Pharmaceutical relationships of three solid state forms of stavudine.

Three solid state forms of stavudine designated forms I, II and III have been identified and characterized. Forms I and II are anhydrous polymorphs whereas form III is hydrated and is pseudopolymorphic with forms I and II. Physico-chemical and thermodynamic properties of the three solid state forms have been characterized. Solid-state stability and potential for interconversion of the forms to aid in the selection of preferred form for development and commercialization has been studied. Conditions of recrystallization governing the formation of thermodynamically most stable polymorphic form I devoid of other forms was identified.

Stabilization of chimeric BR96-doxorubicin immunoconjugate.

Chimeric BR96-doxorubicin conjugate (BR96-DOX) is an immunoconjugate designed to specifically target and kill certain tumor cells. The linker between the chimeric BR96 antibody and DOX is an acid-labile hydrazone group which was designed to undergo lysosomal hydrolysis to release DOX in vivo. Stability studies indicated that acid-catalyzed hydrazone hydrolysis was the major degradation route in vitro. Even under optimal conditions of pH and temperature, the stability of BR96-DOX in solution was not acceptable for long-term storage. Lyophilization of BR96-DOX in the presence of added sugars, such as lactose or sucrose, and subsequent storage of the lyophile under refrigeration significantly improved the stability. Therefore lyophilization appears to be a viable approach for achieving long-term stabilization of BR96-DOX.

Chemical and physical stability of chimeric L6, a mouse-human monoclonal antibody.

Chimeric L6 is a mouse-human monoclonal antibody specific for tumor cell-associated antigens. The factors affecting the physical and chemical stability of chimeric L6 were assessed at elevated temperatures (30-60 degrees C) and by multiple freezing and thawing. Three routes of degradation were observed: chemical degradation to smaller molecular weight species, irreversible aggregation, and formation of a reversible dimer. The specific pathway depended on the stress condition applied and the pH, with maximal overall stability to both thermal stress and multiple freezing/thawing observed at about pH 5.5. Other factors including antibody concentration, buffer concentration, NaCl concentration, and agitation had minimal influence on the stability. Commonly used sugars, polyhydric alcohols, and amino acids effectively prevented freeze/thaw-induced aggregation.

S-acylation of cysteine by O-acetylsalicylic anhydride: a possible mechanism for aspirin hypersensitivity?

In order to elucidate the possible reaction pathways for the acylation of protein by O-acetylsalicylic anhydride, the mechanism of the reaction between L-cysteine and O-acetylsalicylic anhydride was studied. O-Acetylsalicylic anhydride reacts with L-cysteine via a consecutive kinetic pathway. The thiol anion first reacts with the anhydride to form an intermediate thiol ester which then undergoes an intramolecular rearrangement to form the stable N-(O-acetylsalicyloyl)-2-amino-3-thiopropionic acid, 5. The importance of the free amino group in the intramolecular reaction was established by the observed stability of the S-(O-acetylsalicyloyl) derivative of N-acetylcysteine under similar reaction conditions. The formation of the thiol ester was pH dependent, suggesting that the thiol anion was the attacking species. The acyl transfer to the adjacent amino group was catalyzed by both phosphate and acetate buffers. The results suggest that the reaction of O-acetylsalicylic anhydride with the thiol-containing amino acids of a protein molecule may proceed via formation of an initial thio ester, followed by an S to N intramolecular acyl transfer to form an immunogenic amide.

Degradation kinetics and mechanism of N6-[(dimethylamino)methylene]mitomycin C in aqueous solutions.

The degradation of N6-[(dimethylamino)methylene]mitomycin C, a semisynthetic analogue of mitomycin C, was studied in aqueous solution. The compound degraded rapidly and followed pseudo-first-order kinetics in both acidic (pH less than 5) and basic pH greater than or equal to 9) media. In the near-neutral pH region, however, biphasic kinetics were observed. At the pH of maximum stability (6.5), 10% activity was lost after approximately 6 h at 22 degrees C. Citrate and phosphate species were catalytic at pH 6.5. Spectrophotometric and HPLC methods were used to elucidate the degradation mechanism at pH 7-9. Under these conditions, equilibrium addition of one water molecule into the amidine side chain occurred, followed by parallel formation of mitomycin C and N6-(formyl)mitomycin C. The latter compound subsequently hydrolyzed to mitomycin C.

Liquid crystalline phase formation by cholesterol in aqueous fatty acid salt solutions.

The interaction of cholesterol with fatty acid salt solutions was investigated as a potential method for gallstone dissolution. In the presence of sodium oleate or laurate, crystalline cholesterol was rapidly converted into a lamellar liquid crystalline phase. Analysis of the mesophase showed that it contained approximately equimolar amounts of the lipid components, although water was the major constituent. The rate of conversion of cholesterol to mesophase was determined from a compressed disc immersed in a stirred solution, as if it were a simple dissolution process. The apparent rate of solubilization was independent of oleate concentration in the 2.5 to 10% range and was approximately 200 times faster than micellar dissolution of cholesterol in 5% sodium cholate. A mechanism is proposed in which the rate-limiting step in mesophase formation is dispersion of liquid crystalline fragments into the bulk medium.

Dissolution rate of cholesterol in monooctanoin.

Monooctanoin is used clinically for dissolution of common bile duct cholesterol gallstones. A number of factors influencing the dissolution rate of cholesterol monohydrate in this solvent were investigated. Water increased cholesterol dissolution rate in a manner inconsistent with previous solubility measurements. Dissolution rate increased approximately 50% in the presence of 10 to 15% water in monooctanoin. Further studies on viscosity, the effect of polymers and temperature showed that the solvent viscosity had a dominating influence on dissolution rate. This was thought to be caused by the interaction of cholesterol with the solvent in the dissolving surface layer which caused a decrease in the diffusion coefficient (and dissolution rate) of cholesterol. Reducing viscosity and increasing temperature were identified as possible approaches for increasing cholesterol gallstone dissolution rate in monooctanoin.

Importance of viscosity in the dissolution rate of cholesterol in monooctanoin solutions.

Several factors affecting the dissolution rate of cholesterol in monooctanoin were investigated. This solvent is used clinically for dissolution of residual cholesterol gallstones in the bile duct after cholecystectomy. The effect of added water on dissolution rate, measured using the static- or rotating-disk methods, was not consistent with the previously measured solubility. The discrepancy was found to be due to the decreasing viscosity of the solvent as water was added. Addition of cholesterol, however, increased the viscosity of monooctanoin. The viscosity effect on dissolution rate was investigated further by addition of polymers (povidone and poloxamer 237) which increased solvent viscosity. Dissolution rate was proportional to viscosity to the -0.4 power with these polymers. An equation was derived which predicts that dissolution rate should be proportional to viscosity to the -2/3 power. The predicted exponent was very close to reported experimental values for benzoic acid, but the dissolution rate/viscosity relationship for cholesterol in aqueous monooctanoin was nonlinear with apparent exponents of -0.65 to -2.3. Although the Arrhenius activation energies for viscosity (3.79 kcal/mol) and dissolution rate constant (3.66 kcal/mol) were almost equal for benzoic acid, a nonlinear relationship was again observed for cholesterol in aqueous monooctanoin with approximate Ea values of 5.6-10 kcal/mol. The strong influence of viscosity on dissolution rate in this system is attributed to the viscosity-increasing effect of cholesterol in the diffusion layer. The increased viscosity at higher cholesterol concentrations reduces the diffusion coefficient of cholesterol and causes the dissolution rate to be slower even though solubility may have been higher.(ABSTRACT TRUNCATED AT 250 WORDS)

Liquid crystal solubilization of cholesterol: potential method for gallstone dissolution.

Solubilization rate and phase equilibrium studies were conducted for cholesterol in aqueous sodium oleate solutions. The components interacted to form a lamellar liquid crystalline phase, and this phenomenon was investigated as a potential method for cholesterol gallstone dissolution. Phase equilibria data for cholesterol-sodium oleate-water showed that the mesophase contained approximately equimolar amounts of cholesterol and oleate with large amounts of water. The cholesterol solubilization rate from a static pellet in sodium oleate solutions was much faster than dissolution in sodium cholate solutions and was independent of oleate concentration from 2.5 to 10%. In these experiments, the medium became a cloudy dispersion of liquid crystalline phase in the micellar solutions. The rate-limiting step in the solubilization process appears to be dispersion of fragments from the liquid crystalline layer on the cholesterol surface. This hypothesis was consistent with the kinetic effects of viscosity, stirring rate, and oleate concentration. By converting cholesterol to a liquid crystalline phase, the solubilization process avoids the limitations for micellar solubility and interfacial resistance which control cholesterol dissolution in bile salt-containing media.

Kinetics and mechanism of hydrolysis of labile quaternary ammonium derivatives of tertiary amines.

The kinetics and mechanism of hydrolysis of N-(4-hydroxy-3,5-dimethylbenzyl)pyridinium bromide and similar quaternary derivatives of niacinamide, N,N-dimethylaniline, and trimethylamine were investigated. pH-Rate profiles at 25 degrees for formation of tertiary amine and 4-hydroxymethyl-2,6-dimethylphenol indicated that the zwitterionic quaternary phenoxide was the reactive species in alkaline solution. The apparent rate of hydrolysis was strongly inhibited by addition of small amounts of product tertiary amine, which is consistent with the presence of an intermediate in the reaction pathway. A mechanism was proposed for the hydrolysis and methanolysis of these compounds involving the reversible formation of the quinone methide, 4-methylene,-2,6-dimethyl-2,5-cyclohexadien-1-one, followed by a trapping reaction with solvent or nucleophiles. Replacement of the phenolic hydrogen with methyl or acetyl groups greatly stabilized the molecule which is in agreement with the proposed mechanism. For the ester, the rate of amine release was limited by specific base catalyzed hydrolysis of the ester group. Compounds of this type may be useful in prodrug design for tertiary amines. The possibility of quinone methine intermediates in the degradation of structurally similar drugs, such as epinephrine, was discussed.

Common ion equilibria of hydrochloride salts and the Setschenow equation.

A simple equation was derived to describe the relationship between the aqueous solubility of sparingly soluble salts (S0) and the empirical Setschenow salting-out constant (k): k = 0.217/S0. This relationship and the Setschenow equation were found to be valid only at low concentrations of added salt. This equation agreed with recently published data when compared for the effect of the chloride ion on the solubility of a series of drug hydrochloride salts. The theoretical treatment also predicts the curvature which has been reported in literature Setschenow plots at higher salt concentrations. As the concentration of added salt increases, the apparent k value is not constant but is dependent on solubility and the rate of change of solubility with added salt concentration. It was concluded that the Setschenow treatment is generally inappropriate for description and analysis of common ion equilibria.

Phase equilibria of nafcillin sodium-water.

The phase diagram for the binary system nafcillin sodium-water, was determined using differential scanning calorimetry (DSC) and polarized light microscopy. In the temperature range of -20-30 degrees, three crystalline forms and an amphiphilic liquid crystalline phase were detected. The stable crystalline form of nafcillin sodium (form alpha) and water exhibit a eutectic mixture containing 28% drug (w/w) at -1 degrees. In more dilute solutions a lower temperature eutectic (-5 degrees) occurs. The composition and form of nafcillin sodium in this eutectic are not known. The form -5 degrees eutectic was found to be metastable above 28% concentration and converted to form alpha. Two other crystalline forms were observed at 9 degrees (form beta) and 22 degrees (form gamma) at concentrations above 40%. The crystalline forms could not be further characterized due to their transient nature and existence in highly concentrated mixtures. A lamellar mesophase is present near ambient temperature in mixtures containing more than 55% nafcillin sodium. The phase equilibria were highly susceptible to supercooling. Temperature cycling methods were devised which gave reproducible DSC data and allowed construction of the phase diagram.

Dissolution rates of doxycycline free base and hydrochloride salts.

The dissolution rates of doxycycline monohydrate, hyclate, and hydrochloride dihydrate crystal forms were investigated using the static pellet method. Solubility product equilibria with chloride ion strongly suppressed the dissolution rate of the hydrochloride dihydrate salt. This form dissolved about fourfold slower in 0.1 N HCl than in water, which was consistent with its solubility in these media. Specificity for chloride was demonstrated by the rapid dissolution rate for the hydrochloride dihydrate in 0.1 N methanesulfonic acid. The dissolution rates of the hyclate, a solvated hydrochloride salt, and the free base were not sensitive to chloride ion. The results show that common ion equilibria with chloride can strongly reduce the dissolution rate of a thermodynamically stable hydrochloride salt form, while the free base or a metastable hydrochloride salt are not similarly affected.

Solubility of doxycycline in aqueous solution.

The solubility of doxycyline monohydrate and doxycycline hydrochloride dihydrate was investigated in aqueous solution. The hydrochloride dihydrate salt was isolated and identified from solutions initially containing doxycycline hyclate in water. The pKa' = 3.09 (mu = 0.1 and 25 degrees) for protonation of doxycycline was determined spectrophotometrically. The pH-solubility profiles were determined for doxycycline monohydrate in water and in 1.0 M NaNO3-HNO3 and NaCl-HCl. The pH-solubility profile at 25 degrees for doxycycline in aqueous hydrochloric acid without added salt reached a sharp maximum fo 50 mg/ml at pH 2.16. Added chloride ion strongly suppressed the solubility of the hydrochloride dihydrate salt. The apparent solubility product was not constant but decreased as the concentration of added salt increased. A theoretical model was developed involving dimerization of doxycycline and applied to the experimental data. The dimerization constant, Kd = 24 M-1, and true solubility product, K0sp = 1.8 X 10(-3) M2, were calculated. The effect of concentration on NMR and visible spectra indicated that dimerization resulted from intermolecular hydrogen bonding of the phenolic beta-diketone portion of the molecule.