Kinetic Modeling of ¹⁸F-(2S,4R)4-Fluoroglutamine in Mouse Models of Breast Cancer to Estimate Glutamine Pool Size as an Indicator of Tumor Glutamine Metabolism

Abstract

The PET radiotracer 18 F-(2S,4R)4-Fluoroglutamine ( 18 F-Gln) reflects glutamine transport and can be used to infer glutamine metabolism. Mouse xenograft studies have demonstrated that 18 F-Gln uptake correlates directly with glutamine pool size and is inversely related to glutamine metabolism through the glutaminase enzyme. To provide a framework for the analysis of 18 F-Gln-PET, we have examined 18 F-Gln uptake kinetics in mouse models of breast cancer at baseline and after inhibition of glutaminase. We describe results of the pre-clinical analysis and computer simulations with the goal of model validation and performance assessment in anticipation of human breast cancer patient studies. Methods: Triplenegative breast cancer and receptor-positive xenografts were implanted in athymic mice. PET mouse imaging was performed at baseline and after treatment with a glutaminase inhibitor or a vehicle solution for a total of four mouse groups. Dynamic PET images were obtained for one hour beginning at the time of intravenous injection of 18 F-FGln. Kinetic analysis and computer simulations were performed on a representative time-activity curves (TAC), testing 1-and 2-compartment models to describe kinetics.Results: Dynamic imaging for one hour captured blood and tumor TACs indicative of largely reversible uptake of 18 F-FGln in tumors. Consistent with this observation, a two-compartment model indicated a relatively low estimate of trapping (k 3 ) , suggesting that the one-compartment model is preferable. Logan plot graphical analysis demonstrated late linearity, supporting reversible kinetics and modeling with a single compartment. Analysis of the mouse data and simulations suggests that estimates of glutamine pools size, specifically the V D for 18 F-FGln, were more reliably estimated using the one-compartment reversible model compared to the two-compartment irreversible model. Tumor-to-blood ratios, a more practical potential proxy of V D , demonstrated good correlation with volume of distribution from singlecompartment models and Logan analyses. Conclusions: Kinetic analysis of dynamic 18 F-Gln-PET images demonstrated the ability to measure V D to estimate glutamine pool size, a key indicator of cellular glutamine metabolism, by both a one-compartment model and Logan analysis. Changes in V D with glutaminase inhibition supports the ability to assess response to glutamine metabolism-targeted therapy.Concordance of kinetic measures with tumor-to-blood ratios provides a clinically feasible approach for human imaging.

[18F](2S,4R)-4-Fluoroglutamine as a New Positron Emission Tomography Tracer in Myeloma

“…Previous studies suggested that [ 18 F]4-FGln labels the intracellular Gln pool and, indirectly, is a glutaminolytic marker (8). This was recently supported by a kinetic analysis applied on murine breast cancer models (21) and confirmed here by the demonstration that [ 18 F]4-FGln uptake in JJN3 tumors is described by a one-compartment, reversible tissue model with no trapping. Thus, [ 18 F]4-FGln proves a dynamic tool for research and potential clinical use in MM.…”

“…In tumor, [ 18 F]4-FGln uptake peaked at 25 min slowly declining thereafter (Figure 2B). The Logan plot (Figure 2C) confirmed linearity starting at 15 min from injection, consistent with largely reversible tracer exchange (21). The R 2 value for the fits of data from JJN3-grafted mice was 0.9989.…”

“…Growing evidence suggests the possible use of [ 18 F]4-FGln as a new tracer in cancer patients taking advantage by the Glndependent metabolic profile of several tumors. Some preclinical and clinical studies have been published on in vivo testing of [ 18 F]4-FGln (8,9,21,(30)(31)(32), supporting its possible use as imaging biomarker. Moreover, [ 18 F]4-FGln could be used to study the metabolic profile and the Gln addiction of MM cells in order to design a metabolic-based therapeutic approach.…”

“…All images were calibrated with a dedicated phantom, corrected for the radionuclide half-life decay, and then quantified with PMOD 3.2 (Zurich, Switzerland). To set acquisition time and verify reversibility of [ 18 F]4-FGln (21), we applied the simplified Logan analyses using muscle as input function (21,22) to three JJN3 mice. Regions of interest (ROIs) were drawn on tumor and thorax muscle and data expressed as %ID/g to obtain timeactivity curves (TACs) of [ 18 F]4-FGln.…”

Kinetic and Static Analysis of Poly-(Adenosine Diphosphate-Ribose) Polymerase-1–Targeted 18F-Fluorthanatrace PET Images of Ovarian Cancer

“…Sensitivity curves were calculated as percent change in model output corresponding to 1% change in each parameter. Repeatability of kinetic parameter estimation was analyzed using simulated data based on ranges of individual kinetic parameters via methods of Viswanath et al (14). Bias was averaged across each of 100 noise realizations, and averaged across 200 runs.…”