Case studies compare efficacy testing of preservatives for topical formulations with probiotic actives.
In recent years, many topical probiotic personal care products have been launched into the market (1). In an August 2016 review for Dermatology Times (2), dermatologist Patricia Farris concluded, “The studies reviewed suggest that topical prebiotics, probiotics, and bacterial cell lysates do provide demonstrable skin benefits …At this time, it appears that more studies are warranted to determine if these products are really worth the hype.” These scientific reviews are quick to point out that well-crafted, vehicle-controlled clinical trial results are not generally available for topical semisolids containing live microorganisms. One reason that topical probiotic therapies have not advanced beyond the personal care “post-marketing surveillance regulatory environment” into the controlled clinical trial “new drug approval regulatory environment” is the difficulty in reconciling FDA microbiological requirements for a product containing live microorganisms. More specifically, how can a topical suspension containing more than 50,000 colony forming units (CFU) of probiotic active pharmaceutical ingredient (API) pass United States Pharmacopeia (USP) microbial enumeration testing (USP <61>) (3), tests for specified organisms (USP <62>) (4), and antimicrobial preservative effectiveness (USP <51>) (5)?
Two case studies are being presented here to explore the strategy of adequately preserving the formulation, but using a preservative of sufficiently narrow spectrum to maintain viability/potency of the probiotic active. The first case study uses a probiotic strain of Propionibacterium acnes (P. acnes). P. acnes is a lipophilic, gram-positive anaerobic bacillus that resides in the pilosebaceous unit of human skin. Hundreds of different strains of P. acnesexist. The lipases, proteases, and hyaluronidases secreted by certain strains of P. acnes injure the lining of the pilosebaceous unit and activate production of proinflammatory cytokines that in turn lead to acne vulgaris. Other strains of P. acnes produce no inflammatory response and thus do not induce the symptoms of acne vulgaris. One of these non-inflammatory strains of P. acneshas been chosen to be used as an “active ingredient” in a topical formulation product. In the second case study, the microorganism is a bacteriophage (phage), which is a virus that infects and replicates within a bacterium, ultimately causing bacterial death. The phage used in this study was isolated from the follicular casts obtained from volunteers with facial comedones. Bacteriophages were identified and isolated from the comedones and were then propagated using an amplification process and plated against different P. acnes strains to determine breadth of efficacy to assure the selected phage was suitable to infect and eradicate pathogenic (inflammatory) P. acnes strains.
Formulating a living microorganism is fundamentally different from formulating a small-molecule active topical product. Because the P. acnes probiotic or phage products will be dosed as suspensions, active solubility, solvent compatibility, and penetration across the stratum corneum do not factor in to the development of a topical probiotic. In contrast, the aqueous probiotic formulation does require that pH and osmolality be adjusted to values that assure a favourable environment for the microorganisms to remain viable. In these two case studies, eight different formulations containing preservatives were tested for live microorganism viability over six weeks after compounding.
Material and methods
The low immunogenic strain of P. acnes (probiotic bacterial active) and phage that attacks pathogenic P. acnes (microbiome editor) were provided by Phi Therapeutics. Two gelled aqueous products were made with the intent of formulating cosmetically elegant products: hydroxyethyl cellulose (HEC) at 1.5% w/w and polyacrylic acid polymer or carbomer (Carbopol 980, Lubrizol) at 0.75% w/w, both titrated with propylene glycol until isosmotic (~285 mOsm/k). Both gels were prepared with the following preservative systems: preservative free, methylparaben (0.1%) and propylparaben (0.02%), phenoxyethanol (1.0%), and potassium sorbate (0.2%). In addition, two solution products were made: an 80:20 water:propylene glycol blend (w/w) and an 80:20 water:ethanol blend (w/w). Both solution blends were considered self-preserving. The pH of all the products were taken, but no pH modifiers were added. Active viability and stability testing were conducted every two weeks after addition of active microorganism by diluting the formulated products to a known concentration of active microorganisms and plating them out. P. acnes concentration was calculated using diluted growth promotion plating and back calculating; the number of phage was calculated by plating the dilutions onto bacterial lawns and looking for the presence of kill zones
Did you know that one of A&C’s preservatives 2-Phenoxyethanol is used in the case study? Please click https://www.acggp.com/ to learn more about 2-Phenoxyethanol.
** This article was written by Jason Carbol, Pia Isabel Tan, Yug Varma, David W. Osbornefrom Pharmaceutical Technology..**