Cationic Microbubbles Research Articles

Abstract 19114: Ultrasound-Targeted Delivery of microRNA-126 for Therapeutic Angiogenesis

Introduction: MicroRNA-126 (miR-126) is an endothelial-specific microRNA that regulates angiogenesis by blocking endogenous inhibitors of vascular endothelial growth factor (VEGF), Sprouty-related protein (SPRED1) and phosphoinositol-3 kinase (PI3K). We hypothesized that ultrasound-mediated delivery of miR-126 would result in improved angiogenesis in the setting of chronic ischemia. Methods: Naked mature miR-126 was synthesized for transfection of rat specific miR-126 (22bp). Unilateral hindlimb ischemia was created by left femoral artery ligation in F-344 rats (n=45). At day 14 post-ligation, microvascular blood flow (MBF) in the distal hindlimb muscles were assessed by contrast-enhanced ultrasound (CEU). Ultrasound-mediated gene delivery (UMGD) of miR-126 (1x109 cationic microbubbles + 2 μ g of miR-126) (n=12) or scrambled miR (miR-SCR) (n=12) was performed, with control animals (n=11) receiving no treatment. An additional group (n=10) received UMGD of miR-126 (2 ug) three times every 48hours (day14, day 16 and day18). CEU perfusion was re-assessed at day 28, and transfection was assessed by real time PCR. Results: At day 14, prior to miR-126 delivery, normalized MBF for the ischemic muscle was similarly reduced in all treatment groups (55-60% of normal). At day 28, MBF remained unchanged in control and miR-SCR treated groups (0.61±0.08 vs 0.62±0.09, and 0.59±0.06 vs 0.63±0.07, respectively. In miR-126 treated animals, MBF improved (0.59±0.07 vs 0.78±0.06, p < 0.001 versus control and miR-SCM). After triple delivery of miR-126, further improvement in perfusion was noted (0.58±0.04 vs 0.88±0.03, p < 0.001 versus control and miR-SCM, and p <0.01 versus single miR-126). Real time PCR showed a 14.3±3.4 fold increase over control at 3h post transfection of miR-126 that persisted to day 3 post-UMGD, and normalized by day7 (n=3 per time point). Conclusions: UMGD of miR-126 results in improved perfusion in the setting of chronic hindlimb ischemia. Multi-gene delivery of miR-126 showed an incremental angiogenic response as compared to single gene delivery.

Acoustic Radiation Force for Vascular Cell Therapy: In Vitro Validation

Cell-based therapeutic approaches are attractive for the restoration of the protective endothelial layer in arteries affected by atherosclerosis or following angioplasty and stenting. We have recently demonstrated a novel technique for the delivery of mesenchymal stem cells (MSCs) that are surface-coated with cationic lipid microbubbles (MBs) and displaced by acoustic radiation force (ARF) to a site of arterial injury. The objective of this study was to characterize ultrasound parameters for effective acoustic-based delivery of cell therapy. In vitro experiments were performed in a vascular flow phantom where MB-tagged MSCs were delivered toward the phantom wall using ARF generated with an intravascular ultrasound catheter. The translation motion velocity and adhesion of the MB-cell complexes were analyzed. Experimental data indicated that MSC radial velocity and adhesion to the vessel phantom increased with the time-averaged ultrasound intensity up to 1.65 W/cm2, after which no further significant adhesion was observed. Temperature increase from baseline near the catheter was 5.5 ± 0.8°C with this setting. Using higher time-averaged ultrasound intensities may not significantly benefit the adhesion of MB-cell complexes to the target vessel wall (p = NS), but could cause undesirable biologic effects such as heating to the MB-cell complexes and surrounding tissue. For the highest time-averaged ultrasound intensity of 6.60 W/cm2, the temperature increase was 11.6 ± 1.3°C.

535 Development of a Novel microRNA Delivery Platform Using Targeted Ultrasound and Carrier Microbubbles

BACKGROUND: Efficient and targeted delivery strategies for microRNA (miR) are currently under investigation. Ultrasoundmediated gene delivery (UMGD) is a method for delivery of plasmid DNA and siRNA in numerous pre-clinical models; however, UMGD has not been used with microRNA (miR). We aimed to develop a method of conjugating microRNA to cationic microbubbles, and to test it for the purpose of ultrasound-mediated microRNA delivery in vivo. METHODS: Cationic microbubbles were incubated with increasing concentrations (0, 1, 2.5, 5 ig/ml) of miR-126. After incubation, conjugated microbubbles were assessed for clumping, size and amount of bound microRNA. Time course of circulating miR-126 concentration was then assessed using qRTPCR, without and with microbubble conjugation at various time points after intravenous injection in rats. Ultrasound-mediated gene delivery (UMGD) of miR-126 (1x109 cationic microbubbles 2.5 ig of miR-126) (n 6) or scrambled miR (miR-SCR) (n 4) was performed in a subset of animals. Control animals (n 4) receiving no treatment. RESULTS: Binding of miR-126 to microbubbles (1x109) saturated at approximately 120,000 miR-126 molecules per microbubble with the addition of 2.5ig miR (n 5). Upon visual inspection and size analysis, there was no microbubble clumping before or after miR-126 conjugation. Upon intravenous administration, greater concentrations of miR-126 were found in the plasma after injection of miR-126 conjugated microbubbles as compared to miR-126 alone, microbubbles alone or sham. PCR results for miR-126 levels after UMGD into the distal hindlimb showed an approximate 15-fold increase over control at 3h, with persistent expression up to day3 and normalizing by day7 (n 4 per time point). MiR-126 targets (SPRED1, PIK3R2) were knocked down at 3h (SPRED1; 72 10% and PIK3R2 73 13%, p 0.05 v. Control), day1 (SPRED1; 48 5% and PIK3R2 44 11%, p 0.01 v. Control) and day3 (SPRED1; 69 13% and PIK3R2 80 20%, p 0.05 v. Control) with normalization by day 7. MiR-SCR delivery had no effect on SPRED1 or PIK3R2 levels as compared to controls and there was no detectable transfection seen in remote organs (liver, lung, kidney and spleen). CONCLUSION: MiR-126 had avid binding to cationic microbubbles, and miR-conjugated microbubbles led to prolonged circulation times of miR-126. In vivo transfection data shows robust transfection of miR-126 in hindlimb skeletal muscle after UMGD, with significant downregulation of targets, SPRED1 and PIK3R2. UMGD of miR results in targeted transfection and is a promising in vivo miR delivery technique. Canadian Cardiovascular Society (CCS) CCS246 Oral HERITABLE ARRHYTHMIAS AND SUDDEN DEATH Monday, October 29, 2012

526 Targeted Delivery of MicroRNA-126 Improves Perfusion in Chronic Hindlimb Ischemia

MicroRNA-126 (miR-126) is an endothelial-specific microRNA that regulates angiogenesis, acting as an inhibitor of endogenous inhibitors of vascular endothelial growth factor (VEGF), namely Sprouty-related protein (SPRED1) and phosphoinositol-3 kinase (PI3K). In this study we examined the in vivo angiogenic effect of targeted delivery of miR-126 in chronic hindlimb ischemia in rats. Naked mature miR-126 was synthesized for transfection of rat specific miR-126 (22bp). Unilateral hindlimb ischemia was created by left femoral artery ligation in F-344 rats (n=35). At day 14 post-ligation, microvascular blood flow (MBF) in the distal hindlimb muscles were assessed by contrast-enhanced ultrasound (CEU). Ultrasound-mediated gene delivery (UMGD) of miR-126 (1x109 cationic microbubbles + 2 ìg of miR-126) (n=7) or scrambled miR (miR-SCR) (n=6) was performed, with control animals (n=3) receiving no treatment. An additional group (n=10) received UMGD of miR-126 (2 ug) three times every 48hours (day14, day 16 and day18). CEU perfusion was re-assessed at day 28, and transfection was assessed by real time PCR. At day 14, prior to miR-126 delivery, normalized MBF for the ischemic muscle was similarly reduced in all treatment groups (∼55% of normal). At day 28, MBF remained unchanged in control and miR-SCR treated groups (0.59±0.06 vs 0.56±0.04, and 0.59±0.07 vs 0.58±0.05, respectively. In miR-126 treated animals, MBF improved (0.60±0.08 vs 0.78±0.06, p < 0.01 versus control and miR-SCM). After triple delivery of miR-126, further improvement in perfusion was noted (0.57±0.04 vs 0.88±0.03, p < 0.01 versus control and miR-SCM, and p <0.05 versus single miR-126). Real time PCR showed a 14.3±3.4 fold increase over control at 3h post transfection of miR-126 that persisted to day 3 post-UMGD, and normalized by day7 (n=3 per time point). UMGD of miR-126 results in improved perfusion in the setting of chronic hindlimb ischemia. Multi-gene delivery of miR-126 showed an incremental angiogenic response as compared to single gene delivery.

Cationic versus Neutral Microbubbles for Ultrasound-mediated Gene Delivery in Cancer

To test whether plasmid-binding cationic microbubbles (MBs) enhance ultrasound-mediated gene delivery efficiency relative to control neutral MBs in cell culture and in vivo tumors in mice. Animal studies were approved by the institutional animal care committee. Cationic and neutral MBs were characterized in terms of size, charge, circulation time, and DNA binding. Click beetle luciferase (CBLuc) reporter plasmids were mixed with cationic or neutral MBs. The ability of cationic MBs to protect bound plasmids from nuclease degradation was tested by means of a deoxyribonuclease (DNase) protection assay. Relative efficiencies of ultrasound-mediated transfection (ultrasound parameters: 1 MHz, 1 W/cm(2), 20% duty cycle, 1 minute) of CBLuc to endothelial cells by using cationic, neutral, or no MBs were compared in cell culture. Ultrasound-mediated gene delivery to mouse hind limb tumors was performed in vivo (n = 24) with insonation (1 MHz, 2 W/cm(2), 50% duty cycle, 5 minutes) after intravenous administration of CBLuc with cationic, neutral, or no MBs. Tumor luciferase activity was assessed by means of serial in vivo bioluminescence imaging and ex vivo analysis. Results were compared by using analysis of variance. Cationic MBs (+15.8 mV; DNA binding capacity, 0.03 pg per MB) partially protected bound DNA from DNase degradation. Mean CBLuc expression of treated endothelial cells in culture was 20-fold higher with cationic than with neutral MBs (219.0 relative light units [RLUs]/µg protein ± 92.5 [standard deviation] vs 10.9 RLUs/µg protein ± 2.7, P = .001) and was significantly higher (P < .001) than that in the no MB and no ultrasound control groups. Serial in vivo bioluminescence of mouse tumors was significantly higher with cationic than with neutral MBs ([5.9 ± 2.2] to [9.3 ± 5.2] vs [2.4 ± 0.8] to [2.9 ± 1.1] × 10(4) photons/sec/cm(2)/steradian, P < .0001) and versus no MB and no ultrasound controls (P < .0001). Results of ex vivo analysis confirmed these results (ρ = 0.88, P < .0001). Plasmid-binding cationic MBs enhance ultrasound-mediated gene delivery efficiency relative to neutral MBs in both cell culture and mouse hind limb tumors.

A novel formulation of lipid microbubbles coated with stearic acid modified polyethylenimine to enhance ultrasound-mediated gene delivery in vitro

The aim of this study was to assess the properties of a new designed cationic microbubbles as gene carriers and the relative gene transfection efficacy with ultrasound triggered microbubble destruction. Polyethylenimine as a high efficient gene transfection agent has higher cell toxicity with molecular weight increasing. Stearic acid was used to modify branched polyethylenimine to change its hydrophilic properties so that it can be assembled onto the lipid shell of the microbubbles and simultaneously decrease its cell toxicity. Cationic microbubbles was prepared by encapsulating perfluoropropane into phospholipids and stearic acid modified polyethylenimine hybrid shell using mechanical vibration method. The mean, median size and zeta potential of the microbubbles were measured 1.84±1.62um, 1.60um and 54mv respectively. Hoechst 33258 was used to stain the green fluorescent protein reporter plasmid which was charge-coupled to cationic microbubbles, and microbubbles was observed emitting blue light under a fluorescence microscope. About 4ug plasmid loaded by 108 microbubles that contain 5% mole ratio stearic acid modified polyethylenimine was measured by gel electrophoresis. A 1.25MHz single element transducer was used to mediate the gene transfection to MCF-7 cell by using the cationic microbubbles and enhancement of green fluorescent protein expression was observed.

Sustained Improvement in Perfusion and Flow Reserve After Temporally Separated Delivery of Vascular Endothelial Growth Factor and Angiopoietin-1 Plasmid Deoxyribonucleic Acid

The aim of this study was to compare temporally separated vascular endothelial growth factor (VEGF) and angiopoietin (Ang)-1 delivery with concomitant delivery or single VEGF delivery, for therapeutic angiogenesis in chronic ischemia. Single gene delivery of VEGF results in immature neovessels that ultimately regress. Endogenously, VEGF acts early to initiate angiogenesis, whereas Ang-1 acts later to induce vessel maturation. Timing VEGF and Ang-1 gene delivery to mimic endogenous angiogenesis might be more effective for sustained neovascularization. Unilateral hindlimb ischemia was induced in 170 rats. Ultrasound-mediated gene delivery was performed with cationic microbubbles and plasmid deoxyribonucleic acid. Groups included VEGF at 2 weeks, VEGF/Ang-1 at 2 weeks, VEGF at 2 weeks with Ang-1 at 4 weeks, and untreated control subjects. At 2, 4, and 8 weeks after ligation, blood flow and flow reserve (FR) were assessed by contrast-enhanced ultrasound. Vascular density, organization, and supporting cell coverage were assessed by fluorescent microangiography and immunohistochemistry. In untreated control subjects, blood flow, FR, and vessel density remained reduced. The VEGF delivery improved flow and vessel density at 4 weeks; however, FR remained low, supporting cell coverage was poor, and flow and vessel density regressed by 8 weeks. The VEGF/Ang-1 co-delivery marginally increased flow and vessel density; however, FR and supporting cell coverage improved. After temporally separated VEGF and Ang-1 delivery, blood flow, vessel density, and FR increased and were sustained, with improved pericyte coverage at 8 weeks. In conclusion, temporally separated VEGF and Ang-1 gene therapy results in sustained and functional neovascularization.

Ultrasound-Mediated Gene Delivery with Cationic Versus Neutral Microbubbles: Effect of DNA and Microbubble Dose on In Vivo Transfection Efficiency

Objective: To assess the effect of varying microbubble (MB) and DNA doses on the overall and comparative efficiencies of ultrasound (US)-mediated gene delivery (UMGD) to murine hindlimb skeletal muscle using cationic versus neutral MBs.Materials and Methods: Cationic and control neutral MBs were characterized for size, charge, plasmid DNA binding, and ability to protect DNA against endonuclease degradation. UMGD of a codon optimized firefly luciferase (Fluc) reporter plasmid to endothelial cells (1 MHz, 1 W/cm², 20% duty cycle, 1 min) was performed in cell culture using cationic, neutral, or no MBs. In vivo UMGD to mouse hindlimb muscle was performed by insonation (1 MHz, 2 W/cm², 50% duty cycle, 5 min) after intravenous administration of Fluc combined with cationic, neutral, or no MBs. Gene delivery efficiency was assessed by serial in vivo bioluminescence imaging. Efficiency of in vivo UMGD with cationic versus neutral MBs was systematically evaluated by varying plasmid DNA dose (10, 17.5, 25, 37.5, and 50 µg) while maintaining a constant MB dose of 1x108 MBs and by changing MB dose (1x107, 5x107, 1x108, or 5x108 MBs) while keeping a constant DNA dose of 50 µg.Results: Cationic and size-matched control neutral MBs differed significantly in zeta potential with cationic MBs being able to bind plasmid DNA (binding capacity of 0.03 pg/MB) and partially protect DNA from nuclease degradation while neutral MBs could not. Cationic MBs enhanced UMGD compared to neutral MBs as well as no MB and no US controls both in cell culture (P < 0.001) and in vivo (P < 0.05). Regardless of MB type, in vivo UMGD efficiency increased dose-dependently with DNA dose and showed overall maximum transfection with 50 µg DNA. However, there was an inverse correlation (ρ = -0.90; P = 0.02) between DNA dose and the degree of enhanced UMGD efficiency observed with using cationic MBs instead of neutral MBs. The delivery efficiency advantage associated with cationic MBs was most prominent at the lowest investigated DNA dose (7.5-fold increase with cationic versus neutral MBs at a DNA dose of 10 µg; P = 0.02) compared to only a 1.4-fold increase at a DNA dose of 50 µg (P < 0.01). With increasing MB dose, overall in vivo UMGD efficiency increased dose-dependently with a maximum reached at a dose of 1x108 MBs with no further significant increase with 5x108 MBs (P = 0.97). However, compared to neutral MBs, cationic MBs enhanced UMGD efficiency the most at low MB doses. Relative enhancement of UMGD efficiency using cationic over neutral MBs decreased from a factor of 27 for 1x107 MBs (P = 0.02) to a factor of 1.4 for 1x108 MBs (P < 0.01) and no significant difference for 5x108 MBs.Conclusions: Cationic MBs enhance UMGD to mouse skeletal muscle relative to neutral MBs but this is dependent on MB and DNA dose. The enhancement effect of cationic MBs on UMGD efficiency is more evident when lower doses of MBs or DNA are used, whereas the advantage of cationic MBs over neutral MBs is substantially reduced in the presence of excess MBs or DNA.

PHD2-shRNA interference by ultrasound targeted microbubble destruction facilitates angiogenesis and enhances myocardial function in ischemic/reperfusion injury in rats

ObjectiveHypoxia-inducible factor-1α (HIF-1α), a transcription factor, is naturally degraded by prolyl hydroxylase-2 (PHD2). During ischemic and reperfusion injury, upregulation of HIF-1α activates downstream angiogenic genes. We hypothesise inhibition of HIF-1α...

Studies on neutral, cationic and biotinylated cationic microbubbles in enhancing ultrasound-mediated gene delivery in vitro and in vivo

Ultrasound-mediated gene transfer is emerging as a practical means of facilitating targeted gene expression and is significantly enhanced in the presence of exogenously added microbubbles. This study explores the influence of microbubble surface modifications on their interaction with plasmid DNA and target cells, and the functional consequences of those interactions in terms of ultrasound-mediated gene transfer. Polyethylene glycol-stabilized, lipid-shelled microbubbles with neutral (SDM201), cationic (SDM202) and biotinylated cationic (SDM302) surfaces were compared in terms of their abilities to interact with a luciferase-encoding reporter plasmid DNA and with target cells in vitro. The results demonstrate that the biotinylated cationic microbubble>cationic microbubble>neutral microbubble, in terms of their abilities to interact with target cells and to enhance ultrasound-mediated gene transfer, particularly at low microbubble concentration. The presence of a net positive charge on both cationic microbubbles promoted the formation of microbubble–nucleic acid complexes, although preformation of the complexes prior to addition to target cells inhibited the interaction between the microbubbles and target cells in vitro. The impact of these findings on potential in vitro or ex vivo therapeutic applications of microbubble-enhanced ultrasound-mediated gene transfer is discussed. All three microbubble preparations could be used to facilitate gene transfer in vivo and the potential advantages associated with the use of the cationic microbubbles for targeted gene delivery are discussed.

Abstract No. 19: Plasmid-bearing cationic microbubbles potentiate ultrasound-mediated transvascular gene delivery in cell culture and mouse tumor models

Ultrasound (US)-mediated gene delivery (UMGD) with microbubbles (MBs) is a promising strategy for transvascular gene therapy. Systemically administered cationic MBs (+MBs), engineered to directly bind, transport, and protect anionic DNA, may increase plasmid DNA at the site of sonoporation. To test whether +MBs potentiate UMGD, efficiency of UMGD using +MBs versus control neutral MBs (ntMBs) were compared in cell culture and mouse tumor models.

Vascular Endoluminal Delivery of Mesenchymal Stem Cells Using Acoustic Radiation Force

Restoration of functional endothelium is a requirement for preventing late stent thrombosis. We propose a novel method for targeted delivery of stem cells to a site of arterial injury using ultrasound-generated acoustic radiation force. Mesenchymal stem cells (MSCs) were surface-coated electrostatically with cationic gas-filled lipid microbubbles (mb-MSC). mb-MSC was characterized microscopically and by flow cytometry. The effect of ultrasound (5 MHz) on directing mb-MSC movement toward the vessel wall under physiologic flow conditions was tested in vitro in a vessel phantom. In vivo testing of acoustic radiation force-mediated delivery of mb-MSCs to balloon-injured aorta was performed in rabbits using intravascular ultrasound (1.7 MHz) during intra-aortic infusion of mb-MSCs. Application of ultrasound led to marginalization and adhesion of mb-MSCs to the vessel phantom wall, whereas no effect was observed on mb-MSCs in the absence of ultrasound. The effect was maximal when there were 7±1 microbubbles/cell (n=6). In rabbits (n=6), adherent MSCs were observed in the ultrasound-treated aortic segment 20 min after the injection (334±137 MSCs/cm(2)), whereas minimal adhesion was observed in control segments not exposed to ultrasound (2±1 MSCs/cm(2), p<0.05). At 24 h after mb-MSC injection and ultrasound treatment, the engrafted MSCs persisted and spread out on the luminal surface of the artery. The data demonstrate proof of principle that acoustic radiation force can target delivery of therapeutic cells to a specific endovascular treatment site. This approach may be used for endoluminal cellular paving and could provide a powerful tool for cell-based re-endothelialization of injured arterial segments.

Ultrasound-triggered microbubble destruction in combination with cationic lipid microbubbles enhances gene delivery

This study aimed to examine the preparation of cationic lipid microbubble (CLM), and evaluate its physical and chemical properties and toxicity, measure the gene transfection efficiency by ultrasound triggered microbobble destruction (UTMD) in combination with CLM. The CLM was prepared by the method of the thin film hydration, and its morphology was observed under the electron microscopy at 1st, 3rd, 7th, 10th, and 14th day after preparation, respectively. The size, Zeta potential and stability of CLM were tested. The acute toxicity of CLM was assessed. The green fluorescent protein gene (EGFP) transfection efficiency was evaluated. The experiment grouping was as follows: naked plasmid group (P group), ultrasonic irradiation plus naked plasmid group (P-US group), naked plasmid plus CLM group (P-CLM group), naked plasmid plus ultrasound and CLM group (UTMD group). The expression of EGFP was detected by fluorescent microscopy and flow cytometry. The results showed that CLMs were spherical in shape, with the similar size and good distribution degree under the light and electron microscopies. The size of CLMs was varied from 250.4±88.3 to 399.0±99.8 nm and the Zeta potential of CLMs from 18.80±4.97 to 20.1±3.1 mV. The EGFP expression was the strongest in the UTMD group, followed by the P-CLM group, P-US group and P group. Flow cytometry results were consistent with those of fluorescent microscopy. The transfection efficiency was substantially increased in the P-US group, P-CLM group and UTMD group as compared with that in the P group, almost 7 times, 10 times and 30 times higher than that in the P group respectively. It is suggested that CLMs prepared by the method of thin film hydration are uniform in diameter, and proved non-toxic. UTMD combined with CLM can significantly increase the transfection efficiency of EGFP to targeted cells.

Analysis of in vitro Transfection by Sonoporation Using Cationic and Neutral Microbubbles

The objective of the study was to examine the role of acoustic power intensity and microbubble and plasmid concentrations on transfection efficiency in HEK-293 cells using a sonoporator with a 1-MHz transducer. A green fluorescent protein (GFP) reporter plasmid was delivered in as much as 80% of treated cells, and expression of the GFP protein was observed in as much as 75% of cells, using a power intensity of 2 W/cm 2 with a 25% duty cycle. In addition, the relative transfection abilities of a lipid noncationic and cationic microbubble platform were investigated. As a positive control, cells were transfected using Lipofectamine reagent. Cell survival and transfection efficiency were inversely proportional to acoustic power and microbubble concentration. Our results further demonstrated that high-efficiency transfection could be achieved, but at the expense of cell loss. Moreover, direct conjugation of plasmid to the microbubble did not appear to significantly enhance transfection efficiency under the examined conditions, although this strategy may be important for targeted transfection in vivo. (E-mail: mbl2a@virginia.edu)

Targeted Gene Transfection from Microbubbles into Vascular Smooth Muscle Cells Using Focused, Ultrasound-Mediated Delivery

We investigated a method for gene delivery to vascular smooth muscle cells using ultrasound triggered delivery of plasmid DNA from electrostatically coupled cationic microbubbles. Microbubbles carrying reporter plasmid DNA were acoustically ruptured in the vicinity of smooth muscle cells in vitro under a range of acoustic pressures (0 to 950 kPa) and pulse durations (0 to 100 cycles). No effect on gene transfection or viability was observed from application of microbubbles, DNA or ultrasound alone. Microbubbles in combination with ultrasound (500-kPa, 1-MHz, 50-cycle bursts at a pulse repetition frequency [PRF] of 100 Hz) significantly reduced viability both with DNA (53 ± 27%) and without (19 ± 8%). Maximal gene transfection (∼1% of cells) occurred using 50-cycle, 1-MHz pulses at 300 kPa, which resulted in 40% viability of cells. We demonstrated that we can locally deliver DNA to vascular smooth muscle cells in vitro using microbubble carriers and focused ultrasound. (E-mail: jh7fj@virginia.edu)

DNA and Polylysine Adsorption and Multilayer Construction onto Cationic Lipid-Coated Microbubbles

We report on a novel application of the layer-by-layer (LbL) assembly technique to attach multiple layers of DNA and poly-l-lysine (PLL) onto preformed lipid-coated microbubbles to increase the DNA loading capacity. We first measured the effects of the cationic lipid fraction and salt concentration on the microbubble stability. Microbubble production and stability were robust up to a cationic lipid fraction of 40 mol % in 10 mM NaCl. DNA adsorption was heterogeneous over the microbubble shell and occurred primarily on the condensed phase domains. The amount of adsorbed DNA, and subsequently adsorbed PLL, increased linearly with the fraction of cationic lipid in the shell. DNA loading was further enhanced by the LbL assembly method to construct polyelectrolyte multilayers (PEMs) of DNA and PLL. PEM buildup was demonstrated by experimental results from zeta potential analysis, fluorescence microscopy, UV spectroscopy, and flow cytometry. The PEMs exhibited two growth stages and were heterogeneously distributed over the microbubble surface. The DNA loading capacity onto the microbubbles was enhanced by over 10-fold by using five paired layers. However, the PEM shell did not prevent oscillation or destruction during ultrasound insonification. These results suggest that the surface can be compartmentalized to make multifunctional, high-payload ultrasound contrast agents for targeted gene therapy.

Targeted retroviral gene delivery using ultrasound

Achieving specificity of delivery represents a major problem limiting the clinical application of retroviral vectors for gene therapy, whilst lack of efficiency and longevity of gene expression limit non-viral techniques. Ultrasound and microbubble contrast agents can be used to effect plasmid DNA delivery. We therefore sought to evaluate the potential for ultrasound/microbubble-mediated retroviral gene delivery. An envelope-deficient retroviral vector, inherently incapable of target cell entry, was combined with cationic microbubbles and added to target cells. The cells were exposed to pulsed 1 MHz ultrasound for 5 s and subsequently analysed for marker gene expression. The acoustic pressure profile of the ultrasound field, to which transduction efficiency was related, was determined using a needle hydrophone. Ultrasound-targeted gene delivery to a restricted area of cells was achieved using virus-loaded microbubbles. Gene delivery efficiency was up to 2% near the beam focus. Significant transduction was restricted to areas exposed to > or = 0.4 MPa peak-negative acoustic pressure, despite uniform application of the vector. An acoustic pressure-dependence was demonstrated that can be exploited for targeted retroviral transduction. The mechanism of entry likely involves membrane perturbation in the vicinity of oscillating microbubbles, facilitating fusion of the viral and cell membranes. We have established the basis of a novel retroviral vector technology incorporating favourable aspects of existing viral and non-viral gene delivery vectors. In particular, transduction can be controlled by means of ultrasound exposure. The technology is ideally suited to targeted delivery following systemic vector administration.

Microbubble-enhanced ultrasound to deliver an antisense oligodeoxynucleotide targeting the human androgen receptor into prostate tumours

We have shown recently that downregulation of the androgen receptor (AR), one of the key players in prostate tumor cells, with short antisense oligodeoxynucleotides (ODNs) results in inhibition of prostate tumor growth. Particularly with regard to an application of these antisense drugs in vivo, we now investigated the usefulness of microbubble-enhanced ultrasound to deliver these ODNs into prostate cancer cells. Our short antisense AR ODNs were loaded onto the lipid surface of cationic gas-filled microbubbles by ion charge binding, and delivered into the cells by bursting the loaded microbubbles with ultrasound. In vitro experiments were initially performed to show that this kind of delivery system works in principle. In fact, transfection of prostate tumor cells with antisense AR ODNs using microbubble-enhanced ultrasound resulted in 49% transfected cells, associated with a decrease in AR expression compared to untreated controls. In vivo, uptake of a digoxigenin-labelled ODN was found in prostate tumour xenografts in nude mice following intratumoral or intravenous injection of loaded microbubbles and subsequent exposure of the tumour to ultrasound, respectively. Our results show that ultrasound seems to be the driving force of this delivery system. Uptake of the ODN was also observed in tumors after treatment with ultrasound alone, with only minor differences compared to the combined use of microbubbles and ultrasound.

Targeted tissue transfection with ultrasound destruction of plasmid-bearing cationic microbubbles

The aim of this study was to assess the relative efficacy and mechanism of gene transfection by ultrasound (US) destruction of plasmid-bearing microbubbles. Luciferase reporter plasmid was charge-coupled to cationic lipid microbubbles. Rat hindlimb skeletal muscle was exposed to intermittent high-power US during dose-adjusted intra-arterial (IA) or IV administration of plasmid-bearing microbubbles via the carotid artery or jugular vein, respectively. At 4 days, luciferase activity in US-exposed skeletal muscle was 200-fold greater with IA than with IV administration of plasmid-bearing microbubbles, and was similar to transfection achieved by IM injection of plasmid (positive control). No transfection occurred with US and IA injection of plasmid alone. Intravital microscopy of the cremaster muscle in mice following administration of microbubbles and US exposure demonstrated perivascular deposition of fluorescent plasmid, the extent of which was twofold greater for IA compared to IV injection. Electron microscopy demonstrated a greater extent of myocellular microporations in US-exposed muscle after IA injection of microbubbles. We conclude that muscle transfection by US destruction of plasmid-bearing cationic microbubbles is amplified by IA, rather than IV, injection of microbubbles due to greater extravascular deposition of plasmid and to greater extent of myocellular microporation. (E-mail: jlindner@virginia.edu)

Ultrasound-mediated transfection of canine myocardium by intravenous administration of cationic microbubble-linked plasmid DNA

We tested the hypothesis that targeted disruption of cationic microbubble-linked plasmid DNA, using diagnostic ultrasound, may aid transfection of large animal myocardium. Plasmid DNA encoding for CAT (pCAT, chloramphenicol acetyltransferase) was bound to a novel cationic microbubble containing MRX-225 for intravenous administration, and 16 dogs in 4 groups variously received this conjugate or plasmid only, or were exposed to ultrasound. Histochemical staining and enzyme-linked immunosorbent assay analysis showed CAT activity in the myocardium of only those animals that received microbubble-linked DNA and were exposed to ultrasound. Thus, disruption of cationic-linked, low-dose plasmid systems by diagnostic ultrasound may facilitate transfection of large animal hearts. (J Am Soc Echocardiogr 2002;15:214-8.)