Interactions between carbamazepine and polyethylene glycol (PEG) 6000: characterisations of the physical, solid dispersed and eutectic mixtures

The influence of a hydrophilic carrier (PEG 6000) on the polymorphism of carbamazepine, an antiepileptic drug, was investigated in binary physical mixtures and solid dispersions by means of differential scanning calorimetry (DSC), thermal gravimetry, hot-stage microscopy (HSM), and X-ray diffractometry, respectively. This study provides also an attempt to develop a method to calculate more precisely the eutectic composition. In rather ideal physical mixtures, carbamazepine was found as monoclinic Form III. In solid dispersions, the drug was found to crystallize as trigonal Form II; a eutectic invariant in the PEG 6000-rich composition domain (6% of carbamazepine mass) was evidenced by DSC experiments and confirmed by HSM observations. In the binary phase diagram the ideal carbamazepine liquidus curve was located at temperatures higher than the respective experimental ones. This suggests that drug can be maintained in the liquid state in the temperature–mass fraction (T–x) region between the two carbamazepine liquidus curves. This indicates in turn that attractive interactions occur between carbamazepine and PEG 6000-chains. These interactions have been also claimed to prevent carbamazepine from degradation into iminostilbene (a compound resulting from the chemical degradation of carbamazepine which is postulated to be responsible for the idiosyncratic toxicity of the drug) and thought to lead to the crystallization of metastable Carbamazepine II from melt. The negative excess entropy for eutectic mixtures indicated that the drug crystals are finely dispersed in the bulk of polymer chains.     

Solubility and stability of anhydrate/hydrate in solvent mixtures

In this paper, the thermodynamics of the anhydrate/dihydrate carbamazepine (CBZA/CBZH) in ethanol-water mixtures was studied by measuring the solubility of anhydrate and dihydrate carbamazepine at 0-60 degrees C. Both stable form solubility and metastable form solubility were measured, the latter with the assistance of Raman immersion probe. The thermodynamic properties of the anhydrate/dihydrate system, such as the relative stability, and enthalpy and entropy of dissolution, were estimated by plotting the measured solubility data according to the van't Hoff equation. The anhydrate/dihydrate carbamazepine showed an enantiotropic relationship in the studied mixtures and temperature ranges. It was shown that at a certain temperature, there was an equilibrium water activity value at which the anhydrate and dihydrate carbamazepine were in equilibrium. This equilibrium water activity value depends significantly on the temperature. The lower the temperature, the smaller is the water activity value needed to attain equilibrium between anhydrate and dihydrate. The obtained results are useful in determining crystallization parameters to achieve a desired anhydrate or hydrate phase. The approach can be applied to other anhydrate and hydrate systems.     

Thermoanalytic determination of the enantiomeric purity of selegiline]

The thermal behaviour of Selegiline (a chiral, non racemic pharmaceutical used in the therapy of Parkinson disease) was studied by differential scanning calorimetry (DSC) for determining the enantiomeric purity (e.p.) of the bulk substance. It has been found, that (i) the binary phase diagram (melting point phase diagram) of the R/S enantiomer mixtures is characteristic to that of the true racemic compounds; (ii) the melting behaviour of the R/S binary mixtures follows the thermodynamic laws (i.e. the Schr?der-Van Laar and the Prigogine-Defay equations); (iii) the e.p. of the highly purified bulk substance can be expressed as "DSC purity" (this latter is obtained from the Van't Hoff equation) and the microcalorimetric method as above gives good reproducibility; (iv) due to the minor impurities (other than the S(-) enantiomer) the obtained e.p. (expressed as DSC purity) can be higher but not lower than the actual e.p. of the investigated substance.     

Optimization of glibenclamide tablet composition through the combined use of differential scanning calorimetry and D-optimal mixture experimental design

A systematic analysis of the influence of different proportions of excipients on the stability of a solid dosage form was carried out. In particular, a d-optimal mixture experimental design was applied for the evaluation of glibenclamide compatibility in tablet formulations, consisting of four classic excipients (natrosol as binding agent, stearic acid as lubricant, sorbitol as diluent and cross-linked polyvinylpyrrolidone as disintegrant). The goal was to find the mixture component proportions which correspond to the optimal drug melting parameters, i.e. its maximum stability, using differential scanning calorimetry (DSC) to quickly obtain information about possible interactions among the formulation components. The absolute value of the difference between the melting peak temperature of pure drug endotherm and that in each analysed mixture and the absolute value of the difference between the enthalpy of the pure glibenclamide melting peak and that of its melting peak in the different analyzed mixtures, were chosen as indexes of the drug-excipient interaction degree.     

Heat of fusion measurement of a low melting polymorph of carbamazepine that undergoes multiple-phase changes during differential scanning calorimetry analysis.

Two polymorphs of carbamazepine that melt at 176 and 189 degrees C and are known to be enantiotropic have been characterized more fully than in previous reports. For the first time, a value for the heat of fusion (29.3 kJ/mol) of the lower melting form is estimated, even though single melting DSC endotherms were not obtained. A method is described for summing the heat flows of intermediate transitions that occur when low melting carbamazepine is heated to a temperature where it will remain melted. The method should have general applicability in studies of polymorphs. Other characteristics of the carbamazepine enantiotropic pair are reported, including the transition temperature (71 degrees C).     

Preparation of extruded carbamazepine and PEG 4000 as a potential rapid release dosage form.

The aim of this research was to use a ram extruder to prepare directly a fast release dosage form with uniform shape and density, containing carbamazepine (C) as a water-insoluble drug and polyethylene glycol 4000 (PEG) as a low melting binder. The potential inclusion of lactose (L) as a hydrophilic filler was also considered. The temperature suitable to ensure a successful extrusion process of several formulations containing PEG in different percentages was found to be below the melting point of the PEG. The influence of composition on the extrusion process of different ram speeds was checked by measuring the pressure at the steady state, the apparent shear rate and the apparent shear stress of a range of mixtures of drug, lactose and PEG. The physical-mechanical properties of extrudates, including tensile strength and Young's modulus, prepared with different ram velocities were also determined. The solid-state physical structure by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) was established. The dissolution of the extrudates and their corresponding physical mixtures were compared. The mixtures were found to be shear thinning when extruded; the tensile strength of extrudates was dependent on the composition but not the extrusion rate, while the value of Young's modulus was strongly influenced by the rate of extrusion, but less affected by the composition of the extrudates. The results of DSC and XRD indicated that the solid structure of the extrudates corresponded to that of a physical mixture of the components, hence there had been no change in the physical form of the drug induced by extrusion. In terms of dissolution, the rate of the extrusion process did not influence the performance of the products, whereas the composition did. The extruded mixtures of an equivalent composition exhibited a more rapid release than a simple physical mixture. The addition of lactose reduced the dissolution rate.     

Calorimetric investigation of protein/amino acid interactions in the solid state

Possible protein/amino acid interactions and the physical states of amino acids after freeze-drying have been studied using isoperibol calorimetry and differential scanning calorimetry (DSC). Good linear correlations (R² = 0.99) between the enthalpies of solution and the percentage of antibody in all physical mixtures, as well as unchanging melting temperatures of amino acids for physical mixtures demonstrated that there is no interaction between the antibodies and amino acids studied upon physical mixing. On the other hand, positive deviations for antibody/histidine and antibody/arginine freeze-dried samples obtained from the isoperibol calorimetry results demonstrated that molecular level interactions, such as ion–dipole or electrostatic interactions or hydrogen bonding, occur between antibodies and histidine or arginine. The values of ΔH_interaction for antibody/histidine (1:1, w/w) and antibody/arginine (1:1, w/w) lyophilized samples were approximately 8 kJ/mol. These interactions were also confirmed by decreased and/or the disappearance of melting temperatures of the amino acids with DSC measurements. A negative deviation from linearity was detected for antibody/aspartic acid lyophilized samples which indicated partial amorphization of aspartic acid. No deviation from linearity as well as similar melting temperatures of antibody/glycine lyophilized samples indicated the absence of interactions between the antibodies and glycine upon freeze-drying.     

Polymorphism and thermodynamics of m-hydroxybenzoic acid

Solution and solid-state properties of m-hydroxybenzoic acid have been investigated. Two polymorphs were found where the monoclinic modification exhibits a higher stability than the orthorhombic form. The solubility of the monoclinic polymorph was determined between 10 and 50 °C in methanol, acetonitrile, acetic acid, acetone, water and ethyl acetate. The solubility of the orthorhombic polymorph was determined between 10 and 50 °C in acetonitrile, acetic acid, acetone and ethyl acetate. A thermodynamic analysis revealed a marked correlation between the molar solubility and the van’t Hoff enthalpy of solution at constant temperature. In addition, in each solvent increased temperature resulted in increased van’t Hoff enthalpy of solution. It is shown that the solubility data can be used to estimate melting properties for both polymorphs. The solubility ratio of the two forms and the DSC thermogram of the orthorhombic form strongly suggest that the system is monotropic. However, according to the polymorph rules of Burger and Ramberger, the estimated higher melting enthalpy and lower melting temperature of the orthorhombic form points towards an enantiotropic system. Hence, this system appears to be an exception to the Burger and Ramberger melting enthalpy rule, and the probable reason for this is found in the difference in the heat capacity of the two solid forms.     

差示扫描量热法检测那格列奈多晶型

目的建立控制那格列奈B型晶体限度的DSC法。方法测定一系列混合晶型浓度梯度组成的或某一确定组成的那格列奈对照品的ΔH值以及待测样品的ΔH值,以工作曲线法或比较法确定待测样品的B型组成。结果在0～15% B晶组成时,重复性0.61%,回收率86.3%～127%。结论此法可作为那格列奈混晶中B晶限度的检测方法。     

Solubility of lovastatin in a family of six alcohols: Ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-octanol

Accurate experimental determination of solubility of active pharmaceutical ingredients (APIs) in solvents and its correlation, for solubility prediction, is essential for rapid design and optimization of isolation, purification, and formulation processes in the pharmaceutical industry. An efficient material-conserving analytical method, with in-line reversed HPLC separation protocol, has been developed to measure equilibrium solubility of lovastatin in ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-octanol between 279 and 313K. Fusion enthalpy DeltaH(fus), melting point temperature, Tm, and the differential molar heat capacity, DeltaC(P), were determined by differential scanning calorimetry (DSC) to be 43,136J/mol, 445.5K, and 255J/(molK), respectively. In order to use the regular solution equation, simplified assumptions have been made concerning DeltaC(P), specifically, DeltaC(P)=0, or DeltaC(P)=DeltaS. In this study, we examined the extent to which these assumptions influence the magnitude of the ideal solubility of lovastatin, and determined that both assumptions underestimate the ideal solubility of lovastatin. The solubility data was used with the calculated ideal solubility to obtain activity coefficients, which were then fitted to the van't Hoff-like regular solution equation. Examination of the plots indicated that both assumptions give erroneous excess enthalpy of solution, H(infinity), and hence thermodynamically inconsistent activity coefficients. The order of increasing ideality, or solubility of lovastatin was butanol>1-propanol>1-pentanol>1-hexanol>1-octanol.     

Physical stability of solid dispersions of the antiviral agent UC-781 with PEG 6000, Gelucire 44/14 and PVP K30

This paper describes the physical stability of solid dispersions of UC-781 with PEG 6000, Gelucire 44/14 and PVP K30 prepared by the solvent and melting methods. The concentration of the drug in the solid dispersions ranged from 5 to 80% w/w. The solid dispersions were stored at 4-8 and 25 degrees C (25% RH), then their physicochemical properties were analysed by differential scanning calorimetry (DSC), X-ray powder diffraction and dissolution studies as a function of storage time. The DSC curves of solid dispersions of UC-781 with PVP K30 did not show any melting peaks corresponding to UC-781 after storage, indicating no recrystallization of the drug. The DSC data obtained from PEG 6000 and Gelucire 44/14 showed some variations in melting peak temperatures and enthalpy of fusion of the carriers. It was shown that the enthalpy of fusion of PEG 6000 in the dispersions increased after storage; it was more pronounced for samples stored at 25 degrees C compared to those at 4-8 degrees C indicating the reorganization of the crystalline domains of the polymer. Similarly, the enthalpy of fusion of Gelucire 44/14 in the solid dispersions increased as a function of time. Dissolution of UC-781 from all solid dispersions decreased as a function of storage time. While these observations concurred with the DSC data for all solid dispersions, they were not reflected by X-ray powder diffraction data. It was concluded that it is the change of the physical state of the carriers and not that of the drug, which is responsible for the decreased dissolution properties of the solid dispersions investigated.     

Enthalpy Relaxation in Binary Amorphous Mixtures Containing Sucrose

Purpose. To compare the enthalpy relaxation of amorphous sucrose and co-lyophilized sucrose-additive mixtures near the calorimetric glass transition temperature, so as to measure the effects of additives on the molecular mobility of sucrose. Methods. Amorphous sucrose and sucrose-additive mixtures, containing poly(vinylpyrrolidone) (PVP), poly(vinylpyrrolidone-co-vinyl-acetate) (PVP/VA) dextran or trehalose, were prepared by lyophilization. Differential scanning calorimetry (DSC) was used to determine the area of the enthalpy recovery endotherm following aging times of up to 750 hours for the various systems. This technique was also used to compare the enthalpy relaxation of a physical mixture of amorphous sucrose and PVP. Results. Relative to sucrose alone, the enthalpy relaxation of co-lyophilized sucrose-additive mixtures was reduced when aged for the same length of time at a comparable degree of undercooling in the order: dextran PVP > PVP/VA > trehalose. Calculated estimates of the total enthalpy change required for sucrose and the mixtures to relax to an equilibrium supercooled liquid state (H) were essentially the same and were in agreement with enthalpy changes measured at longer aging times (750 hours). Conclusions. The observed decrease in the enthalpy relaxation of the mixtures relative to sucrose alone indicates that the mobility of sucrose is reduced by the presence of additives having a T_g that is greater than that of sucrose. Comparison with a physically mixed amorphous system revealed no such effects on sucrose. The formation of a molecular dispersion of sucrose with a second component, present at a level as low as 10%, thus reduces the mobility of sucrose below T_g, most likely due to the coupling of the molecular motions of sucrose to those of the additive through molecular interactions.     

Contribution of variable domains to the stability of humanized IgG1 monoclonal antibodies

Temperature-induced unfolding of three humanized IgG1 monoclonal antibodies and their Fab and Fc fragments was monitored by differential scanning calorimetry at neutral pH. With some exceptions, the thermogram of the intact antibody presents two peaks and the transition with the larger experimental enthalpy contains the contribution from the Fab fragments. Although the measured enthalpy was similar for all three Fab fragments studied, the apparent melting temperatures were found to vary significantly, even for Fab fragments originating from the same human germline. Therefore, we propose to use the measured enthalpy of unfolding as the key parameter to recognize the unfolding events in the melting profile of an intact IgG1 antibody. If the variable domain sequences, resulting from complementarity determining regions (CDRs) grafting and humanization, destabilize the Fab fragment with respect to the CH3 domain, the first transition represents the unfolding of the Fab fragment and the CH2 domain, while the second transition represents CH3 domain unfolding. Otherwise, the first transition represents CH2 domain unfolding, and the second transition represents the unfolding of the Fab fragment and the CH3 domain. In some cases, the DSC profile may present three transitions, with the Fab unfolding occurring at distinct temperatures compared to the melting of the CH2 and CH3 domains. If the DSC profile of a humanized IgG1 monoclonal antibody cannot be described by the model above, the result may be an indication of significant structural heterogeneity and/or of disruption of the Fab cooperative unfolding. Low stability or heterogeneity of the Fab fragment may prove problematic for long-term storage or consistency of production. Therefore, understanding the features of a DSC profile is important for clone selection and process maturation in the early stages of development of therapeutic monoclonal antibodies.     

Tween protects recombinant human growth hormone against agitation-induced damage via hydrophobic interactions

In the absence of surfactants, recombinant human growth hormone (rhGH) rapidly forms insoluble aggregates during agitation. The nonionic surfactant Tween 20, when present at Tween:protein molar ratios >4, effectively inhibits this aggregation. Differential scanning calorimetry (DSC) of rhGH solutions showed melting transitions that decreased by ca. 2 degrees C in the presence of Tween. Circular dichroism (CD) studies of the same thermal transition showed that the decrease is specific to the relatively high protein concentrations required for DSC. CD studies showed melting transitions that decreased with lower protein concentrations. Tween has an insignificant effect on the melting transition of rhGH at lower protein concentrations (0.18 mg/mL). Injection titration microcalorimetry showed that the interaction of Tween with rhGH is characterized by a weak enthalpy of binding. For comparison, interferon-g, another protein which has been shown to bind Tween, also shows weak enthalpy of binding. Fluorescent probe binding studies and infrared spectroscopic investigations of rhGH secondary structure support suggestions in the literature (Bam, N. B.; Cleland, J. L., Randolph, T. W. Molten globule intermediate of recombinant human growth hormone: stabilization with surfactants. Biotechnol. Prog. 1996. 12, 801-809) that Tween binding is driven by hydrophobic interactions, with little perturbation of protein secondary structure.     

Use of Enthalpy and Gibbs Free Energy to Evaluate the Risk of Amorphous Formation

The control of crystalline and amorphous phases is important during the development of a new drug candidate. Our approach begins with an understanding of the thermodynamics of these two phases. We have developed a quantitative yet practical work flow consisting of three steps towards the analysis of the risk of amorphous material formation. First, we derive the thermodynamic equations to calculate the enthalpy, Gibbs free energy, and the solubility of each phase and their differences as a function of temperature. The enthalpy for each crystalline drug substance at its melting point is selected as the reference state to enable a consistent approach for all analysis. Second, we use data from DSC measurements and the derived thermodynamic equations to construct the enthalpy, Gibbs free energy and solubility diagrams so as to compare the characteristics of these two phases. Finally, we use the results of these calculations to evaluate the potential risk of crystalline-to-amorphous phase conversion during processing of either the drug substance or the drug product. In addition, the impact of amorphous formation on solubility is evaluated. Two drug candidates are used to illustrate this workflow for risk analysis. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:4096–4105, 2010     

Preparation, characterization and in vivo evaluation of ibuprofen binary solid dispersions with poloxamer 188

Ibuprofen-Poloxamer 188 (P 188) binary solid dispersions (SD) with different drug loadings were prepared, characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), and evaluated for solubility, in vitro release, and oral bioavailability of ibuprofen in rats. Loss of their individual surface properties during melting and solidification as revealed by SEM micrographs indicated the formation of effective SDs. Absence or shifting towards the lower melting temperature of the drug peak in SDs and physical mixtures in DSC study indicated the possibilities of its interactions with P 188. However, no such interactions in the solid state were confirmed by FTIR spectra which showed the presence of drug crystalline in SDs. Immediate and complete release of ibuprofen from SDs might be because of the reduction in the drug crystalline due to eutectic formation, and their dosing to fasted rats resulted in a significant increase in the area under curve (AUC) of the plasma concentration versus time curve and the maximum plasma concentration (Cmax), and a significant decrease in the time to reach Cmax (Tmax) over ibuprofen and physical mixtures.     

Effects of Grinding and Humidification on the Transformation of Conglomerate to Racemic Compound in Optically Active Drugs

The effects of grinding and humidification on the transformation of conglomerate to racemic compound have been investigated by X-ray powder diffraction (XPD), differential scanning calorimetry (DSC) and infrared (IR) spectroscopy for leucine, norleucine, valine, serine, tartaric acid and malic acid. Racemic physical mixtures were prepared by physical mixing of equimolar quantities of d and l crystals using a mortar and pestle. Ground mixtures were obtained by grinding the physical mixtures with a vibrational mill. Humidification was performed by storing the physical mixtures and the ground mixtures in a desiccator containing saturated aqueous salt solutions at 40°C. When physical mixtures of malic acid, tartaric acid and serine were ground, the XPD peaks of the racemic compounds were observed. The XPD patterns of humidified physical mixtures of these compounds also showed the formation of the racemic compounds. This indicated that grinding or humidification of malic acid, tartaric acid and serine induced the transformation of conglomerate to racemic compound crystals. When, on the other hand, the physical mixtures of valine, leucine and norleucine were ground, peaks of racemic compounds were not detected in the XPD pattern. After humidification of the ground mixtures of valine, leucine and norleucine, however, the XPD peaks of racemic compounds were observed. DSC and IR studies revealed consistent results. We concluded that grinding or humidification of malic acid, tartaric acid and serine could induce the transformation of a conglomerate to racemic compound. In contrast, humidifying after grinding was needed to bring about the transformation in leucine, norleucine and valine.     

Study of the Interaction of Dithranol with Heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin in Solution and in the Solid State

The interaction between dithranol and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TMBCyD) has been investigated in aqueous solution containing isoascorbic acid (0·2% w/v) as antioxidant and in the solid state. The interaction in the solid state was studied by differential scanning calorimetry (DSC), infrared spectroscopy (IR), X-ray powder diffractometry (XPD) and a dissolution-rate method. The extent of complexation between the two substances was poor, as indicated by the low value of the slope of the linear part of the solubility curve. A phase diagram was constructed by measuring the thermal behaviour of various re-solidified physical mixtures of dithranol and of TMBCyD previously subjected to heating until melting of the TMBCyD. The loss of dithranol, owing to sublimation and degradation caused by the thermal treatment used, was less than 10%. In keeping with XPD and IR data, the phase diagram indicated that a complex was formed containing 13·7% dithranol (molar ratio 1:1) which had a congruent melting point at 164°C. The drug dissolution rate from the 1:1 complex was measurable, unlike that of the corresponding physical mixture, and was significantly increased when the complex was dispersed in the glassy matrix of TMBCyD, as it was in re-solidified mixtures containing 2–7% dithranol. The results show that the solubility of dithranol is increased significantly as a consequence of its interaction with TMBCyD, despite the low extent of complexation between the two substances.     

Preformulation studies to meet the challenges in the manufacture of betaine solid dosage form

The objectives were (1). to perform solid-state characterization of anhydrous betaine (A) and betaine monohydrate (M), (2). to develop a pressure differential scanning calorimetric (DSC) technique for the quantification of M when present as a minor component in a mixture with A and, (3). to study the effect of annealing of A on the kinetics of A --> M transition. X-ray powder diffractometer (XRD), DSC, thermogravimetric analyzer (TGA), and an automated moisture sorption apparatus were used to characterize the phases. DSC at an elevated pressure of 200 psi enabled quantification of M in mixtures of A and M. Humidity-controlled TGA allowed study of the kinetics of A --> M transition. Automated moisture studies showed that A has a strong tendency to sorb water (at RH >or= 20%, 25 degrees C) and convert to M. When M was subjected to DSC at ambient pressure, the endotherms due to dehydration and vaporization of water overlapped. Pressure DSC enabled separation of these two thermal events. In mixtures of A and M, the enthalpy of dehydration (deltaH(d)) of M could be used for its quantification. A linear relationship was obtained when deltaH(d) was plotted as a function of the weight fraction of M in the mixture. The limits of detection and quantification of M in A were 0.15% and 1.5% w/w, respectively. The kinetics of water uptake by the annealed as well as the unannealed A, could be best described by the Avrami-Erofeev model (three-dimensional nucleation and growth). The calculated rate constant (k) of unannealed A (0.075 +/- 0.002 min(-1)) was significantly higher than that of annealed A (0.052 +/- 0.004 min(-1)). DSC at elevated pressure was a sensitive technique for quantification of M when present as a mixture with A. Annealing of A decelerated the A --> M phase transition reaction, possibly by increasing the degree of crystallinity of A.     

A system of differential scanning calorimetry for the investigation of medicinal preparations

A new system for differential scanning calorimetry (DSC) is described, which comprises a DSC setup linked via an interface to an IBM PC provided with specially developed software. The DSC instrument operation is based on the measurement of a local temperature difference with the aid of a disk thermocouple. The new system has been tested by the analysis of model drug mixtures. The results show the expediency and high potential of using DSC for the identification and quality control of drugs. The proposed system allows samples weighing 1–50 mg to be heated at a controlled rate within 0.1–64 K/min in the temperature range from −150 to 500°C. The only limitation is that the DSC method is applicable to substances not exhibiting decomposition at temperatures below the melting point. __________ Translated from Khimiko-Farmatsevticheskii Zhurnal, Vol. 39, No. 11, pp. 46–49, November, 2005.