Tailor-made solvents for pharmaceutical use? Experimental and computational approach for determining solubility in deep eutectic solvents (DES)

Henrik Palmelund,Martin P Andersson,Camilla J Asgreen,Ben J Boyd,Jukka Rantanen,Korbinian Löbmann

doi:10.1016/j.ijpx.2019.100034

Abstract

A deep eutectic solvent (DES) is a mixture of two or more chemicals that interact via hydrogen bonding and has a melting point far below that of the individual components. DESs have been proposed as alternative solvents for poorly soluble active pharmaceutical ingredients (API). In this study, the solvation capacities of six deep eutectic solvents were compared to water and three conventional pharmaceutical solvents (PEG 300, ethanol and glycerol) for 11 APIs. The experimentally determined solubilities were compared to computational solubilities predicted by the Conductor-like Screening Model for Real Solvents (COSMO-RS). While the conventional pharmaceutical solvents PEG 300 and ethanol were the best solvents for the majority of the studied APIs, API-DES combinations were identified, which exceeded the API solubility found in the conventional pharmaceutical solvents. Furthermore, it was also possible to obtain high solubilities in the DESs relative to water, suggesting DESs to be potential solvents for poorly water soluble APIs. In addition, the relative increase in solubility found in the experimental data could be well predicted ab initio using COSMO-RS. Hence, COSMO-RS may in the future be used to reduce the experimental screening of potential DESs for a given API.

Highlights

In 2003, a new type of solvent called deep eutectic solvent (DES) with good solvation properties was described by (Abbott et al, 2003)
UV-spectroscopy A known amount of DES-active pharmaceutical ingredients (API) solution was diluted in a sufficient volume of ethanol and water (1:1 v/v).The concentration of the API dissolved in the DESs was quantified by UV-spectroscopy (UV-1800, Shimadzu, Japan)
For four out of 11 APIs, the solubility was higher in a DES than any of the conventional pharmaceutical solvents

Summary

Introduction

In 2003, a new type of solvent called deep eutectic solvent (DES) with good solvation properties was described by (Abbott et al, 2003). DESs are formed by mixing a hydrogen bond acceptor (HBA), typically a quaternary ammonium salt, and a hydrogen bond donor (HBD) (Smith et al, 2014). The first reported DES was a 1:2 molar mixture of choline chloride and urea. Despite melting points of 133 °C and 302 °C for urea and choline chloride, respectively, the mixture was liquid at room temperature with a freezing point of 12 °C (Abbott et al, 2003). After the discovery of the first DES, many combinations of HBA and HBDs such as sugars, polyols, carboxylic acids, and amino acids have been reported to form DESs (Dai et al, 2013; Smith et al, 2014)

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