Targeted Anti-Cancer Provascular Therapy Using Ultrasound-Stimulated Microbubbles

Hypoxia is a recognized mechanism of resistance to radiation therapy in various solid tumors. It has recently been shown that ultrasound (US) stimulated microbubbles (MBs) cavitation can increase blood perfusion in muscles, by triggering the signaling of nitric oxide. We hypothesized that Ultrasound Targeted Microbubbles Cavitation (UTMC) can radiosensitize solid tumors by increasing blood perfusion and thus reduce tumor hypoxia. A therapeutic transducer (1 MHz, A303S, Olympus) was used to transmit long US pulses (5000 cycles) to the tissue of interest, during the injection of MBs (Definity, Lantheus) via the tail vein (2-5 μL/min). Different US pressure values were tested, ranging from 125 to 750 kPa. UTMC consisted of 60 therapeutic pulses, given at a pulse interval adjusted to allow microbubble replenishment as guided by US contrast imaging (CPS 7MHz, 15L8 probe, Sequoia, Siemens), typically given in about 15 mins. The increase in perfusion in the muscle and in the tumor was quantified by burst replenishment imaging allowing longitudinal quantification of blood perfusion (A×B). In muscle, the increase in blood perfusion following various UTMC treatments was strong and very rapid. For all pulses tested, perfusion increased from 0.5 ± 0.2 dB/s before treatment to 5.4 ± 1.4 dB/s after treatment and typically lasted for at least 10 after UTMC treatment. In the periphery of tumors, there was a significant difference (*** p<; 0.005, T-Test) between tumors subject to low pressure (2.10 +/-0.88 times the pre-treatment value for pressures of 125 and 250 kPa [n=25]) and tumors subject to high pressure (1.30 +/- 0.61 times the pre-treatment value for pressures of 375, 500 and 750 kPa [n=31]). The difference with the negative control group was also significant with both the 125 kPa (*** p<; 0.005) and 250 kPa (** p<; 0.01) groups (post-hoc Tukey's HSD test), but not with the higher pressure groups. Additionally in the periphery, the effect of UTMC was stronger in the big tumors (relative change of 1.83 ± 0.71 times the pre-treatment value for tumors over 335 mm
³
[+++ p<; 0.005, T-Test]), compared to no significant relative change in perfusion for tumors under 335 mm
³
. The difference between small and big tumors was also significant (*** p<; 0.005, T-Test). The increase in perfusion persisted for 10 to 25 minutes post therapy, which may be sufficient for radiotherapy treatment. These results suggest that it is possible to increase tumor blood perfusion by UTMC in solid tumors.