Published online Mar 7, 2020. doi: 10.3748/wjg.v26.i9.904
Peer-review started: November 17, 2019
First decision: December 23, 2019
Revised: January 12, 2020
Accepted: February 15, 2020
Article in press: February 15, 2020
Published online: March 7, 2020
The robust fibroinflammatory stroma characteristic of pancreatic ductal adenocarcinoma (PDA) impedes effective drug delivery. Pulsed focused ultrasound (pFUS) can disrupt this stroma and has improved survival in an early clinical trial. Non-invasive methods to characterize pFUS treatment effects are desirable for advancement of this promising treatment modality in larger clinical trials.
In this study, our objective was to identify non-invasive MRI methods that can be used to assess pFUS treatment effects for PDA, based on data derived from three murine models of PDA, including a genetic model. These methods have translational relevance to future, larger clinical trials that might help to advance pFUS therapy as a valuable supplement to traditional treatment modalities for patients with PDA.
Our primary objective was to identify promising, non-invasive pre-clinical imaging methods to characterize acute pFUS treatment effects for in vivo models of PDA. Robust pre-clinical data such as this builds critical foundation to facilitate efficient clinical trials. Knowledge of reliable methods to characterize the acute phase of treatment also helps to inform selection of methods to characterize long-term treatment follow up assessments in future studies.
We utilized quantitative MRI methods at 14 tesla in three mouse models of PDA (subcutaneous, orthotopic and transgenic - KrasLSL-G12D/+, Trp53LSL-R172H/+, Cre or “KPC”) to assess immediate tumor response to pFUS treatment (VIFU 2000 Alpinion Medical Systems; 475 W peak electric power, 1 millisecond pulse duration, 1 Hz, duty cycle 0.1%) vs sham therapy, and correlated our results with histochemical data. These pFUS treatment parameters were previously shown to enhance tumor permeability to chemotherapeutics. T1 and T2 relaxation maps, high (126, 180, 234, 340, 549) vs low (7, 47, 81) b-value apparent diffusion coefficient (ADC) maps, magnetization transfer ratio (MTR) maps, and chemical exchange saturation transfer (CEST) maps for the amide proton spectrum (3.5 parts per million or “ppm”) and the glycosaminoglycan spectrum (0.5-1.5 ppm) were generated and analyzed pre-treatment, and immediately post-treatment, using ImageJ. Animals were sacrificed immediately following post-treatment imaging. The whole-tumor was selected as the region of interest for data analysis and subsequent statistical analysis. T-tests and Pearson correlation were used for statistical inference.
Mean high-b value ADC measurements increased significantly with pFUS treatment for all models. Mean glycosaminoglycan CEST and T2 measurements decreased significantly post-treatment for the KPC group. Mean MTR and amide CEST values increased significantly for the KPC group. Hyaluronic acid focal intensities in the treated regions were significantly lower following pFUS treatment for all animal models. The MRI changes observed acutely following pFUS therapy likely reflect: (1) Sequelae of variable degrees of microcapillary hemorrhage (T1, MTR and amide CEST); (2) Lower PDA glycosaminoglycan content and associated water content (glycosaminoglycan CEST, T2 and hyaluronic acid focal intensity); and (3) Improved tumor diffusivity (ADC) post pFUS treatment.
T2, glycosaminoglycan CEST, and ADC maps proved to be reliable means of quantifying pFUS treatment effects in murine models of PDA, and may provide reliable, non-invasive quantitation of acute pFUS treatment effects for patients with PDA in future clinical trials.
We have identified specific MRI methods as reliable non-invasive means of quantitating acute pFUS treatment effects for murine models of PDA. Future studies of long-term post-treatment disease burden may also benefit from employing the methods we describe. Clinical trials of pFUS therapy for PDA will be more easily accomplished if similar non-invasive methods of tracking immediate treatment endpoints can replace potentially morbid biopsies of this highly sensitive anatomic area. pFUS therapy may also be more efficacious for certain subpopulations of patients with PDA, and the methods we describe may help to non-invasively select enriched patient populations that will derive the greatest benefit from pFUS treatments in future studies.