The spatial distribution of the ligand on targeted microbubbles influences the binding efficacy

Abstract

An important trait of targeted microbubbles is their binding to biomarkers for ultrasound molecular imaging and drug delivery. Using organic solvents in phospholipid-coated microbubble production results in a homogeneous ligand distribution, while dispersion of lipids directly into aqueous medium results in a heterogeneous ligand distribution. In this study, we compared the binding efficacy of ανβ3-targeted microbubbles with a homogeneous or heterogeneous ligand distribution in vitro and in vivo using confocal microscopy. For in vitro studies, human umbilical vein endothelial cells grown statically and under physiological flow were used. For in vivo studies, chicken embryos (day 5) were used. Microbubbles having a homogeneous ligand distribution bound 1.55× more than microbubbles having a heterogeneous ligand distribution in vitro statically, while in vitro under flow this was 1.49× more at 1.25 dyn/cm2 and 1.56× more at 2.22 dyn/cm2; in vivo this was 1.25× more. The dissociation rate in vitro was lower for bound microbubbles with a homogeneous than heterogeneous ligand distribution at low shear stresses (1–5 dyn/cm2). In conclusion, when producing phospholipid-coated targeted microbubbles using organic solvents is preferable over directly dispersing phospholipids in aqueous medium for optimal binding. [Funding by the Phospholipid Research Center (No. KKO-2017-057/1-1) and NWO (VIDI Project No.17543) is gratefully acknowledged.]