Targeted drug delivery: Effects of grafted polyethylene glycol on ligand-receptor binding under flow

The major challenge in developing clinically useful drug carriers lies in achieving specific binding with intended targets while at the same time avoiding recognition by the immune system. Targeted delivery would greatly impact the pharmaceutical industry by allowing the use of drugs currently shelved because of detrimental side effects. Although a polyethylene glycol (PEG) coating prolongs carrier circulation time, steric hindrance from PEG also interferes with targeted binding. Traditionally drug carriers have been tested in either animal models or cell culture static binding studies. We developed a model in vitro system to investigate PEGylation conditions that improve targeted binding of drug delivery carriers under shear flow while using PEG concentrations (5 mol% total) that confer reasonable circulation times.; Two key and novel aspects of our flow system to compare binding properties for various carrier surface compositions are high-throughput screening and quantitative trajectory analysis. To mimic the surface of liposomal delivery vehicles, parallel lanes of supported lipid bilayers are formed using vesicle fusion in microfluidic channels. Micron-scale patterns enable direct comparison of receptor-coated bead binding to desired surface compositions under identical flow conditions. Image analysis software automatically tracks beads, detects arrests, and calculates velocity before arrest, binding frequency, and arrest duration.; Our results demonstrate that the two most important surface composition parameters conferring stable attachment under flow are ligand accessibility and ligand concentration. Ligand accessibility depends on the relative molecular weights (MW) of liganded and unliganded PEGs. Unliganded PEG of MW 5000 (PEG5000) greatly reduces the accessibility of liganded PEG2000. Increasing the tether length from PEG2000 to PEG3350 has no significant effect on velocity before arrest, binding frequency, or arrest duration when used with unliganded PEG2000. A 5-fold drop in ligand concentration results in a 3-fold drop in binding frequency and a 30% drop in the number of terminal arrests. Brownian motion of beads increases the observed binding distance range beyond the maximum extended tether lengths (>85% of binding occurs within 100 nm of surface, >95% within 200 nm). In summary, we find that while ligand accessibility and concentration promote targeted binding under flow, particle dynamics set the binding distance range.