Unsaturated cyclic ureas as new non-toxic biodegradable transdermal penetration enhancers. II. Evaluation study

Improved Topical Drug Delivery: Role of Permeation Enhancers and Advanced Approaches

The delivery of drugs via transdermal routes is an attractive approach due to ease of administration, bypassing of the first-pass metabolism, and the large skin surface area. However, a major drawback is an inability to surmount the skin's stratum corneum (SC) layer. Therefore, techniques reversibly modifying the stratum corneum have been a classical approach. Surmounting the significant barrier properties of the skin in a well-organised, momentary, and harmless approach is still challenging. Chemical permeation enhancers (CPEs) with higher activity are associated with certain side effects restricting their advancement in transdermal drug delivery. Furthermore, complexity in the interaction of CPEs with the skin has led to difficulty in elucidating the mechanism of action. Nevertheless, CPEs-aided transdermal drug delivery will accomplish its full potential due to advancements in analytical techniques, synthetic chemistry, and combinatorial studies. This review focused on techniques such as drug-vehicle interaction, vesicles and their analogues, and novel CPEs such as lipid synthesis inhibitors (LSIs), cell-penetrating peptides (CPPs), and ionic liquids (ILs). In addition, different types of microneedles, including 3D-printed microneedles, have been focused on in this review.

Recent Advances in Nanomaterials for Dermal and Transdermal Applications

The stratum corneum, the most superficial layer of the skin, protects the body against environmental hazards and presents a highly selective barrier for the passage of drugs and cosmetic products deeper into the skin and across the skin. Nanomaterials can effectively increase the permeation of active molecules across the stratum corneum and enable their penetration into deeper skin layers, often by interacting with the skin and creating the distinct sites with elevated local concentration, acting as reservoirs. The flux of the molecules from these reservoirs can be either limited to the underlying skin layers (for topical drug and cosmeceutical delivery) or extended across all the sublayers of the epidermis to the blood vessels of the dermis (for transdermal delivery). The type of the nanocarrier and the physicochemical nature of the active substance are among the factors that determine the final skin permeation pattern and the stability of the penetrant in the cutaneous environment. The most widely employed types of nanomaterials for dermal and transdermal applications include solid lipid nanoparticles, nanovesicular carriers, microemulsions, nanoemulsions, and polymeric nanoparticles. The recent advances in the area of nanomaterial-assisted dermal and transdermal delivery are highlighted in this review.

Transdermal delivery systems in cosmetics

Transdermal delivery systems have been intensively studied over the past 2 decades, with the focus on overcoming the skin barrier for more effective application of pharmaceutical and cosmetic products. Although the cosmeceutical industry has made a substantial progress in the development and incorporation of new and effective actives in their products, the barrier function of the skin remains a limiting factor in the penetration and absorption of these actives. Enhancement via modification of the stratum corneum by hydration, acting of chemical enhancers on the structure of stratum corneum lipids, and partitioning and solubility effects are described. This review summarizes the advances in the development and mechanisms of action of chemical components that act as permeation enhancers, as well as the advances in appropriate vehicles, such as gels, emulsions, and vesicular delivery systems, that can be used for effective transdermal delivery.

Synthesis and anti-melanogenic effects of lipoic acid-polyethylene glycol ester

To develop a new potent anti-melanogenic agent, we have conjugated lipoic acid (LA) to poly (ethylene) glycol (PEG) of molecular weight 2000 and examined the effects on inhibition of tyrosinase activity and melanin synthesis in B16F10 melanoma cells. The water-soluble LA-PEG 2000 was synthesized from LA and methylated PEG by an esterification reaction in the presence of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. Synthetic LA-PEG 2000 was confirmed by IR and 1H-NMR spectroscopy. The new conjugate is a highly water-soluble molecule, which has lower cell cytotoxicity than LA. Treatment with LA-PEG 2000 significantly suppressed the biosynthesis of melanin by up to 63% at 0.25 mm and reduced tyrosinase activity by up to 80% at 0.50 mm in B16F10 melanoma cells. Furthermore, Western blot and RT-PCR studies indicated that treatment with LA-PEG 2000 decreased the level of tyrosinase, which is a melanogenic enzyme. Taken together, these results suggest that LA-PEG 2000 may inhibit melanin biosynthesis by down-regulating levels and expression of tyrosinase activity. Therefore, LA-PEG 2000 can be used effectively as a new agent to inhibit mel-anogenesis, with lower cytotoxicity than LA (parent molecule) in B16F10 melanoma cells.

The effect of terpene concentrations on the skin penetration of diclofenac sodium

Terpenes and sesquiterpenes have been suggested as promising non-toxic, non-irritating transdermal penetration enhancers. This investigation aimed to study the effect of terpene concentration on the transdermal absorption of diclofenac sodium from ethanol:glycerin:phosphate buffer solution (60:10:30). Therefore, enhancing effects of various terpenes (menthone, limonenoxide, carvone, nerolidol and farnsol) with different concentrations (0.25, 0.5, 1, 1.5 and 2.5%, v/v) on the permeation of diclofenac sodium were evaluated using Franz diffusion cells fitted with rat skin. Furthermore, solubility of diclofenac sodium in the vehicle in presence of different concentrations of terpenes was determined. The results showed that despite the negligible effect of terpenes on the drug solubility, there was a profound skin penetration enhancement effect, although the terpene enhancers varied in their ability to enhance the flux of diclofenac sodium. The results showed that at the highest concentration of terpene (2.5%, v/v) the rank order of enhancement effect for diclofenac sodium was nerolidol>farnesol>carvone>methone>limonenoxide, whereas at the low concentration of 0.25% the rank order was farnesol>carvone>nerolidol>menthone>limonenoxide. No direct relationship existed between terpene concentration and the permeation rate. The most outstanding penetration enhancer was nerolidol, providing an almost 198-fold increase in permeability coefficient of diclofenac sodium, followed by farnesol with a 78-fold increase.

Penetration enhancers

One long-standing approach for improving transdermal drug delivery uses penetration enhancers (also called sorption promoters or accelerants) which penetrate into skin to reversibly decrease the barrier resistance. Numerous compounds have been evaluated for penetration enhancing activity, including sulphoxides (such as dimethylsulphoxide, DMSO), Azones (e.g. laurocapram), pyrrolidones (for example 2-pyrrolidone, 2P), alcohols and alkanols (ethanol, or decanol), glycols (for example propylene glycol, PG, a common excipient in topically applied dosage forms), surfactants (also common in dosage forms) and terpenes. Many potential sites and modes of action have been identified for skin penetration enhancers; the intercellular lipid matrix in which the accelerants may disrupt the packing motif, the intracellular keratin domains or through increasing drug partitioning into the tissue by acting as a solvent for the permeant within the membrane. Further potential mechanisms of action, for example with the enhancers acting on desmosomal connections between corneocytes or altering metabolic activity within the skin, or exerting an influence on the thermodynamic activity/solubility of the drug in its vehicle are also feasible, and are also considered in this review.

Ketoprofen 1-alkylazacycloalkan-2-one esters as dermal prodrugs: in vivo and in vitro evaluations

Six new 1-alkylazacycloalkan-2-one esters of ketoprofen (1-6) were synthesized and evaluated as potential dermal prodrugs of ketoprofen. Their lipophilicity by both experimental lipophilicity indices (log k') and calculated ClogP was also determined. In vitro experiments were carried out to evaluate the chemical and enzymatic stability and permeation through excised human skin of these new ketoprofen derivatives. Furthermore, we investigated the in vivo topical anti-inflammatory activity of ester 5, which showed the best in vitro profile, evaluating the ability of this compound to inhibit methyl nicotinate-induced skin erythema on healthy human volunteers. Esters 1-6 showed increased lipophilicity compared with the parent drug (ketoprofen), good stability in phosphate buffer pH 7.4, and were readily hydrolyzed by porcine esterase. Results from in vitro percutaneous absorption studies showed that, among all esters synthesized, only for esters 1 and 5 did a higher cumulative amount of drug penetrate through the skin, compared with that obtained after topical application of ketoprofen. In vivo results showed an interesting delayed and sustained activity of ester 5, compared with the parent drugs.

New oligoethylene ester derivatives of 5-iodo-2'-deoxyuridine as dermal prodrugs: synthesis, physicochemical properties, and skin permeation studies

Five new oligoethylene ester derivatives (9-13) of 5-iodo-2'-deoxyuridine (IDU) were synthesized and assayed to determine their lipophilicity by both experimental lipophilicity indices (log K') and calculated partition coefficients (CLOGP). In vitro experiments were carried out to evaluate the chemical and enzymatic stability and fluxes through excised human skin of these new IDU derivatives. Esters 9-13 showed increased lipophilicity compared with the parent drug (IDU), had good stability in phosphate buffer (pH 7.4), and were readily hydrolyzed by porcine esterase. No correlation between lipophilicity and skin permeation fluxes of synthesized esters 9-13 was observed. Results from in vitro percutaneous absorption studies showed that, among all esters synthesized, only esters 9 and 10 significantly increased the cumulative amount of IDU that penetrated through excised human skin compared with the parent drug (IDU).

In vitro and in vivo evaluation of polyoxyethylene esters as dermal prodrugs of ketoprofen, naproxen and diclofenac

Novel polyoxyethylene esters of ketoprofen (1(a-e)), naproxen (2(a-e)) and diclofenac (3(a-e)) were synthesized and evaluated as potential dermal prodrugs of naproxen, ketoprofen and diclofenac. These esters were obtained by coupling these drugs with polyoxyethylene glycols by a succinic acid spacer. The aqueous solubilities, lipophilicities and hydrolysis rates of esters 1(a-e), 2(a-e) and 3(a-e) were determined in a buffered solution and in porcine esterase. The permeation of these prodrugs through excised human skin was studied in vitro. Furthermore we investigated the in vivo topical anti-inflammatory activity of esters 1(d), 2(e) and 3(e), which showed the best in vitro profile, evaluating the ability of these compounds to inhibit methyl nicotinate (MN)-induced skin erythema on healthy human volunteers. Esters 1(a-e), 2(a-e) and 3(a-e) showed good water stability and rapid enzymatic cleavage and their hydrolysis rates, both chemical and enzymatic, were not significantly affected by the length of the polyoxyethylenic chain used as promoiety. Concerning in vitro percutaneous absorption studies, only esters 1(d-e), 2(d-e) and 3(c-e) showed an increased flux through stratum corneum and epidermis membranes compared to their respective parent drugs. In vivo results showed an interesting delayed and sustained activity of esters 1(d) and 3(e) compared to the parent drugs. In conclusion polyoxyethylene glycols could prove to be suitable promoieties for ketoprofen, naproxen and diclofenac design since esters 1(d-e), 2(d-e) and 3(c-e) showed some requirements (chemical stability, enzymatic lability and an increased skin permeation) needed to obtain successful dermal prodrugs. Furthermore, was observed an appreciable and sustained in vivo topical anti-inflammatory activity of esters 1(d) and 3(e), compared to the parent drugs, using MN-induced erythema in human volunteers as inflammation model.

Effect of menthol and related terpenes on the percutaneous absorption of propranolol across excised hairless mouse skin

The potential use of terpenes/terpenoids as penetration enhancers in the transdermal delivery of propranolol hydrochloride (PL) was investigated. PL was chosen for the reasons of its extensive first-pass metabolism and short elimination half-life. The terpenes studied included L-menthol, (+)-limonene, (+/-)-linalool, and carvacrol at 1%, 5%, and 10% w/v concentrations. The diffusion of PL across excised hairless mouse skin was determined using side-by-side diffusion cells. Flux, permeability coefficient (Pm), and lag time (tL) were calculated. PL showed comparable lag times with menthol at all three concentration levels. At a 1% level of carvacrol, PL exhibited a 2.4- and 2.2-fold increase in lag time compared with 5 and 10% levels of enhancer, respectively. In the presence of limonene, PL had shown maximum lag time (between 3.0 and 3.3 h) at all three levels. In the case of linalool, the lag times for PL with 5 and 10% levels of enhancer were 7.0- and 5.2-fold less compared with 1% level. A significant (p < 0.05) concentration effect was observed only with linalool. Hydrogel-based patches were formulated with or without menthol as enhancer. Release profiles from the hydrogel formulations obeyed zero-order kinetics. The permeability of propranolol was significantly higher (p < 0.05) from the test patch than the control (no enhancer) patch across the mouse skin. The mechanism of permeation enhancement of menthol could involve its distribution preferentially into the intercellular spaces of stratum corneum and the possible reversible disruption of the intercellular lipid domain. The results suggest the potential use of menthol as effective penetration enhancer in the delivery of significant amounts of PL through skin.

Mechanism of skin penetration-enhancing effect by laurocapram

In order to clarify the mechanism of action of laurocapram (Azone) on the skin permeation of drugs, the following experiments were done. First, the effect of Azone on the skin components was compared with that of other penetration enhancers. Azone markedly fluidized liposomal lipids (as a model lipid system) compared with other enhancers. Ethanol extracted large amounts of the stratum corneum lipids, whereas Azone did not. These results suggest that the effect of Azone on the lipids in the stratum corneum is not the same as that of ethanol. In addition, ethanol increased the amount of free sulfhydryl (SH) group of keratin in the stratum corneum, whereas Azone did not directly affect the stratum corneum protein. Azone increased water content in the stratum corneum, as measured by skin conductance. This effect might be a reason for the action of Azone. For further understanding, the enhancing effects of Azone on the skin permeation of several model compounds (alcohols, sugars, and inorganic ions) were compared with the effects of pretreatment with distilled water, which was thought to increase water-holding capacity, and pretreatment with ethanol, which was thought to affect the lipids and protein in the skin barrier (i.e., stratum corneum). Pretreatment with water or ethanol enhanced skin permeation of hydrophilic compounds, whereas they decreased that of octanol, a hydrophobic compound. The tendency of Azone to increase or decrease the skin permeation rate of most compounds was similar to that of pretreatment with water or ethanol. However, the effect of Azone on the skin permeation of inorganic ions was relatively low, whereas that of pretreatment with water or ethanol was high.(ABSTRACT TRUNCATED AT 250 WORDS)