Combined PET/MRI: Global Warming-Summary Report of the 6th International Workshop on PET/MRI, March 27-29, 2017, Tübingen, Germany

The 6th annual meeting to address key issues in positron emission tomography (PET)/magnetic resonance imaging (MRI) was held again in Tübingen, Germany, from March 27 to 29, 2017. Over three days of invited plenary lectures, round table discussions and dialogue board deliberations, participants critically assessed the current state of PET/MRI, both clinically and as a research tool, and attempted to chart future directions. The meeting addressed the use of PET/MRI and workflows in oncology, neurosciences, infection, inflammation and chronic pain syndromes, as well as deeper discussions about how best to characterise the tumour microenvironment, optimise the complementary information available from PET and MRI, and how advanced data mining and bioinformatics, as well as information from liquid biomarkers (circulating tumour cells and nucleic acids) and pathology, can be integrated to give a more complete characterisation of disease phenotype. Some issues that have dominated previous meetings, such as the accuracy of MR-based attenuation correction (AC) of the PET scan, were finally put to rest as having been adequately addressed for the majority of clinical situations. Likewise, the ability to standardise PET systems for use in multicentre trials was confirmed, thus removing a perceived barrier to larger clinical imaging trials. The meeting openly questioned whether PET/MRI should, in all cases, be used as a whole-body imaging modality or whether in many circumstances it would best be employed to give an in-depth study of previously identified disease in a single organ or region. The meeting concluded that there is still much work to be done in the integration of data from different fields and in developing a common language for all stakeholders involved. In addition, the participants advocated joint training and education for individuals who engage in routine PET/MRI. It was agreed that PET/MRI can enhance our understanding of normal and disrupted biology, and we are in a position to describe the in vivo nature of disease processes, metabolism, evolution of cancer and the monitoring of response to pharmacological interventions and therapies. As such, PET/MRI is a key to advancing medicine and patient care.

Silencing of the glycerophosphocholine phosphodiesterase GDPD5 alters the phospholipid metabolite profile in a breast cancer model in vivo as monitored by (31) P MRS

Abnormal choline phospholipid metabolism is an emerging hallmark of cancer, which is implicated in carcinogenesis and tumor progression. The malignant metabolic phenotype is characterized by high levels of phosphocholine (PC) and relatively low levels of glycerophosphocholine (GPC) in aggressive breast cancer cells. Phosphorus ((31) P) MRS is able to non-invasively detect these water-soluble metabolites of choline as well as ethanolamine phospholipid metabolism. Here we have investigated the effects of stably silencing glycerophosphoester diesterase domain containing 5 (GDPD5), which is an enzyme with glycerophosphocholine phosphodiesterase activity, in MDA-MB-231 breast cancer cells and orthotopic tumor xenografts. Tumors in which GDPD5 was stably silenced with GDPD5-specific shRNA contained increased levels of GPC and phosphoethanolamine (PE) compared with control tumors.

Abstract PD05-10: HOXB7 as a Key Regulator in the Development of Tamoxifen Resistance

Multiple factors including long term treatment with tamoxifen are involved in the development of selective estrogen receptor modulator resistance of ERα positive breast cancer. It is well known in the endocrinology field that increased levels of ErbB/HER family members can also directly alter the cellular response to tamoxifen, but the mechanisms underlying increased expression of ErbB/HERs are not clear. In this report we show that HOXB7 is an ERα responsive gene and the downregulation of HOXB7 expression by estradiol is abrogated by tamoxifen in MCF7 cells. HOXB7 overexpression renders MCF-7 cells resistant to tamoxifen via cross-talk between receptor tyrosine kinases and ERα signaling. EGFR and HER2 expression is upregulated by overexpression of HOXB7, while knockdown of HOXB7 with siRNA in tamoxifen-resistant cell lines causes decrease of EGFR and HER2 expression and loss of tamoxifen resistance. In addition, our work demonstrates that to mediate upregulation of EGFR and HER2, HOXB7 binds to ER-alpha and promotes HER2 transcription activity through direct binding to the HER2 enhancer region by competing with other cofactors. Also, HOXB7 enhances EGFR transcription activity via binding to EGFR promoter region. However, depletion of HOXB7 affects the binding affinity of HOXB7 and the cofactors to the HER2 enhancer element. Higher expression levels of HOXB7 significantly correlated with poorer disease free survival in ERα positive breast cancer patients on adjuvant tamoxifen monotherapy. These findings implicate overexpression of HOXB7 as a key event in the initiation and maintenance of tamoxifen resistance. These studies suggest that HOXB7 acts as a key regulator orchestrating two major groups of target molecules in the oncogene heirarchy. Functional antagonism of HOXB7 could circumvent tamoxifen resistance.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD05-10.

Molecular and functional imaging of breast cancer

Despite several major advances in breast cancer diagnosis and treatment, the American Cancer Society has estimated that in the US alone 43300 women and 400 men will die from breast cancer in 2007. Breast cancer typically is a multi-focal, multi-faceted disease, with the major cause of mortality being complications due to metastasis. Whereas a decade ago genetic alterations were the primary focus in cancer research, it is now apparent that the physiological tumor microenvironment, interactions between cancer cells and stromal cells such as endothelial cells, fibroblasts and macrophages, the extracellular matrix, and a multitude of secreted factors and cytokines influence progression, aggressiveness, and response of the disease to treatment. Prevention, early diagnosis, and treatment are the three broad challenges for MR molecular and functional imaging in reducing mortality from this disease. Multi-parametric molecular and functional MRI provides unprecedented opportunities for identifying novel targets for imaging and therapy at the bench, as well as for accurate diagnosis and monitoring response to therapy at the bedside. Here we provide an overview of the current status of molecular and functional MRI of breast cancer, outlining some key developments, as well as identifying some of the important challenges facing this field in the future.

The physiological environment in cancer vascularization, invasion and metastasis

One of the most lethal aspects of cancer arises from its ability to invade and metastasize. Determining the factors that promote cancer cell invasion and metastasis is therefore critically important in treating this disease. The tumour physiological environment is uniquely different from normal tissue, and exhibits hypoxia, acidic extracellular pH and high levels of lactate. This environment, dictated largely by abnormal tumour vasculature and metabolism, in turn also promotes angiogenesis. The physiological environment, tumour metabolism, angiogenesis and vascularization are therefore inextricably linked. We have developed and applied non-invasive magnetic resonance (MR) imaging (I) and spectroscopy (S) techniques to understand the role of vascular, physiological and metabolic properties in cancer invasion and metastasis. These MR studies are performed with human breast and prostate cancer cells maintained in culture or grown as solid tumours in immune-suppressed mice. We have detected significant differences in vascular, physiological and metabolic characteristics of metastatic and non-metastatic human breast and prostate cancer models with MRI and MRS. Using a combined MRI/MRS approach we are currently acquiring metabolic, extracellular pH and vascular images from the same localized regions within a solid tumour to further understand the dynamics between these parameters and their role in cancer invasion and metastasis.

Combined vascular and extracellular pH imaging of solid tumors

The unique physiological environment of solid tumors, frequently characterized by areas of poor flow, hypoxia, high lactate and low extracellular pH (pHe), influences vascularization, invasion and metastasis. Thus, vascularization and the physiological and metabolic environment play permissive (and conversely preventive) roles in invasion and metastasis. By using a multi-parametric approach of combined vascular and spectroscopic imaging, we can begin to evaluate which combinations of vascular, metabolic and physiological regions in a solid tumor represent the highest 'metastatic threat'. Here, we present measurements of pHe, vascular volume and permeability from co-localized regions within a solid tumor. These studies were performed for a group of metastatic (MDA-MB-231) and non-metastatic (MCF-7) human breast cancer xenografts. In this study, we have demonstrated the feasibility of such an approach, and presented methods of analyses to detect differences in patterns of combined parameters obtained from spatially co-registered regions in a solid tumor.

Real-time measurements of cellular oxygen consumption, pH, and energy metabolism using nuclear magnetic resonance spectroscopy

Changes in molecular expression or apoptotic behavior, induced by malignant transformation or anticancer treatment, are frequently reflected in cellular metabolism and oxygen consumption. A technique to monitor oxygen consumption, cell physiology, and metabolism noninvasively would provide a better understanding of interactions between molecular changes and metabolism in malignant transformation and following cancer treatment. Such a system was developed in this study by adapting multinuclear MRI and spectroscopic techniques to an isolated cell perfusion system. The system was evaluated by studying the effects of two agents, carbonyl cyanide m-chlorophenylhydrazone (CCCP) which is an uncoupler of oxidative phosphorylation, and antimycin, an inhibitor of oxidative phosphorylation, on the oxygen consumption and metabolism of MCF-7 and MatLyLu cancer cell lines.

Detection of increased choline compounds with proton nuclear magnetic resonance spectroscopy subsequent to malignant transformation of human prostatic epithelial cells

In this study, a panel of normal human prostate cells (HPCs) and tumor cells derived from metastases were studied by (1)H NMR spectroscopy to determine whether the malignant transformation of HPCs results in the elevation of choline compounds. Although an elevated choline signal has been observed previously in clinical studies, the contribution of the different Cho compounds to this elevation, as well as their quantification, has not been established until now. Here we have shown that HPCs derived from metastases exhibit significantly higher phosphocholine as well as glycerophosphocholine levels compared with normal prostate epithelial and stromal cells. Thus the elevation of the choline peak observed clinically in prostate cancer is attributable to an alteration of phospholipid metabolism and not simply to increased cell density, doubling time, or other nonspecific effects. Androgen deprivation of the androgen receptor-positive cell lines resulted in a significant increase of choline compounds after chronic androgen deprivation of the LNCaP cell line and in a decrease of choline compounds after a more acute androgen deprivation of the LAPC-4 cell line. These data strongly support the use of proton magnetic resonance spectroscopic imaging to detect the presence of prostate cancer for diagnosis, to detect response subsequent to androgen ablation therapy, and to detect recurrence.

Magnetic resonance pharmacoangiography to detect and predict chemotherapy delivery to solid tumors

Detection and prediction of drug delivery to the tumor interstitium are of critical importance in cancer chemotherapy. Prediction of drug delivery derived from standard pharmacokinetic models is frequently inadequate because of the complex nature of tumor blood flow and the microenvironment. Although drug concentrations can be directly sampled with microdialysis or in biopsy samples, we currently lack methods capable of detecting and/or predicting drug delivery to tumors noninvasively. In this study, we describe a novel magnetic resonance (MR) technique to directly detect the drug, and we present the correlation between delivery of drug and the delivery of MR contrast agents to the tumor. Experiments were performed with tumor xenografts in severe combined immunodeficient mice. Three-dimensional maps of the drug distribution within the tumors were obtained with 13C spectroscopic MR imaging with a spatial resolution of 2 x 2 x 2 mm, using signals of the 13C-labeled anticancer agent phenylacetate. Three-dimensional maps of uptake of gadolinium-diethylenetriaminepentaacetic acid (GdDTPA) contrast agent were obtained for the same tumors using dynamic MR imaging. Experimental data were analyzed for correlation between delivery of the drug and the contrast. Histological analysis was performed for excised tumors. Experimental data demonstrated a significant spatial correlation (r = 0.59 with P < 0.001) between the parameter representing delivery of the contrast to tumor interstitium, determined from the kinetic curves of GdDTPA, and integral tissue drug concentrations for two different tumor models. The method is designed to probe extravasation of the drug molecules from the bloodstream into the tumor interstitium. Although therapeutic efficiency of the drug will also depend upon drug retention in the tumor and the ability of the molecules to cross cellular membranes, inefficient drug transfer from plasma to tissue can be a major impediment in achieving effective tumor chemotherapy. The results of this study demonstrate that the uptake kinetics of a low molecular weight MR contrast agent can be used to predict delivery of drug molecules of similar size to the interstitium of solid tumors.

Vascular differences detected by MRI for metastatic versus nonmetastatic breast and prostate cancer xenografts

Several studies have linked vascular density, identified in histologic sections, to "metastatic risk." Functional information of the vasculature, not readily available from histologic sections, can be obtained with contrast-enhanced MRI to exploit for therapy or metastasis prevention. Our aims were to determine if human breast and prostate cancer xenografts preselected for differences in invasive and metastatic characteristics established correspondingly different vascular volume and permeability, quantified here with noninvasive MRI of the intravascular contrast agent albumin-GdDTPA. Tumor vascular volume and permeability of human breast and prostate cancer xenografts were characterized using MRI. Parallel studies confirmed the invasive behavior of these cell lines. Vascular endothelial growth factor (VEGF) expression in the cell lines was measured using ELISA and Western blots. Metastasis to the lungs was evaluated with spontaneous as well as experimental assay. Metastatic tumors formed vasculature with significantly higher permeability or vascular volume (P<.05, two-sided unpaired t test). The permeability profile matched VEGF expression. Within tumors, regions of high vascular volume usually exhibited low permeability whereas regions of low vascular volume exhibited high permeability. We observed that although invasion was necessary, without adequate vascularization it was not sufficient for metastasis to occur.

Imaging prostate cancer invasion with multi-nuclear magnetic resonance methods: the Metabolic Boyden Chamber

The physiological milieu within solid tumors can influence invasion and metastasis. To determine the impact of the physiological environment and cellular metabolism on cancer cell invasion, it is necessary to measure invasion during well-controlled modulation of the physiological environment. Recently, we demonstrated that magnetic resonance imaging can be used to monitor cancer cell invasion into a Matrigel layer [Artemov D, Pilatus U, Chou S, Mori N, Nelson JB, and Bhujwalla ZM (1999). Dynamics of prostate cancer cell invasion studied in vitro by NMR microscopy. Mag Res Med 42, 277-282.]. Here we have developed an invasion assay ("Metabolic Boyden Chamber") that combines this capability with the properties of our isolated cell perfusion system. Long-term experiments can be performed to determine invasion as well as cellular metabolism under controlled environmental conditions. To characterize the assay, we performed experiments with prostate cancer cell lines preselected for different invasive characteristics. The results showed invasion into, and degradation of the Matrigel layer, by the highly invasive/metastatic line (MatLyLu), whereas no significant changes were observed for the less invasive/metastatic cell line (DU-145). With this assay, invasion and metabolism was measured dynamically, together with oxygen tensions within the cellular environment and within the Matrigel layer. Such a system can be used to identify physiological and metabolic characteristics that promote invasion, and evaluate therapeutic interventions to inhibit invasion.

Regulation of tumor angiogenesis by p53-induced degradation of hypoxia-inducible factor 1alpha

The switch to an angiogenic phenotype is a fundamental determinant of neoplastic growth and tumor progression. We demonstrate that homozygous deletion of the p53 tumor suppressor gene via homologous recombination in a human cancer cell line promotes the neovascularization and growth of tumor xenografts in nude mice. We find that p53 promotes Mdm2-mediated ubiquitination and proteasomal degradation of the HIF-1alpha subunit of hypoxia-inducible factor 1 (HIF-1), a heterodimeric transcription factor that regulates cellular energy metabolism and angiogenesis in response to oxygen deprivation. Loss of p53 in tumor cells enhances HIF-1alpha levels and augments HIF-1-dependent transcriptional activation of the vascular endothelial growth factor (VEGF) gene in response to hypoxia. Forced expression of HIF-1alpha in p53-expressing tumor cells increases hypoxia-induced VEGF expression and augments neovascularization and growth of tumor xenografts. These results indicate that amplification of normal HIF-1-dependent responses to hypoxia via loss of p53 function contributes to the angiogenic switch during tumorigenesis.

Applications of magnetic resonance in model systems: tumor biology and physiology

A solid tumor presents a unique challenge as a system in which the dynamics of the relationship between vascularization, the physiological environment and metabolism are continually changing with growth and following treatment. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) studies have demonstrated quantifiable linkages between the physiological environment, angiogenesis, vascularization and metabolism of tumors. The dynamics between these parameters continually change with tumor aggressiveness, tumor growth and during therapy and each of these can be monitored longitudinally, quantitatively and non-invasively with MRI and MRS. An important aspect of MRI and MRS studies is that techniques and findings are easily translated between systems. Hence, pre-clinical studies using cultured cells or experimental animals have a high connectivity to potential clinical utility. In the following review, leaders in the field of MR studies of basic tumor physiology using pre-clinical models have contributed individual sections according to their expertise and outlook. The following review is a cogent and timely overview of the current capabilities and state-of-the-art of MRI and MRS as applied to experimental cancers. A companion review deals with the application of MR methods to anticancer therapy.

Dynamics of prostate cancer cell invasion studied in vitro by NMR microscopy

Understanding the dynamics and pathogenesis of invasion is vital for developing strategies to prevent cancer metastasis. Conventional invasion assays provide information for a single time point. NMR microscopic imaging as used in the current study to measure cell invasion in vitro provides a nondestructive method for scoring cell invasion thus offering a unique possibility to study this process dynamically. An additional advantage is that cells can be retrieved for metabolic and physiological characterization. Two prostate cancer cell lines, DU-145 and Mat-Ly-Lu, preselected for differences in invasive behavior, were studied. Cells were seeded in 12-mm culture plate inserts containing a 15-microm-thick porous membrane with 3.0 microm pore size that was coated with a 100 microm Matrigel layer. Cell invasion in the Matrigel layer was obtained from the profile of intracellular water measured with diffusion-weighted 1D imaging. Additional experiments were also performed with confocal microscopy to validate the NMR results. Significant differences were detected between the invasive behavior of DU-145 and Mat-Ly-Lu cells. The obtained results show that NMR microscopy can be used to dynamically study invasion by cancer cells. The noninvasive nature of NMR microscopy permits determination of cell migration dynamically for any given sample, which is especially important if cell availability is limited to the unique sample, such as for biopsy specimens. Magn Reson Med 42:277-282, 1999.

Nm23-transfected MDA-MB-435 human breast carcinoma cells form tumors with altered phospholipid metabolism and pH: a 31P nuclear magnetic resonance study in vivo and in vitro

Nm23 genes are involved in the control of the metastatic potential of breast carcinoma cells. To understand the impact of nm23 genes on tumor physiology and metabolism, a 31P nuclear magnetic resonance (NMR) spectroscopic study was performed on tumors formed in the mammary fat pad of severe combined immunodeficiency mice by MDA-MB-435 human breast carcinoma cells transfected with cDNA encoding wild type nm23-H1 and nm23-H2 proteins. Tumors formed by MDA-MB-435 cells transfected with vector alone were used as controls. All transgene tumors exhibited significantly higher levels of phosphodiester (PDE) compounds relative to phosphomonoester (PME) compounds in vivo compared with control tumors. Similar differences in PDE and PME also were observed for spectra obtained from cells growing in culture. Intracellular pH was significantly lower and extracellular pH was significantly higher for transgene tumors compared with control tumors. Histologic analysis of lung sections confirmed reductions in incidence, number, and size of metastatic nodules for animals bearing transgene tumors. These results suggest that nm23 genes may affect suppression of metastasis through phospholipid-mediated signaling and cellular pH regulation.

Enhancement of chemotherapy by manipulation of tumour pH

The extracellular (interstitial) pH (pHe) of solid tumours is significantly more acidic compared to normal tissues. In-vitro, low pH reduces the uptake of weakly basic chemotherapeutic drugs and, hence, reduces their cytotoxicity. This phenomenon has been postulated to contribute to a 'physiological' resistance to weakly basic drugs in vivo. Doxorubicin is a weak base chemotherapeutic agent that is commonly used in combination chemotherapy to clinically treat breast cancers. This report demonstrates that MCF-7 human breast cancer cells in vitro are more susceptible to doxorubicin toxicity at pH 7.4, compared to pH 6.8. Furthermore 31P-magnetic resonance spectroscopy (MRS) has shown that the pHe of MCF-7 human breast cancer xenografts can be effectively and significantly raised with sodium bicarbonate in drinking water. The bicarbonate-induced extracellular alkalinization leads to significant improvements in the therapeutic effectiveness of doxorubicin against MCF-7 xenografts in vivo. Although physiological resistance to weakly basic chemotherapeutics is well-documented in vitro and in theory, these data represent the first in vivo demonstration of this important phenomenon.

Nm23-transfected MDA-MB-435 human breast carcinoma cells form tumors with altered phospholipid metabolism and pH: a 31P nuclear magnetic resonance study in vivo and in vitro

Nm23 genes are involved in the control of the metastatic potential of breast carcinoma cells. To understand the impact of nm23 genes on tumor physiology and metabolism, a 31P nuclear magnetic resonance (NMR) spectroscopic study was performed on tumors formed in the mammary fat pad of severe combined immunodeficiency mice by MDA-MB-435 human breast carcinoma cells transfected with cDNA encoding wild type nm23-H1 and nm23-H2 proteins. Tumors formed by MDA-MB-435 cells transfected with vector alone were used as controls. All transgene tumors exhibited significantly higher levels of phosphodiester (PDE) compounds relative to phosphomonoester (PME) compounds in vivo compared with control tumors. Similar differences in PDE and PME also were observed for spectra obtained from cells growing in culture. Intracellular pH was significantly lower and extracellular pH was significantly higher for transgene tumors compared with control tumors. Histologic analysis of lung sections confirmed reductions in incidence, number, and size of metastatic nodules for animals bearing transgene tumors. These results suggest that nm23 genes may affect suppression of metastasis through phospholipid-mediated signaling and cellular pH regulation.

Tumor angiogenesis, vascularization, and contrast-enhanced magnetic resonance imaging

Angiogenesis, the process by which new blood vessels are generated, occurs during wound healing, in the female reproductive system during ovulation and gestation, and during embryonic development. The process is carefully controlled with positive and negative regulators, because several vital physiological functions require angiogenesis. The consequences of abnormal angiogenesis are either excessive or insufficient blood vessel growth. Ulcers, strokes, and heart attacks can result from the absence of angiogenesis normally required for natural healing, whereas excessive blood vessel proliferation may favor tumor growth and dissemination, blindness, and arthritis. In this review, the process of angiogenesis and the characteristics of the resulting tumor vasculature are outlined. Contrast-enhanced magnetic resonance imaging techniques that currently are available for basic research and clinical applications to study various aspects of tumor angiogenesis and neovascularization are discussed.

In vivo imaging of extracellular pH using 1H MRSI

Tumor pH is physiologically important since it influences a number of processes relevant to tumorigenesis and therapy. Hence, knowledge of localized pH within tumors would contribute to understanding these processes. The destructiveness, poor spatial resolution, and poor signal-to-noise ratio (SNR) of current technologies (e.g., microelectrodes, 31P magnetic resonance spectroscopy) have limited such studies. An extrinsic chemical extracellular pH (pHe) probe is described that is used in combination with 1H magnetic resonance spectroscopic imaging to yield pHe maps with a spatial resolution of 1 x 1 x 4 mm3. The principle of the technique is demonstrated on a phantom. Further data are shown to demonstrate its application in vivo, and results agree with previously reported pH values. The accuracy of the reported pH measurements is <0.1 pH units, as derived from a detailed analysis of the errors associated with the technique, the description of which is included.

Switchable multicoil array for MR micro-imaging of breast lesions

High-resolution breast imaging may improve differentiation between benign and malignant lesions and may be important for refining treatment strategy. This article presents a new, flexible design of a breast-imaging coil capable of providing breast images of a high level of spatial resolution. Referred to as a switchable coil array, the design uses small-diameter surface coils that provide high sensitivity of detection, which, combined with a relatively small field of view, affords a high degree of spatial resolution (up to 200 microm). Remote selection of the coil pair closest to the position of the lesion in the breast permits coverage of the whole breast without changing the position of the coils or the patient. High-resolution MR images of phantom and volunteer patients with benign and malignant breast lesions are presented.

Plasmalemmal pH-gradients in drug-sensitive and drug-resistant MCF-7 human breast carcinoma xenografts measured by 31P magnetic resonance spectroscopy

31p Magnetic resonance spectroscopy (MRS) was employed to investigate tumor pH in xenografts of drug-sensitive and drug-resistant MCF-7 human breast carcinoma cells. Measured extracellular pH values were found to be lower than the intracellular pH in all three tumor types investigated. The magnitude of this acid-outside plasmalemmal pH gradient increased with increasing tumor size in tumors of two drug-resistant variants of MCF-7 cells, but not in tumors of the parent (drug-sensitive) cells. The partitioning of weak-base or weak-acid drug molecules across the plasma membrane of a tumor cell is dependent upon the acid-dissociation constant (pKa) of the drug as well as the plasmalemmal pH gradient. A large acid-outside pH gradient, such as those seen in MCF-7 xenografts, can exert a protective effect on the cell from weak-base drugs such as anthracyclines and Vinca alkaloids, which have pKa values of 7.5 to 9.5. The possibility of enhancing the therapeutic efficacy of weak-base drugs by dietary or metabolic manipulation of the extracellular pH, in order to reduce or reverse the plasmalemmal pH gradient, deserves investigation.

Hypoxic cell cytotoxin tirapazamine induces acute changes in tumor energy metabolism and pH: a 31P magnetic resonance spectroscopy study

Tirapazamine is a hypoxic cell cytotoxin in phase II/III trials. To further understand its mechanism of action in vivo, we examined the effect of tirapazamine on tumor energy metabolism and pH. RIF-1 and SCCVII tumors were grown subcutaneously in the flanks of C3H mice. Tumor energy metabolism, expressed as the ratio of inorganic phosphate to nucleotide triphosphate (Pi/NTP), and intracellular pH (pHi), were measured by 31P magnetic resonance spectroscopy (MRS). In RIF-1 and SCCVII tumors, tirapazamine increased the Pi/NTP ratio by 2.6-fold and 3-fold, respectively, within the first hour after an intraperitoneal dose of 0.3 mmol/kg. A corresponding decrease in pHi from 7.05+/-0.07 to 6.48+/-0.06, and 7.21+/-0.09 to 6.45+/-0.02 in RIF-1 and SCCVII tumors, respectively, was observed. The decrease in tumor 31P bioenergetics and pH was reversible, as exemplified by RIF-1 tumors, which showed a further increase in Pi/NTP ratio of 3.5-fold by 5-8 hr, returning to normal range at 24 hr. Corresponding pHi of RIF-1 tumors was 6.88+/-0.05 at 5-8 hr and 7.16+/-0.05 at 24 hr. We concluded that tirapazamine induces acute changes in tumor energy metabolism and pHi. These findings are relevant to the rational selection and optimal timing of coadministered therapy.

Malignant transformation alters membrane choline phospholipid metabolism of human mammary epithelial cells

Transduction of mitogenic signals in cells can be mediated by molecules derived from the synthesis and breakdown of the major membrane phospholipid, phosphotidylcholine. Studies were performed on human mammary epithelial cells in culture to understand the impact of malignant transformation and progression on membrane phospholipid metabolism. In the model system used here, phosphocholine levels and total choline-containing phospholipid metabolite levels increased with progression from normal to immortalized to oncogene-transformed to tumor-derived cells. These changes occurred independently of cell doubling time. A "glycerophosphocholine to phosphocholine switch" was apparent with immortalization. This alteration in phenotype of increased phosphocholine relative to glycerophosphocholine was observed in oncogene-transformed and for all human breast tumor cell lines analyzed. The results demonstrate that progression of human mammary epithelial cells from normal to malignant phenotype is associated with altered membrane choline phospholipid metabolism.

Two-compartment model for determination of glycolytic rates of solid tumors by in vivo 13C NMR spectroscopy

Carbon-13 NMR spectroscopy of 13C enriched substrates is useful for non-invasively determining metabolic fluxes of cells and tissues. Our study demonstrates that for RIF-1 tumor cells, examined under monolayer culture with continuous perfusion and also grown as solid subcutaneously (sc) implanted tumors in vivo, the levels of intracellular glucose and intermediates of the glycolytic pathway are below the level of detection by NMR spectroscopy. For these tumors, glucose transport into the cell is the most probable rate limiting step of the glycolytic pathway. Under these limiting conditions a simple two-compartment model of glycolysis applies. This model yields two parameters: the average rate of glycolysis and the rate of lactate clearance through the vasculature. For the RIF-1 tumor these parameters were 0.022 +/- 0.01 and 0.034 +/- 0.006 min(-1), respectively.

Intratumoral conversion of 5-fluorocytosine to 5-fluorouracil by monoclonal antibody-cytosine deaminase conjugates: noninvasive detection of prodrug activation by magnetic resonance spectroscopy and spectroscopic imaging

The monitoring of antibody-directed enzyme-prodrug therapies requires evaluation of drug activation within the tissues of interest. We have demonstrated the feasibility of noninvasive magnetic resonance spectroscopy and spectroscopic imaging (chemical shift imaging) to detect activation of the prodrug 5-fluorocytosine (5-FCyt) to the cytotoxic species 5-fluorouracil (5-FU) by monoclonal antibody-cytosine deaminase (CD) conjugates. In vitro, L6-CD but not 1F5-CD selectively metabolized 5-FCyt to 5-FU on H2981 human lung adenocarcinoma cells because of the presence and absence of cell surface L6 and CD20 antigens, respectively. After pretreatment of H2981 tumor-bearing mice with L6-CD, in vivo metabolism of 5-FCyt to 5-FU within the tumors was detected by 19F magnetic resonance spectroscopy; the chemical shift separation between 5-FCyt and 5-FU resonances was approximately 1.2 ppm. 5-FU levels were 50-100% of 5-FCyt levels in tumors 10-60 min after 5-FCyt administration. Whole body 19F chemical shift imaging (6 x 6 mm in-plane resolution) of tumor-bearing mice demonstrated the highest signal intensity of 5-FU within the tumor region. This study supports further development of noninvasive magnetic resonance methods for preclinical and clinical monitoring of CD enzyme-prodrug therapies.

Estimations of intra- and extracellular volume and pH by 31P magnetic resonance spectroscopy: effect of therapy on RIF-1 tumours

Quantification of metabolite or drug concentrations in living tissues requires determination of intra- and extracellular volumes. This study demonstrates how this can be achieved non-invasively by 31P magnetic resonance spectroscopy (MRS) employing dimethyl methylphosphonate (DMMP) as a marker of total water space, 3-aminopropylphosphonate (3-APP) as a marker of extracellular space and P and 3-APP as markers of intracellular pH (pH) and extracellular pH (pHe) respectively. The MRS measurements of the tumour volumes were validated by classic radiolabelling methods using 3H2O and [14C]inulin as markers of total and extracellular space respectively. The extracellular volume fraction measured by radiolabelling of RIF-1 tumours was 23 +/- 0.83% (mean +/- s.e.m. n = 9), not significantly different (P > 0.1) from that found by MRS (27 +/- 2.9%, n = 9, London, and 35 +/- 6.7, n = 14, Baltimore). In untreated RIF-1 tumours, pH was about 0.2 units higher than pHe (P < 0.01). 5-Fluorouracil (5FU) treatment (165 mg kg(-1)) caused no significant changes in either pHe or per cent extracellular volume. However significant increases in pH, 48 h after treatment (P < 0.01) correlated with decreased tumour size and improved bioenergetic status [NTP/inorganic phosphate (Pi) ratio]. This study shows the feasibility of an MR method (verified by a 'gold standard') for studying the effects of drug treatment on intra- and extracellular spaces and pH in solid tumours in vivo.

Evaluation of lactate as a 1H nuclear magnetic resonance spectroscopy index for noninvasive prediction and early detection of tumor response to radiation therapy in EMT6 tumors

In a recent study (Int. J. Radiat. Oncol. Biol. Phys. 36, 635-639, 1996), 1H nuclear magnetic resonance (NMR) spectroscopy was used to demonstrate significant decreases in lactate levels after gamma irradiation of radiosensitive RIF-1 tumors in vitro. For comparison, we have examined the effects of gamma radiation on lactate levels in the more radioresistant EMT6 tumor. Single-slice (5-6 mm thick) localized 1H spectra of subcutaneous RIF-1 (untreated) and EMT6 tumors (pretreatment, 24 and 48 h postirradiation with 4, 10 or 20 Gy of gamma radiation) were measured by the selective multiple quantum coherence transfer method (Sel-MQC, approximately 4 min acquisition time). Both pretreatment lactate levels and pretreatment lactate dehydrogenase (LDH) activities were found to be similar in RIF-1 and EMT6 tumors, suggesting that steady-state lactate levels are unlikely to be reliable indices for predicting response to radiation therapy. After 10 Gy gamma irradiation, EMT6 tumors showed a 21% decrease relative to pretreatment lactate levels at 48 h (1.04 +/- 0.22 to 0.82 +/- 0.16; P = 0.06); after 20 Gy a 40% decrease was observed at 48 h (1.34 +/- 0.27 to 0.81 +/- 0.10; P = 0.07). No significant changes in lactate levels were observed in control EMT6 tumors or in tumors treated with 4 Gy of gamma radiation, in contrast to changes detected previously in RIF-1 tumors, which showed a significant decrease in lactate by 48 h for both 2 and 4 Gy. The decreased effect of radiation on lactate levels in EMT6 compared to RIF-1 tumors may be attributed to the higher hypoxic fraction and lower radiosensitivity of EMT6 tumors (Int. J. Radiat. Oncol. Biol. Phys. 10, 695-712, 1984). The decrease in lactate levels did not, however, strictly reflect the extent of the response to therapy for the high dose of 20 Gy. This study together with our earlier study (Int. J. Radiat. Oncol. Biol. Phys. 36, 635-639, 1996) provides evidence to support the hypothesis that changes in steady-state tumor lactate levels may serve as sensitive early indices of tumor response to gamma radiation at doses of the order of 2 to 4 Gy.

Detection of tumor response to chemotherapy by 1H nuclear magnetic resonance spectroscopy: effect of 5-fluorouracil on lactate levels in radiation-induced fibrosarcoma 1 tumors

The aim of this study was to evaluate the ability of noninvasive 1H magnetic resonance spectroscopic imaging to detect the response of radiation-induced fibrosarcoma 1 tumors to treatment with 5-fluorouracil (5-FU). Parallel magnetic resonance studies of tumor extracts and assays of apoptosis and necrosis in tumor sections were performed to elucidate the mechanism underlying the changes detected in spectra in vivo. Cell death in tumors after a single dose of 5-FU (165 mg/kg, i.p.) was characterized by increased apoptosis, decreased necrotic fraction, and tumor shrinkage within 48 h. No significant change in normalized trimethylamine and lactate levels was observed during 3 days of untreated tumor growth. Following treatment with 5-FU, normalized intensities of both trimethylamine and lactate decreased significantly from pretreatment levels within 24 h and continued to decline at 48 h. The decrease in lactate levels determined by spectroscopic imaging in vivo was also observed in perchloric acid extracts of radiation-induced fibrosarcoma 1 tumors. Possible mechanisms for the decrease of tumor lactate levels include increased blood flow and decreased glycolytic rate. Unlike lactate, changes in normalized trimethylamine levels observed in vivo were not observed in tumor extracts. The mechanism underlying the anomalous decrease in the in vivo trimethylamine level is under investigation. These findings demonstrate that lactate is a reliable and sensitive indirect indicator of response to 5-FU in at least one tumor model and point to the possible clinical utility of this resonance as an index of clinical tumor response to chemotherapy.

Detection of tumor response to radiation therapy by in vivo proton MR spectroscopy

PURPOSE

These studies were performed to investigate the effects of radiation on levels of metabolites such as lactate and choline compounds detected by 1H magnetic resonance spectroscopy (MRS). The purpose was to determine the ability of spatially localized 1H MRS to detect tumor response to radiation therapy.

METHODS AND MATERIALS

Proton spectroscopic images were obtained from RIF-1 tumors with voxel spatial resolutions of 8-16 mm3 before and at 24 and 48 h following 2, 4, and 20 Gy of gamma-radiation.

RESULTS

Lactate levels decreased significantly for all doses by 48 h. Tumors irradiated with 2 and 4 Gy showed a significant decrease by 48 h, but not at 24 h. A group of sham-irradiated control animals demonstrated no significant changes in lactate over the period of observation.

CONCLUSIONS

Changes in lactate observed in this study are consistent with an increased blood flow observed in previous studies (15) in the same tumor model following 20 Gy X-irradiation. These studies point to the feasibility of detecting response to clinical doses of fractionated radiation therapy by 1H MRS.

Effects of blood flow modifiers on tumor metabolism observed in vivo by proton magnetic resonance spectroscopic imaging

Perfusion plays a key role in tumor proliferation and therapeutic response. Tumor heterogeneity necessitates use of the highest spatial resolution to monitor metabolic correlates of blood flow changes. This is best achieved with 1H NMR spectroscopy, which permits noninvasive acquisition of high resolution spectroscopic images (SI) of subcutaneous tumors in a relatively short scan time (e.g., 12-25 microliters voxels with signal-to-noise ratio 7:1 in 30 min at 4.7 T). This study seeks to identify 1H spectroscopic indices of tumor blood flow. Proton SI of subcutaneous murine RIF-1 tumors were recorded (a) before and after administration of nicotinamide (1 g/kg) to increase blood flow, and (b) before and after hydralazine (10 mg/kg) to decrease flow. Nicotinamide produced a significant decrease in the total choline peak amplitudes, which subsequent high resolution NMR spectroscopy of tumor extracts revealed to be due to decreases in phosphocholine and glycerophosphocholine. The deamidation of nicotinamide to nicotinic acid, which is known to have hypolipidemic effects and to stimulate the formation of prostaglandins, may have sufficiently altered lipid metabolism to affect the in vivo concentration of the NMR-visible choline-containing compounds. The main effect of hydralazine was a significant increase of lactate, which is consistent with a reduction of tumor blood flow.

Proton detection of choline and lactate in EMT6 tumors by spin-echo-enhanced selective multiple-quantum-coherence transfer

An extension of the Sel-MQC pulse sequence--SEE-SelMQC (spin-echo-enhanced selective multiple-quantum coherence transfer)--that completely suppresses lipid and water in tissues containing mobile lipid in a single scan and detects 1H resonances of multiple metabolites is described. As in the Sel-MQC lactate-editing experiments [Q. He et al., J Magn. Reson. B 106,203 (1995)], SEE-SelMQC acquires lactate from its ZQ --> DQ coherence-transfer pathway; in addition, the method recovers signal from other metabolites by selective generation of additional spin echoes using extra frequency-selective pulses and gradients. This method introduces no loss of lactate signal intensity beyond the 50% that is lost through the multiple-quantum coherence-transfer process. The spatial distributions of choline and lactate with a phantom and in vivo, in subcutaneously implanted murine EMT6 tumors, have been simultaneously mapped.

Pharmacokinetics of the 13C labeled anticancer agent temozolomide detected in vivo by selective cross-polarization transfer

The anticancer agent temozolomide labeled with 13C (8-Carbamoyl-3-13C-methylimidazo-[5,1-d]-1,2,3,5-tetrazin-4-(3H)-o ne), was noninvasively detected in subcutaneous RIF-1 tumors by a selective cross polarization 13C NMR method, at a field strength of 9.4T. Pharmacokinetics of the drug, at a dose of 150 mg/kg, were determined for intravenous and intraperitoneal models of administration (three animals per mode). The half-life of the drug in the tumors was approximately 60 min. The uptake and clearance of the drug, however, varied significantly between individual hosts, for both modes of administration. These results demonstrate the feasibility of obtaining pharmacokinetics of anticancer agents for individual tumors without the need for a label that might modify drug activity (e.g., fluorine). The variability of the in vivo measurements, even within the same tumor model, demonstrates the necessity of directly monitoring the tumor to evaluate drug pharmacokinetics.

Single-scan in vivo lactate editing with complete lipid and water suppression by selective multiple-quantum-coherence transfer (Sel-MQC) with application to tumors

A novel single-scan selective homonuclear multiple-quantum coherence-transfer technique, Sel-MQC, is presented that achieves lactate editing with complete lipid and water suppression. The method is suitable for studying tissues with high fat content and those subject to substantial motion. Frequency-selective excitation is employed to selectively prepare lactate into its multiple-quantum states; lipid and water are left in the single-quantum modes and eliminated by the multiple-quantum selection gradients. The efficiency of lipid suppression is monitored by a 2D Sel-MQC experiment which separates lipid and lactate along the multiple-quantum-evolution dimension. The spatial distribution of lactate can be imaged by the spectroscopic imaging version of Sel-MQC. Sel-MQC sequences were demonstrated both in phantoms and in vivo, using subcutaneously implanted murine EMT6 tumors.

Proton NMR observation of the antineoplastic agent Iproplatin in vivo by selective multiple quantum coherence transfer (Sel-MQC)

We have noninvasively detected the proton signal of an antineoplastic agent Iproplatin in vivo by selective multiple quantum coherence transfer (Sel-MQC). Without isotopic labeling or chemical modification, the Sel-MQC method labels Iproplatin by its intrinsic proton multiple quantum coherences and, hence, differentiates the Iproplatin signal from the intensive overlapping resonances of lipid and lactate. This proton NMR method should also be applicable to study other drugs with appropriate spin coupling patterns.

In vivo selective measurement of (1-13C)-glucose metabolism in tumors by heteronuclear cross polarization

Selective detection of (1-13C)-glucose and its glycolytic product, (3-13C)-lactate, was achieved by selective 13C NMR spectroscopy with 1H cross polarization. The total sensitivity of conventional broadband experiments was retained, and peak intensities were at least equivalent to those obtained with the inverse detection technique (i.e., 1H(13C)) for single proton resonances. A key advantage of the method is that it maintains the specific absorption rate (SAR) within FDA limits of 5 W/kg by reducing power deposition during decoupling. In this study we have monitored the kinetics of metabolism of 13C-labeled glucose to lactate following intravenous infusion of 0.55 ml of 0.18 M labeled glucose. Physiological effects were minimized by a) maintaining total plasma glucose concentrations below 20 mM throughout the course of NMR experiment and b) by avoiding significant heating of the tumor.

Glucose metabolism in RIF-1 tumors after reduction in blood flow: an in vivo 13C and 31P NMR study

Low pH appears to enhance the effectiveness of therapeutic hyperthermia. 13C and 31P NMR spectroscopy have been employed to examine the possibility that elevating glucose in a solid tumor while simultaneously reducing tumor blood flow would induce a more profound acidosis than either treatment alone. When blood flow in RIF-1 tumors was acutely reduced by administration of hydralazine and additional glucose was delivered locally by intratumoral injection, tumor acidosis (as determined by 31P NMR spectroscopy) during the period of reduced blood flow was not enhanced, relative to administration of hydralazine alone. Tumor NTP/P1 ratios decreased significantly within 20 min of hydralazine administration, whether or not glucose was injected, although NTP/P1 ratios were slightly higher in tumors that received extra glucose. Tumor lactate concentrations were not significantly different in glucose-supplemented tumors, despite glucose concentrations that were 4 to 5 times higher. When the added glucose was labeled with 13C, no correlation was detected between the pH in an individual tumor and the intensity of the 3-[13C]-lactate resonance in the same tumor.

Determination of absolute phosphate metabolite concentrations in RIF-1 tumors in vivo by 31P-1H-2H NMR spectroscopy using water as an internal intensity reference

The absolute metabolite quantification method of Thulborn and Ackerman [J. Magn. Reson. 55, 357 (1983)] in which the tissue water proton signal is used as an internal intensity standard and its more recent variation in which NMR peak intensities are referenced to that of the natural abundance deuterium signal of water [Li et al., SMRM Abstr. 2, 825 (1988); Song et al., Magn. Reson. Med. 25, 45 (1992) have been implemented to obtain absolute phosphate metabolite concentrations in subcutaneous RIF-1 tumors during untreated growth and following treatment with 5-fluorouracil. The equivalence of these two hydrogen isotopes as intensity standards and the validity of their use in the determination of absolute metabolite concentrations in vivo by NMR has been demonstrated. On matched in vivo and extract tumor samples (n = 5), excellent agreement has been obtained between nucleoside triphosphate concentrations determined by NMR and those derived by HPLC analysis for the control tumors. Following 3 days of untreated growth, absolute concentrations of phosphate metabolites in RIF-1 tumors (n = 10) decreased significantly, except for the Pi concentration which did not vary. For the treated tumors (n = 10) there were no changes in metabolite concentrations except for a decrease in the PCr and, possibly, Pi concentrations. The PCr/Pi ratio in the latter tumors did not change. These observations suggest that changes in absolute metabolite concentrations may be more sensitive indices of response to therapy than changes in metabolite peak amplitude ratios, a parameter commonly used to express in vivo NMR data.

Spatial heterogeneity of the metabolic response of RIF-1 tumors to a vasoactive agent evaluated in vivo by one-dimensional 31P chemical-shift imaging

Localized 31P NMR spectroscopy was used to evaluate the spatial heterogeneity of the metabolic response of RIF-1 tumors to hydralazine. Volume localized 31P spectra were obtained from subcutaneous RIF-1 tumors using one-dimensional chemical-shift imaging, before and 20 min after treatment with 5 mg/kg hydralazine, administered intravenously. Following treatment all of the tumors showed an overall decrease in the ratio of nucleoside triphosphate (NTP) to inorganic phosphate (Pi) and a decrease in pH. However, spatial localization revealed that the reduction in NTP/Pi was not uniform within some tumors. This was partly due to regional differences in the levels of metabolites existing before treatment. Normal tissue adjacent to the tumor did not show a significant decrease in high-energy metabolites or pH.

Energy metabolism, pH changes, and lactate production in RIF-1 tumors following intratumoral injection of glucose

The metabolic consequences of increased glucose availability were examined in subcutaneous RIF-1 tumors in vivo, using 13C and 31P NMR spectroscopy. Significant increases in the levels of nucleotide triphosphates and phosphocreatine relative to low energy phosphates and in tumor pH were observed within 30 min following injection of 1 g/kg of glucose directly into the tumor. These changes did not occur following an equivalent intratumoral dose of the non-metabolizable sugar alcohol, mannitol. When [1-13C]-glucose was administered, [3-13C]-lactate and [3-13C]-alanine were the only labeled metabolites detected in the in vivo 13C NMR spectra during the period of bioenergetic improvement. Biochemical analysis revealed a substantial increase in tumor and plasma glucose concentration, but no increase in either tumor or plasma lactate, consistent with the absence of acidosis. Evaluation of the distribution of glucose in the tumor by quantitative autoradiography of [1-14C]-2-deoxyglucose administered with the glucose indicated that, on average, 7 mM of the added glucose distributed over the entire tumor within 10 min. The significant improvement in overall metabolic status of the tumors following glucose administration is attributed to the existence of substrate limited regions within the tumor.

The energy metabolism of RIF-1 tumours following hydralazine

Phosphorus-31 Magnetic Resonance Spectroscopy (MRS) was used to observe the effect of two doses of the vasodilator hydralazine on the energy status of RIF-1 tumours. An intravenous dose of 5 mg/kg hydralazine reduced the high energy phosphate metabolites PCr and ATP, lowered pHMRS and raised the levels of inorganic phosphate of tumours within 20 min of administering the drug. The levels of high energy metabolites continued to decrease for at least 24 h. Normal muscle spectra obtained up to 1 h after drug administration remained unchanged. An intravenous dose of 0.5 mg/kg hydralazine also reduced NTP/Pi and PCr/Pi levels of tumours up to at least 5 h after drug administration, but the effect was smaller than for the higher dose. Blood flow measurements and measurements of systemic blood pressure demonstrated that 5 mg/kg of hydralazine produced a reduction in both systemic blood pressure and tumour blood flow relative to most normal tissues investigated. It is concluded that the changes in the P-31 MRS spectra of tumours were due to a reduction in tumour vascular perfusion following administration of hydralazine.

Phosphorus-31 magnetic resonance spectroscopy and blood perfusion of the RIF-1 tumor following X-irradiation

Phosphorus-31 magnetic resonance spectra were obtained from the RIF-1 tumor in C3H mice before and up to 2 days after various doses of X rays. Parallel studies were performed to measure relative changes in tumor blood perfusion using [14C]iodo-antipyrine and changes in % tumor necrosis using Chalkley's method. Tumor ratios of phosphocreatine to inorganic phosphate (PCr/Pi) and nucleotide triphosphates to inorganic phosphate (NTP/Pi) as well as pH as measured by 31P-MRS increased significantly at most time points after irradiation with doses of 5, 10, and 20 Gy. Tumor blood perfusion was found to significantly improve after a dose of 20 Gy but not after a dose of 2 Gy. Percent tumor necrosis increased to about 3 times its control level at 1 day after a dose of 20 Gy and then declined to about twice its control value at 2 days. The magnitude of the changes in the 31P-MRS parameters makes it unlikely that any of them are entirely due to radiation-induced changes in the radiobiologically hypoxic fraction of these tumors. Changes in the necrotic fraction did not appear to influence the tumor spectra. However, the observed improvement in tumor blood perfusion may have resulted in an increase in oxidative phosphorylation of the whole tumor population as well as a clearance of inorganic phosphate and acid metabolites, so that 31P-MRS changes may indirectly reflect changes in tumor blood perfusion.