Effect of hydrodynamic environment on tablets dissolution rate.

The main objective of this research was to develop an experimental method to apply well-defined flow fields to solid dosage forms, to study the rate process underlying tablet dissolution, and to better understand the role of external hydrodynamic condition on mass transfer rate and film thickness during dissolution. Two drugs models, Theophylline (class 1) and Naproxen (class 2), were selected and formulated into conventional tablets containing 105 mg Theophylline or 300 mg Naproxen using the wet granulation method. Tablets were tested for dissolution using both the basket and paddle methods at different rotational speed of 25, 50, 75, and 100 rpm. In general, the paddle method gave higher dissolution rates than the basket method and as the velocity of rotation was increased, drug release was also increased. Six different paddles and a tablet holder were designed and used to test dissolution rate. The rate of dissolution was dependent on the tablet surface area exposed to the dissolution medium, and on the shape, diameter, and area of the paddles used. Theophylline tablets showed increased mass transfer rate and decreased film thickness as basket rotation speed was increased. At 25 rpm, the mass transfer coefficient was 0.684 x 10(-4) cm/sec and film thickness was 12.003 x 10(-2) cm; at 100 rpm, the mass transfer coefficient was 3.884 x 10(-4) cm/sec and film thickness was 2.114 x 10(-2) cm. Paddle values tested at the same speed showed higher mass transfer coefficient and lower film thickness for Theophylline and Naproxen tablets. P values obtained by modification of the Stokes-Einstein equation showed that diffusion is the rate-limiting step to drug release and not mass transfer. This study demonstrated that hydrodynamic condition, type of dissolution testing used, and design of the paddles have an effect on dissolution rate, mass transfer rate, and the film thickness underlying the dissolution process.

Prediction of tablet hardness and porosity using near-infrared diffuse reflectance spectroscopy as a nondestructive method.

The main objective of this research is to use the near-infrared diffuse reflectance method to evaluate and quantify the effects of hardness and porosity on the near-infrared spectras of tablets. To develop a model equation, validate the model and test the model predictive ability. Seven theophylline tablet formulations of the same composition but with seven different hardness values (3, 6, 8, 10, 12, 15, and 17 kp) were prepared. Another seven theophylline tablets formulation with seven different porosity values (57.4, 50.3, 41.9, 40.3, 39.9, 37.3, and 35.1%) were prepared. Five placebo tablets formulation with different hardness and porosity values were also prepared. Laboratory hardness and porosity values were compared to near-infrared diffuse reflectance data. Linear regression, quadratic, cubic and partial least square techniques were used to determine the relationship between hardness, porosity and the near-infrared spectras. The results demonstrated that an increase in tablet hardness and a decrease in tablets porosity produced an increase in near-infrared absorbance. Series of model equations depending on the mathematical technique used for regression were developed from the calibration of hardness and porosity data using laboratory equipment vs. the near-infrared diffuse reflectance for each formulations. The results of near-infrared hardness and porosity predictions were very similar to laboratory hardness and porosity tests. The near-infrared diffuse reflectance spectroscopy method is an alternative nondestructive method for measurement of hardness and porosity of tablets.

Relationship between internal phase volume and emulsion stability: the cetyl alcohol/stearyl alcohol system.

The main objective of this study was to optimize the stability of cetyl alcohol/stearyl alcohol emulsions in terms of percentage of internal phase volume, emulsifier type and concentration, and amount of external phase (water). Creams (o/w emulsions) were prepared by phase inversion and physical properties as particle size of the internal phase, apparent viscosity, and sedimentation volume evaluated. Stability was performed at room temperature, 40 degrees C, 50 degrees C, and under stress conditions. High hydrophilic lipophilic balance (HLB) nonionic surfactants as tween 80, tween 20, Myrj 52, Brij 35, and low HLB span 60 were used as emulsifying agents. The percentage of internal phase components (cetyl alcohol and stearyl alcohol), percentage of emulsifying agents, and percentage of aqueous external phase were varied, and stability was investigated. As the level of emulsifier agent (tween 80 and span 60) increased from 3% to 15%, and the percent of the internal phase remained constant at 30%, the particle size of the internal phase decreased and the cream became more stable. Formulations of the same composition, but prepared using Myrj 53 and tween 20 as emulsifiers, showed a larger particle size than formulations prepared using tween 80 and Brij 35. As the level of the internal phase volume increased and consequently the amount of water decreased, emulsion viscosity increased. The best formulation containing 30% internal phase (50% cetyl alcohol, 35% stearyl alcohol), 15% emulsifying agents (tween 80/span 60 ratio of 3:1), and 70% water was selected, and effects of process temperature and cooling rate on emulsion stability investigated. This formulation was further investigated in terms of stability of a 1% hydrocortisone addition by varying the percentage (30%, 35%, 40%, and 45%) of internal phase and percentage of water (70%, 65%, 60%, and 55%). The best formulation contained 45% internal phase (22.5 g cetyl alcohol, 15.75 g stearyl alcohol, 15% emulsifying agent, which is equivalent to 5 g tween 80 and 1.7 g span 60), and 55% w/w water, was manufactured under different manufacturing processes. Emulsions prepared by homogenization at the beginning of the process of emulsification were stable with small internal phase particle diameter. This study demonstrates that at every cetyl alcohol/stearyl alcohol ratio there is a phase volume/emulsifier HLB ratio, which results in optimum stability.

An examination of the moisture sorption characteristics of commercial magnesium stearate.

The objective of this study was to characterize the moisture sorption of magnesium stearate and the morphological changes, if any, resulting from moisture sorption. Six samples of commercial magnesium stearate USP were examined. Moisture sorption isotherms were obtained at 25 degrees C and 5% to 98% relative humidity (RH) using a moisture balance. Changes in crystal form resulting from moisture sorption were determined by x-ray diffraction. There were differences in the shape of the isotherm, reversibility of moisture uptake, and shape of the hysteresis loop in the isotherms of crystalline and amorphous magnesium stearates. The isotherm of crystalline magnesium stearate was almost parallel to the pressure axis until an RH of ~80% was reached, when there was desorption of practically all of the adsorbed water. The isotherm of the amorphous sample was characterized by continuous uptake of water over the entire range of RH. Exposure of amorphous magnesium stearate to RH greater than 70% resulted in the formation of the trihydrate. The trihydrate was converted into the anhydrous form when heated to a temperature of 100 degrees C to 105 degrees C. The trihydrate could be generated by exposing the anhydrate to RH higher than 70%.

The effect of particle morphology on the physical stability of pharmaceutical powder mixtures: the effect of surface roughness of the carrier on the stability of ordered mixtures.

The effect of particle morphology of the components on the physical stability of ordered mixtures was determined for a model system comprised of a mixture of micronized aspirin and a monodisperse carrier. Spray-dried lactose, crystallized lactose, microcrystalline cellulose, and dextrate were used as carriers. The surface texture of the carriers was quantified in terms of the ratio of the perimeter of the particles to that of an idealized shape at a constant magnification. Mixtures containing highly textured carriers segregated to a lesser extent than those containing smoother textured carriers. This was postulated to be due to the presence of a higher concentration of surface asperities on the coarse carriers that can constitute potentially strong adhesion sites for the fine component because of their higher energy relative to adjacent areas on the surface. The effect of the addition of a ternary component, magnesium stearate, on the stability of the above mixtures was studied. The observed differences in the segregation response were attributed to electrostatic charge effects.

Hydrotropic solubilization--mechanistic studies.

PURPOSE: This study examines the mechanism of hydrotropic solubilization using the riboflavin-nicotinamide system. The most commonly proposed mechanism for hydrotropic solubilization is complexation, and therefore, is investigated. Additionally, since nicotinamide and several other hydrotropic agents self-associate in aqueous solution, the possibility that self-association of the hydrotropic agent is important mechanistically is examined by studying the effect of temperature on hydrotropic ability. Researchers have shown that the degree of self association decreases with increasing temperature. Therefore, if temperature affects the solubilizing capacity of nicotinamide, self-association must be mechanistically significant. METHODS: The complexation hypothesis is tested by looking at nicotinamide's ability to quench riboflavin fluorescence and by examining changes in the UV/Vis spectrum of riboflavin upon addition of nicotinamide. The solubility of riboflavin in nicotinamide solutions as a function of temperature is determined to assess the impact of self-association on hydrotropicity. RESULTS: Nicotinamide does not alter the intrinsic fluorescence of riboflavin nor are changes indicative of complexation observed in the UV/Vis spectrum Temperature does have an effect on the hydrotropic ability of nicotinamide. Specifically, as temperature increases, the solubilizing capacity of nicotinamide decreases. CONCLUSIONS: Because nicotinamide is unable to quench riboflavin fluorescence, and does not produce significant spectral changes, complexation of hicotinamide and riboflavin does not occur. However, since increasing temperature causes a decrease in the hydrotropic ability of nicotinamide and in its degree of self-association, it is proposed here that the self-association of nicotinamide impacts the hydrotropic mechanism.

Effect of nicotinamide and urea on the solubility of riboflavin in various solvents.

Hydrotropy is a solubilization process whereby addition of large amounts of a second solute results in an increase in the aqueous solubility of another solute. Past investigations have focused on the potential interaction of the hydrotropic agent with the solubilized solute. Conversely, this study proposes that at least some hydrotropic agents exert their solubilizing effect predominately by interacting with the solvent. To that end, the effect of two hydrotropic agents, nicotinamide and urea, on riboflavin solubility in aqueous and nonaqueous systems was examined. The term "solutropy" is introduced to describe solubilization by addition of large amounts of a second solute in any solvent. The nonaqueous solvents used included methanol, N-methylformamide, dimethyl sulfoxide, and acetone. In water, methanol, and N-methylformamide, riboflavin solubility was found to increase with increasing nicotinamide concentration; however, riboflavin solubility decreased with increasing nicotinamide concentration in dimethyl sulfoxide and acetone, thus establishing the solvent-dependent nature of solutropy. An examination of solvent properties revealed that the solvent's ability to be both a proton donor and acceptor is important mechanistically, while dielectric constant and polarity are not. The same solvent-dependency was observed with urea, although urea is a poorer solutrope than nicotinamide. This study proposes that some solutropic agents act by changing the nature of the solvent, specifically by altering the solvent's ability to participate in structure formation via intermolecular hydrogen bonding.

Self-association of nicotinamide in aqueous solution: light-scattering and vapor pressure osmometry studies.

Nicotinamide is a hydrotropic agent that has been reported to self-associate in aqueous solution. The objective of this study is to characterize the self-association of nicotinamide with regard to the extent of self-association as well as association constants using light-scattering and vapor pressure osmometry. Both methods allow calculation of association constants; however, while light-scattering measurements depend on the size of particles in solution, vapor pressure osmometry depends on the number of particles in solution. Using light-scattering, nicotinamide was found to associate primarily as dimers and trimers. Higher order aggregates can be characterized by an average aggregation number of 4.37. The association constants were 9.99 L/mol and 13.1 L/mol for dimerization and trimerization, respectively. From vapor pressure osmometry data were calculated a dimerization constant of 0.203 L/mol and a trimerization constant of 14.1 L/mol. In comparison, the trimerization constants are in good agreement, while the dimerization constants differ by an order of magnitude. Since light-scattering measurements are less reliable for small molecules like nicotinamide at low concentrations, it is felt that the dimerization constant calculated from vapor pressure osmometry is the more accurate.

Pulmonary delivery of free and liposomal insulin.

The effects of oligomerization and liposomal entrapment on pulmonary insulin absorption were investigated in rats using an intratracheal instillation method. The results indicated that both dimeric and hexameric insulins can be rapidly absorbed into the systemic circulation, producing a significant hypoglycemic response. Intratracheal instillation of insulin in two different oligomerized states has not resulted in any significant difference in the duration of hypoglycemic effect. However, the initial hypoglycemic response (first 10 min) obtained from intratracheal administration of 25 IU/kg hexameric insulin appears to be slower than that from the 25 IU/kg dimeric insulin, thereby suggesting that hexameric insulin may have a lower permeability coefficient across alveolar epithelium than the dimeric insulin. Intratracheal administration of insulin liposomes (dipalmitoylphosphatidyl choline:cholesterol, 7:2) led to facilitated pulmonary uptake of insulin and enhanced the hypoglycemic effect. Nevertheless, similar insulin uptake and pharmacodynamic response were obtained from both the physical mixture of insulin and blank liposomes and liposomally entrapped insulin.

Pulmonary biotransformation of insulin in rat and rabbit.

In vitro biodegradation of insulin in rabbit and rat lung homogenates was investigated. Insulin can be sequentially metabolized into two primary fragments in rabbit lung homogenate by an aminopeptidase. The amino acid sequences of the fragments were found to be the des-Phe-InsulinB1 (Metabolite I) and des-Phe-Val-InsulinB1-2 (Metabolite II). However, only the former metabolite (Metabolite I) was identified in the rat lung homogenate. The km and Vm values associated with rabbit lung homogenate were 0.29 +/- 0.14 mM and 16.4 +/- 6.9 microM/hr/mg protein, respectively, whereas those for a rabbit lung preparation containing both microsomes and cytosol were 0.22 +/- 0.07 mM and 17.9 +/- 5.4 microM/hr/mg protein, respectively. The km and Vm associated with the cytosolic fraction of rabbit lung were 0.32 +/- 0.16 and 20.6 +/- 6.1 microM/hr/mg protein, respectively. The results indicate that the lung aminopeptidase may be a cytosolic enzyme. The degradation of dimeric insulin in the lung homogenate was faster than that of hexameric insulin due to the difference in collision frequency between the enzyme and insulin aggregates. The major metabolites in the lungs reportedly retain almost the same bioactivity of insulin, suggesting that the pulmonary route of insulin delivery will not adversely affect its hypoglycemic activity.

Insulin aggregation in aqueous media and its effect on alpha-chymotrypsin-mediated proteolytic degradation.

Self-association of zinc-insulin monomers into dimers and hexamers may lead to enhanced protection of the peptide from proteolytic degradation. The present study has been undertaken to investigate the relationship, if any, between the rate of enzymatic degradation of insulin by a protease, alpha-chymotrypsin, and the extent of insulin aggregation in aqueous solutions. Insulin solutions (0.6 mg/ml) containing varying proportions of dimer and hexamer were obtained by adding ethylene diamine tetraacetic acid (EDTA) within a concentration range of 0.005 to 0.040 mM. As the EDTA concentration was increased above 0.040 mM, a complete dissociation of hexamers to dimers occurred and the rate of enzymatic degradation reached its maximum. The overall first-order rate constants appeared to be linearly related to the square of EDTA concentrations. The apparent first-order rate constants for dimer and hexamer degradation obtained from a linear plot of rate constant versus EDTA squared concentration were found to be 0.02800 +/- 0.00065 and 0.00798 +/- 0.00075 min-1, respectively. Two major insulin degradation products were also detected and the kinetics of product appearance agreed well with the disappearance kinetics of insulin. The results indicated that the degradation of insulin dimers by alpha-chymotrypsin is about 3.5 times faster than the degradation of the hexamer. The second-order dependency of degradation rate on EDTA concentration might be due to the fact that insulin hexamers contain two zinc ions which are sequestered by two EDTA molecules. Chelation of zinc ions by EDTA lead to hexamer deaggregation to dimers as was evidenced from a circular dichroism study.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermorheologic properties of aqueous solutions and gels of Tetronic 1508.

The rheologic properties of aqueous solutions and gels of Tetronic 1508 were investigated as a function of polymer concentration and temperature utilizing rotational viscometry. Below the sol-gel transition temperature the polymer solutions were low in viscosity and exhibited Newtonian rheologic behavior even at concentrations of 20 and 25% (w/w). Upon sol-gel transition, the more concentrated polymer solutions underwent a dramatic four- to five-orders of magnitude increase in viscosity, resulting in the formation of a rigid gel structure. Above the sol-gel transition temperature, the gels displayed pseudoplastic and plastic rheologic properties and the viscosity of the gels remained at a relatively constant value over a wide temperature range. Eventually, the thermal energy of the system exceeded the bonding forces within the gel structure, resulting in a gel-sol transition which was marked by a dramatic decrease in the viscosity of the system. Above the gel-sol transition temperature, the system reverted to a Newtonian fluid with viscosities very similar to those found for the Tetronic 1508 solutions at temperatures below the sol-gel transition temperature. The thermodynamic implications of the gel-sol transition are discussed.

Beta-cyclodextrin/steroid complexation: effect of steroid structure on association equilibria.

Molecular associations of beta-cyclodextrin (beta-CyD) with four steroids (cortisone, hydrocortisone, progesterone, and testosterone) have been studied using phase-solubility and spectroscopic techniques. Phase solubility diagrams could be categorized as B type. The complexes are formed at the stoichiometric ratios of 1:2 (drug:beta-CyD). A mathematical model has been proposed to calculate the apparent stability constants. The results suggest that the inclusion of a steroid molecule into the first beta-CyD cavity is thermodynamically more favorable over the association of 1:1 complex with the second beta-CyD molecule except for cortisone, which exhibits anomalous behavior. A mechanism of complexation has been proposed based on the apparent stability constants and chemical structures of the steroids and beta-CyD. It suggests that complexation is first brought about by inclusion of the five-member cyclopentane ring of the steroid molecule into the first beta-CyD cavity. The 1:1 complex subsequently binds with the second beta-CyD to form the 1:2 complex. The association constants of steroid/beta-CyD complexes are of the following order: progesterone greater than cortisone greater than testosterone greater than hydrocortisone. The order of aqueous solubilities of the complexes is hydrocortisone greater than cortisone greater than testosterone greater than progesterone.

Preparation and characterization of methotrexate-immunoglobulin conjugates.

In the targeting of chemotherapeutic agents to specific cells via covalent conjugation of the agent to monoclonal antibodies, conjugation methods which lead to maximum binding capacity with minimum loss of activity (for both drug and antibody) are essential. A model system for evaluation of various conjugation procedures is described. Rabbit anti-bovine serum albumin immunoglobulin (IgG) was used to represent the antibody, and methotrexate (MTX) was used as the targeting agent. The procedures evaluated were coupling via water soluble 1-ethyl-3 (3'-dimethylaminopropyl) carbodiimide (EDCI), or via prior activation of MTX's carboxyl groups involving active ester (N-hydroxysuccinimide) or mixed anhydride (isobutylchloroformate) formation. The formation of covalent conjugates in all three cases was verified by (a) the lack of incorporation of MTX into IgG when the reactions were performed in the absence of EDCI, or when MTX was reacted without prior activation, and (b) the presence of a constant amount of MTX in the conjugates after exhaustive dialysis of gel-filtration through Sephadex G-50. High incorporation of MTX and high retention of antifolate activity resulted from the EDCI coupling reaction, but there was very low recovery of protein (conjugate) and antibody activity. The biological and physicochemical properties of the conjugates prepared by the two pre-activation methods did not show any significant difference, but the conjugates prepared by the active ester method had a longer shelf-life. Therefore, of the three reactions investigated, the active ester method is our method of choice for conjugating MTX to IgG.

Reversal of acetaminophen intoxication with an N-acetylcysteine-liposome preparation.

An N-acetyl-l-cysteine (NAC)-liposome drug delivery system was assessed for its ability to reverse acetaminophen toxicity. Positively charged NAC-liposomes, as compared to neutral and negatively charged preparations, were highly effective in reversing acetaminophen-induced lethality in mice. The positively charged liposome preparation, when administered at a dose equivalent to 50 mg/kg NAC, increased the LD50 of acetaminophen from 840 to 1507 mg/kg; free NAC (50 mg/kg) did not significantly alter the LD50 (829 mg/kg). At this dose, only the liposome entrapped NAC protects against acetaminophen-induced lethality suggesting that positively charged liposomes enhance the delivery of NAC to hepatic parenchymal cells, the site of acetaminophen-induced necrosis.

Paired-ion high-performance liquid chromatographic assay for sulfinpyrazone in plasma.

A specific and sensitive liquid chromatographic method is reported for the assay of sulfinpyrazone in plasma utilizing ion pairing between the tetrabutylammonium cation and the sulfinpyrazone anion. The method is rapid in that conventional extraction procedures are avoided in favor of using disposable cartridges packed with an octade-cylsilane bonded phase as a means of separating the drug from plasma. The samples were chromatographed on a C18 reversed-phase column using a mobile phase consisting of 0.005 M tetrabutylammonium phosphate in methanol-water (56:44). The coefficient of variation obtained was 4.5% and the response was linear over a range of 0.2-80 micrograms/ml.

Nonisothermal aqueous calorimetry: computation of process-dependent temperature change and aspects of calorimeter design.

A general method for determining the process-dependent (intrinsic) temperature change in a nonisothermal calorimeter is presented. The nonisothermal approach to calorimetric investigations requires an estimate of the magnitude of the process independent (extrinsic) temperature change during the reaction period. The proposed method can be applied to any calorimeter whose output is a discrete or continuous temperature--time profile. It is based on a first-derivative transformation of the temperature--time profile and the partitioning of the observed temperature variation into two components: pure extrinsic variation, which occurs outside the reaction period, and the combined extrinsic and intrinsic effects during the reaction period. Close examination of the pure extrinsic variation was considered essential, since it provided the basis for identifying the form of a descriptive mathematical function consistent with the observed extrinsic behavior. Once a suitable function was selected, parameters for the equation were determined through a linear regression procedure. The resulting equation was used to predict the extrinsic variation within the reaction period. Subtraction of predicted extrinsic variation from the observed total variation and integration over the time course of the experiment provide an estimate of the process-dependent temperature change. The differential approach was examined for processes performed in a calorimeter of simple design. Aspects of calorimeter design and advantages of the proposed method of data analysis are discussed.

Application of immersional calorimetry to investigation of solid-liquid interactions: microcrystalline cellulose-water system.

A comprehensive characterization of the specific solid-liquid interaction for microcrystalline cellulose and water is presented. The procedure consisted of a conjoint vapor adsorption and immersional wetting experiment. The following information was obtained with respect to the solid. Estimates of the total surface are (138 m2/g) and the external surface (9.2 m2/g) were calculated from the adsorption and immersion data, respectively. Existence of an energetically homogeneous surface was verified by a linear decrease in the heat of immersion of samples containing adsorbed moisture approximately up to monolayer capacity. Integral and differential free energy, enthalpy, and entropy changes accompanying the adsorption process were calculated, and a lack of swelling was substantiated by comparison with a similar study of cellulose fibers. Immersional hysteresis was observed, and its magnitude suggested that sorption hysteresis was of enthalpic as well as entropic origin. The experimental method is potentially valuable for routine characterization of hydrophilic powders.