Ultrasound-induced mild hyperthermia has advantages for noninvasive, localized and controlled drug delivery. In this study, a tissue-mimicking agarose-based phantom with a thermally sensitive indicator was developed for studying the spatial drug delivery profile using ultrasound-induced mild hyperthermia.
Agarose powder, regular evaporated milk, Dulbecco's phosphate-buffered saline (DPBS), n-propanol, and silicon carbide powder were homogeneously mixed with low temperature sensitive liposomes (LTSLs) loaded with a self-quenched near-infrared (NIR) fluorescent dye. A dual-mode linear array ultrasound transducer was used for insonation at 1.54 MHz with a total acoustic power and acoustic pressure of 2.0 W and 1.5 MPa, respectively. After insonation, the dye release pattern in the phantom was quantified based on optical images, and the three-dimensional release profile was reconstructed and analyzed. A finite-difference time-domain-based algorithm was developed to simulate both the temperature distribution and spatial dye diffusion as a function of time. Finally, the simulated dye diffusion patterns were compared to experimental measurements.
Self-quenching of the fluorescent dye in DPBS was substantial at a concentration of 6.25 × 10−2 mM or greater. The transition temperature of LTSLs in the phantom was 35 °C, and the release reached 90% at 37 °C. The simulated temperature for hyperthermia correlated with the thermocouple measurements with a mean error between 0.03 ± 0.01 and 0.06 ± 0.02 °C. The R2 value between the experimental and simulated spatial extent of the dye diffusion, defined by the half-peak level in the elevation, lateral and depth directions, was 0.99 (slope = 1.08), 0.95 (slope = 0.99), and 0.80 (slope = 1.04), respectively, indicating the experimental and simulated dye release profiles were similar.
The combination of LTSLs encapsulating a fluorescent dye and an optically transparent phantom is useful for visualizing and modeling drug releasein vitro following ultrasound-induced mild hyperthermia. The coupled temperature simulation and dye-diffusion simulation tools were validated with the experimental system and can be used to optimize the thermal dose and spatial and temporal dye release pattern.
The authors would like to thank Dr. Charles L. Bevins and Patricia Castillo of Medical Microbiology and Immunology Department in the University of California at Davis for their kind offer of the ultracentrifuge in the milk analysis. The authors also appreciate the assistance from Dr. Anthony G. Passerini and J. Sherrod DeVerse of the Department of Biomedical Engineering in the University of California at Davis for the SDS-PAGE analysis. For the analysis of regular and fat-free evaporated milk, the authors want to thank Dr. Daniela Barile, Dr. Jennifer Smilowitz, and Deborah Gho of the Food Science and Technology Department in the University of California at Davis for their kind assistance with the Fourier Transform Infrared (FTIR) spectrometer. They would also like to thank the NIH for their support for this project under NIH R01CA103828, R01CA134659, and R21EB009902.
II. MATERIALS AND METHODS
II.A. Synthesis and calibration of SIDA fluorescence dye
II.C. Phantom production
II.D. LTSL release tests
II.D.1. LTSL release in DPBS
II.D.2. LTSL release in the phantom
II.E. Calibration of SIDA fluorescence as a free dye in the phantom
II.F. Ultrasound-induced dye release
II.F.1. Ultrasoundsystem design and experimental setup
II.F.2. Optical imaging and reconstruction
II.G. Ultrasound beam simulation
II.H. Simulation of temperature and dye diffusion
III.A. Characterization of SIDA in DPBS after synthesis
III.B. Characterization of LTSLs
III.C. LTSL performance with alternative phantom constituents
III.D. Simulation of the acoustic beam
III.E. Optical imaging, reconstruction, and analysis
III.F. Temperature simulation
III.G. Dye diffusion simulation
III.H. Relationship between experiments and simulation
IV. DISCUSSION AND CONCLUSIONS
IV.A. Self-quenching effect
IV.B. Properties and effect of the phantom composition
IV.C. Low-temperature agarose gel
IV.D. Simulation of the temperature and resulting release
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