-, 6-(pyrrolidin-1-yl)-, 6- (piperidin-1-yl) and 6-(morpholin-1-yl)hexanoic acids and alkylamides of 6-dimethylamino-, 6-(piperidin-1-yl) and 6-(morpholin-1-yl)hexanoic acids, containing 8-12 carbon atoms in the alkyl chain, were prepared by methods of classical organic synthesis. The appropriate secondary amine was alkylated with ethyl-6-bromohexanoate to give ester of ?-substituted hexanoic acid, except of ethyl-6-dimethylaminohexanoate (1), which was prepared by Eschweiler-Clarke methylation of 6-aminohexanoic acid followed by direct esterification with ethanol. The resulted esters of ?-substituted hexanoic acids underwent direct transesterification with long chain alkanols to yield the desired amino esters, or they were treated with long-chain alkylamines to prepare secondary amides of the appropriate heterocyclic hexanoic acids. These products were in vitro tested on their activity as transdermal permeation enhancers on the strips of the excised human skin with theophylline as the model permeant. The activity was evaluated using parameter enhancement ratio (ER), defined as the ratio between the overall amount of the permeant passing through the skin with the tested enhancer and that without tested substance. Decyl 6-(pyrrolidin- 1-yl)hexanoate (16) with ER = 30 showed the highest activity. The enhancing effects of the esters were generally better than those of the amides.

Transdermal delivery of carvedilol in rats: probing the percutaneous permeation enhancement mechanism of soybean extract-chitosan mixture

BACKGROUND

This study was designed for investigating the effect of soybean (SS) extract and chitosan (CTN) in facilitating the permeation of carvedilol (CDL) across rat epidermis.

METHOD

Transdermal flux of carvedilol through heat-separated rat epidermis was investigated in vitro using vertical Keshary-Chien diffusion cells. Biophysical and microscopic manifestations of epidermis treated with SS-extract, CTN, and SS extract-CTN mixture were investigated by using DSC, TEWL, SEM, and TEM. Biochemical estimations of cholesterol, sphingosine, and triglycerides were carried out for treated excised as well as viable rat epidermis. The antihypertensive activity of the patches in comparison to that after oral administration of carvedilol was studied in deoxycorticosterone acetate-induced hypertensive rats.

RESULTS

The solubility of CDL was found to be maximum in the presence of 1% (w/v) SS extract. The K(IPM/PB) of CDL decreased with increase in concentration of SS extract. The in vitro permeation of CDL across rat epidermis increased and was maximum with combination of SS extract and chitosan (CTN). Biochemical and microscopic studies revealed the initiation of reversal of barrier integrity after 12 hours. Furthermore, the application of patches containing SS extract-CTN mixture resulted in sustained release of carvedilol, which was able to control the hypertension in deoxycorticosterone acetate (DOCA) induced hypertensive rats through 24 hours. CTN was found to potentiate the permeation enhancing activity of SS extract.

CONCLUSION

The developed transdermal patches of CDL containing SS extract-CTN mixture exhibited better performance as compared to oral administration in controlling hypertension in rats.

Microemulsions containing medium-chain glycerides as transdermal delivery systems for hydrophilic and hydrophobic drugs

We evaluated the ability of microemulsions containing medium-chain glycerides as penetration enhancers to increase the transdermal delivery of lipophilic (progesterone) and hydrophilic (adenosine) model drugs as well as the effects of an increase in surfactant blend concentration on drug transdermal delivery. Microemulsions composed of polysorbate 80, medium-chain glycerides, and propylene glycol (1:1:1, w/w/w) as surfactant blend, myvacet oil as the oily phase, and water were developed. Two microemulsions containing different concentrations of surfactant blend but similar water/oil ratios were chosen; ME-lo contained a smaller concentration of surfactant than ME-hi (47:20:33 and 63:14:23 surfactant/oil/water, w/w/w). Although in vitro progesterone and adenosine release from ME-lo and ME-hi was similar, their transdermal delivery was differently affected. ME-lo significantly increased the flux of progesterone and adenosine delivered across porcine ear skin (4-fold or higher, p < 0.05) compared to progesterone solution in oil (0.05 +/- 0.01 microg/cm(2)/h) or adenosine in water (no drug was detected in the receptor phase). The transdermal flux of adenosine, but not of progesterone, was further increased (2-fold) by ME-hi, suggesting that increases in surfactant concentration represent an interesting strategy to enhance transdermal delivery of hydrophilic, but not of lipophilic, compounds. The relative safety of the microemulsions was assessed in cultured fibroblasts. The cytotoxicity of ME-lo and ME-hi was significantly smaller than sodium lauryl sulfate (considered moderate-to-severe irritant) at same concentrations (up to 50 microg/mL), but similar to propylene glycol (regarded as safe), suggesting the safety of these formulations.