Glioblastoma PET/MRI: kinetic investigation of [18F]rhPSMA-7.3, [18F]FET and [18F]fluciclovine in an orthotopic mouse model of cancer

Abstract

Purpose: Glioblastoma multiforme (GBM) is the most common glioma and standard therapies can only slightly prolong the survival. Neo-vascularization is a potential target to image tumor microenvironment, as it defines its brain invasion. We investigate [¹⁸F]rhPSMA-7.3 with PET/MRI for quantitative imaging of neo-vascularization in GBM bearing mice and human tumor tissue and compare it to [¹⁸F]FET and [¹⁸F]fluciclovine using PET pharmacokinetic modeling (PKM).

Methods: [¹⁸F]rhPSMA-7.3, [¹⁸F]FET, and [¹⁸F]fluciclovine were i.v. injected with 10.5 ± 3.1 MBq, 8.0 ± 2.2 MBq, 11.5 ± 1.9 MBq (n = 28, GL261-luc2) and up to 90 min PET/MR imaged 21/28 days after surgery. Regions of interest were delineated on T2-weighted MRI for (i) tumor, (ii) brain, and (iii) the inferior vena cava. Time-activity curves were expressed as SUV mean, SUVR and PKM performed using 1-/2-tissue-compartment models (1TCM, 2TCM), Patlak and Logan analysis (LA). Immunofluorescent staining (IFS), western blotting, and autoradiography of tumor tissue were performed for result validation.

Results: [¹⁸F]rhPSMA-7.3 showed a tumor uptake with a tumor-to-background-ratio (TBR) = 2.1-2.5, in 15-60 min. PKM (2TCM) confirmed higher K1 (0.34/0.08, p = 0.0012) and volume of distribution V_T (0.24/0.1, p = 0.0017) in the tumor region compared to the brain. Linearity in LA and similar k3 = 0.6 and k4 = 0.47 (2TCM, tumor, p = ns) indicated reversible binding. K1, an indicator for vascularization, increased (0.1/0.34, 21 to 28 days, p < 0.005). IFS confirmed co-expression of PSMA and tumor vascularization. [¹⁸F]fluciclovine showed higher TBR (2.5/1.8, p < 0.001, 60 min) and V_S (1.3/0.7, p < 0.05, tumor) compared to [¹⁸F]FET and LA indicated reversible binding. V_T increased (p < 0.001, tumor, 21 to 28 days) for [¹⁸F]FET (0.5-1.4) and [¹⁸F]fluciclovine (0.84-1.5).

Conclusion: [¹⁸F]rhPSMA-7.3 showed to be a potential candidate to investigate the tumor microenvironment of GBM. Following PKM, this uptake was associated with tumor vascularization. In contrast to what is known from PSMA-PET in prostate cancer, reversible binding was found for [¹⁸F]rhPSMA-7.3 in GBM, contradicting cellular trapping. Finally, [¹⁸F]fluciclovine was superior to [¹⁸F]FET rendering it more suitable for PET imaging of GBM.

Keywords: Amino acid transport; Glioblastoma; Neo-vascularization; PET/MRI; PSMA.

Fig. 1

A T2-weighted FSE 7, 14, 21, and 28 days after GL261-luc2 cell inoculation visible in the left caudate putamen and its growth curve. B [¹⁸F]FET, [¹⁸F]fluciclovine, and [¹⁸F]rhPSMA-7.3 PET/MRI data of tumor bearing mice at different time points after tumor cell inoculation. Clear tumor uptake is visible at 28 days for all radiotracers

Fig. 2

Dynamic uptake (SUVmean ± SD) following i.v. injection of [¹⁸F]FET (n = 4), [¹⁸F]fluciclovine (n = 4), and [¹⁸F]rhPSMA-7.3 (n = 6) after 28 days (A–D) or 21 days (E–G) of tumor growth in A, E the tumor region and B, F the contra lateral hemisphere. C, G TBR based on SUVmean using the contra lateral hemisphere as reference. D Heatmap of Pearson’s correlation (r) comparing tumor uptake of [¹⁸F]FET, [¹⁸F]fluciclovine, and [¹⁸F]rhPSMA-7.3. ^*p < 0.05, ^**p < 0.01, ^***p < 0.001, t test

Fig. 3

Kinetic parameters at 21 or 28 days post tumor cell inoculation using the 1TCM/2TCM according to the optimal evaluation parameters (Table S2/S3)

Fig. 4

Overview over study design and suggested pathways. [¹⁸F]FET and [¹⁸F]fluciclovine are correlated with tumor growth and proliferation, and [¹⁸F]rhPSMA-7.3 is associated with endothelial cells of neo-vascularization. A summary of all PKM parameters is presented for its change over investigation time

Fig. 5

A H&E staining, autoradiography, T2-weighted MRI, and PET of GL261-luc2 tumor following injection of [¹⁸F]rhPSMA-7.3. B Immunofluorescent microscopy of GL261-luc2 brain tumor tissue (blue, DAPI/nuclei; red, CD31; green, PSMA), scale bar 50 μm, 40 × or C 2.5 × magnification showing the whole brain tumor. D Western blotting of cell lines and brain/tumor tissue, confirming PSMA expression in GBM tumor but not in GBM cell lines or healthy brain tissue. E MRI and corresponding immunofluorescent imaging of tumor tissue (blue, DAPI/nuclei; red, CD31; green, PSMA) of a patient with GBM