Sonoporation describes the opening of biological barriers, such as cell membranes or entire cell layers, among others, when they are exposed to ultrasound-responsive, artificially stabilized, micro-meter sized gas bubbles, known as microbubbles (MBs). Over the years, various types of MBs have been created and excited at different ultrasound settings. Depending on the ultrasound settings, such as the applied frequency and the amplitude of the acoustic pressure, different MB responses are stimulated that perturb their direct surrounding. These responses can range from small vibrations, to stable volumetric oscillations and violent MB destructions. The type of MB response strongly influences the extent to which they affect neighboring structures. Sonoporation is being extensively explored to facilitate local drug delivery. It represents a non-invasive method to reduce off-target effects, increase bioavailability and thereby improve the therapeutic outcome of cancer patients. However, adjusting ultrasound settings to precisely control and tailor MB responses in order to stimulate specific biological effects is challenging, particularly for broadly size-distributed MB types. At fixed ultrasound settings, the response of MBs is also dependent on their size, which in turn makes the overall ultrasound response of broadly size distributed MBs comparably heterogeneous and poorly controllable. With this in mind, we set out to characterize narrow size-distributed poly (n-butyl cyanoacrylate) (PBCA) MBs in this thesis in order to address this issue and help make sonoporation procedures more controllable. To this end, the ultrasound response of the PBCA MBs was investigated. Furthermore, the in vitro performance of ultrasound-activated PBCA MBs (PBCA MBs and ultrasound) was compared to a commonly used MB type in inducing sonoporation in cell monolayers. Further experiments were performed to validate our insights by delivering drug-like macromolecules into different cells or across the cell layer. Finally, tumor-bearing mice were treated with PBCA MBs and ultrasound to increase the extravasation of a drug-like macromolecule into the tumor tissue and promote the anti-cancer effect of a commercial chemotherapeutic agent. The experiments demonstrated that PBCA MBs were responsive to ultrasound at acoustic pressures above 165 kPa and started to burst at 500 kPa. The MBs mainly compressed during insonation and showed complete shell fragmentation when destroyed. Next to that, PBCA MBs induced significant biological effects during in vitro experiments. When compared to the other MB type, differences in their size distribution and ultrasound responses accounted for a slightly stronger effect of the reference MB type at intermediate acoustic pressures. The results indicated that the compression-driven ultrasound response of PBCA MBs, if not being destroyed, induced significant biological effects in cell monolayers. Moreover, PBCAMBs and ultrasound exposure of tumor bearing mice boosted the extravasation of macromolecules into tumor tissue up to a distance of 36 µm from the vessels and did not result in significant vascular ablation. Finally, the therapeutic effect of the administered commercial therapeutic agent was promoted when combined with PBCA MBs and ultrasound treatment, effectively inhibiting tumor growth without causing adverse effects. Taken together, PBCA MBs are well suited for sonoporation procedures and are qualified to compete with other commonly used MB types. They allow the ultrasound parameters to be precisely adjusted according to the application, providing more control over the induced biological effects. The knowledge gained may contribute to render sonoporation interventions more efficient and safer to pave the way for PBCA MBs into the clinic.
Julia Karoline Blöck (Wed,) studied this question.