Two-phase partition profiling of drugs and ionisable compounds

Abstract

Lipophilicity of compounds with therapeutic interest represents one of the most important physicochemical parameters in predicting and interpreting transport processes across biological barriers. This property is commonly measured as a drug | membrane partitioning by its distribution in a biphasic system, such as a water | oil system. The lipophilicity is a key parameter used in a variety of fields that may predict absorption and other transport properties of drugs in the human body. For a long time, the understanding of the lipophilicity of ionisable and ionic drugs was limited by the lack of reliable experimental methods to estimate the partition coefficients. Recently the introduction of cyclic voltammetry has filled this gap, opening new perspectives for its use in drug design transfer through a mimic of a biological membrane. The aim of this thesis was to develop new systems for the study of pH-lipophilicity profiles of neutral and ionisable compounds and to illustrate the various contributions of those in order to extend the applications for drug studies. The existing electrochemical methodology has revealed a number of limitations, which calls for further development of existing procedures. In particular, it has proved difficulty to investigate the partitioning of highly hydrophobic and hydrophilic compounds. Consequently, new systems with various phase ratios have been developed in order to extend the electrochemical measurements to medicinal chemistry. In the present work, two systems named using a small aqueous phase and membrane using a small organic phase have been developed to study charge transfer reactions. The drop system is based on the simple deposition of an aqueous drop containing a redox couple onto the surface of a platinum electrode and immersed into an organic electrolyte solution. The electrode | water | oil interfaces are in series. The voltammetric response depends on the coupling of the redox reactions on the solid electrode with the charge transfer reactions at the interface between two immiscible electrolyte solutions (ITIES), being either ion transfer (IT) or electron transfer (ET). This new drop system ensures a rapid equilibrium between the phases and has been used to study the lipophilic molecules. The liquid membrane system consists in a small organic phase either supported by a hydrophobic membrane or hold between two cellulose membranes placed between two aqueous phases, leading also to two interfaces in series. The flowing of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid | liquid interfaces in series. This new methodology is described as a useful tool to study the transfer of highly hydrophilic ions, such as amino acids. Furthermore, a fundamental work on the miniaturization of liquid | liquid partition chromatography in a microchannel has been completed in order to offer a better understanding of the drug transport mechanisms. In the present work, a stationary liquid phase was immobilized in a microchannel using a porous polyvinylidene difluoride (PVDF) membrane. The final goal is to develop a high-throughput method for logP measurements in microchips based on partition chromatography. A computer simulation was performed using a finite element method. The model is validated by comparison of the simulated data with both analytical and experimental data. Finally, transfer reactions across an adsorbed phospholipid monolayer (constituent of biological membrane) at the water | DCE interface were studied by quasi-elastic light scattering (QELS). This technique consists in measuring the interfacial tension at different potential step. The transfer of hydrophilic cations, like amino acids was found to be facilitated by the formation of a complex between the cation or proton with the phospholipid, followed by desorption of the phospholipid monolayer from the interface. The method allows to understand the drug interaction with biological membranes within the transport process across them and to access to the different association constants in the organic phase. The work presented in this thesis demonstrates how the analytical electrochemistry at liquid | liquid interfaces can be extended to evaluate the fundamental thermodynamic parameters of various ionic compounds. The systems used herein are examples of new ways to study the lipophilicity of drugs with applications in drug screening pharmaceutical research and development.