Radiosynthesis,biological evaluation and preliminary microPET study of 18F-labeled 5-resorcinolic triazolone derivative based on ganetespib targeting HSP90

Heat-shock protein 90 (HSP90) is a molecular chaperone that activates oncogenic transformation in several solid tumors, including lung and breast cancers. Ganetespib, a most promising candidate among several HSP90 inhibitors under clinical trials, has entered Phase III clinical trials for cancer therapy. Despite numerous evidences validating HSP90 as a target of anticancer, there are few studies on PET agents targeting oncogenic HSP90. In this study, we synthesized and biologically evaluated a novel ¹⁸F-labeled 5-resorcinolic triazolone derivative (1, [¹⁸F]PTP-Ganetespib) based on ganetespib. [¹⁸F]PTP-Ganetespib was labeled by click chemistry of Ganetespib-PEG-Alkyne (10) and [¹⁸F]PEG-N₃ (11) with 37.3?±?5.11% of radiochemical yield and 99.7?±?0.09% of radiochemical purity. [¹⁸F]PTP-Ganetespib showed proper LogP (0.96?±?0.06) and good stability in human serum over 97% for 2?h. [¹⁸F]PTP-Ganetespib showed high uptakes in breast cancer cells containing triple negative breast cancer (TNBC) MDA-MB-231 and Her2-negative MCF-7 cells, which are target breast cancer cell lines of HSP90 inhibitor, ganetespib, as an anticancer. Blocking of HSP90 by the pretreatment of ganetespib exhibited significantly decreased accumulation of [¹⁸F]PTP-Ganetespib in MDA-MB-231 and MCF-7 cells, indicating the specific binding of [¹⁸F]PTP-Ganetespib to MDA-MB-231 and MCF-7 cells with high HSP90 expression. In the biodistribution and microPET imaging studies, the initial uptake into tumor was weaker than in other thoracic and abdominal organs, but [¹⁸F]PTP-Ganetespib was retained relatively longer in the tumor than other organs. The uptake of [¹⁸F]PTP-Ganetespib in tumors was not sufficient for further development as a tumor-specific PET imaging agent by itself, but this preliminary PET imaging study of [¹⁸F]PTP-Ganetespib can be basis for developing new PET imaging agents based on HSP90 inhibitor, ganetespib.     

Synthesis and preliminary evaluation of 18F-icotinib for EGFR-targeted PET imaging of lung cancer

Epidermal growth factor receptor (EGFR) has emerged as an attracting target in the field of imaging and treatment for non-small cell lung cancer (NSCLC). Radiolabeled EGFR-tyrosine kinase inhibitors (EGFR-TKIs) specifically targeting EGFR are deemed as promising probes for the imaging of NSCLC. This study aimed to label icotinib (one kind of EGFR-TKI) with ¹⁸F through click reaction to develop a new EGFR-targeting PET probe-¹⁸F-icotinib. ¹⁸F-icotinib was obtained in 44.81% decay-corrected yield in 100?min synthesis time with 34?GBq/μmol specific activity and >99% radiochemical purity at the end of synthesis. The identity of the product was confirmed by co-injection with ¹⁸F-icotinib and ¹⁹F-icotinib. The Log P was 1.28?±?0.04 (n?=?6). The tracer displayed excellent stability after incubation for 4?h in vitro. ¹⁸F-icotinib showed satisfying binding ability to A549 NSCLC cells, which could be inhibited by icotinib. PET imaging studies demonstrated a specific uptake of the radiotracer (0.90?±?0.24% ID/g) in A549 tumor-bearing mice, while lower uptake was observed in heart, lung and spleen at 1.5?h post injection. Inmunohistochemical staining confirmed that the A549 tumor was EGFR-positive. Therefore, we considered that ¹⁸F-icotinib was a highly promising compound for EGFR-based tumor PET imaging.     

Synthesis and evaluation of 18F-hexafluorophosphate as a novel PET probe for imaging of sodium/iodide symporter in a murine C6-glioma tumor model

Noninvasive imaging of iodide uptake via the sodium/iodide symporter (NIS) has received great interest for evaluation of thyroid cancer and reporter imaging of NIS-expressing viral therapies. In this study, we investigate ¹⁸F-labeled hexafluorophosphate (HFP or PF₆^?) as a high-affinity iodide analog for NIS imaging. ¹⁸F-HFP was synthesized by radiofluorination of phosphorus pentafluoride·N-methylpyrrolidine complex and evaluated in human NIS (hNIS)-expressing C6 glioma cells and a C6 glioma xenograft mouse model. ¹⁸F-HFP was obtained in radiochemical yield of 10?±?5%, radiochemical purity of >96% and specific radioactivity of 604?±?18?MBq/µmol. Specific uptake of ¹⁸F-HFP and high affinity of ¹⁹F-HFP were observed in hNIS+ C6-glioma cells. PET imaging showed robust uptake of ¹⁸F-HFP in NIS-expressing tissues (thyroid, stomach, and hNIS+ C6 glioma xenografts), and the uptake of ¹⁸F-HFP was blocked by NaClO₄ pretreatment. Specific accumulation in hNIS-expressing xenograft (hNIS+) was observed relative to isogenic control tumor (hNIS?). Clearance of ¹⁸F-HFP was predominantly through renal excretion. The biodistribution showed consistent results with PET imaging. Minimal bone uptake was observed over 2?h period post-injection, indicating excellent in vivo stability of ¹⁸F-HFP. Although improvement in specific radioactivity is desirable, the results indicate that ¹⁸F-HFP is a promising candidate radiotracer for further evaluation for NIS imaging.     

Acetazolamide-based [18F]-PET tracer: In vivo validation of carbonic anhydrase IX as a sole target for imaging of CA-IX expressing hypoxic solid tumors

Carbonic anhydrase IX is overexpressed in many solid tumors including hypoxic tumors and is a potential target for cancer therapy and diagnosis. Reported imaging agents targeting CA-IX are successful mostly in clear cell renal carcinoma as SKRC-52 and no candidate was approved yet in clinical trials for imaging of CA-IX. To validate CA-IX as a valid target for imaging of hypoxic tumor, we designed and synthesized novel [¹⁸F]-PET tracer (1) based on acetazolamide which is one of the well-known CA-IX inhibitors and performed imaging study in CA-IX expressing hypoxic tumor model as 4T1 and HT-29 in vivo models other than SKRC-52. [¹⁸F]-acetazolamide (1) was found to be insufficient for the specific accumulation in CA-IX expressing tumor. This study might be useful to understand in vivo behavior of acetazolamide PET tracer and can contribute to the development of successful PET imaging agents targeting CA-IX in future. Additional study is needed to understand the mechanism of poor targeting of CA-IX, as if CA-IX is not reliable as a sole target for imaging of CA-IX expressing hypoxic solid tumors.     

Synthesis and evaluation of radiolabeled AGI-5198 analogues as candidate radiotracers for imaging mutant IDH1 expression in tumors

Mutations in the metabolic enzyme isocitrate dehydrogenase 1 (IDH1) are commonly found in gliomas. AGI-5198, a potent and selective inhibitor of the mutant IDH1 enzyme, was radiolabeled with radioiodine and fluorine-18. These radiotracers were evaluated as potential probes for imaging mutant IDH1 expression in tumors with positron emission tomography (PET). Radioiodination of AGI-5198 was achieved using a tin precursor in 79?±?6% yield (n?=?9), and ¹⁸F-labeling was accomplished by the Ugi reaction in a decay-corrected radiochemical yield of 2.6?±?1.6% (n?=?5). The inhibitory potency of the analogous nonradioactive compounds against mutant IDH1 (IDH1-R132H) was determined in enzymatic assays. Cell uptake studies using radiolabeled AGI-5198 analogues revealed somewhat higher uptake in IDH1-mutated cells than that in wild-type IDH1 cells. The radiolabeled compounds displayed favorable tissue distribution characteristics in vivo, and good initial uptake in IDH1-mutated tumor xenografts; however, tumor uptake decreased with time. Radioiodinated AGI-5198 exhibited higher tumor-to-background ratios compared with ¹⁸F-labeled AGI-5198; unfortunately, similar results were observed in wild-type IDH1 tumor xenografts as well, indicating lack of selectivity for mutant IDH1 for this tracer. These results suggest that AGI-5198 analogues are not a promising platform for radiotracer development. Nonetheless, insights gained from this study may help in design and optimization of novel chemical scaffolds for developing radiotracers for imaging the mutant IDH1 enzyme.     

Radiosynthesis and preclinical evaluation of [18F]FEM as a potential novel PET probe for tumor imaging

In the 21st century, the incidence and mortality of cancer, one of the most challenging diseases in the world, have rapidly increased. The purpose of this study was to develop 2-(2-[¹⁸F]fluoroethoxy)ethyl 4-methylbenzenesulfonate ([¹⁸F]FEM) as a positron emission tomography (PET) agent for tumor imaging. In this study, [¹⁸F]FEM was synthesized with a good radiochemical yield (45.4 ± 5.8%), high specific radioactivity (over 25 GBq/μmol), and commendable radiochemical purity (over 99%). The octanol/water partition coefficient of [¹⁸F]FEM was 1.44 ± 0.04. The probe demonstrated good stability in vitro (phosphate-buffered saline (PBS) and mouse serum (MS)), and binding specificity to five different tumor cell lines (A549, PC-3, HCC827, U87, and MDA-MB-231). PET imaging of tumor-bearing mice showed that [¹⁸F]FEM specifically accumulated at the tumor site of the five different tumor cell lines. The average tumor-to-muscle (T/M) ratio was over 2, and the maximum T/M values reached about 3.5. The biodistribution and dynamic PET imaging showed that most probes were metabolized by the liver, whereas a small part was metabolized by the kidney. Moreover, dynamic brain images and quantitative data showed [¹⁸F]FEM can quickly cross the blood brain barrier (BBB) and quickly fade out, thereby suggesting it may be a promising candidate probe for the imaging of brain tumors. The presented results demonstrated that [¹⁸F]FEM is a promising probe for tumor PET imaging.     

PET imaging of cardiomyocyte apoptosis in a rat myocardial infarction model

Cardiomyocyte apoptosis has been observed in several cardiovascular diseases and contributes to the subsequent cardiac remodeling processes and progression to heart failure. Consequently, apoptosis imaging is helpful for noninvasively detecting the disease progression and providing treatment guidance. Here, we tested ¹⁸F-labeled 2-(5-fluoropentyl)-2-methyl-malonic acid (¹⁸F-ML-10) and ¹⁸F-labeled 2-(3-fluoropropyl)-2-methyl-malonic acid (¹⁸F-ML-8) for apoptosis imaging in rat models of myocardial infarction (MI) and compared them with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). MI was induced in Sprague-Dawley rats by permanent left coronary artery ligation. Procedural success was confirmed by echocardiography and positron emission tomography (PET) imaging with ¹⁸F-FDG. In vivo PET imaging with ¹⁸F-ML-10 and ¹⁸F-ML-8 was performed in the MI models at different time points after operation. Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assays and immunohistochemical analyses were used to evaluate myocardial apoptosis. In vitro cell binding assays were performed to validate ¹⁸F-ML-8 binding to apoptotic cardiomyocytes. PET imaging demonstrated high ¹⁸F-ML-10 and ¹⁸F-ML-8 uptake where ¹⁸F-FDG uptake was absent. The focal accumulation of the two tracers was high on days 1 and 3 but was not notable on days 5 and 7 after surgery. The infarct-to-lung uptake ratio was 4.29?±?0.30 for ¹⁸F-ML-10 and 3.51?±?0.18 for ¹⁸F-ML-8 (n?=?6, analyzed by averaging the uptake ratios on postoperative days 1 and 3, P?<?0.05). The TUNEL results showed that myocardial cell apoptosis was closely related to the focal uptake of the apoptotic tracers in the infarct area. In addition, the apoptosis rates calculated from the TUNEL results were better correlated with ¹⁸F-ML-8 uptake than with ¹⁸F-ML-10 uptake. Ex vivo cell binding assays demonstrated that ¹⁸F-ML-8 accumulated in apoptotic cells but not in necrotic or normal cells. PET imaging using ¹⁸F-ML-10 or ¹⁸F-ML-8 allows the noninvasive detection of myocardial apoptosis in the early phase. In addition, ¹⁸F-ML-8 may be better than ¹⁸F-ML-10 for apoptosis imaging. We propose that PET imaging with ¹⁸F-ML-10 or ¹⁸F-ML-8 combined with ¹⁸F-FDG is an alternative for detecting and assessing MI.     

Positron labeled muscarinic acetylcholine receptor antagonist: 2- and 4-[18F]fluorodexetimide. Syntheses and biodistribution

Two ¹⁸F-labeled analogues of dexetimides, 2-[¹⁸F]fluorodexetimide (2-FDEX) and 4-[¹⁸F]fluorodexetimide (4-FDEX), were prepared and evaluated in vivo as possible agents for the study of the muscarinic acetylcholine receptor (mAChR) with PET. Two synthetic approaches, a 2-step reductive alkylation procedure and a 4-step alkylation approach, were investigated. The alkylation approach with higher overall radiochemical yields was used to prepare 2- and 4-FDEX for biodistribution studies. The overall synthesis time for both compounds was 2.5 h and the overall radiochemical yield at end-of-synthesis was 12%. The specific activity was found to be greater than 600 mCi/μmol. Biodistribution studies of 2-FDEX in rats produced striatum-to-cerebellum and cortex-to-cerebellum ratios of 8.6 ± 1.1 and 8.4 ± 1.0 at 1 h after injection, and 12.1 ± 2.1 and 10.7 ± 2.2 at 3 h, respectively. Substantial radioactivity detected in bone indicated the in vivo defluorination of 2-FDEX. The striatum-to-cerebellum ratio for 4-FDEX was slightly lower at 1 h (5.9 ± 0.9) but equally high at 3 h (12.3 ± 2.0) when compared to 2-FDEX, and there was little bone uptake. The uptake of both 2-FDEX and 4-FDEX into mAChR rich brain regions (e.g. striatum, cortex) was blocked by a dose of dexetimide (5 mg/kg). Our results suggest 4-FDEX is a potential PET agent for study mAChR in vivo.     

Fluorine-18 labeling of an anti-HER2 VHH using a residualizing prosthetic group via a strain-promoted click reaction: Chemistry and preliminary evaluation

In a previous study, we evaluated a HER2-specific single domain antibody fragment (sdAb) 2Rs15d labeled with ¹⁸F via conjugation of a residualizing prosthetic agent that was synthesized by copper-catalyzed azide-alkyne cycloaddition (CuAAC). In order to potentially increase overall efficiency and decrease the time required for labeling, we now investigate the use of a strain-promoted azide-alkyne cycloaddition (SPAAC) between the 2Rs15d sdAb, which had been pre-derivatized with an azide-containing residualizing moiety, and an ¹⁸F-labeled aza-dibenzocyclooctyne derivative. The HER2-targeted sdAb 2Rs15d and a nonspecific sdAb R3B23 were pre-conjugated with a moiety containing both azide- and guanidine functionalities. The thus derivatized sdAbs were radiolabeled with ¹⁸F using an ¹⁸F-labeled aza-dibenzocyclooctyne derivative ([¹⁸F]F-ADIBO) via SPAAC, generating the desired conjugate ([¹⁸F]RL-II-sdAb). For comparison, unmodified 2Rs15d was labeled with N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB), the prototypical residualizing agent for radioiodination. Radiochemical purity (RCP), immunoreactive fraction (IRF), HER2-binding affinity and cellular uptake of [¹⁸F]RL-II-2Rs15d were assessed in vitro. Paired label biodistribution of [¹⁸F]RL-II-2Rs15d and [¹²⁵I]SGMIB-2Rs15d, and microPET/CT imaging of [¹⁸F]RL-II-2Rs15d and the [¹⁸F]RL-II-R3B23 control sdAb were performed in nude mice bearing HER2-expressing SKOV-3 xenografts. A radiochemical yield of 23.9?±?6.9% (n?=?8) was achieved for the SPAAC reaction between [¹⁸F]F-ADIBO and azide-modified 2Rs15d and the RCP of the labeled sdAb was >95%. The affinity (K_d) and IRF for the binding of [¹⁸F]RL-II-2Rs15d to HER2 were 5.6?±?1.3?nM and 73.1?±?22.5% (n?=?3), respectively. The specific uptake of [¹⁸F]RL-II-2Rs15d by HER2-expressing BT474M1 breast carcinoma cells in vitro was 14–17% of the input dose at 1, 2, and 4?h, slightly higher than seen for co-incubated [¹²⁵I]SGMIB-2Rs15d. The uptake of [¹⁸F]RL-II-2Rs15d in SKOV-3 xenografts at 1?h and 2?h p.i. were 5.54?±?0.77% ID/g and 6.42?±?1.70% ID/g, respectively, slightly higher than those for co-administered [¹²⁵I]SGMIB-2Rs15d (4.80?±?0.78% ID/g and 4.78?±?1.39% ID/g). MicroPET/CT imaging with [¹⁸F]RL-II-2Rs15d at 1–3?h p.i. clearly delineated SKOV-3 tumors while no significant accumulation of activity in tumor was seen for [¹⁸F]RL-II-R3B23. With the exception of kidneys, normal tissue levels for [¹⁸F]RL-II-2Rs15d were low and cleared rapidly. To our knowledge, this is the first time SPAAC method has been used to label an sdAb with ¹⁸F, especially with residualizing functionality.     

Synthesis and evaluation of indium-111-labeled imidazothiadiazole sulfonamide derivative for single photon emission computed tomography imaging targeting carbonic anhydrase-IX

Carbonic anhydrase-IX (CA-IX) is a zinc enzyme overexpressed in the hypoxic regions of many types of solid tumors; therefore, in vivo imaging of CA-IX may contribute to cancer diagnosis. In this study, we newly designed and synthesized an ¹¹¹In-labeled CA-IX imaging agent based on an imidazothiadiazole sulfonamide (IS) scaffold conjugated with a chelating moiety, DO3A ([¹¹¹In]DO3A-IS1), and evaluated its utility for imaging of CA-IX high-expressing tumors. [¹¹¹In]DO3A-IS1 was successfully synthesized at a 76% radiochemical yield by reacting its precursor with ¹¹¹InCl₃ in acetate buffer. In in vitro assays, [¹¹¹In]DO3A-IS1 showed marked stability in murine plasma and greater binding to CA-IX high-expressing (HT-29) cells (118 ± 21% initial dose/mg protein) than CA-IX low-expressing (MDA-MB-231) cells (1.4 ± 0.3% initial dose/mg protein). Moreover, in an in vivo biodistribution assay, [¹¹¹In]DO3A-IS1 showed marked accumulation in the HT-29 tumor (8.71 ± 1.41% injected dose/g at 24 h postinjection). In addition, in a single photon emission computed tomography (SPECT) study, [¹¹¹In]DO3A-IS1 clearly and selectively visualized the HT-29 tumor as compared with the MDA-MB-231 tumor. These results indicate that [¹¹¹In]DO3A-IS1 may serve as a useful SPECT imaging agent with the novel scaffold targeting CA-IX.     

Synthesis and evaluation of a [18F]formyl–Met–Leu–Phe derivative: A positron emission tomography imaging probe for bacterial infections

The tripeptide formyl–Met–Leu–Phe (fMLF) is a prototype of N-formylated chemotactic peptides for neutrophils owing to its ability to bind and activate the G protein-coupled formyl peptide receptor (FPR). Here, we developed an ¹⁸F-labeled fMLF derivative targeting FPR as a positron emission tomography (PET) imaging probe for bacterial infections. The study demonstrates that the fMLF derivative fMLFXYk(FB)k (X?=?Nle) has a high affinity for FPR (Ki?=?0.62?±?0.13?nM). The radiochemical yield and purity of [¹⁸F]fMLFXYk(FB)k were 16% and >96%, respectively. The in vivo biodistribution study showed that [¹⁸F]fMLFXYk(FB)k uptake was higher in the bacterial infected region than in the non-infected region. We observed considerably higher infection-to-muscle ratio of 4.6 at 60?min after [¹⁸F]fMLFXYk(FB)k injection. Furthermore, small-animal PET imaging studies suggested that [¹⁸F]fMLFXYk(FB)k uptake in the bacterial infected region was clearly visualized 60?min after injection.     

Synthesis and preliminary biological evaluation of O-2((2-[18F]fluoroethyl)methylamino)ethyltyrosine ([18F]FEMAET) as a potential cationic amino acid PET tracer for tumor imaging

Amino acid transport is an attractive target for oncologic imaging. Despite a high demand of cancer cells for cationic amino acids, their potential as PET probes remains unexplored. Arginine, in particular, is involved in a number of biosynthetic pathways that significantly influence carcinogenesis and tumor biology. Cationic amino acids are transported by several cationic transport systems including, ATB^0,+ (SLC6A14), which is upregulated in certain human cancers including cervical, colorectal and estrogen receptor-positive breast cancer. In this work, we report the synthesis and preliminary biological evaluation of a new cationic analog of the clinically used PET tumor imaging agent O-(2-[¹⁸F]fluroethyl)-l-tyrosine ([¹⁸F]FET), namely O-2((2-[¹⁸F]fluoroethyl)methylamino)ethyltyrosine ([¹⁸F]FEMAET). Reference compound and precursor were prepared by multi-step approaches. Radiosynthesis was achieved by no-carrier-added nucleophilic [¹⁸F]fluorination in 16–20 % decay-corrected yields with radiochemical purity >99 %. The new tracer showed good stability in vitro and in vivo. Cell uptake assays demonstrated that FEMAET and [¹⁸F]FEMAET accumulate in prostate cancer (PC-3) and small cell lung cancer cells (NCI-H69), with an energy-dependent mechanism. Small animal PET imaging with NCI-H69 xenograft-bearing mice revealed good tumor visualization comparable to [¹⁸F]FET and low brain uptake, indicating negligible transport across the blood–brain barrier. In conclusion, the non-natural cationic amino acid PET probe [¹⁸F]FEMAET accumulates in cancer cells in vitro and in vivo with possible involvement of ATB^0,+.     

Synthesis and biological evaluation of 2-(3,4-dimethoxyphenyl)-6-(2-[18F]fluoroethoxy)benzothiazole ([18F]FEDBT) for PET imaging of breast cancer

Given the ever-present demand for improved PET radiotracer in oncology imaging, we have synthesized 2-(3,4-dimethoxyphenyl)-6-(2-[¹⁸F]fluoroethoxy)benzothiazole ([¹⁸F]FEDBT), a fluorine-18-containing fluoroethylated benzothiazole to explore its utility as a PET imaging tracer. [¹⁸F]FEDBT was prepared via kryptofix-mediated nucleophilic substitution of the tosyl group precursor. Fractionated ethanol-based solid-phase (SPE cartridge-based) purification afforded [¹⁸F]FEDBT in 60% radiochemical yield (EOB), with radiochemical purity in excess of 98% and the specific activity was 35 GBq/μmol. The radiotracer displayed clearly higher cellular uptake ratio in various breast cancer cell lines MCF7, MDA-MB-468 and MDA-MB-231. However, both biodistribution and microPET studies have showed an higher abdominal accumulation of [¹⁸F]FEDMBT and the tumor/muscle ratio of 1.8 was observed in the MDA-MB-231 xenograft tumors mice model. Further the lipophilic improvement is needed for the reducement of hepatobilliary accumulation and to promote the tumor uptake for PET imaging of breast cancer.     

[18F]DAA1106: Automated radiosynthesis using spirocyclic iodonium ylide and preclinical evaluation for positron emission tomography imaging of translocator protein (18 kDa)

DAA1106 (N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide), is a potent and selective ligand for the translocator protein (18?kDa, TSPO) in brain mitochondrial fractions of rats and monkey (K_i?=?0.043 and 0.188?nM, respectively). In this study, to translate [¹⁸F]DAA1106 for clinical studies, we performed automated syntheses of [¹⁸F]DAA1106 using the spirocyclic iodonium ylide (1) as a radiolabelling precursor and conducted preclinical studies including positron emission tomography (PET) imaging of TSPO in ischemic rat brains. Radiofluorination of the ylide precursor 1 with [¹⁸F]F^?, followed by HPLC separation and formulation, produced the [¹⁸F]DAA1106 solution for injection in 6% average (n?=?10) radiochemical yield (based on [¹⁸F]F^?) with >98% radiochemical purity and molar activity of 60–100?GBq/μmol at the end of synthesis. The synthesis time was 87?min from the end of bombardment. The automated synthesis achieved [¹⁸F]DAA1106 with sufficient radioactivity available for preclinical and clinical use. Biodistribution study of [¹⁸F]DAA1106 showed a low uptake of radioactivity in the mouse bones. Metabolite analysis showed that >96% of total radioactivity in the mouse brain at 60?min after the radiotracer injection was unmetabolized [¹⁸F]DAA1106. PET study of ischemic rat brains visualized ischemic areas with a high uptake ratio (1.9?±?0.3) compared with the contralateral side. We have provided evidence that [¹⁸F]DAA1106 could be routinely produced for clinical studies.     

Synthesis and evaluation of 4-(2-fluoro-4-[11C]methoxyphenyl)-5-((2-methylpyridin-4-yl)methoxy)picolinamide for PET imaging of the metabotropic glutamate receptor 2 in the rat brain

Metabotropic glutamate receptor 2 (mGluR2) has been suggested as a therapeutic target for treating schizophrenia-like symptoms arising from increased glutamate transmission in the human forebrain. However, no reliable positron emission tomography (PET) radiotracer allowing for in vivo visualization of mGluR2 in the human brain is currently available. In this study, we synthesized 4-(2-fluoro-4-[¹¹C]methoxyphenyl)-5-((2-methylpyridin-4-yl)methoxy)picolinamide ([¹¹C]1) and evaluated its potential as a PET tracer for imaging mGluR2 in the rodent brain. Compound 1, a negative allosteric modulator (NAM) of mGluR2, showed high in vitro binding affinity (IC₅₀: 26?nM) for mGluR2 overexpressed in human cells. [¹¹C]1 was synthesized by O-[¹¹C]methylation of the phenol precursor 2 with [¹¹C]methyl iodide. After the reaction, HPLC purification and formulation, [¹¹C]1 of 7.4?±?2.8?GBq (n?=?8) was obtained from [¹¹C]carbon dioxide of 22.5?±?4.8?GBq (n?=?8) with >99% radiochemical purity and 70?±?32?GBq/μmol (n?=?8) molar activity at the end of synthesis. In vitro autoradiography for rat brains showed that [¹¹C]1 binding was heterogeneously distributed in the cerebral cortex, striatum, hippocampus, and cerebellum. This pattern is consistent with the regional distribution pattern of mGluR2 in the rodent brain. The radioactivity was significantly reduced by self- or MNI-137 (a mGluR2 NAM) blocking. Small-animal PET studies indicated a low in vivo specific binding of [¹¹C]1 in the rat brain. The brain uptake was increased in a P-glycoprotein and breast cancer resistant protein double knockout mouse, when compared to a wild-type mouse. While [¹¹C]1 presented limited potential as an in vivo PET tracer for mGluR2, we suggested that it can be used as a lead compound for developing new radiotracers with improved in vivo brain properties.     

Radiosynthesis and biological evaluation of 18F-labeled 4-anilinoquinazoline derivative (18F-FEA-Erlotinib) as a potential EGFR PET agent

Epidermal growth factor receptor (EGFR) has gained significant attention as a therapeutic target. Several EGFR targeting drugs (Gefitinib and Erlotinib) have been approved by US Food and Drug Administration (FDA) and have received high approval in clinical treatment. Nevertheless, the curative effect of these medicines varied in many solid tumors because of the different levels of expression and mutations of EGFR. Therefore, several PET radiotracers have been developed for the selective treatment of responsive patients who undergo PET/CT imaging for tyrosine kinase inhibitor (TKI) therapy. In this study, a novel fluorine-18 labeled 4-anilinoquinazoline based PET tracer, 1N-(3-(1-(2-¹⁸F-fluoroethyl)-1H-1,2,3-triazol-4-yl)phenyl)-6,7-bis(2-methoxyethoxy)quinazolin-4-amine (¹⁸F-FEA-Erlotinib), was synthesized and biological evaluation was performed in vitro and in vivo. ¹⁸F-FEA-Erlotinib was achieved within 50 min with over 88% radiochemical yield (decay corrected RCY), an average specific activity over 50 GBq/μmol, and over 99% radiochemical purity. In vitro stability study showed no decomposition of ¹⁸F-FEA-Erlotinib after incubated in PBS and FBS for 2 h. Cellular uptake and efflux experiment results indicated the specific binding of ¹⁸F-FEA-Erlotinib to HCC827 cell line with EGFR exon 19 deletions. In vivo, Biodistribution studies revealed that ¹⁸F-FEA-Erlotinib exhibited rapid blood clearance both through hepatobiliary and renal excretion. The tumor uptake of ¹⁸F-FEA-Erlotinib in HepG2, HCC827, and A431 tumor xenografts, with different EGFR expression and mutations, was visualized in PET images. Our results demonstrate the feasibility of using ¹⁸F-FEA-Erlotinib as a PET tracer for screening EGFR TKIs sensitive patients.     

Noninvasive Visualization of the Activated αvβ3 Integrin in Cancer Patients by Positron Emission Tomography and [18F]Galacto-RGD

BackgroundThe integrin αvβ3 plays an important role in angiogenesis and tumor cell metastasis, and is currently being evaluated as a target for new therapeutic approaches. Several techniques are being studied to enable noninvasive determination of αvβ3 expression. We developed [¹⁸F]Galacto-RGD, a ¹⁸F-labeled glycosylated αvβ3 antagonist, allowing monitoring of αvβ3 expression with positron emission tomography (PET).ConclusionsMolecular imaging with [¹⁸F]Galacto-RGD and PET can provide important information for planning and monitoring anti-angiogenic therapies targeting the αvβ3 integrins and can reveal the involvement and role of this integrin in metastatic and angiogenic processes in various diseases.     

Synthesis and evaluation of a 18F-curcumin derivate for β-amyloid plaque imaging

IntroductionCurcumin is a neuroprotective compound that inhibits the formation of amyloid oligomers and fibrils and binds to β-amyloid plaques in Alzheimer’s disease (AD). We aimed to synthesize an ¹⁸F-labeled curcumin derivate ([¹⁸F]4) and to characterize its positron emission tomography (PET) tracer-binding properties to β-amyloid plaques in a transgenic APP23 mouse model of AD.MethodsWe utilized facile one-pot synthesis of [¹⁸F]4 using nucleophilic ¹⁸F-fluorination and click chemistry. Binding of [¹⁸F]4 to β-amyloid plaques in the transgenic APP23 mouse brain cryosections was studied in vitro using heterologous competitive binding against PIB. [¹⁸F]4 uptake was studied ex vivo in rodents and in vivo using PET/computed tomography of transgenic APP23 and wild-type control mice.ResultsThe radiochemical yield of [¹⁸F]4 was 21 ± 11%, the specific activity exceeded 1 TBq/μmol, and the radiochemical purity exceeded 99.3% at the end of synthesis. In vitro studies of [¹⁸F]4 with the transgenic APP23 mouse revealed high β-amyloid plaque binding. In vivo and ex vivo studies demonstrated that [¹⁸F]4 has fast clearance from the blood, moderate metabolism but low blood–brain barrier (BBB) penetration.Conclusions[¹⁸F]4 was synthesized in high yield and excellent quality. In vitro studies, metabolite profile, and fast clearance from the blood indicated a promising tracer for Aβ imaging. However, [¹⁸F]4 has low in vivo BBB penetration and thus further studies are needed to reveal the reason for this and to possibly overcome this issue.     

Convergent 18F-labeling and evaluation of N-benzyl-phenethylamines as 5-HT2A receptor PET ligands

Positron emission tomography (PET) investigations of the 5-HT_2A receptor (5-HT_2AR) system can be used as a research tool in diseases such as depression, Alzheimer’s disease and schizophrenia. We have previously developed a ¹¹C-labeled agonist PET ligand ([¹¹C]Cimbi-36), and the aim of this study was to identify a ¹⁸F-labeled analogue of this PET-ligand. Thus, we developed a convergent radiochemical approach giving easy access to 5 different ¹⁸F-labeled ligands structurally related to Cimbi-36 from a common ¹⁸F-labeled intermediate. After intravenous injection, all ligands entered the pig brain. However, since within-scan intervention with ketanserin, a known orthosteric 5-HT_2A receptor antagonist, did not result in significant blocking, the radioligands seem unsuitable for neuroimaging of the 5-HT_2AR in vivo.     

Synthesis and evaluation of Al18F-NODA complex conjugated 2-(4-aminophenyl)benzothiazole as a potential tumor imaging agent

The objective of the study was to prepare and evaluate a ¹⁸F-radiolabled tracer (Al¹⁸F-5), derivated from the antitumor agent 2-(4-aminophenyl)benzothiazole, as a PET probe for tumor imaging. Al¹⁸F-5 was successfully prepared with approx. 40% radiochemical yield in aqueous phase. In in vitro cell uptake experiments and competition assay, Al¹⁸F-5 displayed good tumor-binding ability and specificity in HeLa cells (24.7 ± 0.9% ID/10⁶ cells, IC₅₀ = 63.8 ± 13.6 nM) and MCF-7 cells (6.8 ± 0.3% ID/10⁶ cells, IC₅₀ = 331.1 ± 33.7 nM). The nonradioactive compound, Al¹⁹F-5, visibly marked HeLa cells and MCF-7 cells but did not stain HEB cells in florescent staining, which further indicated the tumor-binding ability of Al¹⁸F-5. In in vivo PET imaging, HeLa and MCF-7 tumors were clearly delineated by specific accumulation of Al¹⁸F-5 in model mice. In biodistribution study, Al¹⁸F-5 exhibited good tumor uptake (4.66 ± 0.13% ID/g and 3.69 ± 0.56% ID/g, respectively), moderate tumor-to-muscle ratio (3.38 and 2.48, respectively) at 1 h post injection, which were in a good consistency with the results of PET imaging. In conclusion, Al¹⁸F-5 might be developed as a candidate PET probe for tumor imaging, though additional optimizations are still needed to improve pharmacokinetics in vivo.