Quantitation of taurolidine decomposition in polymer solutions by chromotropic acid formaldehyde assay method.

A sensitive, selective, and specific assay was needed to study the degradation kinetics of taurolidine and its stabilization by polyvinylpyrrolidone (PVP). The purpose of the present study was to evaluate the usefulness of the chromotropic acid method and other formaldehyde or amine derivatization methods. The methods evaluated included formaldehyde derivatization with chromotropic acid, acetylacetone, 4-amino-5-hydrazino-3-mercapto-1,2,4-triazole, semicarbazide hydrochloride, or 2,4-dinitrophenylhydrazine and taurolidine decomposition product derivatization with dansylchloride or 7-chloro-4-nitrobenz-2-oxa-1,3-diazole chloride. Results indicated that the chromotropic acid method provided sufficient selectivity, reproducibility and sensitivity. It was able to quench taurultam decomposition and avoided PVP interference. The method was optimized by performance based selection of reagent lots, appropriate reagent storage and preparation, and controlled derivatization conditions. In conclusion, the optimized chromotropic method was the most appropriate method for quantitating taurolidine decomposition.

Solid state characterization of dehydroepiandrosterone.

Three polymorphs (forms I-III), a monohydrate (form S2), and three new solvates [4:1 hydrate (form S1), monohydrate (form S3), and methanol half-solvate (form S4)] were isolated and characterized by X-ray powder diffractometry (XRPD), IR spectroscopy, differential scanning calorimetry (DSC), hot stage microscopy, solution calorimetry, and their dissolution rates. A new polymorph, designated as form V, melting at 146.5-148 degrees C, was observed by hot stage microscopy. Our results indicate that only forms I and S4 exhibit reproducible DSC thermograms. Five of the isolated modifications undergo phase transformation on heating, and their DSC thermograms are not reproducible. Interpretation of DSC thermograms was facilitated by use of hot stage microscopy. The identification of each modification is based on XRPD patterns (except forms S3 and S4, for which the XRPD patterns are indistinguishable) and IR spectra. In the IR spectra, a significant difference was observed in the OH stretching region of all seven modifications. In a purity determination study, 5% of a contaminant modification in binary mixtures of several modifications could be detected by use of XRPD. To obtain a better understanding of the thermodynamic properties of these modifications, a series of increasing heating rates and different pan types were used in DSC. According to Burger's rule, forms I-III are monotropic polymorphs with decreasing stability in the order form I > form II > form III. The melting onsets and heats of fusion for forms I-III are 149.1 degrees C, 25.5 kJ/mol; 140.8 degrees C, 24.6 kJ/mol; and 137.8 degrees C, 24.0 kJ/mol, respectively. For form III the heat of fusion was calculated from heat of solution and DSC data. In the case of form S1 the melting point, 127.2 degrees C, was obtained by DSC using a hermetically sealed pan. The relative stabilities of the six modifications stored under high humidity conditions were predicted to be, on the basis of the heat of solution and thermal analysis data, from S2 > form S3 > form S1 > form I > form II > form III. However, the results of the dissolution rate determination were inconsistent with the heat of solution data. The stable form I shows a higher initial dissolution rate than the metastable form II and unstable form III. All modifications were converted into the stable monohydrate, form S2, during the dissolution study, suggesting that the moisture level in solid formulations should be carefully controlled.

The effect of excipients on glass transition temperatures for FK906 in the frozen and lyophilized states.

FK906, a tripeptide, has been used as a model compound to study the effect of excipients on the glass transition temperature for peptides and related compounds in the frozen solution and lyophilized states. Three kinds of excipients were chosen for study, low molecular weight sugars, high molecular weight polymers and salts. Our results indicate that the Gordon-Taylor equation can be used to predict the glass transition temperature of FK906 in the presence of sucrose, lactose, trehalose and maltose in the frozen solutions and in lyophilized products. Dextrans having different molecular weights can have different effects on the glass transition temperature of FK906 in the frozen state. Cooling rate can also have an effect on the glass transition temperature for FK906 frozen solutions in the presence of dextran. Finally, salts have a significant effect on the glass transition temperature of FK906 in the frozen state but not in lyophilized FK906.

Some physicochemical properties of FK906: glass transition and relaxation.

FK906, a tripeptide antihypertensive agent, was used as a model compound to study some physicochemical properties of peptides and peptide-related compounds. Our results indicate that lyophilization, grinding and melting or storing the samples under conditions of high relative humidity affect chemical stability and crystallinity of FK906. In a preliminary study of lyophilization process design, an extended phase diagram for FK906 in aqueous solution was constructed, and the effects of cooling rate and and heating rate on the glass transition temperatures of lyophilized FK906 and the FK906 solution were investigated. Following lyophilization, the DSC thermogram of FK906 exhibits an endothermic peak whose area is storage temperature- and time-dependent. The endothermic peak shifts to higher temperatures as the heating rate is increased. Our results indicate that the endothermic peak is related to structural change during incubation of lyophilized FK906.

Formulation, in vitro dissolution, and ocular bioavailability of high- and low-melting phenylephrine oxazolidines.

The in vitro dissolution and the relative ocular bioavailability of high- and low-melting phenylephrine oxazolidines (HMP and LMP) from a nonaqueous suspension (silicone fluid) were compared. Stability-indicating HPLC assays were developed for evaluation of the prototype formulations, in which a normal-phase HPLC method was necessary for analysis of PO, while a reverse-phase HPLC method was required for analysis of the primary degradation product, phenylephrine (PE), following its separation from the formulation using a short silica gel column. PO was formulated as an ophthalmic suspension in silicone fluid (20 cs) because of its property of undergoing rapid hydrolysis in aqueous media. An experimental test system for measuring the dissolution characteristics of a water-immiscible multiparticulate suspension was designed to obtain the dissolution profiles of suspensions of HMP and LMP. The dissolution rates, which were nearly identical for LMP and HMP, were obtained assuming a quasi-infinite reservoir. A reverse-phase HPLC assay with fluorescence detection was used for measuring the concentrations of PE in aqueous humor and corneal samples. Statistical analysis of the bioavailability data showed that suspensions containing HMP and LMP were equal in extent of absorption following a single topical application to the rabbit eye. The results correlated well with the in vitro dissolution rates of the suspensions of HMP and LMP.

Physicochemical characterization of high- and low-melting phenylephrine oxazolidines.

Phenylephrine oxazolidine is a new prodrug of phenylephrine developed for improving ocular absorption and reducing systemic side effects. In the present study, high- and low-melting phenylephrine oxazolidines (HMP and LMP) were characterized in terms of their stereochemistry and crystal properties. It was found that the molecular configuration of the prodrug in the crystals of either HMP or LMP is identical (5R/2R). The two crystals were shown to have the same IR spectra and X-ray diffraction patterns but different crystal habits, thermal properties, solubilities and intrinsic dissolution rates. Single crystal X-ray structure analysis indicates that crystals of both HMP and LMP are orthorhombic and belong to the P2(1)2(1)2(1) space group with four molecules in a unit cell (a = 20.697 A, b = 7.065 A, and c = 9.304 A). The molecules in the crystal are held together by an intermolecular hydrogen bonding interaction between N(3) and O(13). The different physical properties observed for LMP result from crystal imperfections caused by the presence of trace amounts (often at levels < 0.5%) of an unidentified, structurally related synthetic impurity which can be dispersed in the prodrug. It was observed that both HMP and LMP can sustain thermal and mechanical treatment in the solid state. However, LMP was partially converted to HMP when suspended in certain solvents.

A high-performance liquid chromatographic method for the quantitative enantioselective analysis of mefloquine stereoisomers.

A rapid quantitative, enantioselective HPLC method for the analysis of the four stereoisomers, (+) and (-) erythro and (+) and (-) threo forms, of mefloquine has been developed using a Chiralpak Ad analytical column containing amylose tris-3,5-dimethylphenyl carbonate coated on silica gel and hexane/ethanol/diethylamine (96:4:0.1, v/v%) as the mobile phase. This method made it possible to quantitate small amounts of threo form in the presence of the erythro form of mefloquine, the form which is used as the active ingredient in commercial mefloquine tablets. Tablets from three sources were studied to estimate their optical purity, and it was found that tablets from one source contain 0.27 w/w% of the (-)-threo and 0.25 w/w% of the (+)-threo form, tablets from the second source contain 0.056 and 0.042 w/w% (-)- and (+)-threo, respectively, and tablets from the third source contain 0.052 w/w% (+)-threo, with the remainder erythro.

Structural effects on the binding of amine drugs with the diphenylmethyl functionality to cyclodextrins. II. A molecular modeling study.

Molecular modeling has been used to study the complexation between alpha, beta, or gamma-cyclodextrin (CD) and a group of amine compounds having the diphenylmethyl functionality. The computer program SYBYL 5.3 and the Tripos force field (version 5.2) were used for all the calculations. Three-dimensional structures of 13 amine compounds were built individually from their atoms, and CDs were built based on the X-ray crystallographic coordinates. The diphenylmethyl derivative-CD complexes were constructed and optimized. Based on the calculated binding energies accompanying the inclusion process, the preferred method of approach of the compounds to the cavities of the CD molecules, and the structural effects on the binding between amine compounds and three CDs were explored. The calculated binding energies exhibited a good correlation with the stability constants obtained from solution calorimetric titrations. The present study shows that for similar ligand molecules, the molecular modeling technique should enable us to visualize the structure of the inclusion complexes and will also assist us in determining the ability of a potential drug molecule to form a stable complex with CDs.

Structural effects on the binding of amine drugs with the diphenylmethyl functionality to cyclodextrins. I. A microcalorimetric study.

Solution calorimetry has been employed to evaluate the stability constants and enthalpy changes associated with complex formation between alpha-, beta, or gamma-cyclodextrin (CD) and a group of amine compounds having the diphenylmethyl functionality. Data from thermal titrations of the compounds were analyzed using nonlinear least squares. The standard free energy decrease accompanying the formation of inclusion complexes is generally due to a negative standard enthalpy change (delta H degrees). The standard entropy change (delta S degrees) was negative, except in the case of complexes formed with gamma-CD. Of the 13 compounds studied, only 2 formed complexes with 1:2 (compound:beta-CD) stoichiometry, terfenadine . HCl and cinnarizine . 2HCl. All the others formed 1:1 complexes. The structural effect on the stability constants, thermodynamics, and inclusion geometry was explored by relating the calorimetric results to the chemical structures of the guest molecules and the cavity sizes of the CD molecules. The results suggest that one of the phenyl groups of the diphenylmethyl functionality resides in the CD cavity and is in van der Waals contact with the inside wall of the CD cavity. In the case of alpha- and beta-CDs, van der Waals interaction dominates in the stabilization. On the other hand, the interaction between these compounds and gamma-CD is largely entropically driven. Adiphenine . HCl forms a more stable complex with beta-CD than proadifen . HCl, suggesting that hydrogen bonding to the carbonyl oxygen by the hydroxyl group on the rim of the CD ring can influence the strength of the binding interaction.

Physical properties of the complexes formed between heptakis(2,6-di-O-methyl)-beta-cyclodextrin, beta-cyclodextrin, and chlorambucil.

The solid complex of chlorambucil (CHL) and heptakis-(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB) has been isolated from ethanol and analyzed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and powder X-ray diffractometry. The carbonyl stretching band of the complex, observed in the FTIR spectrum, was shifted to higher frequency, suggesting that intermolecular hydrogen bonds between CHL molecules are broken when the complex is formed. Since no melting endotherm was observed for CHL when the DSC thermogram of the complex was obtained, the crystal lattice of the compound must be disrupted upon complexation. X-ray diffraction patterns of the inclusion complex were different from those of the physical mixture and contained no peaks corresponding to free CHL, thus indicating the formation of a new crystalline material. The 1:1 CHL:beta-cyclodextrin (beta-CD) complex was isolated from aqueous solution at 3 degrees C. Results of analyses using FTIR and DSC were similar to those obtained with the CHL:DIMEB complex. The X-ray diffraction patterns of the complex suggest that its degree of crystallinity is higher than that of the CHL:DIMEB complex.

Microcalorimetric investigation of the complexation between 2-hydroxypropyl-beta-cyclodextrin and amine drugs with the diphenylmethyl functionality.

Solution calorimetry has been employed to evaluate the stability constants and standard-enthalpy changes (delta H degrees) associated with complex formation between 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and a group of amine compounds having the diphenylmethyl functionality in aqueous solution at 298.15 K. Data from microcalorimetric titrations of the compounds were analysed using a nonlinear least-squares method. Of the 12 compounds studied, only terfenadine.HCl formed a 1:2 (compound:HP-beta-CD) complex. All the others formed 1:1 complexes. The standard free energy decrease accompanying the formation of inclusion complexes is generally due to a negative delta H degrees. This exothermic delta H degrees can be interpreted as indicating that the binding forces for complexation include both the hydrophobic effect and strong van der Waals interactions. When a halogen substituent is in the aromatic ring, stability constants are higher and standard-entropy changes (delta S degrees) become positive, suggesting greater hydrophobic interaction. Both adiphenine.HCl and proadifen.HCl form more stable complexes, suggesting that hydrogen bonding to the carbonyl oxygen by the hydroxyl-group on the rim of the CD ring could be an important contributor to the complexation. Substitution on the aliphatic carbon of the diphenylmethyl group was also found to be important in determining the ability of compounds to bind with HP-beta-CD. The independence of the thermodynamic constants on the degree of protonation in the case of bifunctional amines indicates that the amine functional groups do not penetrate into the HP-beta-CD cavity.

Quantitative Fourier transform-infrared/attenuated total reflectance (FT-IR/ATR) analysis of trimethoprim and sulfamethoxazole in a pharmaceutical formulation using partial least squares.

An alternative procedure for the simultaneous determination of trimethoprim and sulfamethoxazole in an intravenous pharmaceutical formulation is presented. Infrared spectra of 14 calibration and 6 validation samples were collected using Fourier transform-infrared/attenuated total reflectance (FT-IR/ATR). Partial least-squares (PLS) analysis of the spectral data yielded an average relative error of prediction of 0.69% for trimethoprim and 0.38% for sulfamethoxazole. Analysis of a commercial formulation gave concentration values for sulfamethoxazole and trimethoprim in close agreement with those obtained by a modification of the high-performance liquid chromatography (HPLC) assay listed in the United States Pharmacopeia (USP).

Heptakis(2,6-di-O-methyl)-beta-cyclodextrin complexation with the antitumor agent chlorambucil.

The effects of heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB) and of beta-cyclodextrin (beta-CD) on the aqueous solubility and stability of chlorambucil (CHL) have been compared. In the presence of 1.3 x 10(-3) M DIMEB, there is a greater than 20-fold increase in the stability of chlorambucil at 37 degrees C, pH 4.13. Aqueous solubility of CHL is increased more than 40-fold in the presence of 1.74 x 10(-2) M DIMEB when the solubility is studied under conditions where degradation is minimized (3.0 degrees C, pH 4.13). In the presence of 1.3 x 10(-3) M beta-CD, there is a fourfold increase in the stability, and with 1.74 x 10(-2) M beta-CD, there is a threefold increase in the aqueous solubility of CHL under similar conditions. Stability constants of the CHL complex with DIMEB were determined kinetically and spectrophotometrically under various experimental conditions assuming 1:1 inclusion complex formation.

Physical characterization of the methanol solvate of urapidil.

The methanol solvate of urapidil was prepared and characterized by means of differential scanning calorimetry, thermogravimetric analysis, infrared spectroscopy, X-ray powder diffraction, intrinsic dissolution rate, and solution calorimetry. The stoichiometry of the urapidil:methanol solvate was found to be 1:1. The crystal and molecular structures were determined from three-dimensional X-ray data. The stability of the solvate under different storage conditions was also determined.

Physical characterization of the hydrates of urapidil.

Three hydrates of urapidil were prepared and characterized by means of differential scanning calorimetry, thermogravimetric analysis, infrared spectroscopy, X-ray powder diffraction, intrinsic dissolution rates, and solution calorimetry. The stoichiometry of the urapidil hydrates was found to be 1:5, 1:3, and 1:1 (urapidil:water). The crystal and molecular structures of urapidil pentahydrate were determined from three-dimensional X-ray data. The stability of the pentahydrate and monohydrate under different storage conditions was also determined.

Physical characterization of solid forms of urapidil.

Polymorphic forms of urapidil were prepared by various techniques. Three polymorphic forms were identified and their solid-state properties were characterized by thermal behaviour, X-ray powder diffraction, IR spectra, intrinsic dissolution rates, solution calorimetry and scanning electron microscopy. Solution calorimetry was used to determine each polymorph in mixtures of Forms I and II, with a reproducibility of +/-3%. The stability of the three polymorphic forms was followed over a period of three months under different conditions and it was concluded that Forms I, II and III of urapidil were enantiotropic.

Physical characterization of solid iopanoic acid forms.

Three solid forms of iopanoic acid were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, thermal microscopy, IR spectroscopy, and dissolution studies. X-ray analysis demonstrated that two solid forms were crystalline and that the third was amorphous. The amorphous form had been reported previously as crystalline. Enthalpies and entropies of transition were calculated using differential scanning calorimetry. A fourth form, a benzene solvate, also was isolated but proved to be too unstable at room temperature to permit conclusive characterization. The amorphous form demonstrated a 10-fold greater intrinsic dissolution rate than the commercially available form (Form I). Form II's intrinsic dissolution rate was 1.5 times greater than that of Form I. In powder dissolution studies, the peak solubilities of the different forms followed the same rank order as their intrinsic dissolution rates. Form II was relatively stable in aqueous saturated solutions, but the amorphous form was rapidly converted to Form I under similar conditions.

Effect of sulfasalazine on digoxin bioavailability.

Low levels of digoxin were noted in a patient receiving digoxin and sulfasalazine (SSA). Discontinuation of SSA resulted in a significant increase in serum digoxin levels. To determine whether or not SSA consistently interfered with the therapeutic effect of digoxin, both drugs were administered to 10 normal subjects in a crossover study. Each received 2 doses of digoxin (0.5 mg, elixir): one dose given alone, and a second dose after 6 days of treatment with SSA. When digoxin was given with SSA, the average area under the serum digoxin curve fell from the control value of 8.79 ng-hr-ml(-1) to 6.66 ng-hr-ml(-1) (p less than 0.05), fell and total urinary excretion decreased from 278 mcg/10 days to 228 mcg/10 days (p less than 0.025). These changes suggest interference with the bioavailability of digoxin by SSA. Studies were conducted to determine whether SSA inhibited digoxin absorption by physically absorbing the glycoside from solution. In vitro tests failed to reveal any significant adsorptive properties for SSA.