KINETICS, ULTRASTRUCTURAL ASPECTS AND MOLECULAR MODELING OF TRANSDERMAL PEPTIDE FLUX ENHANCEMENT BY N-ALKYLAZACYCLOHEPTANONES

Transdermal delivery is an attractive route to administer peptide drugs. The stratum corneum, however, is a barrier for peptides; hence there is a need for agents to enhance peptide transport. The aim of this study was to investigate the enhancement properties of azacycloheptan-2-ones (Azones) as a function of hydrocarbon chain length. Their ability to enhance the percutaneous penetration of desglycinamide arginine vasopressin (DGAVP) was taken as a criterion. The flux of DGAVP through human stratum corneum was measured. For control experiments the stratum corneum was either untreated or immersed in propylene glycol (PG). The non-enhanced peptide flux through human stratum corneum was 1.6 +/- 0.1 nmol/cm2 per h (peptide concentration in the donor 6.0 mM). Pretreatment with PG or hexyl- or octyl-Azone did not change the flux significantly. However, the permeability increased 1.9-fold after pretreatment with decyl-Azone, 3.5-fold with dodecyl-Azone, and 2.5-fold with tetradecyl-Azone. Electron micrographs taken from freeze-fracture replicas of skin samples treated with either PG or dodecyl-Azone suggest that these treatments do not drastically change the lamellar appearance of the intercellular lipids. Taken together, the morphological and kinetical data suggest that the enhancing effect of Azone is caused by interference with the packing arrangement of the intercellular lamellar lipids in the stratum corneum, most likely by insertion of Azones into the lipid bilayers. In order to assess the capability of the Azone molecule to perturb the lipid arrangement within the bilayers, the degree of conformational freedom in Azone's polar head group was investigated using computer-aided molecular modelling. These calculations indicated that, upon turning the carbonyl moiety towards the interlamellar hydrophilic domain, a 'soup-spoon' conformation can be obtained, which is only 1 kcal/mol away from the minimum energy conformation. This 'soup-spoon' conformation would be highly favourable in order to accommodate the Azone molecule at the interface between the hydrocarbon and polar head group regions, respectively. This conformation might strongly perturb the lipid structure, making it more permeable to the penetrant. Such an effect apparently optimizes around a chain length of about 12 C-atoms.