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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Pharmaceutical research 12 (1995), S. 1614-1617 
    ISSN: 1573-904X
    Keywords: pKa ; stratum corneum ; fatty acid ; ionization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Purpose. The apparent pKa of the fatty acids within hydrated (30 % w/w) model human stratum corneum (SC) lipid mixtures should be measured. Methods. The degree of ionisation of the fatty acids was calculated as a function of pH using Fourier transform infra-red spectroscopy. The relative intensity of the stretching bands of the unionized and ionized carboxylic groups was determined and fitted to the relevant expression for ionic equilibrium of a monoprotic acid. The pKa was then calculated for increasing proportion of unsaturated fatty acid in the lipid mixture. Results. Values for pKa in the range 6.2-7.3 were found, increasing with greater proportion of oleic acid. These are some 1.5-3 pH units higher than the pKas of fatty acids in molecular solution. Conclusions. As there exists a pH-gradient across the SC, the degree of ionisation will also vary. In the innermost SC layers, a pH of 7 will produce 90% ionization of the fatty acids and head-group repulsion will be great. At the SC surface, the pH of 5 will cause almost minimal head-group repulsion, tending to increase crystallinity and promote a bilayer structure.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pharmaceutical research 13 (1996), S. 421-426 
    ISSN: 1573-904X
    Keywords: stratum corneum ; barrier function ; diffusion equation ; non steady-state model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Purpose. The diffusion equation should be solved for the non-steady-state problem of drug diffusion within a two-dimensional, biphasic stratum corneum membrane having homogeneous lipid and corneocyte phases. Methods. A numerical method was developed for a brick-and-mortar SC-geometry, enabling an explicit solution for time-dependent drug concentration within both phases. The lag time and permeability were calculated. Results. It is shown how the barrier property of this model membrane depends on relative phase permeability, corneocyte alignment, and corneocyte-lipid partition coefficient. Additionally, the time-dependent drug concentration profiles within the membrane can be observed during the lag and steady-state phases. Conclusions. The model SC-membrane predicts, from purely morphological principles, lag times and permeabilities that are in good agreement with experimental values. The long lag times and very small permeabilities reported for human SC can only be predicted for a highly-staggered corneocyte geometry and corneocytes that are 1000 times less permeable than the lipid phase. Although the former conclusion is reasonable, the latter is questionable. The elongated, flattened corneocyte shape renders lag time and permeability insensitive to large changes in their alignment within the SC. Corneocyte/lipid partitioning is found to be fundamentally different to SC/donor partitioning, since increasing drug lipophilicity always reduces both lag time and permeability.
    Type of Medium: Electronic Resource
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