The adsorption of cellulose ethers in aqueous suspensions of pyrantel pamoate: effects on zeta potential and stability

This work examined the physico-chemical phenomena induced in aqueous suspensions of pyrantel pamoate by two varieties of hydroxypropylmethylcellulose (HPMC) and sodium carboxymethylcellulose (NaCMC) of different molecular weights, and the effects of these phenomena on the physical stability of the suspension. The mechanism of the interfacial adsorption of the polymer was investigated by constructing adsorption isotherms: for the two HPMC varieties, the isotherms were of type L and were fitted with the Langmuir model; of the NaCMCs, only the variety with higher molecular weight was adsorbed, its adsorption isotherm being of type S (sigmoidal). The resulting monolayer films were characterized viscosimetrically, determining their thickness and the number of polymer molecules adsorbed per unit area. The nonionic polymers formed thinner, more continuous monolayers than the NaCMC. Only the nonionic polymers significantly altered the zeta potential of the systems. In the range of conditions studied, all the polymers stabilized the initially flocculated systems, decreasing sedimentation volume and increasing the time necessary to redisperse them (the redispersability value). This stabilization occurred either by the steric mechanism (HPMCs and the high-molecular-weight NaCMC) or by depletion mechanisms (low-molecular-weight NaCMC). Owing to the complexity of these mechanisms, sedimentation volume was not found to be a useful index of the consistency of the sediments obtained from the suspensions.

Nifedipine/atenolol interactions in gastrointestinal absorption and biotransformation

The combination of nifedipine and atenolol is widely used for the treatment of hypertension. In the present study, experiments performed in rats indicated that neither drug affects the gastric or intestinal absorption of the other. In assays of the biotransformation of nifedipine in liver homogenates, breakdown was much more rapid in homogenates from male rats than from female rats. In the presence of atenolol, the breakdown rate was significantly increased in homogenates from male rats, and significantly reduced in homogenates from female rats.

[Elimination of viscoelastic substances from the vitreous cavity. Comparative study of sodium hyaluronate and hydroxypropylmethylcellulose]

We study the elimination of two viscoelastic substances from the vitreous cavity: the Hydroxypropyl Methylcellulose and the Sodium Hyaluronate that are potentially vitreous substitutes. We performed gas-vitrectomy in 116 eyes of 58 rabbits. Three days after surgery we performed gas-viscoelastic substance exchange. We analyzed the concentration of Hydroxypropyl Methylcellulose and Sodium Hyaluronate with diphenylamine reaction at different periods: zero, 1 week, 2 weeks, 1 month till 6 months. In both cases the elimination is fast so that 1 week after the intravitreal injection remains only 60.1% of the Hydroxypropyl Methylcellulose and 100% of Sodium Hyaluronate, and two weeks after 38.5% of Hydroxypropyl Methylcellulose and 62.5% of Sodium Hyaluronate. We conclude that both substances have a short half life-time specially in the case of the Hydroxypropyl Methylcellulose.

Molecular weight dependence of the pharmacokinetic of hydroxypropyl methylcellulose in the vitreous

The influence of molecular weight on the elimination kinetics of the potential vitreous substitute hydroxypropyl methylcellulose (HPMC) was studied using solutions of equal viscosity (15,000 cps) of HPMC of molecular weight 86,000 daltons (HPMC 86,000) and HPMC of molecular weight 120,000 daltons (HPMC 120,000). Three days after gas vitrectomy of the right eyes of 64 rabbits, the gas was replaced by 0.5 ml of HPMC solution (2.5% HPMC 86,000 in 39 and 2% HPMC 120,000 solution in 25); HPMC concentration was determined immediately afterwards and 7, 14, 30, 60 and 90 days later by the diphenylamine method, the left eyes being used as blanks. HPMC elimination exhibited order one kinetics, with a half life of 10.4 days for HPMC 86,000 and 38.4 days for HPMC 120,000. These results suggest that by choosing HPMC of suitable molecular weight it may be possible to control the residence time of the polymer in the vitreous cavity.

Use of the crushing strength parameter for quality control of phenobarbital-microcrystalline cellulose tablets

Three variables of the tablet manufacturing process (duration of mixing with lubricant, maximum compression force and compression rate) were studied for their effects on various properties of direct compression phenobarbital-microcrystalline cellulose tablets. Mixing time and maximum compression force were found to have a marked influence on friability and dissolution rate; crushing strength was found to be a useful parameter for quality control.

Ocular penetration and pharmacokinetics of cefotaxime: an experimental study

Cefotaxime levels in the cornea and the aqueous humour were assayed after local topical application and subconjunctival injection in healthy and infected corneas. The results in infected corneas were analysed pharmacokinetically. The absorption and elimination of cefotaxime within 2 hours after topical administration are adequately predicted by a two-compartment model in which one compartment represents the cornea and the other the aqueous humour, with the elimination taking place from the latter. The transfer coefficient kc (cornea-aqueous) was estimated as 1.452 h-1 and the return transfer coefficient kca (aqueous-cornea) as 0.287 h-1. After subconjunctival injection the levels measured exhibit bi-exponential kinetics, with the apparent absorption constant Ka = 3.816 h-1.

Limitations of the MANOVA of statistical moments in bioequivalence trials of drugs with slow elimination rate: a study with spironolactone

The capacity of the statistical moments for detecting bioavailability changes in bioequivalence studies of drugs with high disposition mean residence time (MRT) is analysed. Two bioavailability trials were carried out with spironolactone in which extent and rate of absorption were modified in a controlled way. The statistical analysis employed was a two-way MANOVA of statistical moments of the urinary excretion profiles of canrenone. The results indicate a loss in the sensitivity of MRT for detecting changes in absorption rate as drug elimination rate decreases.

Application of pelletized sodium borohydride in the spectrophotometric determination of arsenic

The spectrophotometric determination of arsenic using sodium borohydride in pellet form, with excipients as the reducing agent and arseine former, has been studied. The use of pelletized sodium borohydride allows the formation of arsenic diethyldithiocarbamate and enables an accurate application of the method without the use of other reagents. The gradual reducing action of the pelletized sodium borohydride, together with the release of hydrogen thus forming arsine with As(III), is the main advantage of this procedure. The arsine thus formed bubbles through the chloroform solution of silver diethyldithiocarbamate to form the complex that allows the determination of the arsenic by visible spectrophotometry.