This group of users covers a wide range of projects relevant to the translational roles of PET. Here, we highlight one project as an example of how PET studies support translational research (Glutamine metabolism in NMYC-driven neuroblastoma).

Oncogene-driven nutrient addiction provides translational opportunities in cancer imaging and therapeutics. Glucose, the major circulating fuel in mammals, is avidly consumed by many solid tumors, providing a basis for the widespread utility of [¹⁸F]FDG-PET in cancer. Altered cellular signal transduction stimulated by oncogenic mutations imposes enhanced glucose dependence on malignant cells, theoretically providing a therapeutic window for inhibitors of glucose metabolism.

Glutamine is the most abundant amino acid in mammals and serves as a major shuttle of carbon and nitrogen among organs. Cultured cancer cells and some tumors consume glutamine disproportionately to any other amino acid. Glutamine is a highly versatile nutrient in that it can supply carbon to both bioenergetic and biosynthetic pathways while also providing nitrogen for nucleotides, hexosamines, and proteins, all of which are required for cancer cells to proliferate. These observations have raised the possibility that inhibiting glutamine metabolism might provide therapeutic benefit in some types of cancer.

Several studies have suggested that MYC-driven tumors have enhanced rates of glutamine consumption and perhaps enhanced dependence on access to an extracellular source of glutamine. MYC drives expression of glutamine transporters and glutaminase (GLS), the mitochondrial enzyme that converts glutamine to glutamate and ammonia. Inhibition of glutaminase using tool compounds suppresses the growth of MYC-driven tumors in mice. Building from these proof-of-principle studies, highly potent GLS inhibitors have been developed for human cancer. One of these, compound CB-839 from Calithera Biosciences, is already in several Phase 1 trials for both solid tumors and hematological malignancies.

Evidence suggests that an important subset of neuroblastomas may be highly dependent on glutamine metabolism. Neuroblastoma accounts for 10-15% of pediatric cancer deaths, and amplification of the MYCN oncogene (encoding the oncoprotein N-Myc) is a poor prognostic indicator. Cells derived from MYCN-amplified tumors are highly sensitive to glutamine withdrawal or to inhibitors of glutamate dehydrogenase (GDH) and alanine transaminase (ALT), enzymes downstream of GLS in the glutamine degradation pathway. GDH inhibition also reduces the size of MYCN-amplified neuroblastoma xenografts in mice.

The clinical compound CB-839 has not yet been tested in mouse models of neuroblastoma, or in pediatric patients. Demonstrating efficacy of CB-839 in these models would have substantial impact, because the drug could then rapidly be advanced into clinical studies in this important and deadly type of pediatric cancer.

We are using xenografts derived from neuroblastoma cell lines and PDXs to test whether CB-839 limits neuroblastoma growth, and whether efficacy is dependent on MYCN amplification. The compound is being tested in PDXs and cell line-derived xenografts containing or lacking MYCN amplification (at least two of each). We are also using our established methods to introduce glutamine labeled with ¹³C into tumor-bearing mice to assess glutamine-dependent metabolic fluxes in vivo, thereby verifying that glutamine is consumed by these tumors and mapping the pathways supplied by glutamine. PET is being used in two ways.

First, L-[5-¹¹C]glutamine is being used to assess glutamine uptake. Glutamine PET has not previously been reported in neuroblastoma, but this technique is a straightforward approach to translate to humans if we find that L-[5-¹¹C]glutamine uptake in the mouse models predicts sensitivity to CB-839.

Second, we are using PET to image biodistribution and tumor uptake of CB-839. Altogether, this translational study is the first to merge L-[5-¹¹C]glutamine PET, ¹³C-glutamine infusions to discover glutamine-dependent metabolic fluxes, and sensitivity to a GLS inhibitor. All three of these components, including the ¹³C infusions, which we have used extensively in humans, could be advanced to pediatric clinical studies if the mouse work produces encouraging results.

The Components of Chemistry and Radiochemistry and Clinical Radiochemistry interact with this User Group to identify their needs for PET imaging and radiotracers. For instance, many neuroimaging agents (e.g., ¹⁸F-FDOPA, ⁶⁸Ga-DOTATOC, ¹⁸F-MFBG, and [¹¹C]hydroxyephedrine) have been reported with promising results for the detection and treatment response in neuroblastoma in addition to [¹¹C]methionine. Therefore, in the preclinical PDX mouse models, these radiotracers are being used in addition to L-[5-¹¹C]glutamine. The results might reveal other downstream pathways or targets affected by CB-839.

In terms of drug molecules, reported elsewhere or identified by the high throughput drug screening core facility on campus in Biochemistry, the CRCFPO team is designing and synthesizing the precursors based on the structure of drug candidates for radiolabeling with either ¹¹C or ¹⁸F without significantly altering the biological properties of the compound. Radiolabeled drug candidates can be evaluated in relevant animal models to test their designed therapeutic efficacies in pediatric cancer treatment.

Additionally, the desirable in vivo pharmacokinetics and biodistribution profiles of the drug molecules can be obtained for the go or no-go decision making in the process of drug discovery. For example, the Radiochemistry Team can develop four strategies to radiolabel the molecule of CB-839 with either C or ¹⁸F.