Aqueous ethylcellulose dispersion of ethylcellulose. I. Evaluation of coating process variables

Physicomechanical characterizations and in vitro release studies of electrospun ethyl cellulose fibers, solvent cast carboxymethyl cellulose films, and their composites

Frequent change of wound dressings introduces wound inflammation and infections. In this study, we electrospun phenytoin (PHT) loaded ethyl cellulose (EC) microfibers and solvent cast tetracycline hydrochloride (TCH) loaded carboxymethyl cellulose (CMC) films with the aim to demonstrate tailorable in vitro drug release behaviors suitable for long-term use of wound dressings. Results from tensile testing showed a significant decrease in average elastic moduli from 8.8 ± 0.6 to 3.3 ± 0.3 MPa after incorporating PHT into EC fibers. PHT-loaded EC fibers displayed a slow and zero-ordered release up to 80 % of the total drug at 48 h, while TCH-loaded CMC films demonstrated a rapid and complete release within 30 min. Furthermore, drug-loaded EC/CMC composites were fabricated into fiber-in-film and fiber-on-film composites. Fiber-in-film composites showed stage release of TCH and PHT at 8 h, while fiber-on-film composites demonstrated simultaneous release of PHT and TCH with a prolonged release of TCH from CMC films. In general, electrospun PHT-loaded EC microfibers, solvent cast TCH-loaded CMC films, and their composites were studied to provide a fundamental scientific understanding on the novelty of the ability to modulate drug release characteristics based on the composite designs.

The effects of the treatment conditions on the dissolution profile of ethylcellulose coated pellets

Due to the additional particle coalescence in the coating, changes in the dissolution profile occur over time in the formulations coated by aqueous ethylcellulose latex. Dry thermal treatment (DT) of the coating can be used as a prevention of this process. Alternatively, it is advisable to take advantage of the synergistic effect of high humidity during wet treatment (WT), which substantially accelerates the film formation. This can be a problem for time-controlled systems, which are based on the coating rupture due to the penetration of water into the core causing the increase in the system volume. This process can begin already during the WT, which may affect the coating adversely. The submitted work was focused on the stability testing of two pellet core compositions: pellets containing swelling superdisintegrant sodium carboxymethyl starch (CMS) and pellets containing osmotically active polyethylene glycol (PEG). Another objective was to identify the treatment/storage condition effects on the pellet dissolution profiles. These pellets are intended to prevent hypoglycemia for patients with diabetes mellitus and therefore, besides the excipients, pellet cores contain 75% or 80% of glucose. The pellet coating is formed by ethylcellulose-based latex, which provides the required lag time (120-360 min). The sample stability was evaluated depending on the pellet core composition (PEG, CMS) for two types of final pellet coating treatment (DT or WT). Scanning electron microscopy and Raman microspectroscopy revealed the penetration of glucose and polyethylene glycol from the core to the PEG pellet surface after WT. For the CMS sample, significant pellet swelling after WT (under the conditions of elevated humidity) was statistically confirmed by the means of stereomicroscopic data evaluation. Therefore, the acceleration of dissolution rate during the stress tests is caused by the soluble substance penetration through the coating in the case of PEG pellets or by dosage form volume increase in the case of CMS pellets. The observed mechanisms can be generally anticipated during the stability testing of the ethylcellulose coated dosage forms. The aforementioned processes do not occur after DT and the pellets are stable in the environment without increased humidity.

Multivariate analysis as a method to understand variability in a complex excipient, and its contribution to formulation performance

A key part of the Risk Assessment of excipients is to understand how raw material variability could (or does) contribute to differences in performance of the drug product. Here we demonstrate an approach which achieves the necessary understanding for a complex, functional, excipient. Multivariate analysis (MVA) of the certificates of analysis of an ethylcellulose aqueous dispersion (Surelease) formulation revealed low overall variability of the properties of the systems. Review of the scores plot to highlight batches manufactured using the same ethylcellulose raw material in the formulation, indicated that these batches tend to be more closely related than other randomly selected batches. This variability could result in potential differences in the quality of drug product lots made from these batches. Manufacture of a model drug product from Surelease batches coated using different lots of starting material revealed small differences in the release of a model drug, which could be detected by certain model dependent dissolution modeling techniques, but they were not observed when using model-independent techniques. This illustrates that the techniques are suitable for detecting and understanding excipient variability, but that, in this case, the product was still robust.

Implementation of quality by design approach in manufacturing process optimization of dry granulated, immediate release, coated tablets - a case study

The aim of this study was to optimize the process of tablets compression and identification of film-coating critical process parameters (CPPs) affecting critical quality attributes (CQAs) using quality by design (QbD) approach. Design of experiment (DOE) and regression methods were employed to investigate hardness, disintegration time, and thickness of uncoated tablets depending on slugging and tableting compression force (CPPs). Plackett-Burman experimental design was applied to identify critical coating process parameters among selected ones that is: drying and preheating time, atomization air pressure, spray rate, air volume, inlet air temperature, and drum pressure that may influence the hardness and disintegration time of coated tablets. As a result of the research, design space was established to facilitate an in-depth understanding of existing relationship between CPPs and CQAs of intermediate product (uncoated tablets). Screening revealed that spray rate and inlet air temperature are two most important factors that affect the hardness of coated tablets. Simultaneously, none of the tested coating factors have influence on disintegration time. The observation was confirmed by conducting film coating of pilot size batches.

The importance of the molecular weight of ethyl cellulose on the properties of aqueous-based controlled release coatings

Previous investigations of aqueous based ethyl cellulose (EC) latex dispersions have mainly focused on the commercially available viscosity grade 20cps. In this study, dispersions of EC with varying viscosity grades (which correspond to molecular weights), ranging from 4 to 100 cps, were produced and characterised. The dispersions showed particle sizes around 200nm and highly negative ζ-potentials (approx. -100mV), which indicated stable dispersions as confirmed by sedimentation studies. The different latexes were used to produce free-standing film coatings. We hypothesised that the different viscosity grades of EC would result in different properties of the films. We found that an increase in viscosity grade (and higher molecular weight) resulted in lower coalescence between the particles during film formation and thus to higher water permeability than in film coatings of lower molecular weight. After exposure to water the EC 4cps and 20cps film coatings had a more porous structure in the side facing the air during production and drying after immersion in water. Molecular weight is therefore a factor that should be considered when producing pharmaceutical coatings for controlled release.

Comparison of film-coated retarded release pellets manufactured by layering technique or by bed rotor pelletization

In order to investigate the influence of coatings for controlled active pharmaceutical ingredient (API) release, two types of pellets were used. Microcrystalline pellets were coated with a model API using the Wurster fluidized bed technique in laboratory scale (layered Cellets). Another type of pellets consisting of microcrystalline cellulose and model API was manufactured by fluidized bed rotor pelletization (matrix pellets (MP)). Both kinds of pellets were coated in a Wurster fluidized bed process with a polymer mixture of ethylcellulose to achieve retarded API release. With layered Cellets and an increased thickness of the ethylcellulose layer, the lag-time was increased and the release rate was decreased. In the case of MP, retardation was less pronounced probable due to inhomogeneous polymer film formation as a result of the porous particle surface. To reduce the surface roughness, the MP were coated with polyvinylpyrrolidone (PVP) as an intermediate smoothing layer, in a first trial step by step. In a second trial, pelletization and the coating steps were performed in an uninterrupted process. Intermediate PVP coating improved the ethylcellulose film formation and led to a more pronounced retardation of API release. The uninterrupted process of matrix pellet manufacturing and coating results in a product with only low retarded release.

Formulation and in vitro and in vivo evaluation of film-coated montelukast sodium tablets using Opadry® yellow 20A82938 on an industrial scale

Purpose:

The aim of this study was to formulate stable film-coated montelukast sodium (MS) tablets using Opadry^® yellow 20A82938 (Montikast^® tablets) and to evaluate their in vitro and in vivo release profile.

Methods:

MS core tablets were manufactured using a direct compression method. Opadry yellow 20A82938 aqueous coating dispersion was used as the film-coating material. Dissolution of the film-coated tablets was tested in 900 mL of 0.5% sodium lauryl sulfate solution and the bioequivalence of the tablets was tested by comparing them with a reference formulation – Singulair^® tablets. In vitro–in vivo correlation was evaluated. The stability of the obtained film-coated tablets was evaluated according to International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use guidelines.

Results:

The efficiency of the film coating was determined by subjecting the coated tablets to gastric pH and drug release was analyzed using high-performance liquid chromatography. The coated tablets had no obvious defects. MS release met the study criterion of not less than 80% dissolved after 30 minutes in 0.5% sodium lauryl sulfate solution. Statistical comparison of the main pharmacokinetic parameters clearly indicated no significant difference between test and reference in any of the calculated pharmacokinetic parameters. Level A correlation between in vitro drug release and in vivo absorption was found to be satisfactory.

Conclusion:

These findings suggest that aqueous film coating with Opadry yellow 20A82938 is an easy, reproducible, and economical approach for preparing stable MS film-coated tablets without affecting the drug-release characteristics.

Evaluation of droplet size distributions using univariate and multivariate approaches

Pharmaceutically relevant material characteristics are often analyzed based on univariate descriptors instead of utilizing the whole information available in the full distribution. One example is droplet size distribution, which is often described by the median droplet size and the width of the distribution. The current study was aiming to compare univariate and multivariate approach in evaluating droplet size distributions. As a model system, the atomization of a coating solution from a two-fluid nozzle was investigated. The effect of three process parameters (concentration of ethyl cellulose in ethanol, atomizing air pressure, and flow rate of coating solution) on the droplet size and droplet size distribution using a full mixed factorial design was used. The droplet size produced by a two-fluid nozzle was measured by laser diffraction and reported as volume based size distribution. Investigation of loading and score plots from principal component analysis (PCA) revealed additional information on the droplet size distributions and it was possible to identify univariate statistics (volume median droplet size), which were similar, however, originating from varying droplet size distributions. The multivariate data analysis was proven to be an efficient tool for evaluating the full information contained in a distribution.

Characterization of latex particles for aqueous polymeric coating by electroacoustic method

Concentrated dispersions are used as coating dispersions for aqueous polymeric coatings, and characterization of the actual concentrated dispersions is an important in the pharmaceutical industry. The commonly used aqueous coating polymers are acrylic polymers and cellulose derivatives. We conducted a characterization study of polymethacrylate-based aqueous polymeric latex for aqueous coating, Eudragit L30D-55 (A-latex) and Eudragit RL30D (C-latex), by electroacoustic method. Colloidal Vibration Current (CVI) is one of the most important parameters relating to dynamic electrophoretic mobility and zeta potential, so we evaluated this parameter first. Volume fractions var of the latex in concentrated dispersions affect the CVIs according to the theory of Dukhin et al. The A-latex and C-latex CVI(*)s which were corrected with regard to the effect of volume fraction by the theoretical equation were nearly constant independent of phi in the ranges >0.04 and >0.03, respectively. The zeta potentials and colloidal stabilities of the concentrated dispersions were evaluated using an electroacoustic method by altering the pH and salt concentration. A-latex strongly aggregated at and below pH 2.5 and at and beyond 0.06 mol/L of electrolyte concentration. Regarding C-latex, instable dispersion was observed at 0.3 and 1 mol/L of NaCl concentration. The total potential energy of interaction between pairs of latex particles was changed by altering the salt concentration in this dispersion based on DLVO theory. The experimental results of stability in the concentrated latex dispersions can be explained by the total interaction energies.

Colloidal stability of aqueous polymeric dispersions: effect of water insoluble excipients

An important issue in the aqueous coating process is dispersion stability. An unstable dispersion results in aggregation of the colloidal particles, thereby affecting the film coating process. In the coating suspension containing pigment, a latex for aqueous film coating might interact with pigment, resulting in unstable dispersion. We therefore conducted a stability investigation in a mixed dispersion including latexes, EudragitL30D-55 (A-latex), EudragitRL30D (C-latex) and EudragitNE30D (N-latex) and pigments, titanium dioxide and iron oxide yellow. An aggregation of the dispersion containing A-latex was observed at pH 2. Regarding the dispersions with C-latex and N-latex, no aggregation was observed in the range pH 2-11. We calculated total interaction energy between latex-latex particles, pigment-pigment particles and latex-pigment particles based on DLVO theory. The calculated results explained two mechanisms of the stable mixed dispersion. The first was that the individual latex particle and the pigment particle dispersed without aggregation in the mixed dispersion because of the electrostatic interaction. The next was that the latexes adsorbed onto the surface of the pigments, making electrostatically stable heterocoaggregates. We also calculated the binding constant of iron oxide yellow for C-latex at pH 10. The value of the constant was determined to be 1.1 x 10(-2).

Colloidal stability of aqueous polymeric dispersions: Effect of pH and salt concentration

Aqueous film coatings often contain some electrolytes, organic acids, and pigments to give functions of sustained release, time-controlled release, or protection against light. Additions of some electrolytes or organic acids into latex dispersion for an aqueous film coating affect its colloidal stability. We characterized the aqueous polymeric latexes used in the pharmaceutical industry by measuring zeta potential and particle size, and evaluated this colloidal stability using DLVO theory. Three polymethacrylate-based aqueous polymeric latexes, Eudragit L30D-55, Eudragit RS30D and Eudragit NE30D, having anionic, cationic, and neutral polymer, respectively, were used in this study. The Hamaker constant of the polymethacrylate-based latex was determined to be 6.35 x 10(-21) J, and the total potential energy of the latex dispersion was calculated. The total potential energy of interaction between pairs of latex particles changes by altering the salt concentration and pH. The experimental results of stability in the anionic and the cationic latex dispersions can be explained by the total interaction energies. However, the stabilization of the neutral latex did not match the calculated result. The steric interaction produced by the surfactant likely resulted in the stable dispersion of this latex.

Aqueous starch acetate dispersion as a novel coating material for controlled release products

The aim of this study was to evaluate film-formation properties of a novel, organic solvent-free aqueous dispersion of potato starch acetate (SA; degree of substitution 2.8) and its ability to control drug release from a coated tablet. Initially, film-formation mechanisms and drug permeabilities of both organic solvent and dispersion-based SA free films (prepared by cast or spraying techniques) were investigated. The SA dispersion was suitable for the fluid-bed coating process, forming strong films with complete coalescent polymeric spheres. The model compounds predominantly permeated via the micro-pores of SA free films, which resulted from the leaching of water-soluble excipients from the dispersion. Thus, the permeation rate depended on the film structure rather than the physico-chemical properties of the penetrant. In the case of SA-coated tablet, drug release was sustained when the coating level was increased (from 12% to 20%, stated as a weight gain), and also as lipophilicity of the drug increased. When compared to the reference polymer dispersion (Surelease), SA coatings showed better mechanical properties against the osmotic pressure caused by a hydrophilic core tablet. These results clearly demonstrate that SA dispersion has high utility as a novel aqueous coating material for controlled release products.

The influence of drug type on the release profiles from Surelease-coated pellets

The release of metoclopramide hydrochloride (a water-soluble cationic drug) and diclofenac sodium (a sparingly soluble anionic drug) from pellets coated with ethylcellulose from an aqueous ethylcellulose dispersion (Surelease) at different coating loads was investigated. The release rates of each drug decreased as the coating load of Surelease increased. However, despite its lower water solubility, diclofenac sodium was released slightly faster than metoclopramide hydrochloride at equivalent coating loads. Changes in the release rates after curing were more pronounced for metoclopramide hydrochloride and the release rates of diclofenac sodium were lower than those of metoclopramide hydrochloride after curing. Differences between the release behaviour of the two drugs were probably due to an interaction between the cationic metoclopramide and the anionic ammonium oleate present in the Surelease. The slower release of metoclopramide hydrochloride may be due to an in situ formation of a poorly soluble complex of the drug and the ammonium oleate. This complex, because of its large molecular size, may diffuse more slowly through the film, causing a reduction in the release rate of metoclopramide hydrochloride. This interaction may also account for the differences in release characteristics of the drugs after curing. During curing the surfactant, due to its unstable nature in heat, may be converted to its constituent components. The interaction of drug with the surfactant was reduced as the residue of the ammonium oleate decreased during curing. However, a relatively low volume flow rate of air, and therefore, slower removal of ammonia in the modified side-vented Manesty Accela-cota 10 may also have affected the coating process of the pellets.

Influence of type and level of water-soluble additives on drug release and surface and mechanical properties of Surelease films

Ethylcellulose in combination with water-soluble additives has been used in the development of microporous membrane-coated dosage forms. In the present study, application of three types of water-soluble additives, namely polyethylene glycols (PEG 400, 3350, and 8000), maltodextrins (Maltrin M150, M100, and M040 in the order of lower to higher average polymer size and molecular weight; dextrose equivalence 16.9, 11.1, and 4.8, respectively), and xylitol, as porosity modifiers in the films of a commercially available aqueous ethylcellulose dispersion (Surelease/E-7-7060 plasticized with glyceryl tricaprylate/caprate) was investigated. The effect of type and level of these additives on drug release characteristics and surface and mechanical properties of the polymeric films was studied. Each additive was incorporated at 20 and 30% levels in the polymeric dispersion based on its solids content. Ibuprofen tablets were coated using the polymeric dispersion with and without additive at 3% w/w coat level in a fluid-bed equipment. The coated tablets were evaluated for their drug release rate, coat reflectivity (gloss), Brinell hardness, and elastic modulus. Differential scanning calorimetric analysis of the films was performed to determine the physico-chemical changes in the applied film-coats. The rate of drug release, hence film porosity, was observed to be dependent on the type and level of the additive added. The molecular weight of the additive and its concentration in the polymeric dispersion had significant influence on the rate of drug release, hardness, and elasticity of the film-coats.

The formation of colonic digestible films of amylose and ethylcellulose from aqueous dispersions at temperatures below 37 degrees C

The film forming properties of a commercial aqueous ethylcellulose dispersion (Surelease) mixed with a range of ratios of an amylose/butanol complex in the presence of a range of concentrations of a plasticiser has been studied by measuring the minimum film forming temperature (MFFT). Contrary to what was to be anticipated from the literature, it was found that an additional 4% of the plasticiser (dibutyl sebacate), normally present in the standard formulation of the ethyl cellulose dispersion, was sufficient to lower the MFFT to allow the formation of films at 35 degrees C. This was confirmed by assessment of the glass transition temperature of free films prepared by casting and drying at 35 degrees C by the application of dynamic mechanical analysis. This technique also demonstrated that the ethylycellulose and the amylose were not miscible. The ability of faecal slurry to digest the films formed at low temperatures was confirmed by the use of a batch fermenter. The extent of digestion was directly related to the amylose content of the films, ensuring the potential to provide films, which could function as colon specific coatings.

Influence of plasticizer type and coat level on Surelease film properties

Two commercially available formulations of aqueous ethylcellulose dispersion differing in their plasticizer, i.e., Surelease/E-7-7050 containing dibutyl sebacate (DBS) and Surelease/E-7-7060 containing glyceryl tricaprylate/caprate (GTC), were evaluated and compared for their film properties as a function of polymeric coat level. Ibuprofen tablets were coated at 1, 2, 3, and 5% w/w levels using each Surelease formulation, and the coated tablets were evaluated for their drug release characteristics, coat reflectivity (gloss), surface texture, Brinell hardness, and elastic modulus. The drug release was dependent on the coat level and followed Hixson-Crowell cube-root model at 1% coat level. However, at > or = 2% coat levels, the release from tablets coated with GTC plasticized formulation appeared to be best described by non-Fickian release mechanism and that from tablets coated with DBS plasticized formulation appeared to follow apparent zero-order release mechanism. At equal coat levels, tablets coated with GTC plasticized Surelease yielded lower drug release rates, higher reflectivity (gloss), lower surface roughness, higher Brinell hardness, and lower elastic modulus than those coated with DBS plasticized formulation. A good correlation was observed between the drug release rates and the reflectivity and surface texture of the coated tablets. The film-coats of GTC plasticized formulation were harder and more elastic than those of DBS plasticized formulation indicating better mechanical integrity.

Comparative study of drug release from pellets coated with HPMC or Surelease

The release of metoclopramide hydrochloride (very water soluble cationic drug) and diclofenac sodium (sparingly soluble anionic drug) from pellets coated with hydroxypropylmethylcellulose (HPMC; water-soluble polymer) or ethylcellulose aqueous dispersion (Surelease; water-insoluble polymer) at different coating loads was investigated. The release rates of either drug decreased as the coating load of HPMC increased, but overall, the release was fast, and the majority of both drugs released in about 1 hr, even at the highest coating load. The drug release mechanism for either drug was not affected by the coating load of HPMC or by the type of drug used, and it was found to be mainly diffusion controlled. Diclofenac sodium released slightly more slowly than metoclopramide hydrochloride from HPMC-coated pellets. This was attributed to the lower water solubility of the former drug. The release rate of either drug decreased greatly as the coating load of Surelease increased. The release of both drugs was sustained over 12 hr as the coating load of Surelease increased, and only about 70% of either drug was released after this period at the highest coating load (20%). The mechanism of release of metoclopramide hydrochloride was independent of coating load, and it was predominantly diffusion controlled. However, the mechanism of diclofenac sodium release was dependent on the coating load of Surelease. At low coating loads, diffusion of drug was facilitated due to the presence of more pores at the surface of the coated pellets; therefore, the rate of dissolution of the drug particles was the rate-limiting step. However, at high coating loads, drug release was mainly diffusion controlled. Despite its lower water solubility, diclofenac sodium released slightly faster than metoclopramide hydrochloride from Surelease-coated pellets at equivalent coating loads.

Formulation optimization of controlled-release pellets of metoclopramide hydrochloride using dissolution fit factor approach

The purpose of this study was to optimize the formulation variables for the preparation of ethyl cellulose-coated nonpareils loaded with metoclopramide hydrochloride (MCL). The approach to evaluate the effectiveness of formulation parameters was monitored by release rate testing using dissolution fit factors as a tool. The content of ethyl cellulose used in the formulation was based on the drug-loaded weight. The interrelationship of each developed formulation and the reference formulation Gastro-Timelets and their respective dissolution curves were evaluated using Moore's equation: [equation: see text]. The relationship between the ethyl cellulose content in the formulation and the dissolution fit factor f2 can be described as the following regression equation: Y = -0.054X2 + 3.347X - 1.915 (r2 = 0.99). The optimum ethyl cellulose content obtained from the equation was 30.8%. The type and content of plasticizer used in the formulation to achieve the greatest f2 were determined to be Myvacet 9-40 at the concentration of 25%. Results indicated that using the release rate testing approach with the dissolution fit factor as a tool could provide valuable information for formulation optimization.

Film-forming polymer-granulated excipients as the matrix materials for controlled release dosage forms

Lactose and dibasic calcium phosphate (DCP) were granulated with various concentrations of film-forming polymers by a stepwise spraying method to prepare a directly compressible matrix excipient. The film-forming polymeric latex of Eudragit RS-30D, Eudragit RL-30D, and Surelease (ethylcellulose) were used in this study as the source of the granulating materials. Better flowability and compressibility were observed for all the granulated particles than the polymer-free granules. Most tablets prepared from the polymer-granulated particles exhibited satisfactory friability of less than 1% except for those prepared from lactose particles granulated with low concentrations of ethylcellulose and from plain lactose granules. Change in tensile strength and tablet thickness were in good agreement with the plasticity of the granulating polymer. Polymer-granulated lactose and DCP provided for controlled release of captopril from matrix tablets. This investigation suggests that conventional excipients can be modified by a simple granulating procedure to provide better physical properties for being used as a matrix material.

Development and optimization of a solid dispersion hot-melt fluid bed coating method

A new hot-melt fluid bed coating process has been developed, characterized, and optimized. Polyethylene glycol served as the model coating agent and was charged with substrate into the fluid bed chamber in the solid state. The processing stages included: (A) warm-up, (B) preheating, (C) melting-spreading, and (D) cooling-congealing. A central composite design was utilized to characterize and optimize the process. Substrate porosity and density evaluations were conducted by mercury intrusion. The method proved capable of coating nonpareils from 10 to 35 mesh (0.500 to 2.00 mm) and tablets up to 1 g. The nonpareils were coated as individual particles, while particle sizes significantly smaller than 40 mesh (0.420 mm) tended to agglomerate. The porosity and density values of dissimilar nonpareil batches showed a large degree of variation, affecting the method's reproducibility. Additive coatings were achieved by sequential runs using coating agents of diminishing melting points. The method is a viable alternative to hot-melt spray-coating processes. Organic solvents, spraying equipment, steam jackets, and/or heating tape are eliminated from the process.

Processing considerations for an EC latex coating system: influence of curing time and temperature

The influence of curing time and curing temperature for a commercially available ethylcellulose latex coating dispersion (Aquacoat) were evaluated using response surface methodology. Levels for the factor curing time ranged from 30 to 300 minutes while levels for curing temperature ranged from 45 degrees to 75 degrees C. Responses, A, kappa, and gamma, were derived from regression analysis of the dissolution profiles and correspond to the maximum amount of drug released over the 12 hour sampling period, the rate of release, and the inflection point of the dissolution profile, respectively. The nature of the response surface was dramatically influenced by the plasticizer incorporated into the coating formula. When dibutyl sebacate was employed as the plasticizer, faster release resulted (higher A and kappa values, lower gamma values) when samples were exposed to higher curing temperatures or were stored for longer periods of time. Paradoxically, when tributyl citrate was used as the plasticizer, slower release resulted when samples were exposed to more rigorous conditions. Overall, curing temperature had a more dramatic effect than curing time.