Transient disruption of vascular barriers using focused ultrasound and microbubbles for targeted drug delivery in the brain
The physiology of the vasculature in the central nervous system (CNS) which includes the blood-brain-barrier (BBB) and other factors, prevents the transport of most anticancer agents to the brain and restricts delivery to infiltrating brain tumors. The heterogeneous vascular permeability in tumor vessels (blood-tumor barrier; BTB), along with several other factors, creates additional hurdles for drug treatment of brain tumors. Different methods have been used to bypass the BBB/BTB, but they have their own limitations such as being invasive, non-targeted or requiring the formulation of new drugs. Magnetic Resonance Imaging guided Focused Ultrasound (MRIgFUS), when combined with circulating microbubbles, is an emerging noninvasive method to temporarily permeabilize the BBB and BTB. The purpose of this thesis was to use this alternative approach to deliver chemotherapeutic agents through the BBB/BTB for brain tumor treatment in a rodent model to overcome the hinderances encountered in prior approaches tested for drug delivery in the CNS. The results presented in thesis demonstrate that MRIgFUS can be used to achieve consistent and reproducible BBB/BTB disruption in rats. It enabled us to achieve clinically-relevant concentrations of doxorubicin (~ 4.8±0.5 µg/g) delivered to the brain with the sonication parameters (0.69 MHz; 0.55 MPa; 10 ms bursts; 1 Hz PRF; 60 s duration), microbubble concentration (Definity, 10 µl/kg), and liposomoal doxorubicin (Lipo-DOX) dose (5.67 mg/kg) used. The resulting doxorubicin concentration was reduced by 32% when the agent was injected 10 minute after the last sonication. Three weekly sessions of FUS and Lipo-DOX appeared to be safe in the rat brain, despite some minor tissue damage. Importantly, the severe neurotoxicity seen in earlier works using other approaches does not appear to occur with delivery via FUS-BBB disruption. The resuls from three weekly treatments of FUS and Lipo-DOX in a rat glioma model are highly promising since they demonstrated that the method significantly inhibits tumor growth and improves survival. Animals that received three weekly sessions of FUS + Lipo-DOX (N = 8) had a median survival time that was increased significantly (P<0.001) compared to animals who received Lipo-DOX only (N = 6), FUS only (N = 8), or no treatment (N = 7). Median survival for animals that received FUS + Lipo-DOX was increased by 100% relative to untreated controls, whereas animals who received Lipo-DOX alone had only a 16% improvement. Animals who received only FUS showed no improvement. No tumor cells were found in histology in 4/8 animals in the FUS + Lipo-DOX group, and only a few tumor cells were detected in two animals. Tumor doxorubicin concentrations increased monotonically (823±600, 1817±732 and 2432±448 ng/g) in the control tumors at 9, 14 and 17 days respectively after administration of Lipo-DOX. With FUS-induced BTB disruption, the doxorubicin concentrations were enhanced significantly (P<0.05, P<0.01, and P<0.0001 at days 9, 14, and 17, respectively) and were greater than the control tumors by a factor of two or more (2222±784, 3687±796 and 5658±821 ng/g) regardless of the stage of tumor growth. The transfer coefficient Ktrans was significantly (p<0.05) enhanced compared to control tumors only at day 9 but not at day 14 or 17. These results suggest that FUS-induced enhancements in tumor drug delivery for Lipo-DOX are relatively consistent over time, at least in this tumor model. These results are encouraging for the use of large drug carriers, as they suggest that even large/late-stage tumors can benefit from FUS-induced drug enhancement. Corresponding enhancements in Ktrans were found variable in large/late-stage tumors and not significantly different than controls, perhaps reflecting the size mismatch between the liposomal drug (~100 nm) and Gd-DTPA (molecular weight: 938 Da). Overall, this thesis research provides pre-clinical data toward the development of MRIgFUS as a noninvasive method for the delivery of agents such as Lipo-DOX across the BBB/BTB to treat patients with diseases of the central nervous system.