Raj Suryanarayanan

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (101)341.49 Total impact

  • Vishard Ragoonanan, Raj Suryanarayanan
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    ABSTRACT: We hypothesize that ultrasonication can accelerate solute crystallization in freeze-concentrates. Our objective is to demonstrate ultrasonication as a potential predictive tool for evaluating physical stability of excipients in frozen solutions. The crystallization tendencies of lyoprotectants (trehalose, sucrose), carboxylic acid buffers (citric, tartaric, malic, and acetic) and an amino acid buffer (histidine HCl) were studied. Aqueous solutions of buffers, lyoprotectants and mixtures of the two were cooled from room temperature to -20°C and sonicated to induce solute crystallization. The crystallized phases were identified by X-ray diffractometry (laboratory or synchrotron source). Sonication accelerated crystallization of trehalose dihydrate in frozen trehalose solutions. Sonication also enhanced solute crystallization in tartaric (200 mM; pH 5), citric (200 mM pH 4) and malic (200 mM; pH 4) acid buffers. At lower buffer concentrations, longer annealing times following sonication were required to facilitate solute crystallization. The time for crystallization of histidine HCl progressively increased as a function of sucrose concentration. The insonation period required to effect crystallization also increased with sucrose concentration. Sonication can substantially accelerate solute crystallization in the freeze-concentrate. Ultrasonication may be useful in assessing the crystallization tendency of formulation constituents used in long term frozen storage and freeze-drying.
    Pharmaceutical Research 01/2014; · 4.74 Impact Factor
  • Naveen K Thakral, Vishard Ragoonanan, Raj Suryanarayanan
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    ABSTRACT: Model tablet formulations containing thiamine hydrochloride [as a nonstoichiometric hydrate (NSH)] and dicalcium phosphate dihydrate (DCPD) were prepared. In intact tablets, the water released by dehydration of DCPD mediated the transition of NSH to thiamine hydrochloride hemihydrate (HH). The use of an X-ray microdiffractometer with an area detector enabled us to rapidly and simultaneously monitor both the phase transformations. The spatial information, gained by monitoring the tablet from the surface to the core (depth profiling), revealed that both DCPD dehydration and HH formation progressed from the surface to the tablet core as a function of storage time. Film coating of the tablets with ethyl cellulose caused a decrease in both the reaction rates. There was a pronounced lag time, but once initiated, the transformations occurred simultaneously throughout the tablet. Thus the difference in the phase transformation behavior between the uncoated and the coated tablets could not have been discerned without the depth profiling. Incorporation of hydrophilic colloidal silica as a formulation component further slowed down the transformations. By acting as a water scavenger it maintained a very "dry" environment in the tablet matrix. Finally, by coating the NSH particles with hydrophobic colloidal silica, the formation of HH was further substantially decelerated. The microdiffractometric technique not only enabled direct analyses of tablets but also provided the critical spatial information. This helped in the selection of excipients with appropriate functionality to prevent the in situ phase transformations.
    Molecular Pharmaceutics 07/2013; · 4.57 Impact Factor
  • Mehak Mehta, Sunny P Bhardwaj, Raj Suryanarayanan
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    ABSTRACT: A potential drawback with the use of mannitol as a bulking agent is its existence as mannitol hemihydrate (MHH; C6H14O6(.)0.5H2O) in the lyophile. Once formed during freeze-drying, MHH dehydration may require secondary drying under aggressive conditions which can be detrimental to the stability of thermolabile components. If MHH is retained in the lyophile, the water released by MHH dehydration during storage has the potential to cause product instability. We systematically identified the conditions under which anhydrous mannitol and MHH crystallized in frozen systems with the goal of preventing MHH formation during freeze-drying. When mannitol solutions were cooled, the temperature of solute crystallization was the determinant of the physical form of mannitol. Based on low temperature X-ray diffractometry (using both laboratory and synchrotron sources), MHH formation was observed when solute crystallization occurred at temperatures ⩽ -20 °C while anhydrous mannitol crystallized at temperatures ⩾ -10 °C. The transition temperature (anhydrate - MHH) appears to be ∼ -15 °C. The use of a freeze-dryer with controlled ice nucleation technology enabled anhydrous mannitol crystallization at ∼ -5 °C. Thus, ice crystallization followed by annealing at temperatures ⩾ -10 °C can be an effective strategy to prevent MHH formation.
    European journal of pharmaceutics and biopharmaceutics: official journal of Arbeitsgemeinschaft fur Pharmazeutische Verfahrenstechnik e.V 04/2013; · 3.15 Impact Factor
  • Kapildev K Arora, Seema Thakral, Raj Suryanarayanan
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    ABSTRACT: PURPOSE: To demonstrate two sequential solid-state reactions in intact tablets: dehydration of active pharmaceutical ingredient (API), and cocrystal formation between the anhydrous API and a second formulation component mediated by the released water. To evaluate the implication of this in situ phase transformation on the tablet dissolution behavior. METHODS: Tablets containing theophylline monohydrate (TPM) and anhydrous citric acid (CA) were stored at 40°C in sealed polyester pouches and the relative humidity in the headspace above the tablet was continuously measured. Dehydration to anhydrous theophylline (TPA) and the product appearance (TPA-CA cocrystal) were simultaneously monitored by powder X-ray diffractometry. Carbamazepine dihydrate and nicotinamide formed the second model system. RESULTS: The water of crystallization released by TPM dehydration was followed first by deliquescence of citric acid, evident from the headspace relative humidity (~ 68%; 40°C), and then the formation of TPA-CA cocrystal in intact tablets. The noncovalent synthesis resulted in a pronounced decrease in the dissolution rate of theophylline from the tablets. Similarly, the water released by dehydration of carbamazepine dihydrate caused the cocrystallization reaction between anhydrous carbamazepine and nicotinamide. CONCLUSIONS: The water released by API dehydration mediated cocrystal formation in intact tablets and affected dissolution behavior.
    Pharmaceutical Research 04/2013; · 4.74 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The goal was to investigate the correlation between molecular mobility and physical stability and identify the specific mobility mode responsible for instability. The molecular mobility of amorphous itraconazole, in the glassy as well as the supercooled liquid state, was comprehensively characterized using dynamic dielectric spectroscopy. Isothermal frequency sweeps in the 5 - 40 °C temperature range revealed a β-relaxation which exhibited Arrhenius temperature dependence. As the temperature approached Tg, β-relaxation became progressively less resolved due to interference from the high frequency tail of the alpha-relaxation and then transformed into an excess wing. Above Tg, non-linear temperature dependence of the alpha-relaxation was described by the Vogel-Tammann-Fulcher (VTF) model. Itraconazole was found to be a fragile glass former with a VTF strength parameter of ~ 4. Isothermal crystallization kinetics, at several temperatures over the range of 75 to 95 °C, was best described by the 3-dimensional nucleation and growth model. Primary relaxation appeared to be the mobility responsible for the observed physical instability at temperatures above Tg as indicated by the linear correlation of alpha-relaxation with both crystallization onset and kinetics (represented by the inverse of the crystallization rate constant). A strong coupling between global mobility and crystallization onset was evident. However, for growth kinetics, the coupling was less pronounced, indicating the involvement of factors other than global mobility.
    Molecular Pharmaceutics 12/2012; · 4.57 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Solid-state NMR spectroscopy (SSNMR), coupled with powder X-ray diffraction (PXRD), was used to identify the physical forms of gabapentin in samples prepared by recrystallization, spray drying, dehydration, and milling. Four different crystalline forms of gabapentin were observed: form I, a monohydrate, form II, the most stable at ambient conditions, form III, produced by either recrystallization or milling, and an isomorphous desolvate produced from desolvating the monohydrate. As-received gabapentin (form II) was ball-milled for 45 min in both the presence and absence of hydroxypropylcellulose (HPC). The samples were then stored for 2 days at 50°C under 0% relative humidity and analyzed by (13)C SSNMR and PXRD. High-performance liquid chromatography was run on the samples to determine the amount of degradation product formed before and after storage. The (1)H T (1) values measured for the sample varied from 130 s for the as-received unstressed material without HPC to 11 s for the material that had been ball-milled in the presence of HPC. Samples with longer (1)H T (1) values were substantially more stable than samples that had shorter T (1) values. Samples milled with HPC had detectable form III crystals as well. These results suggest that SSNMR can be used to predict gabapentin stability in formulated products.
    AAPS PharmSciTech 11/2012; · 1.58 Impact Factor
  • Sunny P Bhardwaj, Raj Suryanarayanan
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    ABSTRACT: PURPOSE: The work aims at investigating the correlation of water sorption potential with different measures of molecular mobility in an annealed amorphous model compound (trehalose). METHODS: Amorphous trehalose, prepared by freeze-drying, was annealed at 100°C (17°C < T (g)) for up to 120 h. Global molecular mobility was studied using a broadband dielectric spectrometer in the frequency range of 10(6)-10(-2) Hz. Enthalpic recovery was measured by differential scanning calorimetry and water sorption profiles were obtained using an automated vapor sorption balance. RESULTS: As a function of annealing time, there was an increase, both in average α-relaxation time and enthalpic recovery and a decrease in the amount of sorbed water. A strong linear correlation was observed between the water sorption potential and the dielectric relaxation time, indicating a common underlying mechanism of the effect of annealing time on these properties. Enthalpic recovery, which is widely used as a measure of structural relaxation, did not correlate well with the extent of water sorption. CONCLUSIONS: The α-relaxation time can be used as a predictor of the water sorption potential of amorphous trehalose. It will be of interest and value to develop such predictive models for other amorphous compounds of pharmaceutical interest.
    Pharmaceutical Research 10/2012; · 4.74 Impact Factor
  • Sunny P Bhardwaj, Raj Suryanarayanan
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    ABSTRACT: Dielectric spectroscopy was used to comprehensively characterize the molecular mobility in amorphous trehalose, an extensively used bioprotective agent. Isothermal frequency sweeps were carried out at different temperatures in the glassy and supercooled liquid states of freeze-dried trehalose. Two previously reported secondary relaxations were observed at temperatures far below its glass transition temperature (T(g)). At temperatures close to T(g), removal of dc conductivity contribution revealed a relaxation peak. The origin of this peak was evaluated using several diagnostic tests and determined to be the α-relaxation. There was also an excess wing in the high-frequency tail of α-relaxation. Sub-T(g) annealing caused the primary relaxation to shift to lower frequencies, enabling resolution of the excess wing, which was characterized to be the true Johari-Goldstein (JG) relaxation. A qualitatively similar effect of annealing on JG relaxation was also observed. The average relaxation times of the two previously reported secondary relaxations were unaffected by annealing.
    The Journal of Physical Chemistry B 08/2012; 116(38):11728-36. · 3.61 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The purpose of this study was to perform physical characterization of pentamidine isethionate (PI) in frozen and freeze-dried systems and to monitor the phase behavior during all the stages of freeze-drying. Frozen aqueous PI solutions as well as the final lyophiles were characterized by differential scanning calorimetry and X-ray diffractometry. The effect of cosolutes, cosolvents, and processing conditions on the PI crystallization behavior during freeze-drying was evaluated. In frozen aqueous solutions, irrespective of the cooling rate and the initial solute concentration, PI readily crystallized as a trihydrate (C(19) H(24) N(4) O(2) ·3H(2) O). It dehydrated to a poorly crystalline anhydrate upon drying at 100 mTorr. The presence of a readily crystallizing cosolute or an organic cosolvent did not influence the physical form of PI in the final lyophile. On the contrary, even in the absence of cosolutes and cosolvents, the crystalline trihydrate was retained when the chamber pressure was increased to 500 mTorr. By altering the drying conditions, it was possible to obtain either a crystalline trihydrate or a poorly crystalline anhydrate. The stability of PI is dependent on its physical form and only the amorphous PI undergoes discoloration. The PI stability can be enhanced by retaining it in a crystalline state in the lyophile.
    Journal of Pharmaceutical Sciences 01/2012; 101(5):1732-43. · 3.13 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The objective of this article was to monitor phase transformation in thiamine hydrochloride, from a nonstoichiometric hydrate (NSH) to a hemihydrate (HH), in stored tablets, prepared both by direct compression and wet granulation, and to relate the storage-induced phase transformation with changes in tablet microstructure, physical properties, and performance. Raman spectroscopy revealed complete NSH → HH transformation in tablets, within 30 h of storage at 40°C/75% relative humidity. When the tablets were prepared by wet granulation of NSH alone, there was a marked increase in both tablet volume and hardness on storage. However, when microcrystalline cellulose (MCC) was included in granulation, the resulting stored tablets also exhibited a pronounced increase in disintegration time. In contrast, tablets prepared by dry processing via compression of a NSH-MCC physical mixture did not exhibit any changes in properties, despite the in situ solid form conversion. Scanning electron microscopy revealed growth of needle-like HH crystals in all stored tablets and mercury porosimetry revealed considerable changes in the pore size distribution during storage. Longer storage led to crystal growth (Ostwald ripening), causing further gradual but less dramatic changes in properties. The phase transformation and the complex interparticulate associations in the tablet influenced the changes in tablet microstructure, compact physical properties, and product behavior.
    Journal of Pharmaceutical Sciences 12/2011; 101(4):1410-22. · 3.13 Impact Factor
  • Prakash Sundaramurthi, Raj Suryanarayanan
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    ABSTRACT: Lyophilization is a commonly used drying technique for thermolabile pharmaceuticals. Crystallization of formulation components may occur during various stages of the freeze-drying process. In frozen solutions, while crystallization of bulking agents is desirable, both from processing and product-elegance perspectives, buffer salt crystallization can cause a significant pH shift. Lyoprotectants (compounds that protect macromolecules, both during freeze-drying and subsequent storage) are effective only when retained amorphous. This review presents numerous applications of differential scanning calorimetry to characterize pharmaceutical systems in frozen state. These studies are aimed at defining the processing parameters and optimizing the freeze-drying cycle. Low temperature pH measurement and sub-ambient X-ray diffractometry served as excellent complementary tools in the characterization of frozen systems. The phase behavior of the systems during annealing (of frozen solutions), primary and secondary drying were monitored by X-ray diffractometry. Finally, the interplay of formulation composition and processing parameters on the development and optimization of freeze-drying cycles are reviewed.
    Advanced drug delivery reviews 12/2011; 64(5):384-95. · 11.96 Impact Factor
  • Source
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    ABSTRACT: Gabapentin is known to undergo intramolecular cyclization to form a lactam (gaba-l) with concomitant loss of water. Gabapentin was milled in a planetary mill for 15–60 min. Unmilled and milled gabapentin were stored at 50°C with relative humidity ranged between 5% and 90%. The unmilled and milled samples were assayed for gabapentin and gaba-l by reversed phase-high-performance liquid chromatography and also subjected to powder X-ray diffraction, solid-state nuclear magnetic resonance and surface area analyses. The rates of lactamization in the milled gabapentin samples correlated to increased surface area, milling duration, and in-process lactam levels. This effect of milling could not be explained solely by the increase in surface area with increased milling time but was more likely due to increased regions of crystal disorder caused by the mechanical and thermal milling stresses. The lactamization rate of milled gabapentin samples was greatest in the presence of the lowest humidity conditions and dramatically decreased with increasing humidity. In particular, milled gabapentin appeared to be much more stable at humidity levels greater than 31% RH. This finding could not be attributed to the possibility of lactam hydrolysis at high humidity but rather to a competitive annealing process wherein milling-induced crystal defects were lost upon exposure to atmospheric moisture thereby stabilizing the milling-damaged drug substance.
    AAPS PharmSciTech 09/2011; · 1.58 Impact Factor
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    ABSTRACT: The objectives of the current study were to investigate (i) the phase behavior of a PEGylated recombinant human growth hormone (PEG-rhGH, ∼60 kDa) during freeze-drying and (ii) its storage stability. The phase transitions during freeze-thawing of an aqueous solution containing PEG-rhGH and sucrose were characterized by differential scanning calorimetry. Finally, PEG-rhGH and sucrose formulations containing low, medium, and high polyethylene glycol (PEG) to sucrose ratios were freeze-dried in dual-chamber syringes and stored at 4°C and 25°C. Chemical decomposition (methionine oxidation and deamidation) and irreversible aggregation were characterized by size-exclusion and ion-exchange chromatography, and tryptic mapping. PEG crystallization was facilitated when it was covalently linked with rhGH. When the solutions were frozen, phase separation into PEG-rich and sucrose-rich phases facilitated PEG crystallization and the freeze-dried cake contained crystalline PEG. Annealing caused PEG crystallization and when coupled with higher drying temperatures, the primary drying time decreased by up to 51%. When the freeze-dried cakes were stored at 4°C, while there was no change in the purity of the PEG-rhGH monomer, deamidation was highest in the formulations with the lowest PEG to sucrose ratio. When stored at 25°C, this composition also showed the most pronounced decrease in monomer purity, the highest level of aggregation, and deamidation. Furthermore, an increase in PEG crystallinity during storage was accompanied by a decrease in PEG-rhGH stability. Interestingly, during storage, there was no change in PEG crystallinity in formulations with medium and high PEG to sucrose ratios. Although PEG crystallization during freeze-drying did not cause protein degradation, crystallization during storage might have influenced protein stability.
    Journal of Pharmaceutical Sciences 08/2011; 100(8):3062-75. · 3.13 Impact Factor
  • Source
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    ABSTRACT: Gabapentin is known to undergo intramolecular cyclization to form a lactam (gaba-L) with concomitant loss of water. Gabapentin was milled in a planetary mill for 15-60 min. Unmilled and milled gabapentin were stored at 50°C with relative humidity ranged between 5% and 90%. The unmilled and milled samples were assayed for gabapentin and gaba-L by reversed phase-high-performance liquid chromatography and also subjected to powder X-ray diffraction, solid-state nuclear magnetic resonance and surface area analyses. The rates of lactamization in the milled gabapentin samples correlated to increased surface area, milling duration, and in-process lactam levels. This effect of milling could not be explained solely by the increase in surface area with increased milling time but was more likely due to increased regions of crystal disorder caused by the mechanical and thermal milling stresses. The lactamization rate of milled gabapentin samples was greatest in the presence of the lowest humidity conditions and dramatically decreased with increasing humidity. In particular, milled gabapentin appeared to be much more stable at humidity levels greater than 31% RH. This finding could not be attributed to the possibility of lactam hydrolysis at high humidity but rather to a competitive annealing process wherein milling-induced crystal defects were lost upon exposure to atmospheric moisture thereby stabilizing the milling-damaged drug substance.
    AAPS PharmSciTech 07/2011; 12(3):924-31. · 1.58 Impact Factor
  • Kapildev K Arora, Nitin G Tayade, Raj Suryanarayanan
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    ABSTRACT: The water of crystallization released during dehydration of dibasic calcium phosphate dihydrate (DCPD) mediated the cocrystal formation between carbamazepine (CBZ) and nicotinamide (NMA) in intact tablets. The dehydration of DCPD, the disappearance of the reactants (CBZ and NMA) and the appearance of the product (CBZ-NMA cocrystal) were simultaneously monitored by quantitative powder X-ray diffractometry. In a second model system, the water of crystallization released by the dehydration of DCPD caused the chemical decomposition of aspirin. Salicylic acid, one of the decomposition products, reacted with CBZ to form CBZ-salicylic acid cocrystal in tablets. This is the first report of cocrystal formation in intact tablets, demonstrating water mediated noncovalent synthesis in a multicomponent matrix. While the potential implications of such transformations, on both the mechanical and biopharmaceutical properties, can be profound, their characterization, using conventional solution based analytical techniques, can be challenging.
    Molecular Pharmaceutics 06/2011; 8(3):982-9. · 4.57 Impact Factor
  • Sunny P Bhardwaj, Raj Suryanarayanan
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    ABSTRACT: Amorphous trehalose finds extensive use as a stabilizer of biomolecules including proteins and phospholipids. Hypothesizing that molecular mobility is a determinant of its stability, dynamic dielectric spectroscopy (DDS) was used to map the different modes of mobility. Isothermal dielectric relaxation profiles of amorphous trehalose were obtained, over the frequency range of 10(-1)-10(7) Hz, and at temperatures ranging from 30-170 °C. At temperatures close to the glass transition (T(g)), the α-relaxation was not readily discernible due to interference from dc conductivity. We used Kramers-Kronig transformation that enabled not only the complete resolution of α-relaxation but also the identification of an excess wing, in the high frequency tail of α-relaxation. On annealing, this excess wing developed into a partially resolved and hitherto unidentified β-relaxation peak. This peak, due to its position in the dielectric spectrum, its annealing time dependence and the good agreement with the calculated independent relaxation time, was assigned to the Johari-Goldstein process. This work demonstrates the utility of conductivity subtraction coupled with sub-T(g) annealing to successfully study all the modes of mobility in amorphous trehalose. This approach can potentially be extended to situations wherein dc conductivity impedes the complete characterization of molecular mobility.
    Molecular Pharmaceutics 06/2011; 8(4):1416-22. · 4.57 Impact Factor
  • Prakash Sundaramurthi, Raj Suryanarayanan
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    ABSTRACT: Macromolecules and other thermolabile biologicals are often buffered and stored in frozen or dried (freeze-dried) state. Crystallization of buffer components in frozen aqueous solutions and the consequent pH shifts were studied in carboxylic (succinic, malic, citric, tartaric acid) and amino acid (glycine, histidine) buffers. Aqueous buffer solutions were cooled from room temperature (RT) to -25 °C and the pH of the solution was measured as a function of temperature. The thermal behavior of frozen solutions was investigated by differential scanning calorimetry (DSC), and the crystallized phases were identified by X-ray diffractometry (XRD). Based on the solubility of the neutral species of each buffer system over a range of temperatures, it was possible to estimate its degree of supersaturation at the subambient temperature of interest. This enabled us to predict its crystallization propensity in frozen systems. The experimental and the predicted rank orderings were in excellent agreement. The malate buffer system was robust with no evidence of buffer component crystallization and hence negligible pH shift. In the citrate and tartrate systems, at initial pH < pK(a)(2), only the most acidic buffer component (neutral form) crystallized on cooling, causing an increase in the freeze-concentrate pH. In glycine buffer solutions, when the initial pH was ∼3 units < isoelectric pH (pI = 5.9), β-glycine crystallization caused a small decrease in pH, while a similar effect but in the opposite direction was observed when the initial pH was ∼3 units > pI. In the histidine buffer system, depending on the initial pH, either histidine or histidine HCl crystallized.
    The Journal of Physical Chemistry B 06/2011; 115(21):7154-64. · 3.61 Impact Factor
  • Sisir Bhattacharya, Raj Suryanarayanan
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    ABSTRACT: To characterize molecular mobility by dielectric spectroscopy and determine the effect of additives on α- and β-relaxation times in amorphous sucrose solid dispersions. Sucrose was co-lyophilized with either PVP or sorbitol. The lyophiles were subjected to dielectric spectroscopy and differential scanning calorimetry. The additives did not have an appreciable effect on the calorimetric T(g). However, dielectric spectroscopy revealed pronounced effects on global mobility (α-relaxation), which correlated with the crystallization tendency of sucrose. The systems were characterized by two β-relaxations, and the relaxation times as well as their temperature dependence were influenced by the additive. Although sorbitol acted as a plasticizer of sucrose with respect to global mobility, it anti-plasticized sucrose in terms of local motions. PVP, on the other hand, acted as an anti-plasticizer with respect to both global and local mobility. The slower β-relaxation in amorphous sucrose was found to correlate with the α-relaxation and was identified as the Johari-Goldstein relaxation. Amorphous systems with identical calorimetric T(g) could have significantly different mobility and physical stability as revealed by dielectric spectroscopy. Additive effect on global mobility cannot be a predictor of the effects on local mobility. Additives could also be used to inhibit local mobility.
    Pharmaceutical Research 04/2011; 28(9):2191-203. · 4.74 Impact Factor
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    ABSTRACT: Infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopic techniques were used to investigate the nature of molecular aggregation of indomethacin (I) in acetonitrile and ethanol solutions. Spectroscopic data provided no evidence for self-aggregation of I in supersaturated solutions of acetonitrile or ethanol. As the concentration of I was increased by more than 130 fold, from 5 × 10–4 to 0.067 M in acetonitrile, the diffusion coefficients of I and water (residual) measured by pulse field gradient spin echo nuclear magnetic resonance (PFGSE-NMR) showed a decrease of about 5 and 27%, respectively, while the diffusion coefficient of acetonitrile did not change. The chemical shift and Nuclear Overhauser Effect Spectroscopy data confirmed formation of hydrogen bonds between the carboxyl group of I and residual water at low concentrations of I in acetonitrile, while IR-Raman data suggest formation of weak hydrogen bonds between I and acetonitrile at higher concentrations. In contrast, the diffusion coefficients of I and ethanol decreased by up to 15% as the concentration of I in ethanol was increased from 3.6 × 10–4 to 0.3 M. This together with IR and Raman data suggest formation of strong intermolecular hydrogen bonds between I and ethanol. In summary, our data suggest that solute–solvent interactions determine the critical supersaturation at the onset of nucleation (σcr) and thereby control the preferential crystallization of I polymorphs from supersaturated solutions of acetonitrile and ethanol. Furthermore, our data highlight the importance of intermolecular interactions between solute and residual water in aprotic solvents, such as acetonitrile.
    04/2011;
  • Prakash Sundaramurthi, Raj Suryanarayanan
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    ABSTRACT: Selective crystallization of buffer components in frozen solutions is known to cause pronounced pH shifts. Our objective was to study the crystallization behavior and the consequent pH shift in frozen aqueous carboxylic acid buffers. Aqueous carboxylic acid buffers were cooled to -25°C and the pH of the solution was measured as a function of temperature. The thermal behavior of solutions during freezing and thawing was investigated by differential scanning calorimetry. The crystallized phases in frozen solution were identified by X-ray diffractometry. The malate buffer system was robust with no evidence of buffer component crystallization and hence negligible pH shift. In the citrate and tartarate systems, at initial pH <pKa2 , only the most acidic buffer component (neutral form) crystallized on cooling, causing an increase in the freeze-concentrate pH. Carboxylic acid buffers were rank ordered based on their propensity to crystallize in frozen solutions. From the aqueous solubility values of these carboxylic acids, which have been reported over a range of temperatures, it was also possible to estimate the degree of supersaturation at the subambient temperature of interest. This enabled us to predict their crystallization propensity in frozen systems. The experimental and the predicted rank orderings were in excellent agreement. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:1288-1293, 2011.
    Journal of Pharmaceutical Sciences 04/2011; 100(4):1288-1293. · 3.13 Impact Factor

Publication Stats

822 Citations
60 Downloads
341.49 Total Impact Points

Institutions

  • 1997–2013
    • University of Minnesota Duluth
      • College of Pharmacy
      Duluth, Minnesota, United States
  • 1997–2012
    • University of Minnesota Twin Cities
      • • Department of Pharmaceutics
      • • College of Pharmacy
      Minneapolis, MN, United States
  • 2002
    • Creighton University
      • Department of Pharmacy Sciences
      Omaha, NE, United States