BAY-94-9392

We aimed to explore whether the imaging of antiporter system x_C ^- of immune cells with (4S)-4-(3-¹⁸F-fluoropropyl)-l-glutamate (¹⁸F-FSPG) PET can assess inflammatory bowel disease (IBD) activity in murine models and patients (NCT03546868). Methods: ¹⁸F-FSPG PET imaging was performed to assess IBD activity in mice with dextran sulfate sodium-induced and adoptive T-cell transfer-induced IBD and a cohort of 20 patients at a tertiary care center in South Korea. Immunohistochemical analysis of system x_C ^- and cell surface markers was also studied. Results: Mice with experimental IBD showed increased intestinal ¹⁸F-FSPG uptake and xCT expression in cells positive (+) for CD11c, F4/80, and CD3 in the lamina propria, increases positively associated with clinical and pathologic disease activity. ¹⁸F-FSPG PET studies in patients, most of whom were clinically in remission or had mildly active IBD, showed that PET imaging was sufficiently accurate in diagnosing endoscopically active IBD and remission in patients and bowel segments. ¹⁸F-FSPG PET correctly identified all 9 patients with superficial or deep ulcers. Quantitative intestinal ¹⁸F-FSPG uptake was strongly associated with endoscopic indices of IBD activity. The number of CD68⁺xCT⁺ and CD3⁺xCT⁺ cells in 22 bowel segments from patients with ulcerative colitis and the number of CD68⁺xCT⁺ cells in 7 bowel segments from patients with Crohn disease showed a significant positive association with endoscopic indices of IBD activity. Conclusion: The assessment of system x_C ^- in immune cells may provide diagnostic information on the immune responses responsible for chronic active inflammation in IBD. ¹⁸F-FSPG PET imaging of system x_C ^- activity may noninvasively assess the IBD activity.

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.