James J. Fort

Solubility and stability characterization of zileuton in a ternary solvent system

Abstract Zileuton (ABT-077) is a 5-lipoxygenase inhibitor with poor aqueous solubility. Using an extreme vertices experimental design and an empirical response surface analysis, both the solubility and pseudo-first-order degradation rate constants of zileuton were determined in a ternary solvent system consisting of water, ethanol and propylene glycol. The logarithm of the solubility increased curvilinearly with increasing weight fraction ethanol or weight fraction propylene glycol in the solvent system and decreased linearly with increasing dielectric constant of the medium. The degradation rate increased linearly with increasing weight fraction ethanol or weight fraction propylene glycol and the logarithm of the degradation rate constant increased linearly with respect to the reciprocal of the dielectric constant of the medium. The degradation products observed were the hydroxylamine, syn - and anti -oximes of zileuton, implying that a solvolysis reaction at the amide bond occurs in all solvent compositions.

Effects of epidermal/dermal separation methods and ester chain configuration on the bioconversion of a homologous series of methotrexate dialkyl esters in dermal and epidermal homogenates of hairless mouse skin

Abstract A comparison has been made regarding various methods of hairless mouse skin dermal/epidermal separation techniques using criteria such as soluble protein yield and rate of hydrolysis per unit soluble protein, as well as effectiveness of separation, in which histological techniques were employed. Also compared were the relative esterase activities of dermal and epidermal layers, in which the dermis exhibited higher activity. A homologous series of methotrexate dialkyl esters were studied as to their pseudo-first order degradation in homogenates of dermis and epidermis of hairless mouse skin. A logarithmic relationship between pseudo-first order rate constant and alkyl chain length was observed for both dermal and epidermal homogenates. The degree of bioconversion of the diesters was also studied and the enzymatic reaction failed to proceed beyond the level of the monoalkyl esters. Examination of the selectivity for the formation of α- and γ-monoalkyi products revealed that γ-monoester appeared to predominate over α-monoester in the case of the branched-chain dialkyl ester.

Solubility and stability characteristics of a series of methotrexate dialkyl esters

A series of methotrexate (MTX) dialkyl esters were examined for their solubility in an aqueous and a co-solvent medium (20%) (Co-solvent solubility-stability relationship; Dimethylformamide/water co-solvent (DMF)-aqueous buffer) as a function of pH. The pH-solubility relationship showed an unusual increase at pH values above the pK8 of these weak bases. The stability of this series of compounds was also studied in the same 20% DMF medium. The rate constant for diester loss decreased as the series was ascended and additional stability resulted from the branching of the diisopropyl ester. Also, the stability of the MTX methyl ester was studied as a function of pH, with the resulting pH-stability profile showing a typical V-shaped pattern. The maximum aqueous stability of dimethyl ester was shown to be between pH 4 and 5.

Physicochemical properties and chromatographic behavior of a homologous series of methotrexate-α-γ-dialkyl ester prodrugs

Abstract A homologous series of 5 α, γ-dialkylester prodrugs of methotrexate (dimethyl through dipentyl) have been synthesized utilizing an acid-catalyzed direct esterification procedure. A high-performance liquid chromatographic (HPLC) method for separating each diester from its corresponding α- and γ-monoester mixture and methotrexate utilizing a pH 3 buffer solution/acetonitrile combination has been developed. The physicochemical properties of each diester including their chromatographic capacity factors and octanol/dimethyl formamide-water partition coefficients have been determined as well as the correlation between these two parameters. The effect of chain length and mobile phase composition on the capacity factors is shown. The methylene group contribution to both capacity factors and partition coefficients have been calculated. Also, the thermodynamic significance of these findings, based on free energy calculations, is discussed. From the data obtained in these experiments evolves a discussion of the possible application of these compounds to the topical treatment of psoriasis.

Transport and degradation characteristics of methotrexate dialkyl ester prodrugs across tape-stripped hairless mouse skin

Abstract A series of methotrexate dialkyl esters were examined with respect to their permeability across tape-stripped hairless mouse skin. The dialkyl esters showed a parabolic permeability versus side chain length relationship with the dimethyl ester being the most permeable compound. These compounds were also found to undergo an increased degree of degradation with increased ester chain length during the diffusion process, while with substantially reduced degradation occuring with the branched chain diisopropyl ester. No measurable methotrexate was formed during the course of the experiment, apparently due to the chemical and enzymatic stability of the intermediate α- and -γ-monoesters.

Effect of Physicochemical and Formulation Variables on the In Vivo Absorption of ABT-761

ABSTRACT ABT-761 is a 5-lipoxygenase inhibitor developed for the treatment of asthma. The present study was undertaken to evaluate different crystal forms of ABT-761 and their impact on in vitro and in vivo performance in capsule formulations. Two crystal forms of ABT-761, hemihydrate and non-solvate from different sources, were characterized by thermal analysis, x-ray powder diffraction, moisture sorption, and intrinsic dissolution studies. An in vitro test was performed to assess the effect of formulation and drug from different sources on drug release. Crossover design was also used to evaluate oral bioavailability of ABT-761 hemihydrate formulations in beagle dogs. Plasma concentrations of ABT-761 were analyzed using a reverse-phase high-performance liquid chromatography (HPLC) assay. It was found that in vivo oral absorption as well as in vitro dissolution of ABT-761 were influenced by different formulations. Capsule formulations of ABT-761 hemihydrate are bioequivalent to the solution formulation in terms of extent of absorption, but formulation and the method of granulation preparation can have a major impact on the absorption rate. In conclusion, a single crystal form of ABT-761, i.e., hemihydrate, is preferred for subsequent product development. However, exposure of the drug to conditions that may facilitate phase transformation should be avoided.

Transport of Methotrexate Dialkyl Ester Prodrugs Across Full-Thickness Hairless Mouse Skin

AbstractThe permeability coefficients of methotrexate (MTX) and a series of its mono and dialky1 ester prodrugs across full-thickness hairless mouse skin have been determined using an in vitro diffusion method. MTX was found to permeate across this skin model with a mean permeability of 5.6x10-5 cm/hr. Monoesterification only slightly improved MTX permeation and the effect appeared to be invariant with alky1 chain length. Diesterification of MTX, on the other hand, resulted in derivatives prone to simultaneous conversion during permeation by both skin esterase and chemical hydrolysis in the diffusion medium. The contribution of conversion to overall MTX diffusion was treated mathematically in order to calculate the intrinsic permeabilities of MTX dialky1 esters. It was found that the dialky1 esters showed a parabolic permeability versus chain length relationship with the dimethy1 ester being the most permeable compound.

Kinetics of degradation in aqueous solution of Abbott-79175, a potent second generation 5-lipoxygenase inhibitor

Abstract The degradation kinetics of Abbott-79175 in aqueous solution have been studied as a function of pH. Concentration/time plots indicated a pseudo-first order nature of reactions throughout the pH range studied. Additionally, the effects of temperature, ionic strength, and buffer concentration have been examined. From multiple temperature experiments, Arrhenius and activation parameters were calculated. Furthermore, it was determined that upon ionization, Abbott-79175 degradation proceeded independently of ionic strength. These data in addition to the plateau-like nature of the pH-rate constant profile above pH 10 suggest a lack of participation of hydroxide ion during the reaction. This behavior in the neutral and alkaline regions was qualitatively very similar to that of zileuton, a 5-lipoxygenase inhibitor in phase III clinical trials. In addition to allowing the determination of the buffer independent rate constants, kinetic studies as a function of buffer concentration allowed in some of the systems the deduction of which buffer species were catalytic. A multi-parameter model was fitted to the pH buffer independent rate constant data using non-linear regression. This modeling yielded parameters such as the microscopic rate constants and the p K a under the aforementioned conditions. From the pH-rate constant profile, Abbott-79175 was found to be more labile than zileuton throughout the pH range studied. This difference was greater than three orders of magnitude at pH 1. Such acid lability produced a pH profile which had a much narrower region of maximum stability.

Development of a physical model for simultaneous diffusion/bioconversion of methotrexate dialkyl esters: A potential parallel/sequential metabolic scheme☆

Abstract A physical model has been developed which accommodates simultaneous diffusion/parallel-sequential bioconversion process through hairless mouse skin. The model includes both tape-stripped and full-thickness skin to describe the diffusion/bioconversion of methotrexate (MTX) dialkyl esters, in which a potential parallel/sequential esterase metabolic scheme is operative. Simulations are also included to illustrate the behavior of MTX dialkyl esters with the simultaneous diffusion/bioconversion parameters in which the metabolism stops at the level of MTX monoalkyl esters.