In Vivo Elongation of Topically Applied Fatty Acids From a Range of Different Topical Skincare Formulations

The lipid matrix of the stratum corneum (SC) is essential to skin barrier function, and detrimental structural changes may occur in response to human behaviour, disease, and external factors. The application of topical formulations is a common approach to repairing the SC and has been investigated ex vivo and in vitro. We aim to demonstrate the first clinical evidence of deuterated palmitic acid (d31-PA) deposition from topical formulations into the SC of human skin in vivo and its conversion into longer-chain free fatty acids (FFAs) and ceramides. Healthy female participants aged 30 to 50 years old participated in two separate product application studies. Participants were randomized to an application of a leave-on lotion or a syndet bar containing d31-PA in the first study, with the second study having an option of a body wash containing different levels of d31-PA. Skin tapes and solvent extracts were collected at baseline and Day 28. Lipid composition and the presence of deuterated sphingosine were analyzed by liquid chromatography tandem mass spectrometry (LC/MS/MS). After four weeks, d31-PA was sufficiently delivered into the SC by all topical formulations. LC/MS/MS data show that FFAs are elongated up to C26:0 and used in the formation of ceramides and free sphingosine. These data provide the first in vivo evidence that FFAs deposited from a range of different topical formulations are elongated in the SC and provide the building blocks for ceramide synthesis. Together, the evidence shows that commercially available topical formulations can nourish the skin which may contribute to maintaining a healthy skin barrier.