Focused Ultrasound Enabled Trans-Blood Brain Barrier Delivery of Radiolabeled Nanoclusters: Feasibility, Effects of Surface Charge, and Size

1150a Objectives: :Nanoparticles have been widely used for brain tumor imaging and therapy mainly through the enhanced permeability and retention effect. Diffuse intrinsic pontine glioma (DIPG), the most common brainstem tumor of childhood and the single greatest cause of brain tumor-related death in children, has remained a clinical challenge for both imaging and therapy due to its relatively intact blood brain barrier (BBB). Recently, there has been a growing interest in using focused ultrasound (FUS) to improve the trans-BBB delivery of nanoparticles via noninvasive and localized BBB disruption. Herein, we first studied the feasibility of FUS enabled trans-BBB delivery of ultrasmall gold nanoclusters incorporated with 64Cu (64Cu-AuNCs) for PET imaging and then assessed the effects of surface charges of 64Cu-AuNCs on the delivery efficiency into brains. Further, we investigated the feasibility of FUS-enabled delivery into pons and studied the size effects on the delivery efficiency. Methods: : 64Cu-AuNCs (hydrodynamic diameter: ~5 nm) withneutral, positive, and negative surface charges were synthesized through PEGylation with thioctic acid (TA) functionalized ligands, TA-PEG750-OCH3, TA-PEG750-NH2, TA-PEG750-COOH, respectively. Biodistribution studies were carried out in wild-type male CD-1 mice at multiple time points post injection. Based on the pharmacokinetic data, PET imaging of neutrally charged 64CuAuNCs-PEG750-OCH3 was performed on mice treated with FUS on the brains. The trans-BBB penetration efficiency and intrabrain diffusion were quantified and further verified by autoradiography and inductively coupled plasma mass spectrometry (ICP-MS). Histopathology of the treated brains was performed to assess the safety of FUS treatment. We also assessed the feasibility of delivering 64Cu-AuNCs to the pons post FUS treatment and studied the size effect of 64Cu-AuNCs with hydrodynamic diameters ranging from 5 nm to 25 nm. Results: Biodistribution showed efficient renal and gastrointestinal tract clearance of all three 64Cu-AuNCs but lowest organ retention for the neutral 64CuAuNCs-PEG750-OCH3. PET signals at the FUS treated sides of mice brains were clearly visualized at 1, 4, and 24 h post injection for all the 64Cu-AuNCs. Quantitative PET analyses also demonstrated the gradual diffusion of PET signals within brain over time. The pons region showed enhanced delivery efficiency of 3.37, 3.03 and 4.76 folds at 1 h, 4 h, and 24 h compared with non-treated mice, respectively. Autoradiography and ICP-MS of ex vivo brain slices at 24 h post injection verified localized delivery of 64Cu-AuNCs to the FUS-targeted pons region. Histological evaluation of brain slices did not show any histological damage caused by the FUS treatment. Compared to the ultrasmall 64Cu-AuNCs (~ 5 nm), the trans-BBB deliveries of particles with larger hydrodynamic sizes were significantly lower, demonstrated by the uptake ratios of PET signals between FUS treated/non-treated regions in same mice for the 64Cu-AuNCs (5 nm: 2.5 ± 0.5, 20 nm: 1.2 ± 0.1, p