A method is described for screening compounds that inhibit crystallization in solution to enable ... more A method is described for screening compounds that inhibit crystallization in solution to enable more accurate measurement of amorphous drug solubility. Three polymers [polyvinylpyrrolidone, hydroxypropyl methylcellulose, and hydroxypropyl methylcellulose acetate succinate (HPMCAS)] were screened for their ability to inhibit the crystallization of neat amorphous drugs during measurement of solubility of the amorphous form in water. Among the polymers evaluated, HPMCAS was found to be most promising. The use of HPMCAS provided an "apparent solubility" of amorphous drugs that was closer to the theoretically calculated values. With danazol, agreement was essentially quantitative, and for griseofulvin and iopanoic acid, agreement was within a factor of two; these maximum concentrations were sustained for a period of 40-90 min. Dynamic light scattering of filtered samples (0.22 µ) revealed the presence of colloidal drug-polymer assemblies in solution (100-150 nm). The supernatant resulting from this centrifugation gradually decreased in concentration, but remained supersaturated with respect to crystalline drug for several hours. Thus, HPMCAS has been shown to be a useful additive in dissolution media to allow a more accurate determination of the solubility of fast crystallizing neat amorphous drugs, at least for the drugs studied, and it should also serve to retard crystallization in vivo and therefore, facilitate improved bioavailability.
In recent years there has been growing interest in advancing amorphous pharmaceuticals as an appr... more In recent years there has been growing interest in advancing amorphous pharmaceuticals as an approach for achieving adequate solubility. Due to difficulties in the experimental measurement of solubility, a reliable estimate of the solubility enhancement ratio of an amorphous form of a drug relative to its crystalline counterpart would be highly useful. We have developed a rigorous thermodynamic approach to estimate enhancement in solubility that can be achieved by conversion of a crystalline form to the amorphous form. We rigorously treat the three factors that contribute to differences in solubility between amorphous and crystalline forms. First, we calculate the free energy difference between amorphous and crystalline forms from thermal properties measured by modulated differential scanning calorimetry (MDSC). Secondly, since an amorphous solute can absorb significant amounts of water, which reduces its activity and solubility, a correction is made using water sorption isotherm data and the Gibbs–Duhem equation. Next, a correction is made for differences in the degree of ionization due to differences in solubilities of the two forms. Utilizing this approach the theoretically estimated solubility enhancement ratio of 7.0 for indomethacin (amorphous/g-crystal) was found to be in close agreement with the experimentally determined ratio of 4.9.
Solventless photocurable film coating was used to obtain modified release coatings. Different por... more Solventless photocurable film coating was used to obtain modified release coatings. Different pore-forming agents were used to achieve immediate and sustained release of a blue dye contained in the coated pellets (non-pareil beads). A super-disintegrant, sodium starch glycolate, was used to obtain immediate release. When incorporated in the coating, this pore-former swelled and yielded large pores that demonstrated quick release of the marker dye while leaving behind the scaffold provided by the photocured polymer. Simple pore-formers (lactose and sodium chloride) dissolved away without swelling and provided a more sustained release profile. Release can be modified with the choice of material, number of layers and thickness of the coating. When release of sodium chloride and release of marker dye were simultaneously monitored, it was observed that at least 40-50% of the sodium chloride that was incorporated in the coating released before the dye was released through the coating showing that pore-formation preceded the release of the marker dye. The coupling factor between dye release and pore-formation was found to be dependent on the ratio of amount of solid pore-forming agent and volume of liquid monomer (S/L) of the coating. Also, studies demonstrate that the coating is photostable and can withstand handling stress.
Coatings are an essential part in the formulation of pharmaceutical dosage form to achieve superi... more Coatings are an essential part in the formulation of pharmaceutical dosage form to achieve superior aesthetic quality (e.g., color, texture, mouth feel, and taste masking), physical and chemical protection for the drugs in the dosage forms, and modification of drug release characteristics. Most film coatings are applied as aqueous- or organic-based polymer solutions. Both organic and aqueous film coating bring their own disadvantages. Solventless coating technologies can overcome many of the disadvantages associated with the use of solvents (e.g., solvent exposure, solvent disposal, and residual solvent in product) in pharmaceutical coating. Solventless processing reduces the overall cost by eliminating the tedious and expensive processes of solvent disposal/treatment. In addition, it can significantly reduce the processing time because there is no drying/evaporation step. These environment-friendly processes are performed without any heat in most cases (except hot-melt coating) and thus can provide an alternative technology to coat temperature-sensitive drugs. This review discusses and compares six solventless coating methods - compression coating, hot-melt coating, supercritical fluid spray coating, electrostatic coating, dry powder coating, and photocurable coating - that can be used to coat the pharmaceutical dosage forms.
A flow-through dissolution apparatus was designed and evaluated to screen small quantities of pha... more A flow-through dissolution apparatus was designed and evaluated to screen small quantities of pharmaceutical drug compounds early in development. The apparatus was designed to mount on a microscope slide such that a compacted solid drug was positioned flush along one wall and the fluid flow in the apparatus was laminar flow in a rectangular duct. Stereomicroscopic digital images and Raman spectra of the solid were taken during dissolution and the effluent dissolution medium was collected in fractions to determine the dissolution rate by fluorescence or HPLC/UV. Three compounds, triamterene, ketoprofen, and β-naphthoic acid were investigated in the dissolution flow cell at various hydrodynamic conditions. In conditions where no solvent-mediated conversion was expected, there was a decrease in dissolution rate with time in the flow through cell that was associated with surface smoothing. This phenomenon also occurred in rotating disk experiments. In either case, the magnitude and time course of the decrease in dissolution rate with time is generally different enough to distinguish from the decrease in dissolution rate due to solvent-mediated conversion.
Although proteins are often frozen during processing or freeze-dried after formulation to improve... more Although proteins are often frozen during processing or freeze-dried after formulation to improve their stability, they can undergo degradation leading to losses in biological activity during the process. During freezing, the physical environment of a protein changes dramatically leading to the development of stresses that impact protein stability. Low temperature, freeze-concentration, and ice formation are the three chief stresses resulting during cooling and freezing. Because of the increase in solute concentrations, freeze-concentration could also facilitate second order reactions, crystallization of buffer or non-buffer components, phase separation, and redistribution of solutes. An understanding of these stresses is critical to the determination of when during freezing a protein suffers degradation and therefore important in the design of stabilizer systems. With the exception of a few studies, the relative contribution of various stresses to the instability of frozen proteins has not been addressed in the freeze-drying literature. The purpose of this review is to describe the various stages of freezing and examine the consequences of the various stresses developing during freezing on protein stability and to assess their relative contribution to the destabilization process. The ongoing debate on thermodynamic versus kinetic mechanisms of stabilization in frozen environments and the current state of knowledge concerning those mechanisms are also reviewed in this publication. An understanding of the relative contributions of freezing stresses coupled with the knowledge of cryoprotection mechanisms is central to the development of more rational formulation and process design of stable lyophilized proteins.
The crystallization of amorphous drugs during dissolution is a type of solution mediated phase tr... more The crystallization of amorphous drugs during dissolution is a type of solution mediated phase transformation that can reduce the bioavailability enhancement one hoped to gain from the amorphous state. The goal of this study was to explore the effects of processing on the dissolution performance of amorphous indomethacin. The amorphous solids were prepared by four techniques, quench cooling the melted solid, cryogrinding γ indomethacin amorphous for 1 or 3 h and quench cooling the solid followed by 1 h of cryogrinding. Dissolution results assessed in a flow-through intrinsic dissolution apparatus reveal decreases in the dissolution rate of amorphous indomethacin during the experimental time frame indicating that a solution mediated phase transformation has occurred. The amorphous solids prepared by melt quenching and melt quenching followed by cryogrinding showed a significant dissolution rate advantage over the γ form of indomethacin. In contrast, indomethacin that was cryoground amorphous for 1 or 3 h did not show any dissolution rate advantage over the crystalline material. Transformation was confirmed by in situ Raman microscopy and polarized light microscopy with differences seen in the nature of the crystals apparent on the surface of the dissolving solid. A portion of the melt quenched amorphous sample was annealed at 25 °C and 0% relative humidity to induce partial crystallization of γ indomethacin. As crystallinity increased, the dissolution rate decreased. The transformation time of partially amorphous indomethacin was not dependent on the level of crystallinity present, indicating only a small fraction of crystalline material needs to be present to affect the kinetics of crystallization. The solution mediated phase transformation of amorphous indomethacin is affected by the processing method even though all solids were confirmed amorphous by polarized light microscopy and X-ray diffraction. Dissolution may distinguish differences in amorphous solids that other methods cannot discern.
Freeze-thawing is routinely used to study freezing-induced irreversible protein denaturation in t... more Freeze-thawing is routinely used to study freezing-induced irreversible protein denaturation in the formulation characterization and development of lyophilized proteins. In most cases, the temperature profiles of the samples are not fully monitored during freeze-thawing and therefore, the sample thermal histories are largely unknown. The objective of this study was to develop experimental protocols for the study of isothermal protein degradation using a temperature-step apparatus. Freeze-thaw experiments were performed at a freezing rate of 10 degrees C/min and various thawing rates (0.5-3.3 degrees C/min) using a temperature-step apparatus. In our efforts to design validation studies, we encountered anomalies in the recovered enzyme activity data of an enzyme, lactate dehydrogenase at the end of freeze-thawing. The effect of thawing rate was studied to explain the variability in the data. In addition, post-thaw "aging" of freshly frozen and thawed samples was performed at 5 degrees C to reduce the variability in the recovered enzyme activity. Results from these experiments implicate the use of aging of dilute multimeric enzymes at the end of freeze-thawing to control the variability in enzyme assays.
Phase separation in amorphous freeze-dried mixtures is likely in many systems. However, suitable ... more Phase separation in amorphous freeze-dried mixtures is likely in many systems. However, suitable detection methodology has been lacking, as the classical technique, differential scanning calorimetry (DSC), relies upon detection of multiple glass transition temperatures (T(g)), each of which is characteristic of a given amorphous phase. The lack of a detectable glass transition temperature in protein-rich phases, limits the application of DSC. Here, we focus on evaluating new methods for detection of phase separation in amorphous freeze-dried mixtures. A novel Raman mapping technique has been evaluated using model binary polymer mixtures of PVP and dextran known to phase separate. The sensitivity of this Raman technique in detecting phase separation was comparable to DSC. Phase separation was detected in compositions of 1:9 to 3:2 (PVP 10,000/dextran 5000) and 3:7 to 4:1 (PVP 29,000/dextran 10,000) by DSC and Raman. Computational methodologies applied to X-ray powder diffraction (XRPD) data from these systems are also shown to reliably detect the presence of phase separation. However, some differences between techniques were observed in cases lying on the boundary of phase separation. Thus, Raman and XRPD show promise for detecting phase separation in systems, which do not exhibit detectable glass transitions by calorimetry.
Soluble salts can undergo solution-mediated phase transformation to a lower solubility form due t... more Soluble salts can undergo solution-mediated phase transformation to a lower solubility form due to pH gradients in the gastrointestinal tract. Therefore, dissolution rate rather than solubility may be the best predictor of bioavailability for such compounds. The purpose of this project was to examine the kinetics of the conversion of a basic compound, haloperidol, and its salt forms using a flow-through dissolution apparatus and rotating disk method in neutral conditions. The effects of buffer concentration, salt form, dissolution apparatus, and hydrodynamics were examined. Raman microscopy was used to characterize solids after dissolution. Haloperidol mesylate and haloperidol chloride showed a decrease in dissolution rate with time in the dissolution media. Haloperidol mesylate and haloperidol chloride dissolution rates also decreased with increasing buffer capacity. Raman microscopy confirmed phase conversion from the salt forms to the free base form in phosphate buffer. Hydrodynamics did not affect the time course of the solution-mediated phase transformation of salt forms. Dissolution and precipitation appear to be a function of pH close to the surface of the dissolving solid. In situations where equilibrium solubility of salts cannot be assessed experimentally, dissolution experiments are useful for examining the extent and duration of the dissolution rate enhancement.
A method is described for screening compounds that inhibit crystallization in solution to enable ... more A method is described for screening compounds that inhibit crystallization in solution to enable more accurate measurement of amorphous drug solubility. Three polymers [polyvinylpyrrolidone, hydroxypropyl methylcellulose, and hydroxypropyl methylcellulose acetate succinate (HPMCAS)] were screened for their ability to inhibit the crystallization of neat amorphous drugs during measurement of solubility of the amorphous form in water. Among the polymers evaluated, HPMCAS was found to be most promising. The use of HPMCAS provided an "apparent solubility" of amorphous drugs that was closer to the theoretically calculated values. With danazol, agreement was essentially quantitative, and for griseofulvin and iopanoic acid, agreement was within a factor of two; these maximum concentrations were sustained for a period of 40-90 min. Dynamic light scattering of filtered samples (0.22 µ) revealed the presence of colloidal drug-polymer assemblies in solution (100-150 nm). The supernatant resulting from this centrifugation gradually decreased in concentration, but remained supersaturated with respect to crystalline drug for several hours. Thus, HPMCAS has been shown to be a useful additive in dissolution media to allow a more accurate determination of the solubility of fast crystallizing neat amorphous drugs, at least for the drugs studied, and it should also serve to retard crystallization in vivo and therefore, facilitate improved bioavailability.
In recent years there has been growing interest in advancing amorphous pharmaceuticals as an appr... more In recent years there has been growing interest in advancing amorphous pharmaceuticals as an approach for achieving adequate solubility. Due to difficulties in the experimental measurement of solubility, a reliable estimate of the solubility enhancement ratio of an amorphous form of a drug relative to its crystalline counterpart would be highly useful. We have developed a rigorous thermodynamic approach to estimate enhancement in solubility that can be achieved by conversion of a crystalline form to the amorphous form. We rigorously treat the three factors that contribute to differences in solubility between amorphous and crystalline forms. First, we calculate the free energy difference between amorphous and crystalline forms from thermal properties measured by modulated differential scanning calorimetry (MDSC). Secondly, since an amorphous solute can absorb significant amounts of water, which reduces its activity and solubility, a correction is made using water sorption isotherm data and the Gibbs–Duhem equation. Next, a correction is made for differences in the degree of ionization due to differences in solubilities of the two forms. Utilizing this approach the theoretically estimated solubility enhancement ratio of 7.0 for indomethacin (amorphous/g-crystal) was found to be in close agreement with the experimentally determined ratio of 4.9.
Solventless photocurable film coating was used to obtain modified release coatings. Different por... more Solventless photocurable film coating was used to obtain modified release coatings. Different pore-forming agents were used to achieve immediate and sustained release of a blue dye contained in the coated pellets (non-pareil beads). A super-disintegrant, sodium starch glycolate, was used to obtain immediate release. When incorporated in the coating, this pore-former swelled and yielded large pores that demonstrated quick release of the marker dye while leaving behind the scaffold provided by the photocured polymer. Simple pore-formers (lactose and sodium chloride) dissolved away without swelling and provided a more sustained release profile. Release can be modified with the choice of material, number of layers and thickness of the coating. When release of sodium chloride and release of marker dye were simultaneously monitored, it was observed that at least 40-50% of the sodium chloride that was incorporated in the coating released before the dye was released through the coating showing that pore-formation preceded the release of the marker dye. The coupling factor between dye release and pore-formation was found to be dependent on the ratio of amount of solid pore-forming agent and volume of liquid monomer (S/L) of the coating. Also, studies demonstrate that the coating is photostable and can withstand handling stress.
Coatings are an essential part in the formulation of pharmaceutical dosage form to achieve superi... more Coatings are an essential part in the formulation of pharmaceutical dosage form to achieve superior aesthetic quality (e.g., color, texture, mouth feel, and taste masking), physical and chemical protection for the drugs in the dosage forms, and modification of drug release characteristics. Most film coatings are applied as aqueous- or organic-based polymer solutions. Both organic and aqueous film coating bring their own disadvantages. Solventless coating technologies can overcome many of the disadvantages associated with the use of solvents (e.g., solvent exposure, solvent disposal, and residual solvent in product) in pharmaceutical coating. Solventless processing reduces the overall cost by eliminating the tedious and expensive processes of solvent disposal/treatment. In addition, it can significantly reduce the processing time because there is no drying/evaporation step. These environment-friendly processes are performed without any heat in most cases (except hot-melt coating) and thus can provide an alternative technology to coat temperature-sensitive drugs. This review discusses and compares six solventless coating methods - compression coating, hot-melt coating, supercritical fluid spray coating, electrostatic coating, dry powder coating, and photocurable coating - that can be used to coat the pharmaceutical dosage forms.
A flow-through dissolution apparatus was designed and evaluated to screen small quantities of pha... more A flow-through dissolution apparatus was designed and evaluated to screen small quantities of pharmaceutical drug compounds early in development. The apparatus was designed to mount on a microscope slide such that a compacted solid drug was positioned flush along one wall and the fluid flow in the apparatus was laminar flow in a rectangular duct. Stereomicroscopic digital images and Raman spectra of the solid were taken during dissolution and the effluent dissolution medium was collected in fractions to determine the dissolution rate by fluorescence or HPLC/UV. Three compounds, triamterene, ketoprofen, and β-naphthoic acid were investigated in the dissolution flow cell at various hydrodynamic conditions. In conditions where no solvent-mediated conversion was expected, there was a decrease in dissolution rate with time in the flow through cell that was associated with surface smoothing. This phenomenon also occurred in rotating disk experiments. In either case, the magnitude and time course of the decrease in dissolution rate with time is generally different enough to distinguish from the decrease in dissolution rate due to solvent-mediated conversion.
Although proteins are often frozen during processing or freeze-dried after formulation to improve... more Although proteins are often frozen during processing or freeze-dried after formulation to improve their stability, they can undergo degradation leading to losses in biological activity during the process. During freezing, the physical environment of a protein changes dramatically leading to the development of stresses that impact protein stability. Low temperature, freeze-concentration, and ice formation are the three chief stresses resulting during cooling and freezing. Because of the increase in solute concentrations, freeze-concentration could also facilitate second order reactions, crystallization of buffer or non-buffer components, phase separation, and redistribution of solutes. An understanding of these stresses is critical to the determination of when during freezing a protein suffers degradation and therefore important in the design of stabilizer systems. With the exception of a few studies, the relative contribution of various stresses to the instability of frozen proteins has not been addressed in the freeze-drying literature. The purpose of this review is to describe the various stages of freezing and examine the consequences of the various stresses developing during freezing on protein stability and to assess their relative contribution to the destabilization process. The ongoing debate on thermodynamic versus kinetic mechanisms of stabilization in frozen environments and the current state of knowledge concerning those mechanisms are also reviewed in this publication. An understanding of the relative contributions of freezing stresses coupled with the knowledge of cryoprotection mechanisms is central to the development of more rational formulation and process design of stable lyophilized proteins.
The crystallization of amorphous drugs during dissolution is a type of solution mediated phase tr... more The crystallization of amorphous drugs during dissolution is a type of solution mediated phase transformation that can reduce the bioavailability enhancement one hoped to gain from the amorphous state. The goal of this study was to explore the effects of processing on the dissolution performance of amorphous indomethacin. The amorphous solids were prepared by four techniques, quench cooling the melted solid, cryogrinding γ indomethacin amorphous for 1 or 3 h and quench cooling the solid followed by 1 h of cryogrinding. Dissolution results assessed in a flow-through intrinsic dissolution apparatus reveal decreases in the dissolution rate of amorphous indomethacin during the experimental time frame indicating that a solution mediated phase transformation has occurred. The amorphous solids prepared by melt quenching and melt quenching followed by cryogrinding showed a significant dissolution rate advantage over the γ form of indomethacin. In contrast, indomethacin that was cryoground amorphous for 1 or 3 h did not show any dissolution rate advantage over the crystalline material. Transformation was confirmed by in situ Raman microscopy and polarized light microscopy with differences seen in the nature of the crystals apparent on the surface of the dissolving solid. A portion of the melt quenched amorphous sample was annealed at 25 °C and 0% relative humidity to induce partial crystallization of γ indomethacin. As crystallinity increased, the dissolution rate decreased. The transformation time of partially amorphous indomethacin was not dependent on the level of crystallinity present, indicating only a small fraction of crystalline material needs to be present to affect the kinetics of crystallization. The solution mediated phase transformation of amorphous indomethacin is affected by the processing method even though all solids were confirmed amorphous by polarized light microscopy and X-ray diffraction. Dissolution may distinguish differences in amorphous solids that other methods cannot discern.
Freeze-thawing is routinely used to study freezing-induced irreversible protein denaturation in t... more Freeze-thawing is routinely used to study freezing-induced irreversible protein denaturation in the formulation characterization and development of lyophilized proteins. In most cases, the temperature profiles of the samples are not fully monitored during freeze-thawing and therefore, the sample thermal histories are largely unknown. The objective of this study was to develop experimental protocols for the study of isothermal protein degradation using a temperature-step apparatus. Freeze-thaw experiments were performed at a freezing rate of 10 degrees C/min and various thawing rates (0.5-3.3 degrees C/min) using a temperature-step apparatus. In our efforts to design validation studies, we encountered anomalies in the recovered enzyme activity data of an enzyme, lactate dehydrogenase at the end of freeze-thawing. The effect of thawing rate was studied to explain the variability in the data. In addition, post-thaw "aging" of freshly frozen and thawed samples was performed at 5 degrees C to reduce the variability in the recovered enzyme activity. Results from these experiments implicate the use of aging of dilute multimeric enzymes at the end of freeze-thawing to control the variability in enzyme assays.
Phase separation in amorphous freeze-dried mixtures is likely in many systems. However, suitable ... more Phase separation in amorphous freeze-dried mixtures is likely in many systems. However, suitable detection methodology has been lacking, as the classical technique, differential scanning calorimetry (DSC), relies upon detection of multiple glass transition temperatures (T(g)), each of which is characteristic of a given amorphous phase. The lack of a detectable glass transition temperature in protein-rich phases, limits the application of DSC. Here, we focus on evaluating new methods for detection of phase separation in amorphous freeze-dried mixtures. A novel Raman mapping technique has been evaluated using model binary polymer mixtures of PVP and dextran known to phase separate. The sensitivity of this Raman technique in detecting phase separation was comparable to DSC. Phase separation was detected in compositions of 1:9 to 3:2 (PVP 10,000/dextran 5000) and 3:7 to 4:1 (PVP 29,000/dextran 10,000) by DSC and Raman. Computational methodologies applied to X-ray powder diffraction (XRPD) data from these systems are also shown to reliably detect the presence of phase separation. However, some differences between techniques were observed in cases lying on the boundary of phase separation. Thus, Raman and XRPD show promise for detecting phase separation in systems, which do not exhibit detectable glass transitions by calorimetry.
Soluble salts can undergo solution-mediated phase transformation to a lower solubility form due t... more Soluble salts can undergo solution-mediated phase transformation to a lower solubility form due to pH gradients in the gastrointestinal tract. Therefore, dissolution rate rather than solubility may be the best predictor of bioavailability for such compounds. The purpose of this project was to examine the kinetics of the conversion of a basic compound, haloperidol, and its salt forms using a flow-through dissolution apparatus and rotating disk method in neutral conditions. The effects of buffer concentration, salt form, dissolution apparatus, and hydrodynamics were examined. Raman microscopy was used to characterize solids after dissolution. Haloperidol mesylate and haloperidol chloride showed a decrease in dissolution rate with time in the dissolution media. Haloperidol mesylate and haloperidol chloride dissolution rates also decreased with increasing buffer capacity. Raman microscopy confirmed phase conversion from the salt forms to the free base form in phosphate buffer. Hydrodynamics did not affect the time course of the solution-mediated phase transformation of salt forms. Dissolution and precipitation appear to be a function of pH close to the surface of the dissolving solid. In situations where equilibrium solubility of salts cannot be assessed experimentally, dissolution experiments are useful for examining the extent and duration of the dissolution rate enhancement.
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Papers by Robin Bogner