Interaction of povidone with aromatic compounds I: Evaluation of complex formation by factorial analysis

A novel amorphous solid dispersion based on drug-polymer complexation

Rafoxanide (RAF) is a poorly water-soluble drug that forms a complex with povidone K25 (PVP) in a cosolvent system containing acetone and an alkaline aqueous medium. This study aims to investigate the impact of RAF-PVP complexation on in vitro and in vivo release of RAF. We prepared two RAF amorphous solid dispersions (ASDs) spray-dried from these two cosolvents. The first is a complexation-based spray-drying using a 70% 0.1 N NaOH solution with 30% acetone. The second is a traditional spray-dried formulation containing 80% acetone and 20% ethanol. Evidence from multiple solid-state characterization techniques indicated that ASDs spray-dried using both methods were amorphous. However, RAF ASDs based on drug-polymer complexation in the feed solution demonstrated not only faster drug release during dissolution testing but also higher in vivo absorption in an animal model. The improved RAF release in the complexation-based ASD is due to (1) high energy state of RAF owing to the amorphous form, (2) high pH in the microenvironment due to the ionized state of RAF and residual sodium hydroxide, (3) increased apparent solubility of RAF results from RAF-PVP complexation in dissolution media and biological media, and (4) improved wettability.

Investigating the Association Mechanism between Rafoxanide and Povidone

The low aqueous solubility of most hydrophobic medications limits their oral absorption. An approach to solve this problem is to make a drug-polymer association. Herein, we investigated the association between rafoxanide (RAF), a surface-active, poorly water-soluble drug, with a commercial hydrophilic polymer povidone. We found that the association is a function of medium composition and could only take place in polar media, such as water. The association is favored by the hydrogen-bond formation between the amide group in RAF and the carbonyl group in povidone. In addition, the association is also favored by the self-association of RAF through π-π interaction between the benzene rings in adjacent RAF molecules. Two-dimensional nuclear magnetic resonance has been applied to investigate the interactions and has confirmed our hypotheses. Geometry optimization confirmed that RAF exists primarily in the antiparallel configuration in the RAF aggregates. This study provides critical information for designing suitable drug-vehicle complexes and engineering the interactions between them to maximize the oral absorption. Our results shed light on drug design and delivery, drug molecule structure-functionality relationship, as well as efficacy enhancement toward interaction engineering.

Increasing the aqueous solubility of acetaminophen in the presence of polyvinylpyrrolidone and investigation of the mechanisms involved

It was shown that the aqueous solubility of acetaminophen in the presence of polyvinylpyrrolidone (PVP) increased. The solubility at 25 degrees C increased from 14.3 m mL(-1) in the absence of PVP, to 19.7 mg m(-1) in the presence of 4% w/v PVP, and to 26.7mg mL(-1) in the presence of 8% w/v PVP. Dialysis studies indicated that there is a potential of binding between PVP and acetaminophen in their aqueous solutions. Dialysis studies also revealed that the nature of interaction between PVP and acetaminophen is physical and reversible, and there was no strong binding between PVP and acetaminophen in their solutions. Infrared spectroscopy of acetaminophen/PVP solid dispersion indicated that the mechanism of interaction between PVP and acetaminophen is via hydrogen bonding. Therefore, the increase in solubility of acetaminophen in the presence of PVP is probably attributed to its ability to form a water-soluble complex with PVP.

Factorial analysis of the influence of dissolution medium on drug release from carrageenan-diltiazem complexes

This research studied the influence of buffer composition, pH, and ionic strength on the release of diltiazem hydrochloride from a complex of the drug with lambda carrageenan. Two viscosity grades of carrageenan were also compared. A factorial analysis was used to evaluate the influence of individual variables and their interactions. Both the complex solubility, measured as the drug concentration in equilibrium with the solid complex, and the drug release rate from constant surface area were considered. The increase of ionic strength significantly increased complex solubility in all the buffer systems. A significant effect of polymer grade on complex solubility was evidenced only in phosphate buffer with a pH of 6.8, indicating lower solubility of the complex when higher polymer molecular weight was involved. In most cases, drug release rate decreased when high polymer grade was involved in the complex. Ionic strength did not always have a significant effect on drug release rate and was quantitatively less important than for solubility. Ionic strength especially affected the drug release profiles. At higher ionic strength drug release was no longer constant, but decreased with time, probably because of lower polymer solubility.

Cellulose acetate phthalate (CAP): an 'inactive' pharmaceutical excipient with antiviral activity in the mouse model of genital herpesvirus infection

The spread of sexually transmitted infections caused by herpes simplex virus type 2 (HSV-2) has continued unabated. At least 20% of the United States population has been infected with HSV-2 and there is a high probability of further virus transmission by asymptomatic carriers. Given the absence of effective vaccines, this indicates the need to develop prophylactic measures such as topical microbicides that have antiviral activity. Recent studies indicate that cellulose acetate phthalate (CAP), an inactive pharmaceutical excipient commonly used in the production of enteric tablets and capsules, is a broad specificity microbicide against diverse sexually transmitted pathogens. When appropriately formulated in micronized form, it inactivates various viruses, including HSV-2, in vitro. Here we show that CAP inhibits HSV-2 infection in the mouse model of genital HSV-2 infection. Pretreatment with micronized CAP formulated in a glycerol-based cream with colloidal silicone dioxide significantly reduced the proportion of HSV-2-infected mice (10% virus shedding, 0-5% lesion development and 0% fatality for CAP as compared to 84% shedding, 63% lesion development and 63% fatality in saline-treated mice). These differences were significant (P < or = 0.0002 by the test of equality of two proportions). Virus titres in the minority of mice that developed infection were similar to those in untreated mice. HSV-2 infection was not inhibited by treatment with CAP formulated with other inactive ingredients (for example povidone plus crosprovidone) instead of silicone dioxide, presumably reflecting CAP complexation/inactivation. These data suggest that properly formulated, CAP may be an efficacious agent for preventing vaginal transmission of genital herpesvirus infections.

Chitosan microspheres of diclofenac sodium: I. application of factorial design and evaluation of release kinetics

In this study microspheres of diclofenac sodium, an anti-inflammatory agent, were prepared by utilizing a natural polysaccharide, chitosan-H. The objective of this investigation was to sustain the action of diclofenac sodium and to show the effect of various conditions on release kinetics. For this reason factorial design experiments were performed. The independent variables in the 3(3) factorial design were chitosan-H concentration, tripolyphosphate concentration and stabilization time, and in the 3(2) factorial design were chitosan-H and tripolyphosphate concentrations. The dependent variables, t50% and the total drug content were investigated by the polynomial equations. The release profiles were evaluated kinetically and the best fit was obtained by the Higuchi equation.

3(3) factorial design-based optimization of the formulation of nitrofurantoin microcapsules

A microcapsule form of nitrofurantoin was prepared by a simple coacervation method with carboxymethylcellulose and aluminium sulfate. 3(3) factorial design was performed for three independent variables, namely, the particle size of the drug, the size of the microcapsules and the pH of the dissolution medium. The dissolution tests with the formulated microcapsules were carried out according to the United States Pharmacopeia XXII rotating basket method at pH 1.2, 5, and 7.5, which represent the pH of gastrointestinal fluids. Release data were examined kinetically and the ideal kinetic models were estimated and t(63.2) values obtained from RRSBW distribution were used in the factorial design experiment. The influence of the independent variables on the dissolution of nitrofurantoin microcapsules could be expressed as the pH of the dissolution medium > particle size of the microcapsule > particle size of nitrofurantoin. The other aim of this study was to evaluate microcapsule formulation in terms of the United States Pharmacopeia criteria with a minimum of experiments. Our findings suggest that dosage forms which comply with the pharmacopoeia criteria for dissolution can be prepared and selected by factorial design.

Factorial design-based optimization of the formulation of isosorbide-5-mononitrate microcapsules

Sustained action Isosorbide-5-mononitrate (IS-5-MN) microcapsules are prepared in order to overcome the tolerance developed in conventional preparations and to increase patient compliance. For this purpose, factorial design experiments are performed and microcapsules of IS-5-MN are formulated by the organic phase separation method using ethylcellulose with two different viscosities (10 and 45 cp) as coating material. The independent variables in the 2 x 3 x 3 factorial design are core: wall ratio, particle size and pH of the medium. The dependent variable, t50 percent is investigated by the second-order polynomial equation to establish the correlation between independent variables. By using the calculated equations, the response-surface graphs, from which various levels of independent variables could be predicted, are obtained. The in vitro release profiles of the formulated microcapsules and the commercially available preparations are obtained by using the rotating basket method. In vitro release is evaluated by zero-order, first-order, Hixson-Crowell and Higuchi release kinetics. The t50 percent values obtained from the Higuchi equation are used as response in the 2 x 3 x 3 factorial design experiments.

Spectroscopic study of the interaction of coumarin anticoagulant drugs with polyvinylpyrrolidone

The interaction of coumarin anticoagulants with polyvinylpyrrolidone (PVP) was investigated using a fluorescence technique. The fluorescence intensities of warfarin and phenprocoumon were greatly enhanced following binding to PVP, while the fluorescence of 4-hydroxycoumarin was little enhanced in the presence of PVP. The enhanced fluorescence of warfarin and phenprocoumon bound to PVP can be explained by their incorporation into the hydrophobic environment in the PVP and by a decrease in the internal rotation of the alpha-substituted benzyl group in the drugs. The binding parameters of warfarin and phenprocoumon were estimated by the Klotz method; the binding constants for phenprocoumon and warfarin were found to be 2.6 X 10(4) and 2.2 X 10(4) M-1, respectively. The 13C-NMR measurements suggest the lactone moiety in the 4-hydroxycoumarin and the substituted benzene ring play an important role in the binding to PVP.

The effects of interacting variables on the tensile strength, disintegration and dissolution of paracetamol tablets

A factorially designed scheme has been used to analyse the separate and combined effects of packing fraction (P), nature of binder (N) and concentration of binder (C) on the tensile strength, disintegration and dissolution (t50%) times of paracetamol tablets. In general, P has the greatest effect on tensile strength, disintegration and dissolution times followed by C then N. For the variables in combination, the ranking of the effects on tensile strength, for the PVP/gelatin formulations, are P x N greater than N x C greater than P x C and for the PVP/tapioca formulations are P x C = N x C greater than P x N. For disintegration and for dissolution, the ranking for the PVP/gelatin formulations are P x C greater than P x N = N x C and P x N greater than P x C greater than N x C, respectively, and for the PVP/tapioca formulations are P x N greater than N x C = P x C. The results also show that tapioca acts as a binding agent when included in paracetamol tablet formulations, but it is a weaker binder than either PVP or gelatin. It is thus required in a higher concentration to produce tablets of comparable physical properties with those formulated with PVP or gelatin.

Degradation of chlorthalidone in methanol: kinetics and stabilization

The reaction of chlorthalidone with methanol to give the corresponding methyl ether was investigated. The kinetics are pseudo-first-order in chlorthalidone, but the observed pseudo-first-order rate constants show an unexpected dependence on the initial chlorthalidone concentration, attributable to the presence of trace catalytic impurities in commercial chlorthalidone. Evidence is presented to show that trace heavy metals are probably responsible for the primary catalytic effect. Trace quantities of acetic acid are also present and show a smaller secondary catalytic effect. Kinetics in the presence of added heavy metals and acetic acid were examined. EDTA and povidone reduce the degradation rate. Stabilization by EDTA is due to its ability to chelate heavy metals. Stabilization by povidone is also primarily due to its ability to complex heavy metals; complexation data of ferric and nickel ions with povidone and with 1-methyl-2-pyrrolidinone as a monomer model are presented. In addition, complexation constants were calculated for the interaction of povidone with chlorthalidone, which may also play a role in stabilization.

Interaction of povidone with aromatic compounds V: Relationship of binding tendency in a macromolecular solution treated as a pseudo two phase and a monophase

The pseudo-two-phase model is proposed to correlate complex formation of ligand molecules with povidone with partition coefficients (log P or II constants). The conditions which permit the use of the pseudo-two-phase model for binding of ligand onto macromolecules are determined. This model seems to be a more rational choice than the frequently used complex formation model (monophase). This is demonstrated theoretically and confirmed experimentally. The advantages of the use of such a model are also discussed.

Interaction of povidone with aromatic compounds IV: effects of macromolecule molecular weight, solvent dielectric constant, and ligand solubility on complex formation

Complex formation of ligand molecules with povidone was investigated to elucidate the effect of the molecular weights of the macromolecule and the influence of the solvent dielectric constant on the complexing tendency. The higher molecular weight polymers were more effective complexing agents than those with lower degrees of polymerization. When studying complex formation as a function of the dielectric constant (D), a linear relationship was noted between D and log B/F (B/F representing the ratio of bound to free ligand); the use of solvent mixtures to achieve a range of solvent dielectric constants enabled changes of the pH of the solvent, ligand dissociation, and solubility of the ligand and macromolecule. Of the variables under investigation, only the change in ligand solubility seemed to play an important role: a linear relationship was noted between the complexing tendency (log B/F) and the logarithm of the inverse of the ligand molecule solubility in the solvent mixtures (log 1/S). It was concluded that the change in solubility of the ligand was the predominant factor in the decrease of the complexing tendency with decreasing dielectric constant.

Factorial designs in pharmaceutical stability studies

An approach to analyzing and interpreting kinetic data from stability studies using factorial designs is presented. This may be useful for screening purposes or as an aid in identifying significant effects in complex systems. A typical 2n factorial experiment is discussed, and methods of variance estimation and statistical testing are presented. An example of simulated data is used to demonstrate how typical results may be analyzed, as well as the potential and limitations of this design in interpretation and construction of kinetic models.

Chromatographic study of interactions between polyvinylpyrrolidone and drugs

A chromatographic technique for the study of possible interactions of drugs with soluble or insoluble polymer additives is proposed. Crospovidone was used as a stationary phase. The method allowed the rapid determination of interaction constants in the range of greater than 1 M-1 as relevant for applications in practice. The interaction of 39 drugs and model compounds of diverse chemical structure with povidone and crospovidone was studied. The results closely agreed with data obtained from conventional equilibrium dialysis and sorption studies. The complexation reaction was found to be dominated by hydrogen binding. A close correspondence between the strength of interaction and the nature, number, and position of hydrogen-donating functional groups in the active ingredient was observed. The binding tendency was enhanced when the functional groups were connected with aromatic residues. The carboxyl group was more effective than the hydroxide or amino groups. The binding can be quantified by the binding constants, Kp and Ks, respectively, describing the interaction with polyvinylpyrrolidone via independent binding sites. At pH 1, with the exception of tannic acid, all investigated drugs exhibited Kp and/or Ks values well below an upper limit of 10 M-1. Hence, with additive-drug ratios commonly used in pharmaceutical preparations, the bound amount of drug after oral administration can hardly exceed 3%. In view of this already low degree of potential binding and considering its reversible character and its decreasing tendency with increasing pH during GI passage, the presence of polyvinylpyrrolidone in pharmaceutical preparations is not expected to interfere with GI drug absorption.

Interaction of povidone with aromatic compounds II: Evaluation of ionic strength, buffer concentration, temperature, and pH by factorial analysis

The interaction of a series of ligand molecules, all consisting of substituted benzoic and nicotinic acid derivatives, and povidone was studied. The influence of ionic strength, buffer concentration, and temperature was evaluated using factorial analysis. Complex formation was not affected at low ionic strength, but increased considerably at higher values due to dehydration of the macromolecule. Complex formation was enhanced in phosphate solutions, particularly in the presence of dibasic phosphate ions. A linear relationship was found between the logarithm of the percentage of bound ligand and ionic strength and buffer capacity. Increasing the temperature lowered complex formation. Although dehydration of the macromolecule also occurred, the decrease in complex formation could be attributed to the solubility increase of the ligand molecules. The influence of the degree of dissociation of the ligand molecules was investigated by factorial analysis. The compounds mainly interacted to a lesser extent in the dissociated than in the nondissociated state. In addition, a negative effect of a pyridine ring with respect to a phenyl ring was observed. The binding tendency was markedly increased by substituting the aromatic ring structure with hydroxyl functions and by esterification of the carboxyl function attached to the ring. The results suggested that lipophilicity and hydrogen bonding played a predominant role in povidone complexation.