Tuning molecular dynamics by hydration and confinement: antiplasticizing effect of water in hydrated prilocaine nanoclusters

Structural Features of the Glassy State and Their Impact on the Solid-State Properties of Organic Molecules in Pharmaceutical Systems

This paper reviews the structure and properties of amorphous active pharmaceutical ingredients (APIs), including small molecules and proteins, in the glassy state (below the glass transition temperature, Tg). Amorphous materials in the neat state and formulated with excipients as miscible amorphous mixtures are included, and the role of absorbed water in affecting glass structure and stability has also been considered. We defined the term "structure" to indicate the way the various molecules in a glass interact with each other and form distinctive molecular arrangements as regions or domains of varying number of molecules, molecular packing, and density. Evidence is presented to suggest that such systems generally exist as heterogeneous structures made up of high-density domains surrounded by a lower density arrangement of molecules, termed the microstructure. It has been shown that the method of preparation and the time frame for handling and storage can give rise to variable glass structures and varying physical properties. Throughout this paper, examples are given of theoretical, computer simulation, and experimental studies which focus on the nature of intermolecular interactions, the size of heterogeneous higher density domains, and the impact of such systems on the relative physical and chemical stability of pharmaceutical systems.

Amorphous solid dispersions of curcumin in a poly(ester amide): Antiplasticizing effect on the glass transition and macromolecular relaxation dynamics, and controlled release

In order to exploit the pharmacological potential of natural bioactive molecules with low water solubility, such as curcumin, it is necessary to develop formulations, such as amorphous polymer dispersions, which allow a constant release rate and at the same time avoid possible toxicity effects of the crystalline form of the molecule under scrutiny. In this study, polymer dispersions of curcumin were obtained in PADAS, a biodegradable semicrystalline copolymer based on 1,12-dodecanediol, sebacic acid and alanine. The dispersions were fully characterized by means of differential scanning calorimetry and broadband dielectric spectroscopy, and the drug release profile was measured in a simulated body fluid. Amorphous homogeneous binary dispersions were obtained for curcumin mass fraction between 30 and 50%. Curcumin has significantly higher glass transition temperature Tg (≈ 347 K) than the polymer matrix (≈274-277 K depending on the molecular weight), and dispersions displayed Tg's intermediate between those of the pure amorphous components, implying that curcumin acts as an effective antiplasticizer for PADAS. Dielectric spectroscopy was employed to assess the relaxation dynamics of the binary dispersion with 30 wt% curcumin, as well as that of each (amorphous) component separately. The binary dispersion was characterized by a single structural relaxation, a single Johari-Goldstein process, and two local intramolecular processes, one for each component. Interestingly, the latter processes scaled with the Tg of the sample, indicating that they are viscosity-sensitive. In addition, both the pristine polymer and the dispersion exhibited an interfacial Maxwell-Wagner relaxation, likely due to spatial heterogeneities associated with phase disproportionation in this polymer. The release of curcumin from the dispersion in a simulated body fluid followed a Fickian diffusion profile, and 51% of the initial curcumin content was released in 48 h.

Thermal investigation on hydrated co-amorphous systems of nicotinamide and prilocaine

It is generally recognized that water, acting as a plasticizer, increases molecular mobility, leading to a decrease of the glass transition temperature (T_g) in amorphous systems. However, an anti-plasticizing effect of water was recently observed on prilocaine (PRL). This effect might be used in co-amorphous systems to moderate the plasticizing effect of water. Nicotinamide (NIC) can form co-amorphous systems with PRL. In order to investigate the effect of water on these co-amorphous systems, the T_gs and molecular mobility of hydrated co-amorphous NIC-PRL systems were compared with those of the respective anhydrous systems. Molecular mobility was estimated by considering the enthalpic recovery at the T_g using the Kohlrausch-Williams-Watts (KWW) equation. At molar ratios of NIC above 0.2, a plasticizing effect of water on co-amorphous NIC-PRL systems was observed with increasing the NIC concentration. In contrast, at molar ratios of NIC of 0.2 and below, water had an anti-plasticizing effect on the co-amorphous NIC-PRL systems, with increased T_gs and reduced mobility upon hydration.

Influence of Water on Amorphous Lidocaine

Water is generally regarded as a universal plasticizer of amorphous drugs or amorphous drug-containing systems. A decrease in glass-transition temperature (T_g) is considered the general result of this plasticizing effect. A recent study exhibits that water can increase the T_g of amorphous prilocaine (PRL) and thus shows an anti-plasticizing effect. The structurally similar drug lidocaine (LID) might show similar interactions with water, and thus an anti-plasticizing effect of water is hypothesized to also occur in amorphous LID. However, the influence of water on the T_g of LID cannot be determined directly due to the very low stability of LID in the amorphous form. It is possible to predict the T_g of LID from a co-amorphous system of PRL-LID using the Gordon-Taylor equation. Interactions were observed between PRL and LID based on the deviations between the experimental T_gs and the T_gs calculated by the conventional use of the Gordon-Taylor equation. A modified use of the Gordon-Taylor equation was applied using the optimal co-amorphous system as a separate component and the excess drug as the other component. The predicted T_g of fully hydrated LID could thus be determined and was found to be increased by 0.9 ± 0.7 K compared with the T_g of water-free amorphous LID. It could be shown that water exhibited a small anti-plasticizing effect on LID.

Arriving at the most plausible interpretation of the dielectric spectra of glycerol with help from quasielastic γ-ray scattering time-domain interferometry

Glycerol is one of the glass-forming liquids selected by Robert H. Cole in 1950 to start his study of molecular dynamics by dielectric spectroscopy. Seventy-one years have gone by and remarkably no consensus has been reached on the nature and identity of the relaxation processes observed in the dielectric spectra. The macroscopic dielectric relaxation data allow different interpretations to yield contrasting results, and it is not possible to determine which one is most plausible. Coming to the rescue is the application of the nuclear γ-resonance time-domain interferometry (TDI) to glycerol by Saito et al. [Phys. Rev. E 105, L012605 (2022)10.1103/PhysRevE.105.L012605]. Their microscopic TDI data potentially can decide which interpretation of the dielectric spectra of glycerol is most plausible. The attempt was made by Saito et al., but there is a problem in their analysis of the dielectric data of glycerol and hence their conclusion is untenable. In this paper, we critically compare four major interpretations with the TDI data in an effort to identify the most plausible interpretation of the relaxation processes constituting the dielectric spectra of glycerol.

New State-Diagram of Aqueous Solutions Unveiling Ionic Hydration, Antiplasticization, and Structural Heterogeneities in LiTFSI-H2O

Here, we report a new state-diagram for aqueous solutions based on concentration-dependent glass-transition temperatures of concentrated and ice freeze-concentrated solutions. Different from the equilibrium phase diagram, this new state-diagram can provide comprehensive information about the hydration numbers of solutes, nonequilibrium vitrification/cold-crystallization, and vitrification/devitrification processes of aqueous solutions in three distinct concentration zones separated by two critical water-content points of only functions of the hydration number. Based on this new state-diagram, we observe the comparable hydration ability of LiTFSI to LiCl and an atypical concentration-dependent cold-crystallization behavior of the LiTFSI-H₂O system. These results unveil the negligible hydration ability of TFSI^- in a water-rich solution, characterize the antiplasticizing effect of water induced by the strengthened Li⁺-TFSI^--H₂O interaction when only hydration water and confined water are present, and confirm the increasing fraction of water-rich domains with the decrease in water content when the cation and anion become incompletely hydrated on average. These results highlight the novel water-content-mediated interactions among the anion, cation, and H₂O for LiTFSI-H₂O.

The structural α-relaxation times of prilocaine confined in 1 nm pores of molecular sieves: quantitative explanation by the coupling model

The molecular glass-former and pharmaceutical, prilocaine, distinguishes itself by exhibiting seven general and fundamental dynamic and thermodynamic properties [Z. Wojnarowska, et al., J. Phys. Chem. B, 2015, 39, 12699.], all of which have been explained using the coupling model. What has not been studied before are the changes in properties of the structural α-relaxation of prilocaine when subjected to extreme nano-confinement in spaces with a size of about 1 nm. Recently, Ruis et al. [G. N. Ruiz, et al., Phys. Chem. Chem. Phys., 2019, 21, 15576.] measured the α-relaxation times, τ_α,conf(T), of prilocaine confined in 1 nm pores of molecular sieves. They found that τ_α,conf(T) are significantly reduced from those of bulk prilocaine, τ_α,bulk(T), and assume a weaker temperature dependence. The data in toto pose a challenge for any theory of glass transition to explain quantitatively. The coupling model (CM) was applied to this problem to predict the α-relaxation times of prilocaine when cooperativity is removed, which is expected because only a few prilocaine molecules can fit into the 1 nm pores. The results from the CM are in quantitative agreement with the experimental values of τ_α,conf(T) and the temperature dependence. The success is nontrivial because no other extant theory can do the same to the best of our knowledge.