Pilot Preclinical and Clinical Evaluation of (4S)-4-(3-[18F]Fluoropropyl)-L-Glutamate (18F-FSPG) for PET/CT Imaging of Intracranial Malignancies

Imaging at the nexus: how state of the art imaging techniques can enhance our understanding of cancer and fibrosis

Both cancer and fibrosis are diseases involving dysregulation of cell signaling pathways resulting in an altered cellular microenvironment which ultimately leads to progression of the condition. The two disease entities share common molecular pathophysiology and recent research has illuminated the how each promotes the other. Multiple imaging techniques have been developed to aid in the early and accurate diagnosis of each disease, and given the commonalities between the pathophysiology of the conditions, advances in imaging one disease have opened new avenues to study the other. Here, we detail the most up-to-date advances in imaging techniques for each disease and how they have crossed over to improve detection and monitoring of the other. We explore techniques in positron emission tomography (PET), magnetic resonance imaging (MRI), second generation harmonic Imaging (SGHI), ultrasound (US), radiomics, and artificial intelligence (AI). A new diagnostic imaging tool in PET/computed tomography (CT) is the use of radiolabeled fibroblast activation protein inhibitor (FAPI). SGHI uses high-frequency sound waves to penetrate deeper into the tissue, providing a more detailed view of the tumor microenvironment. Artificial intelligence with the aid of advanced deep learning (DL) algorithms has been highly effective in training computer systems to diagnose and classify neoplastic lesions in multiple organs. Ultimately, advancing imaging techniques in cancer and fibrosis can lead to significantly more timely and accurate diagnoses of both diseases resulting in better patient outcomes.

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.

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.

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.

Simplified and highly-reliable automated production of [18F]FSPG for clinical studies

BACKGROUND

(S)-4-(3-¹⁸F-Fluoropropyl)-L-Glutamic Acid ([¹⁸F]FSPG) is a positron emission tomography (PET) tracer that specifically targets the cystine/glutamate antiporter (xc^-), which is frequently overexpressed in cancer and several neurological disorders. Pilot studies examining the dosimetry and biodistribution of [¹⁸F]FSPG in healthy volunteers and tumor detection in patients with non-small cell lung cancer, hepatocellular carcinoma, and brain tumors showed promising results. In particular, low background uptake in the brain, lung, liver, and bowel was observed that further leads to excellent imaging contrasts of [¹⁸F]FSPG PET. However, reliable production-scale cGMP-compliant automated procedures for [¹⁸F]FSPG production are still lacking to further increase the utility and clinical adoption of this radiotracer. Herein, we report the optimized automated approaches to produce [¹⁸F]FSPG through two commercially available radiosynthesizers capable of supporting centralized and large-scale production for clinical use.

RESULTS

Starting with activity levels of 60-85 GBq, the fully-automated process to produce [¹⁸F]FSPG took less than 45 min with average radiochemical yields of 22.56 ± 0.97% and 30.82 ± 1.60% (non-decay corrected) using TRACERlab™ FXFN and FASTlab™, respectively. The radiochemical purities were > 95% and the formulated [¹⁸F]FSPG solution was determined to be sterile and colorless with the pH of 6.5-7.5. No radiolysis of the product was observed up to 8 h after final batch formulation.

CONCLUSIONS

In summary, cGMP-compliant radiosyntheses and quality control of [¹⁸F]FSPG have been established on two commercially available synthesizers leveraging high activity concentration and radiochemical purity. While the clinical trials using [¹⁸F]FSPG PET are currently underway, the automated approaches reported herein will accelerate the clinical adoption of this radiotracer and warrant centralized and large-scale production of [¹⁸F]FSPG.

Simplified and Highly-reliable automated production of [18F]FSPG for clinical studies

Background (S)-4-(3- ¹⁸ F-Fluoropropyl)-L-Glutamic Acid ([ ¹⁸ F]FSPG) is a positron emission tomography (PET) tracer that specifically targets the cystine/glutamate antiporter (xc-), which is frequently overexpressed in cancer and several neurological disorders. Pilot studies examining the dosimetry and biodistribution of ([ ¹⁸ F]FSPG in healthy volunteers and tumor detection in patients with non-small cell lung cancer, hepatocellular carcinoma, and brain tumors showed promising results. In particular, low background uptake in the brain, lung, liver, and bowel was observed that further leads to excellent imaging contrasts of [ ¹⁸ F]FSPG PET. However, reliable production-scale cGMP-compliant automated procedures for [ ¹⁸ F]FSPG production are still lacking to further increase the utility and clinical adoption of this radiotracer. Herein, we report the optimized automated approaches to produce [ ¹⁸ F]FSPG through two commercially available radiosynthesizers capable of supporting centralized and large-scale production for clinical use. Results Starting with activity levels of 60-85 GBq, the fully-automated process to produce [ ¹⁸ F]FSPG took less than 45 minutes with average radiochemical yields of 22.56 ± 0.97% and 30.82 ± 1.60% (non-decay corrected) using TRACERlab™ FXFN and FASTlab™, respectively. The radiochemical purities were > 95% and the formulated [ ¹⁸ F]FSPG solution was determined to be sterile and colorless with the pH of 6.5-7.5. No radiolysis of the product was observed up to 8 hours after final batch formulation. Conclusions In summary, cGMP-compliant radiosyntheses and quality control of [ ¹⁸ F]FSPG have been established on two commercially available synthesizers leveraging high activity concentration and radiochemical purity. While the clinical trials using [ ¹⁸ F]FSPG PET are currently underway, the automated approaches reported herein will accelerate the clinical adoption of this radiotracer and warrant centralized and large-scale production of [ ¹⁸ F]FSPG.

Evaluation of sulfone-labeled amino acid derivatives as potential PET agents for cancer imaging

INTRODUCTION

As one of the most important and frequently used molecular imaging techniques in the clinic, positron emission tomography (PET) features high sensitivity and specificity, which generally involves the use of PET contrast agents. Despite the exceptional promise, the availability of novel PET agents could limit its application and there is a clear need to develop new PET agents to improve our understanding of targets of interest and increase the diagnostic specificity.

METHODS

Based on the fact that amino acid transport and protein anabolism are increased in tumor tissues, a series of ¹⁸F-labeled amino acid analog was labeled with ¹⁸F by using [¹⁸F]fluoro-4-(vinylsulfonyl)benzene as the radionuclide linker. The obtained probes were subjected to in vitro and in vivo evaluation, including stability, cell line transport channel specificity, PET/CT imaging on tumor and inflammation bearing mice, and biodistribution.

RESULTS

Our data shows that [¹⁸F]2a had moderate decay corrected labeling yield (>42 %) and high radiochemical purity (>99 %). When tested in vivo, the uptake of [¹⁸F]2a was 1.5 ± 0.2%ID/g in NCI-H1975 tumors and 1.1 ± 0.2%ID/g in inflammatory tissues. In contrast, the values for [¹⁸F]FDG were 5.7 ± 0.2%ID/g and 4.8 ± 0.1%ID/g, respectively. The inflammatory lesion-to-muscle contrast is 2.4 for [¹⁸F]2a, which is 3.0 for [¹⁸F]FDG.

CONCLUSION

Clearly, [¹⁸F]2a hold the great potential for cancer imaging. Its application in distinguishing tumor from inflammatory lesion would still need to be investigated further.

PET Oncological Radiopharmaceuticals: Current Status and Perspectives

Molecular imaging is the visual representation of biological processes that take place at the cellular or molecular level in living organisms. To date, molecular imaging plays an important role in the transition from conventional medical practice to precision medicine. Among all imaging modalities, positron emission tomography (PET) has great advantages in sensitivity and the ability to obtain absolute imaging quantification after corrections for photon attenuation and scattering. Due to the ability to label a host of unique molecules of biological interest, including endogenous, naturally occurring substrates and drug-like compounds, the role of PET has been well established in the field of molecular imaging. In this article, we provide an overview of the recent advances in the development of PET radiopharmaceuticals and their clinical applications in oncology.

18F-FSPG PET imaging for the evaluation of indeterminate pulmonary nodules

Background

¹⁸F-fluorodeoxyglucose (FDG) PET/CT is recommended for evaluation of intermediate-risk indeterminate pulmonary nodules (IPNs). While highly sensitive, the specificity of FDG remains suboptimal for differentiating malignant from benign nodules, particularly in areas where fungal lung diseases are prevalent. Thus, a cancer-specific imaging probe is greatly needed. In this study, we tested the hypothesis that a PET radiotracer (S)-4-(3-[¹⁸F]-fluoropropyl)-L-glutamic acid (FSPG) improves the diagnostic accuracy of IPNs compared to ¹⁸F-FDG PET/CT.

Methods

This study was conducted at a major academic medical center and an affiliated VA medical center. Twenty-six patients with newly discovered IPNs 7-30mm diameter or newly diagnosed lung cancer completed serial PET/CT scans utilizing ¹⁸F-FDG and ¹⁸F-FSPG, without intervening treatment of the lesion. The scans were independently reviewed by two dual-trained diagnostic radiology and nuclear medicine physicians. Characteristics evaluated included quantitative SUVmax values of the pulmonary nodules and metastases.

Results

A total of 17 out of 26 patients had cancer and 9 had benign lesions. ¹⁸F-FSPG was negative in 6 of 9 benign lesions compared to 7 of 9 with ¹⁸F-FDG. ¹⁸F-FSPG and ¹⁸F-FDG were positive in 14 of 17 and 12 of 17 malignant lesions, respectively. ¹⁸F-FSPG detected brain and intracardiac metastases missed by ¹⁸F-FDG PET in one case, while ¹⁸F-FDG detected a metastasis to the kidney missed by ¹⁸F-FSPG.

Conclusion

In this pilot study, there was no significant difference in overall diagnostic accuracy between ¹⁸F-FSPG and ¹⁸F-FDG for the evaluation of IPNs and staging of lung cancer. Additional studies will be needed to determine the clinical utility of this tracer in the management of IPNs and lung cancer.

18F-FSPG PET/CT Imaging of System xC- Transporter Activity in Patients with Primary and Metastatic Brain Tumors

Background The PET tracer (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate (¹⁸F-FSPG) targets the system x_C^- cotransporter, which is overexpressed in various tumors. Purpose To assess the role of ¹⁸F-FSPG PET/CT in intracranial malignancies. Materials and Methods Twenty-six patients (mean age, 54 years ± 12; 17 men; 48 total lesions) with primary brain tumors (n = 17) or brain metastases (n = 9) were enrolled in this prospective, single-center study (ClinicalTrials.gov identifier: NCT02370563) between November 2014 and March 2016. A 30-minute dynamic brain ¹⁸F-FSPG PET/CT scan and a static whole-body (WB) ¹⁸F-FSPG PET/CT scan at 60-75 minutes were acquired. Moreover, all participants underwent MRI, and four participants underwent fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG) PET imaging. PET parameters and their relative changes were obtained for all lesions. Kinetic modeling was used to estimate the ¹⁸F-FSPG tumor rate constants using the dynamic and dynamic plus WB PET data. Imaging parameters were correlated to lesion outcomes, as determined with follow-up MRI and/or pathologic examination. The Mann-Whitney U test or Student t test was used for group mean comparisons. Receiver operating characteristic curve analysis was used for performance comparison of different decision measures. Results ¹⁸F-FSPG PET/CT helped identify all 48 brain lesions. The mean tumor-to-background ratio (TBR) on the whole-brain PET images at the WB time point was 26.6 ± 24.9 (range: 2.6-150.3). When ¹⁸F-FDG PET was performed, ¹⁸F-FSPG permitted visualization of non-¹⁸F-FDG-avid lesions or allowed better lesion differentiation from surrounding tissues. In participants with primary brain tumors, the predictive accuracy of the relative changes in influx rate constant K_i and maximum standardized uptake value to discriminate between poor and good lesion outcomes were 89% and 81%, respectively. There were significant differences in the ¹⁸F-FSPG uptake curves of lesions with good versus poor outcomes in the primary brain tumor group (P < .05) but not in the brain metastases group. Conclusion PET/CT imaging with (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate (¹⁸F-FSPG) helped detect primary brain tumors and brain metastases with a high tumor-to-background ratio. Relative changes in ¹⁸F-FSPG uptake with multi-time-point PET appear to be helpful in predicting lesion outcomes. Clinical trial registration no. NCT02370563 © RSNA, 2022 Online supplemental material is available for this article.

TSPO imaging in animal models of brain diseases

Over the last 30 years, the 18-kDa TSPO protein has been considered as the PET imaging biomarker of reference to measure increased neuroinflammation. Generally assumed to image activated microglia, TSPO has also been detected in endothelial cells and activated astrocytes. Here, we provide an exhaustive overview of the recent literature on the TSPO-PET imaging (i) in the search and development of new TSPO tracers and (ii) in the understanding of acute and chronic neuroinflammation in animal models of neurological disorders. Generally, studies testing new TSPO radiotracers against the prototypic [11C]-R-PK11195 or more recent competitors use models of acute focal neuroinflammation (e.g. stroke or lipopolysaccharide injection). These studies have led to the development of over 60 new tracers during the last 15 years. These studies highlighted that interpretation of TSPO-PET is easier in acute models of focal lesions, whereas in chronic models with lower or diffuse microglial activation, such as models of Alzheimer's disease or Parkinson's disease, TSPO quantification for detection of neuroinflammation is more challenging, mirroring what is observed in clinic. Moreover, technical limitations of preclinical scanners provide a drawback when studying modest neuroinflammation in small brains (e.g. in mice). Overall, this review underlines the value of TSPO imaging to study the time course or response to treatment of neuroinflammation in acute or chronic models of diseases. As such, TSPO remains the gold standard biomarker reference for neuroinflammation, waiting for new radioligands for other, more specific targets for neuroinflammatory processes and/or immune cells to emerge.

Glucose Metabolic Alteration of Cerebral Cortical Subareas in Rats with Renal Ischemia/Reperfusion Based on Small-Animal Positron Emission Tomography

OBJECTIVE

To investigate glucose metabolic alterations in cerebral cortical subareas using ¹⁸F-labeled glucose derivative fluorodeoxyglucose (FDG) micro-positron emission tomography (PET) scanning in a rat renal ischemia/reperfusion (RIR) model.

METHODS

Small-animal PET imaging in vivo was performed with ¹⁸F-labeled FDG as a PET tracer to identify glucose metabolic alterations in cerebral cortical subregions using a rat model of RIR.

RESULTS

We found that the average standardized uptake value (SUV_average) of the cerebral cortical subareas in the RIR group was significantly increased compared to the sham group (P<0.05). We also found that glucose uptake in different cortical subregions including the left auditory cortex, right medial prefrontal cortex, right para cortex, left retrosplenial cortex, right retrosplenial cortex, and right visual cortex was significantly increased in the RIR group (P<0.05), but there was no significant difference in the SUV_average of right auditory cortex, left medial prefrontal cortex, left para cortex, and left visual cortex between the two groups.

CONCLUSION

The ¹⁸F-FDG PET data suggests that RIR causes a profound shift in the metabolic machinery of cerebral cortex subregions.

SLC7A11 Is a Superior Determinant of APR-246 (Eprenetapopt) Response than TP53 Mutation Status

APR-246 (eprenetapopt) is in clinical development with a focus on hematologic malignancies and is promoted as a mutant-p53 reactivation therapy. Currently, the detection of at least one TP53 mutation is an inclusion criterion for patient selection into most APR-246 clinical trials. Preliminary results from our phase Ib/II clinical trial investigating APR-246 combined with doublet chemotherapy [cisplatin and 5-fluorouracil (5-FU)] in metastatic esophageal cancer, together with previous preclinical studies, indicate that TP53 mutation status alone may not be a sufficient biomarker for APR-246 response. This study aims to identify a robust biomarker for response to APR-246. Correlation analysis of the PRIMA-1 activity (lead compound to APR-246) with mutational status, gene expression, protein expression, and metabolite abundance across over 700 cancer cell lines (CCL) was performed. Functional validation and a boutique siRNA screen of over 850 redox-related genes were also conducted. TP53 mutation status was not consistently predictive of response to APR-246. The expression of SLC7A11, the cystine/glutamate transporter, was identified as a superior determinant of response to APR-246. Genetic regulators of SLC7A11, including ATF4, MDM2, wild-type p53, and c-Myc, were confirmed to also regulate cancer-cell sensitivity to APR-246. In conclusion, SLC7A11 expression is a broadly applicable determinant of sensitivity to APR-246 across cancer and should be utilized as the key predictive biomarker to stratify patients for future clinical investigation of APR-246.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

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.

Real-Time Positron Emission Tomography Evaluation of Topotecan Brain Kinetics after Ultrasound-Mediated Blood-Brain Barrier Permeability

Glioblastoma (GBM) is the most common primary adult brain malignancy with an extremely poor prognosis and a median survival of fewer than two years. A key reason for this high mortality is that the blood-brain barrier (BBB) significantly restricts systemically delivered therapeutics to brain tumors. High-intensity focused ultrasound (HIFU) with microbubbles is a methodology being used in clinical trials to noninvasively permeabilize the BBB for systemic therapeutic delivery to GBM. Topotecan is a topoisomerase inhibitor used as a chemotherapeutic agent to treat ovarian and small cell lung cancer. Studies have suggested that topotecan can cross the BBB and can be used to treat brain metastases. However, pharmacokinetic data demonstrated that topotecan peak concentration in the brain extracellular fluid after systemic injection was ten times lower than in the blood, suggesting less than optimal BBB penetration by topotecan. We hypothesize that HIFU with microbubbles treatment can open the BBB and significantly increase topotecan concentration in the brain. We radiolabeled topotecan with 11C and acquired static and dynamic positron emission tomography (PET) scans to quantify [11C] topotecan uptake in the brains of normal mice and mice after HIFU treatment. We found that HIFU treatments significantly increased [11C] topotecan brain uptake. Moreover, kinetic analysis of the [11C] topotecan dynamic PET data demonstrated a substantial increase in [11C] topotecan volume of distribution in the brain. Furthermore, we found a decrease in [11C] topotecan brain clearance, confirming the potential of HIFU to aid in the delivery of topotecan through the BBB. This opens the potential clinical application of [11C] topotecan as a tool to predict topotecan loco-regional brain concentration in patients with GBMs undergoing experimental HIFU treatments.

Synthesis and analysis of 4-(3-fluoropropyl)-glutamic acid stereoisomers to determine the stereochemical purity of (4S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG) for clinical use

(4S)-4-(3-[¹⁸F]Fluoropropyl)-L-glutamic acid ([¹⁸F]FSPG) is a positron emission tomography (PET) imaging agent for measuring the system x_C⁻ transporter activity. It has been used for the detection of various cancers and metastasis in clinical trials. [¹⁸F]FSPG is also a promising diagnostic tool for evaluation of multiple sclerosis, drug resistance in chemotherapy, inflammatory brain diseases, and infectious lesions. Due to the very short half-life (110 min) of ¹⁸F nuclide, [¹⁸F]FSPG needs to be produced on a daily basis; therefore, fast and efficient synthesis and analytical methods for quality control must be established to assure the quality and safety of [¹⁸F]FSPG for clinical use. To manufacture cGMP-compliant [¹⁸F]FSPG, all four nonradioactive stereoisomers of FSPG were prepared as reference standards for analysis. (2S,4S)-1 and (2R,4R)-1 were synthesized starting from protected L- and D-glutamate derivatives in three steps, whereas (2S,4R)-1 and (2R,4S)-1 were prepared in three steps from protected (S)- and (R)-pyroglutamates. A chiral HPLC method for simultaneous determination of four FSPG stereoisomers was developed by using a 3-cm Chirex 3126 column and a MeCN/CuSO_4(aq) mobile phase. In this method, (2R,4S)-1, (2S,4S)-1, (2R,4R)-1, and (2S,4R)-1 were eluted in sequence with sufficient resolution in less than 25 min without derivatization. Scale-up synthesis of intermediates for the production of [¹⁸F]FSPG in high optical purity was achieved via stereo-selective synthesis or resolution by recrystallization. The enantiomeric excess of intermediates was determined by HPLC using a Chiralcel OD column and monitored at 220 nm. The nonradioactive precursor with >98% ee can be readily distributed to other facilities for the production of [¹⁸F]FSPG. Based on the above accomplishments, cGMP-compliant [¹⁸F]FSPG met the acceptance criteria in specifications and was successfully manufactured for human use. It has been routinely prepared and used in several pancreatic ductal adenocarcinoma metastasis-related clinical trials.

Take Advantage of Glutamine Anaplerosis, the Kernel of the Metabolic Rewiring in Malignant Gliomas

Glutamine is a non-essential amino acid that plays a key role in the metabolism of proliferating cells including neoplastic cells. In the central nervous system (CNS), glutamine metabolism is particularly relevant, because the glutamine-glutamate cycle is a way of controlling the production of glutamate-derived neurotransmitters by tightly regulating the bioavailability of the amino acids in a neuron-astrocyte metabolic symbiosis-dependent manner. Glutamine-related metabolic adjustments have been reported in several CNS malignancies including malignant gliomas that are considered ‘glutamine addicted’. In these tumors, glutamine becomes an essential amino acid preferentially used in energy and biomass production including glutathione (GSH) generation, which is crucial in oxidative stress control. Therefore, in this review, we will highlight the metabolic remodeling that gliomas undergo, focusing on glutamine metabolism. We will address some therapeutic regimens including novel research attempts to target glutamine metabolism and a brief update of diagnosis strategies that take advantage of this altered profile. A better understanding of malignant glioma cell metabolism will help in the identification of new molecular targets and the design of new therapies.

Initial evaluation of (4S)-4-(3-[18F]fluoropropyl)-L-glutamate (FSPG) PET/CT imaging in patients with head and neck cancer, colorectal cancer, or non-Hodgkin lymphoma

Purpose

(4S)-4-(3-[¹⁸F]Fluoropropyl)-l-glutamic acid ([¹⁸F]FSPG) measures system x_C⁻ transporter activity and shows promise for oncologic imaging. We present data on tumor uptake of this radiopharmaceutical in human subjects with head and neck cancer (HNC), colorectal cancer (CRC), and non-Hodgkin lymphoma (NHL).

Methods

A total of 15 subjects with HNC (n = 5), CRC (n = 5), or NHL (n = 5) were recruited (mean age 66.2 years, range 44–87 years). 301.4 ± 28.1 MBq (8.1 ± 0.8 mCi) of [¹⁸F]FSPG was given intravenously to each subject, and 3 PET/CT scans were obtained 0–2 h post-injection. All subjects also had a positive [¹⁸F]FDG PET/CT scan within 1 month prior to the [¹⁸F]FSPG PET scan. Semi-quantitative and visual comparisons of the [¹⁸F]FSPG and [¹⁸F]FDG scans were performed.

Results

[¹⁸F]FSPG showed strong uptake in all but one HNC subject. The lack of surrounding brain uptake facilitated tumor delineation in the HNC patients. [¹⁸F]FSPG also showed tumor uptake in all CRC subjects, but variable uptake in the NHL subjects. While the absolute [¹⁸F]FDG SUV values were comparable or higher than [¹⁸F]FSPG, the tumor-to-background SUV ratios were greater with [¹⁸F]FSPG than [¹⁸F]FDG.

Conclusions

[¹⁸F]FSPG PET/CT showed promising results across 15 subjects with 3 different cancer types. Concordant visualization was mostly observed between [¹⁸F]FSPG and [¹⁸F]FDG PET/CT images, with some inter- and intra-individual uptake variability potentially reflecting differences in tumor biology. The tumor-to-background ratios were greater with [¹⁸F]FSPG than [¹⁸F]FDG in the cancer types evaluated. Future studies based on larger numbers of subjects and those with a wider array of primary and recurrent or metastatic tumors are planned to further evaluate the utility of this novel tracer.

Clinical Evaluation of (4S)-4-(3-[18F]Fluoropropyl)-L-glutamate (18F-FSPG) for PET/CT Imaging in Patients with Newly Diagnosed and Recurrent Prostate Cancer

PURPOSE

(4S)-4-(3-[¹⁸F]Fluoropropyl)-L-glutamic acid (¹⁸F-FSPG) is a radiopharmaceutical for PET imaging of system x_C ^- activity, which can be upregulated in prostate cancer. We present data on the first evaluation of patients with newly diagnosed or recurrent prostate cancer with this radiopharmaceutical.

EXPERIMENTAL DESIGN

Ten patients with primary and 10 patients with recurrent prostate cancer were enrolled in this prospective multicenter study. After injection of 300 MBq of ¹⁸F-FSPG, three whole-body PET/CT scans were obtained. Visual analysis was compared with step-section histopathology when available as well as other imaging studies and clinical outcomes. Metabolic parameters were measured semiquantitatively. Expression levels of xCT and CD44 were evaluated by IHC for patients with available tissue samples.

RESULTS

¹⁸F-FSPG PET showed high tumor-to-background ratios with a relatively high tumor detection rate on a per-patient (89%) and per-lobe (87%) basis. The sensitivity was slightly higher with imaging at 105 minutes in comparison with 60 minutes. The maximum standardized uptake values (SUV_max) for cancer was significantly higher than both normal (P < 0.005) and benign pathology (P = 0.011), while there was no significant difference between normal and benign pathology (P = 0.120). In the setting of recurrence, agreement with standard imaging was demonstrated in 7 of 9 patients (78%) and 13 of 18 lesions (72%), and revealed true local recurrence in a discordant case. ¹⁸F-FSPG accumulation showed moderate correlation with CD44 expression.

CONCLUSIONS

¹⁸F-FSPG is a promising tumor imaging agent for PET that seems to have favorable biodistribution and high cancer detection rate in patients with prostate cancer. Further studies are warranted to determine the diagnostic value for both initial staging and recurrence, and how it compares with other investigational radiotracers and conventional imaging modalities.

Insight into the Development of PET Radiopharmaceuticals for Oncology

While the development of positron emission tomography (PET) radiopharmaceuticals closely follows that of traditional drug development, there are several key considerations in the chemical and radiochemical synthesis, preclinical assessment, and clinical translation of PET radiotracers. As such, we outline the fundamentals of radiotracer design, with respect to the selection of an appropriate pharmacophore. These concepts will be reinforced by exemplary cases of PET radiotracer development, both with respect to their preclinical and clinical evaluation. We also provide a guideline for the proper selection of a radionuclide and the appropriate labeling strategy to access a tracer with optimal imaging qualities. Finally, we summarize the methodology of their evaluation in in vitro and animal models and the road to clinical translation. This review is intended to be a primer for newcomers to the field and give insight into the workflow of developing radiopharmaceuticals.

Radiosynthesis of novel N-18 F-labeled 18 F-FHex-α-l-Glu and 18 F-FHex-β-Glu

N-¹⁸ F-labeled amino acids are important substitutes for new positron emission tomography (PET) imaging tracers complementing the deficiency of ¹⁸ F-fluorodeoxyglucose (¹⁸ F-FDG). In this work, two novel N-6-¹⁸ F-alkyl amino acid imaging agents, ¹⁸ F-FHex-α-l-Glu and ¹⁸ F-FHex-β-Glu, were designed and synthesized as potential probes for PET imaging of tumors. ¹⁸ F-FHex-α-l-Glu was synthesized using the precursor 6 from ¹⁸ F-F^- with the yield of 16 ± 4% (n = 5, uncorrected) within about 50 minutes. The specific activity was 14.5 GBq/μmol, and the radiochemical purity was more than 95%. ¹⁸ F-FHex-β-Glu was synthesized using the precursor 12 based on ¹⁸ F-F^- with the yield of 11 ± 3% (n = 3, uncorrected) in about 60 minutes. The specific activity was 9.1 GBq/μmol, and the radiochemical purity was more than 95%.

Radiochemistry: A Useful Tool in the Ophthalmic Drug Discovery

Positron Emission Tomography (PET) and Single Photon Emission Computerized Tomography (SPECT) are ultra-sensitive, fully translational and minimally invasive nuclear imaging techniques capable of tracing the spatiotemporal distribution of positron (PET) or gamma (SPECT) emitter-labeled molecules after administration into a living organism. Besides their impact in the clinical diagnostic, PET and SPECT are playing an increasing role in the process of drug development, both during the evaluation of the pharmacokinetic properties of new chemical entities as well as in the proof of concept, proof of mechanism and proof of efficacy studies. However, they have been scarcely applied in the context of ophthalmic drugs. In this paper, the basics of nuclear imaging and radiochemistry are briefly discussed, and the few examples of the use of these imaging modalities in ophthalmic drug development reported in the literature are presented and discussed. Finally, in a purely theoretical exercise, some labeling strategies that could be applied to the preparation of selected ophthalmic drugs are proposed and potential applications of nuclear imaging in ophthalmology are projected.

Targeting Glutamine Addiction in Gliomas

The most common malignant brain tumors are those of astrocytic origin, gliomas, with the most aggressive glioblastoma (WHO grade IV) among them. Despite efforts, medicine has not made progress in terms of the prognosis and life expectancy of glioma patients. Behind the malignant phenotype of gliomas lies multiple genetic mutations leading to reprogramming of their metabolism, which gives those highly proliferating cells an advantage over healthy ones. The so-called glutamine addiction is a metabolic adaptation that supplements oxidative glycolysis in order to secure neoplastic cells with nutrients and energy in unfavorable conditions of hypoxia. The present review aims at presenting the research and clinical attempts targeting the different metabolic pathways involved in glutamine metabolism in gliomas. A brief description of the biochemistry of glutamine transport, synthesis, and glutaminolysis, etc. will forego a detailed comparison of the therapeutic strategies undertaken to inhibit glutamine utilization by gliomas.

Design, synthesis and evaluation of a novel glutamine derivative (2S,4R)-2-amino-4-cyano-4-[18F]fluorobutanoic acid

A new glutamine derivative (2S,4R)-2-amino-4-cyano-4-[¹⁸F]fluorobutanoic acid (2S,4R)-4-[¹⁸F]FCABA ([18F]1) and its labeled precursor can be converted into (2S,4R)-4-[¹⁸F]FGln and (2S,4R)4-[¹⁸F]FGlu by changing the labeling conditions.

The Role of PET/CT in the Imaging of Pancreatic Neoplasms

Pancreas cancer is a complex disease and its prognosis is related to the origin of the tumor cell as well as the stage of disease at the time of diagnosis. Pancreatic adenocarcinomas derive from the exocrine pancreas and are the fourth leading cause of cancer-related deaths in the United States, while well-differentiated pancreatic neuroendocrine tumors (pNETs) derived from the endocrine part of the pancreas are rare and characterized by a slow growth and good life expectancy. Surgery is the only curative treatment approach, and an accurate assessment of resectability is of paramount importance in order to avoid futile procedures. The role of molecular imaging with positron emission tomography and computed tomography ranges from indispensable for pNETs to controversial for certain scenarios in pancreatic adenocarcinomas. This review article aims to overview molecular pancreatic imaging.

Glutamine Metabolism in Brain Tumors

Altered metabolism is a hallmark of cancer cells. Tumor cells rewire their metabolism to support their uncontrolled proliferation by taking up nutrients from the microenvironment. The amino acid glutamine is a key nutrient that fuels biosynthetic processes including ATP generation, redox homeostasis, nucleotide, protein, and lipid synthesis. Glutamine as a precursor for the neurotransmitter glutamate, and plays a critical role in the normal functioning of the brain. Brain tumors that grow in this glutamine/glutamate rich microenvironment can make synaptic connections with glutamatergic neurons and reprogram glutamine metabolism to enable their growth. In this review, we examine the functions of glutamate/glutamine in the brain and how brain tumor cells reprogram glutamine metabolism. Altered glutamine metabolism can be leveraged to develop non-invasive imaging strategies and we review these imaging modalities. Finally, we examine if targeting glutamine metabolism could serve as a therapeutic strategy in brain tumors.

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.

The 'Achilles Heel' of Metabolism in Renal Cell Carcinoma: Glutaminase Inhibition as a Rational Treatment Strategy

An important hallmark of cancer is 'metabolic reprogramming' or the rewiring of cellular metabolism to support rapid cell proliferation [1-5]. Metabolic reprogramming through oncometabolite-mediated transformation or activation of oncogenes in renal cell carcinoma (RCC) globally impacts energy production as well as glucose and glutamine utilization in RCC cells, which can promote dependence on glutamine supply to support cell growth and proliferation [6, 7]. Novel inhibitors of glutaminase, a key enzyme in glutamine metabolism, target glutamine addiction as a viable treatment strategy in metastatic RCC (mRCC). Here, we review glutamine metabolic pathways and how changes in cellular glutamine utilization enable the progression of RCC. This overview provides scientific rationale for targeting this pathway in patients with mRCC. We will summarize the current understanding of cellular and molecular mechanisms underlying anti-tumor efficacy of glutaminase inhibitors in RCC, provide an overview of clinical efforts targeting glutaminase in mRCC, and review approaches for identifying biomarkers for patient stratification and detecting therapeutic response early on in patients treated with this novel class of anti-cancer drug. Ultimately, results of ongoing clinical trials will demonstrate whether glutaminase inhibition can be a worthy addition to the current armamentarium of drugs used for patients with mRCC.

Prospective comparison of (4S)-4-(3-18F-fluoropropyl)-L-glutamate versus 18F-fluorodeoxyglucose PET/CT for detecting metastases from pancreatic ductal adenocarcinoma: a proof-of-concept study

PURPOSE

(4S)-4-(3-¹⁸F-Fluoropropyl)-L-glutamate (FSPG) positron emission tomography (PET) reflects system x_C^- transporter (xCT) expression. FSPG PET has been used to detect brain, lung, breast and liver cancer with only modest success. There is no report on the use of FSPG PET in pancreatic ductal adenocarcinoma (PDAC), presumably because of normal xCT expression in the pancreas. Nonetheless, the tissue-specific expression of xCT in the pancreas suggests that FSPG PET may be ideal for identifying metastasized PDAC.

METHODS

The performance of FSPG in detecting PDAC metastases was compared with that of ¹⁸F-fluorodeoxyglucose (FDG) in small-animal PET studies in seven PDAC tumour-bearing mice and in prospective PET/computed tomography (CT) studies in 23 patients with tissue-confirmed PDAC of stage III or stage IV. All PET/CT results were correlated with the results of histopathology or contrast-enhanced CT (ceCT) performed 3 and 6 months later.

RESULTS

In the rodent model, FSPG PET consistently found more PDAC metastases earlier than FDG PET. FSPG PET showed a trend for a higher sensitivity, specificity and diagnostic accuracy than FDG PET in detecting PDAC metastases in a patient-based analysis: 95.0%, 100.0% and 95.7%, and 90.0%, 66.7% and 90.0%, respectively. In a lesion-based analysis, FSPG PET identified significantly more PDAC metastases, especially in the liver, than FDG PET (109 vs. 95; P = 0.0001, 95% CI 4.9-14.6). The tumour-to-background ratios for FSPG and FDG uptake on positive scans were similar (FSPG 4.2 ± 4.3, FDG 3.6 ± 3.0; P = 0.44, 95% CI -1.11 to 0.48), despite a lower tumour maximum standardized uptake value in FSPG-avid lesions (FSPG 4.2 + 2.3, FDG 7.7 + 5.7; P = 0.002, 95% CI 0.70-4.10). Because of the lower physiological activity of FSPG in the liver, FSPG PET images of the liver are more easy to interpret than FDG PET images, and therefore the use of FSPG improves the detection of liver metastasis.

CONCLUSION

FSPG PET is superior to FDG PET in detecting metastasized PDAC, especially in the liver.

Investigational PET tracers for high-grade gliomas

High-grade gliomas (HGGs) are the most common primary malignant tumors of the brain, with glioblastoma (GBM) constituting over 50% of all the gliomas in adults. The disease carries very high mortality, and even with optimal treatment, the median survival is 2-5 years for anaplastic tumors and 1-2 years for GBMs. Neuroimaging is critical to managing patients with HGG for diagnosis, treatment planning, response assessment, and detecting recurrent disease. Magnetic resonance imaging (MRI) is the cornerstone of imaging in neuro-oncology, but molecular imaging with positron emission tomography (PET) can overcome some of the inherent limitations of MRI. Additionally, PET has the potential to target metabolic and molecular alterations in HGGs relevant to prognosis and therapy that cannot be assessed with anatomic imaging. Many classes of PET tracers have been evaluated in HGG including agents that target cell membrane biosynthesis, protein synthesis, amino acid transport, DNA synthesis, the tricarboxylic acid (TCA) cycle, hypoxic environments, cell surface receptors, blood flow, vascular endothelial growth factor (VEGF), epidermal growth factor (EGFR), and the 18-kDa translocator protein (TSPO), among others. This chapter will provide an overview of PET tracers for HGG that have been evaluated in human subjects with a focus on tracers that are not yet in widespread use for neuro-oncology.

Standard OSEM vs. regularized PET image reconstruction: qualitative and quantitative comparison using phantom data and various clinical radiopharmaceuticals

We investigated the block sequential regularized expectation maximization (BSREM) algorithm. ACR phantom measurements with different count statistics and 60 PET/CT research scans from the GE Discovery 600 and 690 scanners were reconstructed using BSREM and the standard-of-care OSEM algorithm. Hot concentration recovery and cold contrast recovery were measured from the phantom data. Two experienced nuclear medicine physicians reviewed the clinical images blindly. Liver SNR liver and SUV_max of the smallest lesion detected in each patient were also measured. The relationship between the maximum and mean hot concentration recovery remained monotonic below 1.5 maximum concentration recovery. The mean cold contrast recovery remained stable even for decreasing statistics with a highest absolute difference of 4% in air and 2% in bone for each reconstruction method. The D600 images resulted in an average 30% higher SNR than the D690 data for BSREM; there was no difference in SNR results between the two scanners with OSEM. The small lesion SUV_max values on the BSREM images with β of 250, 350 and 450, respectively were on average 80%, 60% and 43% (D690) and 42%, 29%, and 21% (D600) higher than in the case of OSEM. In conclusion, BSREM can outperform OSEM in terms of contrast recovery and organ uniformity over a range of PET tracers, but a task dependent regularization strength parameter (beta) selection may be necessary. To avoid image noise and artifacts, our results suggest that using higher beta values (at least 350) may be appropriate, especially if the data has low count statistics.

Cell-surface marker discovery for lung cancer

Lung cancer is the leading cause of cancer deaths in the United States. Novel lung cancer targeted therapeutic and molecular imaging agents are needed to improve outcomes and enable personalized care. Since these agents typically cannot cross the plasma membrane while carrying cytotoxic payload or imaging contrast, discovery of cell-surface targets is a necessary initial step. Herein, we report the discovery and characterization of lung cancer cell-surface markers for use in development of targeted agents. To identify putative cell-surface markers, existing microarray gene expression data from patient specimens were analyzed to select markers with differential expression in lung cancer compared to normal lung. Greater than 200 putative cell-surface markers were identified as being overexpressed in lung cancers. Ten cell-surface markers (CA9, CA12, CXorf61, DSG3, FAT2, GPR87, KISS1R, LYPD3, SLC7A11 and TMPRSS4) were selected based on differential mRNA expression in lung tumors vs. non-neoplastic lung samples and other normal tissues, and other considerations involving known biology and targeting moieties. Protein expression was confirmed by immunohistochemistry (IHC) staining and scoring of patient tumor and normal tissue samples. As further validation, marker expression was determined in lung cancer cell lines using microarray data and Kaplan–Meier survival analyses were performed for each of the markers using patient clinical data. High expression for six of the markers (CA9, CA12, CXorf61, GPR87, LYPD3, and SLC7A11) was significantly associated with worse survival. These markers should be useful for the development of novel targeted imaging probes or therapeutics for use in personalized care of lung cancer patients.

Fluorine-18 labeled amino acids for tumor PET/CT imaging

Tumor glucose metabolism and amino acid metabolism are usually enhanced, ¹⁸F-FDG for tumor glucose metabolism PET imaging has been clinically well known, but tumor amino acid metabolism PET imaging is not clinically familiar. Radiolabeled amino acids (AAs) are an important class of PET/CT tracers that target the upregulated amino acid transporters to show elevated amino acid metabolism in tumor cells. Radiolabeled amino acids were observed to have high uptake in tumor cells but low in normal tissues and inflammatory tissues. The radionuclides used in labeling amino acids include ¹⁵O, ¹³N, ¹¹C, ¹²³I, ¹⁸F and ⁶⁸Ga, among which the most commonly used is ¹⁸F [1]. Available data support the use of certain ¹⁸F-labeled AAs for PET/CT imaging of gliomas, neuroendocrine tumors, prostate cancer and breast cancer [2, 3]. With the progress of the method of ¹⁸F labeling AAs [4-6], ¹⁸F-labeled AAs are well established for tumor PET/CT imaging. This review focuses on the current status of key clinical applications of 18F-labeled AAs in tumor PET/CT imaging.

Brain Tumor Imaging

Modern imaging techniques, particularly functional imaging techniques that interrogate some specific aspect of underlying tumor biology, have enormous potential in neuro-oncology for disease detection, grading, and tumor delineation to guide biopsy and resection; monitoring treatment response; and targeting radiotherapy. This brief review considers the role of magnetic resonance imaging and spectroscopy, and positron emission tomography in these areas and discusses the factors that limit translation of new techniques to the clinic, in particular, the cost and difficulties associated with validation in multicenter clinical trials.

Role of Imaging in the Era of Precision Medicine

Precision medicine is an emerging approach for treating medical disorders, which takes into account individual variability in genetic and environmental factors. Preventive or therapeutic interventions can then be directed to those who will benefit most from targeted interventions, thereby maximizing benefits and minimizing costs and complications. Precision medicine is gaining increasing recognition by clinicians, healthcare systems, pharmaceutical companies, patients, and the government. Imaging plays a critical role in precision medicine including screening, early diagnosis, guiding treatment, evaluating response to therapy, and assessing likelihood of disease recurrence. The Association of University Radiologists Radiology Research Alliance Precision Imaging Task Force convened to explore the current and future role of imaging in the era of precision medicine and summarized its finding in this article. We review the increasingly important role of imaging in various oncological and non-oncological disorders. We also highlight the challenges for radiology in the era of precision medicine.

Metabolic Imaging of Glutamine in Cancer

Glucose and glutamine are the most abundant nutrients for producing energy and building blocks in normal and tumor cells. Increased glycolysis in tumors, the Warburg Effect, is the basis for ¹⁸F-FDG PET imaging. Cancer cells can also be genetically reprogrammed to use glutamine. 5-¹¹C-(2S)-glutamine and ¹⁸F-(2S,4R)4-fluoroglutamine may be useful complementary tools to measure changes in tumor metabolism. In glioma patients, the tracer ¹⁸F-(2S,4R)4-fluoroglutamine showed tumor-to-background contrast different from that of ¹⁸F-FDG and differences in uptake in glioma patients with clinical progression of disease versus stable disease (tumor-to-brain ratio > 3.7 in clinically active glioma tumors, minimal or no specific uptake in clinically stable tumors). These preliminary results suggest that ¹⁸F-(2S,4R)4-fluoroglutamine PET may be a new tool for probing in vivo metabolism of glutamine in cancer patients and for guiding glutamine-targeted therapeutics. Further studies of uptake mechanism, and comparison of kinetics for ¹⁸F-(2S,4R)4-fluoroglutamine versus the ¹¹C-labeled native glutamine, will be important and enlightening.

PET Imaging with [(18)F]FSPG Evidences the Role of System xc(-) on Brain Inflammation Following Cerebral Ischemia in Rats

In vivo Positron Emission Tomography (PET) imaging of the cystine-glutamate antiporter (system xc^-) activity with [¹⁸F]FSPG is meant to be an attractive tool for the diagnosis and therapy evaluation of brain diseases. However, the role of system xc^- in cerebral ischemia and its involvement in inflammatory reaction has been scarcely explored. In this work, we report the longitudinal investigation of the neuroinflammatory process following transient middle cerebral artery occlusion (MCAO) in rats using PET with [¹⁸F]FSPG and the translocator protein (TSPO) ligand [¹⁸F]DPA-714. In the ischemic territory, [¹⁸F]FSPG showed a progressive binding increase that peaked at days 3 to 7 and was followed by a progressive decrease from days 14 to 28 after reperfusion. In contrast, [¹⁸F]DPA-714 evidenced maximum binding uptake values over day 7 after reperfusion. Ex vivo immnunohistochemistry confirmed the up-regulation of system xc^- in microglial cells and marginally in astrocytes. Inhibition of system xc^- with sulfasalazine and S-4-CPG resulted in increased arginase (anti-inflammatory M2 marker) expression at day 7 after ischemia, together with a decrease in TSPO and microglial M1 proinflammatory markers (CCL2, TNF and iNOS) expression. Taken together, these results suggest that system xc^- plays a key role in the inflammatory reaction underlying experimental stroke.