How microbubbles work
In many experimental applications, microbubbles have proven to be suitable for drug targeting. The recent generation of microbubbles
can be administered into the vein by infusion or by a bolus injection, and they possess a considerable half life of more than
15 min. (see Figure 2). During administration, diagnostic ultrasound is externally applied to the site of disease as during
a common imaging procedure. Microbubbles scatter the ultrasound back and serve as contrast agents, thus allowing the clinician
to find the target precisely and focus the ultrasound beam. Furthermore, the intensity of the scattered signal provides information
about the amount of microbubbles that reside at the target site.
Figure 2: The mechanism of action of ultrasound-active carriers (UACs). After being applied to the patient, UACs can be excited
on the target site using low-intensity ultrasound (black solid lines) and visualized through the backscattered signal (gray
dashed lines). Clinicians burst the UACs by increasing the ultrasound intensity (red solid lines). The cavitation energy perforates
the capillaries and increases cell permeability for the released drug (red dots). Adapted from Ref. 33. (IMAGE IS COURTESY
OF THE AUTHOR)
The clinician then applies a pulse sequence of ultrasound with a higher intensity to excite the microbubbles beyond their
cavitation threshold. The drug carriers burst into fragments of drug-bearing shell debris and small gas bubbles, which dissolve
and are exhaled through the lungs.
The applied acoustic energy of ultrasound waves is transformed into cavitation energy and causes hydrodynamic phenomena such
as microjetting, microswirling, and shock waves. These cavitation forces can cause the sonoporation of biological barriers
by opening transient wall gaps and making cells and tissues permeable to the drug. After treatment is complete, the cells
and vessels recover within seconds or minutes, and therapeutic effects begin.
The size range of drug molecules or drug-loaded particles that can be administered by sonoporation is limited. Capillaries
and the blood-brain barrier can be opened for particles as large as several hundred nanometers (e.g., liposomes and polyplexes)
The permeability of cell membranes, however, is restricted to particles of 2–3 MDa in mass (e.g., micelles, proteins, and
plasmids) (17). Secondary-carrier-associated microbubbles provide one probable way to circumvent this restriction (see Figure
1d). These hybrid constructs combine site targeting, the ability of UACs to open capillaries, and the capacity of nanoparticles
to enter cells by phagocytosis. The microbubbles carry the nanoparticles to the target site and open the capillary wall. The
released nanoparticles penetrate the tissue and are taken up by the cells through phagocytosis, thus acting as secondary carriers
for the drug (see Figure 3). Within the cell, the nanoparticles break the endosome defense mechanisms (e.g., by osmotic rupture
or the proton-sponge effect) and release the plasmid into the nucleus.
Figure 3: Plasmid DNA uptake and release after capillary break and extravasation into the tissue. The secondary carriers (e.g.,
polyplexes) are attached to the microbubble wall (bottom left corner). After ultrasound-mediated microbubble destruction the
polyplexes leave the vessel and penetrate the tissue. They are taken up by cells and escape enzymatic digestion. The therapeutic
plasmid is then released to the nucleus and expressed. Adapted from Ref. 33. (IMAGE IS COURTESY OF THE AUTHOR)
Sonoporation allows the efficient targeting of delicate organs such as the brain and the heart. Unlike liposomes, ultrasound-mediated
targeting is independent from the availability of capillary fenestration or specific cell ligands, which are necessary for
targeting by enhanced permeability and retention or for affinity targeting.
Current applications of ultrasound drug targeting
Ultrasound-active drug carriers have been applied in numerous fields of medical research. To date, the areas of greatest interest
appear to be gene delivery and tumor therapy.
Microbubbles provide a method to focus the action of chemotherapeutic drugs onto the tumor without requiring an invasive intervention.
Anthracyclines are perhaps the mostly widely endorsed candidates for targeted chemotherapy. They have been included in formulations
such as poly(lactic acid) microcapsules, soft-shelled phospholipid microbubbles, secondary-carrier-associated microbubbles,
and as phase-shift nanoemulsions in microbubbles (8, 9, 14–15, 18–19). Delivery of taxanes such as paclitaxel and docetaxel
has also been investigated (9, 11). Microbubbles also have been used to overcome the blood-brain barrier with promising results
(16, 20–21). Before studies on large animals can begin, however, appropriate medical ultrasound devices must be developed.
Some gene therapy applications use mild diagnostic ultrasound to treat sensitive tissues such as the myocardium. Ultrasound-targeted
gene therapy mostly focuses on promoting myocardium revascularization after ischemic stroke, or on antisense therapies of
vascular hyperplasia (22, 23–24). Studies indicate that microbubbles are likely to become an effective tool for preventing
or treating ischemic stroke. Furthermore, microbubbles have successfully targeted and destroyed blood thrombi, thus helping
to restore the perfusion of the affected myocardium (25).