Evgenyi Y. Shalaev

Hydrolysis in pharmaceutical formulations.

This literature review presents hydrolysis of active pharmaceutical ingredients as well as the effects on dosage form stability due to hydrolysis of excipients. Mechanisms and measurement methods are discussed and recommendations for formulation stabilization are listed.

Polyamorphism: a pharmaceutical science perspective

The occurrence of polymorphic forms of crystalline drugs and excipients is well known to pharmaceutical scientists (Byrn et al 1999), and at several recent scienti® c meetings the possible occurrence of polymorphic forms of amorphous pharmaceutical materials has been proposed. `̀ Polyamorphism ’’ is an intriguing concept from both a scienti® c and commercial perspective, and worthy of further comment in the pages of this journal. This is especially so because of the signi® cant impact amorphous character can have upon the performance of pharmaceutical materials (Hancock & Parks 2000), and the potential opportunities that might arise to exploit (and maybe patent) new and improved forms of existing pharmaceutical materials. Polyamorphism, that is, the possible existence of two distinct amorphous states of the same material separated by a clear phase transition, has been discussed for over twenty years (Angell & Sare 1970). In the most well known example it has been noted, based on a thermodynamic analysis of the heat capacity of water and ice, that there are diŒerences in the properties of amorphous ice samples formed by vapor-deposition and by quenchcooling from the liquid state. By this de® nition, polyamorphs are similar to crystalline polymorphs in that they represent diŒerent and discrete phases from a thermodynamic perspective. Several other examples of such polyamorphic materials which exhibit clear phase transitions between amorphous phases of the same chemical composition have been reported (Grimsditch 1984 ; Mishima et al 1984 ; Aasland & McMillan 1994). For the most part these materials are inorganic substances (Shalaev & Zogra® 2002), and the use of the term polyamorphism has been restricted to describe only those systems where multiple super-cooled thermodynamic liquid states have been shown to exist. The occurrence of true polyamorphs thus appears to be quite rare for typical pharmaceutical materials, so why all the interest in such systems by pharmaceutical scientists ? Over the past decade there have been several anecdotal reports of apparently diŒerent forms of amorphous pharmaceutical materials with readily discernible physical and chemical characteristics, and some marked diŒerences in their pharmaceutical performance. Examples include an antibiotic prepared by lyophilization (Pikal et al 1978) and glasses of an anti-in ̄ ammatory agent produced by fast cooling of the molten material (Yoshioka et al 1994). For these particular materials, even though the amorphous samples had signi® cantly diŒerent physical properties, there was no direct evidence of polyamorphism according to the strict thermodynamic de® nition provided above. Close inspection of the relevant literature reveals that most apparently polyamorphic amorphous pharmaceutical materials have been isolated and}or stored below their calorimetric glass transition temperatures. Such `̀ glassy ’ ’ amorphous materials are by de® nition not at energetic equilibrium with their surroundings and their properties re ̄ ect the conditions under which they were isolated and subsequently stored. As a result of their departure from equilibrium and the very long time that it takes glasses to spontaneously relax back to the equilibrium super-cooled liquid state, it appears that it is possible to isolate amorphous materials with distinct physical and chemical properties which are not true polyamorphs.

Investigation of Design Space for Freeze-Drying: Use of Modeling for Primary Drying Segment of a Freeze-Drying Cycle

In this work, we explore the idea of using mathematical models to build design space for the primary drying portion of freeze-drying process. We start by defining design space for freeze-drying, followed by defining critical quality attributes and critical process parameters. Then using mathematical model, we build an insilico design space. Input parameters to the model (heat transfer coefficient and mass transfer resistance) were obtained from separate experimental runs. Two lyophilization runs are conducted to verify the model predictions. This confirmation of the model predictions with experimental results added to the confidence in the insilico design space. This simple step-by-step approach allowed us to minimize the number of experimental runs (preliminary runs to calculate heat transfer coefficient and mass transfer resistance plus two additional experimental runs to verify model predictions) required to define the design space. The established design space can then be used to understand the influence of critical process parameters on the critical quality attributes for all future cycles.

Biopolymer mediated trehalose uptake for enhanced erythrocyte cryosurvival.

A biopolymer has been shown to facilitate efficient delivery of trehalose, a bioprotectant normally impermeable to the phospholipid bilayer, into ovine erythrocytes. Cellular uptake of trehalose was found to be dependent on polymer pendant amino acid type and degree of grafting, polymer concentration, pH, external trehalose concentration, incubation temperature and time. Optimization of these parameters yielded an intracellular trehalose concentration of 123 +/- 16 mM and concomitant improvement of erythrocyte cryosurvival of up to 20.4 +/- 5.6%. Intracellular trehalose was shown to impart cellular osmoprotection up to an external osmolarity of 230 mOsm and increased osmotic sensitivity above this threshold. Biopolymer mediated membrane permeability was shown to be rapidly and completely reversible via washing with phosphate buffered saline.

A Procedure to Optimize Scale-Up for the Primary Drying Phase of Lyophilization

This article describes a procedure to facilitate scale-up for the primary drying phase of lyophilization using a combination of empirical testing and numerical modeling. Freeze dry microscopy is used to determine the temperature at which lyophile collapse occurs. A laboratory scale freeze-dryer equipped with manometric temperature measurement is utilized to characterize the formulation-dependent mass transfer resistance of the lyophile and develop an optimized laboratory scale primary drying phase of the freeze-drying cycle. Characterization of heat transfer at both lab and pilot scales has been ascertained from data collected during a lyophilization cycle involving surrogate material. Using the empirically derived mass transfer resistance and heat transfer data, a semi-empirical computational heat and mass transfer model originally developed by Mascarenhas et al. (Mascarenhas et al., 1997, Comput Methods Appl Mech Eng 148: 105-124) is demonstrated to provide predictive primary drying data at both the laboratory and pilot scale. Excellent agreement in both the sublimation interface temperature profiles and the time for completion of primary drying is obtained between the experimental cycles and the numerical model at both the laboratory and pilot scales. Further, the computational model predicts the optimum operational settings of the pilot scale lyophilizer, thus the procedure discussed here offers the potential to both reduce the time necessary to develop commercial freeze-drying cycles by eliminating experimentation and to minimize consumption of valuable pharmacologically active materials during process development.

Ionization States in the Microenvironment of Solid Dosage Forms: Effect of Formulation Variables and Processing

PurposeEvaluation of the effect of formulation composition and processing variables on the microenvironment in solid dosage forms, based on ionization of indicator probes.Materials and MethodsSulfonephthalein indicators were intimately mixed with individual excipients, binary excipient mixtures or multi-component blends by the solvent deposition method. Diffuse reflectance visible spectroscopy of these solids provided a measure of indicator ionization extent. Indicator solution studies yielded equations relating solution pH to the ratio of the absorbance signals of the ionized to that of the unionized form, for each indicator. These equations and the spectral data of the indicator-treated solids were used to calculate an acidity function, ‘pHeq’ for the solids. The ionization of incorporated probes was also monitored during various stages of simulated pharmaceutical processing viz. wet and dry mixing.ResultsThe pHeq provided a measure of the physicochemical environment experienced by the probe in the solid. The surface nature of formulation components and their surface area available for interaction influenced the overall properties of the final blend. The extent of probe ionization varied at different stages of a simulated wet mixing–drying process. The pH of the excipient suspension was not a good predictor of the probe ionization in the final dried solid. Indicator ionization is expected to be influenced by the microenvironmental acidity, polarity and ionic strength. Individual excipient properties contributed to the overall microenvironment in powder mixtures even when dry mixed at low water contents.ConclusionsThe environment experienced by a drug in the final solid dosage form will be influenced by the nature of the excipients, the extent of their surfaces available for interaction, surface modification during processing and the amount and nature of solvent used.

Effect of Water on the Chemical Stability of Amorphous Pharmaceuticals: I. Small Molecules

Amorphous states, ubiquitous in pharmaceutical products, possess higher tendency for chemical degradation in comparison to crystalline materials. This instability can be further enhanced by water, which is present even in nominally dry systems. It has been increasingly recognized that in addition to the plasticizing effect of lowering the glass transition temperature, water could influence the degradation rates through medium effects (e.g., through change in solvation of the reactants and the transition state) as well as by direct participation in solid-state hydrolytic degradation processes. In the current review, the impact of water on the chemical stability of small molecules is examined, with emphasis on hydrolysis reactions in freeze-dried materials remaining in the glassy state. Quantitative relationships between water content and stability are discussed, including molecular mobility (global and local) and solution-like mechanisms, using the medium effects concept that has been developed for liquid-state reactions. Further progress in this field requires the development of quantitative and mechanistic understanding of the relationship between local mobility and chemical reactivity in amorphous solids, as well as incorporating the learning from solution chemistry on the role of reaction media in chemical processes.

Calorimetric and Diffractometric Evidence for the Sequential Crystallization of Buffer Components and the Consequential pH Swing in Frozen Solutions

Sequential crystallization of succinate buffer components in the frozen solution has been studied by differential scanning calorimetry and X-ray diffractometry (both laboratory and synchrotron sources). The consequential pH shifts were monitored using a low-temperature electrode. When a solution buffered to pH < pK(a)(2) was cooled from room temperature (RT), the freeze-concentrate pH first increased and then decreased. This was attributed to the sequential crystallization of succinic acid, monosodium succinate, and finally disodium succinate. When buffered to pH > pK(a)(2), the freeze-concentrate pH first decreased and then increased due to the sequential crystallization of the basic (disodium succinate) followed by the acidic (monosodium succinate and succinic acid) buffer components. XRD provided direct evidence of the crystallization events in the frozen buffer solutions, including the formation of disodium succinate hexahydrate [Na(2)(CH(2)COO)(2).6H(2)O]. When the frozen solution was warmed in a differential scanning calorimeter, multiple endotherms attributable to the melting of buffer components and ice were observed. When the frozen solutions were dried under reduced pressure, ice sublimation was followed by dehydration of the crystalline hexahydrate to a poorly crystalline anhydrate. However, crystalline succinic acid and monosodium succinate were retained in the final lyophiles. The pH and the buffer salt concentration of the prelyo solution influenced the crystalline salt content in the final lyophile. The direction and magnitude of the pH shift in the frozen solution depended on both the initial pH and the buffer concentration. In light of the pH-sensitive nature of a significant fraction of pharmaceuticals (especially proteins), extreme care is needed in both the buffer selection and its concentration.

Pronounced Microheterogeneity in a Sorbitol-Water Mixture Observed through Variable Temperature Neutron Scattering

In this study, the structure of concentrated d-sorbitol-water mixtures is studied by wide- and small-angle neutron scattering (WANS and SANS) as a function of temperature. The mixtures are prepared using both deuterated and regular sorbitol and water at a molar fraction of sorbitol of 0.19 (equivalent to 70% by weight of regular sorbitol in water). Retention of an amorphous structure (i.e., absence of crystallinity) is confirmed for this system over the entire temperature range, 100-298 K. The glass transition temperature, Tg, is found from differential scanning calorimetry to be approximately 200 K. WANS data are analyzed using empirical potential structure refinement, to obtain the site-site radial distribution functions (RDFs) and coordination numbers. This analysis reveals the presence of nanoscaled water clusters surrounded by (and interacting with) sorbitol molecules. The water clusters appear more structured compared to bulk water and, especially at the lowest temperatures, resemble the structure of low-density amorphous ice (LDA). Upon cooling to 100 K the peaks in the water RDFs become markedly sharper, with increased coordination number, indicating enhanced local (nanometer-scale) ordering, with changes taking place both above and well below the Tg. On the mesoscopic (submicrometer) scale, although there are no changes between 298 and 213 K, cooling the sample to 100 K results in a significant increase in the SANS signal, which is indicative of pronounced inhomogeneities. This increase in the scattering is partly reversed during heating, although some hysteresis is observed. Furthermore, a power law analysis of the SANS data indicates the existence of domains with well-defined interfaces on the submicrometer length scale, probably as a result of the appearance and growth of microscopic voids in the glassy matrix. Because of the unusual combination of small and wide scattering data used here, the present results provide new physical insight into the structure of aqueous glasses over a broad temperature and length scale, leading to an improved understanding of the mechanisms of temperature- and water-induced (de)stabilization of various systems, including proteins, pharmaceuticals, and biological objects.

Raffinose crystallization during freeze-drying and its impact on recovery of protein activity.

No HeadingPurpose.To study i) phase transitions in raffinose solution in the frozen state and during freeze-drying and ii) evaluate the impact of raffinose crystallization on the recovery of protein activity in reconstituted lyophiles.Methods.X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) were used to study the frozen aqueous solutions of raffinose pentahydrate. Phase transitions during primary and secondary drying were monitored by simulating the entire freeze-drying process, in situ, in the sample chamber of the diffractometer. The activity of lactate dehydrogenase (LDH) in reconstituted lyophiles was determined spectrophotometrically.Results.Raffinose formed a kinetically stable amorphous freeze-concentrated phase when aqueous solutions were frozen at different cooling rates. When these solutions were subjected to primary drying without annealing, raffinose remained amorphous. Raffinose crystallized as the pentahydrate when the solutions were annealed at a shelf temperature of −10°C. Primary drying of these annealed systems resulted in the dehydration of raffinose pentahydrate to an amorphous phase. The phase separation of the protein from the amorphous raffinose in these two systems during freeze-drying resulted in a significant reduction in the recovery of LDH activity, even though the lyophile was amorphous.Conclusions.Annealing of frozen aqueous raffinose solutions can result in solute crystallization, possibly as the pentahydrate. The crystalline pentahydrate dehydrates during primary drying to yield an amorphous lyophile. Raffinose crystallization during freeze-drying is accompanied by a significant loss of protein activity.