A new study by research chemists at the University of Warwick has challenged a century old rule of pharmacology that defined how quickly key chemicals can pass across cell walls. The new observations of the Warwick researchers suggest that the real transport rates could be up to a hundred times slower than predicted by the century old "Overton's Rule". This could have major implications for the development and testing of many future drugs.
Overton's rule says that the easier it is for a chemical to dissolve in a lipid (fat) the easier and faster it will be transported into a cell. The Rule was first outlined in the 1890s by Ernst Overton of the University of Zürich. He declared that substances that dissolve in lipids pass more easily into a cell than those that dissolve in water. He then set forth an equation that predicted how fast that diffusion would happen. One of the key parameters in that equation is K which defines the lipophilicity (oil-liking nature) of the chemical. The higher the value of K, the faster the predicted cell permeation rate. For over a century, medicinal chemists have used this relationship to shape their studies and clinical trials.
A team of electrochemists from the University of Warwick used a combination of a confocal microscope and an ultramicroelectrode to study what really happens when a chemical crosses a cell membrane. Advances in technology enabled them to position an ultramicroelectrode incrediblely close to the membrane boundary (roughly 20 microns away; ca. 1/3rd the thickness of a human hair) where it was used to generate a range of acids that should be able to diffuse relatively easily into a cell. These techniques allowed every step of the diffusion process to be directly examined. Previous studies had not been able to observe every step of the process and often required artificial stirring of the solutions.