The Activation Energy at T g and the Fragility Index of Indomethacin,Predicted from the Influence of the Heating Rate on the Temperature Position and on the Intensity of Thermally Stimulated Depolarization Current Peak

Purpose. The purpose of this study was to estimate the activation energy at the glass transition temperature (and the fragility index) of amorphous indomethacin from the influence of heating rate on the features of the relaxation peaks obtained by thermally stimulated depolarization currents (TSDC) and to compare the obtained results with those obtained by other procedures based on TSDC data. Methods. The glass transition temperature region of amorphous indomethacin was characterized at different heating rates by TSDC in a way similar to that used to determine the kinetics of the glass transition relaxation by differential scanning calorimetry. The features of a thermal sampled TSDC peak, namely the temperature location and the intensity, depend on the heating rate. Results. The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating rate dependence of the TSDC peak location, T _m, and of the maximum intensity of the TSDC peak, I(T _m). Conclusions. The methods for determining the activation energy for structural relaxation and fragility of indomethacin from TSDC data obtained with different heating rates were compared with other procedures previously proposed. TSDC, which is not a very familiar technique in the community of pharmaceutical scientists, proved to be a very convenient technique to study molecular mobility and to determine the fragility index in glass-forming systems. The value of 60 appears as a reasonable value of the fragility index of indomethacin.     

Estimation of the fragility index of indomethacin by DSC using the heating and cooling rate dependency of the glass transition

In this study we have investigated the features of the glass transition relaxation of indomethacin using Differential Scanning Calorimetry (DSC). The purpose of this work is to provide an estimation of the activation energy at the glass transition temperature, as well as of the fragility index, of amorphous indomethacin from DSC data. To do so, the glass transition temperature region of amorphous indomethacin was characterized in both cooling and heating regimes. The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating and cooling rate dependence of the location of the DSC profile of the glass transition. The obtained results were similar in the heating and in the cooling modes. The results on the fragility index of indomethacin obtained in the present study, m = 60 in the cooling mode and m = 56 in the heating mode, are compared with other values previously published in the literature.     

A Pragmatic Test of a Simple Calorimetric Method for Determining the Fragility of Some Amorphous Pharmaceutical Materials

Purpose. To evaluate a simple calorimetric method for estimating the fragility of amorphous pharmaceutical materials from the width of the glass transition region. Methods. The glass transition temperature regions of eleven amorphous pharmaceutical materials were characterized at six different heating and cooling rates by differential scanning calorimetry (DSC). Results. Activation energies for structural relaxation (which are directly related to glass fragility) were estimated from the scan rate dependence of the glass transition temperature, and correlations between the glass transition widths and the activation energies were examined. The expected correlations were observed, and the exact nature of the relationship varied according to the type of material under consideration. Conclusions. The proposed method of determining the fragility of amorphous materials from the results of simple DSC experiments has some utility, although "calibration of the method for each type of materials is necessary. Further work is required to establish the nature of the relationships for a broad range of amorphous pharmaceutical materials.     

Effect of thermal history on the glassy state of indapamide

The effects of thermal history, e.g. cooling rate, annealing, etc., on the thermal behaviour of indapamide glass were determined by differential scanning calorimetry (DSC). The glass was prepared by heating indapamide crystals (m.p. 162 degrees C) to 180 degrees C, and then cooling the melt to room temperature. The glass transition temperature (Tg) of the material was 98 degrees C. An endotherm, due to thermal relaxation of the glass, was observed in the DSC thermogram when indapamide glass was prepared by slow cooling or was annealed isothermally at a temperature below Tg. Such enthalpy relaxation may be observed during ageing of pharmaceutical glasses and might influence their physico-chemical properties.     

Characterization of glassy itraconazole: a comparative study of its molecular mobility below T(g) with that of structural analogues using MTDSC

The objective of the present study was to estimate the molecular mobility of glassy itraconazole below the glass transition, in comparison with structural analogues (i.e. miconazole and ketoconazole).Glassy itraconazole and miconazole were prepared by cooling from the melt. The glassy state of the drug was investigated with modulated temperature DSC using the following conditions: amplitude +/-0.212 K, period 40 s, underlying heating rate 2 K/min. The glass transition was determined from the reversing heat flow and occurred at 332.4 (+/-0.5) K and 274.8 (+/-0.4) K for itraconazole and miconazole, respectively. The jump in heat capacity at the glass transition was 303.42 (+/-3.43) J/mol K for itraconazole and 179.35 (+/-0.89) J/mol K for miconazole. The influence of the experimental conditions on the position of the glass transition of itraconazole was investigated by varying the amplitude from +/-0.133 to +/-0.292 K and the period from 25 to 55 s, while the underlying heating rate was kept constant at 2 K/min. Glass transition temperature, T(g), was not significantly influenced by the frequency of the modulation nor by the cooling rate. However, the relaxation enthalpy at the glass transition increased with decreasing cooling rate indicating relaxation during the glass formation process. To estimate the molecular mobility of the glassy materials, annealing experiments were performed from T(g)--10 to T(g)--40 K for periods ranging from 15 min to 16 h.Fitting the extent of relaxation of glassy itraconazole to the Williams--Watts decay function and comparing the obtained values with those of amorphous miconazole and ketoconazole indicated that the molecular mobility is influenced by the complexity of the molecular structure. The more complex the structure, the more stable the amorphous state.     

Evaluation of glassy-state dynamics from the width of the glass transition: results from theoretical simulation of differential scanning calorimetry and comparisons with experiment

The purpose of this study is to investigate the quantitative relationship between the width of the glass transition, DeltaTg, and glass fragility or activation energy for structural relaxation. The ultimate objective is the estimation of structural relaxation time as a function of temperature from the width of the glass transition region, allowing characterization of glass dynamics by a single simple measurement. The Moynihan correlation indicates that activation energy for structural relaxation should be inversely proportional to the width of the glass transition, but recent experimental studies suggest this relationship is a poor approximation for glasses of pharmaceutical interest. The present study is an effort to better understand the validity of the Moynihan correlation by selected experimental studies and a theoretical analysis of those factors that impact the glass transition width. Experimental data for glass transition widths for (poly)vinylpyrrolidone, sucrose, and trehalose are obtained using a variety of procedures, and relaxation time data are obtained using the thermal activity monitor. The theoretical analysis begins by simulating the temperature dependence of the heat capacity by breaking the cooling and heating scans into a large number of temperature steps followed by isothermal holds, during which relaxation of the material is calculated. Here, the modified VTF equation is used for relaxation time and the generalized Kohlraush-Williams-Watts stretched exponential function is used to describe the relaxation kinetics. Simulations are performed for materials of varying fragility and varying "stretched exponential" constants, beta, and the width of the glass transition region, DeltaTg, is evaluated from the simulated heat capacity versus temperature curves as one would do with experimental data. Agreement between the theoretical simulations and experimental DeltaTg data is excellent. The simulations demonstrate that although the Moynihan correlation is not valid for variable beta, a modification of the Moynihan correlation which includes variation in beta is a good approximation. Thus, an estimate of fragility may be obtained from glass transition width data provided an estimate of beta is available. Furthermore, it is shown that a first approximation for beta may be obtained from the magnitude (i.e., height) of the differential scanning calorimetry thermal overshoot. We also find that using the modified VTF equation to evaluate the temperature dependence of the structural relaxation time at the glass transition, and integrating this expression to lower temperatures, it is possible to obtain an evaluation of the relaxation time constant, tau(beta), in the glass at any temperature, using only the DeltaTg and beta values obtained from a single differential scanning calorimetry scan. These estimated time constants correlate very well with the values directly measured with the thermal activity monitor.     

Characterization of amorphous ketoconazole using modulated temperature differential scanning calorimetry

The objective of the present study was to characterize the glassy state of ketoconazole and to calculate its molecular mobility below the glass transition, with a view to further developing the use of modulated temperature differential scanning calorimetry (MTDSC) as a means of studying relaxation behavior. Particular emphasis is placed on identifying the influence of the choice of experimental parameters on the measured values of both the glass transition temperature (T(g)) and the relaxation enthalpy magnitude. Amorphous ketoconazole was studied using an amplitude of +/-0.212 K, a period of 40 s, and an underlying heating rate of 2 K/min. The correction required for the calculation of the relaxation endotherm magnitude (the "T(g) shift effect") was demonstrated and is discussed in terms of the mechanism underpinning this phenomenon. Similarly, the influence of the choice of MTDSC experimental parameters on the measured T(g) was studied by varying the amplitude from +/-0.011 to +/-0.424 K and the period from 25 to 50 s. The influence of the cooling rate from the melt on the magnitude of the relaxation endotherm and position of the glass transition was investigated. It was noted that the magnitude of the relaxation endotherm increased with slower cooling rates, this being ascribed to a combination of annealing during the cooling and heating cycle and a further facet of the T(g) shift effect. Annealing experiments were performed at aging temperatures T(g)-12--T(g)-42 K for periods ranging from 10 min up to 16 h. The relaxation behavior was characterized by fitting the calculated extent of relaxation to the Williams-Watts equation. Overall, the study has highlighted theoretical and experimental issues that need to be considered when using both DSC and MTDSC for the calculation of relaxation times.     

Comparative Investigation by Two Analytical Approaches of Enthalpy Relaxation for Glassy Glucose,Sucrose, Maltose,and Trehalose

No HeadingPurpose. In an effort to understand the stability of glassy sugars such as glucose, sucrose, maltose, and trehalose, the molecular mobility below the glass transition temperature (T_g) was investigated by an enthalpy relaxation measurement with differential scanning calorimetry (DSC).Methods. The glassy sample was aged over several days at (T_g – 10) K to (T_g – 30) K, before a DSC heating scan was taken. The relaxed enthalpy (H_relax) was estimated from the endothermic peak area. The enthalpy relaxation time was analyzed from the time course of H_relax using two different approaches; Kohlrausch-Williams-Watts (KWW) and extended Adam-Gibbs (exAG).Results. ^KWW, which is defined as the mean average enthalpy relaxation time in a distribution, and ^eff₀ and ^eff, which correspond to the enthalpy relaxation time of the initial minimum and final maximum cooperative rearrangement region, were estimated by KWW and exAG, respectively. And three activation energies for enthalpy relaxation were calculated from the Arrhenius plot.Conclusions. Although these Es originated from different theoretical backgrounds, almost the same trend was observed for a comparison of the values of the four sugars. The finding that the Es of glassy trehalose were the largest among the four sugars may support the reason that glassy trehalose is an effective stabilizer.     

Molecular Mobility and Fragility in Indomethacin: A Thermally Stimulated Depolarization Current Study

Purpose. To show that thermally stimulated depolarization currents (TSDC), which is a dielectric experimental technique relatively unknown in the pharmaceutical scientists community, is a powerful technique to study molecular mobility in pharmaceutical solids, below their glass transition temperature (T_g). Indomethacin (T_g = 42°C) is used as a model compound. Methods. TSDC is used to isolate the individual modes of motion present in indomethacin, in the temperature range between –165°C and +60°C. From the experimental output of the TSDC experiments, the kinetic parameters associated with the different relaxational modes of motion were obtained, which allowed a detailed characterization of the distribution of relaxation times of the complex relaxations observed in indomethacin. Results. Two different relaxational processes were detected and characterized: the glass transition relaxation, or -process, and a sub-T_g relaxation, or secondary process. The lower temperature secondary process presents a very low intensity, a very low activation energy, and a very low degree of cooperativity. The fragility index (Angell's scale) of indomethacin obtained from TSDC data is m = 64, which can be compared with other values reported in the literature and obtained from other experimental techniques. Conclusions. TSDC data indicate that indomethacin is a relatively strong glass former (fragility similar to glycerol but lower than sorbitol, trehalose, and sucrose). The high-resolution power of the TSDC technique is illustrated by the fact that it detected and characterized the secondary relaxation in indomethacin, which was not possible by other techniques.     

Crystallization Behavior of Mannitol in Frozen Aqueous Solutions

Purpose. To study the effect of cooling rate, the influence of phosphate buffers and polyvinylpyrrolidone (PVP) on the crystallization behavior of mannitol in frozen aqueous solutions. Methods. Low-temperature differential scanning calorimetry and powder X-ray diffractometry were used to characterize the frozen solutions. Results. Rapid cooling (20°C/min) inhibited mannitol crystallization, whereas at slower cooling rates (10°C and 5°C/min) partial crystallization was observed. The amorphous freeze-concentrate was characterized by two glass transitions at -32°C and -25°C. When the frozen solutions were heated past the two glass transition temperatures, the solute crystallized as mannitol hydrate. An increase in the concentration of PVP increased the induction time for the crystallization of mannitol hydrate. At concentrations of 100 mM, the buffer salts significantly inhibited mannitol crystallization. Conclusions. The crystallization behavior of mannitol in frozen solutions was influenced by the cooling rate and the presence of phosphate buffers and PVP.     

Measurement of Glass Transition Temperatures in Freeze Concentrated Solutions of Non-Electrolytes by Electrical Thermal Analysis

The electrical resistance (R) of frozen aqueous solutions was measured as a function of temperature in order to determine whether this technique can be applied for determination of glass transition temperatures of maximally freeze concentrated solutions (T_g) of non-electrolytes which do not crystallize during freezing. Electrical thermal analysis (ETA) thermograms of frozen solutions containing the solute alone show a gradual change in slope over the temperature range of interest, with no inflection point which corresponds to T_g. However, addition of low levels (about 0.1%) of electrolyte changes the shape of the thermogram into a biexponential function where the intersection of the two linear portions of the log (R) vs. T plot corresponds to the glass transition region. The total change in log (R) over the temperature range studied increases as the ionic radius of the reporter ion increases. The sharpest inflection points in the log (R) vs T curves, and the best correlation with DSC results, were obtained with ammonium salts. T_g values measured by ETA were compared with values measured by DSC. DSC thermograms of solutes with and without electrolyte (0.1%) show that the electrolyte decreases T_g by about 0.5 to 1.0°C. However, T_g values measured by ETA are somewhat higher than those measured by DSC, and difference between the two methods seems to increase as T_g decreases. T_g as measured by ETA is less heating rate dependent than DSC analysis, and ETA is a more sensitive method than DSC at low solute concentrations and at low heating rates. Results of electrical thermal analysis of frozen solutions are compared and contrasted with the electrical resistance vs. temperature behavior of polymer-electrolytes. ETA appears to be a useful complementary technique to DSC for characterizing formulations intended for freeze drying.     

A Mechanistic Investigation of An Amorphous Pharmaceutical and Its Solid Dispersions,Part II: Molecular Mobility and Activation Thermodynamic Parameters

Purpose. The ability of TSDC to characterize further amorphous materials beyond that possible with DSC was presented in part I (16) of this work. The purpose of part II presented here is to detect and quantitatively characterize time-scales of molecular motions (relaxation times) in amorphous solids at and below the glass transition temperature, to determine distributions of relaxation times associated with different modes of molecular mobility and their temperature dependence, and to determine experimentally the impact upon these parameters of combining the drug with excipients (i.e., solid dispersions at different drug to polymer ratios). The knowledge gleaned may be applied toward a more realistic correlation with physical stability of an amorphous drug within a formulation during storage. Methods. Preparation of amorphous drug and its solid dispersions with PVPK-30 was described in part I (16). Molecular mobility and dynamics of glass transition for these systems were studied using TSDC in the thermal windowing mode. Results. Relaxation maps and thermodynamic activation parameters show the effect of formulating the drug in a solid dispersion on converting the system (drug alone) from one with a wide distribution of motional processes extending over a wide temperature range at and below Tg to one that is homogeneous with very few modes of motion (20% dispersion) that becomes increasingly less homogeneous as the drug load increases (40% dispersion). This is confirmed by the high activation enthalpy (due to extensive intra- and intermolecular interactions) as well as high activation entropy (due to higher extent of freedom) for the drug alone vs. a close to an ideal system (lower enthalpy), with less extent of freedom (low entropy) especially for the 20% dispersion. The polymer PVPK-30 exhibited two distinct modes of motion, one with higher values of activation enthalpies and entropy corresponding to -relaxations, the other with lower values corresponding to -relaxations characterized by local noncooperative motional processes. Conclusions. Using thermal windowing, a distribution of temperature-dependent relaxation times encountered in real systems was obtained as opposed to a single average value routinely acquired by other techniques. Relevant kinetic parameters were obtained and used in mechanistically delineating the effects on molecular mobility of temperature and incorporating the drug in a polymer. This allows for appropriate choices to be made regarding drug loading, storage temperature, and type of polymer that would realistically correlate to physical stability.     

Characterisation of the Glass Transition of an Amorphous Drug Using Modulated DSC

Purpose. The use of modulated differential scanning calorimetry (MDSC) as a novel means of characterising the glass transition of amorphous drugs has been investigated, using the protease inhibitor saquinavir as a model compound. In particular, the effects of measuring variables (temperature cycling, scanning period, heating mode) have been examined. Methods. Saquinavir samples of known moisture content were examined using a TA Instruments 2920 MDSC at a heating rate of 2°C/min and an amplitude of ± 0.159°C with a period of 30 seconds. These conditions were used to examine the effects of cycling between - 50°C and 150°C. A range of periods between 20 and 50 seconds were then studied. Isothermal measurements were carried out between 85°C and 120°C using an amplitude of ± 0.159°C with a period of 30 seconds. Results. MDSC showed the glass transition of saquinavir (0.98 ± 0.05%w/w moisture content) in isolation from the relaxation endotherm to give an apparent glass transition temperature of 107.0° C ± 0.4C. Subsequent temperature cycling gave reproducible glass transition temperatures of approximately 105°C for both cooling and heating cycles. The enthalpic relaxation peak observed in the initial heating cycle had an additional contribution from a Tg 'shift' effect brought about by the difference in response to the glass transition of the total and reversing heat flow signals. Isothermal studies yield a glass transition at 105.9°C ± 0.1°C. Conclusions. MDSC has been shown to be capable of separating the glass transition of saquinavir from the relaxation endotherm, thereby facilitating measurement of this parameter without the need for temperature cycling. However, the Tg 'shift' effect and the number of modulations through the transition should be taken into account to avoid drawing erroneous conclusions from the experimental data. MDSC has been shown to be an effective method of characterising the glass transition of an amorphous drug, allowing the separate characterisation of the Tg and endothermic relaxation in the first heating cycle.     

Phase Transitions of Glycine in Frozen Aqueous Solutions and During Freeze-Drying

Purpose. To study the solid-state and phase transitions of glycine, (i) in frozen aqueous solutions, and (ii) during freeze-drying. Methods. X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) were used to analyze the frozen systems. In situ freeze-drying in the sample chamber of the diffractometer enabled characterization of phase transitions during freeze-drying. Results. Transitions in frozen systems. Rapid (20°C/min) or slow (2°C/min) cooling of aqueous solutions of glycine (15% w/w) to –70°C resulted in crystallization of -glycine. Annealing at –10°C led to an increase in the amount of the crystalline phase. When quench-cooled by immersing in liquid nitrogen, glycine formed an amorphous freeze-concentrate. On heating, crystallization of an unidentified phase of glycine occurred at \-65°C which disappeared at –55°C, and the peaks of -glycine appeared. Annealing caused a transition of - to the - form. The extent of this conversion was a function of the annealing temperature. Slower cooling rates and annealing in frozen solutions increased the crystalline -glycine content in the lyophile. Freeze-drying of quench-cooled solutions led to the formation of -glycine during primary drying resulting in a lyophile consisting of a mixture of - and -glycine. The primary drying temperature as well as the initial solute concentration significantly influenced the solid-state of freeze-dried glycine only in quench-cooled systems. Conclusions. The cooling rate, annealing conditions and the primary drying temperature influenced the solid-state composition of freeze-dried glycine.     

An Evaluation of the Use of Modulated Temperature DSC as a Means of Assessing the Relaxation Behaviour of Amorphous Lactose

Purpose. To evaluate the use of Modulated Temperature DSC(MTDSC) as a means of assessing the relaxation behaviour ofamorphous lactose via measurement of the heat capacity, glasstransition (Tg) and relaxation endotherm. Methods. Samples of amorphous lactose were prepared by freezedrying. MTDSC was conducted using a TA Instruments 2920 MDSCusing a heating rate of 2°C/minute, a modulation amplitude of ±0.3°Cand a period of 60 seconds. Samples were cycled by heating to 140°Cand cooling to a range of annealing temperatures between 80°C and100°C, followed by reheating through the Tg region. Systems werethen recooled to allow for correction of the Tg shift effect. Results. MTDSC enabled separation of the glass transition from therelaxation endotherm, thereby facilitating calculation of the relaxationtime as a function of temperature. The relative merits of using MTDSCfor the assessment of relaxation processes are discussed. In addition,the use of the fictive temperature rather than the experimentally derivedTg is outlined. Conclusions. MTDSC allows assessment of the glass transitiontemperature, the magnitude of the relaxation endotherm and the valueof the heat capacity, thus facilitating calculation of relaxation times.Limitations identified with the approach include the slow scanningspeed, the need for careful choice of experimental parameters and theTg shift effect.     

Cooling-induced contraction of trachea isolated from normal and sensitized guinea-pigs

Summary Fast (–7°C/min) cooling of guinea-pig isolated trachea produced a rapidly developing, transient contraction followed by relaxation. Cooling-induced contraction was dependent on temperature (30, 20 or 10°C) and responses in trachea obtained from actively sensitized guinea pigs were significantly greater (20 and 10°-C) than those observed in normal trachea. Cooling to 20°C was selected for subsequent experiments. Pre-treatment with sufficient concentrations of atropine, clemastine, cromoglycate, indomethacin, or nordihydroguaiaretic acid did not depress contraction to cooling in either normal or sensitized trachea. This indicates a direct effect of cooling. The contraction. produced by cooling was resistant to verapamil (1 mol/l) or dantrolene (0.3 mmol/l). Calmodulin antagonists (trifluoperazine, W-7 and calmidazolium; all of them at 10–100 mol/l) inhibited contraction in sensitized and normal trachea. Activators of protein kinase C (phorbol 12,13-diacetate, 1 mol/l) enhanced while inhibitors (H-7, 20 mol/l; staurosporine, 10 mol/l) depressed cooling-induced contraction in both normal and sensitized tissues. Incubation (20 min) in a Ca²⁺ -free solution inhibited cooling-induced contraction in normal but not in sensitized trachea. Exposure to a low Na⁺ (25 mmol/l) or a K⁺-free medium abolished contraction to cooling in normal and sensitized trachea. Ouabain (0.1–10 mol/l) and vanadate (0.01–5 mmol/l) inhibited cooling-induced-contraction to a greater extent in normal than in sensitized trachea. Polymyxin B (0.5 mmol/l) selectively depressed responses to cooling in sensitized trachea. In a separate series of experiments, it was shown that sensitized trachea was hyperresponsive to ouabain and vanadate. Previous cooling to 20°C abolished responses to ouabain but only attenuated those to vanadate. These results are compatible with an enhancement of Na⁺,K⁺-ATPase and Ca²⁺-ATPase activities in sensitized trachea and further support the notion that intracellularly stored Ca²⁺ plays a decisive role in the activation of sensitized tracheal muscle. Send offprint requests to J. L. Ortiz at the above address     

An investigation into the low temperature thermal behaviour of vitamin E preparation USP using differential scanning calorimetry and low frequency dielectric analysis

The thermal and dielectric responses of Vitamin E Preparation USP have been examined to further understand the melting and solidification of this material. A TA Instruments 2920 Differential Scanning Calorimeter was used to examine the thermal response of the sample at a range of scanning speeds. Isothermal dielectric studies were performed using a Novocontrol Dielectric Spectrometer over a range of temperatures down to -70 degrees C and a frequency range of 10(6)-10(-2) Hz. The differential scanning calorimetry (DSC) studies showed an anomalous response whereby at slow heating rates (2 degrees C min(-1)) a small exotherm followed immediately by an endotherm was observed. This response was considerably diminished in magnitude at higher rates (5 degrees C min(-1)) and was not observed at the fastest heating rate of 10 degrees C min(-1). No thermal events were seen on cooling the sample to -60 degrees C. It was suggested that the material formed a glass on cooling, with a predicted transition temperature of approximately -100 degrees C. Further studies using a liquid nitrogen cooling system indicated that the system did indeed exhibit a glass transition, albeit at a higher temperature than predicted (ca -63 degrees C). Low frequency dielectric analysis showed a clear relaxation peak in the loss component, from which the relaxation time could be calculated using the Havriliak-Negami model. The relationship between the relaxation time and the temperature was studied and was found to follow the Vogel-Tammann-Fulcher (VTF) modification of the Arrhenius equation. It is therefore concluded that Vitamin E Preparation USP is a glass-forming material that exhibits kinetically-hindered recrystallisation and melting behaviour. The study has also indicated that DSC and low frequency dielectric analysis may be powerful complementary tools in the study of the low temperature behaviour of pharmaceuticals.     

The site of action of muscle relaxant purine nucleosides

Summary The effects of a series of purine nucleosides, including the novel marine natural product 1-methylisoguanosine, have been examined on muscle relaxation in conscious animals and on spinal reflexes and neuromuscular transmission in mice anaesthetized with sodium pentobarbitone. 1-Methylisoguanosine (5–15 mol kg^–1) and 2-chloroadenosine (1–5 mol kg^–1), both of which cause muscle relaxation in conscious animals, depressed both mono- and polysynaptic spinal reflexes but did not affect neuromuscular transmission. At much higher doses (300 mol kg^–1) both compounds did depress neuromuscular transmission. Adenosine and 1-methyladenosine did not produce muscle relaxation in conscious animals and only slightly depressed polysynaptic reflexes at the highest doses tested (300 mol kg^–1). Theophylline 50 mol kg^–1 enhanced polysynaptic reflexes and antagonized the depression of these reflexes by 1-methylisoguanosine. Neither adenosine nor 1-methylisoguanosine affected the development of tension by isolated diaphragm muscles in vitro. It is concluded that the muscle relaxant purine nucleosides 2-chloroadenosine and 1-methylisoguanosine produce their effects primarily by depressing activity in the central nervous system. Transmission at the neuromuscular junction is not affected at doses in the range of those producing muscle relaxation.     

Cardiovascular effects of clonidine in an avian species,Larus argentatus

Summary Blood pressure and heart rate were recorded in the sea gull, Larus argentatus, under light pentobarbitone anaesthesia. Clonidine 10^–7 and 10^–8 mol·kg^–1 (27 and 2.7 g·kg^–1) i.v. produced a biphasic effect on blood pressure, a brief initial increase being followed by a prolonged hypotensive response. There was an immediate reduction in heart rate which persisted throughout the hypotensive phase. After spinal transection at the level of C 4, clonidine administration elicited hypertension and bradycardia.Bilateral vagotomy abolished the effect of clonidine on heart rate but did not alter the blood pressure response.Vagotomy in combination with spinal transection abolished the effect of clonidine on heart rate but the hypertensive response was accentuated.Yohimbine 10^–7 or 10^–6 mol·kg^–1 (0.039 or 0.39 mg·kg^–1) given 5 min after clonidine 10^–7 mol·kg^–1 (27 g·kg^–1) effectively antagonized the cardiovascular effects of clonidine, while prazosin 10^–7 or 10^–6 mol·kg^–1 (0.042 or 0.42 mg·kg^–1) had no such effect.We conclude that clonidine acts in the central nervous system of the sea gull to produce a lowering of blood pressure and heart rate. These effects are mediated by central inhibition of sympathetic activity and, in the case of the heart rate, mostly by central activation of vagal activity to the heart. This central action of clonidine involves activation of -adrenoceptors which are blocked by yohimbine but not by prazosin and therefore may belong to the ₂ subtype.     

Molecular Mobility in Raffinose in the Crystalline Pentahydrate Form and in the Amorphous Anhydrous Form

Purpose The aims of the study are to characterize the slow molecular mobility in solid raffinose in the crystalline pentahydrate form, as well as in the anhydrous amorphous form (T_g = 109°C at 5°C/min), and to analyze the differences and the similarities of the molecular motions in both forms.Methods Thermally stimulated depolarization current (TSDC) is used to isolate the individual modes of motion present in raffinose, in the temperature range between −165 and +60°C. From the experimental output of the TSDC experiments, the kinetic parameters associated with the different relaxational modes of motion were obtained, which allowed a detailed characterization of the distribution of relaxation times of the complex relaxations observed in raffinose. The features of the glass transition relaxation in raffinose were characterized by differential scanning calorimetry (DSC).Results A complex mobility was found in the crystalline form of raffinose. From the analysis of the TSDC data, we conclude that these molecular motions are local and noncooperative. A sub-T_g relaxation, or secondary process, was also detected and analyzed by TSDC in the amorphous phase. It has low activation energy and low degree of cooperativity. The glass transition was studied by DSC. The fragility index (Angell’s scale) of raffinose obtained from DSC data is m = 148.Conclusions TSDC proved to be an adequate technique to study the molecular mobility in the crystalline pentahydrate form of raffinose. In the amorphous form, on the other hand, the secondary relaxation was analyzed by TSDC, but the study of the glass transition relaxation was not possible by this experimental technique as a consequence of conductivity problems. The DSC study of the glass transition indicates that raffinose is an extremely fragile glass former.