Effect of Structurally Related Compounds on Desupersaturation Kinetics of Indomethacin.

PURPOSE: The pharmaceutical literature contains examples wherein desupersaturation from high concentrations does not proceed to equilibrium concentration of the thermodynamically most stable form but remains above equilibrium. The purpose of the current research was to investigate the effect of structurally related compounds on desupersaturation kinetics as a possible explanation for a higher than equilibrium solubility after crystal growth of γ-indomethacin (γ-IMC). METHODS: Three structurally related compounds (SRC) - cis-sulindac (c-SUL), trans-sulindac (t-SUL) and indomethacin-related compound-A (IMC-A) -were investigated. Desupersaturation kinetics to the most stable γ-IMC, in the presence of c-SUL, t-SUL or IMC-A, was measured at pH 2.0. RESULTS: The SRCs c-SUL and t-SUL were effective crystallization inhibitors of IMC, while IMC-A was not a potent crystallization inhibitor of IMC. Among the sulindac isomers, t-SUL was a stronger crystallization inhibitor. The apparent solubility of γ-IMC crystals grown from supersaturated solutions in the presence of SRCs matched the equilibrium solubility of γ-IMC. During crystallization of IMC in the presence of IMC-A, the concentration of IMC-A declined initially but rebounded as supersaturation and crystallization rate of IMC declined, suggesting that IMC-A itself became incorporated in the IMC crystal lattice at higher degrees of IMC supersaturation. CONCLUSIONS: The results suggest that high apparent solubility after crystallization of IMC reported by several authors is not related to the presence of IMC-A impurity. The greater IMC crystal growth rate inhibition by t-SUL than by c-SUL was consistent with the proposed orientation of SUL molecules adsorbed on the IMC crystal, providing a mechanistic understanding of the inhibition.

Use of a Design of Experiments (DoE) Approach to Optimize Large-Scale Freeze-Thaw Process of Biologics.

Large volumes of protein solutions are commonly stored in a frozen state before further drug product fill and finish. This study aimed to establish a design space to perform large-scale freeze-thaw (F/T) processes of biotherapeutics without inducing protein destabilization. A response surface model was designed to evaluate the following main factors and interactions: fill volume of the protein solution in 1-L containers, distance among nine containers during both F/T, freezer set temperature, and a novel forced air flow methodology during thawing. The analysis from 46 experimental runs indicated over 4-fold increase in the freezing rate by lowering the freezing temperature from -20 to -80°C, and the forced air flow at 98 fpm doubled the thawing rate. Furthermore, multivariate linear regression modeling revealed the significant impact of all main factors investigated on lactate dehydrogenase (LDH) quality attributes. The factor that most strongly affected the retention of LDH activity was the loading distance: ≥ 5 cm among containers positively affected the LDH activity response in 50.6%. The factor that most strongly retained the LDH tetramers was the set freezer temperature towards the lower range of -80°C (2.2% higher tetramer retention compared to -20°C freezing, due to faster freezing rate). In summary, this DoE-based systematic analysis increased F/T process understanding at large scale, identified critical F/T process parameters, and confirmed the feasibility of applying faster freezing and forced air thawing procedures to maintain the stability of LDH solutions subject to large-scale F/T.

A Compact Model for Lyophilizer Equipment Capability Estimation.

This work presents a compact model for the equipment capability limit of a common configuration of pharmaceutical lyophilizers, a product chamber separated from the condenser by a duct and isolation valve, at a wide range of design parameters. The equipment capability limit is one of the most important characteristics determining the lyophilization design space for a particular product, container, and equipment combination. Experimental measurements of equipment capability are time-consuming and expensive, especially at the production scale. Numerical modeling using computational fluid dynamics may reduce the number of experiments and provide insights into the physics of the process with high resolution. The computational fluid dynamics (CFD) modeling has been used in this work to develop a compact model for lyophilizer equipment capability. This eliminates the need for end users to create a full CFD model of the equipment and process. Full CFD and compact model simulations for laboratory and pilot-scale lyophilizers have been compared with tunable diode laser absorption spectroscopy measurements of the water vapor mass flow during ice slab tests. The compact model results average deviation from the experimental data is within 10%, whereas the full CFD simulations are within 5%. The compact model is based on several key parameters which are the main characteristics of a lyophilizer affecting the equipment capability curve. These parameters are discussed, and their effect on the modeling results is shown.

A Software Tool for Lyophilization Primary Drying Process Development and Scale-up Including Process Heterogeneity, I: Laboratory-Scale Model Testing.

Freeze-drying is a deceptively complex operation requiring sophisticated design of a robust and efficient process that includes understanding and planning for heterogeneity across the batch and shifts in parameters due to vial or lyophilizer changes. A software tool has been designed to assist in process development and scale-up based on a model that includes consideration of the process heterogeneity. Two drug formulations were used to test the ability of the new tool to develop a freeze-drying cycle and correctly predict product temperatures and drying times. Model inputs were determined experimentally, and the primary drying heterogeneous freeze-drying model was used to design drying cycles that provided data to verify the accuracy of model-predicted product temperature and primary drying time. When model inputs were accurate, model-predicted primary drying times were within 0.1 to 15.9% of experimentally measured values, and product temperature accuracy was between 0.2 and 1.2°C for three vial locations, center, inner edge, and outer edge. However, for some drying cycles, differences in vial heat transfer coefficients due to changes in shelf and product temperature as well as altered product resistance due to product temperature-dependent microcollapse increased inaccuracy (up to 28.6% difference in primary drying time and 5.1°C difference in product temperature). This highlights the need for careful determination of experimental conditions used to calculate model inputs. In future efforts, full characterization of location- and shelf temperature-dependentKv as well as location- and product temperature-dependentRp will enhance the accuracy of the predictions by the model within the user-friendly software.

Mechanisms for the Slowing of Desupersaturation of a Weak Acid at Elevated pH.

Supersaturating drug delivery systems are used to achieve higher oral bioavailability for poorly soluble drugs. However, supersaturated solutions often decline to lower concentrations by precipitation and crystallization. The purpose of the current research is to provide a mechanistic understanding of drug crystallization as a function of pH, using indomethacin (IMC, pKa 4.18) as a model compound. Desupersaturation kinetics to the γ-form of IMC was measured at pH 2.0, 3.0, 4.0, and 4.5 from an initial degree of supersaturation of 2.5-6. At equivalent levels of supersaturation, crystal growth rates decreased with an increase in solution pH. Two mechanisms for this phenomenon, reactive diffusion (resulting in a higher surface pH as compared to bulk pH) and inhibition of crystallization by structurally similar ionized IMC at higher pH, were explored. Non-steady-state models for reactive diffusion showed that the surface pH was only 0.01 units above that of the bulk solution pH. Mass transport models for reactive diffusion during crystallization could not explain the decrease in desupersaturation kinetics at higher pH. However, zeta potentials as high as -70 mV suggested that IMC- is adsorbed on the surface of the particles. A mathematical model for inhibition of crystal growth by IMC- accounted for the pH effect suggesting that ionized IMC acts as an effective crystallization inhibitor of IMC.

Evaluation of Predictors of Protein Relative Stability Obtained by Solid-State Hydrogen/Deuterium Exchange Monitored by FTIR.

PURPOSE: Hydrogen/deuterium (H/D) exchange over a range of temperatures suggests a protein structural/mobility transition in the solid state below the system glass transition temperature (Tg). The purpose of this study was to determine whether solid-state protein stability correlates with the difference between storage temperature and apparent Td where an abrupt change in mobility occurs, or alternatively, the extent of H/D exchange at a single temperature correlates directly to protein stability in lyophilized solids. METHODS: Solid-state H/D exchange was monitored by FTIR spectroscopy to study the extent of exchange and the apparent transition temperature in both pure recombinant human serum albumin (rHSA) and rHSA formulated with sucrose or trehalose. H/D exchange of freeze-dried formulations at 11% RH and temperatures from 30 to 80°C was monitored. Protein stability against aggregation at 40°C/11% RH for 6 months was assessed by size exclusion chromatography (SEC). RESULTS: Both sucrose and trehalose showed equivalent protection of protein secondary structure by FTIR. The rHSA:sucrose formulation showed superior long-term stability at 40°C by SEC over the trehalose formulation, but the apparent Td determined from H/D exchange was much higher in the trehalose formulation. Instead, the extent of H/D exchange (X∞) was lower in the sucrose formulation at the temperature of the stability studies (40°C) than found for the trehalose formulation, which was consistent with better stability in the sucrose formulation. CONCLUSIONS: While apparent Td did not correlate with protein stability for rHSA, the extent of H/D exchange, X∞, did.

Impact of Natural Variations in Freeze-Drying Parameters on Product Temperature History: Application of Quasi Steady-State Heat and Mass Transfer and Simple Statistics.

Inter- and intra-batch variability in heat and mass transfer during the drying phase of lyophilization is well recognized. Heat transfer variability between individual vials in the same batch arise from both different positions in the vial array and from variations in the bottom contour of the vials, both effects contributing roughly equally to variations in the effective heat transfer coefficient of the vials, Kv. Both effects can be measured in the laboratory, and variations in average Kv values as a function of vial position in the array for lab and production can be calculated by use of the simple steady-state heat and mass transfer theory. Typically, in the laboratory dryer, vials on the edge of the array, "edge vials," run 2-4°C warmer than "center vials," but differences between laboratory and manufacturing temperatures are modest. The variability in mass transfer can be assigned to major variations in ice nucleation temperature (both intra-batch and inter-batch), including major differences between laboratory and manufacturing. The net effect of all random variations, for each class of vial, can be evaluated by a simple statistical model-propagation of error, which then allows prediction of the distribution in product temperatures and drying times, and therefore prediction of percent of vials dry and percent of vials collapsed and proximity to the edge of failure for a given process. Good agreement between theoretical and experimentally determined maximum temperatures in primary drying and percent collapsed product demonstrates the calculations have useful accuracy.

Oral Ingestion and Intraventricular Injection of Curcumin Attenuates the Effort-Related Effects of the VMAT-2 Inhibitor Tetrabenazine: Implications for Motivational Symptoms of Depression.

Effort-related choice tasks are used for studying depressive motivational symptoms such as anergia/fatigue. These studies investigated the ability of the dietary supplement curcumin to reverse the low-effort bias induced by the monoamine storage blocker tetrabenazine. Tetrabenazine shifted effort-related choice in rats, decreasing high-effort lever pressing but increasing chow intake. The effects of tetrabenazine were reversed by oral ingestion of curcumin (80.0-160.0 mg/kg) and infusions of curcumin into the cerebral ventricles (2.0-8.0 µg). Curcumin attenuates the effort-related effects of tetrabenazine in this model via actions on the brain, suggesting that curcumin may be useful for treating human motivational symptoms.

Spatial Variation of Pressure in the Lyophilization Product Chamber Part 2: Experimental Measurements and Implications for Scale-up and Batch Uniformity.

Product temperature during the primary drying step of freeze-drying is controlled by a set point chamber pressure and shelf temperature. However, recent computational modeling suggests a possible variation in local chamber pressure. The current work presents an experimental verification of the local chamber pressure gradients in a lab-scale freeze-dryer. Pressure differences between the center and the edges of a lab-scale freeze-dryer shelf were measured as a function of sublimation flux and clearance between the sublimation front and the shelf above. A modest 3-mTorr difference in pressure was observed as the sublimation flux was doubled from 0.5 to 1.0 kg·h-1·m-2 at a clearance of 2.6 cm. Further, at a constant sublimation flux of 1.0 kg·h-1·m-2, an 8-fold increase in the pressure drop was observed across the shelf as the clearance was decreased from 4 to 1.6 cm. Scale-up of the pressure variation from lab- to a manufacturing-scale freeze-dryer predicted an increased uniformity in drying rates across the batch for two frequently used pharmaceutical excipients (mannitol and sucrose at 5% w/w). However, at an atypical condition of shelf temperature of +10°C and chamber pressure of 50 mTorr, the product temperature in the center vials was calculated to be a degree higher than the edge vial for a low resistance product, thus reversing the typical edge and center vial behavior. Thus, the effect of local pressure variation is more significant at the manufacturing-scale than at a lab-scale and accounting for the contribution of variations in the local chamber pressures can improve success in scale-up.

Spatial Variation of Pressure in the Lyophilization Product Chamber Part 1: Computational Modeling.

The flow physics in the product chamber of a freeze dryer involves coupled heat and mass transfer at different length and time scales. The low-pressure environment and the relatively small flow velocities make it difficult to quantify the flow structure experimentally. The current work presents the three-dimensional computational fluid dynamics (CFD) modeling for vapor flow in a laboratory scale freeze dryer validated with experimental data and theory. The model accounts for the presence of a non-condensable gas such as nitrogen or air using a continuum multi-species model. The flow structure at different sublimation rates, chamber pressures, and shelf-gaps are systematically investigated. Emphasis has been placed on accurately predicting the pressure variation across the subliming front. At a chamber set pressure of 115 mtorr and a sublimation rate of 1.3 kg/h/m2, the pressure variation reaches about 9 mtorr. The pressure variation increased linearly with sublimation rate in the range of 0.5 to 1.3 kg/h/m2. The dependence of pressure variation on the shelf-gap was also studied both computationally and experimentally. The CFD modeling results are found to agree within 10% with the experimental measurements. The computational model was also compared to analytical solution valid for small shelf-gaps. Thus, the current work presents validation study motivating broader use of CFD in optimizing freeze-drying process and equipment design.

Amorphization of itraconazole by inorganic pharmaceutical excipients: comparison of excipients and processing methods.

The aim of this study was to investigate the effects of solid carriers and processing routes on the properties of amorphous solid dispersions of itraconazole. Three solid carriers with a range of surface properties were studied, (1) a mesoporous silicate, magnesium aluminum silicate (Neusilin US2), (2) a nonporous silicate of corresponding composition (Veegum) and (3) a non-silicate, inorganic excipient, calcium phosphate dibasic anhydrous (A-TAB). The drug was incorporated via either solvent-deposition or ball milling. Both the maximum drug deposited by solvent-based method that produced an amorphous composite and the time for complete amorphization by co-milling was determined by X-ray powder diffraction (XRPD). Changes in the drug and excipients were monitored by nitrogen adsorption and wettability of the powder. The ability of the excipients to amorphize the drug and enhance its dissolution was related to the powder characteristics. Neusilin provided the fastest amorphization time in the mill and highest drug loading by solvent-deposition, compared with the other two excipients. Solvent-deposition provided greater dissolution enhancement than milling, due to the reduction in Neusilin porosity during high energy milling.This study confirms that substrates as well as the processing routes have notable influence on the drug deposition, amorphization, physical stability and drug in vitro release.

Evaluation of a Raman Chemometric Method for Detecting Protein Structural Conformational Changes in Solution.

Raman scattering shows promise as a powerful routine tool, to determine both secondary and the smaller tertiary structural changes that precede aggregation in both solutions and solids. A method was developed utilizing principal component analysis (PCA) of Raman spectra for detection of small, but meaningful, pH induced changes in tertiary protein structure linked to aggregate formation using α-lactalbumin solutions as a model. The sample preparation and spectral parameters, were optimized for a bulk Raman probe. Analysis of large regions (600-1850 cm-1) yielded principal component (PC) scores useful for semi-quantitative comparison of protein conformation between formulations. PC loadings corresponded to specific structural peaks known to change with solution pH. PCA of circular dichroism (CD) spectra of dilute solutions yielded similar results. Sucrose is a common formulation excipient with a Raman spectrum that overlaps many protein peaks. With sucrose in the protein solution, the ability of PCA to discern protein structural changes from the Raman spectra was somewhat reduced. Analysis of a more limited spectral region (1530-1780 cm-1) with negligible sucrose spectral contribution improved the discrimination of protein conformational states. The new Raman method accurately distinguished differences in protein structure in concentrated solutions. The long-term goal is to explore Raman characterization as a routine monitoring tool of protein stability in both solution and solid states.

Large-Scale Freeze-Thaw of Protein Solutions: Study of the Relative Contributions of Freeze-Concentration and Ice Surface Area on Stability of Lactate Dehydrogenase.

Although bulk biotherapeutics are often frozen during fill finish and shipping to improve their stability, they can undergo degradation leading to losses in biological activity during sub-optimal freeze-thaw (F/T) process. Except for a few small-scale studies, the relative contribution of various F/T stresses to the instability of proteins has not been addressed. Thus, the objective of this study was to determine the individual contributions of freeze-concentration, ice surface area, and processing time to protein destabilization at a practical manufacturing-scale. Lactate dehydrogenase (LDH) in histidine buffer solutions were frozen in 1L containers. The frozen solutions were sliced into representative samples and assessed for the ice specific surface area (SSA) and extent of solutes freeze-concentration. For the first time to our knowledge, ice SSA was measured in dried samples from large-volume protein solutions using volumetric nitrogen adsorption isotherms. SSA measurements of the freeze-dried cakes showed that the ice surface area increased with an increase in the freezing rate. The ice SSA was also impacted by the position of the sample within the container: samples closer to the active cooled surface of the container exhibited smaller ice surface area compared to ice-cored samples from the center of the bottle. The freeze-concentrate composition was determined by measuring LDH concentration in the ice-cored samples. The protein distributed more evenly throughout the frozen solution after fast freezing which also correlated with enhanced protein stability compared to slow freezing conditions. Overall, better protein stability parameters correlated with higher ice SSA and lower freeze-concentration extent which was achieved at a faster freezing rate. Thus, extended residence time of the protein at the freeze-concentrated microenvironment is the critical destabilizing factor during freezing of LDH in bulk histidine buffer system. This study expands the understanding of the relative contributions of freezing stresses which, coupled with the knowledge of cryoprotection mechanisms, is imperative to the development of optimized processes and formulations aiming stable frozen protein solutions.

Impact of controlled ice nucleation and lyoprotectants on nanoparticle stability during Freeze-drying and upon storage.

The freezing step of the lyophilization process can impact nanoparticle stability due to increased particle concentration in the freeze-concentrate. Controlled ice nucleation is a technique to achieve uniform ice crystal formation between vials in the same batch and has attracted increasing attention in pharmaceutical industry. We investigated the impact of controlled ice nucleation on three types of nanoparticles: solid lipid nanoparticles (SLNs), polymeric nanoparticles (PNs), and liposomes. Freezing conditions with different ice nucleation temperatures or freezing rates were employed for freeze-drying all formulations. Both in-process stability and storage stability up to 6 months of all formulations were assessed. Compared with spontaneous ice nucleation, controlled ice nucleation did not cause significant differences in residual moisture and particle size of freeze-dried nanoparticles. The residence time in the freeze-concentrate was a more critical factor influencing the stability of nanoparticles than the ice nucleation temperature. Liposomes freeze-dried with sucrose showed particle size increase during storage regardless of freezing conditions. By replacing sucrose with trehalose, or adding trehalose as a second lyoprotectant, both the physical and chemical stability of freeze-dried liposomes improved. Trehalose was a preferable lyoprotectant than sucrose to better maintain the long-term stability of freeze-dried nanoparticles at room temperature or 40 °C.

Spontaneous crystalline-to-amorphous phase transformation of organic or medicinal compounds in the presence of porous media, part 3: effect of moisture.

PURPOSE: Amorphization of crystalline compounds using mesoporous media is a promising technique to improve the solubility and drug release of poorly-soluble compounds. The objective of this paper is to understand the effect of moisture on the capacity and performance of vapor-phase mediated amorphization. METHODS: Mesoporous silicon dioxide (SiO(2)) and crystalline naphthalene were used as the model system. The effect of moisture on the amorphization capacity of naphthalene was determined using adsorption chambers with various levels of relative humidity. Enthalpy and capacity of water vapor adsorption on SiO(2) were measured using isothermal microcalorimetry and thermogravimetry. RESULTS: Moisture not only suppressed the amorphization capacity of naphthalene, but reversed an already-amorphized formulation as well. On the other hand, through the same competitive interaction, improved drug release and enhanced solubility were obtained. The initial supersaturation was followed by an entropy-driven crystallization. In addition, moisture-induced siloxane bond fracture was found at normal processing conditions, which led to the changes in silica surface chemistry. However, the implication in amorphization has not reached a definitive conclusion. CONCLUSIONS: Humidity during processing and storage must be carefully controlled for this type of amorphous formulation. Further investigation is needed to better understand the moisture-induced changes of silica.

Design and characterization of a laminar flow-through dissolution apparatus: Comparison of hydrodynamic conditions to those of common dissolution techniques.

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 ß-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.

Differential heat of adsorption of water vapor on silicified microcrystalline cellulose (SMCC): an investigation using isothermal microcalorimetry.

A novel dual-shaft configuration in isothermal microcalorimetry was developed to study the interaction of water vapor with pharmaceutical excipients. An instrument performance test is suggested to validate the experimental data. Reliable experimental results can be collected using a single perfusion shaft; however, there was limitation of the dual-shaft configuration, which resulted deviation in the experimental results. A periodic performance test is recommended. Silicified microcrystalline cellulose (SMCC) was used as a model system to study the interaction using the dual-shaft method. Enthalpy of water vapor adsorption on SMCC was determined and compared to literature data. The data collected using the dual-shaft configuration did not reflect the actual physical system. The deviation was most likely due to the lack of flow control caused by viscous resistance. The enthalpy of adsorption was then calculated using isothermal microcalorimetry coupled with a dynamic vapor sorption apparatus. The results, -55 kJ/mol at low relative humidity (RH) to -22 kJ/mol at high RH, were consistent with the physical phenomenon of water vapor adsorption. Enthalpy of adsorption showed surface heterogeneity of SMCC and suggested multilayer condensation of water at approximately 60% RH. However, at high RH, the results showed the moisture-excipient interaction can be more complex than the proposed mechanism.

Aqueous solubility of crystalline and amorphous drugs: Challenges in measurement.

Measurement of drug solubility is one of the key elements of active pharmaceutical ingredient (API) characterization during the drug discovery and development process. This report is a critical review of experimental methods reported in the literature for the measurement of aqueous solubility of amorphous, partially crystalline and crystalline organic compounds. A summary of high-throughput automated methods used in early drug discovery research is also provided in this report. This review summarizes the challenges that are encountered during solubility measurement and the complexities that are often overlooked. Even though there is an advantage in using the amorphous form of a drug due to its higher solubility, measurement of its solubility with useful accuracy is still a practical problem. Therefore, this review provides recommendations of preferred methods and precautions in using these methods to determine the aqueous solubility of amorphous and crystalline new molecular entities, with emphasis on the physico-chemical characterization of the solid state of the test substance.

Solution-mediated phase transformation of haloperidol mesylate in the presence of sodium lauryl sulfate.

Forming a salt is a common way to increase the solubility of a poorly soluble compound. However, the solubility enhancement gained by salt formation may be lost due to solution-mediated phase transformation (SMPT) during dissolution. The SMPT of a salt can occur due to a supersaturated solution near the dissolving surface caused by pH or other solution conditions. In addition to changes in pH, surfactants are also known to affect SMPT. In this study, SMPT of a highly soluble salt, haloperidol mesylate, at pH 7 in the presence of a commonly used surfactant, sodium lauryl sulfate (SLS), was investigated. Dissolution experiments were performed using a flow-through dissolution apparatus with solutions containing various concentrations of SLS. Compacts of haloperidol mesylate were observed during dissolution in the flow-through apparatus using a stereomicroscope. Raman microscopy was used to characterize solids. The dissolution of haloperidol mesylate was significantly influenced by the addition of sodium lauryl sulfate. In conditions where SMPT was expected, the addition of SLS at low concentrations (0.1-0.2 mM) reduced the dissolution of haloperidol mesylate. In solutions containing concentrations of SLS above the critical micelle concentration (CMC) (10-15 mM), the dissolution of haloperidol mesylate increased compared to below the CMC. The solids recovered from solubility experiments of haloperidol mesylate indicated that haloperidol free base precipitated at all concentrations of SLS. Above 5 mM of SLS, Raman microscopy suggested a new form, perhaps the estolate salt. The addition of surfactant in solids that undergo solution-mediated phase transformation can add complexity to the dissolution profiles and conversion.