PET labeling allows investigation of microbubble and nanodroplet contrast agent drainage kinetics in mouse models

Acoustically active perfluorocarbon nanodroplets, smaller in size than conventional microbubbles, show significant potential for enhancing utility of contrast ultrasound imaging and drug delivery beyond the confines of the vasculature. Effective exploitation of these nanoscale agents requires detailed understanding of transit and retention in vivo. However, dependence on droplet vaporization for echogenicity and ultrasound tracking presents significant limitation to imaging to support full understanding of nanodroplet drainage, transit and retention times throughout the body. We propose multimodal [<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup>F]FDG-nanodroplets and demonstrate utility for PET imaging in a xenographic model of colorectal cancer.Microbubbles (MicroMarker, Fujifilm) were prepared for use or condensed to create nanodroplets [1]. Suspensions were diluted 1:5 and held at 36 °C in a micro capillary flow system (20 μL/sec) and subject to increasing acoustic pressures (1 MHz, 10 cycles, PRF 10 ms, 0.25 – 2.5 MPa). Acoustic droplet vaporization was detected by passive cavitation (Precision Acoustics) with signals post-processed in MATLAB. For PET imaging, suspensions were reconstituted with 700 μL [18F]FDG, incubated for 1 hour, centrifuged (350 g, 2 minutes), supernatant discarded and resuspended in 700 μL saline and administered via toe injection to right ([18F]FDG-microbubbles) or left ([18F]FDG-nanodroplets) foot of WT mouse (n=3) and imaged using PET/CT (Medison nano) over 60 minutes. PET-nanodroplets were administered to both feet of a tumor bearing (right) colorectal cancer xenographic mouse (n=1).MicroMarker nanodroplets (diameter 129.2±5.1 nm) were successfully formulated through condensation of microbubbles (1100.0±180.0 nm), and conversion back into microbubbles confirmed at pressures above 1 MPa. [18F]FDG-labelling enabled PET imaging (A) and investigation of microbubble and nanodroplet clearance from injection site (B), indicating that microbubbles drained quicker than nanodroplets following toe injection. PET imaging of [18F]FDG-nanodroplets suggests tumor presence retards nanodroplet clearance from toe injection site as compared to healthy contralateral control (0.6 and 1.8 SUVbw/hour respectively).