Development of a flow microcalorimetry method for the assessment of surface properties of powders

Stereomicroscope with Imaging Analysis: A Versatile Tool for Wetting, Gel Formation and Erosion Rate Determinations of Eutectic Effervescent Tablet

Wettability, gel formation and erosion behaviors could influence the drug release pattern of solid dosage forms. Typically, these parameters are evaluated using a variety of techniques. Nonetheless, there has been no previous research on versatile tool development for evaluating several tablet characteristics with a single tool. The aim of this study was to develop the versatile tool for measuring various physical properties of eutectic effervescent tablets and also investigate the relationship between these parameters with parameters from drug dissolution. Ibuprofen (IBU)-poloxamer 407 (P407) eutectic effervescent tablets were fabricated with a direct compression method. Their wetting properties, gel formation and erosion behaviors were investigated using a stereomicroscope with imaging analysis in terms of the liquid penetration distance, gel thickness and erosion boundary diameter, respectively. In addition, the dissolution rate (k) and disintegration time of eutectic effervescent tablets in 0.1 N HCl buffer pH 1.2 were also determined. Incorporation of P407 into the IBU tablet improved the tablet wetting properties with increasing liquid penetration distance under stereoscope. CO₂ liberation from effervescent agents promoted tablet surface roughness from matrix erosion. The relationship between observed physical properties and disintegration and dissolution parameters suggested that the combination of erosion by effervescent agents and gel formation by P407 had a potential influence on dissolution enhancement of the formulation. Therefore, a developed stereomicroscope with an imaging analysis technique was exhibited as an alternative versatile tool for determining the wetting properties, gel formation and erosion behaviors of pharmaceutical solid dosage forms.

Physical characterization of component particles included in dry powder inhalers. I. Strategy review and static characteristics

The performance of dry powder aerosols for the delivery of drugs to the lungs has been studied extensively in the last decade. The focus for different research groups has been on aspects of the powder formulation, which relate to solid state, surface and interfacial chemistry, bulk properties (static and dynamic) and measures of performance. The nature of studies in this field, tend to be complex and correlations between specific properties and performance seem to be rare. Consequently, the adoption of formulation approaches that on a predictive basis lead to desirable performance has been an elusive goal but one that many agree is worth striving towards. The purpose of this paper is to initiate a discussion of the use of a variety of techniques to elucidate dry particle behavior that might guide the data collection process. If the many researchers in this field can agree on this, or an alternative, guide then a database can be constructed that would allow predictive models to be developed. This is the first of two papers that discuss static and dynamic methods of characterizing dry powder inhaler formulations.

Calorimetric Investigation of the Structural Relaxation of Amorphous Materials: Evaluating Validity of the Methodologies

Although the potential advantages of the amorphous solid state is widely recognized among pharmaceutical researchers, its industrial applications have been mainly limited to freeze-dried injectable formulations where the amorphous form is naturally produced. Applications in oral dosage forms have been limited due, at least in part, to the poor state of knowledge regarding physical properties and stability of amorphous materials. Relaxation behavior is perhaps one of the most important physical characteristics of amorphous materials because relaxation kinetics are closely related to physical and chemical stability. Although recent developments in calorimetry methodology have facilitated detailed characterization of relaxation behavior, some experimental difficulties remain, and quantitative analysis of structural relaxation is still under development. This review focuses on the calorimetric investigation of the structural relaxation of drugs and excipients, and discusses the difficulties in the experimental evaluation of the relaxation time by those methods. We also present an original investigation of the impact of increases in relaxation time during an annealing experiment on the values of relaxation time, tau, and stretched exponential constant, beta, obtained from analysis of the experiment according to the Kohlraush-Williams-Watts kinetic model. Using results from a numerical simulation, we find that the values of tau and beta obtained from the data analysis are too large and too small, respectively, but the value of stretched relaxation time, tau(beta), remains reliable. The time dependence of the relaxation time is likely to play an important role in the non-Arrhenius behavior of pharmaceutical glasses.

Effect of surface modification on hydration kinetics of carbamazepine anhydrate using isothermal microcalorimetry

The purpose of this research was to improve the stability of carbamazepine (CBZ) bulk powder under high humidity by surface modification. The surface-modified anhydrates of CBZ were obtained in a specially designed surface modification apparatus at 60 degrees C via the adsorption of n-butanol, and powder x-ray diffraction, Fourier-Transformed Infrared spectra, and differential scanning calorimetry were used to determine the crystalline characteristics of the samples. The hydration process of intact and surface-modified CBZ anhydrate at 97% relative humidity (RH) and 40 +/-C 1 degrees C was automatically monitored by using isothermal microcalorimetry (IMC). The dissolution test for surface-modified samples (20 mg) was performed in 900 mL of distilled water at 37 +/-C 0.5 degrees C with stirring by a paddle at 100 rpm as in the Japanese Pharmacopoeia XIII. The heat flow profiles of hydration of intact and surface-modified CBZ anhydrates at 97% RH by using IMC profiles showed a maximum peak at around 10 hours and 45 hours after 0 and 10 hours of induction, respectively. The result indicated that hydration of CBZ anhydrate was completely inhibited at the initial stage by surface modification of n-butanol and thereafter transformed into dihydrate. The hydration of surface-modified samples followed a 2-dimensional phase boundary process with an induction period (IP). The IP of intact and surface-modified samples decreased with increase of the reaction temperature, and the hydration rate constant (k) increased with increase of the temperature. The crystal growth rate constants of nuclei of the intact sample were significantly larger than the surface-modified sample's at each temperature. The activation energy (E) of nuclei formation and crystal growth process for hydration of surface-modified CBZ anhydrate were evaluated to be 20.1 and 32.5 kJ/mol, respectively, from Arrhenius plots, but the Es of intact anhydrate were 56.3 and 26.8 kJ/mol, respectively. The dissolution profiles showed that the surface-modified sample dissolved faster than the intact sample at the initial stage. The dissolution kinetics were analyzed based on the Hixon-Crowell equation, and the dissolution rate constants for intact and surface-modified anhydrates were found to be 0.0102 +/-C 0.008 mg(1/3) x min(-1) and 0.1442 +/-C 0.0482 mg(1/3) x min(-1). The surface-modified anhydrate powders were more stable than the nonmodified samples under high humidity and showed resistance against moisture. However, surface modification induced rapid dissolution in water compared to the control.

Physicochemical stability of cimetidine amorphous forms estimated by isothermal microcalorimetry

The effect of humidity on the physicochemical properties of amorphous forms of cimetidine was investigated using differential scanning calorimetry, isothermal microcalorimetry, and x-ray diffraction analysis. Amorphous forms were obtained by the melting (amorphous form M [AM]) and the cotton candy (amorphous form C [AC]) methods. Thermal behaviors of AM and AC with or without seed crystals were measured using an isothermal microcalorimeter under various conditions of relative humidity (RH) and temperature, respectively. The crystallization kinetics of amorphous solids was analyzed based on 10 kinds of solid-state reaction models. AM transformed into form A at 11% RH, 50 degrees C but transformed into a mixture of form A and monohydrate at 51% and 75% RH at 25 degrees C. The mean crystallization times (MCTs) of the heat flow curve of AM and AC at 11% RH, 50 degrees C were 47.82 and 32.00 hours, respectively, but at 11% RH, 25 degrees C both were more than 4320 hours. In contrast, AC transformed into form A under all storage conditions. The MCTs of AC at 51% and 75% RH were 29.61 and 11.81 hours, respectively; whereas the MCTs of AM were 46.79 and 15.52 hours, respectively. The crystallization of amorphous solids followed the three-dimensional growth of nuclei (Avrami equation) with an induction period (IP). The IP for AM at 11% RH, 50 degrees C was more than 2 times that for AC, but the difference in the crystal growth rate constant (CR) between AC and AM was within 10%. The IP for AM at 75% RH, 25 degrees C was reduced to only 10% of the IP at 51% RH with increasing humidity, but the CR did not change significantly. In contrast, the IP for AC was slightly reduced at 75% RH compared with 51% RH, but the CR was about 5 times greater. At 75% RH, 25 degrees C, the IP and CR of AM were about one-fourth the values of AC. This result suggests that the crystallization process consists of an initial stage during which the nuclei are formed and a final stage of growth.

The impact of low levels of amorphous material (<5%) on the blending characteristics of a direct compression formulation

During the manufacture of tablets for registration stability studies, it was observed that blends manufactured using milled active frequently failed the blend content uniformity criteria (actual relative standard deviation (RSD) of 4-15%) at the prelubrication stage, whereas unmilled active batches were consistently giving very good blend uniformity results (RSD<3.5%). The addition of magnesium stearate dramatically improved the blending characteristics of the milled batches, suggesting that milling had altered the surface properties. A hypothesis was presented that amorphous material was created during the milling of the active batches, which subsequently recrystallised over a short period of time e.g. days/hours. Following recrystallisation the batches did not exhibit the same physical properties as the unmilled actives, and this resulted in the drug product batches failing to meet their pre-lubrication acceptance criteria for blend content uniformity. This paper describes the results of a laboratory scale study to investigate this hypothesis and therefore explain the processing issues that were observed during the manufacture of the registration stability batches with milled active batches.

Determination of surface free energy of interactive dry powder liposome formulations using capillary penetration technique

The surface free energy of interactive dry powder formulations consisting of varying ratios of lactose plus liposomal ciprofloxacin were determined using capillary penetration technique. Powder is produced by jet-milling after mixing with lyophilized liposomal ciprofloxacin with inhalation grade lactose powder (Pharmatose 325M). Measurement of the weight gained during intrusion of different liquids in a packed column of powder is combined with dynamic considerations to give the surface free energy, gamma(sv). Confidence in methodology was gained by determining gamma(sv) for PMMA microspheres and comparing to literature values. Values of gamma(sv) are then obtained for unmilled Pharmatose 325M powder (gamma(sv)=54.2 mJ m(-2)), milled Pharmatose 325M (gamma(sv)=54.2 mJ m(-2)) and lipid:lactose formulations with weight ratios of 1:5, 1:10 and 1:20. All the powder liposomal formulations are found to have the same gamma(sv)=48.0 mJ m(-2), suggesting that adhesive forces in the three interactive powders should be similar barring any confounding roughness effects.

Simultaneous determination of the heat and the quantity of vapor sorption using a novel microcalorimetric method

PURPOSE

In this study, instrumentation for measuring vapor sorption enthalpies and sorption uptakes simultaneously with an isothermal microcalorimeter is introduced. Various pharmaceutical model substances undergoing phase transitions when exposed to humid conditions (25 degrees C), were employed to evaluate the usefulness and sensitivity of the constructed experimental method.

METHODS

The sample is placed in the sample vessel of a RH cell and the moisture content of the air flow is controlled. From the RH cell the air flow is conducted into a subsequent perfusion cell in which a saturated salt solution has been loaded. The RH cell and perfusion cells are positioned in the sample sides of two twin calorimetric units. Depending on the moisture content in the outlet flow leaving the preceding RH cell, the heat flow signal from the subsequent perfusion cell will vary. By means of blank measurement with identical settings, the rate of water sorption can be calculated and, by integration, the amount of sorbed water is obtained.

RESULTS

Amorphous lactose and cefadroxil undergo recrystallization when the moisture level in the surroundings exceeds the threshold values specific to each compound. During the sorption phase, heat is evolved fairly linearly as a function of consumed moisture, and also after the recrystallization, the heats indicate linear behavior. The heat values for the desorption phase of amorphous lactose and the adsorption of crystalline lactose coincide. With the different anhydrous forms of theophylline, the hydration takes place more rapidly in the metastable form 1, and generally, the process is more energetic in form 1. In all cases, the gravimetric results agree with the water sorption uptakes calculated from the calorimetric data.

CONCLUSIONS

The technique introduced offers a rapid and sensitive method to gain new insights into the transitions in which vapors are involved. In addition, different kinds of surfaces with various energetics can now be studied more closely.

Application of isothermal microcalorimetry in solid state drug development

Microcalorimetry is an analytical technique that has found numerous applications within the pharmaceutical environment. In the realm of pharmaceutics, especially solid state pharmaceutics, the technique has proved to be an invaluable tool. This review addresses the solid state applications of microcalorimetry within the pharmaceutical industry, with a specific focus on stability, compatibility and amorphicity determinations.

Isothermal microcalorimetry and inverse phase gas chromatography to study small changes in powder surface properties

It is known that processing can alter the surface energetics of powders. In this study a sample of drug has been processed by use of different drying techniques. The samples were then assessed using inverse phase gas chromatography. It was seen that the original material had a highest surface energy and the tray-dried sample had the lowest energy surface, other samples were intermediate. The use of isothermal microcalorimetry to study water sorption to the powders revealed that the surface of the original material was unstable, as the water sorption response changed on repeat cycling. The tray-dried sample did have a stable surface which gave the same sorption response on repeat exposure to water vapour. It was concluded that the drug had minor variations in surface energy, with the as received material being in a high energy unstable state, which could be due to it being partially amorphous. The tray-dried sample had a lower energy stable surface. In certain applications differences in surface energetics could be expected to lead to changes in processing nature of the powder, so these vapour sorption techniques offer a good way of providing an assurance of the same surface energy between batches of nay material which may be at risk.

Surface free energy of ethylcellulose films and the influence of plasticizers

The surface free energy parameters of ethylcellulose (EC) films were determined using the Lifshitz-van der Waals/acid-base approach and the influence of plasticizers on their surface energetics was assessed. Films were prepared by dip-coating glass slides in organic solvents containing EC and the advancing angles of drops of pure liquids on the EC films were measured with a contact angle goniometer using the captive drop technique. EC has lower surface free energy than cellulose. The acid-base (AB) term made only a slight contribution to the total surface free energy and the surfaces exhibited predominantly monopolar electron-donicity. The addition of plasticizer (dibutyl sebacate or dibutyl phthalate) resulted in a small decrease in the total surface free energy. The effects of film forming variables, including solvent system, concentration and post-formation treatment (annealing), on the surface free energy parameters of EC films were also investigated. These data were then used to analyze how the surface energetics affect the interaction of the EC films with other surfaces based on interfacial tension, work of adhesion and spreading coefficient calculations. Lifshitz-van der Waals (LW) interactions provided the major contribution to the work of adhesion for EC with all of the solid substrates analyzed. However, the AB interactions contributed significantly to the work of adhesion for EC with 'bipolar' substrates and to the spreading coefficients of EC over substrates. The consideration of work of adhesion and spreading coefficient based on surface free energy parameters may have potential use in evaluating factors affecting film adhesion and, furthermore, in optimizing pharmaceutical film coating processes.

Real time detection of photoreactivity in pharmaceutical solids and solutions with isothermal microcalorimetry

PURPOSE

In this study an irradiation cell made as an accessory for an isothermal microcalorimeter is introduced, and its suitability for detection photoreactivity in pharmaceutical solutions and solids is demonstrated. The pharmaceuticals employed are chosen as sample materials to evaluate the usefulness and stability of the irradiation cell.

METHODS

An irradiation cell has been constructed and tested in an isothermal microcalorimeter with pharmaceutical solutions and solids known to be sensitive to daylight or UV light. Light is produced with an Xe-arc lamp, split into two parts and introduced into calorimetric vessels with optical light cables. One of the vessels containing the reference sample gives the response to the heat absorbed by the material (radiant power), and the other vessel containing the sample material gives the response also to the photoreaction. The two irradiation cells are positioned in the sample sides of two separate twin microcalorimetric units.

RESULTS

Nifedipine and L-ascorbic acid were found to be photosensitive in solutions and solid states, the extent of the degradation depending on the irradiation intensity and wavelength. The threshold values of the wavelength for the photoreactions, as well as the wavelengths for the maximum reaction rates, were estimated via the scanning irradiation measurements. The ability of photons with different energies to produce heat in the photosensitive reaction of nifedipine was calculated using constant lambda measurements.

CONCLUSIONS

The technique introduced offers a rapid and versatile method to study the photosensitivity of materials in any state. In the measurements, various conditions can be simulated and thus provide information on the real behavior of materials.

Use of Fourier transform infrared (FTIR) spectroscopy to follow the adsorption of heptane and 1,4-dioxane vapors on a zinc oxide surface

Vapor adsorption isotherms of two nonpolar model compounds, heptane and 1,4-dioxane, were determined for a very small particle size zinc oxide (ZnO) powder (median particle size approximately 23 nm) in the lower relative vapor pressure (P/Po) region. The ZnO samples for all adsorption measurements were dried at 400 degrees C for 4 h. A new method, which employed an FTIR spectrometer with a long path gas cell (IR path length of 3.0 m), was developed for the organic vapor adsorption measurements. The amount adsorbed was determined by mass balance. This method allows accurate quantification of organic vapors and is sensitive to very low P/Po values. The heptane and 1, 4-dioxane vapor adsorption isotherms appeared to exhibit the expected Type II behavior. The surface areas obtained for ZnO from BET analyses of the heptane and 1,4-dioxane vapor adsorption isotherms (36.9 and 30.3 m2/g) compared reasonably well to the surface area obtained from BET analysis of the nitrogen vapor adsorption isotherm (32.6 m2/g). The amount of vapor adsorbed by ZnO at P/Po equal to 0.1, in terms of number of moles, was observed to decrease in the order: water10 >> 1,4-dioxane > heptane. It was inferred that, while heptane was only adsorbed via a dipole-induced dipole interaction, 1,4-dioxane was physically adsorbed via an interaction dominated by the oxygen lone-pair orbital. Presumably, this interaction was more comparable to a weak dipole-dipole interaction. These results are consistent with the expected strengths of interaction.

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.

Quantitative characterization of adsorption isotherms using isothermal microcalorimetry

The integral heat of adsorption of water vapor on sodium benzoate samples was determined at various partial vapor pressures using a heat conduction microcalorimeter. An equation is presented to describe the calorimetric integral heat response (mJ/g of solid) as a function of relative humidity. This equation, although similar in principle to the well-known BET equation, relates the heat evolved (rather than volume or mass of gas adsorbed) upon adsorption to the partial pressure of the gas. It qualitatively describes the shape of the calorimetric isotherm and quantitatively allows the calculation of "monolayer capacity" or the apparent surface area with water as the adsorbate. The modified BET equation was applied to the calorimetric adsorption data available in the literature. The surface area or the monolayer coverage values of the solid samples used in these studies were calculated from data-fitted parameter estimates. Good agreement was found between Vm or surface area values obtained by the application of the model to the calorimetric data and those reported by the authors using conventional gravimetric or volumetric measurement of adsorption. The model satisfactorily described the experimental calorimetric data of water vapor adsorption on sodium benzoate. The model equation and the use of isothermal microcalorimetry provide a means to obtain the water adsorption surface area of solid materials. The method may also be useful in comparing the surface properties of drugs and excipients obtained by different methods or from different sources. The microcalorimetric method to characterize adsorption is more sensitive and convenient in comparison with some of the conventional techniques.