Ectoenzymes as promising cell identification structures for the high avidity targeting of polymeric nanoparticles

Pharmacotherapy is often limited by undesired side effects while insufficient drug reaches the site of action. Active-targeted nanotherapy should provide a solution for this problem, by using ligands in the nanoparticle corona for the identification of receptors on the target-cell surface. However, since receptor binding is directly associated with pharmacological responses, today's targeting concepts must be critically evaluated. We hypothesized that addressing ectoenzymes would help to overcome this problem, but it was not clear if particles would show sufficiently high avidity to provide us with a viable alternative to classical ligand-receptor concepts. We scrutinized this aspect by immobilizing the highly selective angiotensin-converting enzyme 2 (ACE2) inhibitor MLN-4760 in the corona of block-copolymer nanoparticles and investigated enzyme binding via microscale thermophoresis and flow cytometry. Excellent avidities with K_d values as low as 243 pM for soluble ACE2 and 306 pM for ACE2-positive cells were obtained. In addition, the inhibitory activity had an IC₅₀ value of 2.88 nM. Reliable target cell identification could be proven in coculture experiments. High avidity is the basis for minimizing material loss to off-target sites and paves the way for a paradigm shift in nanoparticle targeting which does not trigger unintended side effects following target cell identification.