Carbogen breathing differentially enhances blood plasma volume and 5-fluorouracil uptake in two murine colon tumor models with a distinct vascular structure

How to Modulate Tumor Hypoxia for Preclinical In Vivo Imaging Research

Tumor hypoxia is related with tumor aggressiveness, chemo- and radiotherapy resistance, and thus a poor clinical outcome. Therefore, over the past decades, every effort has been made to develop strategies to battle the negative prognostic influence of tumor hypoxia. For appropriate patient selection and follow-up, noninvasive imaging biomarkers such as positron emission tomography (PET) radiolabeled ligands are unprecedentedly needed. Importantly, before being able to implement these new therapies and potential biomarkers into the clinical setting, preclinical in vivo validation in adequate animal models is indispensable. In this review, we provide an overview of the different attempts that have been made to create differential hypoxic in vivo cancer models with a particular focus on their applicability in PET imaging studies.

High-field small animal magnetic resonance oncology studies

This review focuses on the applications of high magnetic field magnetic resonance imaging (MRI) and spectroscopy (MRS) to cancer studies in small animals. High-field MRI can provide information about tumor physiology, the microenvironment, metabolism, vascularity and cellularity. Such studies are invaluable for understanding tumor growth and proliferation, response to treatment and drug development. The MR techniques reviewed here include (1)H, (31)P, chemical exchange saturation transfer imaging and hyperpolarized (13)C MRS as well as diffusion-weighted, blood oxygen level dependent contrast imaging and dynamic contrast-enhanced MRI. These methods have been proven effective in animal studies and are highly relevant to human clinical studies.

Exploring ΔR(2) * and ΔR(1) as imaging biomarkers of tumor oxygenation

PURPOSE

To investigate the combined use of hyperoxia-inducedΔR(2) * and ΔR(1) as a noninvasive imaging biomarker of tumor hypoxia.

MATERIALS AND METHODS

MRI was performed on rat GH3 prolactinomas (n = 6) and human PC3 prostate xenografts (n = 6) propagated in nude mice. multiple gradient echo and inversion recovery truefisp images were acquired from identical transverse slices to quantify tumor R(2) * and R(1)before and during carbogen (95% O2 /5% CO2 ) challenge, and correlates of ΔR(2) * and ΔR(1) assessed.

RESULTS

Mean baseline R(2) * and R(1) were 119 ± 7 s(-1) and 0.6 ± 0.03 s(-1) for GH3 prolactinomas and 77 ± 12 s(-1) and 0.7 ± 0.02 s(-1) for PC3 xenografts, respectively. During carbogen breathing, mean ΔR(2) * and ΔR(1) were -20 ± 8 s(-1) and 0.08 ± 0.03 s(-1) for GH3 and -0.5 ± 1 s(-1) and 0.2 ± 0.08 s(-1) for the PC3 tumors, respectively. A pronounced relationship betweenΔR(2) * and ΔR(1) was revealed.

CONCLUSION

Considering the blood oxygen-hemoglobin dissociation curve, fast R2 * suggested that GH3 prolactinomas were more hypoxic at baseline, and their carbogen response dominated by increased hemoglobin oxygenation, evidenced by highly negative ΔR(2) *. PC3 tumors were less hypoxic at baseline, and their response to carbogen dominated by increased dissolved oxygen, evidenced by highly positive ΔR(1) . Because the two biomarkers are sensitive to different oxygenation ranges, the combination of ΔR(2) * and ΔR(1) may better characterize tumor hypoxia than each alone.

Magnetic resonance spectroscopy of cancer metabolism and response to therapy

Magnetic resonance spectroscopy allows noninvasive in vivo measurements of biochemical information from living systems, ranging from cultured cells through experimental animals to humans. Studies of biopsies or extracts offer deeper insights by detecting more metabolites and resolving metabolites that cannot be distinguished in vivo. The pharmacokinetics of certain drugs, especially fluorinated drugs, can be directly measured in vivo. This review briefly describes these methods and their applications to cancer metabolism, including glycolysis, hypoxia, bioenergetics, tumor pH, and tumor responses to radiotherapy and chemotherapy.

Can localised (19)F magnetic resonance spectroscopy pharmacokinetics of 5FU in colorectal metastases predict clinical response?

BACKGROUND

5-Fluorouracil remains widely used in colorectal cancer treatment more than 40 years after its development. 19F magnetic resonance spectroscopy can be used in vivo to measure 5FU's half-life and metabolism to cytotoxic fluoronucleotides. Previous studies have shown better survival associated with longer 5FU tumour half-life. This work investigated 5FU pharmacokinetics in liver metastases of colorectal cancer.

METHODS

A total of 32 subjects with colorectal cancer undergoing 5FU treatment, 15 of whom had liver metastases, were examined in a 1.5T MRI scanner, using a large coil positioned over the liver. Non-localised spectra were acquired in 1-min blocks for 32 min after injection of a 5FU bolus. The 5FU half-life was measured in each subject, and averaged spectra were examined for the presence of fluoronucleotides. Associations with progression-free survival were assessed.

RESULTS

No association was observed between 5FU half-life, tumour burden and survival. Half-lives were all shorter than those associated with improved survival in the literature. Remarkably, in the group with liver metastases, high levels of fluoronucleotides were associated with poorer survival; this counterintuitive result may be due to the higher levels of fluoronucleotides (whose level is higher in tumour tissue than in normal liver) in patients with higher tumour burdens.

CONCLUSIONS

It is recommended that future studies use chemical shift imaging at higher field strengths to better resolve tumour from normal liver. Non-localised spectroscopy retains prognostic potential by enabling straightforward detection of fluoronucleotides, which are present at very low concentrations distributed throughout the tissue.

Investigating temporal fluctuations in tumor vasculature with combined carbogen and ultrasmall superparamagnetic iron oxide particle (CUSPIO) imaging

A combined carbogen ultrasmall superparamagnetic iron oxide (USPIO) imaging protocol was developed and applied in vivo in two murine colorectal tumor xenograft models, HCT116 and SW1222, with established disparate vascular morphology, to investigate whether additional information could be extracted from the combination of two susceptibility MRI biomarkers. Tumors were imaged before and during carbogen breathing and subsequently following intravenous administration of USPIO particles. A novel segmentation method was applied to the image data, from which six categories of R(2)* response were identified, and compared with histological analysis of the vasculature. In particular, a strong association between a negative ΔR(2)*(carbogen) followed by positive ΔR(2)*(USPIO) with the uptake of the perfusion marker Hoechst 33342 was determined. Regions of tumor tissue where there was a significant ΔR(2)*(carbogen) but no significant ΔR(2)*(USPIO) were also identified, suggesting these regions became temporally isolated from the vascular supply during the experimental timecourse. These areas correlated with regions of tumor tissue where there was CD31 staining but no Hoechst 33342 uptake. Significantly, different combined carbogen USPIO responses were determined between the two tumor models. Combining ΔR(2)*(carbogen) and ΔR(2)*(USPIO) with a novel segmentation scheme can facilitate the interpretation of susceptibility contrast MRI data and enable a deeper interrogation of tumor vascular function and architecture.

Neoplasia: the second decade

This issue marks the end of the 10-year anniversary of Neoplasia where we have seen exciting growth in both number of submitted and published articles in Neoplasia. Neoplasia was first published in 1999. During the past 10 years, Neoplasia has dynamically adapted to the needs of the cancer research community as technologies have advanced. Neoplasia is currently providing access to articles through PubMed Central to continue to facilitate rapid broad-based dissemination of published findings to the scientific community through an Open Access model. This has in part helped Neoplasia to achieve an improved impact factor this past year, demonstrating that the manuscripts published by Neoplasia are of great interest to the overall cancer research community. This past year, Neoplasia received a record number of articles for review and has had a 21% increase in the number of published articles.

Characterisation of tumour vasculature in mouse brain by USPIO contrast-enhanced MRI

To enhance the success rate of antiangiogenic therapies in the clinic, it is crucial to identify parameters for tumour angiogenesis that can predict response to these therapies. In brain tumours, one such parameter is vascular leakage, which is a response to tumour-derived vascular endothelial growth factor-A and can be measured by Gadolinium-DTPA (Gd-DTPA)-enhanced magnetic resonance imaging (MRI). However, as vascular permeability and angiogenesis are not strictly coupled, tumour blood volume may be another potentially important parameter. In this study, contrast-enhanced MR imaging was performed in three orthotopic mouse models for human brain tumours (angiogenic melanoma metastases and E34 and U87 human glioma xenografts) using both Gd-DTPA to detect vascular leakage and ultrasmall iron oxide particles (USPIO) to measure blood volume. Pixel-by-pixel maps of the enhancement in the transverse relaxation rates (Delta R(2) and Delta R(2)(*)) after injection of USPIO provided an index proportional to the blood volume of the microvasculature and macrovasculature, respectively, for each tumour. The melanoma metastases were characterised by a blood volume and vessel leakage higher than both glioma xenografts. The U87 glioblastoma xenografts displayed higher permeability and blood volume in the rim than in the core. The E34 glioma xenografts were characterised by a relatively high blood volume, accompanied by only a moderate blood-brain barrier disruption. Delineation of the tumour was best assessed on post-USPIO gradient-echo images. These findings suggest that contrast-enhanced MR imaging using USPIOs and, in particular, Delta R(2) and Delta R(2)(*) quantitation, provides important additional information about tumour vasculature.

19F NMR in vivo spectroscopy reflects the effectiveness of perfusion-enhancing vascular modifiers for improving gemcitabine chemotherapy

Nuclear magnetic resonance spectroscopy of fluorine-19 ((19)F NMR) has proven useful for evaluating kinetics of fluorinated chemotherapy drugs in tumors in vivo. This work investigated how three perfusion-enhancing vascular modifiers (BQ123, thalidomide, and Botulinum neurotoxin type A [BoNT-A]) would affect the chemotherapeutic efficacy of gemcitabine, a fluorinated drug widely used in human cancer treatment. Murine tumor growth experiments demonstrated that only BoNT-A showed a strong trend to enhance tumor growth inhibition by gemcitabine (1.7 days growth delay, P = 0.052, Student t-test). In accord with these results, (19)F NMR experiments showed that only BoNT-A increased significantly the uptake of gemcitabine in tumors (50% increase, P = 0.0008, Student t-test). Further experiments on gemcitabine kinetics (NMR vs time) and distribution ((19)F MRI) confirmed the uptake-enhancing properties of BoNT-A. The results of this study demonstrate that (19)F NMR can monitor modulation of the pharmacokinetics of fluorinated chemotherapy drugs in tumors. The results also show that (19)F NMR data can give a strong indication of the effectiveness of perfusion-enhancing vascular modifiers for improving gemcitabine chemotherapy in murine tumors. (19)F NMR is a promising tool for preclinical evaluation of such vascular modifiers and may ultimately be used in the clinic to monitor how these modifiers affect chemotherapy.

Tumour metabolomics in animal models of human cancer

Multinuclear Nuclear Magnetic Resonance Spectroscopy (MRS) and mass spectrometry (MS) are the key analytical techniques used in an increasing manner to explore tumor metabolite profiles. Recent work has revealed that metabolite profiles in various tumor preparations (i.e., cultured cells, tissue specimens, and tumors in vivo) show strong correlations with tumor type, proliferation, metabolic activity, and cell death. These data are regarded as highly promising for tumor diagnosis as well as assessment of prognosis and treatment response in a clinical setting. In this pursuit, animal models of human cancer have played a central role. In this short account, we review the potentials of MRS and MS techniques for animal tumor metabolomic work, as well as highlight some interesting applications of these techniques for various animal tumor types.