Surface acidity and solid-state compatibility of excipients with an acid-sensitive API: case study of atorvastatin calcium.

The objectives of this study were to measure the apparent surface acidity of common excipients and to correlate the acidity with the chemical stability of an acid-sensitive active pharmaceutical ingredient (API) in binary API-excipient powder mixtures. The acidity of 26 solid excipients was determined by two methods, (i) by measuring the pH of their suspensions or solutions and (ii) the pH equivalent (pHeq) measured via ionization of probe molecules deposited on the surface of the excipients. The chemical stability of an API, atorvastatin calcium (AC), in mixtures with the excipients was evaluated by monitoring the appearance of an acid-induced degradant, atorvastatin lactone, under accelerated storage conditions. The extent of lactone formation in AC-excipient mixtures was presented as a function of either solution/suspension pH or pHeq. No lactone formation was observed in mixtures with excipients having pHeq > 6, while the lactone levels were pronounced (> 0.6% after 6 weeks at 50°C/20% RH) with excipients exhibiting pHeq < 3. The three pHeq regions (> 6, 3-6, and < 3) were consistent with the reported solution pH-stability profile of AC. In contrast to the pHeq scale, lactone formation did not show any clear trend when plotted as a function of the suspension/solution pH. Two mechanisms to explain the discrepancy between the suspension/solution pH and the chemical stability data were discussed. Acidic excipients, which are expected to be incompatible with an acid-sensitive API, were identified based on pHeq measurements. The incompatibility prediction was confirmed in the chemical stability tests using AC as an example of an acid-sensitive API.

Synchrotron X-ray diffraction investigation of the anomalous behavior of ice during freezing of aqueous systems.

Simple aqueous systems, i.e., phosphate-glycine buffers and pure water, were studied at subambient temperatures by X-ray difractometry using a high-intensity synchrotron radiation source at the Advanced Photon Source of Argonne National Laboratory. Complex X-ray diffraction (XRD) patterns, with two or more poorly resolved peaks in place of each of the four diagnostic peaks of hexagonal ice, 100, 002, 101, and 102, referred as "splitting", were observed in the majority of cases. The splitting of up to 0.05 A (d-spacing) was detected for 100, 002, and 101 peaks, whereas 102 peak was less affected. Deformation of the lattice of hexagonal ice, probably due to local stress created on the ice/ice or ice/container interface during water-to-ice transformation, is proposed as a possible mechanism for the observed splitting of XRD peaks. Using molecular modeling, it was estimated that the observed shifts in the peak positions are equivalent to applying a hydrostatic pressure of 2-3 kbars. The splitting can be used to quantify stresses during freezing, which could improve our understanding of the role of water-to-ice transformation on the destabilization of proteins and other biological systems.

Phase transitions in frozen systems and during freeze-drying: quantification using synchrotron X-ray diffractometry.

PURPOSE: (1) To develop a synchrotron X-ray diffraction (SXRD) method to monitor phase transitions during the entire freeze-drying cycle. Aqueous sodium phosphate buffered glycine solutions with initial glycine to buffer molar ratios of 1:3 (17:50 mM), 1:1 (50 mM) and 3:1 were utilized as model systems. (2) To investigate the effect of initial solute concentration on the crystallization of glycine and phosphate buffer salt during lyophilization. METHODS: Phosphate buffered glycine solutions were placed in a custom-designed sample cell for freeze-drying. The sample cell, covered with a stainless steel dome with a beryllium window, was placed on a stage capable of controlled cooling and vacuum drying. The samples were cooled to -50 degrees C and annealed at -20 degrees C. They underwent primary drying at -25 degrees C under vacuum until ice sublimation was complete and secondary drying from 0 to 25 degrees C. At different stages of the freeze-drying cycle, the samples were periodically exposed to synchrotron X-ray radiation. An image plate detector was used to obtain time-resolved two-dimensional SXRD patterns. The ice, beta-glycine and DHPD phases were identified based on their unique X-ray peaks. RESULTS: When the solutions were cooled and annealed, ice formation was followed by crystallization of disodium hydrogen phosphate dodecahydrate (DHPD). In the primary drying stage, a significant increase in DHPD crystallization followed by incomplete dehydration to amorphous disodium hydrogen phosphate was evident. Complete dehydration of DHPD occurred during secondary drying. Glycine crystallization was inhibited throughout freeze-drying when the initial buffer concentration (1:3 glycine to buffer) was higher than that of glycine. CONCLUSION: A high-intensity X-ray diffraction method was developed to monitor the phase transitions during the entire freeze-drying cycle. The high sensitivity of SXRD allowed us to monitor all the crystalline phases simultaneously. While DHPD crystallizes in frozen solution, it dehydrates incompletely during primary drying and completely during secondary drying. The impact of initial solute concentration on the phase composition during the entire freeze-drying cycle was quantified.

The glass transition and sub-T(g)-relaxation in pharmaceutical powders and dried proteins by thermally stimulated current.

The main goal of the study was to evaluate the applicability of thermally stimulated current (TSC) as a measure of molecular mobility in dried globular proteins. Three proteins, porcine somatotropin, bovine serum albumin, and immunoglobulin, as well as materials with a strong calorimetric glass transition (T(g)), that is, indomethacin and poly(vinypyrrolidone) (PVP), were studied by both TSC and differential scanning calorimetry (DSC). Protein/sugar colyophilized mixtures were also studied by DSC, to estimate calorimetric T(g) for proteins using extrapolation procedure. In the majority of cases, TSC detected relaxation events that were not observed by DSC. For example, a sub-T(g) TSC event (beta-relaxation) was observed for PVP at approximately 120 degrees C, which was not detected by the DSC. Similarly, DSC did not detect events in any of the three proteins below the thermal denaturation temperature whereas a dipole relaxation was detected by TSC in the range of 90-140 degrees C depending on the protein studied. The TSC signal in proteins was tentatively assigned as localized mobility of protein segments, which is different from a large-scale cooperative motions usually associated with calorimetric T(g). TSC is a promising method to study the molecular mobility in proteins and other materials with weak calorimetric T(g).

Comparative rates of freeze-drying for lactose and sucrose solutions as measured by photographic recording, product temperature, and heat flux transducer.

Sublimation from lactose and sucrose solutions has been monitored by temperature measurement, visual observation, heat flux sensing and manometric measurements. Estimates of energy transfer rates to the subliming mass made from visual observations and heat flux measurements are in broad agreement, demonstrating for the first time that heat flux sensors can be used to monitor the progress of lyophilization in individual vials with low sample volumes. Furthermore, it is shown that under identical lyophilization conditions the initial rate of drying for lactose solutions is low with little water sublimation for up to 150 minutes, which contrasts markedly with the much faster initial rate of drying for sucrose solutions. Measurement of the initial heat flux between shelf and vial indicated a lower flux to a 10% lactose solution than to a 10% sucrose solution.

Glycine crystallization in frozen and freeze-dried systems: effect of pH and buffer concentration.

PURPOSE: (1) To determine the effect of solution pH before lyophilization, over the range of 1.5 to 10, on the salt and polymorphic forms of glycine crystallizing in frozen solutions and in lyophiles. (2) To quantify glycine crystallization during freezing and annealing as a function of solution pH before lyophilization. (3) To study the effect of phosphate buffer concentration on the extent of glycine crystallization before and after annealing. MATERIALS AND METHODS: Glycine solutions (10% w/v), with initial pH ranging from 1.5 to 10, were cooled to -50 degrees C, and the crystallized glycine phases were identified using a laboratory X-ray source. Over the same pH range, glycine phases in lyophiles obtained from annealed solutions (0.25, 2 and 10% w/v glycine), were characterized by synchrotron X-ray diffractometry (SXRD). In the pH range of 3.0 to 5.9, the extent of glycine crystallization during annealing was monitored by SXRD. Additionally, the effect of phosphate buffer concentration (50 to 200 mM) on the extent of glycine crystallization during freezing, followed by annealing, was determined. RESULTS: In frozen solutions, beta-glycine was detected when the initial solution pH was < or =4. In the lyophiles, in addition to beta- and gamma-glycine, glycine HCl, diglycine HCl, and sodium glycinate were also identified. In the pH range of 3.0 to 5.9, decreasing the pH reduced the extent of glycine crystallization in the frozen solution. When the initial pH was fixed at 7.4, and the buffer concentration was increased from 50 to 200 mM, the extent of glycine crystallization in frozen solutions decreased with an increase in buffer concentration. CONCLUSION: Both solution pH and solute concentration before lyophilization influenced the salt and polymorphic forms of glycine crystallizing in frozen solutions and in lyophiles. The extent of glycine crystallization in frozen solutions was affected by the initial pH and buffer concentration of solutions. The high sensitivity of SXRD allowed simultaneous detection and quantification of multiple crystalline phases.

Solute crystallization in frozen systems-use of synchrotron radiation to improve sensitivity.

PURPOSE: To demonstrate the sensitivity of low temperature synchrotron X-ray diffractometry (SXRD) for detecting solute crystallization in frozen sodium phosphate buffer solutions. To determine the effect of annealing on solute crystallization in frozen solutions. MATERIALS AND METHODS: Sodium phosphate buffer solutions, at initial buffer concentrations ranging from 1 to 100 mM (pH 7.4) were cooled to -50 degrees C. The crystallization of disodium hydrogen phosphate dodecahydrate (Na(2)HPO(4) *12H(2)O) was monitored using a laboratory as well as a synchrotron source. At selected concentrations, the effect of annealing (at -20 degrees C) was investigated. RESULTS: With the laboratory source, solute crystallization, based on the appearance of one diagnostic peak with a d-spacing of 5.4 A, was evident only when the initial buffer concentration was at least 50 mM. In contrast, using SXRD, crystallization was detected at initial buffer concentrations down to 1 mM. In addition, the use of a high-resolution 2D detector enabled the visualization of numerous diffraction rings of the crystalline solute. At both 10 and 100 mM buffer concentration, there was no increase in solute crystallization due to annealing. CONCLUSION: By using synchrotron radiation, solute crystallization was detected with substantially increased sensitivity, making the technique useful for freeze-drying cycles of practical and commercial importance. Since numerous peaks of the crystalline solute appeared, the technique has potential utility in complex, multi-component systems.