William I. Higuchi

Quantitative mechanistic studies in simultaneous fluid flow and intestinal absorption using steroids as model solutes

The interplay of flow-rate, aqueous boundary layer and membrane permeability coefficients, solute lipophilicity and intestinal length has been quantitatively determined for the in situ situation of bulk fluid flow and concurrent steady-state absorption of steroids in the small intestines of the rat. Seven steroids ranging in 3 orders of magnitude in n-octanol/water partition coefficients were used. The results followed the physical model predictions described by: C(l)C(0)=exp−2πγlQ·Paq1+PaqPm where ClC(0) is the fraction of steroid remaining in the intestinal lumen of length l, r is the effective lumenal radius, Q is the flow-rate, Paq and Pm are the respective aqueous boundary layer and membrane permeability coefficients. The log fraction of steroids remaining in the lumen was linear with intestinal length at various flow rates. The fraction absorbed increased with slower flow-rates at any given length due to the longer residence time. The fraction of steroid absorbed vs log partition coefficient profiles as a function of flow-rate were significantly sigmoidal. The absorption rates of progesterone were aqueous boundary layer-controlled and the less lipophilic hydrocortisone were membrane-controlled. It is significant that the permeability of the aqueous boundary layer is proportional to Q0.44.

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.

Hindered Diffusion of Polar Molecules Through and Effective Pore Radii Estimates of Intact and Ethanol Treated Human Epidermal Membrane

The in vitro passive transport of urea, mannitol, sucrose and raffinose across intact and ethanol treated human epidermal membrane was investigated. The intent of this study was to characterize the barrier properties and permeation pathways of these membranes for polar permeants under passive conditions. Based upon the relative permeabilities of these four solutes and hindered diffusion theory, the experimental data was adequately modeled for both membrane systems according to permeation through a porous membrane. Effective pore radii estimates for intact human epidermal membrane fell between 15 Å to 25 Å while similar estimates fell compactly between 15 Å to 20 Å for ethanol treated human epidermal membrane. Similarities between the relative permeabilities of human epidermal membrane for the four permeants studied and the relative permeabilities of these same permeants through ethanol pretreated human epidermal membrane indicate that significant similarities exist between the permeation pathways for both membrane systems. The results of this study have important implications for transdermal drug delivery in general and more specifically for strategies of designing effective chemical permeation enhancement systems.

Relationships among carbonated apatite solubility, crystallite size, and microstrain parameters.

Abstract. The use of the metastable equilibrium solubility (MES) concept to describe the solubility properties of carbonated apatites (CAPs) and human dental enamel (HE) has been well established in previous studies using a range of CAPs with varying carbonate contents and crystallinities. It was shown in these studies that the mean value of the CAP MES is directly related to the broadening parameter full width at half maximum (FWHM) of the 002 reflection of the X-ray diffraction profile. The apparent solubility of the CAPs increased monotonically with an increase in the broadening of the diffraction peaks, and when this peak broadening was taken into account, carbonate had no additional effect upon the MES. The broadening of the diffraction peaks has been used as an indicator of crystallinity, and is generally influenced by both crystallite size and microstrain. The purpose of the present study was to extract the crystallite size and microstrain parameters separately from the X-ray diffraction peaks and then to determine their relationships to the corresponding MES values. The samples studied were CAPs synthesized by precipitation from Ca(NO3)2 and NaH2PO4 solutions in carbonate containing media at temperatures of 95, 80, and 70°C, and powdered HE. The crystallite size and microstrain parameters were determined simultaneously with the refinement of the structural parameters with the Rietveld method of whole-pattern-fitting structure-refinement. A modified pseudo-Voigt function was used to model the observed peak profiles. The MES distributions for the CAPs and HE were determined by a previously described method. The results of this study showed that the CAPs possessed an MES distribution and therefore provided further support that MES distribution is a common phenomenon, regardless of the method of CAP synthesis. The crystallite size decreased and the microstrain increased with increasing carbonate content and decreasing temperature of synthesis of the CAPs. A plot of the mean of the MES distribution versus the microstrain parameter showed that the apparent solubility of the CAPs and HE correlated very well with the microstrain parameter. On the other hand, a plot of the mean of the MES distribution versus the crystallite size parameter showed a poor correlation between MES and crystallite size. These findings support a view that microstrain, rather than crystallite size, is the dominant factor governing the effective solubility of the CAPs and dental enamel.

Heterogeneity effects on permeability-partition coefficient relationships in human stratum corneum

The relationship between the permeability of solutes undergoing transport via the lipid pathway of the stratum corneum and the degree to which the same solutes partition into the stratum corneum has been explored by measuring the permeability coefficients and stratum corneum/water partition coefficients of a series of hydrocortisone esters varying in lipophilicity. Isolated human stratum corneum, used in both the permeability and the uptake experiments, was shown to resemble full-thickness skin in its overall resistance and selectivity to solute structure. As with full-thickness skin, delipidization destroys the barrier properties of isolated stratum corneum. Although a linear relationship is frequently assumed to exist between permeability coefficients and membrane/water partition coefficients, a log–log plot of permeability coefficients versus the intrinsic stratum corneum/water partition coefficients for the series of hydrocortisone esters studied is distinctly nonlinear. This nonlinearity arises from the fact that the transport of these solutes is rate limited by a lipid pathway in the stratum corneum, while uptake reflects both lipid and protein domains. From the relative permeability coefficients of 21-esters of hydrocortisone varying in acyl-chain structure, group contributions to the free energy of transfer of solute into the rate-limiting barrier microenvironment of the stratum corneum lipid pathway are calculated for a variety of functional groups including the −CH2−, −CONH2, −CON(CH3)2, -COOCH3, −COOH, and −OH groups. These are compared to contributions to the free energies of transfer obtained for the same functional groups in octanol/water, heptane/water, and stratum corneum/water partitioning experiments. The group contributions to transport for polar, hydrogen-bonding functional groups are similar to the values obtained from octanol/water partition coefficients. This similarity suggests that complete loss of hydrogen bonding does not occur in the transition state for passive diffusion via the lipid pathway.

Skin alteration and convective solvent flow effects during iontophoresis: I. Neutral solute transport across human skin

Abstract Overall flux enhancement of ions during iontophoresis is due primarily to the electrochemical potential gradient. However, secondary effects such as convective solvent flow and, in biological membranes, permeability increases due to the applied field, may also contribute to flux enhancement. The modified Nernst-Planck theory includes a solvent flow velocity term and predicts uncharged molecules are enhanced or retarded depending on the polarity of the applied field. In this study, mannitol was employed as a probe permeant and the mannitol flux was used as a measure of the solvent flow contribution during iontophoresis across human epidermal membrane. Membrane alterations due to the applied field were also assessed, as was the extent of reversibility of the membrane changes. Mannitol transport was enhanced in the anode to cathode polarity and retarded in the cathode to anode polarity. This was interpreted to mean that significant solvent flow across human skin occurred during iontophoresis. Solvent flow velocity was found to be proportional to the magnitude of the applied field and independent of the system polarity. Membrane alterations occurred at the highest voltage investigated in this study (i.e., 1000 mV). These changes appeared to reverse over time as indicated by the current and transport data.

Studies on the Effects of Applied Voltage and Duration on Human Epidermal Membrane Alteration/Recovery and the Resultant Effects upon Iontophoresis

The effects of applied voltage and the duration of application upon human epidermal membrane (HEM) alterations and recovery were investigated. All experiments were conducted using a two-chamber diffusion cell with constant DC voltage (250–4000 mV) applied over a predetermined period, and HEM changes were monitored by measuring the electrical resistance before and after voltage termination. The key findings were that the rate of decrease in resistance was strongly dependent upon the applied voltage, the reversible recovery times were dependent upon both the magnitude and the duration of the applied field (frequently were several orders of magnitude greater than times for attaining significant resistance reduction), and reversible recovery times were much longer when lower voltages were applied for longer times to attain the same decrease in electrical resistance than for higher voltages at short times. These findings closely parallel those obtained on electrical breakdown/recovery of bilayer membranes (electroporation). The second part of this work examined the hypothesis that decreases in HEM electrical resistance induced by the applied voltage are accompanied by proportional increases in HEM permeability. A study was designed to test this hypothesis involving a four-stage protocol with HEM: passive transport, 250-mV iontophoresis, 2000-mV iontophoresis for 10 min, then back to 250-mV iontophoresis. The data obtained strongly support the view that the HEM alterations induced by the electric field result in pore formation and in the expected changes in HEM permeability.

The effects of ethanol on the transport of lipophilic and polar permeants across hairless mouse skin: Methods/validation of a novel approach

The influence of ethanol on the in vitro transport behavior of some lipophilic and polar/ionic permeants in hairless mouse skin has been investigated over a 0–100% ethanol/saline concentration range in a two-chamber diffusion cell. The lipophilic permeants were employed in probing the transport enhancing effects of ethanol upon the lipoidal pathway of the stratum corneum, and the polar/ionic permeants were used to quantify the influence of ethanol on thepore pathway of the stratum corneum over the entire range of ethanol concentrations. The following were the important interpretations of the data. The lipophilic permeants (estrone, s-estradiol, and hydrocortisone) were mainly transported via the lipoidal pathway up to around 50% ethanol. The permeation enhancement factor, E, for the lipoidal pathway was calcuklated from the transport data for the three lipophilic permeants. In order to calculate the E values, it was first necessary to establish the validity of Henrys law by comparing the ratios of permeant solubilities (in different ethanol/saline solutions) to ratios of permeant partition coefficients (in hexadecane/ethanol-saline systems). The calculated E values were found to be about the same for all three permeants: E = 7.0 ± 2.0 at 25% ethanol and E = 112 ± 19 at 50% ethanol. These large enhancing effects of ethanol upon the lipoidal pathway were somewhat surprising, and it is suggested that ethanol (< 50%) may work as an effective ‘fluidizing’ agent at some locus in the stratum corneum lipid bilayer at or near the polar head plane, but not in the bilayer hydrocarbon interiors. The polar/ionic permeants (tetraethylammonium bromide, mannitol, estrone ammonium sulfate, and vidarabine) all were transported via the pore pathway at all ethanol concentrations. Ethanol up to around 25% had little effect upon the pore pathway; however, at higher concentrations (∼ 50%), ethanol greatly enhanced pore transport and, at very high ethanol levels (t~ 75%), the pore pathway appeared to dominate the transport of all permeants including the lipophilic permeants.

Combined Effects of Laser Irradiation and Chemical Inhibitors on the Dissolution of Dental Enamel

It has previously been shown that the susceptibility of human teeth to acid dissolution can be reduced by the presence of various chemical agents in the dissolution medium or by pretreatment of the teeth with laser irradiation. Now synergism between these two approaches to improving acid resistance has been demonstrated. Extracted human teeth were irradiated with a continuous-wave carbon dioxide laser at a wavelength of 10.6 microns. Energy doses of either 65 or 130 J/cm2 given over periods of 2 or 4 s, respectively, were applied and the teeth subjected to a severe acid challenge (0.1 M acetate buffer, pH 4.5, no calcium or phosphate common ion present) for 24 h. Mineral loss was assessed by measurement of mineral density profiles with quantitative microradiography. Experiments were carried out in the presence or absence of three chemical inhibitors with distinctly different mechanisms of action: ethane-1-hydroxy-1, 1-diphosphonic acid, fluoride, and dodecylamine HCl. Laser irradiation alone was found to lead to increased resistance of the teeth to acid challenge, with the higher energy dose being more effective than the lower dose. Each of the chemical inhibitors was effective on both lased and unlased teeth, with the percent reduction of dissolution greater when the inhibitors were applied to teeth lased with an energy dose of 130 J/cm2 which were already more resistant to acid challenge than were unlased teeth or teeth lased with a dose of 65 J/cm2.(ABSTRACT TRUNCATED AT 250 WORDS)

Short chain alkanols as transport enhancers for lipophilic and polar/ionic permeants in hairless mouse skin : mechanism(s) of action

The influences of short chain n-alkanols (from C1 to C5) and isopropanol on the transport of lipophilic (β-estradiol and hydrocortisone) and polar/ionic (tetraethylammonium ion) permeants across hairless mouse skin have been investigated. Permeability studies employing a two-chamber diffusion cell were carried out over wide ranges of alkanol (in saline) concentrations with an aim toward quantifying the reversible enhancement effects of the added alkanol upon the lipoidal pathway of the stratum corneum. An enhancement factor, E (for the lipoidal pathway of the stratum corneum), was calculated from permeability coefficient and solubility data, and the E values for β-estradiol and for hydrocortisone were found to be nearly always the same in all instances. A pattern of increasing E values with increasing alkanol chain length up to C5 with these two permeants was found. A nearly semi-logarithmic linear relationship was also obtained between the enhancement potency and the carbon number of the n-alkanols; there was about 4-fold increase in the enhancement potency per n-alkanol methylene group. Pretreatment studies showed that the n-alkanol effetcts at low concentrations were reversible as far the lipoidal pathway of the stratum corneum was concerned. These results demonstrate the general usefulness of this approach for evaluating the action of enhancers on the barrier function of the stratum corneum. It is suggested that the short chain alkanols may work at low concentrations as effective ‘fluidizing’ agents at some locus in the stratum corneum lipid bilayer at or near the polar head plane, but not in the deep bilayer hydrocarbon interiors.