Clinical [18F]FSPG Positron Emission Tomography Imaging Reveals Heterogeneity in Tumor-Associated System xc- Activity

BACKGROUND

(4S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG) positron emission tomography/computed tomography (PET/CT) provides a readout of system xc- transport activity and has been used for cancer detection in clinical studies of different cancer types. As system xc- provides the rate-limiting precursor for glutathione biosynthesis, an abundant antioxidant, [18F]FSPG imaging may additionally provide important prognostic information. Here, we performed an analysis of [18F]FSPG radiotracer distribution between primary tumors, metastases, and normal organs from cancer patients. We further assessed the heterogeneity of [18F]FSPG retention between cancer types, and between and within individuals.

METHODS

This retrospective analysis of prospectively collected data compared [18F]FSPG PET/CT in subjects with head and neck squamous cell cancer (HNSCC, n = 5) and non-small-cell lung cancer (NSCLC, n = 10), scanned at different institutions. Using semi-automated regions of interest drawn around tumors and metastases, the maximum standardized uptake value (SUVmax), SUVmean, SUV standard deviation and SUVpeak were measured. [18F]FSPG time-activity curves (TACs) for normal organs, primary tumors and metastases were subsequently compared to 18F-2-fluoro-2-deoxy-D-glucose ([18F]FDG) PET/CT at 60 min post injection (p.i.).

RESULTS

The mean administered activity of [18F]FSPG was 309.3 ± 9.1 MBq in subjects with NSCLC and 285.1 ± 11.3 MBq in those with HNSCC. The biodistribution of [18F]FSPG in both cohorts showed similar TACs in healthy organs from cancer patients. There was no statistically significant overall difference in the average SUVmax of tumor lesions at 60 min p.i. for NSCLC (8.1 ± 7.1) compared to HNSCC (6.0 ± 4.1; p = 0.29) for [18F]FSPG. However, there was heterogeneous retention between and within cancer types; the SUVmax at 60 min p.i. ranged from 1.4 to 23.7 in NSCLC and 3.1-12.1 in HNSCC.

CONCLUSION

[18F]FSPG PET/CT imaging from both NSCLC and HNSCC cohorts showed the same normal-tissue biodistribution, but marked tumor heterogeneity across subjects and between lesions. Despite rapid elimination through the urinary tract and low normal-background tissue retention, the diagnostic potential of [18F]FSPG was limited by variability in tumor retention. As [18F]FSPG retention is mediated by the tumor's antioxidant capacity and response to oxidative stress, this heterogeneity may provide important insights into an individual tumor's response or resistance to therapy.

Theranostics - a sure cure for cancer after 100 years?

Cancer has remained a formidable challenge in medicine and has claimed an enormous number of lives worldwide. Theranostics, combining diagnostic methods with personalized therapeutic approaches, shows huge potential to advance the battle against cancer. This review aims to provide an overview of theranostics in oncology: exploring its history, current advances, challenges, and prospects. We present the fundamental evolution of theranostics from radiotherapeutics, cellular therapeutics, and nanotherapeutics, showcasing critical milestones in the last decade. From the early concept of targeted drug delivery to the emergence of personalized medicine, theranostics has benefited from advances in imaging technologies, molecular biology, and nanomedicine. Furthermore, we emphasize pertinent illustrations showcasing that revolutionary strategies in cancer management enhance diagnostic accuracy and provide targeted therapies customized for individual patients, thereby facilitating the implementation of personalized medicine. Finally, we describe future perspectives on current challenges, emerging topics, and advances in the field.

mRNA therapy corrects defective glutathione metabolism and restores ureagenesis in preclinical argininosuccinic aciduria

The urea cycle enzyme argininosuccinate lyase (ASL) enables the clearance of neurotoxic ammonia and the biosynthesis of arginine. Patients with ASL deficiency present with argininosuccinic aciduria, an inherited metabolic disease with hyperammonemia and a systemic phenotype coinciding with neurocognitive impairment and chronic liver disease. Here, we describe the dysregulation of glutathione biosynthesis and upstream cysteine utilization in ASL-deficient patients and mice using targeted metabolomics and in vivo positron emission tomography (PET) imaging using (S)-4-(3-18F-fluoropropyl)-l-glutamate ([18F]FSPG). Up-regulation of cysteine metabolism contrasted with glutathione depletion and down-regulated antioxidant pathways. To assess hepatic glutathione dysregulation and liver disease, we present [18F]FSPG PET as a noninvasive diagnostic tool to monitor therapeutic response in argininosuccinic aciduria. Human hASL mRNA encapsulated in lipid nanoparticles improved glutathione metabolism and chronic liver disease. In addition, hASL mRNA therapy corrected and rescued the neonatal and adult Asl-deficient mouse phenotypes, respectively, enhancing ureagenesis. These findings provide mechanistic insights in liver glutathione metabolism and support clinical translation of mRNA therapy for argininosuccinic aciduria.

Imaging the Tumor Antioxidant Response with [18F]FSPG PET

(4S)-4-(3-[18F]Fluoropropyl)-L-glutamic acid ([18F]FSPG) is a flourine-18 labeled glutamate analog that enables the noninvasive in vivo imaging of cellular redox status. [18F]FSPG is transported across the cell membrane by the cystine/glutamate antiporter, system xc-, whose expression is upregulated in multiple cancer types. The requirement of cystine for the biosynthesis of glutathione, a major antioxidant, connects [18F]FSPG tissue retention to the intracellular redox response via system xc- activity. We herein describe the use of [18F]FSPG positron emission tomography (PET) to image the tumor antioxidant response and highlight key methodological considerations.

Imaging the master regulator of the antioxidant response in non-small cell lung cancer with positron emission tomography

Mutations in the NRF2-KEAP1 pathway are common in non-small cell lung cancer (NSCLC) and confer broad-spectrum therapeutic resistance, leading to poor outcomes. The cystine/glutamate antiporter, system xc-, is one of the >200 cytoprotective proteins controlled by NRF2, which can be non-invasively imaged by (S)-4-(3-18F-fluoropropyl)-l-glutamate ([18F]FSPG) positron emission tomography (PET). Through genetic and pharmacologic manipulation, we show that [18F]FSPG provides a sensitive and specific marker of NRF2 activation in advanced preclinical models of NSCLC. We validate imaging readouts with metabolomic measurements of system xc- activity and their coupling to intracellular glutathione concentration. A redox gene signature was measured in patients from the TRACERx 421 cohort, suggesting an opportunity for patient stratification prior to imaging. Furthermore, we reveal that system xc- is a metabolic vulnerability that can be therapeutically targeted for sustained tumour growth suppression in aggressive NSCLC. Our results establish [18F]FSPG as predictive marker of therapy resistance in NSCLC and provide the basis for the clinical evaluation of both imaging and therapeutic agents that target this important antioxidant pathway.

The chicken chorioallantoic membrane as a low-cost, high-throughput model for cancer imaging

Mouse models are invaluable tools for radiotracer development and validation. They are, however, expensive, low throughput, and are constrained by animal welfare considerations. Here, we assessed the chicken chorioallantoic membrane (CAM) as an alternative to mice for preclinical cancer imaging studies. NCI-H460 FLuc cells grown in Matrigel on the CAM formed vascularized tumors of reproducible size without compromising embryo viability. By designing a simple method for vessel cannulation it was possible to perform dynamic PET imaging in ovo, producing high tumor-to-background signal for both 18F-2-fluoro-2-deoxy-D-glucose (18F-FDG) and (4S)-4-(3-18F-fluoropropyl)-L-glutamate (18F-FSPG). The pattern of 18F-FDG tumor uptake were similar in ovo and in vivo, although tumor-associated radioactivity was higher in the CAM-grown tumors over the 60 min imaging time course. Additionally, 18F-FSPG provided an early marker of both treatment response to external beam radiotherapy and target inhibition in ovo. Overall, the CAM provided a low-cost alternative to tumor xenograft mouse models which may broaden access to PET and SPECT imaging and have utility across multiple applications.

Is System xc- a Suitable Target for Tumour Detection and Response Assessment with Imaging?

System xc- is upregulated in cancer cells and can be imaged using novel radiotracers, most commonly with (4S)-4-(3-[18F]fluoropropyl)-L-glutamic acid (18F-FSPG). The aim of this review was to summarise the use of 18F-FSPG in humans, explore the benefits and limitations of 18F-FSPG, and assess the potential for further use of 18F-FSPG in cancer patients. To date, ten papers have described the use of 18F-FSPG in human cancers. These studies involved small numbers of patients (range 1-26) and assessed the use of 18F-FSPG as a general oncological diagnostic agent across different cancer types. These clinical trials were contrasting in their findings, limiting the scope of 18F-FSPG PET/CT as a purely diagnostic agent, primarily due to heterogeneity of 18F-FSPG retention both between cancer types and patients. Despite these limitations, a potential further application for 18F-FSPG is in the assessment of early treatment response and prediction of treatment resistance. Animal models of cancer have shown that changes in 18F-FSPG retention following effective therapy precede glycolytic changes, as indicated by 18F-FDG, and changes in tumour volume, as measured by CT. If these results could be replicated in human clinical trials, imaging with 18F-FSPG PET/CT would offer an exciting route towards addressing the currently unmet clinical needs of treatment resistance prediction and early imaging assessment of therapy response.

A prodrug strategy for the in vivo imaging of aldehyde dehydrogenase activity

Therapy resistance is one of the biggest challenges facing clinical oncology. Despite a revolution in new anti-cancer drugs targeting multiple components of the tumour microenvironment, acquired or innate resistance frequently blunts the efficacy of these treatments. Non-invasive identification of drug-resistant tumours will enable modification of the patient treatment pathway through the selection of appropriate second-line treatments. Here, we have designed a prodrug radiotracer for the non-invasive imaging of aldehyde dehydrogenase 1A1 (ALDH1A1) activity. Elevated ALDH1A1 activity is a marker of drug-resistant cancer cells, modelled here with matched cisplatin-sensitive and -resistant human SKOV3 ovarian cancer cells. The aromatic aldehyde of our prodrug radiotracer was intracellularly liberated by esterase cleavage of the geminal diacetate and specifically trapped by ALDH through its conversion to the charged carboxylic acid. Through this mechanism of action, ALDH-specific retention of our prodrug radiotracer in the drug-resistant tumour cells was twice as high as the drug-sensitive cells. Acylal masking of the aldehyde afforded a modest protection from oxidation in the blood, which was substantially improved in carrier-added experiments. In vivo positron emission tomography imaging of tumour-bearing mice produced high tumour-to-background images and radiotracer uptake in high ALDH-expressing organs but was unable to differentiate between drug-sensitive and drug-resistant tumours. Alternative strategies to protect the labile aldehyde are currently under investigation.

Radiotracer stereochemistry affects substrate affinity and kinetics for improved imaging of system xC - in tumors

Amino acid utilization is perturbed in cancer cells, which rewire their metabolism to support cell survival and proliferation. This metabolic reprogramming can be exploited for diagnostic purposes through positron emission tomography imaging of fluorine-18 labeled amino acids. Despite its promise, little is known regarding transporter-recognition of non-natural amino acid stereoisomers or their utility for cancer imaging. We report here the synthesis and in vivo characterization of a radiolabeled amino acid (R)-4-(3-¹⁸F-fluoropropyl)-ʟ-glutamate ([¹⁸F]FRPG) and compared its tumor imaging properties to the 4S-isomer, [¹⁸F]FSPG.

Methods: [¹⁸F]FRPG and [¹⁸F]FSPG uptake was assessed in H460 lung cancer cells, with efflux measured 30 min after removal of exogenous activity. Specificity of [¹⁸F]FRPG for system x_C^- was further examined following transporter inhibition and blocking studies with system x_C^- substrates. [¹⁸F]FRPG and [¹⁸F]FSPG pharmacokinetics was next quantified in mice bearing subcutaneous A549, H460, VCAP and PC3 tumors, with mice bearing A549 tumors imaged by PET/CT. To better-understand differential tumor retention, radiometabolite analysis was performed on tissue and blood samples after imaging. Next, [¹⁸F]FRPG and [¹⁸F]FSPG retention in lipopolysaccharide-treated lungs were compared to an orthotopic H460 lung cancer model. Finally, the sensitivity of [¹⁸F]FRPG to manipulation of the redox environment was examined in cell and in vivo models.

Results: [¹⁸F]FRPG was specifically transported across the plasma membrane by the cystine/glutamate antiporter system x_C^- and retained at high levels in multiple tumor models. Conversely, [¹⁸F]FRPG was rapidly extracted from the blood and cleared from tissues with low system x_C^- expression. Due to its favorable imaging properties, tumor-to-blood ratios ≥10 were achieved with [¹⁸F]FRPG, which were either equal to or greater than [¹⁸F]FSPG. In addition, [¹⁸F]FRPG retention in orthotopic lung tumors with high system x_C^- expression was 2.5-fold higher than inflamed tissue, allowing for clear tumor visualization. In vivo, [¹⁸F]FRPG and [¹⁸F]FSPG were metabolized to a single species, with [¹⁸F]FRPG showing a higher percentage of parent radiotracer in tumors compared to [¹⁸F]FSPG. [¹⁸F]FRPG was sensitive to redox manipulations and tumor retention was reduced following treatment with liposomal doxorubicin in mice bearing ovarian tumors.

Conclusions: Given the fast clearance and low background retention of [¹⁸F]FRPG throughout the body, this radiotracer holds promise for the imaging of system x_C^- activity and treatment response monitoring in tumors of the thorax, abdomen, and head and neck. [¹⁸F]FRPG PET imaging provides a sensitive noninvasive measure of system x_C^- and excellent properties for cancer imaging.

Latest advances in imaging oxidative stress in cancer

Oxidative stress is the imbalance of harmful reactive oxygen species (ROS) and the action of neutralizing antioxidant mechanisms. If left unchecked, the deleterious effects of oxidative stress results in damage to DNA, proteins, and membranes, ultimately resulting in cell death. Tumors are highly proliferative and consequently generate high levels of mitochondrial ROS. To compensate and maintain redox homeostasis, cancer cells upregulate protective antioxidant pathways, which are further amplified in drug-resistant tumors. This review provides an overview of the latest molecular imaging techniques designed to image oxidative stress in cancer. New probes are now able to assess heterogeneous ROS and antioxidant production within tumors and across lesions. Together, the non-invasive imaging of these dynamic processes holds great promise for treatment response monitoring, prediction of drug resistance, and may provide insight into the metastatic potential of tumors.

Robust and Facile Automated Radiosynthesis of [18F]FSPG on the GE FASTlab

Purpose

(S)-4-(3-¹⁸F-Fluoropropyl)-ʟ-Glutamic Acid ([¹⁸F]FSPG) is a radiolabeled non-natural amino acid that is used for positron emission tomography (PET) imaging of the glutamate/cystine antiporter, system x_C^-, whose expression is upregulated in many cancer types. To increase the clinical adoption of this radiotracer, reliable and facile automated procedures for [¹⁸F]FSPG production are required. Here, we report a cassette-based method to produce [¹⁸F]FSPG at high radioactivity concentrations from low amounts of starting activity.

Procedures

An automated synthesis and purification of [¹⁸F]FSPG was developed using the GE FASTlab. Optimization of the reaction conditions and automated manipulations were performed by measuring the isolated radiochemical yield of [¹⁸F]FSPG and by assessing radiochemical purity using radio-HPLC. Purification of [¹⁸F]FSPG was conducted by trapping and washing of the radiotracer on Oasis MCX SPE cartridges, followed by a reverse elution of [¹⁸F]FSPG in phosphate-buffered saline. Subsequently, the [¹⁸F]FSPG obtained from the optimized process was used to image an animal model of non-small cell lung cancer.

Results

The optimized protocol produced [¹⁸F]FSPG in 38.4 ± 2.6 % radiochemical yield and >96 % radiochemical purity with a molar activity of 11.1 ± 7.7 GBq/μmol. Small alterations, including the implementation of a reverse elution and an altered Hypercarb cartridge, led to significant improvements in radiotracer concentration from <10 MBq/ml to >100 MBq/ml. The improved radiotracer concentration allowed for the imaging of up to 20 mice, starting with just 1.5 GBq of [¹⁸F]Fluoride.

Conclusions

We have developed a robust and facile method for [¹⁸F]FSPG radiosynthesis in high radiotracer concentration, radiochemical yield, and radiochemical purity. This cassette-based method enabled the production of [¹⁸F]FSPG at radioactive concentrations sufficient to facilitate large-scale preclinical experiments with a single prep of starting activity. The use of a cassette-based radiosynthesis on an automated synthesis module routinely used for clinical production makes the method amenable to rapid and widespread clinical translation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-021-01609-w.

The chemical tool-kit for molecular imaging with radionuclides in the age of targeted and immune therapy

Nuclear medicine has evolved over the last half-century from a functional imaging modality using a handful of radiopharmaceuticals, many of unknown structure and mechanism of action, into a modern speciality that can properly be described as molecular imaging, with a very large number of specific radioactive probes of known structure that image specific molecular processes. The advances of cancer treatment in recent decades towards targeted and immune therapies, combined with recognition of heterogeneity of cancer cell phenotype among patients, within patients and even within tumours, has created a growing need for personalised molecular imaging to support treatment decision. This article describes the evolution of the present vast range of radioactive probes – radiopharmaceuticals – leveraging a wide variety of chemical disciplines, over the last half century. These radiochemical innovations have been inspired by the need to support personalised medicine and also by the parallel development in development of new radionuclide imaging technologies – from gamma scintigraphy, through single photon emission tomography (SPECT), through the rise of clinical positron emission tomography (PET) and PET-CT, and perhaps in the future, by the advent of total body PET. Thus, in the interdisciplinary world of nuclear medicine and molecular imaging, as quickly as radiochemistry solutions are developed to meet new needs in cancer imaging, new challenges emerge as developments in one contributing technology drive innovations in the others.

Iron(II)/Persulfate Mediated Newman-Kwart Rearrangement

Herein, we report that iron(II)/ammonium persulfate in aqueous acetonitrile mediates the Newman-Kwart rearrangement of O-aryl carbamothioates. Electron-rich substrates react rapidly under moderate heating to afford the rearranged products in excellent yields. The mild conditions, rapid reaction rates, and suitability for scale up offers immediate practical benefits to access functionalized thiophenols.

One-Pot Radiosynthesis and Biological Evaluation of a Caspase-3 Selective 5-[123,125I]iodo-1,2,3-triazole derived Isatin SPECT Tracer

Induction of apoptosis is often necessary for successful cancer therapy, and the non-invasive monitoring of apoptosis post-therapy could assist in clinical decision making. Isatins are a class of compounds that target activated caspase-3 during apoptosis. Here we report the synthesis of the 5-iodo-1,2,3-triazole (FITI) analog of the PET tracer [18F]ICMT11 as a candidate tracer for imaging of apoptosis with SPECT, as well as PET. Labelling with radioiodine (123,125I) was achieved in 55 ± 12% radiochemical yield through a chelator-accelerated one-pot cycloaddition reaction mediated by copper(I) catalysis. The caspase-3 binding affinity and selectivity of FITI compares favourably to that of [18F]ICMT11 (Ki = 6.1 ± 0.9 nM and 12.4 ± 4.7 nM, respectively). In biodistribution studies, etoposide-induced cell death in a SW1222 xenograft model resulted in a 2-fold increase in tumour uptake of the tracer. However, the tumour uptake was too low to allow in vivo imaging of apoptosis with SPECT.

Imaging of T-cells and their responses during anti-cancer immunotherapy

Immunotherapy has proven to be an effective approach in a growing number of cancers. Despite durable clinical responses achieved with antibodies targeting immune checkpoint molecules, many patients do not respond. The common denominator for immunotherapies that have successfully been introduced in the clinic is their potential to induce or enhance infiltration of cytotoxic T-cells into the tumour. However, in clinical research the molecules, cells and processes involved in effective responses during immunotherapy remain largely obscure. Therefore, in vivo imaging technologies that interrogate T-cell responses in patients represent a powerful tool to boost further development of immunotherapy. This review comprises a comprehensive analysis of the in vivo imaging technologies that allow the characterisation of T-cell responses induced by anti-cancer immunotherapy, with emphasis on technologies that are clinically available or have high translational potential. Throughout we discuss their respective strengths and weaknesses, providing arguments for selecting the optimal imaging options for future research and patient management.

High-Throughput PET/CT Imaging Using a Multiple-Mouse Imaging System

A considerable limitation of current small-animal PET/CT imaging is the low throughput of acquisitions. Consequently, to sufficiently power a study, high costs accumulate. Together with a commercial scanner manufacturer, we developed a 4-bed mouse “hotel” to simultaneously image up to 4 mice, thereby reducing costs and maximizing the efficiency of radiotracer use when compared with scans performed with a single mouse bed. Methods: For physiologic evaluation of the mouse hotel, temperature and anesthesia were tested for uniformity in conjunction with ¹⁸F-FDG PET/CT imaging of mini image-quality phantoms designed to fit the new imaging system. After reconstruction, National Electrical Manufacturers Association NU-4 tests examined uniformity, recovery coefficients, and spillover ratios. To evaluate the mouse hotel under standard in vivo imaging conditions, 4 mice were simultaneously scanned by dynamic ¹⁸F-FDG PET/CT over 60 min, and quantified images were compared with those acquired using a single mouse bed. Results: The mouse hotel maintained a constant temperature of 36.8°C ± 0.4°C, with anesthesia distributed evenly to each nose cone (2.9 ± 0.1 L/min). The National Electrical Manufacturers Association tests revealed values within tolerable limits for uniformity, for recovery coefficients in rods larger than 2 mm, and for spillover ratios in the nonradioactive water- and air-filled chambers. There was low variability in radiotracer uptake in all major organs for the mouse hotel versus the single mouse bed. Conclusion: Analysis of images acquired using the mouse hotel confirmed its utility to increase the throughput of small-animal PET imaging without considerable loss of image quality or quantitative precision. In comparison to a single mouse bed, the cost and time associated with each scan were substantially reduced.

[18F]-SuPAR: A Radiofluorinated Probe for Noninvasive Imaging of DNA Damage-Dependent Poly(ADP-ribose) Polymerase Activity

Poly(ADP ribose) polymerase (PARP) enzymes generate poly(ADP ribose) post-translational modifications on target proteins for an array of functions centering on DNA and cell stress. PARP isoforms 1 and 2 are critically charged with the surveillance of DNA integrity and are the first line guardians of the genome against DNA breaks. Here we present a novel probe ([¹⁸F]-SuPAR) for noninvasive imaging of PARP-1/2 activity using positron emission tomography (PET). [¹⁸F]-SuPAR is a radiofluorinated nicotinamide adenine dinucleotide (NAD) analog that can be recognized by PARP-1/2 and incorporated into the long branched polymers of poly(ADP ribose) (PAR). The measurement of PARP-1/2 activity was supported by a reduction of radiotracer uptake in vivo following PARP-1/2 inhibition with talazoparib treatment, a potent PARP inhibitor recently approved by FDA for treatment of breast cancer, as well as ex vivo colocalization of radiotracer analog and poly(ADP ribose). With [¹⁸F]-SuPAR, we were able to map the dose- and time-dependent activation of PARP-1/2 following radiation therapy in breast and cervical cancer xenograft mouse models. Tumor response to therapy was determined by [¹⁸F]-SuPAR PET within 8 h of administration of a single dose of radiation equivalent to one round of stereotactic ablative radiotherapy.

Measurement of Tumor Antioxidant Capacity and Prediction of Chemotherapy Resistance in Preclinical Models of Ovarian Cancer by Positron Emission Tomography

PURPOSE

Drug resistance is a major obstacle for the effective treatment of patients with high-grade serous ovarian cancer (HGSOC). Currently, there is no satisfactory way to identify patients with HGSOC that are refractive to the standard of care. Here, we propose the system xc - radiotracer (4S)-4-(3-[18F]fluoropropyl)-l-glutamate ([18F]FSPG) as a non-invasive method to measure upregulated antioxidant pathways present in drug-resistant HGSOC.

EXPERIMENTAL DESIGN

Using matched chemotherapy sensitive and resistant ovarian cancer cell lines, we assessed their antioxidant capacity and its relation to [18F]FSPG uptake, both in cells and in animal models of human ovarian cancer. We identified the mechanisms driving differential [18F]FSPG cell accumulation and evaluated [18F]FSPG tumor uptake as predictive marker of treatment response in drug-resistant tumors.

RESULTS

High intracellular glutathione (GSH) and low reactive oxygen species corresponded to decreased [18F]FSPG cell accumulation in drug-resistant versus drug-sensitive cells. Decreased [18F]FSPG uptake in drug-resistant cells was a consequence of changes in intracellular cystine, a key precursor in GSH biosynthesis. In vivo, [18F]FSPG uptake was decreased nearly 80% in chemotherapy-resistant A2780 tumors compared with parental drug-sensitive tumors, with nonresponding tumors displaying high levels of oxidized-to-reduced GSH. Treatment of drug-resistant A2780 tumors with doxorubicin resulted in no detectable change in tumor volume, GSH, or [18F]FSPG uptake.

CONCLUSIONS

This study demonstrates the ability of [18F]FSPG to detect upregulated antioxidant pathways present in drug-resistant cancer. [18F]FSPG may therefore enable the identification of patients with HGSOC that are refractory to standard of care, allowing the transferal of drug-resistant patients to alternative therapies, thereby improving outcomes in this disease.

Assessment of Tumor Redox Status through (S)-4-(3-[18F]fluoropropyl)-L-Glutamic Acid PET Imaging of System xc - Activity

The cell's endogenous antioxidant system is vital to maintenance of redox homeostasis. Despite its central role in normal and pathophysiology, no noninvasive tools exist to measure this system in patients. The cystine/glutamate antiporter system xc - maintains the balance between intracellular reactive oxygen species and antioxidant production through the provision of cystine, a key precursor in glutathione biosynthesis. Here, we show that tumor cell retention of a system xc --specific PET radiotracer, (S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG), decreases in proportion to levels of oxidative stress following treatment with a range of redox-active compounds. The decrease in [18F]FSPG retention correlated with a depletion of intracellular cystine resulting from increased de novo glutathione biosynthesis, shown through [U-13C6, U-15N2]cystine isotopic tracing. In vivo, treatment with the chemotherapeutic doxorubicin decreased [18F]FSPG tumor uptake in a mouse model of ovarian cancer, coinciding with markers of oxidative stress but preceding tumor shrinkage and decreased glucose utilization. Having already been used in pilot clinical trials, [18F]FSPG PET could be rapidly translated to the clinic as an early redox indicator of tumor response to treatment. SIGNIFICANCE: [18F]FSPG PET imaging provides a sensitive noninvasive measure of tumor redox status and provides an early marker of tumor response to therapy.See related commentary by Lee et al., p. 701.

Mapping Aldehyde Dehydrogenase 1A1 Activity using an [18 F]Substrate-Based Approach

Aldehyde dehydrogenases (ALDHs) catalyze the oxidation of aldehydes to carboxylic acids. Elevated ALDH expression in human cancers is linked to metastases and poor overall survival. Despite ALDH being a poor prognostic factor, the non‐invasive assessment of ALDH activity in vivo has not been possible due to a lack of sensitive and translational imaging agents. Presented in this report are the synthesis and biological evaluation of ALDH1A1‐selective chemical probes composed of an aromatic aldehyde derived from N,N‐diethylamino benzaldehyde (DEAB) linked to a fluorinated pyridine ring either via an amide or amine linkage. Of the focused library of compounds evaluated, N‐ethyl‐6‐(fluoro)‐N‐(4‐formylbenzyl)nicotinamide 4 b was found to have excellent affinity and isozyme selectivity for ALDH1A1 in vitro. Following ¹⁸F‐fluorination, [¹⁸F]4 b was taken up by colorectal tumor cells and trapped through the conversion to its ¹⁸F‐labeled carboxylate product under the action of ALDH. In vivo positron emission tomography revealed high uptake of [¹⁸F]4 b in the lungs and liver, with radioactivity cleared through the urinary tract. Oxidation of [¹⁸F]4 b, however, was observed in vivo, which may limit the tissue penetration of this first‐in‐class radiotracer.

Imaging Cancer Metabolism with Positron Emission Tomography (PET)

Positron emission tomography (PET) enables the noninvasive spatiotemporal analysis of cancer metabolism in vivo. Both natural and nonnatural PET tracers have been developed to assess metabolic pathways during tumorigenesis, cancer progression, and metastasis. Here we describe the dynamic in vivo PET/CT imaging of the glucose analogue [18F]fluoro-2-deoxy-D-glucose (FDG), taking into consideration the methodology for alternative metabolic PET substrates.

3'-Deoxy-3'-[18F]Fluorothymidine Uptake Is Related to Thymidine Phosphorylase Expression in Various Experimental Tumor Models

PURPOSE

We recently reported that high thymidine phosphorylase (TP) expression is accompanied by low tumor thymidine concentration and high 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) uptake in four untreated lung cancer xenografts. Here, we investigated whether this relationship also holds true for a broader range of tumor models.

PROCEDURES

Lysates from n = 15 different tumor models originating from n = 6 institutions were tested for TP and thymidylate synthase (TS) expression using western blots. Results were correlated to [18F]FLT accumulation in the tumors as determined by positron emission tomography (PET) measurements in the different institutions and to previously published thymidine concentrations.

RESULTS

Expression of TP correlated positively with [18F]FLT SUVmax (ρ = 0.549, P < 0.05). Furthermore, tumors with high TP levels possessed lower levels of thymidine (ρ = - 0.939, P < 0.001).

CONCLUSIONS

In a broad range of tumors, [18F]FLT uptake as measured by PET is substantially influenced by TP expression and tumor thymidine concentrations. These data strengthen the role of TP as factor confounding [18F]FLT uptake.

[18F]FSPG-PET reveals increased cystine/glutamate antiporter (xc-) activity in a mouse model of multiple sclerosis

BACKGROUND

The cystine/glutamate antiporter (xc-) has been implicated in several neurological disorders and, specifically, in multiple sclerosis (MS) as a mediator of glutamate excitotoxicity and proinflammatory immune responses. We aimed to evaluate an xc-specific positron emission tomography (PET) radiotracer, (4S)-4-(3-[¹⁸F]fluoropropyl)-L-glutamate ([¹⁸F]FSPG), for its ability to allow non-invasive monitoring of xc- activity in a mouse model of MS.

METHODS

Experimental autoimmune encephalomyelitis (EAE) was induced in C57BL/6 mice by subcutaneous injection of myelin oligodendrocyte glycoprotein (MOG_35-55) peptide in complete Freund's adjuvant (CFA) followed by pertussis toxin. Control mice received CFA emulsion and pertussis toxin without MOG peptide, while a separate cohort of naïve mice received no treatment. PET studies were performed to investigate the kinetics and distribution of [¹⁸F]FSPG in naïve, control, pre-symptomatic, and symptomatic EAE mice, compared to ¹⁸F-fluorodeoxyglucose ([¹⁸F]FDG). After final PET scans, each mouse was perfused and radioactivity in dissected tissues was measured using a gamma counter. Central nervous system (CNS) tissues were further analyzed using ex vivo autoradiography or western blot. [¹⁸F]FSPG uptake in human monocytes, and T cells pre- and post-activation was investigated in vitro.

RESULTS

[¹⁸F]FSPG was found to be more sensitive than [¹⁸F]FDG at detecting pathological changes in the spinal cord and brain of EAE mice. Even before clinical signs of disease, a small but significant increase in [¹⁸F]FSPG signal was observed in the spinal cord of EAE mice compared to controls. This increase in PET signal became more pronounced in symptomatic EAE mice and was confirmed by ex vivo biodistribution and autoradiography. Likewise, in the brain of symptomatic EAE mice, [¹⁸F]FSPG uptake was significantly higher than controls, with the largest changes observed in the cerebellum. Western blot analyses of CNS tissues revealed a significant correlation between light chain of xc- (xCT) protein levels, the subunit of xc- credited with its transporter activity, and [¹⁸F]FSPG-PET signal. In vitro [¹⁸F]FSPG uptake studies suggest that both activated monocytes and T cells contribute to the observed in vivo PET signal.

CONCLUSION

These data highlight the promise of [¹⁸F]FSPG-PET as a technique to provide insights into neuroimmune interactions in MS and the in vivo role of xc- in the development and progression of this disease, thus warranting further investigation.

Reporter gene imaging of targeted T cell immunotherapy in recurrent glioma

High-grade gliomas are aggressive cancers that often become rapidly fatal. Immunotherapy using CD8⁺ cytotoxic T lymphocytes (CTLs), engineered to express both herpes simplex virus type 1 thymidine kinase (HSV1-TK) and interleukin-13 (IL-13) zetakine chimeric antigen receptor (CAR), is a treatment strategy with considerable potential. To optimize this and related immunotherapies, it would be helpful to monitor CTL viability and trafficking to glioma cells. We show that noninvasive positron emission tomography (PET) imaging with 9-[4-[¹⁸F]fluoro-3-(hydroxymethyl)butyl]guanine ([¹⁸F]FHBG) can track HSV1-tk reporter gene expression present in CAR-engineered CTLs. [¹⁸F]FHBG imaging was safe and enabled the longitudinal imaging of T cells stably transfected with a PET reporter gene in patients. Further optimization of this imaging approach for monitoring in vivo cell trafficking should greatly benefit various cell-based therapies for cancer.

Rapid Imaging of Tumor Cell Death In Vivo Using the C2A Domain of Synaptotagmin-I

Cell death is an important target for imaging the early response of tumors to treatment. We describe here the validation of a phosphatidylserine-binding agent for detecting tumor cell death in vivo based on the C2A domain of synaptotagmin-I. Methods: The capability of near-infrared fluorophore-labeled and 99mTc- and 111In-labeled derivatives of C2Am for imaging tumor cell death, using planar near-infrared fluorescence imaging and SPECT, respectively, was evaluated in implanted and genetically engineered mouse models of lymphoma and in a human colorectal xenograft. Results: The fluorophore-labeled C2Am derivative showed predominantly renal clearance and high specificity and sensitivity for detecting low levels of tumor cell death (2%-5%). There was a significant correlation (R > 0.9, P < 0.05) between fluorescently labeled C2Am binding and histologic markers of cell death, including cleaved caspase-3, whereas there was no such correlation with a site-directed mutant of C2Am (iC2Am) that does not bind phosphatidylserine. 99mTc-C2Am and 111In-C2Am also showed favorable biodistribution profiles, with predominantly renal clearance and low nonspecific retention in the liver and spleen at 24 h after probe administration. 99mTc-C2Am and 111In-C2Am generated tumor-to-muscle ratios in drug-treated tumors of 4.3× and 2.2×, respectively, at 2 h and 7.3× and 4.1×, respectively, at 24 h after administration. Conclusion: Given the favorable biodistribution profile of 99mTc- and 111In-labeled C2Am, and their ability to produce rapid and cell death-specific image contrast, these agents have potential for clinical translation.

PET imaging of tumor glycolysis downstream of hexokinase through noninvasive measurement of pyruvate kinase M2

Cancer cells reprogram their metabolism to meet increased biosynthetic demands, commensurate with elevated rates of replication. Pyruvate kinase M2 (PKM2) catalyzes the final and rate-limiting step in tumor glycolysis, controlling the balance between energy production and the synthesis of metabolic precursors. We report here the synthesis and evaluation of a positron emission tomography (PET) radiotracer, [(11)C]DASA-23, that provides a direct noninvasive measure of PKM2 expression in preclinical models of glioblastoma multiforme (GBM). In vivo, orthotopic U87 and GBM39 patient-derived tumors were clearly delineated from the surrounding normal brain tissue by PET imaging, corresponding to exclusive tumor-associated PKM2 expression. In addition, systemic treatment of mice with the PKM2 activator TEPP-46 resulted in complete abrogation of the PET signal in intracranial GBM39 tumors. Together, these data provide the basis for the clinical evaluation of imaging agents that target this important gatekeeper of tumor glycolysis.

Synthesis and pre-clinical evaluation of a [18F]fluoromethyl-tanaproget derivative for imaging of progesterone receptor expression

This work features the synthesis and biological evaluation of a novel fluorinated derivative of tanaproget and a carbon-11 labelled analogue, a high affinity non-steroidal PR ligand, as a possible candidate for imaging PR expressionin vivo.

A Systematic Comparison of 18F-C-SNAT to Established Radiotracer Imaging Agents for the Detection of Tumor Response to Treatment

PURPOSE

An early readout of tumor response to therapy through measurement of drug or radiation-induced cell death may provide important prognostic indications and improved patient management. It has been shown that the uptake of (18)F-C-SNAT can be used to detect early response to therapy in tumors by positron emission tomography (PET) via a mechanism of caspase-3-triggered nanoaggregation.

EXPERIMENTAL DESIGN

Here, we compared the preclinical utility of (18)F-C-SNAT for the detection of drug-induced cell death to clinically evaluated radiotracers, (18)F-FDG, (99m)Tc-Annexin V, and (18)F-ML-10 in tumor cells in culture, and in tumor-bearing mice in vivo.

RESULTS

In drug-treated lymphoma cells, (18)F-FDG, (99m)Tc-Annexin V, and (18)F-C-SNAT cell-associated radioactivity correlated well to levels of cell death (R(2) > 0.8; P < 0.001), with no correlation measured for (18)F-ML-10 (R(2) = 0.05; P > 0.05). A similar pattern of response was observed in two human NSCLC cell lines following carboplatin treatment. EL-4 tumor uptake of (99m)Tc-Annexin V and (18)F-C-SNAT were increased 1.4- and 2.1-fold, respectively, in drug-treated versus naïve control animals (P < 0.05), although (99m)Tc-Annexin V binding did not correlate to ex vivo TUNEL staining of tissue sections. A differential response was not observed with either (18)F-FDG or (18)F-ML-10.

CONCLUSIONS

We have demonstrated here that (18)F-C-SNAT can sensitively detect drug-induced cell death in murine lymphoma and human NSCLC. Despite favorable image contrast obtained with (18)F-C-SNAT, the development of next-generation derivatives, using the same novel and promising uptake mechanism, but displaying improved biodistribution profiles, are warranted for maximum clinical utility.

Preclinical evaluation of 3-18F-fluoro-2,2-dimethylpropionic acid as an imaging agent for tumor detection

Deregulated cellular metabolism is a hallmark of many cancers. In addition to increased glycolytic flux, exploited for cancer imaging with (18)F-FDG, tumor cells display aberrant lipid metabolism. Pivalic acid is a short-chain, branched carboxylic acid used to increase oral bioavailability of prodrugs. After prodrug hydrolysis, pivalic acid undergoes intracellular metabolism via the fatty acid oxidation pathway. We have designed a new probe, 3-(18)F-fluoro-2,2-dimethylpropionic acid, also called (18)F-fluoro-pivalic acid ((18)F-FPIA), for the imaging of aberrant lipid metabolism and cancer detection.

METHODS

Cell intrinsic uptake of (18)F-FPIA was measured in murine EMT6 breast adenocarcinoma cells. In vivo dynamic imaging, time course biodistribution, and radiotracer stability testing were performed. (18)F-FPIA tumor retention was further compared in vivo to (18)F-FDG uptake in several xenograft models and inflammatory tissue.

RESULTS

(18)F-FPIA rapidly accumulated in EMT6 breast cancer cells, with retention of intracellular radioactivity predicted to occur via a putative (18)F-FPIA carnitine-ester. The radiotracer was metabolically stable to degradation in mice. In vivo imaging of implanted EMT6 murine and BT474 human breast adenocarcinoma cells by (18)F-FPIA PET showed rapid and extensive tumor localization, reaching 9.1% ± 0.5% and 7.6% ± 1.2% injected dose/g, respectively, at 60 min after injection. Substantial uptake in the cortex of the kidney was seen, with clearance primarily via urinary excretion. Regarding diagnostic utility, uptake of (18)F-FPIA was comparable to that of (18)F-FDG in EMT6 tumors but superior in the DU145 human prostate cancer model (54% higher uptake; P = 0.002). Furthermore, compared with (18)F-FDG, (18)F-FPIA had lower normal-brain uptake resulting in a superior tumor-to-brain ratio (2.5 vs. 1.3 in subcutaneously implanted U87 human glioma tumors; P = 0.001), predicting higher contrast for brain cancer imaging. Both radiotracers showed increased localization in inflammatory tissue.

CONCLUSION

(18)F-FPIA shows promise as an imaging agent for cancer detection and warrants further investigation.

Preclinical assessment of carboplatin treatment efficacy in lung cancer by 18F-ICMT-11-positron emission tomography

Tumour response to therapy is assessed primarily in the clinic by monitoring reductions in tumour size. However, this approach lacks sensitivity since in many cases several weeks may elapse before there is evidence of tumour shrinkage. There is therefore a need to develop non-invasive imaging techniques for monitoring tumour treatment response in the clinic. Here, we assessed the pre-clinical utility of (18)F-ICMT-11 positron emission tomography--a method for detecting caspase 3/7 activation--in non-small cell lung cancer (NSCLC). (18)F-ICMT-11 uptake was compared to molecular biochemical measures of cell death in PC9 and A549 NSCLC cells following treatment with carboplatin in vitro and in vivo. Carboplatin-induced apoptosis in the ERCC1 low/mutant EGFR PC9 cells was characterised by time and dose-related increased caspase-3/7 activation, poly-ADP-ribose polymerase cleavage and Annexin V staining. 18F-ICMT-11 uptake was consequently increased up to 14-fold at 200 µM carboplatin compared to vehicle treated cells (P<0.01). In contrast, necrosis was the predominant death mechanism in ERCC1 high/wt EGFR A549 cells and no change in (18)F-ICMT-11 uptake was detected. In vivo, histological analysis of PC9 tumour xenografts indicated high pre-therapy necrosis. A 4.6-fold increase in cleaved caspase-3/7 was measured in non-necrotic regions of PC9 tumours at 48 h post carboplatin therapy. Average PET-derived tumour (18)F-ICMT-11 uptake was insensitive to changes in apoptosis in the presence of substantial pre-existing necrosis. PET-based voxel intensity sorting however, identified intra-tumoural regions of high (18)F-ICMT-11 uptake, enabling accurate assessment of apoptosis and therefore therapy response. In A549 tumours that lacked high pre-therapy necrosis, carboplatin induced growth inhibition that was only minimally associated with apoptosis and thus not detectable by (18)F-ICMT-11 PET.

A novel radiotracer to image glycogen metabolism in tumors by positron emission tomography

The high rate of glucose uptake to fuel the bioenergetic and anabolic demands of proliferating cancer cells is well recognized and is exploited with (18)F-2-fluoro-2-deoxy-d-glucose positron emission tomography ((18)F-FDG-PET) to image tumors clinically. In contrast, enhanced glucose storage as glycogen (glycogenesis) in cancer is less well understood and the availability of a noninvasive method to image glycogen in vivo could provide important biologic insights. Here, we demonstrate that (18)F-N-(methyl-(2-fluoroethyl)-1H-[1,2,3]triazole-4-yl)glucosamine ((18)F-NFTG) annotates glycogenesis in cancer cells and tumors in vivo, measured by PET. Specificity of glycogen labeling was demonstrated by isolating (18)F-NFTG-associated glycogen and with stable knockdown of glycogen synthase 1, which inhibited (18)F-NFTG uptake, whereas oncogene (Rab25) activation-associated glycogen synthesis led to increased uptake. We further show that the rate of glycogenesis is cell-cycle regulated, enhanced during the nonproliferative state of cancer cells. We demonstrate that glycogen levels, (18)F-NFTG, but not (18)F-FDG uptake, increase proportionally with cell density and G1-G0 arrest, with potential application in the assessment of activation of oncogenic pathways related to glycogenesis and the detection of posttreatment tumor quiescence.

Evaluation of deuterated 18F- and 11C-labeled choline analogs for cancer detection by positron emission tomography

PURPOSE

(11)C-Choline-positron emission tomography (PET) has been exploited to detect the aberrant choline metabolism in tumors. Radiolabeled choline uptake within the imaging time is primarily a function of transport, phosphorylation, and oxidation. Rapid choline oxidation, however, complicates interpretation of PET data. In this study, we investigated the biologic basis of the oxidation of deuterated choline analogs and assessed their specificity in human tumor xenografts.

EXPERIMENTAL DESIGN

(11)C-Choline, (11)C-methyl-[1,2-(2)H(4)]-choline ((11)C-D4-choline), and (18)F-D4-choline were synthesized to permit comparison. Biodistribution, metabolism, small-animal PET studies, and kinetic analysis of tracer uptake were carried out in human colon HCT116 xenograft-bearing mice.

RESULTS

Oxidation of choline analogs to betaine was highest with (11)C-choline, with reduced oxidation observed with (11)C-D4-choline and substantially reduced with (18)F-D4-choline, suggesting that both fluorination and deuteration were important for tracer metabolism. Although all tracers were converted intracellularly to labeled phosphocholine (specific signal), the higher rate constants for intracellular retention (K(i) and k(3)) of (11)C-choline and (11)C-D4-choline, compared with (18)F-D4-choline, were explained by the rapid conversion of the nonfluorinated tracers to betaine within HCT116 tumors. Imaging studies showed that the uptake of (18)F-D4-choline in three tumors with similar radiotracer delivery (K(1)) and choline kinase α expression-HCT116, A375, and PC3-M-were the same, suggesting that (18)F-D4-choline has utility for cancer detection irrespective of histologic type.

CONCLUSION

We have shown here that both deuteration and fluorination combine to provide protection against choline oxidation in vivo. (18)F-D4-choline showed the highest selectivity for phosphorylation and warrants clinical evaluation.

Kinetic modeling of hyperpolarized 13C label exchange between pyruvate and lactate in tumor cells

Measurements of the kinetics of hyperpolarized (13)C label exchange between [1-(13)C]pyruvate and lactate in suspensions of intact and lysed murine lymphoma cells, and in cells in which lactate dehydrogenase expression had been modulated by inhibition of the PI3K pathway, were used to determine quantitatively the role of enzyme activity and membrane transport in controlling isotope flux. Both steps were shown to share in the control of isotope flux in these cells. The kinetics of label exchange were well described by a kinetic model that employed rate constants for the lactate dehydrogenase reaction that had been determined previously from steady state kinetic studies. The enzyme showed pyruvate inhibition in steady state kinetic measurements, which the kinetic model predicted should also be observed in the isotope exchange measurements. However, no such pyruvate inhibition was observed in either intact cells or cell lysates and this could be explained by the much higher enzyme concentrations present in the isotope exchange experiments. The kinetic analysis presented here shows how lactate dehydrogenase activity can be determined from the isotope exchange measurements. The kinetic model should be useful for modeling the exchange reaction in vivo, particularly as this technique progresses to the clinic.

Detection of tumor response to a vascular disrupting agent by hyperpolarized 13C magnetic resonance spectroscopy

Nuclear spin hyperpolarization can dramatically increase the sensitivity of the (13)C magnetic resonance experiment, allowing dynamic measurements of the metabolism of hyperpolarized (13)C-labeled substrates in vivo. Here, we report a preclinical study of the response of lymphoma tumors to the vascular disrupting agent (VDA), combretastatin-A4-phosphate (CA4P), as detected by measuring changes in tumor metabolism of hyperpolarized [1-(13)C]pyruvate and [1,4-(13)C(2)]fumarate. These measurements were compared with dynamic contrast agent-enhanced magnetic resonance imaging (DCE-MRI) measurements of tumor vascular function and diffusion-weighted MRI (DW-MRI) measurements of the tumor cell necrosis that resulted from subsequent loss of tumor perfusion. The rate constant describing flux of hyperpolarized (13)C label between [1-(13)C]pyruvate and lactate was decreased by 34% within 6 hours of CA4P treatment and remained at this lower level at 24 hours. The rate constant describing production of labeled malate from hyperpolarized [1,4-(13)C(2)]fumarate increased 1.6-fold and 2.5-fold at 6 and 24 hours after treatment, respectively, and correlated with the degree of necrosis detected in histologic sections. Although DCE-MRI measurements showed a substantial reduction in perfusion at 6 hours after treatment, which had recovered by 24 hours, DW-MRI showed no change in the apparent diffusion coefficient of tumor water at 6 hours after treatment, although there was a 32% increase at 24 hours (P < 0.02) when regions of extensive necrosis were observed by histology. Measurements of hyperpolarized [1-(13)C]pyruvate and [1,4-(13)C(2)]fumarate metabolism may provide, therefore, a more sustained and sensitive indicator of response to a VDA than DCE-MRI or DW-MRI, respectively.

Detecting treatment response in a model of human breast adenocarcinoma using hyperpolarised [1-13C]pyruvate and [1,4-13C2]fumarate

BACKGROUND

The recent introduction of a dynamic nuclear polarisation technique has permitted noninvasive imaging of tumour cell metabolism in vivo following intravenous administration of (13)C-labelled cell substrates.

METHODS

Changes in hyperpolarised [1-(13)C]pyruvate and [1,4-(13)C(2)]fumarate metabolism were evaluated in both MDA-MB-231 cells and in implanted MDA-MB-231 tumours following doxorubicin treatment.

RESULTS

Treatment of MDA-MB-231 cells resulted in the induction of apoptosis, which was accompanied by a decrease in hyperpolarised (13)C label flux between [1-(13)C]pyruvate and lactate, which was correlated with a decrease in the cellular NAD(H) coenzyme pool. There was also an increase in the rate of fumarate conversion to malate, which accompanied the onset of cellular necrosis. In vivo, the decrease in (13)C label exchange between pyruvate and lactate and the increased flux between fumarate and malate, following drug treatment, were shown to occur in the absence of any detectable change in tumour size.

CONCLUSION

We show here that the early responses of a human breast adenocarcinoma tumour model to drug treatment can be followed by administration of both hyperpolarised [1-(13)C]pyruvate and [1,4-(13)C(2)]fumarate. These techniques could be used, therefore, in the clinic to detect the early responses of breast tumours to treatment.

Magnetization transfer measurements of exchange between hyperpolarized [1-13C]pyruvate and [1-13C]lactate in a murine lymphoma

Measurements of the conversion of hyperpolarized [1-(13)C]pyruvate into lactate, in the reaction catalyzed by lactate dehydrogenase, have shown promise as a metabolic marker for the presence of disease and response to treatment. However, it is unclear whether this represents net flux of label from pyruvate to lactate or exchange of isotope between metabolites that are close to chemical equilibrium. Using saturation and inversion transfer experiments, we show that there is significant exchange of label between lactate and pyruvate in a murine lymphoma in vivo. The rate constants estimated from the magnetization transfer experiments, at specific points during the time course of label exchange, were similar to those obtained by fitting the changes in peak intensities during the entire exchange time course to a kinetic model for two-site exchange. These magnetization transfer experiments may therefore provide an alternative and more rapid way of estimating flux between pyruvate and lactate to serial measurements of pyruvate and lactate (13)C peak intensities following injection of hyperpolarized [1-(13)C]pyruvate.