Raj Suryanarayanan

A rapid thermal method for cocrystal screening

A compound-sparing, automated and ‘green’ differential scanning calorimetric method was developed for rapid cocrystal screening.

Sustained release of nerve growth factor from biodegradable polymer microspheres.

Although grafted adrenal medullary tissue to the striatum has been used both experimentally and clinically in parkinsonism, there is a definite need to augment long-term survival. Infusion of nerve growth factor (NGF) or implantation of NGF-rich tissue into the area of the graft prolongs survival and induces differentiation into neural-like cells. To provide for prolonged, site-specific delivery of this growth factor to the grafted tissue in a convenient manner, we fabricated biodegradable polymer microspheres of poly(L-lactide)co-glycolide (70:30) containing NGF. Biologically active NGF was released from the microspheres, as assayed by neurite outgrowth in a dorsal root ganglion tissue culture system. Anti-NGF could block this outgrowth. An enzyme-linked immunosorbent assay detected NGF still being released in vitro for longer than 5 weeks. In vivo immunohistochemical studies showed release over a 4.5-week period. This technique should prove useful for incorporating NGF and other growth factors into polymers and delivering proteins and other macromolecules intracerebrally over a prolonged time period. These growth factor-containing polymer microspheres can be used in work aimed at prolonging graft survival, treating experimental Alzheimers disease, and augmenting peripheral nerve regeneration.

Local Mobility in Amorphous Pharmaceuticals― Characterization and Implications on Stability

In recent years, considerable effort has been directed towards correlating molecular mobility with the physical as well as chemical stability of amorphous pharmaceuticals. Global mobility (molecular motions associated with glass transition) has been the focus of most of these studies. However, in several instances, global mobility could not explain the instability. It is becoming recognized that local mobility (beta-relaxations), which is significant below the glass transition temperature, could be influencing stability. Generally, information on the mobility of an amorphous pharmaceutical below the glass transition temperature (T(g)) has been obtained by extrapolation of data from above T(g). Such studies, while providing information about overall mobility, are unsuitable for directly characterizing the local mobility. Our overall objective is to highlight the pharmaceutical significance of local motions in amorphous pharmaceuticals, primarily the Johari-Goldstein relaxations. The coupling model, which correlated the local motions with global mobility, has been discussed in order to emphasize the potential impact of local mobility on amorphous phase stability. The influence of additives including water on the local motions in an amorphous matrix, as in molecular dispersions, has been reviewed. Finally, we have provided a brief overview, including the strengths and limitations, of the common instrumental techniques used to characterize local motions.

Effect of Preparation Method on Physical Properties of Amorphous Trehalose

AbstractPurpose. To determine the effect of preparation method on the physical properties of amorphous trehalose. Methods. Amorphous anhydrous trehalose was prepared by four different methods, viz., freeze-drying, spray-drying, dehydration, and melt quenching. The glass transition temperature (Tg), enthalpic relaxation behavior, and crystallization were studied by differential scanning calorimetry, whereas X-ray diffractometry was used for phase identification. The rate and extent of water uptake at different relative humidity values were also obtained. Results. Though the enthalpic relaxation and crystallization behaviors were influenced by the method of preparation of amorphous trehalose, the Tg and fragility were not. The phase prepared by dehydration showed the highest enthalpic recovery at Tg, indicating that aging may have occurred during preparation. Among the four methods used, trehalose prepared by dehydration had the highest tendency to crystallize, whereas there was no crystallization in melt-quenched amorphous trehalose. The method of preparation influenced not only the rate and extent of water sorption but also the phase crystallized. Water vapor sorption removed the effects of structural history in the amorphous phase formed by dehydration. Conclusions. The method of preparation strongly influenced the pharmaceutically relevant properties of amorphous trehalose. The resistance to crystallization can be rank ordered as trehalose prepared by dehydration < freeze-dried mF spray-dried < melt-quenched. The rate of water sorption can be rank ordered as trehalose prepared by dehydration < freeze-dried < spray-dried.

Crystallization Behavior of Mannitol in Frozen Aqueous Solutions

AbstractPurpose. To study the effect of cooling rate, the influence of phosphate buffers and polyvinylpyrrolidone (PVP) on the crystallization behavior of mannitol in frozen aqueous solutions. Methods. Low-temperature differential scanning calorimetry and powder X-ray diffractometry were used to characterize the frozen solutions. Results. Rapid cooling (20°C/min) inhibited mannitol crystallization, whereas at slower cooling rates (10°C and 5°C/min) partial crystallization was observed. The amorphous freeze-concentrate was characterized by two glass transitions at -32°C and -25°C. When the frozen solutions were heated past the two glass transition temperatures, the solute crystallized as mannitol hydrate. An increase in the concentration of PVP increased the induction time for the crystallization of mannitol hydrate. At concentrations of ≥100 mM, the buffer salts significantly inhibited mannitol crystallization. Conclusions. The crystallization behavior of mannitol in frozen solutions was influenced by the cooling rate and the presence of phosphate buffers and PVP.

Effective Inhibition of Mannitol Crystallization in Frozen Solutions by Sodium Chloride

AbstractPurpose. The purpose of this work was to study the possibility of preventing mannitol crystallization in frozen solutions by using pharmaceutically acceptable additives. Methods. Differential scanning calorimetry (DSC) and low-temperature X-ray diffractometry (LTXRD) were used to characterize the effect of additives on mannitol crystallization. Results. DSC screening revealed that salts (sodium chloride, sodium citrate, and sodium acetate) inhibited mannitol crystallization in frozen solutions more effectively than selected surfactants, α-cyclodextrin, polymers, and alditols. This finding prompted further studies of the crystallization in the mannitol-NaCl-water system. Isothermal DSC results indicated that mannitol crystallization in frozen solutions was significantly retarded in the presence of NaCl and that NaCl did not crystallize until mannitol crystallization completed. Low-temperature X-ray diffractometry data showed that when a 10% w/v mannitol solution without additive was cooled at 1°C/min, the crystalline phases emerging after ice crystallization were those of a mannitol hydrate as well as the anhydrous polymorphs. In the presence of NaCl (5% w/v), mannitol crystallization was suppressed during both cooling and warming and occurred only after annealing and rewarming. In the latter case however, mannitol did not crystallize as the hydrate, but as the anhydrous δ polymorph. At a lower NaCl concentration of 1% w/v, the inhibitory effect of NaCl on mannitol crystallization was evident even during annealing at temperatures close to the Tg′ (−40°C). A preliminary lyophilization cycle with polyvinyl pyrrolidone and NaCl as additives rendered mannitol amorphous. Conclusion. The effectiveness of additives in inhibiting mannitol crystallization in frozen solutions follows the general order: salts > alditols > polyvinyl pyrrolidone > α-cyclodextrin > polysorbate 80 ∼ polyethylene glycol ∼ poloxamer. The judicious use of additives can retain mannitol amorphous during all the stages of the freeze-drying cycle.

Determination of the relative amounts of anhydrous carbamazepine (C15H12N2O) and carbamazepine dihydrate (C15H12N2O•2H2O) in a mixture by powder X-ray diffractometry

A powder x-ray diffraction technique has been developed to quantify the relative amounts of anhydrous carbamazepine (C15H12N2O) (I) and carbamazepine dihydrate (C15H12N2O · 2H2O) (II) when they occur as a mixture. The theoretical basis of this technique was developed by Alexander and Klug (Anal. Chem.20:886–889, 1948). The powder x-ray diffraction patterns of I and II revealed that the lines with d-spacings of 6.78 and 9.93 Å were unique to I and II, respectively. The ratio of the integrated intensity of the 6.78 Å line in a mixture of I and II to the intensity of the 6.78 Å line in a sample consisting of only I was calculated as a function of weight fraction of I in the mixture. These ratios were also experimentally determined, and there was a good agreement between the theoretical and the experimental intensity ratios. Similarly good agreements between the theoretical and the experimental intensity ratios for the 9.93 Å line of II were obtained. The samples were compressed into tablets and subjected to x-ray analysis. When compressed to a certain pressure, the particles tended to orient the same way in replicate samples resulting in highly reproducible intensity values. Compression into tablets was necessary because the powder samples yielded unsatisfactory results.

Effect of Aging on the Physical Properties of Amorphous Trehalose

AbstractPurpose. The purpose of this investigation was i) to study the effect of physical aging on crystallization and water vapor sorption behavior of amorphous anhydrous trehalose prepared by freeze-drying, and ii) to determine the effects of water sorption on the relaxation state of the aged material. Methods. Freeze-dried trehalose was aged at 100°C for varying time periods to obtain samples with different degrees of relaxation. The glass transition temperature (Tg) and enthalpic relaxation were determined by differential scanning calorimetry, and the rate and extent of water uptake at different relative humidity values were quantified using an automated vapor sorption balance. Results. Annealing below the Tg caused nucleation in the amorphous trehalose samples, which decreased the crystallization onset temperature on subsequent heating. However, no crystallization was observed below the Tg even after prolonged annealing. Physical aging caused a decrease in the rate and extent of water vapor sorption at low relative humidity values. Moreover, the water sorption removed the effects of physical aging, thus effectively causing enthalpic recovery in the aged samples. This recovery occurred gradually in the glassy phase and was not associated with a glass to rubber transition. We believe this aging reversal to be due to volume expansion during water sorption in the amorphous structure. Conclusions. Thermal history of amorphous materials is a crucial determinant of their physical properties. Aging of amorphous trehalose led to nucleation below the Tg, and decrease in rate and extent of water sorption. Sorption of water resulted in irreversible changes in the relaxation state of the aged material.

In Situ Dehydration of Carbamazepine Dihydrate: A Novel Technique to Prepare Amorphous Anhydrous Carbamazepine

The purposes of this project were to prepare amorphous carbamazepine by dehydration of crystalline carbamazepine dihydrate, and to study the kinetics of crystallization of the prepared amorphous phase. Amorphous carbamazepine was formed and characterized in situ in the sample chamber of a differential scanning calorimeter (DSC), a thermogravimetric analyzer (TGA), and a variable temperature x-ray powder diffractometer (VTXRD). It has a glass transition temperature of 56°C and it is a relatively strong glass with a strength parameter of 37. The kinetics of its crystallization were followed by isothermal XRD, under a controlled water vapor pressure of 23 Torr. The crystallization kinetics are best described by the three-dimensional nuclear growth model with rate constants of 0.014, 0.021, and 0.032 min1 at 45, 50, and 55°C, respectively. When the Arrhenius equation was used, the activation energy of crystallization was calculated to be 74 kJ/mol in the presence of water vapor (23 Torr). On the basis of the Kissinger plot, the activation energy of crystallization in the absence of water vapor (0 Torr water vapor pressure) was determined to be 157 kJ/mol. Dehydration of the dihydrate is a novel method to prepare amorphous carbamazepine; in comparison with other methods, it is a relatively gentle and effective technique.

Correlation between molecular mobility and physical stability of amorphous itraconazole.

The goal was to investigate the correlation between molecular mobility and physical stability in amorphous itraconazole and identify the specific mobility mode responsible for its 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 T(g), β-relaxation became progressively less resolved due to interference from the high frequency tail of the α-relaxation and then transformed into an excess wing. Above T(g), nonlinear temperature dependence of the α-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 T(g) as indicated by the linear correlation of α-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.