Young Investigator Award

Dr. Claire Spitzlei – University Paris-Saclay

Thesis: Formulation and validation of sonosensitive agents for ultrasound brain therapy

Supervisor: Dr. Nicolas Tsapis

The blood–brain barrier (BBB) prevents nearly 98% of drugs from entering the brain, representing a major obstacle in the treatment of brain diseases.To address this challenge, the combination of focused ultrasound and microbubbles (MBs) injection enables localized, reversible, and non-invasive opening of the BBB. This mechanism relies on the oscillations of MBs under ultrasound, which weakens BBB endothelial cell junctions. The field has expanded significantly, with ongoing clinical trials for the treatment of brain tumors, Alzheimer’s disease, and amyotrophic lateral sclerosis. However, their efficacy remains limited by the exclusive use of MBs initially developed for diagnostic purposes (SonoVue® or Definity®) and thus not optimized for therapeutic applications.

Developing optimized agents for brain therapy is a major multidisciplinary challenge. Conventional MBs suffer from rapid gas diffusion due to a low affinity between the gas core and the shell, leading to their clearance from circulation within minutes. In recent years, another type of agent has emerged: nanodroplets (NDs), composed of liquid perfluorocarbons (PFCs) encapsulated within a polymer or lipid shell. NDs can be converted into MBs using a low-amplitude ultrasound wave (acoustic vaporization), and then used to promote drug penetration into the brain. Although their in vivo lifetime is prolonged, NDs require high acoustic pressures to vaporize. To improve the stability and ultrasound sensitivity of these agents (MBs and NDs), it is possible to modify the gas/liquid core or the shell. The latter strategy is employed in this work, which aims to formulate new polymer agents (MBs and NDs) that are more stable, sonosensitive, non-toxic, and suitable for prolonged treatments.

The chosen polymer is derived from poly(malic acid) (PMA), a biodegradable polymer onto which fluorinated chains have been grafted to enhance fluorine-fluorine interactions between the MB (or ND) core and the shell. In the first part of the study, MBs were stabilized using this biodegradable polymer, varying the degree of fluorinated chain grafting. This new generation of fluorinated polymer MBs exhibited exceptional in vivo stability, ultrasound sensitivity, and a favorable safety profile for BBB-opening, making them highly suitable for therapeutic applications and for more efficient drug delivery to the brain. Further modifications to the polymer shell—PEGylation, molar mass, architecture, and lipid incorporation—were explored to investigate their impact on the physical and acoustic properties of the resulting MBs.

The second part of this work focused on NDs obtained by condensation of MBs, yielding particles in the sub-200 nm range. Their in vitro vaporization threshold was tuned based on polymer properties. This threshold was correlated with the effective and safe opening of the BBB in vivo, paving the way for precise control of acoustic thresholds through chemical engineering of the shell. These findings represent the first demonstration of polymer NDs with low-boiling point PFCs.