A procedure to optimize scale-up for the primary drying phase of lyophilization.

This article describes a procedure to facilitate scale-up for the primary drying phase of lyophilization using a combination of empirical testing and numerical modeling. Freeze dry microscopy is used to determine the temperature at which lyophile collapse occurs. A laboratory scale freeze-dryer equipped with manometric temperature measurement is utilized to characterize the formulation-dependent mass transfer resistance of the lyophile and develop an optimized laboratory scale primary drying phase of the freeze-drying cycle. Characterization of heat transfer at both lab and pilot scales has been ascertained from data collected during a lyophilization cycle involving surrogate material. Using the empirically derived mass transfer resistance and heat transfer data, a semi-empirical computational heat and mass transfer model originally developed by Mascarenhas et al. (Mascarenhas et al., 1997, Comput Methods Appl Mech Eng 148: 105-124) is demonstrated to provide predictive primary drying data at both the laboratory and pilot scale. Excellent agreement in both the sublimation interface temperature profiles and the time for completion of primary drying is obtained between the experimental cycles and the numerical model at both the laboratory and pilot scales. Further, the computational model predicts the optimum operational settings of the pilot scale lyophilizer, thus the procedure discussed here offers the potential to both reduce the time necessary to develop commercial freeze-drying cycles by eliminating experimentation and to minimize consumption of valuable pharmacologically active materials during process development.

The nonsteady state modeling of freeze drying: in-process product temperature and moisture content mapping and pharmaceutical product quality applications.

INTRODUCTION: Theoretical models of the freeze-drying process are potentially useful to guide the design of a freeze-drying process as well as to obtain information not readily accessible by direct experimentation, such as moisture distribution and glass transition temperature, Tg, within a vial during processing. Previous models were either restricted to the steady state and/or to one-dimensional problems. While such models are useful, the restrictions seriously limit applications of the theory. An earlier work from these laboratories presented a nonsteady state, two-dimensional model (which becomes a three-dimensional model with an axis of symmetry) of sublimation and desorption that is quite versatile and allows the user to investigate a wide variety of heat and mass transfer problems in both primary and secondary drying. The earlier treatment focused on the mathematical details of the finite element formulation of the problem and on validation of the calculations. The objective of the current study is to provide the physical rational for the choice of boundary conditions, to validate the model by comparison of calculated results with experimental data, and to discuss several representative pharmaceutical applications. To validate the model and evaluate its utility in studying distribution of moisture and glass transition temperature in a representative product, calculations for a sucrose-based formulation were performed, and selected results were compared with experimental data. THEORETICAL MODEL: The model is based on a set of coupled differential equations resulting from constraints imposed by conservation of energy and mass, where numerical results are obtained using finite element analysis. Use of the model proceeds via a "modular software package" supported by Technalysis Inc. (Passage/ Freeze Drying). This package allows the user to define the problem by inputing shelf temperature, chamber pressure, container properties, product properties, and numerical analysis parameters required for the finite element analysis. Most input data are either available in the literature or may be easily estimated. Product resistance to water vapor flow, mass transfer coefficients describing secondary drying, and container heat transfer coefficients must normally be measured. Each element (i.e., each small subsystem of the product) may be assigned different values of product resistance to accurately describe the nonlinear resistance behavior often shown by real products. During primary drying, the chamber pressure and shelf temperature may be varied in steps. During secondary drying, the change in gas composition from pure water to mostly inert gas is calculated by the model from the instantaneous water vapor flux and the input pumping capacity of the freeze dryer. RESULTS: Comparison of the theoretical results with the experiment data for a 3% sucrose formulation is generally satisfactory. Primary drying times agree within two hours, and the product temperature vs. time curves in primary drying agree within about +/-1 degrees C. The residual moisture vs. time curve is predicted by the theory within the likely experimental error, and the lack of large variation in moisture within the vial (i.e., top vs. side vs. bottom) is also correctly predicted by theory. The theoretical calculations also provide the time variation of "Tg-T" during both primary and secondary drying, where T is product temperature and Tg is the glass transition temperature of the product phase. The calculations demonstrate that with a secondary drying protocol using a rapid ramp of shelf temperature, the product temperature does rise above Tg during early secondary drying, perhaps being a factor in the phenomenon known as "cake shrinkage." CONCLUSION: The theoretical results of in-process product temperature, primary drying time, and moisture content mapping and history are consistent with the experimental results, suggesting the theoretical model should be useful in process development and "trouble-shooting" applications.

Effect of initial buffer composition on pH changes during far-from-equilibrium freezing of sodium phosphate buffer solutions.

PURPOSE: This study aims to assess the pH changes induced by salt precipitation during far-from-equilibrium freezing of sodium phosphate buffers as a function of buffer composition, under experimental conditions relevant to pharmaceutical applications-sample volumes larger than a few microliters, experiencing large degrees of undercooling and supersaturation. METHODS: Buffer solutions were prepared by dissolving the monosodium and disodium phosphate salts in the appropriate ratios to obtain initial buffer concentrations in the range of 8-100 mM and pH values between 5.7 and 7.4 at 25 degrees C. Temperature and pH were monitored in situ during cooling to -10 degrees C (at a rate of 0.3 to 0.5 degrees C/min) and for 10-20 min after the sample reached the final temperature. Salt crystallization was confirmed by ion analysis and x-ray powder diffraction. RESULTS: Precipitation of Na2HPO4, 12H2O caused abrupt pH decreases after the onset of ice crystallization, at temperatures between -0.5 and -4.0 degrees C. Decreasing the initial buffer concentration and/or initial pH resulted in higher final pH values at -10 degrees C, farther removed from the equilibrium value of 3.6. At an initial pH of 7.4, the 50 and 100 mM buffer solutions reached a pH of 4.2 +/- 0.1 at -10 degrees C, whereas the 8 mM solutions reached a pH of 5.2 +/- 0.2. Solutions having an initial pH of 5.7 and initial buffer concentrations of 8 and 100 mM experienced less pH shifts upon freezing to -10 degrees C, with final pH values of 5.1 +/- 0.1 and 4.7 +/- 0.1, respectively. CONCLUSIONS: Precipitation-induced pH shifts are dependent on the concentrations (activities) of precipitating ions, and are determined by both initial pH and salt concentration. The ion activity product is a meaningful parameter when describing salt precipitation in solutions prepared by mixing salts containing precipitating and nonprecipitating ions.

The role of electroosmotic flow in transdermal iontophoresis.

Iontophoresis enhances transdermal drug delivery by three mechanisms: (a) the ion-electric field interaction provides an additional force which drives ions through the skin; (b) flow of electric current increases permeability of skin; and (c) electroosmosis produces bulk motion of the solvent itself that carries ions or neutral species, with the solvent 'stream'. The relative importance of electroosmotic flow is the subject of this review. Experimental observations and theoretical concepts are reviewed to clarify the nature of electroosmotic flow and to define the conditions under which electroosmotic flow is an important effect in transdermal iontophoresis. Electroosmotic flow is bulk fluid flow which occurs when a voltage difference is imposed across a charged membrane. Electroosmotic flow occurs in a wide variety of membranes, is always in the same direction as flow of counterions and may either assist or hinder drug transport. Since both human skin and hairless mouse skin are negatively charged above about pH 4, counterions are positive ions and electroosmotic flow occurs from anode to cathode. Thus, anodic delivery is assisted by electroosmosis, but cathodic delivery is retarded. Water carried by ions as 'hydration water' does not contribute significantly to electroosmotic flow. Rather electroosmotic flow is caused by an electrical volume force acting on the mobile counterions. The simple 'limiting law' theory commonly given in textbooks and some research articles is a very poor approximation for transdermal systems. However, several extensions of the limiting law are compatible with each other and with the available experimental data. One of these theories, the Manning theory, has been incorporated into a theory for the effect of electroosmotic flow on iontophoresis, the latter theory being in good agreement with experiment. Both theory and experimental data indicate that electroosmotic flow increases in importance as the size of the drug ion increases. The 'ionic' or Nernst-Planck effect is the largest contributor to flux enhancement for small ions. Increased skin permeability or the skin 'damage effect', is a significant factor for both large and small ions, particularly for experiments at high current density. For monovalent ions with Stokes radii larger than about 1 nm, electroosmotic flow is the dominant flow mechanism. Because of electroosmotic flow, transdermal delivery of a large anion (or negatively charged protein) from the anode compartment can be more effective than delivery from the cathode compartment.

Interpretation of relaxation time constants for amorphous pharmaceutical systems.

The molecular mobility of amorphous pharmaceutical materials is known to be a key factor in determining their stability, reactivity, and physicochemical properties. Usually such molecular mobility is quantified using relaxation time constants. Typically relaxation processes in amorphous systems are non-exponential and relaxation time constants are usually obtained from experimental data using a curve fitting procedure involving the empirical Kohlrausch-Williams-Watts (KWW) equation. In this article we explore the possible relationship between the KWW curve fitting parameters (tau(KWW), beta(KWW)) and common statistical measures of the average and the distribution (e.g., median, standard deviation) of the relaxation time values. This analysis is performed for several common statistical distributions (e.g., normal, lognormal, and Lorentzian), and the results are compared and analyzed in the context of pharmaceutical product stability predictions. The KWW function is able to describe relaxation processes stemming from several different statistical distribution functions. Under some circumstances the "average" relaxation time constant of the KWW equation (tau(KWW)) is significantly different from common statistical measures of the central value of a distribution (e.g., median). Simply knowing the relaxation time constants from the fit of the KWW equation is not sufficient to completely characterize and quantify the molecular mobility of amorphous pharmaceutical materials. An appreciation of the distribution of relaxation times and the resulting effects upon the KWW constants should be considered to be essential when working with amorphous pharmaceutical materials, especially when attempting to use relaxation time constants for predicting their physical or chemical stability.

A pragmatic test of a simple calorimetric method for determining the fragility of some amorphous pharmaceutical materials.

PURPOSE: To evaluate a simple calorimetric method for estimating the fragility of amorphous pharmaceutical materials from the width of the glass transition region. METHODS: The glass transition temperature regions of eleven amorphous pharmaceutical materials were characterized at six different heating and cooling rates by differential scanning calorimetry (DSC). RESULTS: Activation energies for structural relaxation (which are directly related to glass fragility) were estimated from the scan rate dependence of the glass transition temperature, and correlations between the glass transition widths and the activation energies were examined. The expected correlations were observed, and the exact nature of the relationship varied according to the type of material under consideration. CONCLUSIONS: The proposed method of determining the fragility of amorphous materials from the results of simple DSC experiments has some utility, although "calibration" of the method for each type of materials is necessary. Further work is required to establish the nature of the relationships for a broad range of amorphous pharmaceutical materials.

A method for the rapid estimation of sublimation rates of organic compounds at standard temperature and pressure.

A method for the rapid estimation of the sublimation rates of organic compounds at standard temperature and pressure (STP; 298 K and 101.3 kPa, respectively) is presented. Thermogravimetric analysis was used to obtain accelerated sublimation rates for anthracene at a series of elevated temperatures and reduced pressures. An empirical equation relating these parameters was used to extrapolate to the STP sublimation rate. This value is in good agreement with the measured value.

The stability of insulin in crystalline and amorphous solids: observation of greater stability for the amorphous form.

PURPOSE: Generalizations based upon behavior of small molecules have established that a crystalline solid is generally much more stable toward chemical degradation than is the amorphous solid. This study examines the validity of this generalization for proteins using biosynthetic human insulin as the model protein. METHODS: Amorphous insulin was prepared by freeze drying the supernate from a suspension of zinc insulin crystals adjusted to pH 7.1. Storage stability at 25 degrees C and 40 degrees C were compared for the freeze dried material, the dried suspended crystals, and the starting batch of crystals. Samples were equilibrated at selected relative humidities between zero and 75% to obtain samples at various water contents. Assays for dimer formation were performed by size exclusion HPLC and assays for deamidated product were carried out by reverse phase HPLC. Degradation was found to be linear in square root of time, and the slopes from % degradation vs. square root of time were used to define the rate constants for degradation. Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) were used to characterize the state of the protein in the solids. RESULTS: As expected based upon previous results, the primary degradation pathways involve deamidation at the AsnA21 site and co-valent dimer formation, presumably involving the A-21 site. Contrary to expectations, amorphous insulin is far more stable than crystalline insulin under all conditions investigated. While increasing water content increases the rate of degradation of crystalline insulin, rate constants for degradation in the amorphous solid are essentially independent of water content up to the maximum water content studied (approximately 15%). CONCLUSIONS: Based upon the FTIR and DSC data, both crystalline and amorphous insulin retain some higher order structure when dried, but the secondary structure is significantly perturbed from that characteristic of the native solution state. However, neither DSC nor FTIR data provide a clear interpretation of the difference in stability between the amorphous and crystalline solids. The mechanism responsible for the superior stability of amorphous insulin remains obscure.

Evaluation of manometric temperature measurement as a method of monitoring product temperature during lyophilization.

The objective of this study was to evaluate manometric temperature measurement as a non-invasive method of monitoring product temperature during the primary drying phase of lyophilization. This method is based on analysis of the transient response of the chamber pressure when the flow of water vapor from the chamber to the condenser is momentarily interrupted. Manometric temperature measurements (MTM) were compared to product temperature data measured by thermocouples during the lyophilization of water, mannitol, lactose and potassium chloride solutions. The transient pressure response was mathematically modeled by assuming that four mechanisms contribute to the pressure rise: 1) direct sublimation of ice through the dried product layer at a constant temperature, 2) an increase in the temperature at the sublimation interface due to equilibration of the temperature gradient across the frozen layer, 3) an increase in the ice temperature due to continued heating of the frozen matrix during the measurement, and 4) leaks in the chamber. Experimental transient pressure response data were fitted to an equation consisting of the sum of these terms containing three variables corresponding to the vapor pressure of ice, product resistance to vapor flow, and the vial heat transfer coefficient. Excellent fit between the mathematical model and the experimental data was observed, and the value of the variables was calculated from the measured transient pressure response by a least squares method. The product temperature measured by MTM, which measures the temperature at the sublimation interface, was compared with product temperature measured by thermocouples placed in the bottom center of the vials. Manometrically measured temperatures were consistently lower than the thermocouple measurements by about 2 degrees C, this difference being largely accounted for by the temperature gradient across the frozen layer. The resistance of the dried product to mass transfer calculated from MTM was found to agree reasonably well with values measured by a direct vial technique. Product resistance was observed to increase with increasing solute concentration, and to increase continuously as the depth of the dried product layer increases for mannitol and potassium chloride. For lactose, product resistance increases continuously with thickness up to the onset of collapse, at which point the product resistance becomes essentially independent of depth. Scanning electron microscopy was used to explain this observation based on changes in morphology of the solid. The vial heat transfer coefficients obtained from regression analysis were on the order of 10(-3)-10(-4) cal.sec-1. degrees C-1; however, the scatter in the vial heat transfer coefficient data prevents the method from being used for accurate measurement of the vial heat transfer coefficient. The results of the study show that the manometric method shows promise as a process development tool and as an alternative method of in-process product temperature measurement during primary drying.

Intravial distribution of moisture during the secondary drying stage of freeze drying.

Particularly for proteins that may be damaged by overdrying, the distribution of residual moisture in the dried product may be as important as the mean water content. Protein at the top of the cake could be "overdried" even though the mean water content was optimal. Theoretical studies suggest that, particularly for large fill depths, the residual moisture near the top of the cake is much lower than the moisture near the bottom, at least during and immediately after freeze drying. This work is an experimental study of the distribution of moisture within a vial as a function of time in secondary drying (mean residual water content of about 3%-6%) for dextran, human serum albumin, and bovine somatotropin. A core sample of the dried product was taken and sectioned into "top," "middle," and "bottom" sections. These three "core sections" and the remaining "outer" section (i.e., near the vial wall) were assayed for moisture. In general, the moisture content in the top section is less than the moisture content in the bottom section, but the "outer" section is consistently found to be lowest in moisture content. This observation suggests that drying was faster along the walls of the vial than in the core region, resulting in a highly curved ice-vapor interface. It is proposed that faster drying along the vial walls is a result of the observed product shrinkage during drying which provides a low resistance pathway for vapor escape along the vial wall. Product temperature measurements support the above speculation.

Moisture transfer from stopper to product and resulting stability implications.

Since the stability of a freeze-dried product is often sensitive to the level of moisture, control of residual moisture by attention to the secondary drying phase of the freeze-drying process is of considerable importance. However, several reports in the literature as well as our own experience suggest that low residual moisture immediately after manufacture does not ensure low moisture throughout the shelf life of the product. Equilibration of the product with moisture in the stopper can lead to significant increases in product water content. This research is a study of the kinetic and equilibrium aspects of moisture transfer from stopper to product at 5 degrees C, 25 degrees C, and 40 degrees C for two amorphous materials: vancomycin (highly hygroscopic) and lactose (moderately hygroscopic). Stoppers are 13 mm butyl rubber (#1816, West Co.) slotted freeze-drying stoppers which were studied: (a) "U"-with no treatment; (b) "SV1"-steam-sterilized followed by 1 hr vacuum drying; and (c) "SV8"-steam sterilized followed by 8 hrs vacuum drying. No evidence was found for moisture transmission through the stopper. Rather, the product moisture content increases with time and reaches an apparent equilibrium value characteristic of the product, amount of product, and stopper treatment method ("SV1" much greater than "U" greater than "SV1"). As a first approximation, the rate of approach to "equilibrium" depends only on temperature (t1/2 approximately 10 months at 5 degrees C to approximately 4 days at 40 degrees C) with the "equilibrium" water content being independent of temperature. The "equilibrium" moisture content increases as the dose decreases and is larger for vancomycin than for lactose. The "equilibrium" moisture contents range from 5.0% (25 mg vancomycin, "SV1" stoppers) to 0.68% (100 mg lactose, "SV8" stoppers).

Formulation and stability of freeze-dried proteins: effects of moisture and oxygen on the stability of freeze-dried formulations of human growth hormone.

This research presents the results of a series of stability studies on freeze-dried formulations of human growth hormone (hGH). Chemical decomposition via methionine oxidation and asparagine deamidation as well as irreversible aggregation are characterized by HPLC. Water sorption isotherms, DSC thermograms, and pulsed proton NMR data are also obtained. No glass transition temperatures are observed in the temperature range of the stability studies. The pulsed NMR data suggest onset of greater mobility in the solid at a water content slightly higher than BET "monolayer" level. Stability of freeze-dried solids at 25 degrees C and 40 degrees C is studied as a function of residual moisture and exposure to oxygen. Formulations with and without a glycine/mannitol excipient system are studied. Significant levels of chemical decomposition and irreversible aggregation occur under most conditions with the effects of residual water content and "headspace oxygen" strongly dependent on the formulation. At low water content with minimal oxygen in the vial headspace, the glycine/mannitol formulation yields optimum stability. However, for either high water content or high oxygen content in the vial, stability of hGH without excipients is superior. The qualitative effect of residual moisture on stability depends on the temperature of the stability study. Generally, the stability of a sample adjusted to a given water content by desorption (during freeze-drying) is identical to the stability of a sample prepared by sorption of water on to a previously highly dried sample.

The effects of formulation and moisture on the stability of a freeze-dried monoclonal antibody-vinca conjugate: a test of the WLF glass transition theory.

Deacetylvinblastine (DAVLB) hydrazide, a cytotoxic vinca alkaloid, has been linked to the monoclonal antibody, KS1/4, via aldehyde residues of the oxidized carbohydrate groups on the antibody. The resulting KS1/4-DAVLB hydrazide conjugate is unstable in solution with both the acyl hydrazone linkage and the vinca moiety being subject to significant degradation, even at 5 degrees C. This necessitated the development of a freeze-dried formulation of the antibody-drug conjugate. Formulation factors considered were pH, ionic strength, buffer, excipient types, and excipient ratios. A formulation with equal weight ratios of mannitol, glycine, and conjugate in a low ionic strength phosphate buffer at near neutral pH was selected. Stability was studied at various moisture levels (1.4%, 3.0%, and 4.7%) and temperatures (5 degrees C, 25 degrees C, and 40 degrees C). Degradation was measured by size exclusion HPLC (aggregate formation) and by reverse phase HPLC (hydrolysis of hydrazone linkage and vinca decomposition). Differential scanning calorimetry (DSC) indicated that all samples were above their glass transition temperatures, Tg, when stored at 40 degrees C. When stored at 25 degrees C, only the highest moisture sample was initially above its Tg. However, due to crystallization of the excipients during storage and the resulting decrease in Tg, samples stored at 25 degrees C were also above their Tg during much of the storage period. The degradation rate, R, increased sharply with increasing temperature and with increasing moisture level. Degradation kinetics obeyed the Williams-Landel-Ferry relationship, R/Rg = exp[k(T-Tg)], where Rg is the degradation rate at Tg. For all three moisture levels and all three degradation pathways, k = 0.143.

The effects of formulation variables on the stability of freeze-dried human growth hormone.

Formulation often has a dramatic effect on degradation of proteins during the freeze-drying process as well as impacting on the "shelf-life" stability of the freeze-dried product. This research presents the results of a formulation optimization study of the "in-process" and shelf-life stability of freeze-dried human growth hormone (hGH). Chemical decomposition via methionine oxidation and deamidation of asparagine residues as well as irreversible aggregation were characterized by HPLC assay methodology. In-process degradation and stability of low moisture freeze-dried solids were studied at 25 and 40 degrees C in a nominal nitrogen headspace (approximately 0.5% O2). Formulation variables included pH, level of salts, and the nature of the lyoprotectant. Studies of the effect of shear on aggregation in solutions indicated that shear comparable to that experienced during filtration does not induce aggregation. Irreversible changes in hGH during the freeze-drying process were minimal, but chemical decomposition via methionine oxidation and asparagine deamidation and aggregation did occur on storage of the freeze-dried solid. Decomposition via methionine oxidation was significant. A combination of mannitol and glycine, where the glycine remains amorphous, provided the greatest protection against decomposition and aggregation. It is postulated that an excipient system that remains at least partially amorphous is necessary for stabilization. However, the observation that dextran 40 formulations showed poor stability toward aggregation demonstrates that an amorphous excipient system is not a sufficient condition for stability. Stability of the solid was optimal when produced from solutions in the pH range, 7-7.5, with severe aggregation being observed at high pH. The level of sodium phosphate buffer affected stability of the solid, although this relationship was complex. Freeze-drying in the presence of NaCl produced severe aggregation and precipitation during the freeze-drying process as well as acceleration of oxidation and/or deamidation.

Study of the mechanisms of flux enhancement through hairless mouse skin by pulsed DC iontophoresis.

Enhanced iontophoretic transport using pulsed DC is usually explained by citing the observed decrease in skin resistance caused by an increase in AC pulse frequency at very small currents. Alternately, it has been suggested that the "on-to-off" nature of pulsed DC imparts an "impact energy" to the fluid, thereby increasing transport. This report provides a test of these mechanisms for enhanced delivery via pulsed iontophoresis. The DC resistance of hairless mouse skin during continuous and pulsed DC iontophoresis is measured as a function of time for selected pulse frequencies and duty cycles using current densities ranging from 0.1 to 1.0 mA/cm2. As a test of the impact energy mechanism, the iontophoretic transport of 14C-glucose measured with pulsed DC is compared with similar data obtained previously using continuous DC. It is suggested that pulsed current can yield lower resistance and enhanced drug delivery provided that (a) the "steady-state" current during the "on" phase of the pulse is very small and (b) the frequency is low enough to allow depolarization of the skin during the "off" phase of the pulse. The glucose transport results suggest that the "impact energy" concept does not apply to iontophoresis.

Transport mechanisms in iontophoresis. II. Electroosmotic flow and transference number measurements for hairless mouse skin.

Previous studies suggest that bulk fluid flow by electroosmosis is a significant factor in iontophoresis and may provide an explanation for the observed enhanced transport of neutral species. In a charged membrane, the solution carries a net charge and thus experiences a volume force in an electric field, which causes volume flow (Jv) in the direction of counterion flow. Jv data were obtained for hairless mouse skin (HMS) as a function of pH, concentration of NaCl, current density, and time. Volume flow was measured by timing fluid movement in horizontal capillary tubes attached to the anode and cathode (Ag/AgCl) compartments. By convention, the sign of Jv is taken as positive when the volume flow is in the same direction as positive current flow. Experimental mean values were in the range 0 to +37 microliters/cm2 hr, depending on the experimental conditions. Volume flow of this magnitude is large enough to have significant impact on flow of both ions and neutral species. The positive sign for Jv indicates that HMS is negative in the pH range studied (3.8-8.3). Jv decrease with time, decrease with increasing NaCl concentration, are much lower at pH 3.8 than at the higher pH's, and increase with current density. Effective transference numbers, determined from membrane potential measurements, showed significant pH dependence, consistent with a small negative charge on the membrane at mid pH's and charge reversal around pH 4. Both electrical resistance and Jv data indicate changes in transport properties occur when HMS is subjected to an electric field.

Transport mechanisms in iontophoresis. III. An experimental study of the contributions of electroosmotic flow and permeability change in transport of low and high molecular weight solutes.

The objective of this research was to provide in vitro transport data designed to clarify the relative importance of permeability increase and electroosmotic flow in flux enhancement via iontophoresis. Iontophoretic fluxes were measured with both anode and cathode donor cells, and passive fluxes were measured both before iontophoresis (Passive 1) and after iontophoresis (Passive 2). Data were generated for three uncharged low molecular weight solutes (glycine, glucose, and tyrosine) and two high molecular weight anionic species (carboxy inulin and bovine serum albumin). Flux enhancement is greater for anodic delivery than for cathodic delivery, even for the negatively charged molecules, and anodic flux of glucose decreases as the concentration of NaCl increases. Both observations are consistent with a mass transfer mechanism strongly dependent on electroosmotic flow. Steady-state anodic flux at 0.32 mA/cm2, expressed as equivalent donor solution flux (in microliters/hr cm2), ranged from 6.1 for glycine to about 2 for the large anions. As expected, iontophoretic flux is higher at 3.2 mA/cm2 than at 0.32 mA/cm2, and passive flux measured after iontophoresis is about a factor of 10 greater than the corresponding flux measured before the skin was exposed to electric current. There are two mechanisms for flux enhancement relative to passive flux on "fresh" hairless mouse skin: (1) the effect of the voltage in increasing mass transfer over the passive diffusion level, the effect of electroosmotic flow dominating this contribution in the systems studied in this report; and (2) the effect of prior current flow in increasing the "intrinsic permeability" of the skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport mechanisms in iontophoresis. I. A theoretical model for the effect of electroosmotic flow on flux enhancement in transdermal iontophoresis.

Bulk fluid flow or volume flow in the direction of counterion flow is a probable mechanism for enhanced flux of uncharged species by iontophoresis. Both the electrical volume force effect, resulting from the interaction of the "ion atmosphere" and the electric field, and an induced osmotic pressure effect produce volume flow in the same direction as counterion flow through the membrane. Since each of these effects is proportional to the membrane charge and the imposed electric field, we classify both as electroosmotic flow. This research develops a detailed theoretical model which allows the effect of volume flow on flux enhancement to be evaluated. A detailed theoretical result for the electroosmotic flow coefficient also results from the analysis. The model assumes that transport occurs in three types of aqueous pores: positively charged, neutral, and negatively charged. For hairless mouse skin (HMS), pore size, charge, and number are evaluated from transference number, volume flow, and electrical resistance data. The flux enhancement ratio is J1/J1D = sigma Ai alpha i/[1-exp(-alpha i)], where i = pore type, and the summation runs over the three pore types. Ai is the area fraction of pore type i effective for transport; J1 and J1D are flux of species 1 with and without the electric field, respectively; and alpha i is given by alpha i = F(-delta phi/RT)[zeta 1 + (-zeta mi)Bari2Cmi(Gi + F)].(ABSTRACT TRUNCATED AT 250 WORDS)