![]() ![]() 28–30 Moreover, an increase in the particle curvature with a crystal size below 500 nm facilitates the penetration of intact drug particles into the skin through the hair follicles and its subsequent absorption by the surrounding follicular epithelium. 26, 27 Moreover, the increased dissolution rate and saturation solubility intensifies the concentration gradient between the biological membranes including stratum corneum and topical preparation, subsequently allowing higher penetration into skin layers. 24, 25 A decrease in particle size leads to an increase in surface area, which is favorable for the adhesion of drug particles to biological membranes. 21–23 Drug nanocrystals are colloidal dispersions of sub-micronized drug particles stabilized using minimal polymeric or amphiphilic stabilizers in a continuous phase. Recently, drug nanocrystal suspension (NS) has been in the spotlight as a promising topical as well as transdermal delivery system for lipophilic pharmaceuticals and cosmetic substances. Additionally, previous skin absorption-enhancing approaches for CA include high contents of surfactants, fatty acids, or cosolvents as solubility or penetration enhancers, which may cause skin irritation such as itching, rash, and even local inflammation after long-term application. 16, 17 However, even with these solubilization systems, the content of CA in external formulations has been restrained below 1.0% w/v, because of the low solubility of AA or MA in formulations. 16–19 These approaches have been effective in increasing the skin absorption of the active ingredients of CA, with improved pharmacological activities. 16ĭifferent pharmaceutical approaches such as employment of penetration enhancers, lipid nanoparticles, micelles, liposomal, and niosomal vesicle systems have been implemented to enhance the solubility of the water-insoluble ingredients and to promote skin absorption into relevant skin layers. AA and MA are poorly water-soluble lipophilic ingredients with partition coefficient (log P) values of 5.7 and 4.3, respectively. Furthermore, solubilization techniques are additionally required to formulate external preparations and to impede penetration of the hydrophobic aglycones into the skin layer. 14, 15 The high molecular weight (959.1 daltons) and hydrophilicity (log P value of 0.1) of AS prevent the penetration of the pentacyclic triterpene across the stratum corneum. 12, 13 It is composed of 15–20 cell layers of corneocytes that are embedded in a lipid-enriched intercellular space consisted of principally ceramides, free fatty acids, and cholesterol. The stratum corneum, ie, the uppermost layer of the epidermis with a thickness of 10–15 μm, represents the greatest challenge for the penetration of active substances into epidermis and dermis. To initiate tissue regeneration and wound healing activities of CA, proficient delivery of the bioactive ingredients into the relevant skin layers across the stratum corneum is a prerequisite. Abbreviations: AS, asiaticoside AA, asiatic acid MA, madecassic acid. ![]()
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