Pharmaceutical applications of microcalorimetry

Toxicity of nickel to soil microbial community with and without the presence of its mineral collectors-a calorimetric approach

The toxicity of nickel and three of its main collectors, sodium isopropyl xanthate (SIPX), sodium ethyl xanthate (SEX), and potassium ethyl xanthate (PEX) to soil microbial activity, was analyzed, individually and as a binary combination of nickel and each of the collectors. The investigation was performed through the microcalorimetric analysis method. For the single chemicals, all power-time curves exhibited lag, exponential, stationary, and death phases of microbial growth. Different parameters exhibited a significant adverse effect of the analyzed chemicals on soil microbial activity, with a positive relationship between the inhibitory ratio and the chemical dose (p < 0.05 or p < 0.01). A peak power reduction level of 24.23% was noted for 50 μg g^-1 soil in the case of Ni while for the mineral collectors, only 5 μg g^-1 soil and 50 μg g^-1 soil induced a peak power reduction level of over 35 and 50%, respectively, in general. The inhibitory ratio ranged in the following order: PEX > SEX > SIPX > Ni. Similar behavior was observed with the mixture toxicity whose inhibitory ratio substantially decreased (maximum decrease of 38.35%) and slightly increased (maximum increase of 15.34%), in comparison with the single toxicity of mineral collectors and nickel, respectively. The inhibitory ratio of the mixture toxicity was positively correlated (p < 0.05 or p < 0.01) with the total dose of the mixture. In general, the lesser and higher toxic effects are those of mixtures containing SIPX and PEX, respectively.

Schistosoma mansoni: antischistosomal activity of the four optical isomers and the two racemates of mefloquine on schistosomula and adult worms in vitro and in vivo

Recent studies have shown that mefloquine (MQ) reveals interesting antischistosomal properties. We examined the antischistosomal activities of the erythro and threo isomers and racemates of MQ on newly transformed schistosomula (NTS) and adult Schistosoma mansoni in vitro and in mice harbouring adult S. mansoni. The in vitro effects in the presence and absence of haemin were monitored by means of microcalorimetry, scanning electron microscopy and phenotypic evaluation. Incubation of NTS with the erythro derivatives at concentrations of 3 μg/ml and above resulted in convulsions, granularity, decrease in heat flow, and death while NTS incubated with the threo derivatives were only affected at high concentrations (100 μg/ml). Extensive tegumental alterations, decrease in metabolic activity, viability, and death were observed when adult schistosomes had been exposed to 10 μg/ml of the erythro compounds. Moderate tegumental and viability changes but reduced heat production rates were observed with the threo derivatives at 10 μg/ml. In the presence of haemin, all MQ derivatives showed pronounced antischistosomal properties against adult S. mansoni in vitro. In vivo, MQ derivatives achieved statistically significant total and female worm burden reductions ranging between 65.4% and 100%. The highest total worm burden reductions of 93.4% and 90.2% were observed following treatment with the erythro and threo racemates, respectively. In conclusion, the optical isomers and racemates of MQ show only moderate stereoselectivity, in particular in vivo. Our results may enhance our understanding of the mechanism of action and therapeutic profile of MQ derivates on schistosomes.

Thermogravimetric analysis for the determination of water release rate from microcrystalline cellulose dry powder and wet bead systems

The purpose of this work was to determine the total amount of water contained in dry powder and wet bead samples of microcrystalline cellulose, MCC, (Avicel PH-101), taken from various stages of the extrusion/marumerization process used to make beads and to determine the kinetic rates of water release from each sample. These samples were allowed to equilibrate in controlled humidity chambers at 25 degrees C. The total amount of water in each sample, after equilibration, was determined by thermogravimetric analysis (TGA) as a function of temperature. The rates of water release from these samples were determined by using isothermal gravimetric analysis (ITGA) as a function of time. Analysis of the results for these studies suggest that water was released from these systems by several different kinetic mechanisms. The water release mechanisms for these systems include: zero order, second order, and diffusion controlled kinetics. It is believed that all three kinetic mechanisms will occur at the same time, however; only one mechanism will be prominent. The prominent mechanism was based on the amount of water present in the sample.

Evaluation of the moisture sorption behaviour of several excipients by BET, GAB and microcalorimetric approaches

Moisture content of solid-state pharmaceutical products is one of the main factors that affect drug stability, therefore suitable sorption studies need to be performed to assure drug quality throughout their shelf life. The Brunauer-Emmett-Teller (BET) and the Guggenheim-Anderson-de Boer (GAB) models are usually used for this purpose. Using gravimetrically obtained data, both methods were applied in the present work to evaluate the sorption characteristics of several excipients. Microcalorimetric analysis was also performed in order to evaluate the interaction between water and the substances. The results of these experiments show excellent agreement between data and the BET model up to 55% RH and the GAB model over the entire humidity range, confirmed by high values of the statistical determination coefficients. Furthermore, microcalorimetric measurements suggested that the hygroscopicity of solid materials could be estimated approximately using this approach.

Determination of the physical state of norethindrone acetate containing transdermal drug delivery systems by isothermal microcalorimetry, X-ray diffraction, and optical microscopy

Transdermal drug delivery systems (TDDS) enable a controlled drug delivery to the skin. The low permeability of the stratum corneum necessitates a high drug concentration of the polymeric matrix and often requires supersaturation. This, however, promotes crystallisation of supersaturated systems. Isothermal microcalorimetry at 25 degrees C, polarisation light microscopy, and X-ray powder diffraction (XRPD) were used to characterise the crystal growth of norethindrone acetate (NEA). The solubility of NEA in the patches determined by these methods is about 4%. The crystallisation process could be measured reliably and with a high accuracy by microcalorimetry and microscopy. XRPD was considerably less sensitive but was the only method allowing a semi-quantitative determination of the amounts of crystals formed. The drug-associated heat measured by microcalorimetry increased proportionally with increasing NEA concentration in the concentration range of 4-10% demonstrating a constant crystallisation rate. At a higher supersaturation, such as 12% drug content, the crystallisation process was accelerated. The application of Johnson-Mehl-Avrami kinetics for the analysis of nucleation and crystal growth of the punched patches indicated a site-saturated nucleation mechanism and a one-dimensional crystal growth. The crystallisation enthalpy of NEA was -22.8+/-2.6 kJ/mol. The most specific method to observe the crystal formation is polarisation light microscopy. However, the microscopic analysis requires much longer storage times than microcalorimetry to detect crystallisation.

Flow microcalorimetric studies of phenol and its chlorinated derivatives and a theoretical evaluation of their possible inhibition mode on Chromobacterium violaceum respiration

The general belief that chemical structure determines the biological effect of drugs has led to several techniques to establish structure-activity relationships (SAR) that is useful in the development of more active compounds. Predicting toxic effects based on SAR, one can obtain toxicological data with a low cost-benefit ratio. Chlorophenols that represent a class of toxic agents frequently used in industrial processes are not satisfactorily described in the literature in relation to their toxicity. The main objective of this work is to relate the microbial activities of phenol, anisole and their chlorinated derivatives on Chromobacterium violaceum respiration with their physicochemical properties. Anisole and its chlorinated derivatives were used to evaluate the influence of phenol acidity on biological activity. The calculations were carried out at the semi-empirical AM1 and ab initio DFT levels employing the basis sets CEP-31G, CEP-31+Ge CEP-31G** that were parameterized using the continuum-solvation model COSMO for solvent contribution. Both empirical and theoretical properties were evaluated by chemometric analyses (hierarchical cluster analysis (HCA) and principal component analysis (PCA)), to correlate the physicochemical properties of the phenol, anisole and their chlorinated derivatives with their biological activities. The results obtained for the current work indicate that the biological activities of these compounds increase as the n-octanol/water (logP) partition coefficients, ionization energies (IP), melting points (mp) and dissociation constants increase and the solvent effects (SE), enthalpies of formation (DeltafH degrees ) and proton affinities (PA) decrease.

The effect of humidity on samples of microcrystalline cellulose taken from the extrusion/marumerization process

The purpose of this work was to examine the sorption and desorption of water by various samples of microcrystalline cellulose, MCC (Avicel PH-101), taken from the extrusion/marumerization process, and to provide data that may explain how water affects the MCC polymer matrix during the formation of beads. Two isopiestic (humidity) studies were conducted: the first used samples exposed directly to controlled humidity conditions, whereas the second used samples that were freeze-dried before being exposed to controlled humidity conditions. Water sorption and desorption were determined gravimetrically. When both sets of samples were initially exposed to low-humidity conditions, they reached equilibrium by desorbing water. When these samples were initially exposed to high-humidity conditions, the high moisture content samples desorbed water, whereas the low moisture content and the freeze-dried samples sorbed water to reach equilibrium. When the first set of samples was initially exposed to high- and then to low-humidity conditions, they reached the same water content achieved by being equilibrated directly at the low-humidity condition. However, samples that were initially exposed to low- and then to high-humidity conditions had equilibrium water contents that were lower than those achieved by being equilibrated directly at the high-humidity condition. The original MCC systems exhibit a hysteretic effect above 85%, whereas the freeze-dried systems have a broader range hysteretic effect starting at 20% relative humidity. The results suggest that the internal structure of the MCC polymer fibers must change with the sorption and desorption of water, supporting the autohesion theory.

Evaluation of amorphous ursodeoxycholic acid by thermal methods

PURPOSE

The purpose of this study was to characterize the amorphous state of ursodeoxycholic acid (UDCA) samples by using isothermal microcalorimetry, X-ray diffraction, infrared (IR) spectroscopy and solid state carbon 13 nuclear magnetic resonance (13C-NMR) spectroscopy, and to demonstrate the application of the thermal methods (microcalorimetry and differential scanning calorimetry (DSC) for studying the amorphous state and clarifying the dissolution mechanism of UDCA.

METHODS

Amorphous UDCA was prepared by grinding and rapid cooling of the melts. The heat of solution of UDCA was measured by an isothermal heat-conduction twin microcalorimeter at 25.0 degrees C. Some physicochemical properties of amorphous UDCA were also studied.

RESULTS

The intensities of X-ray diffraction peaks of crystalline UDCA decreased with an increase in grinding time. The heat levels of solution of crystalline UDCA and UDCA ground for 1 min were endothermic, and became exothermic with an increase in grinding time. A good correlation was obtained between the heat of solution and the heat of crystallization determined from the peak area in DSC. Although no significant difference was observed in X-ray diffraction patterns of amorphous UDCA prepared by the two methods, significant differences were recognized in DSC, IR and 13C-NMR, and the heat of solution indicated different values among the two samples. The stability of amorphous UDCA samples stored under 74.5% relative humidity at 40 degrees C was found to depend upon the preparation methods.

CONCLUSIONS

Different states of amorphous UDCA were obtained depending on the preparation method. The application of thermal methods to evaluate the amorphous state was demonstrated. The mechanism of dissolution of UDCA was discussed from the results of the heat of solution examination.