Electrolyte-induced changes in glass transition temperatures of freeze-concentrated solutes.

Addition of electrolytes to solutions of non-crystallizing solutes can cause a significant decrease in the glass transition temperature (Tg') of the maximally freeze-concentrated solution. For example, addition of 2% sodium chloride to 10% solutions of dextran, PVP, lactose, and sucrose causes a decrease in Tg' of 14 degrees to 18 degrees C. Sodium phosphate has a smaller effect on Tg' and is unusual in that 1% to 2% sodium phosphate in 10% PVP causes a second glass transition to be observed in the low-temperature thermogram, indicating a phase separation in the freeze concentrate. Comparison of DSC thermograms of fast-frozen solutions of sucrose with and without added sodium chloride shows that electrolyte-induced reduction of Tg' is not caused by a direct plasticizing effect of the electrolyte on the freeze concentrate. Measurement of unfrozen water content as a function of temperature by a pulsed nmr method shows that the most likely mechanism for electrolyte-induced changes in Tg' is by increasing the quantity of unfrozen water in the freeze concentrate, where the unfrozen water acts as a plasticizer and decreases Tg'. The correlation time (tau c) of water in the freeze concentrate is in the range of 10(-7) to 10(-8) seconds. The results underscore the importance of minimizing the amount of added salts to formulations intended for freeze drying.

An improved microscope stage for direct observation of freezing and freeze drying.

A microscope stage for observation of freezing and freeze drying is described. The stage uses thermoelectric (Peltier) heaters configured in two stages, with circulating fluid as a heat sink on the high temperature side. Lowest attainable sample temperature is about -47 degrees C. Principal advantages of this system are closed-loop control of stage temperature, rapid response to changes in temperature set point, and improved documentation of experiments by use of a video recorder system with a character generator which allows display of sample identity and temperature. Accuracy of measuring the sample temperature in the field of view was validated by comparing observed values of eutectic melting with published values for a series of solutes with eutectic temperatures in the range from -2 degrees C to -32 degrees C. Good agreement was obtained throughout this range.

Measurement of glass transition temperatures in freeze concentrated solutions of non-electrolytes by electrical thermal analysis.

The electrical resistance (R) of frozen aqueous solutions was measured as a function of temperature in order to determine whether this technique can be applied for determination of glass transition temperatures of maximally freeze concentrated solutions (Tg') of non-electrolytes which do not crystallize during freezing. Electrical thermal analysis (ETA) thermograms of frozen solutions containing the solute alone show a gradual change in slope over the temperature range of interest, with no inflection point which corresponds to Tg'. However, addition of low levels (about 0.1%) of electrolyte changes the shape of the thermogram into a biexponential function where the intersection of the two linear portions of the log (R) vs. T plot corresponds to the glass transition region. The total change in log (R) over the temperature range studied increases as the ionic radius of the reporter ion increases. The sharpest inflection points in the log (R) vs T curves, and the best correlation with DSC results, were obtained with ammonium salts. Tg' values measured by ETA were compared with values measured by DSC. DSC thermograms of solutes with and without electrolyte (0.1%) show that the electrolyte decreases Tg' by about 0.5 to 1.0 degrees C. However, Tg' values measured by ETA are somewhat higher than those measured by DSC, and difference between the two methods seems to increase as Tg' decreases. Tg' as measured by ETA is less heating rate dependent than DSC analysis, and ETA is a more sensitive method than DSC at low solute concentrations and at low heating rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of glass transition temperatures of freeze-concentrated solutes by differential scanning calorimetry.

Thermal analysis of aqueous solutions in which the solute does not crystallize immediately upon freezing was carried out to define the effects of experimental parameters on thermograms in the glass transition region. The intensity of enthalpy relaxations in the glass transition region is related to both the rate of cooling and the rate of heating through the glass transition region--slow cooling or slow heating increases the extent of structural relaxation in the glassy state and increases the intensity of the endotherm. Plots of the logarithm of heating rate versus 1/Tg' are linear, and activation enthalpies for structural relaxation are in the range of 210-350 kJ/mol. For polymeric solutes, both the activation enthalpies for structural relaxation and the heat capacity change accompanying the glass transition increase with increasing molecular weight of the solute. Molecular weight dependence of the observed midpoint of the glass transition agrees with the Fox-Flory relationship. Results are compared and contrasted with glass transitions in solid polymers and with the glass transition of hyperquenched water. Practical implications for characterization of formulations intended for freeze-drying are discussed.

Pharmacodynamics and biophasic drug levels of methionine enkephalin.

Methionine enkephalin (Met-E) is a naturally occurring pentapeptide. It appears to mediate pain perception by blocking CNS pathways. Using rabbits, a log dose relationship was obtained between Met-E-induced dilation of the pupil (mydriasis) and constant intravenous infusion dose rates (14, 20, 28, 37, 56, 73, and 129 micrograms/min/kg). Steady-state dilation is reached within 9 min. Dose-effect curves (DEC) were fitted by a linear regression analysis of the log dose versus percentage dilation plots. Fitted DEC were used to determine temporal profiles for the relative biophasic drug level.