Noninvasive Stratification of Colon Cancer by Multiplex PET Imaging.

PURPOSE: The current approach for molecular subtyping of colon cancer relies on gene expression profiling, which is invasive and has limited ability to reveal dynamics and spatial heterogeneity. Molecular imaging techniques, such as PET, present a noninvasive alternative for visualizing biological information from tumors. However, the factors influencing PET imaging phenotype, the suitable PET radiotracers for differentiating tumor subtypes, and the relationship between PET phenotypes and tumor genotype or gene expression-based subtyping remain unknown. EXPERIMENTAL DESIGN: In this study, we conducted 126 PET scans using four different metabolic PET tracers, [18F]fluorodeoxy-D-glucose ([18F]FDG), O-(2-[18F]fluoroethyl)-l-tyrosine ([18F]FET), 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT), and [11C]acetate ([11C]ACE), using a spectrum of five preclinical colon cancer models with varying genetics (BMT, AKPN, AK, AKPT, KPN), at three sites (subcutaneous, orthograft, autochthonous) and at two tumor stages (primary vs. metastatic). RESULTS: The results demonstrate that imaging signatures are influenced by genotype, tumor environment, and stage. PET imaging signatures exhibited significant heterogeneity, with each cancer model displaying distinct radiotracer profiles. Oncogenic Kras and Apc loss showed the most distinctive imaging features, with [18F]FLT and [18F]FET being particularly effective, respectively. The tissue environment notably impacted [18F]FDG uptake, and in a metastatic model, [18F]FET demonstrated higher uptake. CONCLUSIONS: By examining factors contributing to PET-imaging phenotype, this study establishes the feasibility of noninvasive molecular stratification using multiplex radiotracer PET. It lays the foundation for further exploration of PET-based subtyping in human cancer, thereby facilitating noninvasive molecular diagnosis.

Magnetic Resonance Imaging Is More Sensitive Than PET for Detecting Treatment-Induced Cell Death-Dependent Changes in Glycolysis.

Metabolic imaging has been widely used to measure the early responses of tumors to treatment. Here, we assess the abilities of PET measurement of [18F]FDG uptake and MRI measurement of hyperpolarized [1-13C]pyruvate metabolism to detect early changes in glycolysis following treatment-induced cell death in human colorectal (Colo205) and breast adenocarcinoma (MDA-MB-231) xenografts in mice. A TRAIL agonist that binds to human but not mouse cells induced tumor-selective cell death. Tumor glycolysis was assessed by injecting [1,6-13C2]glucose and measuring 13C-labeled metabolites in tumor extracts. Injection of hyperpolarized [1-13C]pyruvate induced rapid reduction in lactate labeling. This decrease, which correlated with an increase in histologic markers of cell death and preceded decrease in tumor volume, reflected reduced flux from glucose to lactate and decreased lactate concentration. However, [18F]FDG uptake and phosphorylation were maintained following treatment, which has been attributed previously to increased [18F]FDG uptake by infiltrating immune cells. Quantification of [18F]FDG uptake in flow-sorted tumor and immune cells from disaggregated tumors identified CD11b+/CD45+ macrophages as the most [18F]FDG-avid cell type present, yet they represented <5% of the cells present in the tumors and could not explain the failure of [18F]FDG-PET to detect treatment response. MRI measurement of hyperpolarized [1-13C]pyruvate metabolism is therefore a more sensitive marker of the early decreases in glycolytic flux that occur following cell death than PET measurements of [18F]FDG uptake. SIGNIFICANCE: These findings demonstrate superior sensitivity of MRI measurement of hyperpolarized [1-13C]pyruvate metabolism versus PET measurement of 18F-FDG uptake for detecting early changes in glycolysis following treatment-induced tumor cell death.

[18F]fluoroethyltyrosine-induced Cerenkov Luminescence Improves Image-Guided Surgical Resection of Glioma.

The extent of surgical resection is significantly correlated with outcome in glioma; however, current intraoperative navigational tools are useful only in a subset of patients. We show here that a new optical intraoperative technique, Cerenkov luminescence imaging (CLI) following intravenous injection of O­(2-[18F]fluoroethyl)-L-tyrosine (FET), can be used to accurately delineate glioma margins, performing better than the current standard of fluorescence imaging with 5-aminolevulinic acid (5-ALA). Methods: Rats implanted orthotopically with U87, F98 and C6 glioblastoma cells were injected with FET and 5-aminolevulinic acid (5-ALA). Positive and negative tumor regions on histopathology were compared with CL and fluorescence images. The capability of FET CLI and 5-ALA fluorescence imaging to detect tumor was assessed using receptor operator characteristic curves and optimal thresholds (CLIOptROC and 5-ALAOptROC) separating tumor from healthy brain tissue were determined. These thresholds were used to guide prospective tumor resections, where the presence of tumor cells in the resected material and in the remaining brain were assessed by Ki-67 staining. Results: FET CLI signal was correlated with signal in preoperative PET images (y = 1.06x - 0.01; p < 0.0001) and with expression of the amino acid transporter SLC7A5 (LAT1). FET CLI (AUC = 97%) discriminated between glioblastoma and normal brain in human and rat orthografts more accurately than 5-ALA fluorescence (AUC = 91%), with a sensitivity >92% and specificity >91%, and resulted in a more complete tumor resection. Conclusion: FET CLI can be used to accurately delineate glioblastoma tumor margins, performing better than the current standard of fluorescence imaging following 5-ALA administration, and is therefore a promising technique for clinical translation.

Late Imaging with [1-(11)C]Acetate Improves Detection of Tumor Fatty Acid Synthesis with PET.

UNLABELLED: Tumors are often characterized by high levels of de novo fatty acid synthesis. The kinetics of acetate incorporation into tricarboxylic acid cycle intermediates and into lipids suggest that detection of tumors with [1-(11)C]acetate PET could be improved by imaging at later time points. METHODS: The uptake and metabolism of [1-(11)C], [1-(13)C], and [1-(14)C]acetate were measured in mouse prostate and lung cancer models to investigate the time course of (11)C label incorporation into tumor metabolites. RESULTS: Radioactivity in the lipid fraction, as compared with the aqueous fraction, in extracts of C4-2B human prostate xenografts peaked at 90 min after [1-(14)C]acetate injection, which coincided with peak (13)C label incorporation into the fatty acids palmitate and stearate. Contrast between the tumor and tissues, such as blood and muscle, increased in PET images acquired over a period of 120 min after [1-(11)C]acetate injection, and Patlak plots were linear from 17.5 min after injection. Similar results were obtained in a genetically engineered K-ras(G12D); p53(null) lung cancer model, in which the mean tumor-to-lung ratio at 90 min after [1-(14)C]acetate injection was 4.4-fold higher than at 15 min. CONCLUSION: These findings suggest that when imaging de novo fatty acid synthesis with [1-(11)C]acetate it is preferable to measure uptake at later time points, when the effects of perfusion and (11)C incorporation into tricarboxylic acid cycle intermediates and bicarbonate are declining. The data presented here suggest that future clinical PET scans of tumors should be acquired later than 30 min, when tracer accumulation due to de novo fatty acid synthesis prevails.