Freezing of Aqueous Solutions and Chemical Stability of Amorphous Pharmaceuticals: Water Clusters Hypothesis

Abstract

Molecular mobility has been traditionally invoked to explain physical and chemical stability of diverse pharmaceutical systems. Although the molecular mobility concept has been credited with creating a scientific basis for stabilization of amorphous pharmaceuticals and biopharmaceuticals, it has become increasingly clear that this approach represents only a partial description of the underlying fundamental principles. An additional mechanism is proposed herein to address 2 key questions: (1) the existence of unfrozen water (i.e., partial or complete freezing inhibition) in aqueous solutions at subzero temperatures and (2) the role of water in the chemical stability of amorphous pharmaceuticals. These apparently distant phenomena are linked via the concept of water clusters. In particular, freezing inhibition is associated with the confinement of water clusters in a solidified matrix of an amorphous solute, with nanoscaled water clusters being observed in aqueous glasses using wide-angle neutron scattering. The chemical instability is suggested to be directly related to the catalysis of proton transfer by water clusters, considering that proton transfer is the key elementary reaction in many chemical processes, including such common reactions as hydrolysis and deamidation.

Investigation of the Freezing Phenomenon in Vials Using an Infrared Camera

“…Once the system reaches a specific concentration characteristic of each excipient, the cryo-concentrated solution remains in a supercooled single-phase amorphous state. Water molecules get trapped into the excipient solidified matrix and cannot diffuse and crystallise further [ 47 , 48 ]. For this reason, freezing is usually regarded as complete when the solution reaches the eutectic point ( ) for crystalline solutes or the glass transition temperature ( ) for amorphous ones.…”

“…A slight temperature jump may be observed after one of the solutes becomes supersaturated and releases latent heat of crystallisation [ 15 ]. In many cases, freezing is assumed to be complete when the glass transition temperature (for amorphous solutes) or the eutectic point (for crystalline solutes) is reached [ 47 , 57 ]. However, the solution may continue its freezing process at even lower temperatures [ 15 ].…”

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

“…Neutron scattering 79 and molecular dynamics simulations 80 suggest that glycerol and water form clusters depending on mixing ratios which is directed by intramolecular and intermolecular hydrogen bonding between the two systems. Very recently a hypothesis connecting the freezing of aqueous solutions and chemical stability of amorphous pharmaceuticals have invoked water clusters, 81 and the aforementioned neutron scattering and molecular dynamics calculations were used as evidence in this intriguing suggestion.…”

Observation of high-temperature macromolecular confinement in lyophilised protein formulations using terahertz spectroscopy

“…Using THz-TDS, at the glass transition temperature (Tg) we can observe the Tg,α associated with the α-relaxation process, and below Tg, we can observe Tg,β associated with the β-relaxation process (Yu et al, 2013, Alegria and Colmenero, 2016, Cerveny et al, 2002, Roy and Inglefield, 1990, Barnes et al, 2001, Sibik and Zeitler, 2016). In doing so, THz-TDS can be used to characterise the molecular mobility of a system, which is a key factor when determining the chemical stability of a formulation, as an increase in molecular mobility has been directly linked to an increase in chemical degradation of a material and its storage stability (Shalaev et al, 2019). Thus, using THz-TDS it is possible to investigate the interplay between dielectric and vibrational dynamics and study the molecular mobility and structural dynamics of lyophilised formulations.…”