β-Relaxation of Insulin Molecule in Lyophilized Formulations Containing Trehalose or Dextran as a Determinant of Chemical Reactivity

Purpose

The purpose of this study was to elucidate whether the degradation rate of insulin in lyophilized formulations is determined by matrix mobility, as reflected in glass transition temperature (T_g), or by β-relaxation, as reflected in rotating-frame spin-lattice relaxation time ${\left( {T_{{1{\text{ $ \rho $ }}}} } \right)}$.

Methods

The storage stability of insulin lyophilized with dextran was investigated at various relative humidities (RH; 12–60%) and temperatures (40–90°C) and was compared with previously reported data for insulin lyophilized with trehalose. Insulin degradation was monitored by reverse-phase high-performance liquid chromatography. Furthermore, the ${\left( {T_{{1{\text{ $ \rho $ }}}} } \right)}$of the insulin carbonyl carbon in the lyophilized insulin–dextran and insulin–trehalose systems was measured at 25°C by ¹³C solid-state NMR, and the effect of trehalose and dextran on ${\left( {T_{{1{\text{ $ \rho $ }}}} } \right)}$was compared at various humidities.

Results

The degradation rate of insulin lyophilized with dextran was not significantly affected by the T_g of the matrix, even at low humidity (12% RH), in contrast to that of insulin lyophilized with trehalose. The insulin–dextran system exhibited a substantially greater degradation rate than the insulin–trehalose system at a given temperature below the T_g. The difference in degradation rate between the insulin–dextran and insulin–trehalose systems observed at 12% RH was eliminated at 43% RH. In addition, the ${\left( {T_{{1{\text{ $ \rho $ }}}} } \right)}$of the insulin carbonyl carbon at low humidity (12% RH) was prolonged by the addition of trehalose, but not by the addition of dextran. This difference was eliminated at 23% RH, at which point the solid remained in the glassy state. These findings suggest that the β-relaxation of insulin is inhibited by trehalose at low humidity, presumably as a result of insulin–trehalose interaction, and thus becomes a rate determinant. In contrast, dextran, whose ability to interact with insulin is thought to be less than that of trehalose, did not inhibit the β-relaxation of insulin, and thus, the chemical activational barrier (activation energy) rather than β-relaxation becomes the major rate determinant.

Conclusions

β-Relaxation rather than matrix mobility seems to be more important in determining the stability of insulin in the glassy state in lyophilized formulations containing trehalose and dextran.