Surface Ligand Valency and Immunoliposome Binding: when More Is Not Always Better

Nano-drug delivery systems are designed to contain surface ligands including antibodies for "active targeting". The number of ligands on each nanoparticle, known as the valency, is considered a critical determinant of the "targeting" property. We sought to understand the correlation between valency and binding properties using antibody conjugated liposomes, i.e. immunoliposomes (ILs), as the model.

Anti-CD3 Fab containing a terminal cysteine residue were conjugated to DSPE-PEG-maleimide and incubated with preformed liposomes at 60°C. The un-incorporated antibodies were removed and the obtained ILs were characterized to contain in average 2-22 copies of anti-CD3 Fabs per liposome. The Biolayer Interferometry (BLI) probe surface was coated with various densities of CD3 epsilon&delta heterodimer (CD3D/E) to imitate different CD3 expression levels on target cells. The inference wavelength shifts upon anti-CD3 liposome binding were monitored and analyzed.

The data indicated ILs may bind either monovalently or multivalently, determined mainly by the surface ligand density rather than the ILs antibody valency. The ILs valency indeed correlated with the dissociation rate constant (K_off), but not with the association rate constant (K_on). Their binding capabilities also did not necessarily increase with the surface anti-CD3 valency.

We proposed a model for understanding the binding properties of ILs with different ligand valencies. The binding mode may change when the targeted surfaces had different antigen densities. The model should be important for the designing and optimization of active targeting drug delivery systems to fit different applications.