X-ray Attenuating Vesicles with Neutrophil Extracellular Trap (NET) Specificity: Synthesis and Testing in a Model System.

X-ray attenuating contrast agents for imaging thrombi directly during endovascular thrombectomy (EVT) are urgently needed for shortening the wait time for treatment and for reducing the chances of blood clot fragmentation. Neutrophil extracellular traps (NETs) are a product of an innate immune system response by which neutrophils release decondensed chromatin strands decorated with granule and cytosolic proteins, including neutrophil elastase and citrullinated histone H3 (CitH3). NETs are frequently found within fibrous thrombi in pathology and represent a promising target for thrombi-specific imaging agents due to their common occurrence in human cerebrovascular thrombi. We designed and tested 200 nm lipid vesicles (LV) formulated in the presence of a combination of hydrophilic and hydrophobic computed tomography (CT) contrast agents with resultant efficacy of X-ray attenuation corresponding to 312 ± 54 mg/mL iodine. The LV incorporated -cyclooctene-terminated pegylated distearoylphosphatidylethanolamine (TCO-PEG-DSPE) for rapid conjugation of methyltetrazine(mTz)-modified monoclonal immunoglobulin G (IgG) with anti-citH3 binding specificity. By using differential fluorescent labeling of the antibody and lipid components, we determined that 80 ± 3% of mTz-linked IgG coprecipitated with the LV after conjugation in contrast to 0.1-0.2% of control IgG. The engineered NET-specific LV were tested in vitro using differentiated human HL60 promyeloblasts (dHL60) as a standard model of NETing neutrophils. Using fibrin meshwork-incorporated dHL60 as well as monolayer cell cultures, we determined that anti-citH3 LV showed specific and high-affinity binding to dHL60 cells, which were stimulated to undergo NETosis. This work suggests the high promise of NET-specific agents in providing thrombus-specific imaging contrast during EVT.