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

High-resolution extracellular pH imaging of liver cancer with multiparametric MR using Deep Image Prior

Noninvasive extracellular pH (pHe) mapping with Biosensor Imaging of Redundant Deviation in Shifts (BIRDS) using MR spectroscopic imaging (MRSI) has been demonstrated on 3T clinical MR scanners at 8 × 8 × 10  mm3 spatial resolution and applied to study various liver cancer treatments. Although pHe imaging at higher resolution can be achieved by extending the acquisition time, a postprocessing method to increase the resolution is preferable, to minimize the duration spent by the subject in the MR scanner. In this work, we propose to improve the spatial resolution of pHe mapping with BIRDS by incorporating anatomical information in the form of multiparametric MRI and using an unsupervised deep-learning technique, Deep Image Prior (DIP). Specifically, we used high-resolution T 1 , T 2 , and diffusion-weighted imaging (DWI) MR images of rabbits with VX2 liver tumors as inputs to a U-Net architecture to provide anatomical information. U-Net parameters were optimized to minimize the difference between the output super-resolution image and the experimentally acquired low-resolution pHe image using the mean-absolute error. In this way, the super-resolution pHe image would be consistent with both anatomical MR images and the low-resolution pHe measurement from the scanner. The method was developed based on data from 49 rabbits implanted with VX2 liver tumors. For evaluation, we also acquired high-resolution pHe images from two rabbits, which were used as ground truth. The results indicate a good match between the spatial characteristics of the super-resolution images and the high-resolution ground truth, supported by the low pixelwise absolute error.

Transmembrane pH gradient imaging in rodent glioma models

A unique feature of the tumor microenvironment is extracellular acidosis in relation to intracellular milieu. Metabolic reprogramming in tumors results in overproduction of H+ ions (and lactate), which are extruded from the cells to support tumor survival and progression. As a result, the transmembrane pH gradient (ΔpH), representing the difference between intracellular pH (pHi) and extracellular pH (pHe), is posited to be larger in tumors compared with normal tissue. Controlling the transmembrane pH difference has promise as a potential therapeutic target in cancer as it plays an important role in regulating drug delivery into cells. The current study shows successful development of an MRI/MRSI-based technique that provides ΔpH imaging at submillimeter resolution. We applied this technique to image ΔpH in rat brains with RG2 and U87 gliomas, as well as in mouse brains with GL261 gliomas. pHi was measured with Amine and Amide Concentration-Independent Detection (AACID), while pHe was measured with Biosensor Imaging of Redundant Deviation in Shifts (BIRDS). The results indicate that pHi was slightly higher in tumors (7.40-7.43 in rats, 7.39-7.47 in mice) compared with normal brain (7.30-7.38 in rats, 7.32-7.36 in mice), while pHe was significantly lower in tumors (6.62-6.76 in rats, 6.74-6.84 in mice) compared with normal tissue (7.17-7.22 in rats, 7.20-7.21 in mice). As a result, ΔpH was higher in tumors (0.64-0.81 in rats, 0.62-0.65 in mice) compared with normal brain (0.13-0.16 in rats, 0.13-0.16 in mice). This work establishes an MRI/MRSI-based platform for ΔpH imaging at submillimeter resolution in gliomas.

Printing a cure: A tailored solution for localized drug delivery in liver cancer treatment

Adjuvant chemotherapy is highly recommended for liver cancer to enhance survival rates due to its tendency to recur frequently. Localized drug-eluting implants have gained traction as an alternative to overcome the limitations of systemic chemotherapy. This work describes the development of biodegradable 3D printed (3DP) bilayer films loaded with 5-fluorouracil (5FU) and cisplatin (Cis) with different infill percentages where the 5FU layers were 40%, 30%, and 30% and Cis layers were 10%, 15%, and 10% for films A, B, and C, respectively. The relevant characterization tests were performed, and the drug content of films was 0.68, 0.50, and 0.50 mg of 5FU and 0.39, 0.80, and 0.34 mg of Cis for films A, B, and C, respectively. Cis release was affected by the alterations to the film design, where films A, B, and C showed complete release at 12, 14, and 23 days, respectively. However, 5FU was released over 24 h for all films. The films were stable for up to two weeks after storage at 25 °C/65% relative humidity and four weeks at 4 °C where drug content, tensile strength, FTIR, and thermal analysis results demonstrated negligible alterations. The cytotoxicity of the films was assessed by MTS assays using HepG2 cell lines demonstrating up to 81% reduction in cell viability compared to blank films. Moreover, apoptosis was confirmed by Western Blots and the determination of mitochondrial cell potential, highlighting the potential of these films as a promising approach in adjuvant chemotherapy.

Detection of tissue pH with quantitative chemical exchange saturation transfer magnetic resonance imaging

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as a novel means for sensitive detection of dilute labile protons and chemical exchange rates. By sensitizing to pH-dependent chemical exchange, CEST MRI has shown promising results in monitoring tissue statuses such as pH changes in disorders like acute stroke, tumor, and acute kidney injury. This article briefly reviews the basic principles for CEST imaging and quantitative measures, from the simplistic asymmetry analysis to multipool Lorentzian decoupling and quasi-steady-state reconstruction. In particular, the advantages and limitations of commonly used quantitative approaches for CEST applications are discussed.

Chemical exchange saturation transfer imaging of creatine, phosphocreatine, and protein arginine residue in tissues

Chemical exchange saturation transfer (CEST) MRI has become a promising technique to assay target proteins and metabolites through their exchangeable protons, noninvasively. The ubiquity of creatine (Cr) and phosphocreatine (PCr) due to their pivotal roles in energy homeostasis through the creatine phosphate pathway has made them prime targets for CEST in the diagnosis and monitoring of disease pathologies, particularly in tissues heavily dependent on the maintenance of rich energy reserves. Guanidinium CEST from protein arginine residues (i.e. arginine CEST) can also provide information about the protein profile in tissue. However, numerous obfuscating factors stand as obstacles to the specificity of arginine, Cr, and PCr imaging through CEST, such as semisolid magnetization transfer, fast chemical exchanges such as primary amines, and the effects of nuclear Overhauser enhancement from aromatic and amide protons. In this review, the specific exchange properties of protein arginine residues, Cr, and PCr, along with their validation, are discussed, including the considerations necessary to target and tune their signal effects through CEST imaging. Additionally, strategies that have been employed to enhance the specificity of these exchanges in CEST imaging are described, along with how they have opened up possible applications of protein arginine residues, Cr and PCr CEST imaging in the study and diagnosis of pathology. A clear understanding of the capabilities and caveats of using CEST to image these vital metabolites and mitigation strategies is crucial to expanding the possibilities of this promising technology.

Generalization of quasi-steady-state reconstruction to CEST MRI with two-tiered RF saturation and gradient-echo readout-Synergistic nuclear Overhauser enhancement contribution to brain tumor amide proton transfer-weighted MRI

PURPOSE

While amide proton transfer-weighted (APTw) MRI has been adopted in tumor imaging, there are concurrent APT, magnetization transfer, and nuclear Overhauser enhancement changes. Also, the APTw image is confounded by relaxation changes, particularly when the relaxation delay and saturation time are not sufficiently long. Our study aimed to extend a quasi-steady-state (QUASS) solution to determine the contribution of the multipool CEST signals to the observed tumor APTw contrast.

METHODS

Our study derived the QUASS solution for a multislice CEST-MRI sequence with an interleaved RF saturation and gradient-echo readout between signal averaging. Multiparametric MRI scans were obtained in rat brain tumor models, including T₁ , T₂ , diffusion, and CEST scans. Finally, we performed spinlock model-based multipool fitting to determine multiple concurrent CEST signal changes in the tumor.

RESULTS

The QUASS APTw MRI showed small but significant differences in normal and tumor tissues and their contrast from the acquired asymmetry calculation. The spinlock model-based fitting showed significant differences in semisolid magnetization transfer, amide, and nuclear Overhauser enhancement effects between the apparent and QUASS CEST MRI. In addition, we determined that the tumor APTw contrast is due to synergistic APT increase (+3.5 ppm) and NOE decrease (-3.5 ppm), with their relative contribution being about one third and two thirds under a moderate B₁ of 0.75 μT at 4.7 T.

CONCLUSION

Our study generalized QUASS analysis to gradient-echo image readout and quantified the underlying tumor CEST signal changes, providing an improved elucidation of the commonly used APTw MRI.

Magnetic resonance imaging for characterization of hepatocellular carcinoma metabolism

With a better understanding of the pathophysiological and metabolic changes in hepatocellular carcinoma (HCC), multiparametric and novel functional magnetic resonance (MR) and positron emission tomography (PET) techniques have received wide interest and are increasingly being applied in preclinical and clinical research. These techniques not only allow for non-invasive detection of structural, functional, and metabolic changes in malignant tumor cells but also characterize the tumor microenvironment (TME) and the interactions of malignant tumor cells with the TME, which has hypoxia and low pH, resulting from the Warburg effect and accumulation of metabolites produced by tumor cells and other cellular components. The heterogeneity and complexity of the TME require a combination of images with various parameters and modalities to characterize tumors and guide therapy. This review focuses on the value of multiparametric magnetic resonance imaging and PET/MR in evaluating the structural and functional changes of HCC and in detecting metabolites formed owing to HCC and the TME.

Brain-targeting, acid-responsive antioxidant nanoparticles for stroke treatment and drug delivery

Stroke is the leading cause of death and disability. Currently, there is no effective pharmacological treatment for this disease, which can be partially attributed to the inability to efficiently deliver therapeutics to the brain. Here we report the development of natural compound-derived nanoparticles (NPs), which function both as a potent therapeutic agent for stroke treatment and as an efficient carrier for drug delivery to the ischemic brain. First, we screened a collection of natural nanomaterials and identified betulinic acid (BA) as one of the most potent antioxidants for stroke treatment. Next, we engineered BA NPs for preferential drug release in acidic ischemic tissue through chemically converting BA to betulinic amine (BAM) and for targeted drug delivery through surface conjugation of AMD3100, a CXCR4 antagonist. The resulting AMD3100-conjugated BAM NPs, or A-BAM NPs, were then assessed as a therapeutic agent for stroke treatment and as a carrier for delivery of NA1, a neuroprotective peptide. We show that intravenous administration of A-BAM NPs effectively improved recovery from stroke and its efficacy was further enhanced when NA1 was encapsulated. Due to their multifunctionality and significant efficacy, we anticipate that A-BAM NPs have the potential to be translated both as a therapeutic agent and as a drug carrier to improve the treatment of stroke.

Tumor acidity: From hallmark of cancer to target of treatment

Tumor acidity is one of the cancer hallmarks and is associated with metabolic reprogramming and the use of glycolysis, which results in a high intracellular lactic acid concentration. Cancer cells avoid acid stress major by the activation and expression of proton and lactate transporters and exchangers and have an inverted pH gradient (extracellular and intracellular pHs are acid and alkaline, respectively). The shift in the tumor acid-base balance promotes proliferation, apoptosis avoidance, invasiveness, metastatic potential, aggressiveness, immune evasion, and treatment resistance. For example, weak-base chemotherapeutic agents may have a substantially reduced cellular uptake capacity due to "ion trapping". Lactic acid negatively affects the functions of activated effector T cells, stimulates regulatory T cells, and promotes them to express programmed cell death receptor 1. On the other hand, the inversion of pH gradient could be a cancer weakness that will allow the development of new promising therapies, such as tumor-targeted pH-sensitive antibodies and pH-responsible nanoparticle conjugates with anticancer drugs. The regulation of tumor pH levels by pharmacological inhibition of pH-responsible proteins (monocarboxylate transporters, H+-ATPase, etc.) and lactate dehydrogenase A is also a promising anticancer strategy. Another idea is the oral or parenteral use of buffer systems, such as sodium bicarbonate, to neutralize tumor acidity. Buffering therapy does not counteract standard treatment methods and can be used in combination to increase effectiveness. However, the mechanisms of the anticancer effect of buffering therapy are still unclear, and more research is needed. We have attempted to summarize the basic knowledge about tumor acidity.

Predictive Value of MRI with Serum Lectin-Reactive Alpha-Fetoprotein for Liver Cancer Recurrence after Percutaneous Radiofrequency Ablation

Objective

To explore the predictive value of magnetic resonance imaging (MRI) with serum lectin-reactive alpha-fetoprotein (AFP-L3) for liver cancer recurrence after percutaneous radiofrequency ablation (RFA).

Methods

This study included 94 liver cancer patients admitted for RFA treatment and 82 healthy subjects. MRI was performed to record the apparent diffusion coefficient (ADC). The serum concentrations of AFP-L3 were quantified in all participants. The correlation of the AFP-L3 serum level and ADC value with clinical efficacy following RFA was analyzed. Moreover, the prognostic factors affecting liver cancer recurrence were analyzed, as well as the predictive effect of the ADC value and AFP-L3 on liver cancer recurrence.

Results

The serum AFP-L3 level was higher in liver cancer patients than the healthy controls with a lower ADC value. Besides, the patients with tumor residuals had lower ADC values and higher serum AFP-L3 levels than those with complete ablated tumor. The combined detection of the ADC value and serum AFP-L3 level had a sensitivity of 87.50% and a specificity of 87.18% for diagnosing complete ablation after RFA treatment. The number of tumor nodules, tumor diameter, AFP, AFP-L3, and the presence of liver cirrhosis are all independent risk factors for liver cancer recurrence within one year. Meanwhile, the combined detection of the ADC value and serum AFP-L3 level had a good predictive effect on liver cancer recurrence with the sensitivity of 92.86% and a specificity of 69.62%.

Conclusion

The ADC values combined with serum AFP-L3 detection had good predictive effects on complete ablation and recurrence of liver cancer after RFA treatment.

Methylated tetra-amide derivatives of paramagnetic complexes for magnetic resonance biosensing with both BIRDS and CEST

Paramagnetic agents that utilize two mechanisms to provide physiological information by magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) are described. MRI with chemical exchange saturation transfer (CEST) takes advantage of the agent's exchangeable protons (e.g., -OH or -NH_x , where 2 ≥ x ≥ 1) to create pH contrast. The agent's incorporation of non-exchangeable protons (e.g., -CH_y , where 3 ≥ y ≥ 1) makes it possible to map tissue temperature and/or pH using an MRSI method called biosensor imaging of redundant deviation in shifts (BIRDS). Hybrid probes based upon 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate chelate (DOTA^4- ) and its methylated analog (1,4,7,10-tetraazacyclododecane-α, α', α″, α‴-tetramethyl-1,4,7,10-tetraacetate, DOTMA^4- ) were synthesized, and modified to create new tetra-amide chelates. Addition of several methyl groups per pendent arm of the symmetrical chelates, positioned proximally and distally to thulium ions (Tm³⁺ ), gave rise to favorable BIRDS properties (i.e., high signal-to-noise ratio (SNR) from non-exchangeable methyl proton peaks) and CEST responsiveness (i.e., from amide exchangeable protons). Structures of the Tm³⁺ probes elucidate the influence of methyl group placement on sensor performance. An eight-coordinate geometry with high symmetry was observed for the complexes: Tm-L1 was based on DOTA^4- , whereas Tm-L2 and Tm-L3 were based on DOTMA^4- , where the latter contained an additional carboxylate at the distal end of each arm. The distance of Tm³⁺ from terminal methyl carbons is a key determinant for sustaining BIRDS temperature sensitivity without compromising CEST pH contrast; however, water solubility was influenced by introduction of hydrophobic methyl groups and hydrophilic carboxylate. Combined BIRDS and CEST detection of Tm-L2, which features two high-SNR methyl peaks and a strong amide CEST peak, should enable simultaneous temperature and pH measurements for high-resolution molecular imaging in vivo.

Comparison of metabolic and immunologic responses to transarterial chemoembolization with different chemoembolic regimens in a rabbit VX2 liver tumor model

OBJECTIVES

The goal of this study was to investigate the effects of TACE using Lipiodol, Oncozene™ drug-eluting embolics (DEEs), or LUMI™-DEEs alone, or combined with bicarbonate on the metabolic and immunological tumor microenvironment in a rabbit VX2 tumor model.

METHODS

VX2 liver tumor-bearing rabbits were assigned to five groups. MRI and extracellular pH (pHe) mapping using Biosensor Imaging of Redundant Deviation in Shifts (BIRDS) were performed before and after intra-arterial therapy with conventional TACE (cTACE), DEE-TACE with Idarubicin-eluting Oncozene™-DEEs, or Doxorubicin-eluting LUMI™-DEEs, each with or without prior bicarbonate infusion, and in untreated rabbits or treated with intra-arterial bicarbonate only. Imaging results were validated with immunohistochemistry (IHC) staining of cell viability (PCNA, TUNEL) and immune response (HLA-DR, CD3). Statistical analysis was performed using Mann-Whitney U test.

RESULTS

pHe mapping revealed that combining cTACE with prior bicarbonate infusion significantly increased tumor pHe compared to control (p = 0.0175) and cTACE alone (p = 0.0025). IHC staining revealed peritumoral accumulation of HLA-DR+ antigen-presenting cells and CD3 + T-lymphocytes in controls. cTACE-treated tumors showed reduced immune infiltration, which was restored through combination with bicarbonate. DEE-TACE with Oncozene™-DEEs induced moderate intratumoral and marked peritumoral infiltration, which was slightly reduced with bicarbonate. Addition of bicarbonate prior to LUMI™-beads enhanced peritumoral immune cell infiltration compared to LUMI™-beads alone and resulted in the strongest intratumoral immune cell infiltration across all treated groups.

CONCLUSIONS

The choice of chemoembolic regimen for TACE strongly affects post-treatment TME pHe and the ability of immune cells to accumulate and infiltrate the tumor tissue.

KEY POINTS

• Combining conventional transarterial chemotherapy with prior bicarbonate infusion increases the pHe towards a more physiological value (p = 0.0025). • Peritumoral infiltration and intratumoral accumulation patterns of antigen-presenting cells and T-lymphocytes after transarterial chemotherapy were dependent on the choice of the chemoembolic regimen. • Combination of intra-arterial treatment with Doxorubicin-eluting LUMI™-beads and bicarbonate infusion resulted in the strongest intratumoral presence of immune cells (positivity index of 0.47 for HLADR+-cells and 0.62 for CD3+-cells).

Extracellular pH Mapping as Therapeutic Readout of Drug Delivery in Glioblastoma

An acidic extracellular space is a hallmark of the tumor microenvironment. Acidosis has been postulated to promote the aggressive and invasive characteristics of tumors and also inhibit the therapeutic response, particularly in the context of novel immunotherapies. Therefore, methods to quantitatively measure the extracellular pH (pH_e) are needed. Here we describe a magnetic resonance spectroscopic imaging (MRSI) technique termed Biosensor Imaging of Redundant Deviation in Shifts (BIRDS), which uses the pH-dependent chemical shifts of nonexchangeable protons of lanthanide-based contrast agents to generate quantitative spatial pH_e maps. We assess this method in the context of evaluating the acidic pH_e and therapeutic response in glioblastoma in rodents, where normalization of the pH_e upon therapy can serve as a quantitative readout of successful drug delivery to the tumor.

Molecular Imaging of Tumor Microenvironment to Assess the Effects of Locoregional Treatment for Hepatocellular Carcinoma

Liver cancer is one of the leading causes of cancer deaths worldwide. Among all primary liver cancers, hepatocellular carcinoma (HCC) is the most common type, representing 75%‐85% of all primary liver cancer cases. Median survival following diagnosis of HCC is approximately 6 to 20 months due to late diagnosis in its course and few effective treatment options. Interventional therapy with minimal invasiveness is recognized as a promising treatment for HCC. However, due to the heterogeneity of HCC and the complexity of the tumor microenvironment, the long‐term efficacy of treatment for HCC remains a challenge in the clinic. Tumor microenvironment, including factors such as hypoxia, angiogenesis, low extracellular pH, interstitial fluid pressure, aerobic glycolysis, and various immune responses, has emerged as a key contributor to tumor residual and progression after locoregional treatment for HCC. New approaches to noninvasively assess the treatment response and assist in the clinical decision‐making process are therefore urgently needed. Molecular imaging tools enabling such an assessment may significantly advance clinical practice by allowing real‐time optimization of treatment protocols for the individual patient. This review discusses recent advances in the application of molecular imaging technologies for noninvasively assessing changes occurring in the microenvironment of HCC after locoregional treatment.

Imaging Hallmarks of the Tumor Microenvironment in Glioblastoma Progression

Glioblastoma progression involves multifaceted changes in vascularity, cellularity, and metabolism. Capturing such complexities of the tumor niche, from the tumor core to the periphery, by magnetic resonance imaging (MRI) and spectroscopic imaging (MRSI) methods has translational impact. In human-derived glioblastoma models (U87, U251) we made simultaneous and longitudinal measurements of tumor perfusion (F_p), permeability (K^trans), and volume fractions of extracellular (v_e) and blood (v_p) spaces from dynamic contrast enhanced (DCE) MRI, cellularity from apparent diffusion coefficient (ADC) MRI, and extracellular pH (pH_e) from an MRSI method called Biosensor Imaging of Redundant Deviation in Shifts (BIRDS). Spatiotemporal patterns of these parameters during tumorigenesis were unique for each tumor. While U87 tumors grew faster, F_p, K^trans, and v_p increased with tumor growth in both tumors but these trends were more pronounced for U251 tumors. Perfused regions between tumor periphery and core with U87 tumors exhibited higher F_p, but K^trans of U251 tumors remained lowest at the tumor margin, suggesting primitive vascularization. Tumor growth was uncorrelated with v_e, ADC, and pH_e. U87 tumors showed correlated regions of reduced v_e and lower ADC (higher cellularity), suggesting ongoing proliferation. U251 tumors revealed that the tumor core had higher v_e and elevated ADC (lower cellularity), suggesting necrosis development. The entire tumor was uniformly acidic (pH_e 6.1-6.8) early and throughout progression, but U251 tumors were more acidic, suggesting lower aerobic glycolysis in U87 tumors. Characterizing these cancer hallmarks with DCE-MRI, ADC-MRI, and BIRDS-MRSI will be useful for exploring tumorigenesis as well as timely therapies targeted to specific vascular and metabolic aspects of the tumor microenvironment.

Impact of Inhibition of the Mitochondrial Pyruvate Carrier on the Tumor Extracellular pH as Measured by CEST-MRI

(1) Background: The acidosis of the tumor micro-environment may have profound impact on cancer progression and on the efficacy of treatments. In the present study, we evaluated the impact of a treatment with UK-5099, a mitochondrial pyruvate carrier (MPC) inhibitor on tumor extracellular pH (pHe); (2) Methods: glucose consumption, lactate secretion and extracellular acidification rate (ECAR) were measured in vitro after exposure of cervix cancer SiHa cells and breast cancer 4T1 cells to UK-5099 (10 µM). Mice bearing the 4T1 tumor model were treated daily during four days with UK-5099 (3 mg/kg). The pHe was evaluated in vivo using either chemical exchange saturation transfer (CEST)-MRI with iopamidol as pHe reporter probe or ³¹P-NMR spectroscopy with 3-aminopropylphosphonate (3-APP). MR protocols were applied before and after 4 days of treatment; (3) Results: glucose consumption, lactate release and ECAR were increased in both cell lines after UK-5099 exposure. CEST-MRI showed a significant decrease in tumor pHe of 0.22 units in UK-5099-treated mice while there was no change over time for mice treated with the vehicle. Parametric images showed a large heterogeneity in response with 16% of voxels shifting to pHe values under 7.0. In contrast, ³¹P-NMR spectroscopy was unable to detect any significant variation in pHe; (4) Conclusions: MPC inhibition led to a moderate acidification of the extracellular medium in vivo. CEST-MRI provided high resolution parametric images (0.44 µL/voxel) of pHe highlighting the heterogeneity of response within the tumor when exposed to UK-5099.

Early detection of Alzheimer's disease using creatine chemical exchange saturation transfer magnetic resonance imaging

Detecting Alzheimer's disease (AD) at an early stage brings a lot of benefits including disease management and actions to slow the progression of the disease. Here, we demonstrate that reduced creatine chemical exchange saturation transfer (CrCEST) contrast has the potential to serve as a new biomarker for early detection of AD. The results on wild type (WT) mice and two age-matched AD models, namely tauopathy (Tau) and Aβ amyloidosis (APP), indicated that CrCEST contrasts of the cortex and corpus callosum in the APP and Tau mice were significantly reduced compared to WT counterpart at an early stage (6-7 months) (p < 0.011). Two main causes of the reduced CrCEST contrast, i.e. cerebral pH and creatine concentration, were investigated. From phantom and hypercapnia experiments, CrCEST showed excellent sensitivity to pH variations. From MRS results, the creatine concentration in WT and AD mouse brain was equivalent, which suggests that the reduced CrCEST contrast was dominated by cerebral pH change involved in the progression of AD. Immunohistochemical analysis revealed that the abnormal cerebral pH in AD mice may relate to neuroinflammation, a known factor that can cause pH reduction.

Imaging Extracellular Acidification and Immune Activation in Cancer

Metabolism reveals pathways by which cells, in healthy and disease tissues, use nutrients to fuel their function and (re)growth. However, gene-centric views have dominated cancer hallmarks, relegating metabolic reprogramming that all cells in the tumor niche undergo as an incidental phenomenon. Aerobic glycolysis in cancer is well known, but recent evidence suggests that diverse symbolic traits of cancer cells are derived from oncogene-directed metabolism required for their sustenance and evolution. Cells in the tumor milieu actively metabolize different nutrients, but proficiently secrete acidic by-products using diverse mechanisms to create a hostile ecosystem for host cells, and where local immune cells suffer collateral damage. Since metabolic interactions between cancer and immune cells hold promise for future cancer therapies, here we focus on translational magnetic resonance methods enabling in vivo and simultaneous detection of tumor habitat acidification and immune activation - innovations for monitoring personalized treatments.

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

Given the extraordinary nature of tumor metabolism in hepatocellular carcinoma and its impact on oncologic treatment response, this study introduces a novel high-throughput extracellular pH (pH_e ) mapping platform using magnetic resonance spectroscopic imaging in a three-dimensional (3D) in vitro model of liver cancer. pH_e mapping was performed using biosensor imaging of redundant deviation in shifts (BIRDS) on 9.4 T and 11.7 T MR scanners for validation purposes. 3D cultures of four liver cancer (HepG2, Huh7, SNU475, VX2) and one hepatocyte (THLE2) cell line were simultaneously analyzed (a) without treatment, (b) supplemented with 4.5 g/L d-glucose, and (c) treated with anti-glycolytic 3-bromopyruvate (6.25, 25, 50, 75, and 100 μM). The MR results were correlated with immunohistochemistry (GLUT-1, LAMP-2) and luminescence-based viability assays. Statistics included the unpaired t-test and ANOVA test. High-throughput pH_e imaging with BIRDS for in vitro 3D liver cancer models proved feasible. Compared with non-tumorous hepatocytes (pH_e = 7.1 ± 0.1), acidic pH_e was revealed in liver cancer (VX2, pH_e = 6.7 ± 0.1; HuH7, pH_e = 6.8 ± 0.1; HepG2, pH_e = 6.9 ± 0.1; SNU475, pH_e = 6.9 ± 0.1), in agreement with GLUT-1 upregulation. Glucose addition significantly further decreased pH_e in hyperglycolytic cell lines (VX2, HepG2, and Huh7, by 0.28, 0.06, and 0.11, respectively, all p < 0.001), whereas 3-bromopyruvate normalized tumor pH_e in a dose-dependent manner without affecting viability. In summary, this study introduces a non-invasive pH_e imaging platform for high-yield screening using a translational 3D liver cancer model, which may help reveal and target mechanisms of therapy resistance and inform personalized treatment of patients with hepatocellular carcinoma.

Quasi-steady state chemical exchange saturation transfer (QUASS CEST) analysis-correction of the finite relaxation delay and saturation time for robust CEST measurement

PURPOSE

CEST provides a MR contrast mechanism sensitizing to the exchange between dilute labile and bulk water protons. However, the CEST effect depends on the RF saturation duration and relaxation delay, which need to be long to reach its steady state. Our study aims to estimate the QUAsi-Steady State (QUASS) CEST signal from experiments with shorter saturation and relaxation delay times.

METHODS

The evolution of the CEST signal was modeled as a function of the bulk water longitudinal relaxation rate during the relaxation delay (Td) and spin-lock relaxation rate during the RF saturation (Ts), from which the QUASS CEST effect is solved. Numeric simulations were programmed to compare the apparent CEST and QUASS CEST effects as a function of Ts and Td times. We also performed CEST MRI experiments from a creatine-gel pH phantom under serially varied Ts and Td times.

RESULTS

The numeric simulation showed that although the apparent CEST effect depends on Td and Ts, the QUASS CEST solution has little dependence. Phantom results showed that the routine CEST pH contrast could be described by a nonlinear regression model (ie, Δ C E S T R = Δ C E S T R eq app 1 - e - R 1 ρ app · t ). We had Δ C E S T R eq app = 3.90 ± 0.03 % (P < 5e-8) and R 1 ρ app = 0.62 ± 0.02 s - 1 (P < 5e-6). For the QUASS CEST analysis, we modeled the pH contrast as Δ C E S T R = Δ C E S T R eq QUASS + s · t , using a linear regression model. We had Δ C E S T R eq QUASS = 3.63 ± 0.01 % (P < 5e-9) and s = - 0.02 ± 0.00 % / s (P < 0.01), the slope of which is minimal.

CONCLUSIONS

The QUASS CEST algorithm provides a post-processing solution that facilitates robust CEST measurement.

Brain pH Imaging and its Applications

Acid-base homeostasis and pH regulation are critical for normal tissue metabolism and physiology, and brain tissue pH alters in many diseased states. Several noninvasive tissue pH Magnetic Resonance (MR) techniques have been developed over the past few decades to shed light on pH change during tissue function and dysfunction. Nevertheless, there are still challenges for mapping brain pH noninvasively at high spatiotemporal resolution. To address this unmet biomedical need, chemical exchange saturation transfer (CEST) MR techniques have been developed as a sensitive means for non-invasive pH mapping. This article briefly reviews the basic principles of different pH measurement techniques with a focus on CEST imaging of pH. Emerging pH imaging applications in the tumor are provided as examples throughout the narrative, and CEST pH imaging in acute stroke is discussed in the final section.

Glucometabolic Reprogramming in the Hepatocellular Carcinoma Microenvironment: Cause and Effect

Hepatocellular carcinoma (HCC) is a tumor that exhibits glucometabolic reprogramming, with a high incidence and poor prognosis. Usually, HCC is not discovered until an advanced stage. Sorafenib is almost the only drug that is effective at treating advanced HCC, and promising metabolism-related therapeutic targets of HCC are urgently needed. The “Warburg effect” illustrates that tumor cells tend to choose aerobic glycolysis over oxidative phosphorylation (OXPHOS), which is closely related to the features of the tumor microenvironment (TME). The HCC microenvironment consists of hypoxia, acidosis and immune suppression, and contributes to tumor glycolysis. In turn, the glycolysis of the tumor aggravates hypoxia, acidosis and immune suppression, and leads to tumor proliferation, angiogenesis, epithelial–mesenchymal transition (EMT), invasion and metastasis. In 2017, a mechanism underlying the effects of gluconeogenesis on inhibiting glycolysis and blockading HCC progression was proposed. Treating HCC by increasing gluconeogenesis has attracted increasing attention from scientists, but few articles have summarized it. In this review, we discuss the mechanisms associated with the TME, glycolysis and gluconeogenesis and the current treatments for HCC. We believe that a treatment combination of sorafenib with TME improvement and/or anti-Warburg therapies will set the trend of advanced HCC therapy in the future.

Idarubicin-Loaded ONCOZENE Drug-Eluting Bead Chemoembolization in a Rabbit Liver Tumor Model: Investigating Safety, Therapeutic Efficacy, and Effects on Tumor Microenvironment

PURPOSE

To investigate toxicity, efficacy, and microenvironmental effects of idarubicin-loaded 40-μm and 100-μm drug-eluting embolic (DEE) transarterial chemoembolization in a rabbit liver tumor model.

MATERIALS AND METHODS

Twelve male New Zealand White rabbits with orthotopically implanted VX2 liver tumors were assigned to DEE chemoembolization with 40-μm (n = 5) or 100-μm (n = 4) ONCOZENE microspheres or no treatment (control; n = 3). At 24-72 hours postprocedurally, multiparametric magnetic resonance (MR) imaging including dynamic contrast-enhanced (DCE), diffusion-weighted imaging (DWI), and biosensor imaging of redundant deviation in shifts (BIRDS) was performed to assess extracellular pH (pHe), followed by immediate euthanasia. Laboratory parameters and histopathologic ex vivo analysis included fluorescence confocal microscopy and immunohistochemistry.

RESULTS

DCE MR imaging demonstrated a similar degree of devascularization of embolized tumors for both microsphere sizes (mean arterial enhancement, 8% ± 12 vs 36% ± 51 in controls; P = .07). Similarly, DWI showed postprocedural increases in diffusion across the entire lesion (apparent diffusion coefficient, 1.89 × 10^-3 mm²/s ± 0.18 vs 2.34 × 10^-3 mm²/s ± 0.18 in liver; P = .002). BIRDS demonstrated profound tumor acidosis at baseline (mean pHe, 6.79 ± 0.08 in tumor vs 7.13 ± 0.08 in liver; P = .02) and after chemoembolization (6.8 ± 0.06 in tumor vs 7.1 ± 0.04 in liver; P = .007). Laboratory and ex vivo analyses showed central tumor core penetration and greater increase in liver enzymes for 40-μm vs 100-μm microspheres. Inhibition of cell proliferation, intratumoral hypoxia, and limited idarubicin elution were equally observed with both sphere sizes.

CONCLUSIONS

Noninvasive multiparametric MR imaging visualized chemoembolic effects in tumor and tumor microenvironment following DEE chemoembolization. Devascularization, increased hypoxia, coagulative necrosis, tumor acidosis, and limited idarubicin elution suggest ischemia as the predominant therapeutic mechanism. Substantial size-dependent differences indicate greater toxicity with the smaller microsphere diameter.

Molecular MRI of the Immuno-Metabolic Interplay in a Rabbit Liver Tumor Model: A Biomarker for Resistance Mechanisms in Tumor-targeted Therapy?

Background The immuno-metabolic interplay has gained interest for determining and targeting immunosuppressive tumor micro-environments that remain a barrier to current immuno-oncologic therapies in hepatocellular carcinoma. Purpose To develop molecular MRI tools to reveal resistance mechanisms to immuno-oncologic therapies caused by the immuno-metabolic interplay in a translational liver cancer model. Materials and Methods A total of 21 VX2 liver tumor-bearing New Zealand white rabbits were used between October 2018 and February 2020. Rabbits were divided into three groups. Group A (n = 3) underwent intra-arterial infusion of gadolinium 160 (¹⁶⁰Gd)-labeled anti-human leukocyte antigen-DR isotope (HLA-DR) antibodies to detect antigen-presenting immune cells. Group B (n = 3) received rhodamine-conjugated superparamagnetic iron oxide nanoparticles (SPIONs) intravenously to detect macrophages. These six rabbits underwent 3-T MRI, including T1- and T2-weighted imaging, before and 24 hours after contrast material administration. Group C (n = 15) underwent extracellular pH mapping with use of MR spectroscopy. Of those 15 rabbits, six underwent conventional transarterial chemoembolization (TACE), four underwent conventional TACE with extracellular pH-buffering bicarbonate, and five served as untreated controls. MRI signal intensity distribution was validated by using immunohistochemistry staining of HLA-DR and CD11b, Prussian blue iron staining, fluorescence microscopy of rhodamine, and imaging mass cytometry (IMC) of gadolinium. Statistical analysis included Mann-Whitney U and Kruskal-Wallis tests. Results T1-weighted MRI with ¹⁶⁰Gd-labeled antibodies revealed localized peritumoral ring enhancement, which corresponded to gadolinium distribution detected with IMC. T2-weighted MRI with SPIONs showed curvilinear signal intensity representing selective peritumoral deposition in macrophages. Extracellular pH-specific MR spectroscopy of untreated liver tumors showed acidosis (mean extracellular pH, 6.78 ± 0.09) compared with liver parenchyma (mean extracellular pH, 7.18 ± 0.03) (P = .008) and peritumoral immune cell exclusion. Normalization of tumor extracellular pH (mean, 6.96 ± 0.05; P = .02) using bicarbonate during TACE increased peri- and intratumoral immune cell infiltration (P = .002). Conclusion MRI in a rabbit liver tumor model was used to visualize resistance mechanisms mediated by the immuno-metabolic interplay that inform susceptibility and response to immuno-oncologic therapies, providing a therapeutic strategy to restore immune permissiveness in liver cancer. © RSNA, 2020 Online supplemental material is available for this article.

Molecular Imaging of Extracellular Tumor pH to Reveal Effects of Locoregional Therapy on Liver Cancer Microenvironment

PURPOSE

To establish magnetic resonance (MR)-based molecular imaging paradigms for the noninvasive monitoring of extracellular pH (pH_e) as a functional surrogate biomarker for metabolic changes induced by locoregional therapy of liver cancer.

EXPERIMENTAL DESIGN

Thirty-two VX2 tumor-bearing New Zealand white rabbits underwent longitudinal imaging on clinical 3T-MRI and CT scanners before and up to 2 weeks after complete conventional transarterial chemoembolization (cTACE) using ethiodized oil (lipiodol) and doxorubicin. MR-spectroscopic imaging (MRSI) was employed for pH_e mapping. Multiparametric MRI and CT were performed to quantify tumor enhancement, diffusion, and lipiodol coverage of the tumor posttherapy. In addition, incomplete cTACE with reduced chemoembolic doses was applied to mimic undertreatment and exploit pH_e mapping to detect viable tumor residuals. Imaging findings were correlated with histopathologic markers indicative of metabolic state (HIF-1α, GLUT-1, and LAMP-2) and viability (proliferating cell nuclear antigen and terminal deoxynucleotidyl-transferase dUTP nick-end labeling).

RESULTS

Untreated VX2 tumors demonstrated a significantly lower pH_e (6.80 ± 0.09) than liver parenchyma (7.19 ± 0.03, P < 0.001). Upregulation of HIF-1α, GLUT-1, and LAMP-2 confirmed a hyperglycolytic tumor phenotype and acidosis. A gradual tumor pH_e increase toward normalization similar to parenchyma was revealed within 2 weeks after complete cTACE, which correlated with decreasing detectability of metabolic markers. In contrast, pH_e mapping after incomplete cTACE indicated both acidic viable residuals and increased tumor pH_e of treated regions. Multimodal imaging revealed durable tumor devascularization immediately after complete cTACE, gradually increasing necrosis, and sustained lipiodol coverage of the tumor.

CONCLUSIONS

MRSI-based pH_e mapping can serve as a longitudinal monitoring tool for viable tumors. As most liver tumors are hyperglycolytic creating microenvironmental acidosis, therapy-induced normalization of tumor pH_e may be used as a functional biomarker for positive therapeutic outcome.