The influence of viscosity on the migration of chloramphenicol and 4-hydroxybenzoic acid through glycerogelatin gels

Topical dosage form of valdecoxib: preparation and pharmacological evaluation

Valdecoxib, a selective COX-2 inhibitor, produces serious side effects when given orally. This has led to its withdrawal. Topical application of valdecoxib was formulated and evaluated for its efficacy and safety. Standard procedures were followed and male Wistar albino rats were used to test the anti-inflammatory effect and effect in hyperalgesic conditions. Ointments, creams, and gels containing valdecoxib 1% (m/m) were prepared. These were tested for physical appearance, pH, spreadability, drug content uniformity, in vitro diffusion. Gel prepared using Carbopol 940 (F-X) was selected after the analysis of the results. Formulation F-X was evaluated for acute skin irritancy, anti-inflammatory effect, optimum effective concentration of valdecoxib, effect on hyperalgesia, inhibition of the granulation tissue formation and anti-arthritic effect. Determination of valdecoxib in test animals plasma and determining the blood clotting time and bleeding time were conducted to study the safety of topical valdecoxib. Valdecoxib gel containing 1% (m/m) of the drug was significantly (p < 0.05) more effective in inhibiting hyperalgesia associated with inflammation, compared to placebo gel, but exhibited significantly (p < 0.05) lower suppression of inflammation than commercial rofecoxib gel. Concentration of valdecoxib used in the preparation minimizes the risk of systemic effects, as shown by the analysis of rat plasma for the presence of valdecoxib; hence, this may be the alternative to oral preparations. The bleeding and clotting time showed no significant difference before and after application of F-X.

Microviscosity of hydroxypropylcellulose gels as a basis for prediction of drug diffusion rates

This study investigated the influence of the rheological properties of hydroxypropylcellulose (HPC) gels on the in vitro release of theophylline included in the gel at 0.2 g/l. Experiments were performed with six HPC varieties (mean molecular weight between 5x105 and 1.2x106, nominal viscosity between 100 and 4000 mPa.s) at concentrations of 0-2% (w/w). Theophylline diffusion coefficients at 37 degrees C ranged from 3.5x10-7 to 1.1x10-3 cm2/min, and were in all cases markedly higher than those predicted on the basis of gel macroviscosity as determined by capillary viscometry. In general, the theophylline diffusion coefficient declined exponentially with HPC concentration; in the case of the lowest-molecular-weight HPC, however, the diffusion coefficient remained constant to HPC concentrations of up to 0.8%, probably because of the high entanglement concentration of the HPC. Gel microviscosities as determined by dynamic light scattering (DLS) with latex microspheres (162 nm diameter) were considerably lower than the macroviscosities determined by capillary viscometry, and similar to microviscosities estimated on the basis of theophylline diffusion. Nevertheless, macroviscosity was correlated with microviscosity, suggesting that it is of value for approximate estimates of rates of diffusion of theophylline from HPC gels.

Effect of water mobility on drug hydrolysis rates in gelatin gels

The stability of drugs incorporated in gelatin gels was studied, with a focus on the water mobility in the gels. Trichlormethiazide hydrolysis and kanamycin-catalyzed flomoxef hydrolysis in gelatin gels were chosen as models for apparent first-order and second-order hydrolysis, respectively. The mobility of water in gelatin gels was determined by NMR, ESR, and dielectric relaxation spectroscopies. The amount of bound water in the gels was determined from dielectric relaxation spectra. Spin-lattice relaxation time of water determined by 17O NMR and rotational correlation time of an ESR probe determined by an ESR probing method were useful in determining the microviscosity of the gels. The hydrolysis rate of trichlormethiazide in the gels was found to depend on the amount of free water available for the reaction, while that of flomoxef depended on the microviscosity of the gels, which reflected the mobility of water molecules. Thus the dependence of hydrolysis rates on the water mobility was influenced by the hydrolysis mechanism.

The use of electron spin resonance to measure microviscosity

The viscosities of a series of mixtures of glycerol and water were measured by electron spin resonance (ESR), photon correlation spectroscopy and Ostwald viscometry. Close agreement was obtained between the last two methods but viscosity as measured by ESR was always significantly lower. The difference, the magnitude of which depended on the spin probe used, was attributed to interaction between water and glycerol. Similar results were obtained using water-sorbitol and water-sucrose mixtures.

The effect of gelatin grade and concentration on the migration of solutes into and through glycerogelatin gels

The diffusion of 4-hydroxybenzoic acid and phenobarbitone through glycerogelatin gels was found to be independent of the type of gelatin used. Three types of gelatin, two acid-processed and one alkali-processed were studied, and the bulk viscosities of gels prepared from them was seen to vary considerably. However, the microviscosities of the gels, as measured by ESR, showed no significant differences. Thus microviscosity was the factor governing diffusion. Gelatin concentration in aqueous solutions without glycerol influenced microviscosity and hence diffusion. This is believed to be caused by dissolution of water-soluble fractions of the gelatin. Interstices in the gelatin matrix, though reduced in size when gelatin concentration is raised, are still too large to act as physical barriers to diffusing molecules. It is suggested that hydrated gelatin forms the matrix of glycerogelatin mixtures and that the interstitial fluid, through which migration occurs, consists almost entirely of glycerol and water.