Microbubble-free mechanical sonoporation for MR-guided drug delivery in solid tumours: a proof-of-concept study

Ultrasound (US)-mediated drug delivery is a promising strategy for localised, minimally invasive molecular transport, typically relying on microbubble-assisted sonoporation. However, clinical translation is limited by the short half-life, cost, and cytotoxicity of exogenous microbubbles. In this proof-of-concept study, we present a microbubble-free, mechanically driven sonoporation approach for intracellular delivery of hydrophilic agents, monitored non-invasively by magnetic resonance imaging (MRI). Using a low-intensity, non-focused ultrasound protocol (1 MHz, MI 0.25, 25% duty cycle, 1 Hz PRF) we achieved efficient intracellular internalisation of the MRI contrast agent Gadoteridol (~ 7 × 10⁹ Gd ions/cell) in vitro while maintaining > 70% cell viability and demonstrating complete membrane resealing within 30 min. Co-delivery with propidium iodide (PI) was confirmed by fluorescence microscopy and MRI, showing cytosolic distribution with negligible passive uptake in non-sonoporated controls. Compared to established permeabilisation methods (electroporation, pinocytosis, hypotonic swelling), microbubble-free sonoporation produced higher uptake with lower oxidative stress. In vivo experiments in K562 tumour-bearing mice showed that intravenous co-administration of Gadoteridol and PI followed by local sonoporation yielded significantly enhanced and prolonged T1 MRI contrast and increased intratumoural accumulation of both agents. These findings establish microbubble-free sonoporation as an effective, controllable technique for intracellular delivery, with the added benefit of MRI-based monitoring. This strategy addresses key limitations of microbubble-based methods and holds strong potential for image-guided therapeutic applications in solid tumors.