Effect of Cations and Anions on Glass Transition Temperatures in Excipient Solutions

Abstract

The purpose of this study was to investigate the effects of cations and anions of various electrolytes on the glass transition temperature (Tg') of frozen solutions of excipients commonly used in freeze-drying. The effect of electrolyte concentration on freezable water content was also investigated by measuring the enthalpy of melting (DeltaH) using Differential Scanning Calorimetry (DSC). Cations and anions induce changes in Tg' of frozen solutions of commonly used parenteral excipients. These changes are dependent on the properties of the excipients used. Tg' values of 5% w/v solutions of maltose, trehalose, sucrose, dextran 40, and polyvinylpyrrolidone (PVP, 17K) were determined as a function of sodium chloride (NaCl) or potassium chloride (KCl) concentrations. In general, a significant decrease in Tg' was observed as a function of increasing the electrolyte concentration. For the disaccharide solutions, the decrease in Tg' due to the addition of NaCl or KCl was similar in magnitude, indicating that changing the cation from K+ to Na+ had no effect on Tg'. However, the decrease in Tg' for the PVP solution due to the addition of KCl was greater than that observed by the addition of NaCl . The differences in the electrolyte-induced changes on Tg' between the disaccharides and PVP may be potentially attributed to the formation of complexes between the cations and the properly oriented hydroxyl groups in the sugars leaving the anions (Cl- ions) to exert their effect on Tg'. While zero cation effect would be consistent with these results for the disaccharides, these results do not mean that the cation effects are zero; they only mean that the cation effects are the same. For the PVP solution, K+ and Na+ ions are not engaged in complex formation with PVP due to the lack of hydroxyl groups. We hypothesize that the structure-breaking K+ ions increase the fluidity of water and exert a greater plasticizing effect on Tg', leading to a more significant decrease in Tg' than the structure-making Na+ ions, which increase the viscosity of water. The decrease in Tg' of frozen solutions of pharmaceutical excipients caused by the addition of electrolytes may be primarily attributed to an increase in the unfrozen plasticizing water surrounding the excipient molecules. Formulation scientists should evaluate the use of electrolytes in the formulation development of lyophilized products containing commonly used excipients. Electrolytes are often needed as stabilizers for protein formulations; however, their selection and use should be properly evaluated. Because electrolytes cause a decrease in Tg' as a function of electrolyte concentration, it is recommended that the minimum electrolyte concentration needed to maintain product stability should be used to minimize the effect of the electrolyte on lowering the Tg'.

Impact of matrix properties on the survival of freeze‐dried bacteria

“…The thermal event taking place at − 35 °C, detected for our 10% sucrose solution, may instead be related to the onset of ice dissolution as suggested by Ablett et al 37 A step change could not be observed for the other polymers, which can be explained by the fact that cellulose‐based polymers are strong glass formers, hence having a too small change in heat capacity over the glass transition to be detectable by DSC 39, 40. The addition of the salt was found to decrease the T g ′ of Ficoll by about 9 °C, an effect of that the presence of NaCl increases the amount of water withheld in the freeze concentrate during ice formation 41, 42. In the case of HEC and HPMC an endothermic peak with an onset of − 28 °C and − 27 °C, respectively, appeared when NaCl was included in the formulations, interpreted as the onset of a eutectic melting of NaCl and ice (see Discussion).…”

“…A prerequisite for the eutectic melting behaviour is that the components (NaCl and water) are phase separated in the frozen state. NaCl is known to form complexes with the hydroxyl groups of carbohydrates 41, 53. The NaCl to hydroxyl molar ratio in the dry formulations was 1:11, 1:6 and 1:1 for the Ficoll‐, PVA‐ and HEC‐based formulations, respectively, explaining why no or a smaller fraction of crystallised NaCl was detected in the Ficoll and PVA samples.…”

“…Hence, to accurately compare the survival of cells formulated in the different sucrose and polymer solutions we aimed at keeping the initial osmolality of the different formulations equal by including 150 mmol L −1 of NaCl in all polymer solutions. It is, however, important to appreciate the effect that inclusion of electrolytes such as NaCl have on the T g ′ of the freeze concentrate41, 42 and if needed adjust the freeze‐drying protocol accordingly. Due to high viscosities the amount of polymer in the HEC, HPMC and PVA formulations was lower than in the Ficoll solution.…”

Using Excimeric Fluorescence to Study How the Cooling Rate Determines the Behavior of Naphthalenes in Freeze-Concentrated Solutions: Vitrification and Crystallization

“…90 Additionally, the presence of salts decreases the T g ' even further. 60,112 On the other hand, a special mechanism where the water behaves like an antiplasticizing agent is also known. 113…”

Formulations for Freeze-drying of Bacteria and Their Influence on Cell Survival

“…No crystalline material was detected in the Ficoll based formulation. NaCl can form complexes with the hydroxyl moieties in carbohydrates 9,10 . In Ficoll and HEC-based formulations the NaCl to hydroxyl moiety molar ratio was 1:11 and 1:1, respectively, explaining why no crystallization was detected in the Ficoll-based formulation.…”