The antimicrobial activity of liposomal lauric acids against Propionibacterium acnes

Abstract

This study evaluated the antimicrobial activity of lauric acid (LA) and its liposomal derivatives against Propionibacterium acnes (P. acnes), the bacterium that promotes inflammatory acne. First, the antimicrobial study of three free fatty acids (lauric acid, palmitic acid and oleic acid) demonstrated that LA gives the strongest bactericidal activity against P. acnes. However, a setback of using LA as a potential treatment for inflammatory acne is its poor water solubility. Then the LA was incorporated into a liposome formulation to aid its delivery to P. acnes. It was demonstrated that the antimicrobial activity of LA was not only well maintained in its liposomal derivatives but also enhanced at low LA concentration. In addition, the antimicrobial activity of LA-loaded liposomes (LipoLA) mainly depended on the LA loading concentration per single liposomes. Further study found that the LipoLA could fuse with the membranes of P. acnes and release the carried LA directly into the bacterial membranes, thereby killing the bacteria effectively. Since LA is a natural compound that is the main acid in coconut oil and also resides in human breast milk and liposomes have been successfully and widely applied as a drug delivery vehicle in the clinic, the LipoLA developed in this work holds great potential of becoming an innate, safe and effective therapeutic medication for acne vulgaris and other P. acnes associated diseases.

Introduction

Acne vulgaris (commonly called acne) is a skin disease that is most common during adolescence, afflicting more than 85% of teenagers and over 40 million people in the United States alone [1], [2]. Acne is inflammatory and associates with the immune response to Propionibacterium acnes (P. acnes), a Gram-positive bacterium that colonizes sebum-rich follicles [3]. The entire genome analysis of P. acnes has revealed numerous genes that regulate products involved in degrading host molecules and triggering inflammation [4]. It has been reported that P. acnes releases chemoattractants that attract the immune system cells such as neutrophils, monocytes and lymphocytes [5], [6]. Previous studies have also found that P. acnes stimulates the production of pro-inflammatory cytokines such as interleukins-1β, -8, -12, and tumor necrosis factor-α [7]. Besides acne, the overgrowth of P. acnes in human is also associated with many other diseases such as endocarditis and toxic shock syndrome [8].

Antimicrobial agents and antibiotics have been used epicutaneously to treat acne for several decades and are still widely prescribed for acne patients. The oxidizing agent benzoyl peroxide (BPO) has been one of the most frequently used epicutaneous medications to decrease P. acnes population in patients suffering from mild to moderate acne [9]. However, several side effects of BPO have been reported including erythema, scaling, burning, and flare [10]. In contrast, we recently demonstrated that lauric acid (LA), one of the typical free fatty acids found in the human sebum, shows stronger antimicrobial activity than BPO while not inducing any cytotoxicity to human sebocytes [11]. Nevertheless, LA is poorly water soluble and a solvent such as dimethylsulfoxide (DMSO) is required to dissolve LA into topical dosage forms. DMSO is a penetration enhancer that improves the transport rate through the skin barrier; however, its irritative and toxic side effects have been reported [12]. Furthermore, the conventional dosage forms such as cream, gel, and ointment have some major limitations, for example, they do not penetrate through the pilosebaceous unit efficiently and the effective concentration of drug is not sustained [12].

Liposomes have been extensively studied as a drug carrier since the early 1980s [13]. They have shown great potential to act as a topical delivery system for carrying drugs and skin care products. Liposomes can transport drugs to target sites and maintain a higher drug concentration than conventional dosage forms. As a result, the therapeutic effectiveness of liposomal drugs can be enhanced for several folds [14]. Because of the similarity in lipid composition to the epidermis, liposomes can also enhance dermal and transdermal drug delivery while reducing systemic absorption [15]. The study on liposomes for targeting drugs into the pilosebaceous units has suggested that liposomes are potent drug delivery systems for treating hair follicle-associated disorders such as acne [13]. In fact, Lieb et al. has proved that liposomes deliver much higher drug concentrations to the pilosebaceous unit than conventional drug formulations [16]. During the past a few decades, liposomes have been used as carriers to enhance clinical efficacy for a large number of drugs. Pevaryl Lipogel as the first topical liposomal drug in the market was launched in 1988 [13]. In addition, antiacne drug-loaded liposomes such as tretinoin, clindamycin, salicylic acid, and tea tree oil-loaded liposomes have been recently reported [17], [18].

Here we use liposomes to encapsulate LA and deliver it to P. acnes without using any solvents such as DMSO. LA is an amphiphilic molecule consisting of a hydrophobic hydrocarbon chain and a hydrophilic carboxylic acid headgroup. This structure makes it a good candidate drug to be incorporated into the bilayered wall of liposomes that provides an amphiphilic environment. The present study focuses on the preparation, characterization, antimicrobial activity, and drug delivery mechanism of LA-loaded liposomes (LipoLA) against P. acnes bacteria.