Percutaneous absorption of urea

Valence topological charge-transfer indices for dipole moments: percutaneous enhancers

Valence topological charge-transfer (CT) indices are applied to the calculation of dipole moments. The algebraic and vector semisum CT indices are defined. The combination of CT indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moments of homologous series of percutaneous enhancers (phenyl alcohols and 4-alkylanilines). Linear and quadratic correlation models are obtained. CT indices improve the multivariable quadratic regression equations for the dipole moment. The variance decreases 97% (4-alkylanilines). No superposition of the corresponding G(k)-J(k)and G(k)(V)-J(k)(V) pairs is observed in the fits, which diminishes the risk of co-linearity. The inclusion of the heteroatom in the pi-electron system is beneficial for the description of the dipole moment, owing to either the role of the additional p orbitals provided by the heteroatom or the role of steric factors in the pi-electron conjugation. Inclusion of a conjugated double bond in the alkyl chain lends to more rigid structures with dipole moment variations lower than 1%.

Shed snake skin and hairless mouse skin as model membranes for human skin during permeation studies

Difficulties in obtaining and using human skin have tempted many workers to employ animal membranes for percutaneous absorption studies. We have investigated the suitability of two species of snake (Elaphe obsoleta, Python molurus) for this purpose and compared our in vitro experimental results for human skin and for hairless mouse, a currently popular model. The effects of long-term hydration on the membranes were investigated over 8 d using tritiated water as a model permeant. The initial permeability coefficients of all the membranes were similar (0.74-2.2 X 10(-3) cm 2h-1). Although the human and squamate skins did not change significantly over the test period, the permeability of hairless mouse skin increased 37 times. The actions of typical enhancers on the permeabilities of the membranes to a model penetrant 5-fluorouracil (5-FU) were tested using 3% Azone in Tween 20/saline, propylene glycol (PG), 2% Azone in PG, and 5% oleic acid in PG. While the data from snake membranes tended to underestimate the enhancer effects, those from hairless mouse skin greatly overestimated the changes. None of the membranes was a completely reliable model for assessing human percutaneous absorption as modified by accelerants. Pretreatment with acetone did not significantly change the permeability of human or squamate skins to 5-FU, although that of hairless mouse increased twentyfold. An overall conclusion is that, wherever possible, human skin should be used in absorption studies and not hairless mouse or snake skin; otherwise, misleading results may be obtained.

Limitations of hairless mouse skin as a model for in vitro permeation studies through human skin: hydration damage

Hairless mouse skin currently provides a popular model membrane for studies in human percutaneous absorption. Although some similarities between the two skin types have been demonstrated, the effects of prolonged hydration on hairless mouse skin have not previously been rigorously examined. We have measured in vitro the effects of hydration at 31 degrees C on the permeabilities of hairless mouse skin and human abdominal and scalp skin to a model polar compound (water) and a lipid material (hexanol). The permeability of hairless mouse skin rose dramatically, especially to water (fiftyfold increase), whereas the human skin was more stable. We also compared the effects of stripping the stratum corneum with the effects of 8-d hydration for hairless mouse and human abdominal skin. Hydration of hairless mouse skin was as effective as tape-stripping in eliminating the stratum corneum barrier, whereas stripping human skin was far more damaging than hydration, suggesting that prolonged hydration mechanically disrupted mouse skin but not human skin. Histological examination of fresh and hydrated tissues confirmed this suggestion. We therefore recommend that hairless mouse skin is not used as a model for human tissue during in vitro permeation studies under conditions of long-term hydration, i.e., greater than three days.