Physicochemical properties and membrane interactions of anti-apoptotic derivatives 2-(4-fluorophenyl)-3-(pyridin-4-yl)imidazo[1,2-a]pyridine depending on the hydroxyalkylamino side chain length and conformation: an NMR and ESR study.

Three imidazo[1,2-a]pyridine derivatives 3a-c have been synthesized from p38 kinase inhibitor structures and evaluated as anti-apoptosis agents. These drugs were designed to interact with nucleic acids and membrane interactions by varying the chain length in position 6, from hydroxyethylamino (3a), to hydroxybutylamino (3b) and hydroxyhexylamino (3c). First experiments showed that 3a and 3b were insoluble in water while 3c could be solubilized in water despite its partition coefficient (logP=3.2). This latter feature was explained by the formation of a fifth intramolecular cycle thus allowing supramolecular structure formation (NMR and MD calculations). The interactions with membranes have been studied using (1)H, (2)H, (31)P Nuclear Magnetic Resonance (NMR), Electron Spin Resonance (ESR) and High Resolution-Magic Angle Spinning (HR-MAS). Despite the insolubility of 3a and 3b in water, these derivatives could be partially solubilized by synthetic phospholipidic model membranes (small unilamellar vesicles, SUV). (1)H NMR paramagnetic broadening experiments performed on the same models showed that 3a was located in the external layer, probably close to the surface while 3b only formed external superficial adducts. Supplementary (31)P, (2)H NMR and ESR experiments on phospholipid dispersions confirmed the location of 3a close to the polar headgroup of the external layer of the membrane, this resulting in a 2K lowering of the transition temperature. Moreover, no significant interaction was detected on the deep part of the layer ((2)H NMR and 16NS ESR experiments). This binding was also found in the presence of cell cultures, as revealed by HR-MAS NMR experiments. Conversely, no significant interaction with membranes was found with 3b or 3c. From both the unexpected solubility of 3c and 3a interactions with membranes, further chemical modifications were finally proposed.