PET Imaging with S-[11C]Methyl-L-Cysteine and L-[Methyl-11C]Methionine in Rat Models of Glioma, Glioma Radiotherapy, and Neuroinflammation

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Optimization of Precursor Synthesis Conditions of (2S,4S)4–[18F]FPArg and Its Application in Glioma Imaging

Although the tracer (2S,4S)4–[¹⁸F]FPArg is expected to provide a powerful imaging method for the diagnosis and treatment of clinical tumors, it has not been realized due to the low yield of chemical synthesis and radiolabeling. A simple synthetic method for the radiolabeled precursor of (2S,4S)4–[¹⁸F]FPArg in stable yield was obtained by adjusting the sequence of the synthetic steps. Furthermore, the biodistribution experiments confirmed that (2S,4S)4–[¹⁸F]FPArg could be cleared out quickly in wild type mouse. Cell uptake experiments and U87MG tumor mouse microPET–CT imaging experiments showed that the tumor had high uptake of (2S,4S)4–[¹⁸F]FPArg and the clearance was slow, but (2S,4S)4–[¹⁸F]FPArg was rapidly cleared in normal brain tissue. MicroPET–CT imaging of nude mice bearing orthotopic HS683–Luc showed that (2S,4S)4–[¹⁸F]FPArg can penetrate blood–brain barrier and image gliomas with a high contrast. Therefore, (2S,4S)4–[¹⁸F]FPArg is expected to be further applied in the diagnosis and efficacy evaluation of clinical glioma.

Inhibition of EMMPRIN by microRNA-124 suppresses the growth, invasion and tumorigenicity of gliomas

MicroRNAs (miR) are a group of non-coding, small RNAs, 18-20 nucleotides in length, that are frequently involved in the development of a variety of different types of cancer, including glioma, which is a type of severe tumor in the brain. Previous studies reported that miR-124 levels were downregulated in glioma specimens; however, the potential role of miR-124 in glioma currently remains unclear. The present study performed experiments, including dual-luciferase reporter assay (DLRA), MTT assay, transwell assay and flow cytometry, with the aim of elucidating the molecular mechanism of miR-124 in glioma. The results indicated that miR-124 expression was decreased in glioma tissues, accompanied by the increased expression of extracellular matrix metalloproteinase inducer (EMMPRIN). The expression of EMMPRIN was inhibited by miR-124 transfection. The DLRA results revealed that EMMPRIN directly targets miR-124. Furthermore, upon overexpression of miR-124 in the U87 cells, cell proliferation was significantly inhibited, apoptosis was increased, and cell migration and invasion were decreased. Furthermore, tumor growth was blocked by miR-124 in mice. Based on these results, the present study concluded that miR-124 is critical for amelioration of glioma by targeting EMMPRIN, thereby acting as a tumor suppressor. Thus, miR-124/EMMPRIN constitutes a plausible basis for the treatment of glioma.

11C-Methionine PET Identifies Astroglia Involvement in Heart-Brain Inflammation Networking After Acute Myocardial Infarction

Acute myocardial infarction (MI) triggers a local and systemic inflammatory response. We recently showed microglia involvement using translocator protein imaging. Here, we evaluated whether ¹¹C-methionine provides further insight into heart-brain inflammation networking. Methods: Male C57BL/6 mice underwent permanent coronary artery ligation followed by ¹¹C-methionine PET at 3 and 7 d (n = 3). In subgroups, leukocyte homing was blocked by integrin antibodies (n = 5). The cellular substrate for PET signal was identified using brain section immunostaining. Results: ¹¹C-methionine uptake (percentage injected dose/cm³) peaked in the MI region on day 3 (5.9 ± 0.9 vs. 2.4 ± 0.5), decreasing to the control level by day 7 (4.3 ± 0.6). Brain uptake was proportional to cardiac uptake (r = 0.47, P < 0.05), peaking also on day 3 (2.9 ± 0.4 vs. 2.4 ± 0.3) and returning to baseline on day 7 (2.3 ± 0.4). Integrin blockade reduced uptake at every time point. Immunostaining on day 3 revealed colocalization of the l-type amino acid transporter, with glial fibrillary acidic protein-positive astrocytes but not CD68-positive microglia. Conclusion: PET imaging with ¹¹C-methionine specifically identifies an astrocyte component, enabling further dissection of the heart-brain axis in post-MI inflammation.

Synthesis of enantiopure 18F-trifluoromethyl cysteine as a structure-mimetic amino acid tracer for glioma imaging

Although 11C-labelled sulfur-containing amino acids (SAAs) including L-methyl-[11C]methionine and S-[11C]-methyl-L-cysteine, are attractive tracers for glioma positron emission tomography (PET) imaging, their applications are limited by the short half-life of the radionuclide 11C (t1/2 = 20.4 min). However, development of 18F-labelled SAAs (18F, t1/2 = 109.8 min) without significant structural changes or relying on prosthetic groups remains to be a great challenge due to the absence of adequate space for chemical modification. Methods: We herein present 18F-trifluoromethylated D- and L-cysteines which were designed by replacing the methyl group with 18F-trifluoromethyl group using a structure-based bioisosterism strategy. These two enantiomers were synthesized stereoselectively from serine-derived cyclic sulfamidates via a nucleophilic 18F-trifluoromethylthiolation reaction followed by a deprotection reaction. Furthermore, we conducted preliminary in vitro and in vivo studies to investigate the feasibility of using 18F-trifluoromethylated cysteines as PET tracers for glioma imaging. Results: The two-step radiosynthesis provided the desired products in excellent enantiopurity (ee > 99%) with 14% ± 3% of radiochemical yield. In vitro cell study demonstrated that both enantiomers were taken up efficiently by C6 tumor cells and were mainly transported by systems L and ASC. Among them, the D-enantiomer exhibited relatively good stability and high tumor-specific accumulation in the animal studies. Conclusion: Our findings indicate that 18F-trifluoromethylated D-cysteine, a new SAA tracer, may be a potential candidate for glioma imaging. Taken together, our study represents a first step toward developing 18F-trifluoromethylated cysteines as structure-mimetic tracers for PET tumor imaging.