Two-phase partition profiling of drugs and ionisable compounds

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 "drop system" using a small aqueous phase and "liquid membrane system" 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 t