DSPC or DPPC as main shell component influences ligand distribution and binding area of lipid‐coated targeted microbubbles

Abstract

Ultrasound contrast agents (UCA) consist of gas‐filled coated microbubbles with diameters of 1–10 µm. Targeted UCA can bind to biomarkers associated with disease through coating‐incorporated ligands, making ultrasound molecular imaging possible. The aim of our research was to compare the ligand distribution, binding area, and bound microbubble shape of 1,2‐distearoyl‐ sn ‐glycero‐3‐phosphocholine (DSPC) based and 1,2‐dipalmitoyl‐ sn ‐glycero‐3‐phosphocholine (DPPC) based lipid‐coated microbubbles using super‐resolution microscopy. Ligand distribution was studied by conjugating the fluorescent streptavidin Oregon Green 488 to the biotinylated microbubbles. An inhomogeneous streptavidin distribution was found when DSPC was the main coating lipid. When DSPC was replaced by DPPC, a more homogeneous streptavidin distribution was observed. Binding area of targeted microbubbles was studied using biotinylated microbubbles bound to a streptavidin‐coated surface. DSPC microbubbles had a significantly smaller binding area than DPPC microbubbles. Whereas the bound DSPC microbubbles remained spherical, the DPPC microbubbles were dome‐shaped. This study reveals that lipid‐coated microbubbles differ in ligand distribution, binding area, and bound microbubble shape solely on the basis of their main lipid component. Practical applications: A homogeneous ligand distribution, larger binding area and domed shape upon binding could be advantageous for binding of targeted microbubbles, thereby favoring DPPC over DSPC as main lipid for UCA for ultrasound molecular imaging. The findings of the present study can be used for the design of targeted microbubbles with improved binding capabilities and for the ongoing research to acoustically distinguish bound from unbound microbubbles. Targeted biotinylated DSPC and DPPC‐based microbubbles bound to streptavidin‐coated surface. Left graph: binding area; right panels: microbubbles (red fluorescent) bound to streptavidin‐coated surface (green fluorescent).