A microfluidic vesicle screening platform: monitoring the lipid membrane permeability of tetracyclines

For many drugs including antibiotics such as tetracyclines it is crucial that the molecule has the ability to quickly and passively permeate lipid membranes. Hence, the understanding of the permeability in relation to the molecular structure is an important aspect to rationally design novel pharmaceutically active compounds with high bioavailability. Here, we present a versatile method to study the kinetics of tetracycline permeation across liposome membranes on a microchip. Liposomes are immobilized onto the glass surface in a stripe pattern via an avidin-biotin bond and covered by microchannels to allow continuous delivery of tetracycline and buffer. The fluid flow provides a constant concentration profile and thereby resembles the drug transport via blood in the human body. Total internal reflection fluorescence (TIRF) microscopy was used to image the formation of a fluorescent drug-europium complex inside the liposomes. The permeation rates of various tetracyclines were investigated and the results compared to a conventional method (water-octanol partitioning). The findings largely confirm the correlation between membrane permeability and lipophilicity of the permeating molecules (Overton's rule). However, slight deviations reveal that lipophilicity is an important but not the exclusive parameter for the prediction of permeation. The method is fast enough to study the permeation of unstable tetracyclines such as rolitetracycline. Additionally, with the use of different cholesterol concentrations, the influence of membrane composition on the permeation rate can be investigated conveniently. The microfluidic approach can be easily applied to investigate the kinetics of other processes such as ligand-membrane receptor association and dissociation, provided that the process can be visualized by means of fluorescence spectroscopy.