The influence of blood on targeted microbubbles

Nested Nanobubbles for Ultrasound-Triggered Drug Release

Because of their size (1-10 μm), microbubble-based drug delivery agents suffer from confinement to the vasculature, limiting tumor penetration and potentially reducing the drug efficacy. Nanobubbles (NBs) have emerged as promising candidates for ultrasound-triggered drug delivery because of their small size, allowing drug delivery complexes to take advantage of the enhanced permeability and retention effect. In this study, we describe a simple method for production of nested-nanobubbles (Nested-NBs) by encapsulation of NBs (∼100 nm) within drug-loaded liposomes. This method combines the efficient and well-established drug-loading capabilities of liposomes while utilizing NBs as an acoustic trigger for drug release. Encapsulation was characterized using transmission electron microscopy with an encapsulation efficiency of 22 ± 2%. Nested-NBs demonstrated echogenicity using diagnostic B-mode imaging, and acoustic emissions were monitored during high-intensity focused ultrasound (HIFU) in addition to monitoring of model drug release. Results showed that although the encapsulated NBs were destroyed by pulsed HIFU [peak negative pressure (PNP) 1.54-4.83 MPa], signified by loss of echogenicity and detection of inertial cavitation, no model drug release was observed. Changing modality to continuous wave (CW) HIFU produced release across a range of PNPs (2.01-3.90 MPa), likely because of a synergistic effect of mechanical and increased thermal stimuli. Because of this, we predict that our NBs contain a mixed population of both gaseous and liquid core particles, which upon CW HIFU undergo rapid phase conversion, triggering liposomal drug release. This hypothesis was investigated using previously described models to predict the existence of droplets and their phase change potential and the ability of this phase change to induce liposomal drug release.

In Vitro Sonothrombolysis Enhancement by Transiently Stable Microbubbles Produced by a Flow-Focusing Microfluidic Device

Therapeutic approaches that enhance thrombolysis by combining recombinant tissue plasminogen activator (rtPA), ultrasound, and/or microbubbles (MBs) are known as sonothrombolysis techniques. To date, sonothrombolysis approaches have primarily utilized commercially available MB formulations (or derivatives thereof) with diameters in the range 1-4 µm and circulation lifetimes between 5 and 15 min. The present study evaluated the in vitro sonothrombolysis efficacy of large diameter MBs (d MB ≥ 10 µm) with much shorter lifetimes that were produced on demand and in close proximity to the blood clot using a flow-focusing microfluidic device. MBs with a N2 gas core and a non-crosslinked bovine serum albumin shell were produced with diameters between 10 and 20 µm at rates between 50 and 950 × 103 per second. Use of these large MBs resulted in approximately 4.0-8.8 fold increases in thrombolysis rates compared to a clinical rtPA dose and approximately 2.1-4.2 fold increases in thrombolysis rates compared to sonothrombolysis techniques using conventional MBs. The results of this study indicate that the large diameter microbubbles with transient stability are capable of significantly enhanced in vitro sonothrombolysis rates when delivered directly to the clot immediately following production by a flow focusing microfluidic device placed essentially in situ adjacent to the clot.

The Use of Acoustic Radiation Force Decorrelation-Weighted Pulse Inversion for Enhanced Ultrasound Contrast Imaging

OBJECTIVES

The use of ultrasound imaging for cancer diagnosis and screening can be enhanced with the use of molecularly targeted microbubbles. Nonlinear imaging strategies such as pulse inversion (PI) and "contrast pulse sequences" (CPS) can be used to differentiate microbubble signal, but often fail to suppress highly echogenic tissue interfaces. This failure results in false-positive detection and potential misdiagnosis. In this study, a novel acoustic radiation force (ARF)-based approach was developed for superior microbubble signal detection. The feasibility of this technique, termed ARF decorrelation-weighted PI (ADW-PI), was demonstrated in vivo using a subcutaneous mouse tumor model.

MATERIALS AND METHODS

Tumors were implanted in the hindlimb of C57BL/6 mice by subcutaneous injection of MC38 cells. Lipid-shelled microbubbles were conjugated to anti-VEGFR2 antibody and administered via bolus injection. An image sequence using ARF pulses to generate microbubble motion was combined with PI imaging on a Verasonics Vantage programmable scanner. ADW-PI images were generated by combining PI images with interframe signal decorrelation data. For comparison, CPS images of the same mouse tumor were acquired using a Siemens Sequoia clinical scanner.

RESULTS

Microbubble-bound regions in the tumor interior exhibited significantly higher signal decorrelation than static tissue (n = 9, P < 0.001). The application of ARF significantly increased microbubble signal decorrelation (n = 9, P < 0.01). Using these decorrelation measurements, ADW-PI imaging demonstrated significantly improved microbubble contrast-to-tissue ratio when compared with corresponding CPS or PI images (n = 9, P < 0.001). Contrast-to-tissue ratio improved with ADW-PI by approximately 3 dB compared with PI images and 2 dB compared with CPS images.

CONCLUSIONS

Acoustic radiation force can be used to generate adherent microbubble signal decorrelation without microbubble bursting. When combined with PI, measurements of the resulting microbubble signal decorrelation can be used to reconstruct images that exhibit superior suppression of highly echogenic tissue interfaces when compared with PI or CPS alone.

Understanding the dynamics of superparamagnetic particles under the influence of high field gradient arrays

The aim of this study was to characterize the behaviour of superparamagnetic particles in magnetic drug targeting (MDT) schemes. A 3-dimensional mathematical model was developed, based on the analytical derivation of the trajectory of a magnetized particle suspended inside a fluid channel carrying laminar flow and in the vicinity of an external source of magnetic force. Semi-analytical expressions to quantify the proportion of captured particles, and their relative accumulation (concentration) as a function of distance along the wall of the channel were also derived. These were expressed in terms of a non-dimensional ratio of the relevant physical and physiological parameters corresponding to a given MDT protocol. The ability of the analytical model to assess magnetic targeting schemes was tested against numerical simulations of particle trajectories. The semi-analytical expressions were found to provide good first-order approximations for the performance of MDT systems in which the magnetic force is relatively constant over a large spatial range. The numerical model was then used to test the suitability of a range of different designs of permanent magnet assemblies for MDT. The results indicated that magnetic arrays that emit a strong magnetic force that varies rapidly over a confined spatial range are the most suitable for concentrating magnetic particles in a localized region. By comparison, commonly used magnet geometries such as button magnets and linear Halbach arrays result in distributions of accumulated particles that are less efficient for delivery. The trajectories predicted by the numerical model were verified experimentally by acoustically focusing magnetic microbeads flowing in a glass capillary channel, and optically tracking their path past a high field gradient Halbach array.

Halbach arrays consisting of cubic elements optimised for high field gradients in magnetic drug targeting applications

A key challenge in the development of magnetic drug targeting (MDT) as a clinically relevant technique is designing systems that can apply sufficient magnetic force to actuate magnetic drug carriers at useful tissue depths. In this study an optimisation routine was developed to generate designs of Halbach arrays consisting of multiple layers of high grade, cubic, permanent magnet elements, configured to deliver the maximum pull or push force at a position of interest between 5 and 50 mm from the array, resulting in arrays capable of delivering useful magnetic forces to depths past 20 mm. The optimisation routine utilises a numerical model of the magnetic field and force generated by an arbitrary configuration of magnetic elements. Simulated field and force profiles of optimised arrays were evaluated, also taking into account the forces required for assembling the array in practice. The resultant selection for the array, consisting of two layers, was then constructed and characterised to verify the simulations. Finally the array was utilised in a set of in vitro experiments to demonstrate its capacity to separate and retain microbubbles loaded with magnetic nanoparticles against a constant flow. The optimised designs are presented as light-weight, inexpensive options for applying high-gradient, external magnetic fields in MDT applications.

Magnetic targeting of microbubbles against physiologically relevant flow conditions

The localization of microbubbles to a treatment site has been shown to be essential to their effectiveness in therapeutic applications such as targeted drug delivery and gene therapy. A variety of different strategies for achieving localization has been investigated, including biochemical targeting, acoustic radiation force, and the incorporation of superparamagnetic nanoparticles into microbubbles to enable their manipulation using an externally applied magnetic field. The third of these strategies has the advantage of concentrating microbubbles in a target region without exposing them to ultrasound, and can be used in conjunction with biochemical targeting to achieve greater specificity. Magnetic microbubbles have been shown to be effective for therapeutic delivery in vitro and in vivo. Whether this technique can be successfully applied in humans however remains an open question. The aim of this study was to determine the range of flow conditions under which targeting could be achieved. In vitro results indicate that magnetic microbubbles can be retained using clinically acceptable magnetic fields, for both the high shear rates (approx. 10⁴ s⁻¹) found in human arterioles and capillaries, and the high flow rates (approx. 3.5 ml s⁻¹) of human arteries. The potential for human in vivo microbubble retention was further demonstrated using a perfused porcine liver model.