Feasibility of [18F]FSPG PET for Early Response Assessment to Combined Blockade of EGFR and Glutamine Metabolism in Wild-Type KRAS Colorectal Cancer

Early response assessment is critical for personalizing cancer therapy. Emerging therapeutic regimens with encouraging results in the wild-type (WT) KRAS colorectal cancer (CRC) setting include inhibitors of epidermal growth factor receptor (EGFR) and glutaminolysis. Towards predicting clinical outcome, this preclinical study evaluated non-invasive positron emission tomography (PET) with (4S)-4-(3-[¹⁸F]fluoropropyl)-L-glutamic acid ([¹⁸F]FSPG) in treatment-sensitive and treatment-resistant WT KRAS CRC patient-derived xenografts (PDXs). Tumor-bearing mice were imaged with [¹⁸F]FSPG PET before and one week following the initiation of treatment with either EGFR-targeted monoclonal antibody (mAb) therapy, glutaminase inhibitor therapy, or the combination. Imaging was correlated with tumor volume and histology. In PDX that responded to therapy, [¹⁸F]FSPG PET was significantly decreased from baseline at 1-week post-therapy, prior to changes in tumor volume. In contrast, [¹⁸F]FSPG PET was not decreased in non-responding PDX. These data suggest that [¹⁸F]FSPG PET may serve as an early metric of response to EGFR and glutaminase inhibition in the WT KRAS CRC setting.

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.

[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.

Utility of [18F]FSPG PET to Image Hepatocellular Carcinoma: First Clinical Evaluation in a US Population. Mol Imaging Biol. 2016;18:924-934. [PMID: 27677886 DOI: 10.1007/s11307-016-1007-0]

PURPOSE

Non-invasive imaging is central to hepatocellular carcinoma (HCC) diagnosis; however, conventional modalities are limited by smaller tumors and other chronic diseases that are often present in patients with HCC, such as cirrhosis. This pilot study evaluated the feasibility of (4S)-4-(3-[¹⁸F]fluoropropyl)-L-glutamic acid ([¹⁸F]FSPG) positron emission tomography (PET)/X-ray computed tomography (CT) to image HCC. [¹⁸F]FSPG PET/CT was compared to standard-of-care (SOC) magnetic resonance imaging (MRI) and CT, and [¹¹C]acetate PET/CT, commonly used in this setting. We report the largest cohort of HCC patients imaged to date with [¹⁸F]FSPG PET/CT and present the first comparison to [¹¹C]acetate PET/CT and SOC imaging. This study represents the first in a US HCC population, which is distinguished by different underlying comorbidities than non-US populations.

PROCEDURES

x_C- transporter RNA and protein levels were evaluated in HCC and matched liver samples from The Cancer Genome Atlas (n = 16) and a tissue microarray (n = 83). Eleven HCC patients who underwent prior MRI or CT scans were imaged by [¹⁸F]FSPG PET/CT, with seven patients also imaged with [¹¹C]acetate PET/CT.

RESULTS

x_C- transporter RNA and protein levels were elevated in HCC samples compared to background liver. Over 50 % of low-grade HCCs and ~70 % of high-grade tumors exceeded background liver protein expression. [¹⁸F]FSPG PET/CT demonstrated a detection rate of 75 %. [¹⁸F]FSPG PET/CT also identified an HCC devoid of typical MRI enhancement pattern. Patients scanned with [¹⁸F]FSPG and [¹¹C]acetate PET/CT exhibited a 90 and 70 % detection rate, respectively. In dually positive tumors, [¹⁸F]FSPG accumulation consistently resulted in significantly greater tumor-to-liver background ratios compared with [¹¹C]acetate PET/CT.

CONCLUSIONS

[¹⁸F]FSPG PET/CT is a promising modality for HCC imaging, and larger studies are warranted to examine [¹⁸F]FSPG PET/CT impact on diagnosis and management of HCC. [¹⁸F]FSPG PET/CT may also be useful for phenotyping HCC tumor metabolism as part of precision cancer medicine.