Towards a universal dissolution medium for carbamazepine

The aim of this study was to develop a dissolution medium for assessment of various carbamazepine (CBZ) formulations with different strengths. The design of a system inhibiting transformation of the anhydrous CBZ (CBZ A) to the dihydrate form (CBZ D), with minimum surface-active properties and suitable sink was investigated. The effect of pH, different concentrations of sodium lauryl sulphate (SLS), polyvinylpyrrolidone (PVP), and methyl cellulose (MC) on dissolution rate, solubility, dissolution solubility, and polymorphic transformation of CBZ was assessed. Solution-mediated transformation of CBZ A into CBZ D was monitored using optical microscopy, Fourier transform infrared spectroscopy and differential scanning calorimetry. Results showed that different strengths (100, 200, 400 mg) of the same CBZ tablet formulation exhibited different dissolution patterns, in 1% SLS (USP system). Such differences were reduced in 0.5% SLS solution which provided sufficient sink for up to 200 mg CBZ. It was also shown that solubility of CBZ A could not be detected in the media under study (water, SGF, SIF, and SLS solutions) due to its rapid transformation into CBZ D. The use of 3% PVP solution protected CBZ A from conversion for 75 min, while 0.01% MC completely inhibited the transformation up to 24 h. Therefore, a medium consisting of 0.5% SLS and 0.01% MC was selected. The medium provided: a) protection against transformation of CBZ A to CBZ D, b) increased solubility of CBZ A (204 mg % compared to 128 mg % of CBZ D in 0.5% SLS), c) suitable sink for up to 400 mg CBZ and d) overlapping dissolution profiles of various strengths of the same CBZ formulation. The suggested system may be a step in the way of solving CBZ dissolution problems that forced the USP to specify two similar dissolution tests with two different limits for conventional 200 mg CBZ tablets.

Study of In‐Vitro Release Characteristics of Carbamazepine Extended Release Semisolid Matrix Filled Capsules Based on Gelucires

Various extended release carbamazepine (CBZ) formulations have been developed previously, in order to reduce the frequency of dosing in chronic therapy and to decrease the variability in drug plasma concentration. In the present study, the suitability of different grades of Gelucires (G, glyceride based excipients) to formulate CBZ extended release capsules by the application of semisolid matrix (SSM) filling capsule technology was investigated. The possible modification of CBZ release kinetics by using Gelucire blends or inclusion of hydrophilic additives in the SSM was studied. The effect of ageing on some selected formulations was also evaluated, using scanning electron microscopy and differential thermal analysis. Twenty-one capsule formulations were prepared and assessed for their release characteristics. The mechanism of drug release from the test formulations was studied. The following results were obtained: a) Release data could not be correlated to the melting point (mp) of Gelucires used, pointing to relative lipophilicity of the base as a more important determinant of drug release. Among Gelucire grades having melting points higher than 37 degrees C, the release rate proved to be highly dependent on the HLB value and matrix composition. b) CBZ release occurred by different mechanisms, including matrix disintegration, diffusion and or erosion depending on the vehicle employed. c) Zero order release profiles of CBZ were obtained from SSM-based on G50/13, G53/10 and their blends in ratios higher than 1:1 and G53/10 containing croscarmellose sodium. d) The ageing study revealed that these latter formulations, except those based on G50/13, also showed high dissolution stability during one year of shelf ageing. e) PVP, as a polymorphic transformation inhibitor, can be used to reduce the storage-induced changes of some grades of Gelucires. From the above data, it can be concluded that different grades of Gelucires and their blends as well as hydrophilic additives could be successfully used to formulate CBZ extended release SSM filled capsules with various release kinetics.

Semisolid matrix filled capsules: an approach to improve dissolution stability of phenytoin sodium formulation

Seven semisolid fill bases were selected for the formulation of 24 capsule formulations, each containing 100 mg of phenytoin sodium. The fill materials were selected based on the water absorption capacity of their mixtures with phenytoin sodium. The fill matrices included lipophilic bases (castor oil, soya oil, and Gelucire (G) 33/01), amphiphilic bases (G 44/14 and Suppocire BP), and water-soluble bases (PEG 4000 and PEG 6000). The drug:base ratio was 1:2. Excipients such as lecithin, docusate sodium, and poloxamer 188 were added to some formulations. The dissolution rate study indicated that formulations containing lipophilic and amphiphilic bases showed the best release profiles. These are F4 (castor oil-1% docusate sodium); F10 (castor oil-3% poloxamer 188); F14 (G33/01-10% lecithin); F17 (G33/01-1% docusate sodium), and F20 (Suppocire BP). Further, the dissolution stability of the five formulations above was assessed by an accelerated stability study at 30 degrees C and 75% RH using standard Epanutin capsules for comparison. The study included the test and standard capsules either packed in the container of marketed Epanutin capsules (packed) or removed from their outer pack (unpacked). Release data indicated superior release rates of castor oil based formulations (F4 and F10) relative to standard capsules in both the unpacked and packed forms. For instance, the extent of drug release at 30 min after 1 month was 91% for F4 and F10 and 20% for standard capsules. Drug release from packed capsules after 6 months storage was 88% for both formulations F4 and F10 and 35% for standard capsules. In conclusion, the pharmaceutical quality of phenytoin sodium capsules can be improved by using a semisolid lipophilic matrix filled in hard gelatin capsules.

Examination of the Sephadex G10 beads uptake method for determination of bile salt monomer concentration in taurocholate-lecithin solutions

Experiments have been undertaken to examine the validity of the method of Ammon and Walter for the determination of bile acid monomer concentrations in taurocholate-lecithin (TC-L) solutions. The procedure involves, basically, the uptake of the bile acid monomer by Sephadex G10 beads (exclusion limits 700 Da) in the presence of a suitable aqueous phase marker (raffinose). Several of the experimental variables involved in the bead-washing technique have been critically studied, and good precision in data (SD in the range of +/- 2-5% of the mean value) has been obtained. Monomer concentrations determined in the absence of lecithin over a wide range (10-70 mM) of TC were found to be in agreement with those obtained by Ammon. Most important, however, is the finding that TC monomer concentrations at constant NMBS were found to be constant in all solutions containing or not containing lecithin [here, the nonmixed micelle bile salt (NMBS) is the total bile salt concentration, but not including species involving lecithin, and is determined by dialysis equilibrium experiments].

Aspirin stability in solid dispersion binary systems

The stability of aspirin in its solid dispersion with urea or povidone was investigated at two accelerated storage conditions. The observed aspirin degradation in both systems followed the first-order rate equation. The water sorption ability of the two carriers as well as the alkalinity imparted by urea could possibly be the most important factors responsible for the observed acceleration of aspirin decomposition. The results also showed that the temperature effect was more pronounced than the humidity effect. Generally, coprecipitated samples exhibited slightly higher degradation rates than physically mixed ones.

Effect of magnesium trisilicate and citric acid on the biovailability of tetracycline in man

Oral absorption of tetracycline hydrochloride by human subjects was compared with its absorption when coadministered with magnesium trisilicate, citric acid or magnesium trisilicate-citric acid mixture. Evaluation of the absorption rate was done by means of urinary excretion measurements. The concomitant administration of magnesium trisilicate with tetracycline hydrochloride resulted in a dramatic reduction of the excretion rate of the drug. Ingestion of citric acid with tetracycline showed no significant effect on its absorption. Citric acid administration simultaneously with tetracycline hydrocloride-magnesium trisilicate combination failed to improve the absorption of tetracycline in the presence of magnesium trisilicate contrarily to preveious in vitro results. Conclusions were made that in vitro experiments are not always successful in the prediction of in vivo findings.

The effect of dihydroxypropyl theophylline on the solubility and stability of menadione (vitamin K3)

The effect of dihydroxypropyl theophylline on the solubility and stability of menadione was investigated. An increase of about 4-fold in the solubility of menadione with 0.1 m dihydroxypropyl theophylline was observed at 30 degrees C. Thermodynamic paremeters were calculated by conducting the experiments at different temperatures. The rate of base-catalyzed degradation of menadione in the presence and absence of dihydroxypropyl theophylline was found to be first order with respect to the vitamin at all conditions of pH, temperature and concentration. Dihydroxypropyl theophylline leads to an accelerated rate of decomposition of the drug at relatively high pH values. This adverse effect decreases considerably as the pH of the solution approaches neutrality. At pH = 7.5, dihydroxypropyl theophylline exerts a rather stabilizing effect on menadione solution. The enhancement of the rate of base-catalyzed degradation is directly proportional to the concentration of dehydroxypropyl theophylline at pH = 9.2. The base-catalyzed degradation of menadione in the absence and presence of dihydroxypropyl theophylline follows Arrhenius' equation for the thermal activation of molecules. No noticeable variation in the energy of activation of both systems was detected. The addition of 4% dihydroxypropyl theophylline to menadione solution nearly completely suppressed the aerobic photodegradation of menadione.

The in-vitro adsorption of some antirheumatics on antacids

The adsorption of sodium salicylate, salicylamide, acetylsalicylic acid, paracetamol, mefenamic acid, flufenamic acid, phenylbutazone, oxyphenbutazone, phenazone, aminophenazone, indometacin and methiazinic acid on some antacids was studied. The antacids used were magnesium trisilicate, magnesium oxide, aluminium hydroxide, bismuth oxycarbonate, calcium carbonate and kaolin. Magnesium oxide, followed by aluminium hydroxide and bismuth oxycarbonate showed a fairly high adsorptive capacity for salicylates, mefenamic acid, flufenamic acid, methiazinic acid, indometacin and to a lesser extent for phenylbutazone and oxyphenbutazone. On the other hand, magnesium trisilicate exhibited a tendency to adsorb phenazone, aminophenazone, indometacin and methiazinic acid. Kaolin was found to be a good adsorbent for anthranilic acid derivatives, indometacin and methiazinic acid. Calcium carbonate showed a weak adsorptive capacity for all drugs tested. The adsorption of phenylbutazone and salicylates on magnesium oxide, aluminium hydroxide and/or bismuth oxycarbonate obeyed the Freundlich adsorption isotherm. Elution study showed that salicylates and anthranilic acid derivatives were tenaciously held by magnesium oxide while magnesium trisilicate showed an intermediate retention power for phenazone and aminophenazone. Sodium hydrogen carbonate solution gave, in general, a higher eluting power than hydrochloric acid solution. A marked reduction in the apparent partition coefficients of all drugs tested was observed in the presence of magnesium trisilicate or aluminium hydroxide. Careful in vitro and in vivo testing of drug availability is advisable prior to the concomitant administration of antirheumatics with antacids or other adsorbents.

Physicochemical study of drug binary systems. Part 3: Tolbutamide-urea and tolbutamide-mannitol systems

The phase diagram of the tolbutamide-urea system, constructed by data obtained from the microheating table apparatus, was a peritectic type. It showed the possibility of the formation of two molecular compounds with incongruent m. p. at 112 degrees C and 123 degrees C. Metastable and stable eutectic points were observed at 63 degrees C and 92 degrees C respectively. The phase diagram of tolbutamide-mannitol system revealed that in addition to eutectic liquefaction at 102 degrees C and 6% (w/w) mannitol, there was a region [40--80% (w/w) mannitol] in which the two components were not completely miscible in the liquid state. Solubility studies showed an increase of about 2.5 fold in the solubility of tolbutamide with 0.5 g% urea and suggested the possibility of the formation of a low-solubility complex in addition to a soluble higher complex of the two components. No obvious increase in the solubility of tolbutamide was detected in the presence of mannitol. A full description of the nature and composition of the final solidified melts of the two present systems could prove to be helpful in the explanation of the dissolution behaviour of the different studied samples and their corresponding physical mixtures. The fused 90% urea and mannitol solid dispersions showed a fast rate of dissolution in the order of 15 and 10 times (respectively) greater than pure tolbutamide.