Optical Molecular Imaging of Ultrasound-mediated Drug Delivery

Abstract

The goal of this PhD project was to develop optical molecular imaging methods to study drug delivery facilitated by ultrasound waves (US) and hyperthermia. Fibered confocal fluorescence microscopy (FCFM), together with dedicated image analysis, was used in vitro on a cell monolayer, and in vivo at the tissue scale, to monitor in real time and assess model drug and drug distribution. To this end, setups were designed that allowed ultrasound exposure or hyperthermia conditions, and that would present geometrical constraints in conventional optical imaging systems. However, these were largely overcome by the fiber based design of the microscope. In chapter 2, the feasibility to monitor in real-time US- and microbubble-mediated uptake of a cell-impermeable fluorescent model drug, i.e., SYTOX Green, is evaluated. An in vitro setup was designed that combined a mono-element US transducer, a cell culture chamber containing a monolayer of tumor cells together with SonoVue® microbubbles, and FCFM. The sequences showed a remaining plasma membrane permeability after the end of US exposure. To improve the accuracy of uptake kinetic parameter estimates of SYTOX Green, a post-processing method including cell tracking was presented in chapter 3. Using a two-compartment model representing the extracellular space and the cellular compartment, separated by a plasma membrane, the statistical analysis of the population kinetic data showed a median time constant of 2 minutes 19 seconds. Using the setup described in chapter 2 and the post-processing pipeline developed in chapter 3, we investigated in chapter 4 whether endocytosis is involved in US- and microbubbles- mediated delivery of small molecules using chlorpromazine, an inhibitor of clathrin-mediated endocytosis, or genistein, an inhibitor of caveolae-mediated endocytosis. During the real-time monitoring of SYTOX Green uptake, the cells in the presence of SonoVue® microbubbles were exposed to 1.4 MHz US waves at a 0.2 MPa peak-negative pressure. Both inhibitors were observed to slow down the US-mediated uptake of SYTOX Green, with a significant 2.5-fold increase of the uptake time constant with chlorpromazine, and a 1.1-fold increase with genistein. The impact of photobleaching on uptake rate estimates measured by FCFM was evaluated in chapter 5 to correct for it and improve the accuracy of pharmacokinetic parameter estimates. To model photobleaching of SYTOX Green, a photobleaching rate was added to the current two-compartment model describing cell uptake. Using this three-compartment model, an uptake rate of 6.0 10-3 s-1, independent of the applied laser power, was measured. In chapter 6, a feasibility study is described to evaluate tissue penetration of doxorubicin after its intravascular release from the thermo-sensitive liposomes ThermodoxTM using FCFM, and in a normal physiological environment. To this end, rat R1 rhabdomyosarcoma tumor pieces were implanted subcutaneously in the hind leg of 9 nude rats. A setup combining a water bath with a platform was designed to create local hyperthermia and thus trigger the release of doxorubicin from ThermodoxTM injected intravenously. In the tumor microenvironment, real-time simultaneous monitoring of doxorubicin penetration (488-nm excitation channel) succeeded in 2 of the 9 rats. A strong heterogeneity of doxorubicin distribution in the tumor was observed, which likely limited the success rate of the real-time monitoring.