Understanding ultrasound induced sonoporation: Definitions and underlying mechanisms

Abstract

In the past two decades, research has underlined the potential of ultrasound and microbubbles to enhance drug delivery. However, there is less consensus on the biophysical and biological mechanisms leading to this enhanced delivery. Sonoporation, i.e. the formation of temporary pores in the cell membrane, as well as enhanced endocytosis is reported. Because of the variety of ultrasound settings used -and corresponding microbubble behavior, a clear overview is missing. Therefore, in this review, the mechanisms contributing to sonoporation are categorized according to three ultrasound settings: i) low intensity ultrasound leading to stable cavitation of microbubbles, ii) high intensity ultrasound leading to inertial cavitation with microbubble collapse, and iii) ultrasound application in the absence of microbubbles. Using low intensity ultrasound, the endocytotic uptake of several drugs could be stimulated, while short but intense ultrasound pulses can be applied to induce pore formation and the direct cytoplasmic uptake of drugs. Ultrasound intensities may be adapted to create pore sizes correlating with drug size. Small molecules are able to diffuse passively through small pores created by low intensity ultrasound treatment. However, delivery of larger drugs such as nanoparticles and gene complexes, will require higher ultrasound intensities in order to allow direct cytoplasmic entry.2

Ultrasound and microbubble mediated drug delivery: Acoustic pressure as determinant for uptake via membrane pores or endocytosis

“…In mechanically injured cells, small lesions were found to be resealed by endocytosis [50]. The same processes may apply for ultrasound exposed cells, since they are also wounded in a mechanical way [8]. However, our confocal recordings rather support the cytoskeletal stimulated endocytosis assumption.…”

“…When using higher ultrasound intensities, inertial cavitation occurs with larger oscillations and eventually microbubble implosion. This is accompanied by more violent phenomena, such as microjets and shock waves [8].…”

The potential of antigen and TriMix sonoporation using mRNA-loaded microbubbles for ultrasound-triggered cancer immunotherapy

“…Two possible mechanisms might explain this sonoporation-induced maturation. First of all, sonoporation is a physical transfection method, where pores are created as a result from shear forces and violent microbubble movements [28]. DCs are known to be sensitive to shear forces, and can already upregulate maturation markers upon stress due to pipetting and centrifugation [29].…”

“…Thus, the shear stress induced by sonoporation could contribute to the observed phenotypical changes. In addition, sonoporation was reported to influence Ca 2+ signaling within cells [28,30]. Importantly, increased intracellular Ca 2+ concentrations were also reported as one of the early events after LPS stimulation of mouse bone marrow-derived DCs [31].…”

Acoustic Cluster Therapy (ACT) — pre-clinical proof of principle for local drug delivery and enhanced uptake

“…In brief, local US insonation of a pathologic condition (e.g. tumor) containing microbubbles in vascular compartments induces bubble oscillations (stable or inertial cavitation) which lead to a variety of biomechanical effects that enhance extravasation and distribution of drug molecules to target tissue [9][10][11][12][13][14]. This approach may hence be explored to improve on the local specificity of a drug, either co-injected with, or attached to microbubbles.…”