Transscleral Iontophoresis for Noninvasive Ocular Drug Delivery of Macromolecules

Purpose: The objectives were to investigate the effect of transscleral iontophoresis of macromolecules in vitro and in vivo, to study the importance of electroosmosis on macromolecules of low charge to mass ratio, and to evaluate transscleral iontophoresis efficacy in a choroidal neovascularization (CNV) animal model. Methods: Through in vitro transport experiments, the permeability coefficients of macromolecules [eg, immunoglobulin G (IgG), dextran 70 kDa] were determined under different conditions. The effect of ionic strength formulations and iontophoretic conditions was studied on the distribution of IgG and bevacizumab into the eye in vivo. Magnetic resonance imaging (MRI) was utilized to evaluate in vivo real time distribution of gadolinium-labeled albumin (Galbumin) following iontophoresis. The efficacy between no treatment, intravitreal injection (IVT), and iontophoresis of bevacizumab on a CNV model of subretinal injection of adeno-associated virus encoding human VEGF-165 was investigated. Results: The permeability data suggested a significant effect of ionic strength on the iontophoretic transport of macromolecules. Transscleral iontophoresis of IgG at 4 mA with a low ionic strength formulation was about 600 times greater than passive diffusion and 14-fold over a conventional formulation in vitro. Approximately 0.6 mg of bevacizumab can be delivered into the rabbit eye in vivo with a 20-min treatment of iontophoresis. MRI showed that Galbumin was in the posterior tissues after iontophoresis. In the CNV model, the iontophoresis and IVT methods of bevacizumab delayed retinal neovascularization by 4 and 8 weeks, respectively. Conclusions: Transscleral iontophoresis is capable of delivering macromolecule drugs through the conjunctiva and sclera, eventually exposing the retina/choroid to the drugs.

Novel Dexamethasone Sodium Phosphate Treatment (DSP-Visulex) for Noninfectious Anterior Uveitis: A Randomized Phase I/II Clinical Trial

PURPOSE

Frequent steroid drops represent a challenge in patient compliance. This study evaluated the safety and efficacy of 5 minute topical dexamethasone sodium phosphate-Visulex (DSP-Visulex) treatment regimen (two applications on the first week then weekly after) compared to daily prednisolone acetate 1% (PA) for noninfectious anterior uveitis.

MATERIALS AND METHODS

Forty-four patients were randomized to 8% DSP-Visulex with placebo eye drops (8% group, n = 14), 15% DSP-Visulex with placebo eye drops (15% group, n = 15), or Vehicle-Visulex with PA eye drops (PA group, n = 15). Patients received daily eye drops and Visulex treatments on days 1, 3, 8, and 15 with an optional treatment on day 22. Efficacy measures were change in anterior chamber cell (ACC) count from baseline and proportion of patients with zero ACC count at days 8, 15, and 29. Safety measures were adverse events (AEs), visual acuity, ocular symptoms, and intraocular pressure (IOP).

RESULTS

ACC resolution over time was similar among the three groups. The percentage of patients with clear ACC was 18%, 22%, and 15% on day 8; 27%, 56%, and 54% on day 15; and 90%, 88%, and 77% on day 29 for the 8%, 15%, and PA groups, respectively. The numbers of reported AEs were 10, 36, and 12 for the 8%, 15%, and PA groups, respectively. Ten patients among all groups experienced treatment-related AEs, which included headache, eye pain, corneal abrasion, conjunctival/corneal staining, conjunctivitis, visual acuity reduction, and keratitis all of which were resolved during the timeframe of patients' participation in the study. IOP elevation was noted in the PA group throughout the study, whereas IOP elevation in the DSP-Visulex groups was observed at day 3 but not thereafter.

CONCLUSIONS

The efficacy of the DSP-Visulex applications was comparable to the daily PA drops in the treatment of noninfectious anterior uveitis. Both 8% and 15% DSP-Visulex treatments were safe and well tolerated.

Ocular Drug Distribution and Safety of a Noninvasive Ocular Drug Delivery System of Dexamethasone Sodium Phosphate in Rabbit

PURPOSE

To determine the ocular toxicity, systemic exposure, and amounts of dexamethasone sodium phosphate (DSP) in ocular tissues after administration of DSP with the Visulex system (DSP-Visulex).

METHODS

DSP-Visulex was applied onto healthy rabbit eyes. DSP concentrations (4%, 8%, 15%, and 25%) and treatment durations (5, 10, and 20 min) were evaluated for the amounts of DSP in the ocular tissues and in plasma after single administrations of DSP-Visulex. The drug in eye tissues and plasma was analyzed by high-performance liquid chromatography-UV/VIS and by liquid chromatography-mass spectrometry, respectively. The safety and tolerability were ascertained based on clinical observations and histopathological examinations from repeat weekly DSP-Visulex treatments (4%, 8%, 15%, and 25% for 20 min) for 12 weeks.

RESULTS

Significant amounts of DSP (ie, higher than 1 μg/g) were found in the anterior chamber, retina-choroid, cornea, vitreous, conjunctiva, and sclera after single applications of DSP-Visulex. The DSP concentrations in the ocular tissues and in plasma increased with increased DSP concentrations in the Visulex applicator and with increased application times. Systemic DSP was rapidly detected. The plasma half-life was 2-3 h. C_max was 148 and 1,844 ng/mL, and the area under the plasma drug concentration versus time curve (AUC) was 418 and 3,779 ng · h/mL for the low dose (4% DSP-Visulex for 5 min) and the high dose (15% DSP-Visulex for 20 min), respectively. Ocular findings over 12 weeks were mostly conjunctival injection and eye discharge. These were transient and mild. Histopathological examinations indicated the eyes to be normal.

CONCLUSIONS

DSP can be administered safely and effectively into the rabbit eye with the Visulex system. Treatment duration and DSP concentration are important factors in achieving therapeutic levels. Repeat applications of DSP-Visulex are safe and well tolerated for weekly administrations over 4-12 weeks. DSP-Visulex has clinical potential for the noninvasive treatment of ocular diseases.

Noninvasive Ocular Drug Delivery System of Dexamethasone Sodium Phosphate in the Treatment of Experimental Uveitis Rabbit

PURPOSE

To investigate the efficacy and safety of dexamethasone sodium phosphate administered through Visulex system (DSP-Visulex) in treating experimental uveitis.

METHODS

Uveitis was induced in rabbits by subcutaneous injections of complete Freund's adjuvant and an intravitreal injection of H37RA antigen. After induction, the animals of the control group received no treatment and the others received various treatment regimens of DSP-Visulex. Each regimen was different in DSP strength (4%, 8%, and 15%), application time, or treatment frequency. Efficacy and safety of DSP-Visulex were evaluated by ophthalmic observations and histopathological examinations for ocular inflammations and pathology.

RESULTS

The control group exhibited panuveitis with significant inflammation in the vitreous, choroid, and retina, but less in the conjunctiva, cornea, and anterior chamber. The uveitis occurred within 24 h after induction and persisted throughout the study in the control group. All treatments showed some reduction in inflammation in the vitreous, choroid, and retina. The higher dose regimens generally showed more rapid and higher degree of resolution than the lower dose regimens. The posterior eye tissues of the 15% and 8% DSP-Visulex appeared normal with minimal or no inflammation, whereas the untreated eye and the 4% DSP-Visulex eyes showed minimal response.

CONCLUSIONS

All DSP-Visulex regimens suppressed the signs of inflammation and were well tolerated over the course of a 29-day study. The 8% and 15% DSP-Visulex treatment regimens were safe and efficacious for anterior, intermediate, and posterior uveitis. On the other hand, the 4% DSP-Visulex regimen may only be considered for anterior and intermediate uveitis.

Quantitative determination of lattice fluoride effects on the solubility and crystallinity of carbonated apatites with incorporated fluoride

The purpose of this study was to evaluate quantitatively the effects of fluoride on the solubility and crystallinity of carbonated apatites (CAPs) after its incorporation into the crystal lattice using the metastable equilibrium solubility (MES) distribution method. Fluoride-incorporated CAPs (F-CAPs) of two different carbonate levels (3 and 5%) and fluoride contents from 0 to 20,000 µg/g were synthesized. X-ray diffraction experiments and Rietveld analysis were conducted to obtain crystallite microstrain and unit cell parameters. Acetate buffer MES solution media were prepared at two solution fluoride concentrations (0.2 and 2.0 mg/l) and at two pHs (5.0 and 5.7). The unit cell a-axis values of the F-CAPs were found to decrease as the fluoride content increased, consistent with the fluoride being incorporated into the crystal lattice. The fluoride concentrations in the MES solution media were high enough to provide a 'swamping' effect such that the fluoride released from the F-CAPs during dissolution was minimal in changing the solution fluoride concentration. Employing the MES distribution superposition method, it was shown that the surface complex possessing the fluorapatite (FAP) stoichiometry [Ca10(PO4)6F2] accounted for the MES distribution behavior of all experiments. In addition, the mean pIFAP [the value of -log(aCa(10)aPO4(6)aF(2)) calculated from the ionic activity product based on FAP stoichiometry of the MES dissolution media in which 50% of the F-CAPs had dissolved] correlated well with the crystallite microstrain parameters of the F-CAPs. The incorporated fluoride in the F-CAPs showed only modest effects on F-CAP crystallinity and solubility.

Effect of geometrical structure on the in vivo quality change of a three-dimensionally perforated porous bone cell scaffold made of apatite/collagen composite

Biodegradable artificial bone blocks with interconnective pores were prepared using a self-setting apatite/collagen composite cement as a cell scaffold for bone regenerative medicine. The biological behavior of the blocks was tested in rats, and the change in their properties after implantation was measured. One cubic block [10 mm X 10 mm X 10 mm; porous composite (PC)] was obtained from apatite cement (apatite/collagen cement; 80% of apatite:20% of collagen) with 60 interconnecting holes, 500 um in diameter. The other blocks (NC and NN) without holes were obtained from the apatite/collagen and plain apatite cements, respectively. All blocks were implanted in the rats for 56 days. Changes in the amount and density (block mineral mass and block mineral density) of the blocks were evaluated based on dual energy X-ray absorptiometry images, and the order of biodegradation was PC < NC < NN. After implantation, the blocks were removed, and subjected to an X-ray diffraction (XRD) analysis, Fourier-transformed infrared (FT-IR) spectroscopy and thermogravimetry (TG). The XRD peaks of all blocks increased significantly. TG revealed that the amount of carbonated apatite also increased with time. However, the organic component of PC depended on the implantation period, consistent with the FT-IR results. Because PC had interconnective macro- and micropores in the apatite/collagen matrices, the results indicated that soft tissue penetrated the block carbonated apatite was generated, bone remodeling was accelerated in the implant.

Determination of binding affinity for chenodeoxycholate in equilibrium with sulfobutylether-β-cyclodextrin

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 K₁ and K₂, the binding affinity for the solutions was quantified at the best curve fitting using a least-squares technique. The very high binding affinity of K₁ and the very low (i.e., negligible) binding affinity of K₂ indicate the formation of 1:1 inclusion complexes. In addition, the values of K₁ and K₂ 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.

Effects of oxygen-containing terpenes as skin permeation enhancers on the lipoidal pathways of human epidermal membrane

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.

Ion-exchange membrane assisted transdermal iontophoretic delivery of salicylate and acyclovir

The presence of endogenous competing counterions is a main reason for the generally low efficiency of transdermal iontophoretic drug delivery. The objective of the present study was to test the hypothesis that the incorporation of an ion-exchange membrane (Ionac) in an iontophoresis system to hinder transdermal transport of these counterions can enhance iontophoretic delivery. The properties of Ionac were characterized in passive and iontophoretic transport experiments. Iontophoretic transport across human epidermal membrane (HEM) and across HEM in series with Ionac was then studied. To assess the effect of HEM electrical resistance upon Ionac-assisted iontophoresis, HEM resistance was reduced in the iontophoresis experiments with alternating current (AC). Salicylate (SA) was the negatively charged permeant first tested in this study. Mannitol was the model permeant to examine the effects of electroosmosis. At the completion of the SA study, experiments were performed with acyclovir (ACV), an antiviral drug with limited water solubility. When Ionac was used to enhance SA transdermal fluxes, higher SA fluxes were observed with HEM of lower resistances in Ionac-assisted iontophoresis. Up to a four-fold flux enhancement was achieved when the electrical resistance of HEM was reduced using an AC iontophoresis method. For ACV, two-fold flux enhancement was observed in Ionac-assisted iontophoresis compared with the conventional iontophoresis baseline. In all experiments, the contribution of electroosmosis to drug transport was less than 10%. The present study has demonstrated the potential of a new approach using a positively charged ion-exchange membrane to enhance transdermal iontophoretic transport of negatively charged drugs.

Passive and oxymetazoline-enhanced delivery with a lens device: pharmacokinetics and efficacy studies with rabbits

PURPOSE

The aims of this study were to assess the trans-scleral delivery of dexamethasone phosphate (DexP) with a prototype lens device and a formulation comprising a vasoconstrictor and to determine the efficacy of this delivery system in treating experimentally induced uveitis in a rabbit model.

METHODS

Passive trans-scleral delivery was performed on New Zealand white rabbits in vivo, using the lens device and a formulation of 0.034 M oxymetazoline (OMZ, the vasoconstrictor) and 0.5 M of dexamethasone sodium phosphate (DexNaP). Trans-scleral delivery of DexP without OMZ was the control. The amounts of DexP delivered into the eye and its distributions in the eye were determined by dissection of the eye and high-performance liquid chromatography assay in the pharmacokinetics study. The efficacy of the DexP delivery system in treating lipopolysaccharide-induced uveitis was also evaluated in the rabbit model in vivo. The effect of OMZ upon DexP delivery and its treatment efficacy was studied by comparing the DexP results with and without OMZ.

RESULTS

In the pharmacokinetics study, the amounts of DexP delivered into the eye using the lens system with OMZ were significantly higher than those without OMZ. The results in the efficacy study showed a better treatment outcome with OMZ to relieve the symptoms of endotoxin-induced uveitis in rabbits.

CONCLUSIONS

The potential of vasoconstrictors to enhance eye disease treatments in passive trans-scleral drug delivery was demonstrated. The higher DexP level in the eye and the improvement of the outcome in the efficacy study in the presence of the vasoconstrictor are consistent with the hypothesis that the vasoconstrictor enhances drug delivery by decreasing clearance.

Effects of alternating current frequency and permeation enhancers upon human epidermal membrane

Previous studies have demonstrated the ability of AC iontophoresis to control skin resistance in different transdermal iontophoresis applications. The objectives of the present study were to (a) identify the alternating current (AC) frequency for the optimization of AC pore induction of human epidermal membrane (HEM) and (b) determine the effects of chemical permeation enhancers upon the extent of pore induction under AC conditions. Experiments with a synthetic membrane system were first conducted as the control. In these synthetic membrane experiments, the electrical resistance of the membrane remained essentially constant, suggesting constant electromobility of the background electrolyte ions under the AC conditions studied. In the HEM experiments, the electrical resistance data showed that higher applied voltages were required to induce the same extent of pore induction in HEM at AC frequency of 1kHz compared with those at 30Hz. Even higher voltages were needed at AC frequencies of 10kHz and higher. AC frequency also influenced the recovery of HEM electrical resistance after AC iontophoresis application. An optimal AC frequency region for effective pore induction and least sensation was proposed. Permeation enhancers were shown to enhance pore induction in HEM during AC iontophoresis. The enhancers reversibly reduced the AC voltage required to sustain a constant state of pore induction in HEM during AC iontophoresis, consistent with the mechanism of lipid lamellae electroporation in the stratum corneum.

Transscleral iontophoretic and intravitreal delivery of a macromolecule: study of ocular distribution in vivo and postmortem with MRI

The distribution and clearance of macromolecules in ocular delivery are not well understood. It has been hypothesized that iontophoresis can enhance transscleral delivery of macromolecules. The objective of this study was to investigate the ocular distribution of a macromolecule after transscleral iontophoretic delivery and intravitreal injection in vivo using nuclear magnetic resonance imaging (MRI) and to compare these results. Experiments of constant current transscleral iontophoresis of 4mA or intravitreal injection were performed on New Zealand white rabbits in vivo. Iontophoresis experiments were also performed on rabbits postmortem. Galbumin (Gd-labeled albumin) was the model permeant surrogate to clinical therapeutic agents. MRI was used to monitor the distribution of the molecule in the eye after ocular iontophoresis and intravitreal injection. In addition, the conjunctiva, sclera, choroid, and retina were extracted in the transscleral iontophoresis study to determine the amounts of Galbumin in these tissues using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The results show that iontophoresis enhanced the ocular delivery of Galbumin. The macromolecule was mainly delivered into the conjunctiva and sclera in microgram quantities and then diffused towards the posterior section in the upper hemisphere of the eye in vivo. Both in vivo and postmortem studies show that the iontophoretic delivery of Galbumin into the vitreous was below the detection limit. In the intravitreal injection study, the diffusion coefficient of Galbumin in the vitreous humor was estimated to be close to that of free aqueous diffusion.

Examination of barriers and barrier alteration in transscleral iontophoresis

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/cm(2)) 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 (Mn(2+)) and manganese ethylenediaminetetraacetic acid complex (MnEDTA(2-)) 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.

Iontophoretic transport across a multiple membrane system

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.

Estimation of cholesterol solubilization by a mixed micelle binding model in aqueous tauroursodeoxycholate:lecithin:cholesterol solutions

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.

Systematic studies on the paracellular permeation of model permeants and oligonucleotides in the rat small intestine with chenodeoxycholate as enhancer

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.

Efficacy of the injectable calcium phosphate ceramics suspensions containing magnesium, zinc and fluoride on the bone mineral deficiency in ovariectomized rats

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 (p < 0.05). 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.

Silicone Elastomer Uptake Method for Determination of Free 1‐Alkyl‐2‐Pyrrolidone Concentration in Micelle and Hydroxypropyl‐β‐Cyclodextrin Systems Used in Skin Transport Studies

Previous investigations in our laboratory demonstrated how the polar head group and alkyl chain of amphiphilic chemical skin permeation enhancers contribute to enhancer potency. In those studies enhancers with n-alkyl chain lengths of eight or less were investigated. In order to investigate enhancers with longer n-alkyl chain lengths, enhancer-solubilizing agents should be considered. Corticosterone (CS) flux enhancement along the lipoidal pathway of hairless mouse skin (HMS) was determined with the enhancers 1-hexyl- (HP), 1-octyl- (OP), 1-decyl- (DP), and 1-dodecyl-2-pyrrolidone (DoP) solubilized in 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine-N-[methoxy(polyethylene glycol-2000] (DSPE) micelles or in hydroxypropyl-beta-cyclodextrin (HPbetaCD). The free CS, HP, OP, DP, and DoP aqueous concentrations in the DSPE micelle and HPbetaCD systems were determined using a partitioning method. Comparisons of the enhancer potencies based on the free concentration of the enhancers revealed a nearly semi-logarithmic linear relationship between enhancer potency and the carbon number of the alkyl chain length with a slope of approximately 0.55. The observed n-alkyl chain length dependency in the aqueous phase is consistent with the hydrophobic effect. This study shows that longer chain enhancers may be studied by employing a solubilizing system, and free enhancer concentration in these systems can be determined with the aid of the silicone elastomer uptake method.

Comparison of the effects of chemical permeation enhancers on the lipoidal pathways of human epidermal membrane and hairless mouse skin and the mechanism of enhancer action

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 beta-estradiol, it was found that the amounts of enhancers taken up and the partitioning of beta-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.

Noninvasive measurement of phenylalanine by iontophoretic extraction in patients with phenylketonuria

Phenylketonuria is an autosomal recessive disorder characterized by elevated concentrations of phenylalanine. Elevated phenylalanine concentrations can impair intellectual functions and the disease is treated with a lifelong diet and frequent monitoring of plasma phenylalanine concentrations. Previous in vitro studies have demonstrated the feasibility of iontophoretically enhanced transdermal transport of phenylalanine. Here we evaluate the feasibility of transdermal iontophoretic extraction of phenylalanine in vivo. Phenylalanine was iontophoretically extracted from the skin of healthy volunteers and of patients with phenylketonuria for up to 6 h and concentrations were compared with those measured in plasma. The amount of phenylalanine iontophoretically extracted from the skin declined over time, suggesting contribution of phenylalanine from the skin in the initial extraction. Phenylalanine iontophoretically extracted from skin correlated with plasma phenylalanine levels at plasma levels above 300 micromol/L. This correlation supports the feasibility of iontophoretic phenylalanine extraction for monitoring phenylketonuria.

Osteotropic Peptide that differentiates functional domains of the skeleton

HPMA copolymer-d-aspartic acid octapeptide (D-Asp8) conjugates have been found to target the entire skeleton after systemic administration. In a recent study using the ovariectomized rat model of osteoporosis, we surprisingly discovered that D-Asp8 would favorably recognize resorption sites in skeletal tissues, while another bone-targeting moiety, alendronate (ALN), directs the delivery system to both formation and resorption sites. Atomic force microscopy (AFM) analyses reveal that ALN has a stronger binding force to hydroxyapatite (HA) than D-Asp8. In vitro HA binding studies indicate that D-Asp8 is more sensitive to change of HA crystallinity than ALN. Because the bone apatite in the newly formed bone (formation sites) usually has lower crystallinity than the resorption sites (mainly mature bone), we believe that the favorable recognition of D-Asp8 to the bone resorption sites could be attributed to its relatively weak binding to apatite, when compared to bisphosphonates, and the different levels of crystallinity of bone apatite at different functional domains of the skeleton.

Alternating current (AC) iontophoretic transport across human epidermal membrane: effects of AC frequency and amplitude

PURPOSE

As a continuing effort to understand the mechanisms of alternating current (AC) transdermal iontophoresis and the iontophoretic transport pathways in the stratum corneum (SC), the objectives of the present study were to determine the interplay of AC frequency, AC voltage, and iontophoretic transport of ionic and neutral permeants across human epidermal membrane (HEM) and use AC as a means to characterize the transport pathways.

MATERIALS AND METHODS

Constant AC voltage iontophoresis experiments were conducted with HEM in 0.10 M tetraethyl ammonium pivalate (TEAP). AC frequencies ranging from 0.0001 to 25 Hz and AC applied voltages of 0.5 and 2.5 V were investigated. Tetraethyl ammonium (TEA) and arabinose (ARA) were the ionic and neutral model permeants, respectively. In data analysis, the logarithm of the permeability coefficients of HEM for the model permeants was plotted against the logarithm of the HEM electrical resistance for each AC condition.

RESULTS

As expected, linear correlations between the logarithms of permeability coefficients and the logarithms of resistances of HEM were observed, and the permeability data were first normalized and then compared at the same HEM electrical resistance using these correlations. Transport enhancement of the ionic permeant was significantly larger than that of the neutral permeant during AC iontophoresis. The fluxes of the ionic permeant during AC iontophoresis of 2.5 V in the frequency range from 5 to 1,000 Hz were relatively constant and were approximately 4 times over those of passive transport. When the AC frequency decreased from 5 to 0.001 Hz at 2.5 V, flux enhancement increased to around 50 times over passive transport.

CONCLUSION

While the AC frequency for achieving the full effect of iontophoretic enhancement at low AC frequency was lower than anticipated, the frequency for approaching passive diffusion transport at high frequency was higher than expected from the HEM morphology. These observations are consistent with a transport model of multiple barriers in series and the previous hypothesis that the iontophoresis pathways across HEM under AC behave like a series of reservoirs interconnected by short pore pathways.

Examination of penetration routes and distribution of ionic permeants during and after transscleral iontophoresis with magnetic resonance imaging

Previously, transscleral and transcorneal iontophoretic delivery was studied and compared to passive delivery and intravitreal injection using nuclear magnetic resonance imaging (MRI). The objective of the present study was to employ MRI to further investigate the factors affecting transscleral iontophoretic delivery. In the present study, anodal and cathodal constant current transscleral iontophoresis were conducted with excised sclera in side-by-side diffusion cells in vitro and with rabbits in vivo. The total current and duration of application were 2 and 4mA (current density 10 and 20mA/cm(2)) and 20-60min, respectively. The delivery and distribution of the model permeants manganese ion (Mn(2+)) and manganese ethylenediaminetetraacetic acid complex (MnEDTA(2-)) into the eye during iontophoresis were determined with MRI and compared with the results obtained in previous studies of subconjunctival injection and passive delivery. Both anodal and cathodal iontophoresis provided significant enhancement in ocular delivery compared to passive transport in the in vitro and in vivo experiments. Transscleral iontophoretic delivery was related to the position and duration of the iontophoresis application in vivo. Permeants were observed to be delivered primarily into the anterior segment of the eye when the pars plana was the application site. Extending the duration of iontophoresis at this site allowed the permeants to be delivered into the vitreous more deeply and to a greater extent than when the application site was at the back of the eye near the fornix. The present results show that electrode placement was an important factor in transscleral iontophoresis, and the ciliary body (pars plana) was determined to be the pathway of least resistance for iontophoretic transport. These new findings continue to support the utility of MRI as a noninvasive technique in ocular drug delivery research and testing.

Effect of nanostructure on biodegradation behaviors of self-setting apatite/collagen composite cements containing vitamin K2 in rats

Apatite cement and collagen were combined by a mechanochemical method to create a new self-setting apatite/collagen composite cement, and menatetrenone (VK2) was loaded into a drug-delivery system to test biocompatibility in rats. Powder X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and electron probe microanalyzer (EPMA) were performed to characterize the physicochemical properties of apatite/collagen composite cements. The XRD results suggested that ground apatite/collagen cement was completely transformed into bone-like hydroxyapatite, but that without grinding was incomplete. The SEM and EPMA results suggested that ground apatite/collagen cement was homogeneously dispersed of nanoapatite crystals in collagen matrices, similar to that in natural bone. In contrast, the cement without grinding was heterogeneously distributed. To evaluate in-vivo cement density (CMM), microradiograms were measured for 72 days after implanting apatite/collagen composite cements in intramuscular tissue on the backs of rats, and cross sections of the cements and surrounding soft tissues were observed by microscope. The CMM results of the apatite/collagen composite cements suggested that the biodegradation rate was dependent on the cement quality and nanogeometrical structure. The CMM result of VK2-loaded apatite/collagen cements suggested that the biodegradation rates of the cements were significantly dependent on their formulation. The CMM of ground apatite/collagen cement increased until 7 days and then decreased, and bone-like cells penetrated deeply in the center. The microphotograph and CMM results of apatite/collagen without grinding indicated that a lot of bone-like cells penetrated into the cement and the cement shape was totally deformed.

Enhanced Transscleral Iontophoretic Transport with Ion-Exchange Membrane

PURPOSE

Transscleral iontophoresis has been recently re-examined for drug delivery to the back of the eye. In conventional iontophoresis, due to the relatively high electromobility of the endogenous competing ions (counterions) relative to that of the drug ion in the tissue barrier, the efficiency of iontophoretic drug delivery is generally low. The objective of the present study was to examine ion-exchange membrane-enhanced transscleral iontophoretic transport in which the ion-exchange membrane in series with the sclera can hinder the transport of the competing counterions and selectively allow the transport of the permeant across the sclera.

METHODS

The physical properties of the Ionac ion-exchange membrane and excised rabbit sclera were determined in equilibrium uptake experiments and in passive and iontophoretic transport experiments with salicylate, tetraethylammonium, urea, and mannitol. Transscleral experiments with the ion-exchange membrane were conducted with salicylate and excised rabbit sclera in vitro. The contribution of electroosmosis to electrotransport during transscleral iontophoresis was assessed with urea and mannitol.

RESULTS

The ion-exchange membrane is highly positively charged and has a small effective pore size. The sclera is relatively porous with a large effective pore size and low pore tortuosity. The sclera is also net negatively charged but this does not significantly affect the transport of small ions. A three-fold steady-state transscleral flux enhancement of salicylate was observed in ion-exchange membrane-enhanced iontophoresis over conventional transscleral iontophoresis without the membrane. Such enhancement was relatively independent of the applied electric current density and the thickness of the studied ion-exchange membrane assembly. Although the ion-exchange membrane altered transscleral electroosmosis, the contribution of electroosmosis to electrotransport was not significant.

CONCLUSIONS

The present study has demonstrated the potential of ion-exchange membranes for enhancing iontophoretic transport and drug delivery.

Evaluation of constant current alternating current iontophoresis for transdermal drug delivery

Previous studies in our laboratory have demonstrated that alternating current (AC) iontophoresis can significantly decrease skin electric resistance and enhance the transport of charged permeants across skin. Flux variability of neutral permeants during AC iontophoresis was also found to be less than that of conventional direct current (DC) iontophoresis. The objectives of the present study were to evaluate flux enhancement of constant current AC transdermal iontophoresis and compare the AC flux with that of constant current DC iontophoresis. Iontophoresis studies of AC amplitude of 1, 2, and 5 mA were conducted in side-by-side diffusion cells with donor solution of 0.015, 0.15, and 1.0 M tetraethylammonium (TEA) chloride and receiver solution of phosphate buffered saline (PBS) using human epidermal membrane (HEM). Conventional constant current DC iontophoresis of 0.2 mA was also performed under similar conditions. TEA and mannitol were the model permeants. The following are the major findings in the present study. The flux of TEA increased proportionally with the AC current for all three TEA chloride concentrations and at the AC frequency used in the present study. When the permeant and its counter ion were the only ionic species in the donor chamber, the fluxes during DC iontophoresis were weakly dependent of its donor concentration. The fluxes of TEA during constant current AC iontophoresis were moderately related to the donor concentration with the highest TEA flux observed under the 1.0 M TEA chloride condition although the relationship between flux and donor concentration was not linear. A trend of decreasing electroosmotic transport with increasing donor TEA chloride concentration was observed with significant sample-to-sample variability during DC iontophoresis. Mannitol permeability was also observed to decrease with increasing TEA chloride concentration in the donor under the AC conditions, but data variability under AC was significantly smaller than that under DC. The results in the present study indicate that constant current AC iontophoresis under conditions tolerable to human (2 and 5 mA) can provide predictable fluxes that were lower than but of comparable magnitude as those of conventional constant current DC iontophoresis (0.2 mA).

A comparative study of the metastable equilibrium solubility behavior of high-crystallinity and low-crystallinity carbonated apatites using pH and solution strontium as independent variables

Using solution strontium and pH as independent variables, the metastable equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 degrees C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 degrees C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca(10-n)Sr(n)(PO4)6(OH)2 (n = 0-10)) and the calcium/hydroxide deficient hydroxyapatite template (Ca(9-n)Sr(n)(HPO4)(PO4)5OH (n = 0-9)).

FINDINGS

(a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.

Model analysis of flux enhancement across hairless mouse skin induced by chemical permeation enhancers

Previous permeant partitioning studies with hairless mouse skin (HMS) in the presence of several chemical skin permeation enhancers have revealed that, when such enhancers induce significant skin permeability coefficient enhancement, it is accompanied by significant enhancement in the equilibrium uptake (partitioning) of the permeant into the intercellular lipid component of the stratum corneum (SC). Particularly, it was found that the 1-alkyl-2-pyrrolidones and the 1-alkyl-2-azacycloheptanones, at aqueous solution concentrations that gave skin permeation enhancement (E) of 10 for corticosterone (CS, the permeant), enhanced the equilibrium uptake of beta-estradiol (E2beta, a surrogate permeant) from the aqueous phase into the intercellular lipids of HMS SC by a factor of 5-7. This finding raised the question of whether this uptake enhancement induced by the permeation enhancer under equilibrium conditions would be essentially the same as that determined kinetically from time-dependent permeation experiments utilizing appropriate SC membrane models and Fick's laws of diffusion to treat the data. HMS transport experiments were conducted with CS as the permeant and 1-octyl-2-pyrrolidone (OP) and 1-hexyl-2-azacyloheptanone (HAZ) as the enhancers. In treating the experimental data, a one-layer skin transport model (SC only) and a two-layer model (SC layer and the epidermis/dermis layer) were both investigated. Both the partition coefficient enhancement (E(K)) and the diffusion coefficient enhancement (E(D)) were deduced from the data treatment. The results showed that when the total transport enhancement of CS was around 11, E(K) was in the range of 6-8 and E(D) was in the range of 1.5-1.9 using both the one-layer and the two-layer models. This E(K) value was found to be in good agreement with the E2beta partition enhancement obtained directly under equilibrium conditions in previous studies. This indicates that (a) the rate-limiting domain for the transport of the lipophilic permeants across HMS and the HMS SC intercellular lipid domain probed in the equilibrium partitioning experiments are essentially the same, and (b) the total flux enhancement (E) of lipophilic permeants across HMS was driven mainly by enhancing the partitioning of the permeant into the rate-limiting domain (E(K)) and secondarily by enhancing the diffusion coefficients (E(D)) of the permeant in the domain. Comparison of the one-layer and two-layer skin model results revealed that non-steady-state transport of lipophilic compounds across HMS was better described by the two-layer model because the dermis/viable epidermis played a significant role in lipophilic permeant binding.

Influence of Asymmetric Donor–Receiver ion Concentration Upon Transscleral Iontophoretic Transport

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/cm2). 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.

Effects of electrophoresis and electroosmosis during alternating current iontophoresis across human epidermal 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.

Mechanistic study of chemical skin permeation enhancers with different polar and lipophilic functional groups

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.

Quantitative study of electrophoretic and electroosmotic enhancement during alternating current iontophoresis across synthetic membranes

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.

Correlation of transdermal iontophoretic phenylalanine and mannitol transport: test of the internal standard concept under DC iontophoresis and constant resistance AC iontophoresis conditions

The purpose of this study was to test the feasibility of using an internal standard approach to predict the transdermal flux of phenylalanine (Phe) across human epidermal membrane (HEM) in vitro during iontophoresis of constant direct current (DC), constant skin resistance alternating current (AC) with DC offset (AC-plus-DC), symmetric constant resistance AC (AC-only), and constant current DC with the application of a polyelectrolyte. Under the constant current DC and AC-plus-DC conditions, a linear relationship between the permeation of Phe and that of mannitol was observed with a slope close to unity. The ratios of the Phe permeability coefficients to those of mannitol during iontophoresis with different HEM samples were essentially constant with significantly smaller inter-sample and intra-sample variabilities than those of the raw Phe permeability coefficient data. This demonstrates that mannitol is a good internal standard for predicting the transdermal iontophoretic flux of Phe under these two conditions. On the other hand, the correlations of the permeability coefficients of Phe and mannitol under the AC-only iontophoresis and iontophoresis with the polyelectrolyte are poorer than those observed during DC and AC-plus-DC iontophoresis. The poor correlations are believed to be related to Phe being a zwitterion and its anionic form in HEM. Previously, iontophoretic transport of glucose has been demonstrated to be essentially the same as that of mannitol. The good correlations between Phe and mannitol transport across HEM during DC and AC-plus-DC iontophoresis in the present study therefore suggest that glucose and its extraction can be used as the internal standard for iontophoretic Phe monitoring.

Mechanistic studies of flux variability of neutral and ionic permeants during constant current dc iontophoresis with human epidermal membrane

Although constant current iontophoresis is supposed to provide constant transdermal transport, significant flux variability and/or time-dependent flux drifts are observed during iontophoresis with human skin in vitro and human studies in vivo. The objectives of the present study were to determine (a) the causes of flux variability in constant current dc transdermal iontophoresis and (b) the relationships of flux variabilities among permeants of different physicochemical properties. Changes in the human epidermal membrane (HEM) effective pore size and/or electroosmosis during constant current dc iontophoresis were examined. Tetraethylammonium ion (TEA), urea, and mannitol were the model permeants. For the neutral permeants, the results in the present study showed a significant increase of fluxes with time in a given experiment and large HEM sample-to-sample variability. Although both effective pore size and pore charge density variations contributed to the time-dependent flux drifts observed in electroosmotic transport, the significant flux drifts observed were found to be primarily a result of the time-dependent increase in effective pore charge density. For the ionic permeant, the observed flux variability was smaller than that of the neutral permeants and was believed to be primarily due to effective pore size alteration in HEM during iontophoresis as suggested in a previous study. The different extents of flux variability observed between neutral and ionic permeants are consistent with the different iontophoretically enhanced transport mechanisms for the neutral and ionic permeants (i.e. electroosmosis and electrophoresis, respectively). The results of the present study also demonstrate that flux variability of two neutral permeants are inter-related, so the flux of one neutral permeant can be predicted if the permeability coefficient of the other neutral permeant is known.

Mechanistic Studies of Branched-Chain Alkanols as Skin Permeation Enhancers

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 (E2beta), 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 E2beta 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 E2beta 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.

Metastable equilibrium solubility behavior of carbonated apatite in the presence of solution strontium

The purpose of this study was to use the concept of metastable equilibrium solubility (MES) to describe the anomalous solubility behavior of carbonated apatite (CAP) in the presence of solution strontium. A CAP sample (4.8 wt% CO(3), synthesized at 70 degrees C) was prepared by precipitation. Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. In order to assess the influence of strontium, MES profiles were then determined in a similar fashion with 20, 30, 40, 50, 60, 70, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. From the compositions of the equilibrating buffer solutions, ion activity products (IAPs) of the form Ca(10-n)Sr(n)(PO(4))(6)(OH)(2) (n = 0-10) were calculated in an attempt to determine the correct function governing the dissolution of the CAP preparation. The results demonstrate the following important findings: (a) at high solution strontium/calcium ratios (i.e., when 60% or more of the solution calcium was replaced by strontium), the MES profiles in all the experiments were found to be essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca(6)Sr(4)(PO(4))(6)(OH)(2), and (b), at low solution strontium/calcium ratios (i.e., when 40% or less of the solution calcium was replaced by strontium), the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.

Effect of controlled zinc release on bone mineral density from injectable Zn-containing ?-tricalcium phosphate suspension in zinc-deficient diseased rats

The purpose of this study was to evaluate the efficacy of zinc (Zn)-containing beta-tricalcium phosphate (Zn-TCP) in correcting the bone mineral deficiency noted in osteoporosis using ovariectomized rat model. Four rats were used for each of the four experimental groups: D0, D10, D20, and N10. The rats in D0, D10, and D20 groups were ovariectomized, and fed a vitamin D-, Ca-, and Zn-deficient diet, and induced Zn-deficient osteoporoses for 9 weeks. In contrast, the N10 group was the normal rats fed normal healthy diet for 9 weeks. D0 group was injected with pure beta-TCP suspension, D10 and D20 groups were injected with suspensions containing 10 mg of 10 mol % (6.17 wt % Zn) and 20 mol % (12.05 wt % Zn) Zn-TCP, respectively, and the healthy group, N10 were injected with 10 mol %. Zn-TCP suspensions. Injections were administered intramuscularly in the left thigh once a week in all rats, and fed a vitamin D- and Zn-deficient diet for 9 weeks. The plasma calcium (Ca) and Zn levels, plasma alkaline phosphatase activity (ALP) and bone mineral density (BMD) of the lumbar vertebra and femora were measured. The plasma Zn levels in all the rats were between 1.1 and 2.8 microg/mL. The areas under the curves for the Ca, Zn, and ALP (Ca-AUC, Zn-AUC, and ALP-AUC) levels between 0 and 63 days were calculated. Results for the AUCs were as follows: (1) the Zn-AUCs were in the order of N10 = D20 > D10 > D0; (2) the Ca-AUCs for D0, D10 groups were significantly lower than that for the N10 group; (3) the ALP-AUCs for the D10 and D20 groups were significantly higher than that for the N10 group, and that of the D0 group was in between those. The body weight of D10 and D20 groups significantly increased with time, that of the D0 group increased slightly, and that of the N10 group remained unchanged for the entire experimental period. The BMD of the lumbar vertebrae of the D10 and D20 groups (about 100 mg/cm(2)) was significantly higher than that of the D0 group but lower than that of the N10 group. The BMD of the left femur increased more than that of the right femur with the increase in the amount of Zn in the suspension. The results of this study suggest that the local effect on BMD was more pronounced than the effect on the whole body.