Characterization of dynamics in complex lyophilized formulations: II. Analysis of density variations in terms of glass dynamics and comparisons with global mobility, fast dynamics, and Positron Annihilation Lifetime Spectroscopy (PALS)

Abstract

Amorphous HES/disaccharide (trehalose or sucrose) formulations, with and without added polyols (glycerol and sorbitol) and disaccharide formulations of human growth hormone (hGH), were prepared by freeze drying and characterized with particular interest in methodology for using high precision density measurements to evaluate free volume changes and a focus on comparisons between "free volume" changes obtained from analysis of density data, fast dynamics (local mobility), and PALS characterization of "free volume" hole size. Density measurements were performed using a helium gas pycnometer, and fast dynamics was characterized using incoherent neutron scattering spectrometer. Addition of sucrose and trehalose to hGH decreases free volume in the system with sucrose marginally more effective than trehalose, consistent with superior pharmaceutical stability of sucrose hGH formulations well below Tg relative to trehalose. We find that density data may be analyzed in terms of free volume changes by evaluation of volume changes on mixing and calculation of apparent specific volumes from the densities. Addition of sucrose to HES decreases free volume, but the effect of trehalose is not detectable above experimental error. Addition of sorbitol or glycerol to HES/trehalose base formulations appears to significantly decrease free volume, consistent with the positive impact of such additions on pharmaceutical stability (i.e., degradation) in the glassy state. Free volume changes, evaluated from density data, fast dynamics amplitude of local motion, and PALS hole size data generally are in qualitative agreement for the HES/disaccharide systems studied. All predict decreasing molecular mobility as disaccharides are added to HES. Global mobility as measured by enthalpy relaxation times, increases as disaccharides, particularly sucrose, are added to HES.

Keywords: Density; Free volume; Freeze drying; Glass dynamics; PALS; Pharmaceutical stability.

Fig. 1

Decrease in apparent specific volume at room temperature $({\Phi }_{\nu }^{\prime })$ of 1% polyol on mixing with hydroxyethylstarch/trehalose systems, as calculated from densities in Table 1: difference between amorphous polyol specific volume and apparent specific volume of mixture (apparent loss of specific volume of polyol on mixing). Error bars reflect estimated standard errors evaluated from corresponding standard errors in density using propagation of errors.

Fig. 2

Analysis of density data: volume change on mixing amorphous polyols with hydroxyethylstarch/trehalose (HES) systems. The volume decrease on mixing 1% polyol with HES/trehalose is calculated from density data (Table 1) using density of amorphous polyol to evaluate initial volume. Error bars represent standard errors of the mean calculated from corresponding density errors using propagation of errors.

Fig. 3

Volume change on mixing hydroxyethylstarch (HES) with amorphous disaccharides at room temperature evaluated from density data in Table 1. Error bars represent standard errors as estimated from corresponding errors in density measurements using propagation of errors. Open triangles = HES/trehalose; filled squares = HES/sucrose.

Fig. 4

Effect of disaccharide content on density of human growth hormone (hGH) formulations: comparison of experimental data for sucrose and trehalose systems with calculated effect of changing atomic density with added disaccharide. The effect of variable average atomic density was estimated from calculated van der Waals densities. Relative normalized density means density normalized to the value obtained for hGH without saccharide. All disaccharide containing systems contained sodium phosphate buffer, but the density measurement for hGH alone did not contain buffer. Thus, the value of density for hGH (zero disaccharide) was evaluated from the measured value for pure hGH (1.282) and handbook literature values for density of phosphate salts to give the value for zero disaccharide used in the plot (1.327). Error bars indicated standard errors evaluated from density errors using propagation of errors. Error estimate in the van der Waals density is about 2.5%. ANOVA comparing sucrose with trehalose systems gave a P value of 0.20 (Tukey’s HSD).

Fig. 5

Comparison of relaxation dynamics (ln τ β) [τ in h], reciprocal of fast dynamics amplitude (1/〈u²〉), % decrease in PALS lifetime from base hydroxyethylstarch (HES) for HES/disaccharide systems, and loss of volume on mixing 1:1 systems (µL/g). Relaxation times at 40 °C were taken from a previous report (Ref. [25]), fast dynamics refers to the mean square of the amplitude of motion (Ų) evaluated from neutron scattering at 40 °C, and PALS refers to positron annihilation lifetimes (ns). Error bars indicate estimated standard errors. Note that ΔV_mix for trehalose systems is essentially zero so all that is shown on the plot is the error bar.

Scheme 1

van der Waals radii, relative volume (to H), and atomic weight