Unsaturated cyclic ureas as new nontoxic biodegradable transdermal penetration enhancers I: Synthesis

Design of an Ante-enhancer with an Azone-Mimic Structure using Ionic Liquid

PURPOSE

Laurocapram (Azone) was broadly examined as a representative enhancer of skin penetration in the 1980s. However, it was not approved for treatment because it caused skin irritation following its penetration into the epidermis through the stratum corneum. In the present study, a so-called ante-enhancer with an Azone-mimic structure was designed based on an ante-drug with negligible systemic toxic effects following its permeation through the skin.

METHODS

The ante-enhancer was designed using ionic liquid technology: an ionic liquid-type ante-enhancer (IL-Azone) with an Azone-mimic structure was prepared from ε-caprolactam and myristic acid as cationic and anionic substances, respectively. The enhancing effects of IL-Azone on the permeation by the following model drugs through pig skin were examined: isosorbide 5-mononitrate (ISMN), antipyrine (ANP), and fluorescein isothiocyanate dextran (FD-4). Skin irritation by IL-Azone was assessed using the Draize method.

RESULTS

The primary irritation index (P.I.I.) of IL-Azone by the Draize method was markedly lower than that of Azone (6.9). Although the ability of IL-Azone to enhance skin penetration was not as high as Azone, IL-Azone moderately increased skin permeation by the model compounds tested (ISMN: 4.7 fold, ANP: 4.5 fold, FD-4: 4.0 fold).

CONCLUSIONS

These results suggest the usefulness of designing a skin penetration enhancer using ionic liquid technology. Further trials on the ionic liquid design with an Azone-mimic structure using other cations and anions may lead to the development of better ante-enhancers.

Effect of Different Carriers on in vitro Permeation of Meloxicam through Rat Skin

The ability of β-cyclodextrin, hydroxypropyl-β-cyclodextrin, polyvinyl pyrrolidone and urea to influence the percutaneous absorption of meloxicam through isolated rat skin was evaluated. Carrier complex were prepared by kneading method in 1:1 and 1:2 in molar ratios for β-cyclodextrin and hydroxypropyl-β-cyclodextrin and in 1:1, 1:3 and 1:5 in weight ratios for polyvinyl pyrrolidone and urea. The complexes were characterized by IR, DSC and evaluated for solubility, dissolution and skin permeability. The solubility, dissolution and permeability of meloxicam were enhanced by using the carriers. The influence of cyclodextrins, polyvinyl pyrrolidone and urea on in vitro permeation of meloxicam through rat skin was investigated by incorporation of prepared carrier complex in 1% carbopol gel. The prepared gel was evaluated for drug content, pH and viscosity and in vitro permeation. All the percutaneous parameters like flux (Jss), amount permeated (Q₆), diffusivity (D), permeability coefficient (K_p), partition coefficient (K) and release rate constant (k) were calculated statistically. In vitro permeation study showed the trend that the penetration flux and enhancement factor increases with increasing concentration of β-cyclodextrin and hydroxypropyl-β-cyclodextrin and then decrease dramatically in case of hydroxypropyl-β-cyclodextrin gel formulation with the increase to 1:2 ratio. Similar changes in pattern of permeation were also observed with polyvinyl pyrrolidone and urea carrier complex. These findings concluded that the carriers cyclodextrins, polyvinyl pyrrolidone and urea could be used as transdermal permeation enhancer in topical preparation of meloxicam.

Percutaneous Penetration Enhancers: An Overview

Transdermal drug delivery is the controlled release of drugs through the skin to obtain therapeutic levels systematically. Several technological advances have been made in the recent decades to enhance percutaneous drug penetration. This overview focuses on the physical, biochemical, and chemical means of penetration enhancement, as well as the classification and mechanisms of chemical penetration enhancers, their application in transdermal drug delivery, and trends and development in penetration enhancement.

Overcoming the Stratum Corneum: The Modulation of Skin Penetration

It is preferred that topically administered drugs act either dermally or transdermally. For that reason they have to penetrate into the deeper skin layers or permeate the skin. The outermost layer of the human skin, the stratum corneum, is responsible for its barrier function. Most topically administered drugs do not have the ability to penetrate the stratum corneum. In these cases modulations of the skin penetration profiles of these drugs and skin barrier manipulations are necessary. A skin penetration enhancement can be achieved either chemically, physically or by use of appropriate formulations. Numerous chemical compounds have been evaluated for penetration-enhancing activity, and different modes of action have been identified for skin penetration enhancement. In addition to chemical methods, skin penetration of drugs can be improved by physical options such as iontophoresis and phonophoresis, as well as by combinations of both chemical and physical methods or by combinations of several physical methods. There are cases where skin penetration of the drug used in the formulation is not the aim of the topical administration. Penetration reducers can be used to prevent chemicals entering the systemic circulation. This article concentrates on the progress made mainly over the last decade by use of chemical penetration enhancers. The different action modes of these substances are explained, including the basic principles of the physical skin penetration enhancement techniques and examples for their application.

Efficient approach to acyloxymethyl esters of nalidixic acid and in vitro evaluation as intra-ocular prodrugs

Various alkylcarbonyloxymethyl esters of nalidixic acid ranging from 3 to 15 carbon units in the pro-moiety have been prepared and assessed as potential prodrugs. Their chromatographic retention factors k', silicone oil solubilities and in vitro conversion to nalidixic acid by a commercial esterase were determined together with their in vitro antimicrobial activity and cytotoxicity. The preliminary results suggest that silicone oil may have potential for the intra-ocular delivery of antibacterial compounds. Moreover, the in vitro release rate can be controlled by the lipophilicity of the prodrug.

Veterinary drug delivery: potential for skin penetration enhancement

A range of topical products are used in veterinary medicine. The efficacy of many of these products has been enhanced by the addition of penetration enhancers. Evolution has led to not only a highly specialized skin in animals and humans, but also one whose anatomical structure and skin permeability differ between the various species. The skin provides an excellent barrier against the ingress of environmental contaminants, toxins, and microorganisms while performing a homeostatic role to permit terrestrial life. Over the past few years, major advances have been made in the field of transdermal drug delivery. An increasing number of drugs are being added to the list of therapeutic agents that can be delivered via the skin to the systemic circulation where clinically effective concentrations are reached. The therapeutic benefits of topically applied veterinary products is achieved in spite of the inherent protective functions of the stratum corneum (SC), one of which is to exclude foreign substances from entering the body. Much of the recent success in this field is attributable to the rapidly expanding knowledge of the SC barrier structure and function. The bilayer domains of the intercellular lipid matrices within the SC form an excellent penetration barrier, which must be breached if poorly penetrating drugs are to be administered at an appropriate rate. One generalized approach to overcoming the barrier properties of the skin for drugs and biomolecules is the incorporation of suitable vehicles or other chemical compounds into a transdermal delivery system. Indeed, the incorporation of such compounds has become more prevalent and is a growing trend in transdermal drug delivery. Substances that help promote drug diffusion through the SC and epidermis are referred to as penetration enhancers, accelerants, adjuvants, or sorption promoters. It is interesting to note that many pour-on and spot-on formulations used in veterinary medicine contain inert ingredients (e.g., alcohols, amides, ethers, glycols, and hydrocarbon oils) that will act as penetration enhancers. These substances have the potential to reduce the capacity for drug binding and interact with some components of the skin, thereby improving drug transport. However, their inclusion in veterinary products with a high-absorbed dose may result in adverse dermatological reactions (e.g., toxicological irritations) and concerns about tissue residues. These are important considerations when formulating a veterinary transdermal product when such compounds are added, either intentionally or otherwise, for their penetration enhancement ability.

Synthesis and in vitro transdermal penetration enhancing activity of lactam N-acetic acid esters

A homologous series of N-acetic acid esters of 2-pyrrolidinone and 2-piperidinone has been prepared and evaluated for its ability to enhance the skin content and flux of hydrocortisone 21-acetate in hairless mouse skin in vitro. Enhancement ratios (ER) were determined for flux (J), 24-hour diffusion cell receptor cell concentrations (Q24), and 24-h full-thickness mouse skin steroid content (SC) and compared to control values (no enhancer present). In addition, in an attempt to abrogate toxicity, these dermal penetration enhancers were designed to have the potential for biodegradation by dermal esterases. 2-Oxopyrrolidine-alpha acetic acid dodecyl ester (5) showed the highest enhancement ratios for J (ER 67.33) and Q24 (ER 180.66). 2-Oxopiperidine-alpha-acetic acid decyl ester (10) showed a high Q24 (ER 162.07) but a lower J (ER 12.67). 2-Oxopyrrolidine-alpha-acetic acid decyl ester (3) showed the highest enhancement ratio for SC (ER 8.7). The ER Q24 for 3, 5 and 10, as well as other lactam N-acetic acid esters in this work, were significantly higher than the ER found using Azone as enhancer.

Synthesis and enhancing effect of dodecyl 2-(N,N-dimethylamino)propionate on the transepidermal delivery of indomethacin, clonidine, and hydrocortisone

The biodegradable transdermal penetration enhancer, dodecyl 2-(N,N-dimethylamino)propionate (II; DDAIP), was prepared by reacting dodecyl 2-bromopropionate (I), obtained by reaction of n-dodecanol with 2-bromopropionyl halogenide, with dimethylamine. The penetration enhancing effects of DDAIP on the transport of indomethacin, clonidine, and hydrocortisone across shed snake skin (Elaphe obsoleta) were evaluated. Azone and lauryl alcohol, a possible decomposition product of DDAIP, were used as standard enhancers for comparison. In terms of flux, DDAIP showed 4.7 and 7.5 times the promoting effect for indomethacin compared to azone and lauryl alcohol, respectively. With clonidine this effect was 1.7 and 3.1 times, whereas with hydrocortisone it was 2.4 and 2.8 times higher, respectively. In vitro biodegradability of DDAIP was demonstrated in the presence of porcine esterase. The results indicate that DDAIP increases markedly the transepidermal delivery of several types of drug substances.

Enhancing effect of combining two pyrrolidone vehicles on transdermal drug delivery

The enhancing effect of combining 1-methyl-2-pyrrolidone (MP) and 1-lauryl-2-pyrrolidone (LP) as the vehicles for transdermal penetration of phenolsulphonphthalein (phenol red) has been investigated by using an in-vitro technique with excised rat skin. LP had a higher enhancing effect on the penetration of phenol red than MP, but there was a long lag time before steady-state penetration was attained. A potent effect with a shorter lag time was obtained when MP and LP were used together. This potentiation was maintained when the concentration of MP was decreased by 95%. The combined vehicle also enhanced the skin accumulation of phenol red. MP promoted the rapid penetration of LP into the skin and potentiated the enhancing effect of LP on the penetration of phenol red and thereby shortened the lag time. The combined vehicle also enhanced the penetration of the hydrophilic anticancer agent, 5-fluorouracil.

New alkyl N,N-dialkyl-substituted amino acetates as transdermal penetration enhancers

New alcohol derivatives of N,N-disubstituted amino acids with a low toxicity have been synthesized and evaluated for their transdermal penetration enhancing effects on the transport of indomethacin from petrolatum ointments across shed skin of black rat snake (Elaphe obsoleta). The derivatives show excellent penetration enhancement of indomethacin, as high as 3.8 times that of Azone, with decyl N,N-dimethylamino acetate as the lead compound in the series. The release of indomethacin from an ointment containing 1% indomethacin, 5% dodecyl N,N-dimethylamino acetate, and 94% petrolatum was 3.15 micrograms/min1/2/cm2. Saturation studies performed by incorporating varying concentrations of indomethacin, from 0.1 to 10%, into the ointments and determination of the fluxes of indomethacin demonstrated that the saturated concentration of indomethacin in petrolatum base was approximately 1%. Penetration fluxes of indomethacin (1%) through snake skin increased linearly as the concentration of dodecyl N,N-dimethylamino acetate increased from 2.5 to 15%. Experiments involving the pretreatment of the snake skins with dodecyl N,N-dimethylamino acetate indicated that pretreatment of the skin increased the skin permeability significantly. Electron micrograph studies on the snake skin treated with dodecyl N,N-dimethylamino acetate show clearly that the enhancer interacted with both the lipid-rich layer (mesos phase) and the keratin-rich layers (both alpha and beta phases).