Development of Microbubble Contrast Agents for Left Ventricle Opacification

Abstract

Microbubble contrast agents for ultrasound/ echocardiography are used to increase the echogenicity of the target tissues thereby raising the contrast resolution of the resultant image. They are core-shell microbubbles (MBs) with a core of low solubility gas and shell made up of stabilizing material mostly lipid, polymers, and proteins. Their echogenic properties lie in their dynamic response in the acoustic field. Lipid-based microbubbles are flexible therefore, provide better oscillations resulting in high echogenicity. While polymer-based microbubbles have high elastic modulus, thereby undergo oscillations at higher frequencies making them unfit for diagnostic purposes but provides stable response compared to lipids. Attractive aspects of Poly Glycerol Sebacate (PGS) to be used as ultrasound contrast agent lie in its excellent viscoelastic properties i.e. low elastic modulus. This can give it acoustic properties of lipid microbubbles while keeping a stable response of polymeric microbubbles. Since, pre PGS is insoluble, in this study we have synthesized pre PGS, performed its rheological characterization, and theoretically estimated its response in simulated acoustic field. Based on this information, we synthesized our pre-PGS MBs via solvent displacement method and checked its in vitro-ultrasound enhancement and safety. In vivo analysis enhancement analysis and safety assessment was then performed in rats and dogs. Multi modal potential of these MBs was also checked. Pre-PGS MBs gave much better oscillations in the simulated acoustic field with higher scattering cross-sectional area than standard Sonovue® microbubbles. Upon in vitro analysis microbubbles showed excellent enhancement (up to 100 times) and imaging time up to 13 minutes much more than Sonovue® MBs. Microbubbles showed excellent hemocompatibility, cell viability and do not triggered any immune response in vitro. Upon in vivo administration, pre-PGS-PFP microbubbles resulted in opacification of both ventricles with much more contrast effect than Sonovue® MBs. Biochemical and histopathology analysis showed no systemic and organ toxicity justifying the safety of these microbubbles. Microbubbles also enhanced liver and kidneys of the dog which shows its potential to be used in abdominal imaging. Iron oxide loaded pre-PGS MBs showed excellent enhancement (up to 120 times) on in vitro ultrasound and susceptibility effect on T2- weighted MRI resulting in a reduction in signal intensity. Another chance interesting finding was a multimodal potential of unloaded pre-PGS MBs as T1 MR contrast agent and ultrasound contrast agent. These results suggest the excellent potential of pre-PGS-PFP microbubbles as safe, non-immunogenic, biocompatible contrast agent for left ventricle opacification.