Receptor-Targeted Liposomal Delivery of Boron-Containing Cholesterol Mimics for Boron Neutron Capture Therapy (BNCT)

Abstract

Liposomes have been a main focus of tumor-selective boron delivery strategies in boron neutron capture therapy (BNCT), a binary method for the treatment of cancer that is based on the nuclear reaction between boron atoms and low-energy thermal neutrons. Three novel carboranyl cholesterol derivatives were prepared as lipid bilayer components for the construction of nontargeted and receptor-targeted boronated liposomes for BNCT. A major structural feature of these novel boronated cholesterol mimics is the replacement of the B and the C ring of cholesterol with a carborane cluster. Computational analyses indicated that all three boronated compounds have structural features and physicochemical properties that are very similar to those of cholesterol. One of the synthesized boronated cholesterol mimics was stably incorporated into non-, folate receptor (FR)-, and vascular endothelial growth factor receptor-2 (VEGFR-2)-targeted liposomes. No major differences were found in appearance, size distribution, and lamellarity between conventional dipalmitoylphosphatidylcholine (DPPC)/cholesterol liposomes, nontargeted, and FR-targeted liposomal formulations of this carboranyl cholesterol derivative. FR-targeted boronated liposomes were taken up extensively in FR overexpressing KB cells in vitro, and the uptake was effectively blocked in the presence of free folate. In contrast, a boronated cholesterol mimic incorporated into nontargeted liposomes showed significantly lower cellular uptake. There was no apparent in vitro cytotoxicity in FR overexpressing KB cells and VEGFR-2 overexpressing 293/KDR cells when these were incubated with boronated FR- and (VEGFR-2)-targeted liposomes, respectively, although the former accumulated extensively in KB cells and the latter effectively interacted with VEGFR-2 by causing autophosphorylation and protecting 293/KDR cells from SLT (Shiga-like toxin)-VEGF cytotoxicity.

scVEGF Microbubble Ultrasound Contrast Agents

“…The Cys-tagged scVEGF has been conjugated to a variety of payloads,13,26,27 although it has not been used for ultrasound imaging. scVEGF has been incorporated into NIRF, SPECT, and PET probes for molecular imaging of VEGF receptors during tumor angiogenesis 28.…”

Enhanced fluorescence diffuse optical tomography with indocyanine green-encapsulating liposomes targeted to receptors for vascular endothelial growth factor in tumor vasculature

“…Although our experiments did not distinguish between VEGFR-2 bound and internalized scVEGF-Lip/ICG, VEGFR-2mediated internalization was reported previously for other scVEGF-driven liposomes and dendrimers. 23,31 Taken together, three in vitro assays indicate that scVEGF-Lip/ICG displays a significant receptor-binding ability of scVEGF and are capable of accumulating in endothelial cells expressing high levels of VEGFRs.…”

“…For targeting, Lip/ICG were decorated with previously described scVEGF-PEG-DSPE, an engineered single-chain version of vascular endothelial growth factor (scVEGF), site-specifically derivatized with PEGylated lipid for coupling to liposomes. 22,23 We have previously described scVEGF-based nuclear and fluorescent tracers, as well as lipid microbubbles-based ultrasound (US) tracers, for VEGFR imaging in angiogenic vasculature in various pathologies. [22][23][24][25][26][27] High-affinity binding of these tracers to VEGFRs has been validated by positron emission tomography, single-photon emission computed tomography, US, and near-infrared fluorescence imaging of VEGFR in various cancer, cardiovascular, and inflammation models.…”

“…22,23 We have previously described scVEGF-based nuclear and fluorescent tracers, as well as lipid microbubbles-based ultrasound (US) tracers, for VEGFR imaging in angiogenic vasculature in various pathologies. [22][23][24][25][26][27] High-affinity binding of these tracers to VEGFRs has been validated by positron emission tomography, single-photon emission computed tomography, US, and near-infrared fluorescence imaging of VEGFR in various cancer, cardiovascular, and inflammation models. 25,[27][28][29][30] Here, we used scVEGF-Lip/ICG for FDOT imaging of VEGFRs in tumor-bearing mice with orthotopic mammary carcinoma.…”

Noninvasive Imaging of Liposomal Delivery of Superparamagnetic Iron Oxide Nanoparticles to Orthotopic Human Breast Tumor in Mice

“…Decoration of Lip(Gd/Fe), Lip(ICG), and Doxil ® (clinically used doxorubicin-containing STEALTH liposomes) with scVEGF was achieved by post-insertion of scVEGF-PEG 3400 -DSPE conjugate into a liposomal membrane, as previously described for different liposomal formulations [13, 14]. Briefly, the entire reaction mixture of [C4]-monothiol-scVEGF with DSPE-PEG 3400 -maleimide was added to purified liposomes at an average scVEGF-to-phospholipid molar ratio of 1:150, and incubated for 12-14 hours at 37°C.…”