Lionel Mignion

Hypoxia imaging with the nitroimidazole 18F-FAZA PET tracer: A comparison with Oxylite, EPR Oximetry and 19F-MRI Relaxometry

BACKGROUND AND PURPOSE (18)F-FAZA is a nitroimidazole PET tracer that can provide images of tumor hypoxia. However, it cannot provide absolute pO(2) values. To qualify (18)F-FAZA PET, we compared PET images to pO(2) measured by OxyLite, EPR oximetry and (19)F-MRI. MATERIALS AND METHODS Male WAG/Rij rats grafted with rhabdomyosarcoma were used. Tumor oxygenation was modified by gas breathing (air or carbogen). The same day of PET acquisition, the pO(2) was measured in the same tumor either by OxyLite probes (measurement at 10 different sites), EPR oximetry using low frequency EPR or (19)F-relaxometry using 15C5 on an 11.7T MR system. RESULTS There was a good correlation between the results obtained by PET and EPR (R = 0.93). In the case of OxyLite, although a weaker correlation was observed (R = 0.55), the trend for two values to agree was still related to the inverse function theoretically predicted. For the comparison of (18)F-FAZA PET and (19)F-MRI, no change in T(1) was observed. CONCLUSIONS A clear correlation between (18)F-FAZA PET image intensities and tumor oxygenation was demonstrated, suggesting that (18)F-FAZA PET is a promising imaging technique to guide cancer therapy.

Hexafluorobenzene in comparison with perfluoro-15-crown-5-ether for repeated monitoring of oxygenation using19F MRI in a mouse model

Hexafluorobenzene (HFB) and perfluoro‐15‐crown‐5‐ether (15C5) were compared as fluorine reporter probes of tissue oxygenation using 19F MRI for dynamic assessment of muscle oxygenation, with special focus on muscle tissue toxicity of the probes, and consecutive alteration of animal behavior. The latter were also compared in terms of sensitivity to changes in oxygenation as well as of signal‐to‐noise ratio for accurate pO2 measurements. For that purpose, mouse muscles were imaged at 11.7 T, at 2‐ and 36‐h after intramuscular injection of HFB or 15C5. Histological analysis of the muscle tissue revealed a lack of toxicity for 15C5 from 2 up to 36‐h postinjection, whereas HFB induced tissue necrosis, blood clots and thrombosis as soon as 24‐h postinjection. This muscle toxicity led to a limitation in mice mobility 24‐h after injection of HFB as evidenced by behavioral testing (open‐field, grip strength, and catwalk tests), which was not the case after 15C5 intramuscular injection. Finally, pO2 measurements assessed 2‐h postinjection showed consistent values with both probes, evidencing cross‐validation of the 19F MRI oximetry technique for acute measurements. However, the measurement at 36‐h was hampered for HFB, which showed significant lower values of muscle pO2, whereas 15C5 was able to reliably assess muscle pO2 at 36‐h postinjection. Magn Reson Med, 2013.

Mapping of oxygen by imaging lipids relaxation enhancement: A potential sensitive endogenous MRI contrast to map variations in tissue oxygenation

Because of its paramagnetic properties, oxygen may act as an endogenous magnetic resonance imaging contrast agent by changing proton relaxation rates. Changes in tissue oxygen concentrations have been shown to produce changes in relaxation rate R1 of water. The aim of the study was to improve the sensitivity of oxygen enhanced R1 imaging by exploiting the higher solubility of oxygen in lipids (as compared with water) to sensitively monitor changes in tissue oxygen levels by selectively measuring the R1 of lipids.

Monitoring Chemotherapeutic Response by Hyperpolarized 13C-Fumarate MRS and Diffusion MRI

Targeted chemotherapeutic agents often do not result in tumor shrinkage, so new biomarkers that correlate with clinical efficacy are needed. In this study, we investigated noninvasive imaging protocols to monitor responses to sorafenib, a multikinase inhibitor approved for treatment of renal cell and hepatocellular carcinoma. Healthy cells are impermeable to fumarate, so conversion of this metabolite to malate as detected by (13)C-magnetic resonance spectroscopy (MRS) has been suggested as one marker for cell death and treatment response in tumors. Diffusion MRI also has been suggested as a measure of therapy-induced cytotoxic edema because viable cells act as a diffusion barrier in tissue. For these reasons, we assessed sorafenib responses using hyperpolarized (13)C-fumarate, diffusion-weighted MRI (DW-MRI) in a xenograft model of human breast cancer in which daily administration of sorafenib was sufficient to stabilize tumor growth. We detected signals from fumarate and malate following intravenous administration of hyperpolarized fumarate with a progressive increase in the malate-to-fumarate (MA/FA) ratio at days 2 to 5 after sorafenib infusion. The apparent diffusion coefficient (ADC) measured by DW-MRI increased in the treated group consistent with cytotoxic edema. However, the MA/FA ratio was a more sensitive marker of therapeutic response than ADC, with 2.8-fold versus 1.3-fold changes, respectively, by day 5 of drug treatment. Histologic analyses confirmed cell death in the sorafenib-treated cohort. Notably, (13)C-pyruvate-to-lactate conversion was not affected by sorafenib in the breast cancer model examined. Our results illustrate how combining hyperpolarized substrates with DW-MRI can allow noninvasive monitoring of targeted therapeutic responses at relatively early times after drug administration.

Tumor reoxygenation following administration of Mitogen-Activated Protein Kinase inhibitors: A rationale for combination with radiation therapy

BACKGROUND AND PURPOSE The relevance of Mitogen Activated Protein Kinase (MAPK) inhibitors as co-treatments for radiation therapy is investigated, with special focus on a potential link between the MAPK pathway and tumor hypoxia, which is a critical determinant for response to therapy. MATERIALS AND METHODS The effects of two MAPK inhibitors, Sorafenib and PD0325901, were monitored daily using in vivo EPR (Electron Paramagnetic Resonance) oximetry in FSaII and TLT tumor models in order to identify a window of reoxygenation, during which tumor blood flow, oxygen consumption and radiation sensitivity were assessed. RESULTS Reoxygenation was shown after two days of treatments with Sorafenib or PD0325901 in two tumor models, which was further successfully exploited with Sorafenib for improving the radiation response of FSaII tumors by a factor of 1.5. The increase in tumor oxygenation was shown to be the result of two major factors: (i) an increase in blood flow for Sorafenib, that might be linked to its anti-angiogenic effect (vascular normalization), and (ii) a decrease in oxygen consumption for Sorafenib and PD0325901, due to an alteration of the mitochondrial activity. CONCLUSION We evidenced tumor reoxygenation in vivo following MAPK inhibition and suggest a rationale for the combination of radiation therapy with Sorafenib.

Multimodal imaging of tumor response to sorafenib combined with radiation therapy: comparison between diffusion-weighted MRI, choline spectroscopy and 18F-FLT PET imaging

The purpose of this study was to determine the value of different imaging modalities, that is, magnetic resonance imaging/spectroscopy (MRI/MRS) and positron emission tomography (PET), to assess early tumor response to sorafenib with or without radiotherapy. Diffusion-weighted (DW)-MRI, choline (1)H MRS at 11.7 T, and (18)F-FLT PET imaging were used to image fibrosarcoma (FSaII) tumor-bearing mice over time. The imaging markers were compared with apoptosis cell death and cell proliferation measurements assessed by histology. Anti-proliferative effects of sorafenib were evidenced by (1)H MRS and (18)F-FLT PET after 2 days of treatment with sorafenib, with no additional effect of the combination with radiation therapy, results that are in agreement with Ki67 staining. Apparent diffusion coefficient calculated using DW-MRI was not modified after 2 days of treatment with sorafenib, but showed significant increase 24 h after 2 days of sorafenib treatment combined with consecutive irradiation. The three imaging markers were able to show early tumor response as soon as 24 h after treatment initiation, with choline MRS and (18)F-FLT being sensitive to sorafenib in monotherapy as well as in combined therapy with irradiation, whereas DW-MRI was only sensitive to the combination of sorafenib with radiotherapy.

Application of MOBILE (Mapping of Oxygen By Imaging Lipids relaxation Enhancement) to Study Variations in Tumor Oxygenation

The aim of the study was to sensitively monitor changes in tumor oxygen using the MOBILE (mapping of oxygen by imaging lipids relaxation enhancement) technique. This method was applied in mammary tumor mouse models on an 11.7T Bruker MRI system. MOBILE was compared with functional imaging R2*, R1 of water and with pO2 measurements (using EPR oximetry and O2-dependent fluorescence quenching measurements). MOBILE was shown to be capable to monitor changes in oxygenation in tumor tissues.

Non-invasive in vivo imaging of early metabolic tumor response to therapies targeting choline metabolism.

The cholinic phenotype, characterized by elevated phosphocholine and a high production of total‐choline (tCho)‐containing metabolites, is a metabolic hallmark of cancer. It can be exploited for targeted therapy. Non‐invasive imaging biomarkers are required to evaluate an individuals response to targeted anticancer agents that usually do not rapidly cause tumor shrinkage. Because metabolic changes can manifest at earlier stages of therapy than changes in tumor size, the aim of the current study was to evaluate 1H‐MRS and diffusion‐weighted MRI (DW‐MRI) as markers of tumor response to the modulation of the choline pathway in mammary tumor xenografts. Inhibition of choline kinase activity was achieved with the direct pharmacological inhibitor H‐89, indirect inhibitor sorafenib and down‐regulation of choline‐kinase α (ChKA) expression using specific short‐hairpin RNA (shRNA). While all three strategies significantly decreased tCho tumor content in vivo, only sorafenib and anti‐ChKA shRNA significantly repressed tumor growth. The increase of apparent‐diffusion‐coefficient of water (ADCw) measured by DW‐MRI, was predictive of the induced necrosis and inhibition of the tumor growth in sorafenib treated mice, while the absence of change in ADC values in H89 treated mice predicted the absence of effect in terms of tumor necrosis and tumor growth. In conclusion, 1H‐choline spectroscopy can be useful as a pharmacodynamic biomarker for choline targeted agents, while DW‐MRI can be used as an early marker of effective tumor response to choline targeted therapies. DW‐MRI combined to choline spectroscopy may provide a useful non‐invasive marker for the early clinical assessment of tumor response to therapies targeting choline signaling.

Interruption of lactate uptake by inhibiting mitochondrial pyruvate transport unravels direct antitumor and radiosensitizing effects

Lactate exchange between glycolytic and oxidative cancer cells is proposed to optimize tumor growth. Blocking lactate uptake through monocarboxylate transporter 1 (MCT1) represents an attractive therapeutic strategy but may stimulate glucose consumption by oxidative cancer cells. We report here that inhibition of mitochondrial pyruvate carrier (MPC) activity fulfils the tasks of blocking lactate use while preventing glucose oxidative metabolism. Using in vitro 13C-glucose and in vivo hyperpolarized 13C-pyruvate, we identify 7ACC2 as a potent inhibitor of mitochondrial pyruvate transport which consecutively blocks extracellular lactate uptake by promoting intracellular pyruvate accumulation. Also, while in spheroids MCT1 inhibition leads to cytostatic effects, MPC activity inhibition induces cytotoxic effects together with glycolysis stimulation and uncompensated inhibition of mitochondrial respiration. Hypoxia reduction obtained with 7ACC2 is further shown to sensitize tumor xenografts to radiotherapy. This study positions MPC as a control point for lactate metabolism and expands on the anticancer potential of MPC inhibition.Tumor cells can fuel their metabolism with lactate. Here the authors show that inhibition of mitochondrial pyruvate carrier (MPC) blocks extracellular lactate uptake by promoting intracellular pyruvate accumulation and inhibits oxidative metabolism, ultimately resulting in cytotoxicity and radiosensitization.

Influence of paramagnetic melanin on the MRI contrast in melanoma: a combined high-field (11.7 T) MRI and EPR study.

Melanoma is the most dangerous form of skin cancer and its incidence is rising each year. Because the current methods of diagnosis based on the visual aspect of the tumor show limitations, several new techniques are emerging to help in this diagnosis, amongst which are magnetic resonance imaging (MRI) and electron paramagnetic resonance (EPR). The origin of the typical contrast pattern observable in melanoma in T1 - and T2 -weighted images remains to be elucidated and is a source of controversy. In addition, melanin could create sufficient magnetic inhomogeneities to allow its visualization on T2 *-weighted images using high-field MRI. In order to elucidate the possible role of melanin in the MRI contrast of melanoma, the present study was designed to correlate the paramagnetic content in melanin pigment to the contrast on T1 -, T2 - and T2 *-weighted images. MR images were obtained in vivo at 11.7 T using four types of experimental tumors with different pigmentations (B16, HBL, LND1 melanomas and KHT sarcomas). The paramagnetic content in melanin pigment was measured by EPR. No significant correlation was observed between the content in melanin and the relaxation times T1 , T2 and T2 *, emphasizing that the presence of pigment alone has negligible effect on the MRI contrast.