Kristine Knutson

Macro- and molecular physical-chemical considerations in understanding drug transport in the stratum corneum

Abstract Traditional permeability studies of full-thickness skin have implied molecules permeated through the skin by various polar or nonpolar pathways depending on the hydrophilicity or lipophilicity of the permeant. However, relatively little is known about the structure of stratum corneum, even though it is considered the primary barrier in transdermal permeation of most permeants. Current macroscopic (permeability, differential scanning calorimetry) and molecular (Fourier transform infrared spectroscopy) investigations of the stratum corneum and its components imply hydration and temperature enhanced permeability of lipophilic molecules through the stratum corneum is associated with transitions involving the hydrocarbon chains of the lipid components.

Percutaneous Absorption Enhancement of an Ionic Molecule by Ethanol–Water Systems in Human Skin

Ethanol–water systems enhance permeation of ionic solutes through human stratum corneum. Optimum enhancement of salicylate ion permeation has been observed with ethanol volume fractions near 0.63. The mechanism of action of ethanol–water systems enhancing skin permeation was investigated by in vitro skin permeation studies combined with Fourier transform infrared spectroscopy experiments. The increased skin permeation of the ionic permeant by the ethanol–water systems may be associated with alterations involving the polar pathway. Polar pathway alterations may occur in either or both the lipid polar head and proteinaceous regions of the stratum corneum. Ion-pair formation may also contribute to increased permeation. However, the decreased permeation of salicylate ion observed at higher volume fractions of ethanol may be attributed to decreased uptake of permeant into the stratum corneum.

Mechanism of ethanol-enhanced estradiol permeation across human skin in vivo.

The influence of ethanol on the permeation of 17β-estradiol (estradiol) across viable human skin in vivo was investigated with the human skin sandwich flap model. Maintaining continuous delivery of a constant concentration of the solute in phosphate-buffered saline, pH 7.4 (PBS), or mixtures of ethanol in PBS to the skin surface revealed that steady-state flux of estradiol was achieved within 30–60 min and maintained throughout 4 hr. The 10-fold decrease in in vivo flux and permeability coefficient (Kp) of tracer estradiol solutions in ethanol or ethanol solutions compared with PBS vehicle reflected the 10-fold difference in the apparent partition coefficients (Km) of estradiol from the respective vehicles into isolated human stratum corneum. Neither the stratum corneum thickness nor the diffusion coefficient of estradiol was significantly different among the vehicles tested. In vivo flux of estradiol in ethanol or ethanol solutions across viable human skin was increased with saturated solutions of estradiol. Further, in vivo flux of estradiol from vehicles such as PBS, ethanol, and ethanol mixtures, which minimally alter the rate-limiting barrier, can be successfully predicted with knowledge of only two physicochemical parameters, the estradiol concentration in the vehicle and the Km of estradiol from the vehicle into isolated human stratum corneum.

Enhanced permeation of polar compounds through human epidermis. I. Permeability and membrane structural changes in the presence of short chain alcohols

The influence of alcohol chain length on polar compound permeation in human skin was investigated to further understand alcohol-enhanced permeation mechanisms. Both thermodynamic and kinetic variables associated with the enhanced permeation of mannitol were ascertained in the presence of high concentrations of short chain alcohols. Permeation of mannitol through human epidermis in the presence of 75% (v/v) alcohol-saline mixtures was determined in symmetric, side-by-side diffusion cells at 32 degrees C. Permeability coefficients increased with increasing alcohol chain length (iso-propanol > ethanol > methanol). Uptake of mannitol into the epidermal tissue increased in the presence of the short chain alcohols, but was independent of alcohol chain length. In addition, mannitol solubility decreased in the presence of the short chain alcohols, but again was independent of alcohol chain length. Therefore, increased mannitol permeability with increasing alcohol chain length could not be attributed to thermodynamic variables. Changes in the amount and conformation of stratum corneum lipids and proteins were determined by Fourier transform infrared (FTIR) spectroscopy. Stratum corneum lipid conformation and mobility was not significantly altered in the presence of the short chain alcohols. However, decreased absorbance of the alkyl chain suggested lipid extraction, which increased with increasing alcohol chain length. Stratum corneum protein conformation was altered in the presence of the short chain alcohols. Decreased infrared absorbance of the Amide I band maximum suggested extraction of stratum corneum proteins, which increased with increased alcohol chain length. These results suggest a correlation between enhanced permeation and extraction of lipids as well as proteins from human skin in the presence of 75% (v/v) aqueous alcohol solutions.

Solvent-mediated alterations of the stratum corneum

Abstract The intercellular lipid multilayers of stratum corneum, the outermost layer of mammalian skin, are generally accepted as the rate-determining pathway of transmembrane diffusion for lipophilic solutes. The effects of a series of saturated and unsaturated octadecanoic acids (saturated stearic acid, cis- and trans-octadecenoic acid) and a series of short-chain alcohols (ethanol, n-propanol, isopropanol and n-butanol) on the thermotropic phase behavior of stratum corneum have been investigated. In conjunction with the thermotropic phase behavior, fatty-acid-induced effects on the permeation of a lipophilic compound have also been studied. The alcohol-induced effects were determined at concentrations that haue given penetration enhancement for the respective alcohols in order to relate the findings to available skin transport data from literature. Permeation enhancement of lipophilic solutes does not necessarily occur with a concomitant decrease in the gel to liquid-crystalline phase transition. The unsaturated fatty acids in the presence of aqueous ethanol solutions have been associated with conformational alterations of the gel phase lipid domain. However, the effect of fatty acids is complicated by the presence of the cosolvent. Therefore, the thermotropic phase behavior of the stratum corneum was also investigated in the presence of a series of short-chain alcohols. Short-chain alcohol enhanced permeation may result from a reduction of lipid polar head interactions, an inter facial transport pathway between limited populations of interdigitated and noninterdigitated gel phases within the membrane or further disordering of possible liquid-crystalline phases within the membrane.

In Situ Study of Insulin Aggregation Induced by Water-Organic Solvent Interface

AbstractPurpose. The aim of this study was to assess insulin stability by monitoring in situ time-course of insulin aggregation induced by a water-organic solvent (o/w) interface that occurs during the microencapsulation process. Methods. Insulin aggregation at a simple o/w interface was monitored spectrophotometrically by detecting the percentage of turbidity changes (%T) at 350 nm. The effects of protein concentration and agitation and the presence of poly (lactic-co-glycolic acid) (PLGA) in methylene chloride (MC) on insulin aggregation were observed. For the 0.72 mg/ml insulin in phosphate-buffered saline (PBS), the effect of nonionic (dodecyl maltoside [DDM]) and anionic (sodium dodecyl sulfate [SDS]) surfactant in PBS were also evaluated at various protein/surfactant mol ratios. The conformation of insulin protected by a 10-fold molar excess of SDS recovered after 1 h of contact with MC was evaluated via circular dichroism (CD) spectroscopy. Results. A typical turbidity-time profile was represented by a sigmoidal curve. Greater change in %T was observed with increasing insulin concentration, in the presence of PLGA in MC and in the presence of agitation. DDM failed to delay insulin aggregation at all ratios used, whereas a less than 10% change in %T was observed in 1 h when a 10- −∼20-fold excess of SDS was used. CD spectra indicated that the presence of insulin in SDS after 1 h of contact with MC qualitatively retained its secondary structure integrity. Conclusions. An experimental method was designed for an in situ assessment of protein stability at the o/w interface.

Physicochemical aspects of transdermal permeation

Abstract The nature and state of lipids are often important regulators in many membrane functions and properties. Current studies probe the physicochemical aspects of the enhanced permeabilities of lipophilic solutes through hairless mouse stratum corneum. The barrier properties of the stratum corneum to lipophilic solutes haue been related to the order and mobility of the hydrocarbon portion of the lipoidal pathway. Temperature enhanced permeabilities of the solutes were associated with the gel to liquid crystalline transition of the lipid hydrocarbon chains. Minor enhancement also resulted from decreased order within pathway. Thermal pretreatments in the presence or absence of hydration also affected the pretransitional order of the lipids, although the gel to liquid crystalline transition remained within the same temperature region during thermal perturbation. Therefore, the physicochemical nature of the lipoidal pathway affects the barrier properties of the stratum corneum.

Ethanol effects on the stratum corneum lipid phase behavior

The stratum corneum is considered to be the diffusional barrier of mammalian skin for water and most solutes. The intercellular lipid multilayer domains of the stratum corneum are believed to be the diffusional pathway for most lipophilic solutes. Fluidization of the lipid multilayers in the presence of ethanol is frequently conceived to result in enhanced permeation. Current investigations address the effect of ethanol on the phase behavior in terms of stratum corneum lipid alkyl chain packing, mobility and conformational order as measured by Fourier transform infrared (FTIR) spectroscopy. Phospholipid multilamellar vesicles were also studied as model systems. There appeared to be no effect of ethanol on either the solid-solid phase transition or the gel phase interchain coupling of the stratum corneum lipids. However, there was a reduction in the mobility of the alkyl chains in the presence of ethanol. Possible mechanistic relationships between the current FTIR spectroscopic results with available literature data of ethanol induced lipophilic solute penetration enhancement through the skin are discussed.

The stratum corneum lipid thermotropic phase behavior

The stratum corneum, the outermost layer of mammalian skin, is considered the least permeable skin layer to the diffusion of water and other solutes. It is generally accepted that the intercellular lipid multilayer domain is the diffusional pathway for most lipophilic solutes. Fluidization of the lipid multilayers is believed to result in the loss of barrier properties of the stratum corneum. Current investigations address the lipid thermotropic phase behavior in terms of lipid alkyl chain packing, mobility and conformational order as measured by Fourier transform infrared (FTIR) spectroscopy. A solid-solid phase transition is observed with increased alkyl chain mobility followed by a gel to liquid-crystalline phase transition near 65 degrees C. These results further elucidate the role of lipid fluidity that may contribute to the transport properties of the stratum corneum.

Dose-dependent enhancement effects of azone on skin permeability

In vitro permeability experiments have been combined with differential scanning calorimetry (DSC) studies in an attempt to address the dose-dependent influence of Azone on the permeability coefficients of solutes for hairless mouse stratum corneum. A spray technique was developed to deliver uniformly and quantitatively small amounts of Azone to the stratum corneum. Permeability data obtained for several model solutes of varying lipophilicity suggest lipid fluidization and polar route enhancement as the mechanisms of action for Azone. Alkanols and steroids, both of which are enhanced primarily by lipid fluidization, had different degrees of relative enhancement. This provides evidence that the stratum corneum barrier is heterogeneous, rather than a homogeneous slab barrier. Two effects of Azone on the stratum corneum were detected by DSC. A decrease in the area and a shift to lower temperatures were noted for the lipid endotherms with increasing doses of Azone. A lipid fluidizing effect would qualitatively account for the increases in the permeability coefficients noted for more lipophilic solutes. The stratum corneum keratin endotherm also appears to be altered in the presence of Azone. It is possible that alteration of the keratin structure could lead to the development of polar routes in the stratum corneum.