Biomedical subjects

H G Ibrahim

Publications and source records attributed to H G Ibrahim.

6 recordsLinked to original sources

PubMed2001

Release of salicylic acid, diclofenac acid and diclofenac acid salts from isotropic and anisotropic nonionic surfactant systems across rat skin.

Release of salicylic acid, diclofenac acid, diclofenac diethylamine and diclofenac sodium, from lyotropic structured systems, namely; neat and middle liquid crystalline phases, across mid-dorsal hairless rat skin into aqueous buffer were studied. Release results were compared with those from the isotropic systems. The donor systems composed of the surfactant polyoxyethylene (20) isohexadecyl ether, HCl buffer of pH 1 or distilled water and the specific drug. High performance liquid chromatography (HPLC) methods were used to monitor the transfer of the drugs across the skin barrier. Results indicated that the rate-determining step in the transport process was the release of the drug from the specified donor system. Further, apparent zero order release was demonstrated with all systems. Except for diclofenac sodium, drug fluxes decreased as the donor medium changed from isotropic to anisotropic. The decrease in fluxes was probably due to the added constrains on the movement of drug molecules. By changing the anisotropic donor medium from neat to middle phase, drug flux decreased in case of salicylic acid and diclofenac sodium. In the mean time, flux increased in case of the diethylamine salt and appeared nearly similar in case of diclofenac acid. Rates of drug transfer across the skin from the anisotropic donors seemed to be largely controlled by the entropy contribution to the transport process. The type and extent of drug-liquid crystal interactions probably influenced the latter.

Animals↗

PubMed1993

Effects of solute characteristics and concentration on a lyotropic liquid crystal: solute-induced phase change.

We investigated the effects of increased concentrations of the solutes, salicylic acid, benzoic acid, and o-, m-, and p-methoxy benzoic acids, on the anisotropic properties of a liquid crystal solvent. The lamellar liquid crystal was composed of 37% polyoxyethylene (20) isohexadecyl ether in aqueous buffer of pH 1. Phase change, transition temperature, refractive index, and specific resistance of the mesophase were studied in the presence of solutes. Transfer rates of the solutes from the bulk mesophase into aqueous buffer across a lipoidal barrier were used to determine their apparent permeability coefficients. The results indicate that a phase change occurred in the liquid crystal from a lamellar to a hexagonal structure, in the case of salicylic, benzoic, and m-methoxy benzoic acids. However, o- and p-methoxy benzoic acids showed no effect on the packing arrangement of the liquid crystal in the concentration range studied. The occurrence of the phase change was both solute and concentration dependent. Relative values of apparent permeability coefficients of solutes reflected the extent of solute-solvent interactions in the systems.

Anisotropy↗

PubMed1989

Release studies from lyotropic liquid crystal systems.

A temperature-composition diagram of systems containing the nonionic surfactant polyoxethylene (20) isohexadecyl ether (1) and water was established. Three different anisotropic areas existed over the concentration range 33-80% of 1. Release of salicylic acid from liquid crystalline phases of 1 into aqueous buffer across lipoidal barriers was also studied. Rates of transfer as a function of percent loading from a neat mesophase containing 37% of 1 and also from systems with different molecular packing were determined. Apparent activation energies of transfer from an ordered solvent containing 37% of 1 and from an isotropic medium of the same chemical composition were found to be 33.3 and 9.4 kcal/mol, respectively. These findings suggest a pronounced effect of medium structure on the interfacial resistance of the barrier.

Chemical Phenomena↗

PubMed1985

Observations on the dissolution behavior of a tablet formulation: effect of compression forces.

Two test drugs, hydrochlorothiazide and phenylbutazone, were separately incorporated into a standard formulation to study their disintegration and dissolution properties as a function of compression force. The increase in dissolution with force was attributed to the manner in which tablets disintegrated while dissolving. At a fixed press setting, tablets from the same compression cycle showed variations in their dissolution which were in agreement with the observed effects of force on dissolution. A linear correlation between dissolution efficiency and the logarithm of force was found to exist over the compression range studied.

Drug Compounding↗

PubMed1977

Polymorphism of phenylbutazone: properties and compressional behavior of crystals.

Data from X-ray diffraction, thermal analysis, IR spectroscopy, and solubility studies were used for the identification and characterization of four crystalline modifications of phenylbutazone. The thermal behavior of the polymorphs under different treatment conditions also was investigated. Compression of the thermodynamically unstable forms, at a compression force of 1590--2040 kg, induced polymorphic changes in the crystals. Similar changes also were produced through grinding. The apparent equilibrium solubilties of polymorphs were determined, as was the dissolution of the polymorphs as compressed disks in an aqueous medium. The small effective surface area possessed by one polymorph resulted in slow dissolution.

Calorimetry, Differential Scanning↗

PubMed1976

Microenvironmental kinetic effects within a lyotropic smectic biophase model: conformational restrictions in Fischer indole cyclization.

The microenvironmental orientation effects, arising from an orderer solvent structure, were studied in a model liquid crystalline biophase for the cyclization of a series of 2-substituted cyclohexanone phenylhydrazones. The magnitude of such solvent-induced intramolecular conformational constraints were determined from a comparison of the kinetics of the Fischer indole rearrangement in a lyotropic smectic liquid crystal versus those in an isotropic liquid of similar chemical composition but lacking the structured nature of the mesophase. Solutions consisting of 50% (w/w) polyoxyethylene 6 tridecyl ether or 44% (w/v) polyethylene glycol in aqueous buffers comprised the smectic or isotropic media, respectively. The apparent dissociation constants of the conjugate acids of the phenylhydrazones were determined kinetically, as were their partition coefficients between lipid and polar isotropic phases approximating the compositions of the smectic lamellae. Intrinsic first-order rate constants, corrected for partitioning within the lamellar mesophase, were used to compute the enthalpies and entropies of activation. The somewhat slower intrinsic rates of cyclization and the accompanying less negative entropies of activation generally observed in the liquid crystalline medium, as opposed to the isotropic system, are attributed to the orienting effects of the lamellar lyotropic mesophase.

Chemical Phenomena↗