Noninvasive positron emission tomography imaging of cell death using a novel small-molecule probe, 18F labeled bis(zinc(II)-dipicolylamine) complex

The synthetic bis(zinc(II)-dipicolylamine) (DPAZn2) coordination complexes are known to have a high specific and selective affinity to target the exposed phosphatidylserine (PS) on the surface of dead and dying cells. An ¹⁸F-labeled DPAZn2 complex (4-¹⁸F-Fluoro-benzoyl-bis(zinc(II)-dipicolylamine), ¹⁸F-FB-DPAZn2) as positron emission tomography (PET) tracer was developed and evaluated for in vivo imaging of tumor treated with a chemical agent. The in vitro cell stain studies revealed that fluorescent DPAZn2 complexes (Dansyl-DPAZn2) stained the same cells (apoptotic and necrotic cells) as fluorescein isothiocyanate (FITC) labeled Annexin V (FITC-Annexin V). The radiosynthesis of ¹⁸F-FB-DPAZn2 was achieved through the amidation the precursor bis(2,2′-dipicolylamine) derivative (DPA2) with the prosthetic group N-succinimidyl-4-[¹⁸F]-fluorobenzoate (¹⁸F-SFB) and chelation with zinc nitrate. In the biodistribution study, the fast clearance of ¹⁸F-FB-DPAZn2 from blood and kidney was observed and high uptake in liver and intestine within 90 min postinjection was also found. For the PET imaging, significantly higher tumor uptake of ¹⁸F-FB-DPAZn2 was observed in the adriamycin (ADM)-treated Hepa1-6 hepatocellular carcinoma-bearing mice than that in the untreated tumor-model mice, while a slightly decreased tumor uptake of ¹⁸F-FDG was found in the ADM-treated tumor-bearing mice. The results indicate that ¹⁸F-FB-DPAZn2 has the similar capability of apoptosis detection as FITC-Annexin V and seems to be a potential PET tracer for noninvasive evaluation and monitoring of anti-tumor chemotherapy. The high uptake of ¹⁸F-FB-DPAZn2 in the abdomen needs to optimize the structure for improving its pharmacokinetics characteristics in the future work.     

Evaluation of 2'-deoxy-2'-flouro-5-methyl-1-beta-D-arabinofuranosyluracil as a potential gene imaging agent for HSV-tk expression in vivo

2'-Deoxy-2'-flouro-5-methyl-1-beta-D-arabinofuranosyluracil (FMAU) has been evaluated in HT-29 cells as a potential positron emission tomography (PET) radiotracer for imaging HSV-tk gene expression in vivo. In vitro experiments demonstrate that the accumulation of [14C]-FMAU in HSV-tk-expressing cells is 2.4-fold (p < .02), 4.0-fold (p < .001), and 5.3-fold (p < .001) higher than the wild-type cells at 1, 3, and 5 hr, respectively. In vivo studies revealed that the tumor uptake in HSV-tk-expressing cells was 2.3-fold (p < .001), 3.0-fold (p < .001), and 5.5-fold (p < .001) higher than the control cells at 1, 2, and 5 hr, respectively. FMAU was found to be more sensitive compared to our earlier studies using 9-[(3-18F-fluoro-1-hydroxy-2-propoxy)methyl]-guanine ([18F]-FHPG) and 9-(4-[18F]-fluoro-3-hydroxy-methylbutyl)guanine ([18F]-FHBG) in the same cell lines, although, the specificity was less than FHBG. These results suggest that while FMAU labeled with PET isotopes may be useful for imaging HSV-tk-expressing tumors in vivo, multitracer studies across additional tumor models are necessary in order to identify an optimal PET radiotracer.     

Synthesis, biological evaluation and radiochemical labeling of a dansylhydrazone derivative as a potential imaging agent for apoptosis

To develop a small molecule-based tracer for in vivo apoptosis imaging, dansylhydrazone (DFNSH) was synthesized in 93% yield in less than 30 min. The biological evaluation showed that DFNSH selectively binds to paclitaxel-induced apoptotic cancer cells. The high magnification fluorescent images demonstrate that DFNSH is localized within the cytoplasm of cells that bound Alexa 488 labeled annexin V on the plasma membrane. [(18)F]-DFNSH ([(18)F]-3) was synthesized and isolated in 50-60% radiochemical yields, based on [K/K(222)](18)F, with a synthesis time of 50 min (EOB). The straightforward preparation of fluorine-18 labeled 3 makes it a promising tracer for PET imaging of apoptosis.     

Multi-modality imaging identifies key times for annexin V imaging as an early predictor of therapeutic outcome

Radiolabeled annexin V may provide an early indication of the success or failure of anticancer therapy on a patient-by-patient basis as an in vivo marker of tumor cell killing. An important question that remains is when, after initiation of treatment, should annexin V imaging be performed. To address this issue, we obtained simultaneous in vivo measurements of tumor burden and uptake of radiolabeled annexin V in the syngeneic orthotopic murine BCL1 lymphoma model using in vivo bioluminescence imaging (BLI) and small animal single-photon emission computed tomography (SPECT). BCL1 cells labeled for fluorescence and bioluminescence assays (BCL1-gfp/luc) were injected into mice at a dose that leads to progressive disease within two to three weeks. Tumor response was followed by BLI and SPECT before and after treatment with a single dose of 10 mg/kg doxorubicin. Biodistribution analyses revealed a biphasic increase of annexin V uptake within the tumor-bearing tissues of mice. An early peak occurring before actual tumor cells loss was observed between 1 and 5 hr after treatment, and a second longer sustained rise from 9 to 24 hr after therapy, which heralds the onset of tumor cell loss as confirmed by BLI. Multimodality imaging revealed the temporal patterns of tumor cell loss and annexin V uptake revealing a better understanding of the timing of radiolabeled annexin V uptake for its development as a marker of therapeutic efficacy.     

Synthesis and radiopharmacological characterization of 2beta-carbo-2'-[18F]fluoroethoxy-3beta-(4-bromo-phenyl)tropane ([18F]MCL-322) as a PET radiotracer for imaging the dopamine transporter (DAT)

The fluoroalkyl-containing tropane derivative 2beta-carbo-2'-fluoroethoxy-3beta-(4-bromo-phenyl)tropane (MCL-322) is a highly potent and moderately selective ligand for the dopamine transporter (DAT). The compound was labeled with the short-lived positron emitter (18)F in a single step by nucleophilic displacement of the corresponding tosylate precursor MCL-323 with no-carrier-added [(18)F]fluoride. The positron emission tomography (PET) radiotracer 2beta-carbo-2'-[(18)F]fluoroethoxy-3beta-(4-bromo-phenyl)tropane [(18)F]MCL-322 was obtained in decay-corrected radiochemical yields of 30-40% at a specific radioactivity of 1.6-2.4Ci/mumol (60-90GBq/mumol) at the end-of-synthesis (EOS). Small animal PET, ex vivo and in vivo biodistribution experiments in rats demonstrated a high uptake in the striatum (3.2% ID/g) 5min after injection, which increased to 4.2% ID/g after 60min. The uptake in the cerebellum was 1.8% ID/g and 0.6% ID/g after 5min and 60min post-injection, respectively. Specific binding to DAT of [(18)F]MCL-322 was confirmed by blocking experiments using the high affinity DAT ligand GBR 12909. The radiopharmacological characterization was completed with metabolite and autoradiographic studies confirming the selective uptake of [(18)F]MCL-322 in the striatum. It is concluded that the simple single-step radiosynthesis of [(18)F]MCL-322 and the promising radiopharmacological data make [(18)F]MCL-322 an attractive candidate for the further development of a PET radiotracer potentially suitable for clinical DAT imaging in the human brain.     

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 evaluation of 18F- and 11C-labeled phenyl-galactopyranosides as potential probes for in vivo visualization of LacZ gene expression using positron emission tomography

3-Hydroxy-2-nitrophenyl 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranoside, a derivative of the chromogenic beta-galactosidase (beta-gal) substrate o-nitrophenyl beta-D-galactopyranoside (ONPG) was synthesized using a Koenigs-Knorr glycosylation reaction. It was alkylated with 2-[(18)F]fluoroethyl triflate or [(11)C]methyl triflate, followed by deacetylation of the sugar hydroxyl groups to obtain radiolabeled 3-(2'-[(18)F]fluoroethoxy)-2-nitrophenyl beta-D-galactopyranoside ([(18)F]-2c) and 3-[(11)C]methoxy-2-nitrophenyl beta- d-galactopyranoside ([(11)C]-3c), which were evaluated as potential reporter probes for in vivo visualization of LacZ gene expression with positron emission tomography (PET). In vitro, [(18)F]- 2c and [(11)C]-3c were good substrates of beta-gal and showed, respectively, a 7.5- and 2.5-fold higher uptake into beta-gal expressing cells (LacZ cells) compared to control cells. However, reversed-phase HPLC analysis of the LacZ cell lysate and supernatant showed that labeled 3-(2'-[(18)F]fluoroethoxy)-2-nitrophenol, the hydrolysis product formed by beta-gal-mediated cleavage of [(18)F]-2c, substantially leaked out of the cells, which would lead to loss of PET signal. In a microPET study of [(18)F]-2c in a mouse with a beta-gal expressing tumor, high retention was observed in liver and kidneys, but only negligible accumulation was seen in the tumor. As a general conclusion, it can be stated that the synthesized PET tracers [ (18)F]-2c and [(11)C]-3c are not suitable for use as LacZ reporter probes. Further structural modifications to improve the diffusion over the tumor cell membrane and to increase retention in beta-gal expressing cells may lead to more favorable in vivo imaging probes.     

Micro-PET imaging of alphavbeta3-integrin expression with 18F-labeled dimeric RGD peptide

The alphav integrins, which act as cell adhesion molecules, are closely involved with tumor invasion and angiogenesis. In particular, alphavbeta3 integrin, which is specifically expressed on proliferating endothelial cells and tumor cells, is a logical target for development of a radiotracer method to assess angiogenesis and anti-angiogenic therapy. In this study, a dimeric cyclic RGD peptide E[c(RGDyK)]2 was labeled with 18F (t(1/2) = 109.7 min) by using a prosthetic 4-[18F]fluorobenzoyl moiety to the amino group of the glutamate. The resulting [18F]FB-E[c(RGDyK)]2, with high specific activity (200-250 GBq/micromol at the end of synthesis), was administered to subcutaneous U87MG glioblastoma xenograft models for micro-PET and autoradiographic imaging as well as direct tissue sampling to assess tumor targeting efficacy and in vivo kinetics of this PET tracer. The dimeric RGD peptide demonstrated significantly higher tumor uptake and prolonged tumor retention in comparison with a monomeric RGD peptide analog [18F]FB-c(RGDyK). The dimeric RGD peptide had predominant renal excretion, whereas the monomeric analog was excreted primarily through the biliary route. Micro-PET imaging 1 hr after injection of the dimeric RGD peptide exhibited tumor to contralateral background ratio of 9.5 +/- 0.8. The synergistic effect of polyvalency and improved pharmacokinetics may be responsible for the superior imaging characteristics of [18F]FB-E[c(RGDyK)]2.     

N-Succinimidyl 4-[(18)F]-fluoromethylbenzoate-labeled dimeric RGD peptide for imaging tumor integrin expression

RGD peptides, radiolabeled with (18)F, have been used in the clinic for PET imaging of tumor angiogenesis in cancer patients. RGD peptides are typically labeled using a prosthetic group such as N-succinimidyl 4-[(18)F]-fluorobenzoate ([(18)F]SFB) or 4-nitrophenyl 2-[(18)F]-fluoropropionate ([(18)F]NPFP). However, the complex radiosynthetic procedures have impeded their broad application in clinical studies. We previously radiolabeled proteins and peptides with the prosthetic group, N-succinimidyl 4-[(18)F]-fluoromethylbenzoate ([(18)F]SFMB), which was prepared in a simple one-step procedure. In this study, we labeled a PEGylated cyclic RGD peptide dimer, PEG(3)-E[c(RGDyK)](2) (PRGD2), using [(18)F]SFMB and evaluated for imaging tumor αvβ3 integrin expression with positron emission tomography (PET). [(18)F]SFMB was prepared in one step using [(18)F]fluoride displacement of a nitrobenzenesulfonate leaving group under mild reaction conditions followed by HPLC purification. The (18)F-labeled peptide, [(18)F]FMBPRGD2 was prepared by coupling PRGD2 with [(18)F]SFMB in pH 8.6 borate buffer and purified with HPLC. The direct labeling on BMBPRGD2 was also attempted. A Siemens Inveon PET was used to image the uptake of the [(18)F]FMBPRGD2 into a U87MG xenograft mouse model. [(18)F]FMBPRGD2, was prepared with a 15% overall radiochemical yield (uncorrected) in a total synthesis time of 90?min, which was considerably shorter than the preparation of [(18)F]SFB- and [(18)F]NPFP-labeled RGD peptides. The direct labeling, however, was not successful. High quality microPET images using [(18)F]FMBPRGD2 clearly visualized tumors by 15?min with good target to background ratio. Early tracer accumulation in the bladder suggests fast renal clearance. No obvious bone uptake can be detected even at 4-h time point indicating that fluorine attachment is stable in mice. In conclusion, N-succinimidyl 4-[(18)F]-fluoromethylbenzoate ([(18)F]SFMB) prosthetic group can be a good alternative for labeling RGD peptides to image αvβ3 integrin expression and for labeling other peptides.     

Comparison study of [18F]FAl-NOTA-PRGD2, [18F]FPPRGD2, and [68Ga]Ga-NOTA-PRGD2 for PET imaging of U87MG tumors in mice

[(18)F]FPPRGD2, an F-18 labeled dimeric cyclic RGDyK peptide, has favorable properties for PET imaging of angiogenesis by targeting the α(v)β(3) integrin receptor. This radiotracer has been approved by the FDA for use in clinical trials. However, the time-consuming multiple-step synthetic procedure required for its preparation may hinder the widespread usage of this tracer. The recent development of a method using an F-18 fluoride-aluminum complex to radiolabel peptides provides a strategy for simplifying the labeling procedure. On the other hand, the easy-to-prepare [(68)Ga]-labeled NOTA-RGD derivatives have also been reported to have promising properties for imaging α(v)β(3) integrin receptors. The purpose of this study was to prepare [(18)F]FPPRGD2 [corrected] , [(18)F]FAl-NOTA-PRGD2, and [(68)Ga]Ga-NOTA-PRGD2 and to compare their pharmacokinetics and tumor imaging properties using small animal PET. All three compounds showed rapid and high tracer uptake in U87MG tumors with high target-to-background ratios. The uptake in the liver, kidneys, and muscle were similar for all three tracers, and they all showed predominant renal clearance. In conclusion, [(18)F]FAl-NOTA-PRGD2 and [(68)Ga]Ga-NOTA-PRGD2 have imaging properties and pharmacokinetics comparable to those of [(18)F]FPPRGD2. Considering their ease of preparation and good imaging qualities, [(18)F]FAl-NOTA-PRGD2 and [(68)Ga]NOTA-PRGD2 are promising alternatives to [(18)F]FPPRGD2 for PET imaging of tumor α(v)β(3) integrin expression.     

Synthesis and investigation of a radioiodinated F3 peptide analog as a SPECT tumor imaging radioligand

A radioiodinated derivative of the tumor-homing F3 peptide, (N-(2-{3-[(125)I]Iodobenzoyl}aminoethyl)maleimide-F3Cys peptide, [(125)I]IBMF3 was developed for investigation as a SPECT tumor imaging radioligand. For this purpose, we custom synthesized a modified F3 peptide analog (F3Cys) incorporating a C-terminal cysteine residue for site-specific attachment of a radioiodinated maleimide conjugating group. Initial proof-of-concept Fluorescence studies conducted with AlexaFluor 532 C(5) maleimide-labeled F3Cys showed distinct membrane and nuclear localization of F3Cys in MDA-MB-435 cells. Additionally, F3Cys conjugated with NIR fluorochrome AlexaFluor 647 C(2) maleimide demonstrated high tumor specific uptake in melanoma cancer MDA-MB-435 and lung cancer A549 xenografts in nude mice whereas a similarly labeled control peptide did not show any tumor uptake. These results were also confirmed by ex vivo tissue analysis. No-carrier-added [(125)I]IBMF3 was synthesized by a radioiododestannylation approach in 73% overall radiochemical yield. In vitro cell uptake studies conducted with [(125)I]IBMF3 displayed a 5-fold increase in its cell uptake at 4 h when compared to controls. SPECT imaging studies with [(125)I]IBMF3 in tumor bearing nude mice showed clear visualization of MDA-MB-435 xenografts on systemic administration. These studies demonstrate a potential utility of F3 peptide-based radioligands for tumor imaging with PET or SPECT techniques.     

Imaging endothelin ET(B) receptors using [18F]-BQ3020: in vitro characterization and positron emission tomography (microPET)

The endothelin (ET) receptor system has been shown to play a role in a number of vascular diseases. We have synthesized 18F-and 11C-labeled radioligands to enable in vivo imaging of the fundamental processes involved in ET receptor pharmacology in normal and diseased tissue using positron emission tomography (PET). One aim is to elucidate the proposed role of the ET(B) subtype as clearing receptor, removing ET-1 from the circulation, and whether this is an important mechanism to limit the detrimental effects caused by upregulated ET-1 in disease. To image ET(B) receptors we have labeled the selective agonist BQ3020 with 18F. In vitro characterization verified that [18F]-BQ3020 bound with a single subnanomolar affinity (K(D) = 0.34 +/- 0.10 nM, B(max) = 9.23 +/- 3.70 fmol/mg protein) to human left ventricle. Binding of [18F]-BQ3020 to human kidney was inhibited by ET-1 and unlabeled BQ3020 but not by the ET(A) selective antagonist FR139317, confirming that selectivity for the ET(B) receptor was retained. In vitro autoradiography revealed, as expected, high levels of ET(B) receptor densities in lung and kidney medulla, whereas kidney cortex and heart showed lower levels of ET(B) receptor densities. Furthermore, a high level of [18F]-BQ3020 binding was found to colocalize to macrophages in atherosclerotic coronary arteries. MicroPET studies demonstrated high uptake of [18F]-BQ3020 in ET(B) receptor-rich tissue, including lung, liver and kidney. The in vivo biodistribution of [18F]-BQ3020 was comparable to that previously obtained for [18F]-ET-1, supporting our hypothesis that the ET(B) receptor plays a significant role in the uptake of ET-1. In conclusion, [18F]-BQ3020 has retained high affinity and selectivity, allowing imaging of ET(B) receptor distributions in vitro and in vivo in human and animal tissue. Furthermore, in vitro data suggest that [18F]-BQ3020 potentially can be used to image atherosclerotic lesions in vivo using PET.     

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.     

Imaging the L-Type Amino Acid Transporter-1 (LAT1) with Zr-89 ImmunoPET

The L-type amino acid transporter-1 (LAT1, SLC7A5) is upregulated in a wide range of human cancers, positively correlated with the biological aggressiveness of tumors, and a promising target for both imaging and therapy. Radiolabeled amino acids such as O-(2-[¹⁸F]fluoroethyl)-L-tyrosine (FET) that are transport substrates for system L amino acid transporters including LAT1 have met limited success for oncologic imaging outside of the brain, and thus new strategies are needed for imaging LAT1 in systemic cancers. Here, we describe the development and biological evaluation of a novel zirconium-89 labeled antibody, [⁸⁹Zr]DFO-Ab2, targeting the extracellular domain of LAT1 in a preclinical model of colorectal cancer. This tracer demonstrated specificity for LAT1 in vitro and in vivo with excellent tumor imaging properties in mice with xenograft tumors. PET imaging studies showed high tumor uptake, with optimal tumor-to-non target contrast achieved at 7 days post administration. Biodistribution studies demonstrated tumor uptake of 10.5 ± 1.8 percent injected dose per gram (%ID/g) at 7 days with a tumor to muscle ratio of 13 to 1. In contrast, the peak tumor uptake of the radiolabeled amino acid [¹⁸F]FET was 4.4 ± 0.5 %ID/g at 30 min after injection with a tumor to muscle ratio of 1.4 to 1. Blocking studies with unlabeled anti-LAT1 antibody demonstrated a 55% reduction of [⁸⁹Zr]DFO-Ab2 accumulation in the tumor at 7 days. These results are the first report of direct PET imaging of LAT1 and demonstrate the potential of immunoPET agents for imaging specific amino acid transporters.     

18F Labeled benzimidazole derivatives as potential radiotracer for positron emission tomography (PET) tumor imaging

This article reported the synthesis and bioevaluation of two [¹⁸F] labeled benzimidazole derivatives, 4-(5-(2-[¹⁸F] fluoro-4-nitrobenzamido)-1-methyl-1H-benzimidazol-2-yl) butanoic acid ([¹⁸F] FNBMBBA, [¹⁸F]a1) and 3-(2-fluoroethyl)-7-methyl-2-propyl-3H-benzimidazole-5-carboxylic acid ([¹⁸F] FEMPBBA, [¹⁸F]b1) for PET tumor imaging. The preparation [¹⁸F] FEMPBBA was completed in 1 h with overall radiochemical yield of 50–60% (without decay corrected). Biodistribution assay in S180 tumor bearing mice of both compounds were carried out, and the results are both meaningful. [¹⁸F] FEMPBBA which can be taken as a revision of [¹⁸F] FNBMBBA got an excellent result, and has significant advantages in some aspects compared with L-[¹⁸F] FET and [¹⁸F]-FDG in the same animal model, especially in tumor/brain uptake ratio. The tumor/brain uptake ratio of [¹⁸F] FEMPBBA gets to 4.81, 7.15, and 9.8 at 30 min, 60 min and 120 min, and is much higher than that of L-[¹⁸F] FET (2.54, 2.92 and 2.95) and [¹⁸F]-FDG (0.61, 1.02, 1.33) at the same time point. The tumor/muscle and tumor/blood uptake ratio of [¹⁸F] FEMPBBA is also higher than that of L-[¹⁸F] FET at 30 min and 60 min. This result indicates compound [¹⁸F] FEMPBBA is a promising radiotracer for PET tumor imaging.     

18F]-2'-Fluoro-5-methyl-1-beta-D-arabinofuranosyluracil (18F-FMAU) in prostate cancer: initial preclinical observations

We hypothesized that imaging-based assessment of cellular proliferation in prostate cancer may improve tumor characterization. We therefore evaluated the biodistribution and effect of androgen on tumor uptake of the cellular proliferation imaging marker [(18)F]-2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil ((18)F-FMAU) in xenograft mouse models of human prostate cancer. Castrated and noncastrated athymic male mice were implanted with androgen-independent PC3 and androgen-sensitive CWR22 human prostate cancer cells. Dynamic micro-positron emission tomography (PET)/computed tomography was performed for 1 hour followed by 10-minute static scans at 2 and 3 hours. Animals were sacrificed after imaging for biodistribution studies and immunohistochemical staining of tumors for androgen receptor and Ki-67/MIB expression. (18)F-FMAU uptake was significantly higher in all major organs of the castrated animals in comparison with noncastrated mice, with the highest uptake in liver and the lowest uptake in muscle and bone. When compared to PC3 tumors, CWR22 xenografts showed significantly higher tumor to muscle (2.56 ± 0.30 vs 1.99 ± 0.30, p = .008) and tumor to liver (1.72 ± 0.12 vs 1.26 ± 0.17, p = .0003) uptake ratios in the noncastrated animal at the 3-hour time point. Androgen receptor and Ki-67/MIB expressions were higher in CWR22 than in PC3 xenografts. Our initial preclinical observations suggest that there may be an association between androgen signaling and thymidine metabolism and that (18)F-FMAU PET may be useful in prostate tumor characterization.     

Al18F labeled sulfonamide-conjugated positron emission tomography tracer in vivo tumor-targeted imaging

An ideal positron emission tomography (PET) tracer should be highly extractable by the tumor tissue or organ that contains low toxicity and can provide high-resolution images in vivo. In this work, the aim was to evaluate the application of Al¹⁸F-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid containing sulfonamide group (¹⁸F-Al-NOTA-SN) as a potential tumor-targeting signal-enhanced radioactive tracer in PET. SN as a tumor-targeting group was incorporated to NOTA to make a ligand. Subsequently, this ligand reacted with Na¹⁸F and AlCl₃ to produce a compound ¹⁸F-Al-NOTA-SN. This compound was further characterized and its property in regard to cell cytotoxicity assay, microPET imaging, biodistribution, cell uptake assay, and tumor selectivity in vitro and in vivo, was also investigated. ¹⁸F-Al-NOTA-SN possessed low cell cytotoxicity and uptake to COS-7 and 293T healthy cells and high cell cytotoxicity and uptake to MDA-MB-231, HepG2, and HeLa tumor cells in vitro. Moreover, ¹⁸F-Al-NOTA-SN showed good tumor-targeting property and high PET signal enhancement of HeLa tumors, liver, and kidneys in mice, as well as the uptake ratios of tumor to blood and tumor to muscle, were 4.98 and 3.87, respectively. ¹⁸F-Al-NOTA-SN can be accepted to be kidney and liver eliminated earlier and show a potential tumor-targeting signal-enhanced radioactive tracer in PET.     

Radiofluorinated rhenium cyclized α-MSH analogues for PET imaging of melanocortin receptor 1

In order to accomplish in vivo molecular imaging of melanoma biomarker melanocortin 1 receptor (MC1R), several α-melanocyte-stimulating hormone (α-MSH) analogues have been labeled with N-succinimidyl-4-1?F-fluorobenzoate (1?)F-SFB) and studied as positron emission tomography (PET) probes in our recent studies. To further pursue a radiofluorinated α-MSH peptide with high clinical translation potential, we utilized 4-nitrophenyl 2-1?F-fluoropropionate (1?F-NFP) to radiofluorinate the transition metal rhenium cyclized α-MSH metallopeptides for PET imaging of MC1R positive malignant melanoma. Metallopeptides Ac-d,Lys-ReCCMSH(Arg11) (two isomers, namely RMSH-1 and RMSH-2) were synthesized using conventional solid phase peptide synthesis chemistry and rhenium cyclization reaction. The two isomers were then conjugated with 1?F-NFP or 1?F-NFP. The resulting cold or radiofluorinated metallopeptides, (1?/1?)F-FP-RMSH-1 and (1?/1?)F-FP-RMSH-2, were further evaluated for their in vitro receptor binding affinities, in vivo biodistribution, and small-animal PET imaging properties. The binding affinities of 1?F-FP-RMSH-1 and 1?F-FP-RMSH-2 were determined to be within low nanomolar range. In vivo studies revealed that both F-labeled metallopeptides possessed good tumor uptake in the B16F10 murine model with high MC1R expression, while possessing much lower uptake in A375M human melanoma xenografts. Moreover, 1?F-FP-RMSH-1 displayed more favorable in vivo performance in terms of higher tumor uptake and much lower accumulation in the kidney and liver, when compared to that of 1?F-FP-RMSH-2 at 2 h postinjection (p.i.). 1?F-FP-RMSH-1 also displayed lower liver and lung uptake when compared with that of the same peptide labeled with 1?F-SFB (named as 1?F-FB-RMSH-1). Small animal PET imaging of 1?F-FP-RMSH-1 in mice bearing B16F10 tumors at 1 and 2 h showed good tumor imaging quality. As expected, much lower tumor uptake and poorer tumor/normal organ contrast were observed for A375M model compared to those of the B16F10 model. 1?F-FP-RMSH-1 also exhibited higher tumor uptake and better tumor retention when compared with 1?F-FB-RMSH-1. 1?F-FP-RMSH-1 demonstrates significant advantages over 1?F-FB-RMSH-1 and 1?F-FP-RMSH-2. It is a promising PET probe for imaging MC1R positive melanoma and MC1R expression in vivo.     

PET Imaging of Leptin Biodistribution and Metabolism in Rodents and Primates

We have determined the systemic biodistribution of the hormone leptin by PET imaging. PET imaging using ¹⁸F- and ⁶⁸Ga-labeled leptin revealed that, in mouse, the hormone was rapidly taken up by megalin (gp330/LRP2), a multiligand endocytic receptor localized in renal tubules. In addition, in rhesus monkeys, 15% of labeled leptin localized to red bone marrow, which was consistent with hormone uptake in rodent tissues. These data confirm a megalin-dependent mechanism for renal uptake in vivo. The significant binding to immune cells and blood cell precursors in bone marrow is also consistent with prior evidence showing that leptin modulates immune function. These experiments set the stage for similar studies in humans to assess the extent to which alterations of leptin's biodistribution might contribute to obesity; they also provide a general chemical strategy for ¹⁸F labeling of proteins for PET imaging of other polypeptide hormones.     

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.