18F-Fluciclovine PET Imaging of Glutaminase Inhibition in Breast Cancer Models.

Aggressive cancers such as triple-negative breast cancer (TNBC) avidly metabolize glutamine as a feature of their malignant phenotype. The conversion of glutamine to glutamate by the glutaminase enzyme represents the first and rate-limiting step of this pathway and a target for drug development. Indeed, a novel glutaminase inhibitor (GLSi) has been developed and tested in clinical trials but with limited success, suggesting the potential for a biomarker to select patients who could benefit from this novel therapy. Here, we studied a nonmetabolized amino acid analog, 18F-fluciclovine, as a PET imaging biomarker for detecting the pharmacodynamic response to GLSi. Methods: Uptake of 18F-fluciclovine into human breast cancer cells was studied in the presence and absence of inhibitors of glutamine transporters and GLSi. To allow 18F-fluciclovine PET to be performed on mice, citrate in the tracer formulation is replaced by phosphate-buffered saline. Mice bearing triple-negative breast cancer (TNBC) xenografts (HCC38, HCC1806, and MBA-MD-231) and estrogen receptor-positive breast cancer xenografts (MCF-7) were imaged with dynamic PET at baseline and after a 2-d treatment of GLSi (CB839) or vehicle. Kinetic analysis suggested reversible uptake of the tracer, and the distribution volume (VD) of 18F-fluciclovine was estimated by Logan plot analysis. Results: Our data showed that cellular uptake of 18F-fluciclovine is mediated by glutamine transporters. A significant increase in VD was observed after CB839 treatment in TNBC models exhibiting high glutaminase activity (HCC38 and HCC1806) but not in TNBC or MCF-7 exhibiting low glutaminase. Changes in VD were corroborated with changes in GLS activity measured in tumors treated with CB839 versus vehicle, as well as with changes in VD of 18F-(2S,R4)-fluoroglutamine, which we previously validated as a measure of cellular glutamine pool size. A moderate, albeit significant, decrease in 18F-FDG PET signal was observed in HCC1806 tumors after CB839 treatment. Conclusion: 18F-fluciclovine PET has potential to serve as a clinically translatable pharmacodynamic biomarker of GLSi.

The Development of 18F Fluorthanatrace: A PET Radiotracer for Imaging Poly (ADP-Ribose) Polymerase-1.

Fluorine 18 (18F) fluorthanatrace (18F-FTT) is a PET radiotracer for imaging poly (adenosine diphosphate-ribose) polymerase-1 (PARP-1), an important target for a class of drugs known as PARP inhibitors, or PARPi. This article describes the stepwise development of this radiotracer from its design and preclinical evaluation to the first-in-human imaging studies and the initial validation of 18F-FTT as an imaging-based biomarker for measuring PARP-1 expression levels in patients with breast and ovarian cancer. A detailed discussion on the preparation and submission of an exploratory investigational new drug application to the Food and Drug Administration is also provided. Additionally, this review highlights the need and future plans for identifying a commercialization strategy to overcome the major financial barriers that exist when conducting the multicenter clinical trials needed for approval in the new drug application process. The goal of this article is to provide a road map that scientists and clinicians can follow for the successful clinical translation of a PET radiotracer developed in an academic setting. Keywords: Molecular Imaging-Cancer, PET, Breast, Genital/Reproductive, Chemistry, Radiotracer Development, PARPi, 18F-FTT, Investigational New Drug © RSNA, 2022.

Metallaphotoredox aryl and alkyl radiomethylation for PET ligand discovery.

Positron emission tomography (PET) radioligands (radioactively labelled tracer compounds) are extremely useful for in vivo characterization of central nervous system drug candidates, neurodegenerative diseases and numerous oncology targets1. Both tritium and carbon-11 radioisotopologues are generally necessary for in vitro and in vivo characterization of radioligands2, yet there exist few radiolabelling protocols for the synthesis of either, inhibiting the development of PET radioligands. The synthesis of such radioligands also needs to be very rapid owing to the short half-life of carbon-11. Here we report a versatile and rapid metallaphotoredox-catalysed method for late-stage installation of both tritium and carbon-11 into the desired compounds via methylation of pharmaceutical precursors bearing aryl and alkyl bromides. Methyl groups are among the most prevalent structural elements found in bioactive molecules, and so this synthetic approach simplifies the discovery of radioligands. To demonstrate the breadth of applicability of this technique, we perform rapid synthesis of 20 tritiated and 10 carbon-11-labelled complex pharmaceuticals and PET radioligands, including a one-step radiosynthesis of the clinically used compounds [11C]UCB-J and [11C]PHNO. We further outline the direct utility of this protocol for preclinical PET imaging and its translation to automated radiosynthesis for routine radiotracer production in human clinical imaging. We also demonstrate this protocol for the installation of other diverse and pharmaceutically useful isotopes, including carbon-14, carbon-13 and deuterium.

Automated synthesis of [11C]L-glutamine on Synthra HCN plus synthesis module.

BACKGROUND: L-Glutamine (L-Gln) is the most abundant amino acid present in the human body and is involved in numerous metabolic pathways. Glutaminolysis is the metabolic process deployed by many aggressive cancers such as triple negative breast cancer (TNBC). Imaging the metabolic pathways of L-glutamine could provide more insights into tumor biology. Reliable and reproducible automated synthesis of [11C]L-glutamine PET (Positron Emission Tomography) radiotracer is critical for these studies. RESULTS: [11C]L-Glutamine ([11C]L-Gln) was reliably and reproducibly synthesized. The automated process involves cleaning and drying of the synthesis module, azeotropic drying of crown ether and cesium bicarbonate, conversion of [11C]CO2 to [11C] CsCN, incorporation of [11C] CN into the starting material, and hydrolysis and deprotection of the corresponding [11C] nitrile to yield [11C]L-glutamine. Starting with approximately 1 Ci of [11C] cesium cyanide ([11C]CsCN), 47-77 mCi (n = 4) of the final product, [11C]L-Gln, was obtained after sterile filtration. The radiochemical purity of the final product was > 90% with almost exclusively L-glutamine isomer. The yield of [11C]L-Gln was 43-52% (n = 4), decay corrected to end of [11C] CsCN trapping in the reaction vessel. CONCLUSIONS: All the steps including drying of the mixture of base and crown ether, preparation of [11C] cyanide, radiochemical synthesis and formulation were accomplished on a single synthesis unit. [11C]L-Gln has been successfully adapted and optimized on an automated synthesis module, Synthra HCN Plus. This process can be readily adapted for clinical research use.

A PET Glutamate Analogue to Measure Cancer Cell Redox State and Oxidative Stress: Promise and Paradox.

[18F]FSPG was shown to provide an indirect measure of the cellular redox state and may be used as an early indicator of therapy response to cancer therapies that cause oxidative stress. A somewhat paradoxical finding was that reduced [18F]FSPG cellular uptake was associated with either lower cellular concentrations of cystine or glutamate, despite opposing the transport of these substances in the Xc- antiporter, for which [18F]FSPG is also a substrate. Further studies of the kinetics of [18F]FSPG will help elucidate the factors mediating a decline in [18F]FSPG with oxidative stress.See related article by McCormick et al, p. 853.