Phase-change nanoparticles using highly volatile perfluorocarbons: toward a platform for extravascular ultrasound imaging

Abstract

Recent efforts using perfluorocarbon (PFC) nanoparticles in conjunction with acoustic droplet vaporization has introduced the possibility of expanding the diagnostic and therapeutic capability of ultrasound contrast agents to beyond the vascular space. Our laboratories have developed phase-change nanoparticles (PCNs) from the highly volatile PFCs decafluorobutane (DFB, bp =-2 °C) and octafluoropropane (OFP, bp =-37 °C ) for acoustic droplet vaporization. Studies with commonly used clinical ultrasound scanners have demonstrated the ability to vaporize PCN emulsions with frequencies and mechanical indices that may significantly decrease tissue bioeffects. In addition, these contrast agents can be formulated to be stable at physiological temperatures and the perfluorocarbons can be mixed to modulate the balance between sensitivity to ultrasound and general stability. We herein discuss our recent efforts to develop finely-tuned diagnostic/molecular imaging agents for tissue interrogation. We discuss studies currently under investigation as well as potential diagnostic and therapeutic paradigms that may emerge as a result of formulating PCNs with low boiling point PFCs.

Keywords: Acoustic droplet vaporization; Phase-change nanoparticles; diagnostic; extravascular imaging; perfluorocarbon; therapeutic.; ultrasound.

Figure 1

Vaporization temperature predictions for perfluorocarbon particles as a function of droplet diameters. Reprinted with permission from Langmuir. Copyright 2011 American Chemical Society.

Figure 2

Pictorial diagram for making phase-change nanoparticles of submicron size distribution using pressurization and slow cooling to condense the gas inside the microbubbles. Reprinted with permission from Langmuir. Copyright 2011 American Chemical Society.

Figure 3

Ultrasound vaporization thresholds (mechanical index) of individual particles as a function of initial particle diameter when exposed to 5 MHz ultrasound (10 cycles). Decafluorobutane droplets require significantly less energy than droplets composed of higher boiling point compounds. Reprinted from Ultrasound in Medicine & Biology. Vol. 37 (9). Sheeran PS, Wong VP, Luois S et al. Decafluorobutane as a Phase-Change Contrast Agent for Low-Energy Extravascular Ultrasonic Imaging. pgs. 1518-1530, Copyright 2011, with permission from Elsevier. *note: rarefactional pressure (in MPa) can be calculated from the mechanical index with

Figure 4

Submicron particle sizing of nanodroplets produced by the microbubble condensation method. Peak sizes of 200 - 300 nm resulted, which appeared to be independent of lipid concentration. Mean droplet size for each curve in order of increasing lipid concentration was 229 ± 120, 345 ± 97, and 325 ± 268 nm, respectively. Mode droplet size was 220, 295, and 255 nm, respectively. Reprinted with permission from Langmuir. Copyright 2011 American Chemical Society.

Figure 5

Microbubble diameter post vaporization as a function of acoustic pressure at 5 MHz frequency and lipid concentration of 3 mg/mL. As pressure increases, the distribution skews towards smaller resultant microbubble diameters. Reprinted with permission from Langmuir. Copyright 2011 American Chemical Society. *note: rarefactional pressure (in MPa) can be calculated from the mechanical index with .

Figure 6

Rarefactional pressure required to vaporize droplets of based on initial diameter, and as a function of PFC composition (decafluorobutane, octafluoropropane and a 50:50 mixture of the two gasses). Measurements taken at room temperature (22°C) using an 8 MHz ultrasound transducer with a 2-cycle pulse length. Note the intermediate pressure required for vaporization of the mixture. Reprinted from Biomaterials. Vol. 33. Sheeran PS, Luois S, Mullin LB et al. Design of ultrasonically-activatable nanoparticles using low boiling point perfluorocarbons. pgs. 3262-68, Copyright 2011, with permission from Elsevier.

Figure 7

Corresponding Microbubble diameters (y-axis) after ultrasound-mediated vaporization of PCNs. Reprinted from Ultrasound in Medicine & Biology. Vol. 37 (9). Sheeran PS, Wong VP, Luois S et al. Decafluorobutane as a Phase-Change Contrast Agent for Low-Energy Extravascular Ultrasonic Imaging. pgs. 1518-1530, Copyright 2011, with permission from Elsevier.

Figure 8

Perfluorocarbon droplet distributions in vitro over a 1 h period: Pure DFB at a) 22 C and b) 37 C; DFB . OFP mixture at c) 22 C and d) 37 C; and pure octafluoropropane at e) 22 C and f) 37 C. The distribution at each time point was scaled to the relative mean concentration (concentration-weighted) to simultaneously reflect changes in concentration over the time period. Reprinted from Biomaterials. Vol. 33. Sheeran PS, Luois S, Mullin LB et al. Design of ultrasonically-activatable nanoparticles using low boiling point perfluorocarbons. pgs. 3262-3268, Copyright 2011, with permission from Elsevier.

Figure 9

Mean scattering intensity as a function of PCN binding to HUVEC cells. Note the significant increase in scattering intensity with targeted PCNs vs. untargeted PCNs, demonstrating that the cyclic RGD ligand targets αvβ3 expression in the HUVEC cells, and that the PCNs are capable of providing quantitative information on molecular expression at the cellular level. © 2012 IEEE. Reprinted, with permission from P.S. Sheeran et al., "Ultrasound molecular imaging with customizable nanoscale phase-change contrast agents: an in-vitro feasibility study", 2012 IEEE International Ultrasonics Symposium (IUS) Proceedings, In Press, 2012.

Figure 10

Overlays of contrast-specific CPS (green) and B-mode (grey) ultrasound scans of HUVEC cells incubated with targeted PCNs: Prior to activation, no contrast-specific echogenicity is detected, suggesting no microbubbles have formed. After exposure to a mechanical index of 1.1 at 8 MHz, targeted droplets vaporize to the highly-echogenic microbubble state and remain in the plane of the HUVEC cells. *note: rarefactional pressure (in MPa) can be calculated from the mechanical index with . © 2012 IEEE. Reprinted, with permission from P.S. Sheeran et al., "Ultrasound molecular imaging with customizable nanoscale phase-change contrast agents: an in-vitro feasibility study", 2012 IEEE International Ultrasonics Symposium (IUS) Proceedings, In Press, 2012.