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The present study describes a physical model approach applicable to understanding the transport of highly lipophilic, ionizable drugs across a lipophilic membrane between two aqueous compartments in the presence of a cyclodextrin in the aqueous phase. Model predictions were compared with experimental results of capric acid (HA) transport across a silicone polymer membrane in the presence and in the absence of 2-hydroxypropyl-β-cyclodextrin (HPB) in the aqueous phase over wide ranges of conditions. Key parameters entering into the physical model calculations were the HA–HPB and the A − –HPB binding constants, the unionized and ionized free and the complexed HA species diffusion coefficients, the HA p K a, the HA intrinsic silicone polymer membrane permeability coefficient, and the aqueous boundary layer thickness. All of these key parameters were determined from independent or essentially independent experiments. The agreement between the model predictions and the experiments were generally quite good over the entire ranges of the studied independent variables. The results of this study provide an approach that is useful in the mechanistic understanding of how cyclodextrins may enhance the passive absorption of highly lipophilic, low solubility drug molecules in the intestinal tract.

The purpose of this study was to characterize changes that occur in the iontophoretic transport of nonionic probe permeants in hairless mouse skin epidermal membrane from the anode to cathode when polystyrene sulfonate (PSS) oligomers are cotransported from the cathode to anode. The experiments were conducted with trace levels of the nonionic probe permeants: urea, mannitol, and raffinose. In order to systematically assess changes that occur as a result of having PSS in the cathodal chamber, the steady-state transport parameters of the membrane and the experimental permeability coefficients of the probe permeants were determined and compared with results obtained from earlier baseline experiments where both the cathodal and anodal chamber media were phosphate buffered saline. In addition, the physicochemical properties of the PSS solutions were determined including the solution viscosity and conductance as well as the mobilities of individual PSS oligomers. The effective pore radii of the transport pathways were calculated using a theoretical expression based on simultaneous diffusion and electroosmosis. Compared with the baseline results, the calculated radii were found to have increased up to around twofold and the iontophoretic fluxes of the probe permeants increased by as much sixfold.

To improve the biocompatibility, inter-connective pore structure, and drug delivery ability of self-setting apatite/collagen composite cement (ACC), a three-dimensionally perforated porous apatite/collagen composite cement (TPPACC) containing 3% indomethacin (IMC) was obtained in an arranged multi-cross with 20, 40, and 60 stainless steel needlelike male dies, and stored and hardened at 37°C and 100% relative humidity for 24 h. The mean radius of micro-pores of the TPPACC was evaluated to be 0.125 µm by mercury porosimetry. X-ray powder diffraction and FT-IR spectroscopy suggested that TPPACC consisted of carbonated apatite and had a structure similar to that of natural rat bone. The IMC release rates from a TPPACC block containing the drug were measured in simulated body fluid. The rate of release increased with the number of macro-pores that from planar surface matrix systems followed the Higuchi equation. The relationship between the Higuchi constant and surface area of TPPACC showed a straight line with K = 0.2123 and R2 = 0.9892. These results indicated that the rate of drug release from TPPACC could be controlled by the number of macro-pores for bone cells. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:286–292, 2010

The present study investigated the effects of oxygen-containing terpenes as skin permeation enhancers on the lipoidal pathways of human epidermal membrane (HEM). The enhancement (E(HEM)) effects of menthol, thymol, carvacrol, menthone, and cineole on the transport of a probe permeant, corticosterone, across HEM were determined. It was found that the enhancer potencies of menthol, thymol, carvacrol, and menthone were essentially the same and higher than that of cineole based on their aqueous concentration in the diffusion cell chamber at E(HEM) = 4. Thymol and carvacrol also had the same E(HEM) = 10 concentration further supporting that they had the same enhancer potency based on the aqueous concentration. The uptake amounts of terpene into the HEM stratum corneum (SC) intercellular lipid under the same conditions indicate that the intrinsic potencies of the studied terpenes are the same based on their concentration in the SC and similar to those of n-alkanol and n-alkylphenyl alcohol. Moreover, they are all better enhancers compared to branched-chain alkanol. The approximately same uptake enhancement of beta-estradiol induced by the studied terpenes and alcohols at E(HEM) conditions into the SC intercellular lipids suggests that the mechanism of enhancement action for the terpenes and those of alcohols are essentially the same.

The flux enhancing mechanisms of transscleral iontophoresis are not well understood. The objective of the present study was to investigate the ocular barrier and barrier alterations in transscleral iontophoretic delivery with magnetic resonance imaging (MRI). Experiments involving constant current transscleral iontophoresis of 2 mA (current density 10 mA/cm2) and subconjunctival injection were conducted with rabbits in vivo and postmortem and with excised sclera in side-by-side diffusion cells in vitro. The postmortem and in vitro experiments were expected to be helpful in clarifying the importance of vascular clearance and other transport barriers in transscleral iontophoresis. Manganese ion (Mn2+) and manganese ethylenediaminetetraacetic acid complex (MnEDTA2−) were the model permeants. The results show that pretreatment of the eye with an electric field by iontophoresis enhanced subconjunctival delivery of the permeants to the anterior segment of the eye in vivo. This suggests that electric field-induced barrier alterations can be an important absorption enhancing mechanism of ocular iontophoresis. Penetration enhancement was magnified in the postmortem experiments with larger amounts of the permeants delivered into the eye and to the back of the eye. The different results observed in the in vivo and postmortem studies can be attributed to ocular clearance in ocular delivery. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:831–844, 2008

The objective of the present study was to investigate the iontophoretic transport behavior across multiple membranes of different barrier properties. Spectra/Por(R) (SP) and Ionac membranes were the synthetic membranes and sclera was the biomembrane in this model study. The barrier properties of SP membranes were determined individually in passive and iontophoresis transport experiments with tetraethylammonium ion (TEA), chloride ion (Cl), and mannitol as the model permeants. Passive and iontophoretic transport experiments were then conducted with an assembly of SP membranes. The contribution of electroosmosis to iontophoresis was assessed using the mannitol data. Model analysis was performed to study the contribution of diffusion and electromigration to electrotransport across the multiple membrane system. The effects of membrane barrier thickness upon ion-exchange membrane-enhanced iontophoresis were examined with Ionac, SP, and sclera. The present study shows that iontophoretic transport of TEA across the membrane system was related to the thicknesses and permeability coefficients of the membranes and the electromobilities of the permeant across the individual membranes in the assembly. Model analysis suggests significant contribution of diffusion within the membranes across the membrane system, and this mechanism is relatively independent of the current density applied across the system in iontophoresis dominant transport.

The objective of this study was to mechanistically and quantitatively analyze chenodeoxycholate-enhanced paracellular transport of polar permeants and oligonucleotides in the rat jejunum and ileum. Micellar chenodeoxycholate solutions were used to perturbate the tight junctions. Supporting studies included assessment of the aqueous boundary layer (ABL) with ABL-controlled permeants, measurements of the permeability coefficients and fluxes of the bile acid in dilute and micellar concentrations, and determinations of pore sizes with paracellular probes (urea, mannitol, and raffinose). The paracellular permeability coefficients, P(para), of two model oligonucleotides (ON3 and ON6; 12- and 24-mers with 11 and 23 negative charges, respectively) were determined. The enhanced permeabilities paralleled the increased fluxes of micellar bile salt solutions into mesenteric blood and the opening of the tight junctions as compared to controls. As the pore radius increased from 0.7 nm to a maximum of 2.4 nm in the jejunum and ileum, the absorption of ON3 was enhanced up to sixfold in the jejunum and about 14-fold in the ileum with P(para) values between 0.5 x 10(-6) and 6 x 10(-6) cm/s, whereas ON6 was enhanced up to twofold in the jejunum and fivefold in the ileum with permeabilities between 0.3 x 10(-6) and 2 x 10(-6) cm/s.

Recent in vitro and in vivo studies have suggested transscleral iontophoresis as a means for non-invasive drug delivery to the eye. However, there remains a lack of information of the iontophoretic transport behavior of the sclera. The objective of the present study was to investigate the effects of permeant concentration upon transscleral iontophoretic transport. Constant current direct current (DC) iontophoresis was conducted with rabbit sclera in vitro at permeant concentration ranging from 0.015 to 1.0 M in the donor chamber without background electrolyte at 0.4–4 mA (current density: 2–20 mA/cm 2 ). PBS (0.15 M) was the receiver solution. Salicylate (SA) and tetraethylammonium (TEA) were the model ionic permeants, and mannitol was the neutral probe permeant. Conductivity experiments of SA and TEA solutions were performed to determine the effects of ion concentration upon SA and TEA electromobilities. Model simulations were carried out and compared with the experimental data. It was found that the fluxes of the ionic permeants increased linearly with the electric current but were relatively independent of their donor concentrations. Electric field-induced convective solvent flow (electroosmosis) in the sclera was observed to be from the anode to cathode, suggesting that the sclera is net negatively charge at neutral pH. For the studied permeants, electrophoresis was the main transport enhancing mechanism with electroosmosis as a secondary effect. No significant interaction between the permeants and sclera was observed that significantly altered electroosmosis in the membrane. Under the asymmetric donor and receiver conditions, the transference of the permeants could not be predicted by the concentrations of the ions in the donor and receiver chambers with the assumption of constant electric field in the membrane. The membrane ion concentrations were different from those in the chambers due to the requirement of charge neutrality in the membrane.

Previous studies in our laboratory have demonstrated that skin electrical resistance can be controlled by an alternating current (AC) electric field. By maintaining constant skin resistance, AC iontophoresis has been shown to reduce the iontophoretic flux variability of neutral permeants. Recently, it was found that symmetric square-wave AC could enhance iontophoretic transport of both neutral and ionic permeants by means of electrophoresis and/or electroosmosis in a synthetic membrane system, and a model was presented to describe the experimental results. The objective of the present study was to assess the effects of AC voltage and frequency and direct current (DC) offset on the flux of neutral and ionic model permeants with human epidermal membrane (HEM). Experiments were conducted under two different conditions: constant AC voltage iontophoresis and iontophoresis using constant HEM resistance with DC offset voltage. The following are the main findings in these experiments. In the constant AC voltage study, when the permeability data were compared at the same HEM electrical resistance, it was demonstrated that AC even at high frequency (approximately 1 kHz) could enhance the transport of the ionic permeant (tetraethylammonium ion) across HEM, but no enhancement was observed for the neutral permeant (arabinose). For the ionic permeant flux enhancement, the higher the applied AC voltage, the greater the flux enhancement. There was little or no AC frequency dependence of the flux enhancement in the frequency range of 50-1000 Hz. In the constant HEM resistance study of AC with DC offset, approximately linear relationships were observed between flux enhancement and the DC offset voltage for both the neutral and ionic permeants, and these results were found to be consistent with predictions of the modified Nernst-Planck model for conventional constant voltage DC iontophoresis. When the DC offset voltage was increased, the AC component of the flux enhancement for the ionic permeant decreased, eventually appearing to contribute negligibly to the total flux enhancement at high DC offset voltages.

One of the primary safety and tolerability limitations of direct current iontophoresis is the potential for electrochemical burns associated with the necessary current densities and/or application times required for effective treatment. Alternating current (AC) transdermal iontophoresis has the potential to eliminate electrochemical burns that are frequently observed during direct current transdermal iontophoresis. Although it has been demonstrated that the intrinsic permeability of skin can be increased by applying low-to-moderate AC voltages, transdermal transport phenomena and enhancement under AC conditions have not been systematically studied and are not well understood. The aim of the present work was to study the fundamental transport mechanisms of square-wave AC iontophoresis using a synthetic membrane system. The model synthetic membrane used was a composite Nuclepore membrane. AC frequencies ranging from 20 to 1000 Hz and AC fields ranging from 0.25 to 0.5 V/membrane were investigated. A charged permeant, tetraethyl ammonium, and a neutral permeant, arabinose, were used. The transport studies showed that flux was enhanced by increasing the AC voltage and decreasing AC frequency. Two theoretical transport models were developed: one is a homogeneous membrane model; the other is a heterogeneous membrane model. Experimental transport data were compared with computer simulations based on these models. Excellent agreement between model predictions and experimental data was observed when the data were compared with the simulations from the heterogeneous membrane model.

In a previous study, the enhancement effects on the transport of a steroidal permeant along the hairless mouse skin (HMS) stratum corneum (SC) lipoidal pathway were investigated for two homologous series of chemical enhancers: the 1‐alkyl‐2‐pyrrolidones and the 1‐alkyl‐2‐azacycloheptanones. The objective of the present study was to extend this investigation to a broader range of enhancers in order that generalizations with regard to the mechanistic aspects of enhancer function might be established. Specific questions to be addressed included: (a) what is the nature of the microenvironment of the enhancer site of action? (b) what is the extent of the equilibrium uptake of the enhancer from its E = 10 aqueous enhancer solution (the aqueous concentration for which the enhancer induces a tenfold transport enhancement) into the HMS SC intercellular lipid “phase”? and (c) are the microenvironment of the enhancer site of action and that for the equilibrium enhancer uptake at E = 10 relatively independent of the molecular characteristics of the enhancers (as suggested by the earlier study)? Enhancers selected for this study included: a wide range of polar head group size and polarity; n ‐alkyl group chain lengths from C 4 to C 12 ; and enhancers in which a double bond is substituted for a single bond in the hydrocarbon chain (3‐alkenols) from C 5 to C 9 . In addition to the main study, an ancillary set of experiments were to be conducted on the partitioning of a surrogate permeant (estradiol) into the intercellular lipid “phase” under E = 10 isoenhancement conditions to assess the extent to which the permeant partition coefficient may contribute to the permeation enhancement. The following were the principal findings of this research. First, there was very good correlation between the E = 10 isoenhancement aqueous enhancer concentrations and K octanol/water for all the studied enhancers. Second, the partitioning of the enhancer from the E = 10 aqueous enhancer solution into the HMS SC intercellular lipid “phase” was found to be relatively independent of the molecular characteristics for all studied enhancers, and the partition coefficients also correlated well with K octanol/water . These results may have the following meanings: both the microenvironment of the enhancer site of action and the SC intercellular lipid “phase” involved in the enhancer partitioning experiments are well mimicked by liquid n ‐octanol, and the “intrinsic” potencies (as assessed by the equilibrium enhancer concentration in the microenvironment at the site of action) of the enhancers are relatively independent of the molecular characteristics of the studied enhancers. Finally, the estradiol partitioning experiments suggest the permeant partitioning into the HMS SC intercellular lipid “phase” is enhanced around five‐ to seven‐fold when permeation is enhanced ten‐fold for most of the studied enhancers; therefore, the enhancement of the permeant partition coefficient rather than the permeant diffusion coefficient seems to be more important in permeation enhancement of the SC barrier lipoidal pathway. © 2004 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:1415–1430, 2004

As part of a long-term effort to understand the structure/function relationship between chemical permeation enhancers and skin permeation enhancement, the present study examined the influence of hydrocarbon chain branching on the effectiveness of skin permeation enhancers of the type that possesses a polar group (e.g., the hydroxyl group) attached to a hydrocarbon chain(s). The effects of x -hexanol, x -heptanol, x -octanol, and x -nonanol (where x is the position of the hydroxyl group ranging from 1 up to 5) on the transport of a probe permeant, corticosterone, across hairless mouse skin (HMS) were investigated. Isoenhancement concentrations are defined as the aqueous concentrations for which different enhancers induce the same extent of permeant transport enhancement, E , across the lipoidal pathway of stratum corneum (SC). The isoenhancement concentrations of 2-alkanol, 3-alkanol, 4-alkanol, and 5-alkanol to induce E = 10 were approximately 1.9-, 2.6-, 3.1-, and 3.9-fold higher, respectively, than those of the 1-alkanols of the same molecular formula. This suggested that the branched-chain alkanols have lower enhancer potency than the 1-alkanols of the same molecular formula; the potency decreases as the hydroxyl group moves from the end of the chain towards the center of the enhancer alkyl chain. To further investigate the mechanism(s) of action of the branched-chain alkanols as skin permeation enhancers, the equilibrium uptake of the enhancers into the hairless mouse skin stratum corneum (HMS SC) from aqueous enhancer solutions of E = 10 was determined. The data from these experiments provided a direct measure of the “intrinsic” potency of the enhancer. In the same experiments, the equilibrium partitioning (distribution) of a surrogate permeant, estradiol (E2β), into the HMS SC was also determined and compared to the partitioning from PBS (no enhancer present). The uptake amounts (micromole/mg SC) for 1-alkanols into the intercellular lipids of the SC were found to be essentially the same at their isoenhancement concentrations. However, at their isoenhancement concentrations, the uptake amounts of the branched-chain alkanols into the intercellular lipids of HMS SC were higher than those of the 1-alkanols. These results support the view that: (1) the intrinsic potencies of the 1-alkanols are essentially the same and independent of their 1-alkyl chain length at their isoenhancement concentrations, (2) the intrinsic potencies of the branched-chain alkanols are lower than those of the normal alkanols, and (3) branching of the alkyl chain reduces the ability of the enhancer to effect lipid fluidization in the SC lipid lamellae at the target site(s). The enhancement effects of the branched-chain alkanols and the 1-alkanols at their isoenhancement concentrations upon E2β partitioning into the SC intercellular lipids were found to be approximately the same and in the range of five- to eight-fold enhancement. The constancy of this enhancement for E2β partitioning suggests that the mechanism of enhancement action for the branched-chain alkanols and the 1-alkanols are the same. Additionally, a good correlation of the intercellular lipid/PBS partition coefficients of both the branched-chain alkanols and the 1-alkanols with the n -octanol/PBS partition coefficients was found. This supports the view that the chemical microenvironment of the polar head group and the alkyl group of the studied enhancers at the site of skin permeation enhancer action in the SC lipid lamellae can be represented by water-saturated n -octanol for both the branched-chain alkanols and the 1-alkanols. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association

The purpose of the present study was to gain quantitative mechanistic insight into the role cyclodextrin carriers may play in the intestinal absorption of highly lipophilic molecules. The physical model approach was employed to investigate capric acid absorption in the rat ileum using the in situ single-pass method with 2-hydroxypropyl-β-cyclodextrin (HPB) present in the perfusate. Two physical models were examined: the flat surface model in which the intestinal wall was treated as a hollow, smooth, circular cylinder, and the villus model in which the intestinal surface allowed for the presence of villi. Capric acid absorption was found to be essentially 100% aqueous boundary layer controlled at low HPB concentrations and increasingly membrane controlled at the higher HPB concentrations. Theoretical calculations based on the experimental data and model parameters were found to be consistent with: at low HPB concentrations, capric acid was mainly absorbed at the villus tips and there was very little capric acid penetration into the intervillus space; in contrast, at 50 mM HPB, there was considerable capric acid penetration into the intervillus space, this corresponding to around a 4.5-fold increase in the accessible area for absorption when compared with 0 mM HPB. © 2014 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 104:2832–2844, 2015

Previous investigations in our laboratory on the influence of the n-alkanols and the 1-alkyl-2-pyrrolidones as skin permeation enhancers for steroid molecules as permeants demonstrated that the enhancer potencies (based on aqueous concentration values) of these two homologous series were the same when compared at the same alkyl chain length; that is, the contribution of the hydroxyl group and that of the pyrrolidone group to enhancer potency were the same. The purpose of the present study was to further investigate what was believed to be a somewhat surprising finding, and two additional homologous series, the 1,2-alkanediols and N,N-dimethylalkanamides, were selected for study as enhancers. Corticosterone (CS) flux enhancement along the lipoidal pathway of hairless mouse skin stratum corneum was determined with 1,2-hexane-, 1,2-octane-, and 1,2-decanediol and with N,N-dimethylhexanamide, N,N- dimethylheptanamide, N,N-dimethyloctanamide, and N,N-dimethylnonanamide as enhancers. The enhancement factor (E) for the lipoidal pathway was calculated from the CS permeability coefficient and the CS solubility data over a 4 to 100 range of E values. Comparisons of the enhancer potencies of all four homologous series revealed that the enhancer potencies of all were very nearly the same when compared at equal alkyl group chain length. Moreover, the contribution of each of the polar head groups toward the enhancer potency was essentially constant, independent of the alkyl group chain length. It was reasoned that this outcome was either the result of the random selection of four polar head groups making the same contribution to enhancer potency or the result of these particular polar head groups not contributing to enhancer potency. To test the hypothesis that the former was more likely than the latter and that a suitable semipolar organic phase may mimic the microenvironment of the polar head group at the site of enhancer action, n-octanol-phosphate buffered saline (PBS) and n-hexane-PBS partition coefficients were determined for all the enhancers. The n-octanol-PBS partition coefficients for the enhancers, but not the n-hexane-PBS partition coefficients, were very nearly the same when compared at equal alkyl group chain lengths; this result supports the hypothesis that each of the four polar head groups likely contributes the same toward the enhancer potency and locates in the semipolar region of the hairless mouse skin stratum corneum lipid bilayers, which is well-approximated by water-saturated n-octanol.

In recent studies we found that the topical effectiveness of acyclovir (ACV) formulations was a single‐valued function of C*— the target site free drug concentration. The topical efficacy was the same when the therapy was initiated 0, 1, or 2 days after intracutaneous herpes simplex virus type‐1 (HSV‐1) inoculation in hairless mice. The purpose of the present study was to examine the hypothesis that the topical effectiveness of cidofovir (HPMPC) would not be a single valued function of C* and that it would be dependent upon when the therapy was initiated relative to the time of viral infection. Formulations of HPMPC and ACV in 95% DMSO as a vehicle were used. Hairless mice intracutaneously infected with HSV‐1 were used, and 20 µL of the test formulation was topically applied twice a day. In protocol A, the treatment was continued until the fourth day after virus inoculation, whereas in protocol B the treatment was terminated on the day of virus inoculation. Treatment was initiated on various days ranging from day −6 to day 4, and the lesions were scored on day 5. Treatment of ACV according to protocol A proved efficacious whether started as early as 6 days before virus inoculation or later, whereas the efficacy of ACV was annihilated if applied following protocol B. For HPMPC, on the other hand, the in vivo efficacies were found to be strongly dependent on how early the therapy was initiated, and significant efficacy was observed even when the treatment was terminated on the day of virus inoculation. This difference was attributed to the virus‐independent intracellular phos‐phorylation of HPMPC and slow clearance of its metabolites from the cell. It was also noted that, similar to ACV, for HPMPC the topical efficacy is likely to be a function of C* for a fixed protocol. However, unlike for ACV, for HPMPC the efficacy was not a single‐valued function of C*.

The present study aimed to investigate new pore induction as a flux-enhancing mechanism in human epidermal membrane (HEM) with low to moderate voltage electric fields. The extent of pore induction and the effective pore sizes of these induced pores were to be assessed using a low frequency (12.5 Hz) low to moderate voltage (2.0 to 4.0 V) square-wave alternating current (ac) “passive” permeation method (ac iontophoresis). This ac approach was to allow for inducing and sustaining a state of pore induction in HEM while permitting no significant transport enhancement via electroosmosis; thus, transport enhancement entirely due to new pore induction (enhanced passive permeation) was to be assessed without any contributions from electroosmosis. Good proportionality between the increase in HEM permeability and its electrical conductance was found with the “passive” transport data obtained during square-wave ac iontophoresis using urea as the model permeant. Typically, at 3.0 to 4.0 V, HEM conductance increases (and permeability increases) ranged from around 3- to 30-fold. These results appear to be the first direct evidence that new pore induction in HEM is a significant flux enhancing mechanism under moderate voltage conditions. The extents of pore induction in HEM under low frequency moderate voltage (2.0 to 3.0 V) ac, pulsed direct current (dc), and continuous dc were also compared. The extents of pore induction from square-wave ac and pulsed dc were generally of the same order of magnitude but somewhat less than that observed during continuous dc iontophoresis at the same applied voltage and duration, suggesting less extent of pore induction with reversing polarity or when a brief delay is provided between pulses to allow for membrane depolarization. The average effective pore sizes calculated for the induced pores from the experimental data with urea and mannitol as probe permeants and the hindered transport theory were 12 ± 2 A, which are of the same order of magnitude as those of preexisting pores determined from conventional passive diffusion experiments.

In a previous study, the enhancing effects of a series of 1-alkyl-2-pyrrolidones (APs; 1-ethyl, 1-butyl, 1-hexyl, and 1-octyl-2-pyrrolidone) on the transport of steroidal permeants across hairless mouse skin were investigated via a parallel pathway skin model. Isoenhancement concentration conditions were deduced under which different APs induce essentially the same transport enhancement for the lipoidal pathway of the stratum corneum (s.c.). As a continuing effort to understand the mechanism of action of permeation enhancers, the influence of the APs on permeant partitioning into hairless mouse s.c. was investigated under the isoenhancement concentration conditions using beta-estradiol (E2 beta) as the model permeant. The amount of E2 beta uptake into s.c. was found to be essentially the same for all the APs under these isoenhancement conditions. This result suggests that inducing a higher partitioning tendency for E2 beta into the lipoidal pathway of hairless mouse s.c. is a principal mechanism of action of the APs in enhancing transdermal transport. The uptake of the APs into s.c. lipoidal domains was also determined, and the results show only a modest (approximately 2-fold) increase in the uptake of the APs in going from 1-ethyl-to 1-octyl-2-pyrrolidone under isoenhancement conditions. This indicates the potency of the APs as permeation enhancers is only very modestly dependent upon the alkyl chain length in this chain length region when compared at concentrations in the microenvironment where the action occurs in the lipid domains.

For the past few years, our laboratory has been involved in the development of a novel approach for predicting topical in vivo efficacy based on the estimation of skin target site free drug concentration ( C *) from in vitro flux data. We have used acyclovir (ACV) as a model drug in the treatment of cutaneous herpes simplex virus type 1 infections in hairless mice. The goal of this study was to rigorously evaluate the applicability of this approach over the entire range of topical efficacy (i.e., from 0 to 100%). We employed a variety of ACV formulations differing in solvent compositions, enhancers, and excipients (and therefore in their efficacies) to achieve this goal. The C * values were estimated from the in vitro flux data obtained in an in vivo - in vitro experimental design that closely approximated the in vivo treatment protocol. For the in vivo antiviral efficacy studies, a finite dose of ACV formulation was applied twice a day, beginning the day after virus inoculation, for 4 days. The lesions were scored on the fifth day, and the efficacies were calculated as described earlier. 1 Our results indicate that, for a variety of formulations over a wide range of efficacies, the predictions based on C * are in good agreement with the observed in vivo efficacies. These findings strongly demonstrate the predictive value of C * over the entire range of topical efficacy, thereby further strengthening its potential for future studies. The findings also indicate that although the excipients in a formulation may alter the rate and extent of available drug at the target site, in these cases, they do not seem to have any effect on the in vivo potency of the drug.

The effects of permeant charge (z) on iontophoretic-enhanced transport were investigated with synthetic Nucleopore membranes and with human epidermal membranes using a four-electrode potentiostat with side-by-side diffusion cells. The modified Nernst-Planck model (Nernst-Planck theory with an additional transport term to correct for the effect of the convective solvent flow due to electroosmosis) was first examined in a Nuclepore membrane system with model permeants calcein (z = -4), salicylate (z = -1), and a series of polystyrene sulfonates (from monomer to molecular weight of approximately 8000 with a z range of -1 to approximately -40). The flux enhancement (E) for each permeant was determined at 470 mV. Mannitol (a neutral molecule) was used as a probe to determine a correction for convective solvent flow under the same applied voltage conditions. Good agreement between the experimental results and the predictions from the modified Nernst-Planck model was found for calcein, salicylate, and polystyrene sulfonates up to molecular weight of approximately 1800 (z approximately -8). The flux enhancements for the higher molecular weight polystyrene sulfonates with greater z values were more than a factor of three lower than theoretical predictions; the electrophoretic effect and counterion binding to the permeants are proposed as possible explanations for these discrepancies between experiment and the modified Nernst-Planck theory. In the studies with human epidermal membranes, iontophoretic flux enhancements for calcein, salicylate, and taurocholate were determined at 250 and/or 470 mV. The flux enhancements were generally consistent with the results calculated from the modified Nernst-Planck model.

A quantitative physical model is presented for the formation of subsurface carious lesions in the presence of low levels of solution fluoride. Calculations using independently determined model parameters are in agreement with mineral density profiles measured in bovine enamel lesions. The proposed mechanism is controlled by fluoride in the following way: as fluoride diffuses into enamel, it is rapidly adsorbed to enamel crystallites, resulting in very low microenvironmental fluoride concentrations, so long as the crystals are not saturated with respect to fluoride adsorption. The result of this saturable adsorption is a widening band of fluoride-saturated crystals near the surface, beneath which the microenvironmental fluoride concentrations are negligible. In the saturated band, the microenvironmental fluoride concentration in the pore solution is high enough to suppress dissolution, while in the deeper, relatively fluoride free region, dissolution can occur. In addition to predicting observed mineral density profiles, the model also predicts the demarcation in solution conditions between the regime where subsurface lesion formation occurs and that where the dissolution pattern is that of surface erosion; and the lack of insensitivity of dissolution rate to hydrodynamics in the presence of low levels of fluoride, as contrasted to the square root of stirring rate dependency observed in the absence of fluoride.

The temperature dependence of in vitro permeation through human epidermal membrane (HEM) was determined for urea, mannitol, tetraethylammonium ion (TEA), and corticosterone. The effect of temperature upon HEM electrical resistance was also measured. The majority of the experiments involved measuring the permeability coefficients of a specific permeant at 27 degrees C and 39 degrees C for a given HEM sample, the electrical resistance was also measured at each temperature. Similar experiments were also conducted with a model synthetic porous membrane. The effect of temperature was quantitated as the ratio of the permeability at 39 degrees C to the permeability at 27 degrees C for each permeant. These ratios observed for HEM with urea, mannitol, and TEA as the permeants were 1.66 +/- 0.05, 1.76 +/- 0.14, and 1.71 +/- 0.11, respectively. The change in temperature was shown to have a similar effect upon the electrical conductance of the HEM samples. The observed ratio for corticosterone permeation was 4.5 +/- 0.4. The experimental ratios observed for the three polar/ionic permeants were shown to approach those obtained from the model porous membrane and differed greatly from the ratio observed for the more lipophilic corticosterone, indicating differences in the effective transport mechanism/pathway for these classes of permeants. The permeability of urea was also observed to be inversely proportional to the electrical resistance of the HEM samples; this relationship was shown to be independent of temperature over the temperature range studied. The temperature dependence data and the observed relationship between urea permeability and electrical resistance strongly support the existence of a porous permeation pathway through the HEM as an operative diffusional route for polar-ionic permeants.

The influences of 1‐ethyl‐, 1‐butyl‐, 1‐hexyl‐, and 1‐octyl‐2‐pyrrolidone in their saline solutions on the transport of β ‐estradiol, corticosterone, and hydrocortisone across hairless mouse skin under in vitro conditions have been investigated by the physical model approach. The experimental data were interpreted with a physical model that treats the stratum coneum as a diffusional barrier with a lipoidal pathway and a pore pathway. Enhancement factors ( E values) for the lipoidal pathway were calculated from the permeability coefficients and solubility data as a function of the 1‐alkyl‐2‐pyrrolione concentration for all three permeants. A pattern of increasing E values with increasing 1‐alkyl‐2‐pyrrolidone chain length was found, and the results were essentially the same for all three steroidal permeants. A nearly semilogarithmic linear relationship was also obtained between the enhancement potency and the carbon number of the alkyl chain; there was about an ∼ 3.5‐fold increase in the enhancement potency per 1‐alkyl‐2‐pyrrolidone methylene group. An important outcome of this research is that the enhancement potencies of the 1‐alkyl‐2‐pyrrolidones were essentially the same as those for the previously studied n ‐alkanols when compared at the same carbon numbers of the alkyl groups. This result is somewhat surprising as it suggests that the enhancer action resides (in its entirely) in the alkyl group, and the nature of the polar head group may not be intrinsically important in transdermal enhancement of the lipoidal pathway within a class of permeation enhancers.

The use of self-setting bioactive calcium phosphate cement containing indomethacin as a model drug in bovine bone was investigated by means of an in vitro drug release test, mercury porosimetry, and scanning electron microscopy (SEM). Calcium phosphate cements containing 2 and 5% indomethacin after being mixed with dilute phosphoric acid were applied to defect sites and the medullary cavity of bovine bone and transformed into hydroxyapatite. The in vitro drug release from the cement loaded on the defect site into a simulated body fluid (SBF) containing 2.5 mM Ca2+ and 1.0 mM HPO4(2+) or 0.1 M phosphate buffer at pH 7.25 and 37 degrees C continued for more than 3 weeks. The release profiles of the drug-loaded cements in phosphate buffer were linear using the Higuchi plot; however, that was not the case for SBF. The drug release in SBF was much lower than that in phosphate buffer. The total pore volume of the cement after the drug release test in SBF was lower than its initial value. However, the pore size of 0.1-0.01 microns after drug release in phosphate buffer was higher than that seen in SBF. The micropore distribution results suggested that hydroxyapatite crystallized from SBF and the pore volume in the cement decreased after drug release. However, in phosphate buffer it appeared to dissolve. The SEM observations for cements loaded on the bone after drug release in phosphate buffer suggested that there was a boundary layer between the cement and natural bone, but this was not the case in SBF, where the cement bonded with the natural bone. The drug release rates from the cement-loaded bone were significantly higher than those from cement loaded on the dissolution holder. The results suggested that cement formation and drug release were affected by the presence of protein from natural bone. The drug release rates from the cement loaded on the defective bone were slower than those from the medullary cavity.

A novel drug delivery device based on a self-setting bioactive calcium phosphate cement formed from tetracalcium phosphate and dicalcium phosphate has been developed and tested in vitro using the anticancer agent 6-mercaptopurine (6-MP) as a model compound. X-ray diffraction results suggest that equimolar mixtures of the calcium phosphate salts were transformed into hydroxyapatite after being mixed with a dilute phosphoric acid solution, even in the presence of various amounts of 6-MP powder. The inclusion of 6-MP in the reaction mixture resulted in the formation of a homogeneous drug-containing cement. Alternatively, the drug was loaded after cement formation to produce a heterogeneous drug-containing pellet. In vitro drug release from both the homogeneous and heterogeneous drug-loaded cement pellets into simulated body fluid (pH 7.25, 37.0 degrees C) was measured using the rotating-disk method. Release from the homogeneous 5% drug-loaded cements did not obey the Higuchi equation. The release rate from the heterogeneous drug-loaded cements of different thicknesses (1, 2, and 3 mm) was a function of thickness, indicating that release kinetics could be controlled by the design of the cement formulation.

The effect of the mixing solution volume was investigated on the in vitro drug-release rate of a novel drug-delivery device based on a self-setting bioactive calcium phosphate cement containing aspirin as a model drug. Equimolar mixtures of metastable calcium phosphate powders containing various proportions (3-40 w/w %) of seed hydroxyapatite crystals transformed into hydroxyapatite after being mixed with dilute phosphoric acid. The drug release from cement pellets in vitro into a 0.1 mol/L phosphate buffer at pH 7.40 and 37 degrees C by the rotating disk method continued for more than 1 week. The drug-release rate from the cement increased with increasing volumes of mixing solution. The relationship between the liquid/powder ratio and the porosity of the cement was a straight line, indicating that the cement porosity depended on the amount of the mixing solution, but was independent of the amount of seed crystals. Drug release from the cement followed the modified Fick's law, with the rate increasing with the amount of mixing solution, since the porosity depended on the amount. The tortuosity of the cements was estimated from the modified Fick's equation, and the relationships between the drug release rate and the tortuosity of the pore in the drug-loaded cement in the plots were nonlinear. The results suggested that the drug-release rates from the cement were controlled by the drug diffusion in the pores.

A novel drug-deliver device based on a self-setting bioactive cement formed from tetracalcium phosphate and dicalcium phosphate has been developed and tested in vitro using bovine insulin and bovine albumin as model polypeptide drugs. Equimolar mixtures of the calcium phosphate powders containing bovine insulin and bovine albumin were transformed into a hydroxyapatite cement after being mixed with a dilute phosphoric acid solution. X-ray diffraction and FT-IR spectra results suggested that the raw materials transformed into lower crystallinity of hydroxyapatite as it hardened. In vitro drug release from cement pellets into a 0.1 mol/L phosphate buffer at pH 7.40 and 37 degrees C continued for more than 3 weeks. Release from the drug-loaded cements followed the Higuchi model equation.

A kinetic dialysis technique together with a radiolabeled chenodeoxycholate (CDC) was used to determine the existence of a relationship between the monomer concentration of CDC and the total CDC concentration in different CDC solutions containing 1 or 5 mM sulfobutylether (SBE)-β-cyclodextrin. On the basis of the nature of the relationship and a binding model with binding constants of K 1 and K 2 , the binding affinity for the solutions was quantified at the best curve fitting using a least-squares technique. The very high binding affinity of K 1 and the very low (i.e., negligible) binding affinity of K 2 indicate the formation of 1:1 inclusion complexes. In addition, the values of K 1 and K 2 were reasonably interpreted. Similar analysis showed that the formation of 1:2 inclusion complexes and the self-association of the SBE-β-cyclodextrin molecules in the solutions are unlikely. The present study provides a basis for investigating the self-association, quantifying the binding affinity, and interpreting the quantified values. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association

The purpose of this study was to evaluate the therapeutic efficacy of a new calcium phosphate (CaP)-based formulation in improving the bone mineral deficiency in ovariectomized (OVX) rats. The ions release experiments for CaP preparations (G2: 0.46% Mg, 5.78% Zn, and 2.5% F; G3:3.1% Mg, 0.03% Zn, and 3.01% F; G4: 1.25% Mg, 1.77% Zn, 1.35% F) and of a Zn-TCP (G1: 6.17% Zn) powders, the initial Mg and Zn ion release rates of MZF-CaPs were performed in acetate buffer at pH 4.5 (37 degrees C). Wistar rats were divided into six groups including a normal (not OVX) group (GN) and a control, OVX group (GC). Rats in groups GC, G1, G2, G3, G4 were OVX. Suspensions consisting of CaP preparations (G2, G3, G4) and of a Zn-TCP (G1) powders were injected in the right thighs of OVX rats in all groups except for GN and GC, once a week for 4 weeks. GN and GC rats were injected with saline solutions. Plasma was analyzed for Zn land alkaline phosphatase levels. The bone mineral density (BMD) was measured using DEXA and the bone (femur) strength determined using three-point-bending analysis. G1 and G2 groups showed high plasma Zn levels. The area under the curve of plasma Zn was significantly greater in the G1, G2, and GN groups than in the G3, G4, and GC groups (p0.05). The BMD and bone mechanical strength of the right femur were significantly higher in the G1, G2, G3, and G4 groups than GC group on day 28. The right femur had significantly greater BMD and bone mechanical strength than the left femur in G1, G2, G3, and G4 groups. However, there was no significant difference in the BMD of the right femur between the G1, G2, G3, and G4 groups. Results indicate that the new injectable CaP formulations are effective in improving bone properties of OVX rats and may be useful in osteoporosis therapy.

In order to interpret the clinical efficacy of conjugated ursodeoxycholate (UDC) in cholesterol (Ch) gallstone patients, the Ch solubilization in mixed micelles in 40:40:32 mM tauroursodeoxycholate (TUDC):taurochenodeoxycholate (TCDC):lecithin (L) and 80:32 mM TUDC:L systems was estimated by using a model of Ch binding to mixed micelles. The Ch solubilization limit in mixed TUDC:L micelles was found to be higher than that in mixed TUDC:TCDC:L micelles. In the 80:32 mM TUDC:L system, the dissolution of the Ch pellet decreased after vesicles (liposomes) formed on the surface of the Ch pellet whereas the dissolution of microcrystalline Ch was rapid before and after vesicle formation in the solution, indicating that the total surface area of solid Ch exposed to the solution may be another important factor in inducing the dissolution of Ch gallstones. These phenomena suggest that although vesicles, occasionally formed in the bile of patients under the therapy of conjugated UDC, make a contribution to the solubilization of Ch gallstones, the model of Ch binding to mixed TUDC:L micelles can be used to estimate Ch solubility in TUDC:L system.

Previously, the effects of chemical permeation enhancers upon the permeability of the lipoidal pathway of hairless mouse skin (HMS) were investigated and a quantitative structure enhancement relationship was established. The present study was to study the effects of these enhancers on human epidermal membrane (HEM) using the same experimental method employed in the previous HMS studies. The effects of enhancers on the permeability coefficients of the lipoidal pathways of HEM and HMS for corticosterone were found to be essentially the same. In the equilibrium uptake studies of the enhancers and β‐estradiol, it was found that the amounts of enhancers taken up and the partitioning of β‐estradiol into the HEM stratum corneum (SC) intercellular lipid under the E = 10 conditions were different from those of HMS. Despite these differences, the HEM data show a correlation between the intercellular lipid/PBS partition coefficients of the enhancers and the enhancer n‐octanol/PBS partition coefficients. This correlation is consistent with the observed chemical microenvironment of the site of enhancer action in the HMS SC in previous studies. Therefore, provided with proper experimental protocols, HMS can be a reliable model for the evaluation of the effects of skin permeation enhancers on the lipoidal pathway of HEM. © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 2310–2326, 2007

Studies were previously conducted in our laboratory on the influence of n ‐alkanols, 1‐alkyl‐2‐pyrrolidones, N , N ‐dimethlyalkanamides, and 1,2‐alkanediols as skin permeation enhancers on the transport of a model permeant, corticosterone (CS). The experiments were conducted with hairless mouse skin (HMS) in a side‐by‐side, two‐chamber diffusion cell, with enhancer present in an aqueous buffer in both chambers. The purpose of the present study was to extend these studies and investigate in greater detail the hypothesis that a suitable semipolar organic phase may mimic the microenvironment of the site of enhancer action, and that the enhancer partitioning tendency into this organic phase may be used to predict the enhancer potency. CS flux enhancement along the lipoidal pathway of HMS stratum corneum was determined with the 1‐alkyl‐2‐azacycloheptanones, 1‐alkyl‐2‐piperidinones, 1,2‐dihydroxypropyl decanoate, 1,2‐dihydroxypropyl octanoate, n ‐alkyl‐β‐ D ‐glucopyranosides, 2‐(1‐alkyl)‐2‐methyl‐1,3‐dioxolanes, 1,2,3‐nonanetriol, and trans ‐hydroxyproline‐ N ‐decanamide‐C‐ethylamide as enhancers. Enhancement factors ( E values) were calculated from the permeability coefficient and solubility data over a range of E values. Comparisons of the enhancer potencies for all studied homologous series and the carbon number of the n ‐alkyl group revealed a nearly semilogarithmic linear relationship with a slope of ∼0.55, which is consistent with the hydrophobic effect. Moreover, comparisons of the enhancer potencies of all the enhancers with the n ‐hexanol–phosphate buffered saline (PBS), n ‐octanol–PBS, n ‐decanol–PBS, and n ‐hexane–PBS partition coefficients showed very good correlations for the n ‐alkanol solvents but not for n ‐hexane. This result supports the interpretation that the enhancer potency is directly related to the ability of the enhancer molecule to translocate to a site of action via its free energy of transfer from the bulk aqueous phase to a semipolar microenvironment in the stratum corneum lipid lamella that is well mimicked by water‐saturated n ‐alkanols. © 2003 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 92:1305–1322, 2003

In situ and in vitro intestinal absorption in the rat ileum was systematically studied and mechanistically quantified in terms of permeability coefficients ( P ) of a series of [ 3 H]steroids as model transcellular permeants, [ 3 H]taurocholate utilizing the active membrane transport systems to define the aqueous boundary layer (ABL), and [ 14 C]urea and [ 14 C]mannitol as pore-hindered paracellular diffusants. In situ single-pass perfusion experiments were performed in isolated ileal segments and blood samples were collected from the cannulated mesenteric vein. For the in vitro experiments, an excised, serosal and muscular layer-removed, ileal tissue was mounted in the Ussing chamber diffusion cells. In situ and in vitro P values versus logarithm of the partition coefficient in n-octanol/water (log K ) of the steroids were characterized by a sigmoidal-shaped curve in which plateau values were attained for the highly lipophilic steroids with log K ≳ 2.5. The in situ and in vitro transport barriers in series were viewed as ABL/mucosal epithelium and ABL/mucosal epithelium/submucosal tissue, respectively. Within this framework and the use of experimental strategies and theoretical reasoning, the transport barriers of the steroids were quantitatively delineated and the rate-determining barriers identified. In the plateau region, the analyses indicate that the in situ absorption of the lipophilic steroids was essentially ABL controlled, whereas the in vitro absorption was about equally controlled by diffusion across the ABL and submucosal tissue. The in situ and in vitro pore radii of the paracellular route were 7.2 and 9.2 A, respectively, and the difference was likely the result of perturbation of the tight junctions during the in vitro preparation of the ileal tissue.

In previous studies (Yoneto et al., 1995. J Pharm Sci 84:312-317; Kim et al., 1992. Int J Pharm 80:17-31; and Warner et al., 2001. J Pharm Sci 90:1143-53), the transport enhancing effects of four homologous series of enhancers-the n-alkanols, 1-alkyl-2-pyrrolidones, 1,2-alkanediols, and N,N-dimethylalkanamides - on the transport of steroidal permeants across hairless mouse skin (HMS) were investigated. Isoenhancement concentrations are defined as the aqueous concentrations for which different enhancers induce the same extent of permeant transport enhancement, E, for the lipoidal pathway of the stratum corneum (SC). Our studies have shown that the E = 10 isoenhancement concentrations of these four homologous series were nearly the same when compared at the same n-alkyl group chain length and therefore that the contribution of the polar head group toward the enhancer potency was found to be essentially constant. In the present study, we have determined the isoenhancement concentrations (E = 10) for the 1-alkyl-2-azacycloheptanone series [1-butyl-2-azacycloheptanone (BAZ), 1-hexyl-2-azacycloheptanone (HAZ), and 1-octyl-2-azacycloheptanone (OAZ)] and compared the results with those of the previously studied four homologous series. We have found that the E = 10 isoenhancement concentrations (aqueous phase concentrations) of the 1-alkyl-2-azacycloheptanones (Azs) are around 10 times lower than those for the previously studied four homologous series when compared at the same alkyl group chain length. This indicates an approximately 10 times higher potency of Azs. This finding was a point of interest because the polar group of Azs is similar to that of 1-alkyl-2-pyrrolidones (Aps). To further probe the nature of the mechanism of action of the Azs and Aps and to better understand the lower E = 10 isoenhancement concentrations found with the Azs, it was decided (a) to determine the equilibrium partitioning (uptake) of the Azs and the Aps from the aqueous phase into the HMS SC at E = 10, and (b) to determine the equilibrium partitioning (uptake) of a surrogate permeant, estradiol (E2beta), into the SC in the absence of and in the presence of Azs and Aps at E = 10. The following were the outcomes from the two partitioning studies. Firstly, at the E = 10 isoenhancement concentrations, the extent of partitioning (uptake) of the Azs and Aps into the intercellular lipids of the HMS SC was found to be approximately the same, even though the E = 10 isoenhancement concentrations (aqueous phase concentrations) of the Aps were around 10 times greater than those of the Azs. We interpret this to mean (whereas the potencies of the Azs are around ten times greater than those of the Aps when related to their aqueous concentrations) that the potencies of the two enhancer series are about the same when expressed in terms of their concentrations in the intercellular lipid phase of the SC. Another outcome of the partitioning studies has been the finding that the extent of partitioning into the intercellular lipids of the SC at E = 10 isoenhancement conditions for both the Azs and Aps is essentially independent of the n-alkyl chain length (from butyl to octyl). A third result from these experiments has been that the partitioning of E2beta (the surrogate permeant) into the HMS SC under E = 10 isoenhancement concentration conditions is approximately the same with the Aps and Azs as enhancers. For both the Aps and Azs, the E2beta SC partitioning enhancement was found to be in the range of 5-6 at E = 10. This comparable partitioning enhancement for E2beta in the presence of Aps and Azs at E = 10 suggests that the same mechanism was involved and that these enhancers act, in part but to a significant extent, by inducing a higher partitioning tendency of the permeant into the transport rate-limiting lipoidal domains of the SC. (c) 2003 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 92:297-310, 2003

The objective of this study was to investigate the transport behavior of a series of oligonucleotides with human epidermal membrane (HEM) and to examine the applicability of the modified NERNST-PLANCK model to transdermal iontophoresis of these macromolecules. Iontophoretic transport experiments were first carried out in a synthetic model membrane system (Nuclepore membranes) with a four-electrode potentiostat to examine the baseline modified NERNST-PLANCK model. The modified NERNST-PLANCK model derived from the Einstein relation and the Stokes-Einstein equation taken from previous work did not hold for the oligonucleotides. Results obtained in the Nuclepore studies were, however, consistent with predictions of the modified NERNST-PLANCK model using the experimentally determined electromobilities and diffusion coefficients. The electromobilities of the oligonucleotides (determined by capillary electrophoresis) were found to be more than a factor of two smaller than expected from the Einstein relation between electromobilities and diffusion coefficients (the latter determined in diffusion cell experiments). A correlation between these electromobilities and the theoretical electromobilities estimated by considering the effects of counterion binding and the effects of mobility reduction according to colloid theory was also observed. These results suggest that the modified NERNST-PLANCK model predictions are satisfactory only when the electromobilities and the effective molecular size of the oligonucleotides are known and are used directly to predict the iontophoretically enhanced transport. Results with the HEM experiments generally agreed with model predictions based on the experimental electromobilities. The oligonucleotide HEM flux data also suggest the existence of pores with effective pore radii greater than the effective radii estimated in previous studies with small molecular weight model permeants.

It has been shown that significant pore induction (electroporation) occurs in human epidermal membrane (HEM) during iontophoresis even at moderate applied voltages (1–10 V). Recent efforts in our laboratory have been aimed at quantifying HEM electroporation by examining the proportionality between flux enhancement due to electroporation and electrical conductance changes during iontophoresis. The specific purpose of the present study was to test the hypothesis that by matching the background electrolyte ion sizes with the permeant ion sizes, the flux enhancement due to electroporation can be quantified by the change in HEM electrical conductance. In this study, radiolabeled tetraethylammonium (TEA+), methylammonium (MA+), and mannitol were the permeants. Potassium chloride (KCl), tetraethylammonium bromide (TEAB), tetraethylammonium pivalate (TEAP), and sodium fluoride (NaF) were the background electrolytes. Iontophoresis experiments were carried out over an applied voltage range of 1 to 3 V. The experimental flux enhancement results were compared with the theoretical predictions from the Nernst–Planck model after corrections were made: (a) for HEM pore induction during iontophoresis based on electrical conductance changes and (b) for electroosmosis employing mannitol as the neutral probe permeant. In experiments where the ion sizes of the background electrolyte and permeant were closely matched (e.g., TEA+ as the permeant and TEAP as the background electrolyte), there was excellent agreement between experimental results and theoretical predictions of the modified Nernst–Planck model, with only modest data scatter. When the electrolyte and permeant sizes were quite different (e.g., TEA+/KCl and MA+/TEAP), the experimental flux data were inconsistent with model predictions and there were large variations in the experimental results. The results of the present study illustrate that permeant flux enhancement can be predicted by the modified Nernst-Planck model even during moderate voltage iontophoresis when electroporation is operative. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:932–942, 2001

The aim of this study was to assess the extent to which a model with pores having only net negative charges would adequately describe transdermal electroosmosis in human epidermal membrane (HEM) at neutral pH. Such information would enhance the predictive value of the modified Nernst-Planck model for transdermal iontophoresis, in addition to providing insights regarding the likelihood of significant pore charge distribution in HEM. Baseline results (the control) obtained from 0.1 to 0.4 V anodal and cathodal electroosmosis experiments with synthetic polycarbonate membranes (Nuclepore membranes), using radiolabeled urea and mannitol as the model permeants, demonstrated that such a membrane system can be modeled by the electrokinetic (electroosmosis) theory with the assumption of the pores possessing only negative charges. The studies with HEM were carried out at low voltage (/=0.5 V) where alterations in the barrier properties of HEM were minimal and at higher voltages (/approximately = 1.0 V) where significant field-induced pore formation in HEM occurred. In both the low and high voltage studies, radiolabeled urea, mannitol, and water were employed as permeants in cathodal and anodal iontophoresis experiments. The results of the low voltage iontophoresis experiments suggest significant pore charge distribution in HEM (a significant deviation between the predictions from the single pore charge type assumption and the experimental data). Under the higher applied voltage conditions (/approximately = 1.0 V), results from anodal and cathodal electroosmosis studies were consistent with the model in which the HEM has only pores that are net negatively charged.

This study focused upon the enhancement effects of ionic surfactants upon passive and electroosmotic transdermal flux. The first phase of the study involved validating theories relating surface properties of a membrane to electroosmotic solvent flow under appropriate experimental conditions using a synthetic model membrane (stack of 50 Nuclepore membranes). Numerical solutions to the Poisson-Boltzmann equation and the equations of fluid motion served as the theoretical basis for the experimental studies. Important outcomes of the model membrane studies were that electroosmotic solvent flow velocity was enhanced by the addition of an anionic surfactant, sodium dodecyl sulfate, and reversed by the addition of a cationic surfactant, dodecyltrimethylammonium bromide. The effective membrane pore wall surface charge densities were determined under a variety of experimental conditions. Adsorption of dodecyl sulfate to the pore wall increased the net negative charge on the pore wall. A reversal of the net pore wall surface charge density resulted from the adsorption of dodecyltrimethylammonium. The interrelationship between electroosmosis, surfactant adsorption, and ionic strength was also evaluated. The second phase of the study was an investigation of the effects of sodium dodecyl sulfate upon the transport of neutral polar permeants through human epidermal membrane (HEM). Fluxes of [14C]urea and [3H]sucrose were simultaneously measured across HEM samples under passive and 250 mV conditions; flux measurements were made before, during, and after HEM exposure to sodium dodecyl sulfate. A systematic analysis of the experimental data made it possible to elucidate the specific contributions of sodium dodecyl sulfate and the applied electric potential to the overall flux enhancement. Sodium dodecyl sulfate enhanced the intrinsic passive permeability of the HEM, and it also enhanced the contribution of electroosmosis to the flux during iontophoresis.

Recently, we reported that the in vivo efficacy of acyclovir (ACV) formulations was a single valued function of skin target site free drug concentration (C) irrespective of the formulation compositions. A long-term objective of this research has been to generalize the C concept using model drugs which are similar to as well as different from ACV in their mechanism of actions. (Bromovinyl)deoxyuridine (BVDU) was selected as a model drug based on the reported similarity in its mechanism of action with ACV. The relationship between the C predictions and the in vivo efficacies for some topical formulations containing different concentrations (0.05-10%) of either ACV or BVDU in 95% DMSO as a vehicle with or without 5% Azone as skin permeation enhancer was examined. Hairless mice infected cutaneously with HSV-1 were used to quantitatively estimate the in vivo topical antiviral efficacy. A finite dose of the test antiviral formulation was applied twice a day for 4 days, starting the day after virus inoculation. On the fifth day, the lesions were scored and the efficacy values were calculated. For each formulation, in vitro flux experiments were performed in an in vivo-in vitro experimental design that closely approximated the in vivo study protocol. As was previously shown, with all ACV formulations, a good correlation was found between the C predictions and the in vivo topical efficacy. With the BVDU formulations, on the other hand, this was found not to be the case. BVDU formulations with 5% Azone were generally much more effective than those without Azone at comparable C values. This finding is believed to be the first of its kind showing that skin "permeation enhancers" may enhance efficacy by more than simply increasing skin permeation rates.

□ This report describes the results of iontophoresis experiments involving the transport of polar nonelectrolytes across human epidermal membrane (HEM) at a moderate applied voltage of 2.0 V and where the data are interpreted via a convective transport model and hindered transport theory. A principal finding is that although HEM iontophoresis at 2.0 V resulted in a large increase in HEM porosity, the pore radii of the newly induced pores in HEM as calculated from the iontophoresis data using the hindered transport theory were found to be in the range of 6–12 A. This supports the view that electroporation at these modest applied voltages results in pores with sizes the same order of magnitude but somewhat smaller than those estimated for the preexisting pores in HEM prior to electroporation. This outcome is also important from a practical standpoint, as flux enhancement for large molecules (such as oligonucleotides and polypeptides) arising from electroporation under these conditions would be expected to be significantly less than if the resulting pore sizes were much greater. Providing a “prepulse” of 4.0, 8.0, and 15 V prior to the 2.0 V iontophoresis generally gave greater increases in HEM conductance (and, therefore, in porosity) but did not significantly change the deduced effective pore radii (around 5–9 A). The alteration during and the recovery of HEM after iontophoresis was also investigated. The recovery behavior was found to be dependent upon both the duration of the applied voltage and the magnitude of its effects: the recovery for a HEM sample that experienced a large increase in electrical conductance during iontophoresis was generally poorer than that for a sample that was more resistant to the electric field. Incomplete recovery was generally observed in experiments with long iontophoresis duration (50 min) and with the higher voltages (4.0, 8.0 V, and 15 V). In these cases, the barrier properties of HEM were more greatly altered as indicated by larger increases in the electrical conductance and passive permeability of HEM after iontophoresis.

This study focused upon quantitatively determining the influence of permeant molecular size upon flux enhancement which results from electroosmosis. The first phase of the study involved validation of a fundamental model describing the molecular size dependence of flux enhancement which results from convective solvent flow. This was accomplished using a model synthetic membrane (stack of 50 Nuclepore membranes) and four model permeants with a molecular weight range of 60–504 (urea, mannitol, sucrose, and raffinose). The steady-state flux of each permeant was determined under passive conditions and applied voltages of 125, 250, 500, and 1000 mV using side-by-side diffusion cells and a four-electrode potentiostat system. On the basis of the permeability enhancement for each permeant at each applied voltage (relative to the passive permeability) it was possible to calculate the effective solvent flow velocity from each permeant at each field strength. An important finding was that the flux enhancement due to electroosmosis was strongly molecular weight dependent (i.e., the flux enhancement ratio was around 4 times greater for raffinose than for urea, with mannitol and sucrose yielding intermediate values), while the calculated effective flow velocity at each voltage was independent of the molecular weight of the permeant. This coupled with a linear correlation between flow velocity and applied voltage served to establish the validity of the method and model. The second phase of the study was an extension of the model to human epidermal membrane (HEM). These experiments involved simultaneously measuring the fluxes of [14C]urea and [3H]sucrose across HEM samples under passive, 250 mV, and 500 mV conditions. Similar to the Nuclepore system, the observed flux enhancement ratios with HEM were approximately 3 times greater for sucrose than for urea. A detailed analysis of the HEM data showed semiquantitative agreement between predictions of the model and experimental results.

Previously, the effects of a series of 1-alkyl-2-pyrrolidones (APs; C2-C8) on the lipoidal pathway of hairless mouse skin (HMS) were studied with a parallel pathway skin model. At their isoenhancement concentrations, these 1-alkyl-2-pyrrolidones induce the same transport enhancement (isoenhancement factor, EHMS) on the lipoidal pathway of the stratum comeum for the probe permeants studied. In the present study, the fluidizing effects of APs upon the stratum comeum lipid liposome (SCLL) bilayer were investigated under these isoenhancement conditions using steady state anisotropy and fluorescence lifetime studies with fluorescent probes 2-, 6-, and 9-(9-anthroyloxy)stearic acids, 16-(9-anthroyloxy)palmitic acid, and 1,6-diphenyl-1,3,5-hexatriene to examine a possible correlation between the fluidizing properties of APs and their enhancement effects on transdermal drug transport. Time-resolved fluorescence decay studies were also conducted to further investigate the fluidizing properties of APs and add support to the steady-state fluorescence results. Under an isoenhancement condition of EHMS = 10, these APs fluidized the alkyl chains of the lipids at intermediate depths (C6-C9) in the SCLL bilayer (a 40-50% decrease in the rotational correlation times) but did not significantly change the fluidity in the deep hydrophobic region of the bilayer. Three rotational correlation times were deduced from the global simultaneous analysis in time-resolved fluorescence decay measurements. The slowest of these (greater than 1000 ns) was attributed to the global motion of SCLLs and is probably related to the static component of steady-state anisotropy. The other two rotational correlation times (on the order of nanoseconds) were in the range expected for the local motion of the fluorophores and may correspond to their vibrational and rotational motions. When the concentrations of APs were increased (increasing the EHMS value), the static component (alpha) decreased. This suggests that APs might induce a general fluidizing effect upon the lipid bilayer (i.e., a decrease in the order of the lipid bilayer). The decrease in the longer rotational correlation time (on the order of nanoseconds) with increasing EHMS value, on the other hand, indicates a possible increase in the "cavity volume" for the hindered motions of the fluorophores (i.e., an increase in the free volume at intermediate depths in the bilayer).

The influence of a series of 1-alkyl-2-pyrrolidones (C 2 -C 8 ) on the transport behavior of lipophilic and polar/ionic permeants across hairless mouse skin was recently investigated by employing a physical model approach that treats the stratum comeum barrier as a diffusional system of parallel lipoidal and pore pathways. In this previous study, the transport enhancement effects (enhancement factor, E HMS ) on the lipoidal pathway of the stratum corneum were found to be essentially the same for all steroidal probe permeants investigated at various concentrations of these 1-alkyl-2-pyrrolidones. In the present research, the relationship between solute transport enhancement in the lipoidal pathway of hairless mouse skin and the transport enhancement in the stratum corneum lipid liposome bilayer was studied by comparing the enhancement factor for the lipoidal pathway in the hairless mouse skin, E HMS , with that for the stratum corneum lipid liposome, E SCLL , at equal solution concentrations of the 1-alkyl-2-pyrrolidones. The release rates of D-mannitol, D-glucose, 3- O -methyl-D-glucose, sucrose, and raffinose from stratum corneum lipid liposomes were determined, and the E SCLL values for these permeants were compared with the E HMS values obtained with hairless mouse skin using the steroidal permeants. An important finding in this study was a semiquantitative correlation between the enhancement effects induced by the 1-alkyl-2-pyrrolidones, except 1-ethyl-2-pyrrolidone, with the liposome bilayer using sugar molecules as permeants and those found with the lipoidal pathway in hairless mouse skin using steroid molecules as permeants. The enhancement effects on the barrier properties of the liposome bilayer were found to be reversible at the levels of the 1-alkyl-2-pyrrolidones used in the present study. The transport mechanism of the sugar molecules in the liposome bilayer was also investigated, and analyses involving hindered diffusion calculations and a permeability vs partition relationship suggest that partitioning followed by diffusion in the liposome bilayer is the likely major mechanism in the transport of the sugar molecules out of stratum corneum lipid liposomes; this finding is consistent with the semiquantitative correlation found between E HMS and E SCLL .

The effects of mixing solution volume (0.25-0.65 mL/g) on in vitro drug release from a self-setting bioactive calcium phosphate cement containing the anticancer agent 6-mercaptopurine (6-MP) as a model compound were investigated. The drug release profiles from isolated planar surfaces as well as the entire surfaces of cement systems containing 5% 6-MP were measured in simulated body fluid at pH 7.25 and 37.0 degrees C. The drug release rate from both cement system geometries increased with increasing mixing solution volume. Drug release profiles from the planar-release and entire-surface-release cement matrix systems were analyzed by and found to agree with the Higuchi and Cobby equations, and the kinetic parameters were estimated with a nonlinear least-squares computer program. Linear relationships were found between the mixing solution volume and the drug release rate constants or time required for 50% drug release for both cement release geometries. Furthermore, the total pore volume of the cements, as measured by mercury porosimetry, increased with increasing mixing solution volume.

A novel drug delivery device based on a self-setting bioactive cement formed from tetracalcium phosphate and dicalcium phosphate has been developed and tested in vitro with indomethacin as a model drug. Equimolar mixtures of the calcium phosphate powders containing 2 and 5% of indomethacin were transformed into a hydroxyapatite after being mixed with a dilute phosphoric acid solution. X-ray diffraction and differential scanning calorimetry results suggested that indomethacin transformed into an amorphous form in the pores of the cement matrix as it hardened. In vitro drug release from cement pellets into a 0.1 mol/L phosphate buffer at pH 7.40 and 37 degrees C continued for > 3 weeks. Release from 2 and 5% drug-loaded cements followed the Higuchi model equation. The drug release profiles of 5% drug-loaded cements with different thicknesses (0.5, 1.0, and 1.5 g) overlapped up to 90% drug release, indicating that the drug concentration gradient in the pore was independent of the thickness of the cement as expected from the model equation.

Earlier work from this laboratory suggested that the cholesterol (Ch) thermodynamic activity is a more meaningful measure of the degree of Ch supersaturation in human bile than the widely known cholesterol saturation index. An early version of a method for determining thermodynamic activity based on Ch uptake from bile salt (BS)-lecithin (LE) solutions into silicone polymer particles, but requiring 12-24 h for reaching equilibrium, was considered unsatisfactory because Ch nucle- ation and crystal formation frequently occurred within a few hours. The aim of the present work was to develop a method that would reduce equilibration times to the order of 1 h. Changing the thickness of the silicone film alone did not result in the desired reduction of equilibration times and it was soon deduced that the uptake of Ch by the silicone film from the BS-LE solution was a surface-controlled transport process involving the transport of Ch by negatively charged BS and BS-LE micelles at the interface. Three different approaches were tried to modify the silicone film to make its surface positively charged, thereby reducing and/or eliminating the presumed electrical repulsion barrier for the interfaciai transport of Ch. The film was treated with different concentrations of aminopropyl methyl-dimethylsiloxane (AMDS) in cyclohexane, octadecyldimethyl-[3-(trimethoxysilyl)-propyl] ammonium chloride (ODTOP) in methanol, and octadecylamine solution in ethanol. Films treated with 1-1.5% ODTOP and 5-10% AMDS reduced the Ch equilibration times for model BS-LE solutions to

A novel approach using a self‐setting hydroxyapatite (HAP) cement as a skeletal drug‐delivery system has been proposed to solve the problem of delivering drugs to skeletal tissue at sufficiently high local concentrations for desirable therapeutic effects. HAP cements loaded with antibiotics can be formed in situ and can be used as bonding materials between bone and prostheses, as well as drug‐release devices. The cement also possesses sufficient mechanical strength to be a potential bone grafting material. Using cephalexin and norfloxacin as model drugs, we observed continuous in vitro release profiles of these compounds from cement pellets loaded 0.9‐4.8% by weight with one of the drugs. This drug‐release pattern correlated well with the Higuchi model. This hydroxyapatite cement drug‐delivery system can be applied in the treatment of osteomyelitis and infected compound fractures.

Transdermal delivery systems of 9-beta-D-arabinofuranosyladenine (ara-A), having controlling membranes of various permeabilities, were developed and applied to Azone-pretreated hairless mouse abdominal skin. It was confirmed that the blood concentrations of ara-A and its metabolite 9-beta-D-arabinofuranosylhypoxanthine (ara-H) in hairless mice are controlled by the permeability of the controlling membrane in the transdermal patch. Furthermore, these blood concentrations were found to closely agree with the values obtained from theoretical model calculations. Finally, but importantly, the "micropharmacokinetic" behavior of ara-A in cutaneous tissue could also be predicted. These results suggest that the transdermal patch may be employed in dermal and transdermal ara-A efficacy studies in the treatment of cutaneous herpes virus infections in hairless mice.

This research investigated the effects of a laser irradiation treatment on the dissolution characteristics of hydroxyapatite (HAP), and the results provide an insight into the relationship between the effects of laser treatment and the two-site dissolution kinetics of HAP samples. The HAP samples prepared by aqueous precipitation and digestion at ~ 100 °C were irradiated with a CO2 laser (20–50 W) with a beam diameter of 14 mm for a total of 10–400 s. Dissolution rates of the laser-treated HAP samples were subsequently determined in acetate buffer (pH = 4.5, μ = 0.50) at various levels of partial saturation (0–24% with respect to the HAP thermodynamic solubility of pK sp = 116). The following were the important findings. The X-ray diffraction and the IR spectroscopy results suggested that the HAP crystalline structure was not changed by laser treatment. Laser treatment of HAP powder at 50 W for 400 s, however, caused an ~ 3.5-fold reduction in the specific surface area of HAP and reduced the initial dissolution rate of HAP in acetate buffer by a factor of ~ 22.9. Also, this laser treatment appeared to reduce the dissolution rate of HAP in 16 and 24% partially saturated acetate buffer from substantial levels to essentially zero. These results may be summarized as follows. Laser treatment of HAP results in a reduction in the dissolution rate and also a reduction in the specific surface area of this material. However, the dissolution rate reduction is significantly greater than the reduction in the specific surface area. Also, the percent reduction in the dissolution rate of laser-treated HAP in partially saturated buffers appears to be much greater than that for non-laser-treated HAP. In order to independently evaluate the effects of laser irradiation on the ion activity product (IAP) of HAP, a rotating disk dissolution study (based on Levich theory) of lased HAP pellets (50 W for 400 s, 13 kJ/cm2) was performed. The ion activity product (K HAP) governing the dissolution of the lased HAP pellet was found to be 1 × 10− 129.2 ± 0.7. This value for K HAP, together with the HAP powder results (above), strongly suggests that laser irradiation converts HAP possessing a two-site character to a HAP of a lower effective solubility (i.e., site #2 only).

The original article to which this Erratum refers was published in Journal of Pharmaceutical Sciences 94, 2005, 847–860: DOI 10.1002/jps.20293.