31P solid-state NMR based monitoring of permeation of cell penetrating peptides into skin

The main objective of the current study was to investigate penetration of cell penetrating peptides (CPPs: TAT, R₈, R₁₁, and YKA) through skin intercellular lipids using ³¹P magic angle spinning (MAS) solid-state NMR. In vitro skin permeation studies were performed on rat skin, and sections (0–60, 61–120, and 121–180 μm) were collected and analyzed for ³¹P NMR signal. The concentration-dependent shift of 0, 25, 50, 100, and 200 mg/ml of TAT on skin layers, diffusion of TAT, R₈, R₁₁, and YKA in the skin and time dependent permeation of R₁₁ was measured on various skin sections using ³¹P solid-state NMR. Further, CPPs and CPP-tagged fluorescent dye encapsulate liposomes (FLip) in skin layers were tagged using confocal microscopy. The change in ³¹P NMR chemical shift was found to depend monotonically on the amount of CPP applied on skin, with saturation behavior above 100 mg/ml CPP concentration. R₁₁ and TAT caused more shift in solid-state NMR peaks compared to other peptides. Furthermore, NMR spectra showed R₁₁ penetration up to 180 μm within 30 min. The results of the solid-state NMR study were in agreement with confocal microscopy studies. Thus, ³¹P solid-state NMR can be used to track CPP penetration into different skin layers.