Molecular Dynamics and Physical Stability of Pharmaceutical Co-amorphous Systems: Correlation Between Structural Relaxation Times Measured by Kohlrausch-Williams-Watts With the Width of the Glass Transition Temperature (ΔTg) and the Onset of Crystallization.

The study aims to characterize the structural relaxation times of quench-cooled co-amorphous systems using Kohlrausch-Williams-Watts (KWW) and to correlate the relaxation data with the onset of crystallization. Comparison was also made between the relaxation times obtained by KWW and the width of glass transition temperature (ΔTg) methods (simple and quick). Differential scanning calorimetry, Fourier-transformed infrared spectroscopy, and polarized light microscopy were used to characterize the systems. Results showed that co-amorphous systems yielded a single Tg and ΔCp, suggesting the binary mixtures exist as a single amorphous phase. A narrow step change at Tg indicates the systems were fragile glasses. In co-amorphous nap-indo and para-indo, experimental Tgs were in good agreement with the predicted Tg. However, the Tg of co-amorphous nap-cim and indo-cim were 20°C higher than the predicted Tg, possibly due to stronger molecular interactions. Structural relaxation times below the experimental Tg were successfully characterized using the KWW and ΔTg methods. The comparison plot showed that KWW data are directly proportional to the ½ power of ΔTg data, after adjusting for a small offset. A moderate positive correlation was observed between the onset of crystallization and the KWW data. Structural relaxation times may be useful predictor of physical stability of co-amorphous systems.

Effect of disaccharide-polyol systems on the thermal stability of freeze-dried Mycobacterium bovis.

Live attenuated Mycobacterium bovis (M. bovis), marketed as Bacille Calmette-Guérin is the only FDA-approved vaccine against tuberculosis. The prerequisite of cold chain storage between 2 and 8 °C hinders the global vaccination effort. The study aims to investigate the effect of trehalose, sucrose and glycerol combinations in enhancing the stability of M. bovis. The bacilli were formulated in various ratios of trehalose-glycerol, sucrose-glycerol, trehalose-sucrose-glycerol systems (test samples) and sodium glutamate (control), freeze-dried and stored for 28 days at 4 °C, 25 °C and 37 °C. Bacteria viability at pre-, post-freeze-drying and after storage were quantified by its density in colony-forming unit per milliliter (CFU/mL) as obtained through the pour plate method. Formulations were characterized using differential scanning calorimetry. Structural collapsed cakes were found on all freeze-dried formulations because of the low Tg'. Comparing between binary and ternary formulations, trehalose-sucrose-glycerol was found to be a superior lyoprotectant. Upon storage, the viability of bacteria in disaccharide-polyol formulations was highest when stored at 4 °C followed by 25 °C. The lowest viability was found after storage at 37 °C. While the ternary disaccharide-polyol system may be used as a thermoprotectant up to 25 °C, sodium glutamate has a superior thermoprotective effect at temperature above 25 °C.

Investigation of physical properties and stability of indomethacin-cimetidine and naproxen-cimetidine co-amorphous systems prepared by quench cooling, coprecipitation and ball milling.

OBJECTIVES: The objective was to characterize the structural behaviour of indomethacin-cimetidine and naproxen-cimetidine co-amorphous systems (1 : 1 molar ratio) prepared by quench cooling, co-evaporation and ball milling. METHODS: Powder X-ray diffraction (PXRD) and DSC were used to characterise the samples. Structural relaxation (i.e. molecular mobility) behaviour was obtained from the Kohlrausch-Williams-Watts (KWW) relationship. KEY FINDINGS: A glass transition temperature (Tg ), on average 20 °C higher than the predicted Tg (calculated from the Fox equation), was observed in all samples. The structural relaxation was dependent on the preparative methods. At a storage temperature of 40 °C, a comparatively higher molecular mobility was observed in indomethacin-cimetidine samples prepared by ball milling (ln τ(ß) = 0.8), while similar molecular mobility was found for the same sample prepared by quench cooling (ln τ(ß) = 2.4) and co-evaporation (ln τ(ß) = 2.5). In contrast, molecular mobility of the naproxen-cimetidine samples followed the order co-evaporation (ln τ(ß) = 0.8), quench cooling (ln τ(ß) = 1.6) and ball milling (ln τ(ß) = 1.8). CONCLUSION: The estimated relaxation times by the DSC-KWW method suggest that different preparative methods resulted in a variation of structural characteristics. Despite the differences in molecular mobility, all sample remained co-amorphous for up to 7 months.

Detecting phase separation of freeze-dried binary amorphous systems using pair-wise distribution function and multivariate data analysis.

The purpose of this study is to investigate the use of multivariate data analysis for powder X-ray diffraction-pair-wise distribution function (PXRD-PDF) data to detect phase separation in freeze-dried binary amorphous systems. Polymer-polymer and polymer-sugar binary systems at various ratios were freeze-dried. All samples were analyzed by PXRD, transformed to PDF and analyzed by principal component analysis (PCA). These results were validated by differential scanning calorimetry (DSC) through characterization of glass transition of the maximally freeze-concentrate solute (Tg'). Analysis of PXRD-PDF data using PCA provides a more clear 'miscible' or 'phase separated' interpretation through the distribution pattern of samples on a score plot presentation compared to residual plot method. In a phase separated system, samples were found to be evenly distributed around the theoretical PDF profile. For systems that were miscible, a clear deviation of samples away from the theoretical PDF profile was observed. Moreover, PCA analysis allows simultaneous analysis of replicate samples. Comparatively, the phase behavior analysis from PXRD-PDF-PCA method was in agreement with the DSC results. Overall, the combined PXRD-PDF-PCA approach improves the clarity of the PXRD-PDF results and can be used as an alternative explorative data analytical tool in detecting phase separation in freeze-dried binary amorphous systems.

Characterization of dynamics in complex lyophilized formulations: II. Analysis of density variations in terms of glass dynamics and comparisons with global mobility, fast dynamics, and Positron Annihilation Lifetime Spectroscopy (PALS).

Amorphous HES/disaccharide (trehalose or sucrose) formulations, with and without added polyols (glycerol and sorbitol) and disaccharide formulations of human growth hormone (hGH), were prepared by freeze drying and characterized with particular interest in methodology for using high precision density measurements to evaluate free volume changes and a focus on comparisons between "free volume" changes obtained from analysis of density data, fast dynamics (local mobility), and PALS characterization of "free volume" hole size. Density measurements were performed using a helium gas pycnometer, and fast dynamics was characterized using incoherent neutron scattering spectrometer. Addition of sucrose and trehalose to hGH decreases free volume in the system with sucrose marginally more effective than trehalose, consistent with superior pharmaceutical stability of sucrose hGH formulations well below Tg relative to trehalose. We find that density data may be analyzed in terms of free volume changes by evaluation of volume changes on mixing and calculation of apparent specific volumes from the densities. Addition of sucrose to HES decreases free volume, but the effect of trehalose is not detectable above experimental error. Addition of sorbitol or glycerol to HES/trehalose base formulations appears to significantly decrease free volume, consistent with the positive impact of such additions on pharmaceutical stability (i.e., degradation) in the glassy state. Free volume changes, evaluated from density data, fast dynamics amplitude of local motion, and PALS hole size data generally are in qualitative agreement for the HES/disaccharide systems studied. All predict decreasing molecular mobility as disaccharides are added to HES. Global mobility as measured by enthalpy relaxation times, increases as disaccharides, particularly sucrose, are added to HES.

Characterization of dynamics in complex lyophilized formulations: I. Comparison of relaxation times measured by isothermal calorimetry with data estimated from the width of the glass transition temperature region.

The purposes of this study are to characterize the relaxation dynamics in complex freeze dried formulations and to investigate the quantitative relationship between the structural relaxation time as measured by thermal activity monitor (TAM) and that estimated from the width of the glass transition temperature (ΔT(g)). The latter method has advantages over TAM because it is simple and quick. As part of this objective, we evaluate the accuracy in estimating relaxation time data at higher temperatures (50 °C and 60 °C) from TAM data at lower temperature (40 °C) and glass transition region width (ΔT(g)) data obtained by differential scanning calorimetry. Formulations studied here were hydroxyethyl starch (HES)-disaccharide, HES-polyol, and HES-disaccharide-polyol at various ratios. We also re-examine, using TAM derived relaxation times, the correlation between protein stability (human growth hormone, hGH) and relaxation times explored in a previous report, which employed relaxation time data obtained from ΔT(g). Results show that most of the freeze dried formulations exist in single amorphous phase, and structural relaxation times were successfully measured for these systems. We find a reasonably good correlation between TAM measured relaxation times and corresponding data obtained from estimates based on ΔT(g), but the agreement is only qualitative. The comparison plot showed that TAM data are directly proportional to the 1/3 power of ΔT(g) data, after correcting for an offset. Nevertheless, the correlation between hGH stability and relaxation time remained qualitatively the same as found with using ΔT(g) derived relaxation data, and it was found that the modest extrapolation of TAM data to higher temperatures using ΔT(g) method and TAM data at 40 °C resulted in quantitative agreement with TAM measurements made at 50 °C and 60 °C, provided the TAM experiment temperature, is well below the Tg of the sample.

An overview of recent studies on the analysis of pharmaceutical polymorphs.

Pharmaceutical solids are well known to be able to exist in different solid-state forms and there are a wide variety of solid-state analytical techniques available to characterize pharmaceutical solids and solid-state transformations. In this review, the commonly used solid-state analytical techniques, the type of information collected, and advantages and disadvantages of each technique are discussed, with the focus on their application in solid-state characterization and monitoring solid-state transformations, such as amorphization, crystallization, hydrate formation/dehydration and cocrystal formation. The information gathered from recent literature is compiled in various tables to aid the reader to get a quick overall picture about what type of phenomena have recently been studied and which analytical technique(s) have been used.

Enhanced dissolution rate and synchronized release of drugs in binary systems through formulation: Amorphous naproxen-cimetidine mixtures prepared by mechanical activation.

Naproxen, a non-steroidal anti-inflammatory drug (NSAID), is a biopharmaceutics classification system (BCS) class II drug whose bioavailability is rate-limited by its dissolution. Cimetidine is sometimes co-administered with naproxen for the treatment of NSAID-induced gastro-intestinal disorders. Hence, there is interest in the design of new formulations that offer (1) concomitant release of both drugs, and (2) an enhanced dissolution rate of naproxen. This study investigates the formation of amorphous binary systems with naproxen and cimetidine. The binary mixtures of all tested molar ratios were found to become amorphous upon co-milling for 60 min at 4 degrees C. In contrast, pure naproxen could not be transformed to the amorphous state by mechanical activation. The 1:1 sample was the most physically stable when stored for 33 days at 40 degrees C, even though it did not have the highest T(g) when compared to the 1:2 sample. The 1:1 sample was further stored for 186 days and remained amorphous under all conditions. Raman spectroscopy suggested a 1:1 solid-state interaction between the imidazole ring of cimetidine and the carboxylic acid moiety of naproxen in the co-milled amorphous sample. Thus, the stabilization of the amorphous binary system is dictated by molecular-level interactions rather than bulk-level phenomena. No recrystallization of either drug in the 1:1 co-milled sample was observed during dissolution testing, with naproxen and cimetidine having a four and two times higher intrinsic dissolution rate, respectively, compared to their crystalline counterparts. Further, the release of the two drugs could be synchronized using this formulation approach.

Physical characterization and stability of amorphous indomethacin and ranitidine hydrochloride binary systems prepared by mechanical activation.

Co-milling of gamma-indomethacin and ranitidine hydrochloride form 2 at various weight ratios (1:2, 1:1 and 2:1) was investigated with a particular interest in the physicochemical properties and the stability of the milled mixed amorphous form. Co-milling was carried out using an oscillatory ball mill for various periods of time up to 60 min in a cold room (4 degrees C). The maximum temperature of the solid material was 42 degrees C during co-milling in a cold room. Results showed that both indomethacin and ranitidine hydrochloride were fully converted into the amorphous state after 60 min of co-milling. In contrast individually milled drugs remained partially crystalline after co-milling under the same conditions. During co-milling, the XRPD characteristic peaks of indomethacin were found to decrease faster than those of ranitidine hydrochloride. DSC results were in agreement with XRPD, and T(g)s of the fully converted amorphous mixtures of 29.3, 32.5 and 34.3 degrees C were measured for the 1:2, 1:1 and 2:1 mixtures, respectively. These T(g) values were in good agreement with the predicted T(g)s of the mixtures using the Gordon-Taylor equation. DRIFTS spectra of the co-milled amorphous samples showed peaks at 1610, 1679 and 1723 cm(-1), that were not present in the individually milled samples and that are indicative of an interaction at the carboxylic acid carbonyl (HO-C=O) and benzonyl amide (N=CO) of the indomethacin molecule with the aci-nitro (C=N) of ranitidine hydrochloride. Upon 30 days of storage, the 1:2 mixtures were found to crystallize; however, the amorphous 2:1 and 1:1 mixtures were stable when milled for 60 min and stored at 4 degrees C (for the 2:1 mixture) and at 4 and 25 degrees C (for the 1:1 mixture), respectively. Although XRPD, DSC and DRIFTS suggested an interaction between the two drugs, co-crystal formation was not observed between indomethacin and ranitidine hydrochloride.

Quantitative solid-state analysis of three solid forms of ranitidine hydrochloride in ternary mixtures using Raman spectroscopy and X-ray powder diffraction.

The aim of the study was to develop a reliable quantification procedure for mixtures of three solid forms of ranitidine hydrochloride using X-ray powder diffraction (XRPD) and Raman spectroscopy combined with multivariate analysis. The effect of mixing methods of the calibration samples on the calibration model quality was also investigated. Thirteen ternary samples of form 1, form 2 and the amorphous form of ranitidine hydrochloride were prepared in triplicate to build a calibration model. The ternary samples were prepared by three mixing methods (a) manual mixing (MM) and ball mill mixing (BM) using two (b) 5 mm (BM5) or (c) 12 mm (BM12) balls for 1 min. The samples were analyzed with XRPD and Raman spectroscopy. Principal component analysis (PCA) was used to study the effect of mixing method, while partial least squares (PLS) regression was used to build the quantification models. PCA score plots showed that, in general, BM12 resulted in the narrowest sample clustering indicating better sample homogeneity. In the quantification models, the number of PLS factors was determined using cross-validation and the models were validated using independent test samples with known concentrations. Multiplicative scattering correction (MSC) without scaling gave the best PLS regression model for XPRD, and standard normal variate (SNV) transformation with centering gave the best model for Raman spectroscopy. Using PLS regression, the root mean square error of prediction (RMSEP) values of the best models were 5.0-6.9% for XRPD and 2.5-4.5% for Raman spectroscopy. XRPD and Raman spectroscopy in combination with PLS regression can be used to quantify the amount of single components in ternary mixtures of ranitidine hydrochloride solid forms. Raman spectroscopy gave better PLS regression models than XRPD, allowing a more accurate quantification.

Formation and physical stability of the amorphous phase of ranitidine hydrochloride polymorphs prepared by cryo-milling.

The effect of cryo-milling on ranitidine hydrochloride polymorphs form 1 and 2 was investigated with particular interest in the formation and the stability of the amorphous phase. Cryo-milling was carried out using an oscillatory ball mill for periods up to 60 min, with re-cooling of the milling chamber with liquid nitrogen at 15 min intervals. Results showed that both ranitidine hydrochloride form 1 and form 2 could be fully converted to the amorphous form as determined by XRPD within 30 min. Upon 14 days storage, the amorphous samples crystallized back to their original forms. In the stability studies of amorphous drug with seeds, significant polymorphic transformation from form 1 to form 2 was not found when amorphous form prepared from form 1 was seeded with form 2 crystals by gentle physical mixing. In contrast, amorphous form prepared from form 1 seeded with form 2 crystals by ball milling for 1 min and simultaneous cryo-milling methods were found to transform amorphous form prepared from form 1 to crystalline form 2 under some storage conditions. The transformation was thought to be facilitated by interaction between seed crystals and amorphous drug and a storage temperature above the Tg. Amorphous form prepared from form 2 did not transform to crystalline form 1 under any conditions used in this study.