The aim of the study was to assess the skin permeation and distribution of a novel cell penetrating peptide (CPP), diketopiperazine (DKP) and HIV-1 Trans-Activator of Transcription (TAT). The effect of DKP and TAT on the skin permeation, distribution and efficacy of diclofenac was assessed.
CPPs were applied as physical admixtures with diclofenac and conjugated to diclofenac. DKP alone, TAT alone and chemical conjugates of DKP-diclofenac (1:1 molar ratio) and TAT-conjugates were synthesized. Ex vivo permeation across human epidermis was assessed (300 µL at 500 µg/mL DKP or TAT alone, physical admixture of DKP and diclofenac, and DKP-diclofenac or TAT-diclofenac conjugates). The content of DKP, TAT, diclofenac, and conjugates in the receptor and epidermal membrane samples was determined using HPLC. COX-2 inhibition by DKP-diclofenac conjugate and diclofenac alone was assessed using a COX fluorescent inhibitor assay concentration range up to 0.3 mM).
The cumulative amount of DKP, TAT and diclofenac permeating human epidermis over 24h was 10.46, 3.43, and 4.06 µg/cm2 respectively. The physical admixture with DKP enhanced permeation of diclofenac (7.61 µg/cm2; >2-fold increase). Chemical conjugation with DKP or TAT enhanced the permeation of diclofenac (7.82 and 7.92 µg/cm2 respectively). The amount of DKP, TAT, diclofenac, DKP-diclofenac and TAT-diclofenac recovered from the epidermal membrane at 24h was 10.43, 13.2, 8.58, 138.07 and 20.25 µg respectively. There was no significant difference in COX-2 inhibition was seen between diclofenac alone and DKP-conjugate.
CPPs enhanced the permeation of diclofenac into and through human epidermis. Diclofenac permeation through the epidermis was enhanced to a similar extent by conjugation to DKP and TAT (≈2-fold). However DKP significantly increased the deposition of diclofenac within the epidermis (≈16-fold). This novel skin permeation enhancement approach may be useful for targeting delivery within the skin.