**14. Conclusions**

However, in the presence of MBs the amount of required power is substantially reduced because the MBs decrease the amount of energy needed for the process of cavitation, leading to MB collapse due to excessive oscillations induced by ultrasound pulses. Furthermore, cavitation of MBs in capillary beds leads to a capillary permeability increase, thus improving local access of the released therapeutic drug [139]. The clinical use of virus-based vectors for gene therapy is limited. This is due to the fact that immune response can be induced by viral proteins in the target tissue and elicit a strong inflammatory activation of endothelial cells. However, it has been shown that when e.g. plasmids and antisense oligonucleotides are used in non-viral delivery vehicles, transfection efficiency is decreased and the expression of the gene product is transient. Commonly used MB preparation Levovist enhanced the beta-Gal gene transfection efficiency more than 6-fold and Optison were used for gene transfer to cardiomyocytes. The transfection efficiencies are significantly higher than that of no MB and

The blood-brain barrier (BBB) is a specialized system of capillary endothelial cells preventing harmful substances in the blood stream from entering the brain. They also supply the brain with nutrients necessary for proper brain functioning. The BBB regulates transport through both tight junctions and metabolic barriers and is often a rate-limiting factor in determining permeation of therapeutic drugs into the brain. It is a significant obstacle affecting delivery of both small molecules and macromolecular agents. Despite the fact that a variety of drugs could be potentially used to treat brain diseases, no method exists that would allow non-invasive-

Ultrasound can be used to locally deliver a drug or gene to a specific region of interest in the brain. The benefit of the MB technique is that it can be used on a very small area of the BBB. MBs are used in combination with ultrasound exposure. The effects of ultrasound can be focused upon the vasculature to reduce the acoustic intensity required for BBB opening production. Several pathways of transcapillary passage after ultrasound sonication have been recognized, including transcytosis, passage through endothelial cell cytoplasmic openings holeyed by "nanoschrapnels" produced by explosion of MB, opening of tight junctions by

The major cause of death in Western countries is thrombo-occlusive cardiovascular disease, with acute ST-elevation myocardial infarction (STEMI) being a serious medical condition leading to high morbidity and mortality. Two main treatment strategies have been developed during the past years. Their aim is to immediately restore coronary blood flow. In the first studies it was shown that fibrinolysis may be accelerated by ultrasound alone. Besides that, fibrinolysis can be even more increased by the addition of MBs which are destroyed by ultrasound induced high pressure. Three mechanisms leading to this increase have been proposed. The first is the occurrence of complete stress based on micro-jets formed after the destruction of the MBs by ultrasound. The second mechanism is the generation of reactive oxygen species in endothelial cells. The third proposed mechanism is a temperature increase,

oscillating MBs and free passage through injured endothelium [136, 140, 141].

it is similar to Levovist [97, 100, 114].

targeted delivery through the BBB.

**13.3. Thrombolysis and thrombus destruction**

**13.2. Blood-brain barrier**

104 Advances in Bioengineering

LNPs and MBs are examples of the successful transfer of basic and applied research into commercial medicinal products. Biocompatibility and biodegradability of LNPs together with versatility in modification and functionalization are features which put together favour their application in medicine. Combinations of LNPs and MBs make best use of both technologies to form new theranostic systems that could form the basis of future advanced therapeutics.
