Extracellular pH mapping of liver cancer on a clinical 3T MRI scanner

Abstract

Purpose-To demonstrate feasibility of developing a noninvasive extracellular pH (pH e) mapping method on a clinical MRI scanner for molecular imaging of liver cancer. Methods-In vivo pH e mapping has been demonstrated on preclinical scanners (e.g., 9.4T, 11.7T) with Biosensor Imaging of Redundant Deviation in Shifts (BIRDS), where the pH e readout by 3D chemical shift imaging (CSI) depends on hyperfine shifts emanating from paramagnetic macrocyclic chelates like TmDOTP 5− which upon extravasation from blood resides in the extracellular space. We implemented BIRDS-based pH e mapping on a clinical 3T Siemens scanner, where typically diamagnetic 1 H signals are detected using millisecond-long radiofrequency (RF) pulses, and 1 H shifts span over ±10 ppm with long transverse (T 2 , 10 2 ms) and longitudinal (T 1 , 10 3 ms) relaxation times. We modified this 3D-CSI method for ultra-fast acquisition with microsecond-long RF pulses, because even at 3T the paramagnetic 1 H shifts of TmDOTP 5− have millisecond-long T 2 and T 1 and ultra-wide chemical shifts (±200 ppm) as previously observed in ultra-high magnetic fields. Results-We validated BIRDS-based pH in vitro with a pH electrode. We measured pH e in a rabbit model for liver cancer using VX2 tumors, which are highly vascularized and hyperglycolytic. Compared to intratumoral pH e (6.8 ± 0.1; P < 10 −9) and tumor's edge pH e (6.9 ± 0.1; P < 10 −7), liver parenchyma pH e was significantly higher (7.2 ± 0.1). Tumor localization was confirmed with histopathological markers of necrosis (hematoxylin and eosin), glucose uptake (glucose transporter 1), and tissue acidosis (lysosome-associated membrane protein 2).

A high‐throughput imaging platform to characterize extracellular pH in organotypic three‐dimensional in vitro models of liver cancer

“…While pH e measurements using BIRDS have already been applied in vivo, this study proves the feasibility of BIRDS in 3D organotypic cell culture models of liver cancer 21‐24 . 3D cell cultures allow for studying concepts of the TME in relation to drug effects on cell viability prior to clinical testing 38 .…”

“…The pH e and temperature determination with BIRDS depends only on TmDOTP 5− chemical shifts, which, when expressed in ppm, are independent of the strength of the magnetic field. Both pH e and temperature sensitivities for BIRDS with TmDOTP 5− were found to be the same over a wide range of magnetic fields from 3 T to 11.7 T in previous studies 18,24 . However, the accuracy of BIRDS measurements depends on the signal‐to‐noise ratio (SNR) of the TmDOTP 5− resonances, which is only slightly lower at low magnetic fields 18 .…”

“…Three well studied and characterized human hepatic cancer cell lines with varying growth characteristics were used: HepG2 (ATCC, Gaithersburg, MD; catalog no HB‐8065, ), Huh7 (CLS Cell Lines Services, Eppelheim, Germany; catalog no 300156/p7178_HuH7, ), and SNU475 (ATCC; catalog no CRL‐2236, ). Additionally, we used a human hepatocyte cell line derived from primary normal liver cells infected with SV40 large T‐antigen, THLE2 (ATCC; catalog no CRL‐10149, ), and a metastatic liver cancer cell line, VX2 (ATCC; catalog no CRL‐6503, ), which was derived from a sarcoma of a female New Zealand White rabbit, commonly used in a translational rabbit liver cancer model 21,24 . All experiments were performed in mycoplasma‐free cells and human cell lines have been authenticated using short tandem repeat genotype profiling within three years prior to experimental use.…”

“…As opposed to many fluorescent, luminescent, or radioactive probes, MRSI provides a non‐invasive approach to quantify functional cancer characteristics 20 . While BIRDS and other MRSI techniques have been tested in various experimental settings in vivo 21‐24 , in vitro models that mimic the TME can be beneficial alternatives that allow measurements of tumor pH e with high throughput in human‐derived cell cultures. This technique bears the potential to test the susceptibility of individual metabolic tumor phenotypes to different oncological drugs, and may help design strategies to mitigate resistance mechanisms, and thus identify a personalized targeted therapy prior to clinical application 11,22 …”

<p>Glucometabolic Reprogramming in the Hepatocellular Carcinoma Microenvironment: Cause and Effect</p>

“…54 Recently scientists have proposed using high-resolution pH mapping to monitor pH e in HCC, which could be a biomarker for metabolic changes and monitoring tumor aggressiveness and therapeutic outcome. [55][56][57] Tumor acidosis is the consequence of lactate and H + ions accumulation, which are produced by glycolysis and oxidative metabolism. Most tumor cells, also called as glycolytic tumor cells, prefer glycolysis rather than OXPHOS, leading to increases in lactate and H + production.…”