Gene activation during high intensity focused ultrasound tumor ablation.

摘要

High intensity focused ultrasound (HIFU) has emerged as a non-invasive non-ioning treatment modality for both benign and malignant cancers by generating coagulative thermal necrosis in tumor tissues. In order to further enhance HIFU therapy efficiency, this work investigates inducible gene expression under the control of a heat sensitive gene promoter (hsp70B) during HIFU tumor ablation. The feasibility of HIFU-induced gene activation was first evaluated in vitro using cancer cell lines (HeLa and R3230Ac) that were transfected with marker gene encoding green fluorescent protein (GFP) under the control of hsp70B promoter. Consistent gene expression was produced under various HIFU exposure conditions and optimal thermal dosage was determined to be 60°C-5s. These exposure conditions are similar to those experienced by the sub-lethally injured cells surrounding HIFU-induced necrosis lesion in tissue. Moreover, simultaneous HIFU tumor ablation and gene activation was investigated in a mice tumor model using an ultrasound imaging-guided HIFU system. Gene (luciferase) activation following 1.1- or 3.3-MHz HIFU exposures using different combination of peak temperature (50 to 80°C), treatment duration (5 to 30s), and scanning strategy were evaluated. Maximum gene expression was produced in a peak temperature range of 65°C to 75°C following a 10-20s exposure. Furthermore, thermal stress rather than non-thermal mechanical stress associated with cavitation was identified as the primary physical mechanism for HIFU-induced gene activation. To better elucidate the underlying physical mechanisms, a cell-embedded tissue-mimicking phantom was developed and characterized to allow for simultaneous assessment of spatial and temporal temperature distribution, HIFU-induced gene expression, and lesion formation. Under a 10s HIFU exposure at 3.3-MHz, the range of peak temperature for the gene activation area along the HIFU lesion boundary was determined to be 54 to 63°C within the sub-lethally injured cell population. Altogether, HIFU-induced gene activation under the control of hsp70B promoter was, for the first time, investigated and optimized both in vitro and in vivo. The present work opens up a new paradigm for combination of HIFU thermal ablation with heat-induced gene therapy to improve the overall quality and effectiveness of cancer therapy.