study, when the application of UTMD combined with PEI, the transfection efficiency for both plasmids in the tumor xenografts could be significantly improved, providing a new strategy for cancer gene therapy. UTMD could facilitate targeted gene therapy, thus significantly enhance gene transfection in vivo. The results of our study showed that, after intravenous injection of plasmids DNA, there was obvious gene expression in the irradiated tumors. And the difference had statistical significance when compared with that of non-irradiated tumors. Similar to our study, Haag et al.  KU-60019 established two tumors BAY 63-2521 mouse in each animal, injected the ODN-loaded microbubbles intravenously, and then exposed only one of the tumors to ultrasound. Their results showed that, digoxigenin staining intensity was significantly stronger in treated tumors (16-49%) that were exposed to ultrasound as compared with the untreated collateral control tumors
(2-18%). Dittmar et al.  applied pulsed high intensity focused ultrasound to expose one tumor while the other tumor served as a control and found that local exposure in tumors could enhance expression of green fluorescent protein (GFP). Moreover, UTMD could transduce plasmids into target tissue when systemic administration rather than direct target organ delivery by catheter-based approaches or operative injection. And this was particularly important in cardiovascular as well as gene therapy of inaccessible tumors. Howard et al.  reported that, systemic delivery of Ad-GFP microbubbles R406 pretreated with complement and injected in the tail vein of nude mice resulted
in high level of transgene within the tumor alone. Both fluorescence microscopy and GFP immunohistochemistry demonstrated UTMD induced specific transduction in the targeted cells only, with no uptake in hearts, lungs or liver. Chen et al.  incorporated plasmids into the phospholipid shell of gas-filled microbubbles, Forskolin which were then infused into rats and destroyed within the pancreatic microcirculation with UTMD technology. They found that UTMD allowed relatively noninvasive delivery of genes to pancreatic islets with efficiency sufficient to modulate the function of β-cell, and a low level of luciferase activity was detected in all organs within the ultrasound beam. Activity of skeletal muscle or right kidney which lie outside the ultrasound beam was not detected in their study. This data illustrated that this technique largely could prevent the problem of hepatic uptake seen with viral vectors. Moreover, study indicated  that the reticuloendothelial system was not a limiting factor for the ultrasound-based gene delivery with these experimental conditions. While Huber et al.  found that, after intratumoral DNA injection, ultrasound induced a 10-fold increase of β-galactosidase positive cells.