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1.  Delta opioidmimetic antagonists: prototypes for designing a new generation of ultraselective opioid peptides. 
Molecular Medicine  1995;1(6):678-689.
BACKGROUND: Tyr-Tic (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) and Tyr-Tic-Ala were the first peptides with delta opioid antagonist activity lacking Phe, considered essential for opioid activity based on the N-terminal tripeptide sequence (Tyr-D-Xaa-Phe) of amphibian skin opioids. Analogs were then designed to restrain the rotational flexibility of Tyr by the substitution of 2,6-dimethyl-L-tyrosine (Dmt). MATERIALS AND METHODS: Tyr and Dmt peptides were synthesized by solid phase and solution methods using Fmoc technology or condensing Boc-Dmt-OH or Boc-Tyr(But)-OH with H-L-Tic-OBut or H-D-Tic-OBut, respectively. Peptides were purified (> 99%) by HPLC and characteristics determined by 1H-NMR, FAB-MS, melting point, TLC, and amino acid analyses. RESULTS: H-Dmt-Tic-OH had high affinity (Ki delta = 0.022 nM) and extraordinary selectivity (Ki mu/Ki delta = 150,000); H-Dmt-Tic-Ala-OH had a Ki delta = 0.29 nM and delta selectivity = 20,000. Affinity and selectivity increased 8700- and 1000-fold relative to H-Tyr-Tic-OH, respectively. H-Dmt-Tic-OH and H-Dmt-Tic-NH2 fitted one-site receptor binding models (eta = 0.939-0.987), while H-Dmt-Tic-ol, H-Dmt-Tic-Ala-OH and H-Dmt-Tic-Ala-NH2 best fitted two-site models (eta = 0.708-0.801, F 18.9-26.0, p < 0.0001). Amidation increased mu affinity by 10- to 100-fold and acted synergistically with D-Tic2 to reverse selectivity (delta-->mu). Dmt-Tic di- and tripeptides exhibited delta antagonist bioactivity (Ke = 4-66 nM) with mouse vas deferens and lacked agonist mu activity (> 10 microM) in guinea-pig ileum preparations. Dmt-Tic analogs weakly interacted with kappa receptors in the 1 to > 20 microM range. CONCLUSIONS: Dmt-Tic opioidmimetic peptides represent a highly potent class of opioid peptide antagonists with greater potency than the nonopioid delta antagonist naltrindole and have potential application as clinical and therapeutic compounds.
PMCID: PMC2229973  PMID: 8529134
2.  N-Succinimidyl 4-[18F]-fluoromethylbenzoate-labeled dimeric RGD peptide for imaging tumor integrin expression 
Amino acids  2011;43(3):1349-1357.
RGD peptides, radiolabeled with 18F, 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-[18F]-fluorobenzoate ([18F]SFB) or 4-nitrophenyl 2-[18F]-fluoropropionate ([18F]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-[18F]-fluoromethylbenzoate ([18F]SFMB), which was prepared in a simple one-step procedure. In this study, we labeled a PEGylated cyclic RGD peptide dimer, PEG3-E[c(RGDy K)]2 (PRGD2), using [18F]SFMB and evaluated for imaging tumor αvβ3 integrin expression with positron emission tomography (PET). [18F]SFMB was prepared in one step using [18F]fluoride displacement of a nitrobenzenesulfonate leaving group under mild reaction conditions followed by HPLC purification. The 18F-labeled peptide, [18F]FMBPR GD2 was prepared by coupling PRGD2 with [18F]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 [18F]FMBPRGD2 into a U87MG xenograft mouse model. [18F]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 [18F]SFB- and [18F]NPFP-labeled RGD peptides. The direct labeling, however, was not successful. High quality microPET images using [18F]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-[18F]-fluoromethylbenzoate ([18F]SFMB) prosthetic group can be a good alternative for labeling RGD peptides to image αvβ3 integrin expression and for labeling other peptides.
PMCID: PMC3577934  PMID: 22209865
Integrin αvβ3; RGD peptide dimer; Positron emission tomography; N-Succinimidyl 4-[18F]-fluoromethylbenzoate ([18F]SFMB)
3.  Further Studies on Lead Compounds Containing the Opioid Pharmacophore Dmt-Tic 
Journal of medicinal chemistry  2008;51(16):5109-5117.
Opioids containing the Dmt-Tic pharmacophore, especially the δ agonists H-Dmt-Tic-Gly-NH-Ph 1 and H-Dmt-Tic-NH-(S)CH(CH2-COOH)-Bid 4 (UFP-512) were evaluated for the influence of the substitution of Gly with aspartic acid, its chirality, and the importance of the – NH-Ph and N1H-Bid hydrogens relative to δ agonism. The results provide the following conclusions: (i) Asp increases δ selectivity by lowering μ affinity; (ii) -NH-Ph and N1H-Bid nitrogen methylation transforms δ agonists into δ antagonists; (iii) substitution of Gly with L-Asp/D-Asp in the δ agonist H-Dmt-Tic-Gly-NH-Ph resulted in δ antagonists, while the same substitution in the δ agonist H-Dmt-Tic-NH-CH2-Bid yielded more selective δ agonists, H-Dmt-Tic-NH-(S)CH(CH2-COOH)-Bid and H-Dmt-Tic-NH-(R)CH(CH2-COOH)-Bid; (iv) L-Asp seems important only for functional bioactivity, not receptor affinity; (v) H-Dmt-Tic-NH-(S)CH(CH2-COOH)-Bid(N1-Me) (10) revealed analgesia similar to 4, which was reversed by naltrindole only in the tail-flick test. Compounds 4 and 10 had opposite behaviours in mice: 4 caused agitation, while 10 gave sedation and convulsions.
PMCID: PMC2812024  PMID: 18680274
4.  An Engineered Knottin Peptide Labeled with 18F for PET Imaging of Integrin Expression 
Bioconjugate chemistry  2009;20(12):2342-2347.
Knottins are small constrained polypeptides that share a common disulfide-bonded framework and a triple-stranded β-sheet fold. Previously, directed evolution of the Ecballium elaterium trypsin inhibitor (EETI-II) knottin led to the identification of a mutant that bound to tumor-specific αvβ3 and αvβ5 integrin receptors with low nanomolar affinity. The objective of this study was to prepare and evaluate a radiofluorinated version of this knottin (termed 2.5D) for microPET imaging of integrin positive tumors in living subjects. Knottin peptide 2.5D was prepared by solid phase synthesis and folded in vitro, and its free N-terminal amine was reacted with N-succinimidyl-4-18/19F-fluorobenzoate (18/19F-SFB) to produce the fluorinated peptide 18/19F-FB-2.5D. The binding affinities of unlabeled knottin peptide 2.5D and 19F-FB-2.5D to U87MG glioblastoma cells were measured by competition binding assay using 125I-labeled echistatin. It was found that unlabeled 2.5D and 19F-FB-2.5D competed with 125I-echistatin for binding to cell surface integrins with IC50 values of 20.3 ± 7.3 and 13.2 ± 5.4 nM, respectively. Radiosynthesis of 18F-FB-2.5D resulted in a product with high specific activity (ca 100 GBq/μmol). Next, biodistribution and positron emission tomography (PET) imaging studies were performed to evaluate the in vivo behavior of 18F-FB-2.5D. Approximately 3.7 MBq 18F-FB-2.5D was injected into U87MG tumor bearing mice via the tail vein. Biodistribution studies demonstrated that 18F-FB-2.5D had moderate tumor uptake at 0.5 h post injection, and co-injection of a large excess of the unlabeled peptidomimetic c(RGDyK) as a blocking agent significantly reduced tumor uptake (1.90 ± 1.15 vs. 0.57 ± 0.14 %ID/g, 70% inhibition, P < 0.05). In vivo microPET imaging showed that 18F-FB-2.5D rapidly accumulated in the tumor and quickly cleared from the blood through the kidneys, allowing excellent tumor-to-normal tissue contrast to be obtained. Collectively, 18F-FB-2.5D allows integrin-specific PET imaging of U87MG tumors with good contrast, and further demonstrates that knottins are excellent peptide scaffolds for development of PET probes with potential for clinical translation.
PMCID: PMC2804269  PMID: 19908826
Kottin Peptide; Integrin; 18F; PET; RGD
5.  Further Studies on the Effect of Lysine at the C-Terminus of the Dmt-Tic Opioid Pharmacophore 
Bioorganic & medicinal chemistry  2007;15(9):3143-3151.
A wide range of bioactivities are induced by Lys when introduced at the C-terminus of the δ-opioid Dmt-Tic pharmacophore through the α-amine group, such as improved δ-antagonism, and presence of μ-agonism and μ-antagonism. We report the synthesis of a new series of compounds with the general formula H-Dmt-Tic-NH-(CH2)4-CH(R)-R’ (R = -NH2, -NH-Ac, -NH-Z; R’ = CO-NH-Ph, -CO-NH-CH2-Ph, -Bid) in which Lys is linked to Dmt-Tic through its amine group side chain. The compounds (1-9) displayed a potent and selective δ-antagonism (pA2 = 7.81-8.27) independent of the functionalized α-amine and carboxylic groups of the C-terminal Lys. This suggests direct application as a prototype intermediate, such as Boc-Dmt-Tic-ε-Lys(Z)-OMe, which could be applied in the synthesis (after Z or methyl ester removal) of unique “designed multiple ligands” containing the pharmacophore of the quintessential δ-antagonist Dmt-Tic and another opioid or biologically active non-opioid ligand.
PMCID: PMC2377021  PMID: 17339114
6.  Quantitative PET Imaging of Tumor Integrin αvβ3 Expression with 18F-FRGD2 
The development of noninvasive methods to visualize and quantify integrin αvβ3 expression in vivo appears to be crucial for the success of antiangiogenic therapy based on integrin antagonism. Precise documentation of integrin receptor levels will allow appropriate selection of patients who will most likely benefit from an antiintegrin treatment regimen. Imaging can also be used to provide an optimal dosage and time course for treatment based on receptor occupancy studies. In addition, imaging integrin expression will be important to evaluate antiintegrin treatment efficacy and to develop new therapeutic drugs with favorable tumor targeting and in vivo kinetics. We labeled the dimeric RGD peptide E[c(RGDyK)]2 with 18F and evaluated its tumor-targeting efficacy and pharmacokinetics of 18F-FB–E[c(RGDyK)]2 (18F-FRGD2).
E[c(RGDyK)]2 was labeled with 18F by conjugation coupling with N-succinimidyl-4-18F-fluorobenzoate (18F-SFB) under a slightly basic condition. The in vivo metabolic stability of 18F-FRGD2 was determined. The diagnostic value after injection of 18F-FRGD2 was evaluated in various xenograft models by dynamic microPET followed by ex vivo quantification of tumor integrin level.
Starting with 18F− Kryptofix 2.2.2./K2CO3 solution, the total reaction time for 18F-FRGD2, including final high-performance liquid chromatography purification, is about 200 ± 20 min. Typical decay-corrected radiochemical yield is 23% ± 2% (n = 20). 18F-FRGD2 is metabolically stable. The binding potential extrapolated from graphical analysis of PET data and Logan plot correlates well with the receptor density measured by sodium dodecyl sulfate polyacrylamide electrophoresis and autoradiography in various xenograft models. The tumor-to-background ratio at 1 h after injection of 18F-FRGD2 also gives a good linear relationship with the tumor tissue integrin level.
The dimeric RGD peptide tracer 18F-FRGD2, with high integrin specificity and favorable excretion profile, may be translated into the clinic for imaging integrin αvβ3 expression. The binding potential calculated from simplified tracer kinetic modeling such as the Logan plot appears to be an excellent indicator of tumor integrin density.
PMCID: PMC4160026  PMID: 16391195
molecular imaging; integrin αvβ3; dimeric RGD peptide; dynamic microPET; Logan plot
7.  PET of Tumor CXCR4 Expression with 4-18F-T140 
Expression of the chemokine receptor CXCR4 by cancers has been shown to correlate with tumor aggressiveness and poor prognosis and may also contribute to metastatic seeding of organs that express its ligand SDF-1. However, fully optimized PET agents for determining CXCR4 expression by tumor cells in vivo are not yet available. This study aims to develop a stable, 18F-labeled peptide that enables in vivo quantification of CXCR4 in cancer.
4-F-benzoyl-TN14003 (4-F-T140), a short peptide antagonist of CXCR4 with 1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl protecting groups on the ε-amino groups of the lysine residues, was labeled with 18F-fluoride via N-succini-midyl-4-18F-fluorobenzoate conjugation, followed by deprotection to give 4-18F-T140 that was exclusively labeled on the α-amine at the N terminus. Cell binding, migration, biodistribution, and small-animal PET studies of 4-18F-T140 were performed.
4-F-T140 was radiolabeled by coupling with N-succinimidyl-4-18F-fluorobenzoate, with an overall decay-corrected radiochemical yield of 15% ± 5% calculated from the start of synthesis. The mean measured specific activity (±SD) was 7 ± 2 GBq/μmol (0.19 ± 0.05 Ci/μmol), and radio-chemical purity was greater than 99%. 4-18F-T140 was found to bind specifically to red blood cells in vitro and in vivo. The binding of 4-18F-T140 to red blood cells was blocked with a small amount of cold 4-F-T140, which led to higher uptake of 4-18F-T140 by Chinese hamster ovarian (CHO)-CXCR4 tumors. Biodistribution experiments at 3 h after injection with the addition of 10 μg of cold 4-F-T140 showed a 3.03 ± 0.31 percentage injected dose per gram uptake in CHO-CXCR4 tumors, with a tumor-to-blood ratio of 27.1 ± 8.7 and a tumor-to-muscle ratio of 21.6 ± 7.1. PET studies demonstrated clear visualization of CXCR4-transfected, but not CXCR4-negative, CHO tumors.
4-18F-T140 can be used as a PET tracer to image tumor expression of CXCR4, with a high tumor-to-background ratio. The knowledge of whether tumors express or do not express CXCR4 might be beneficial in determining appropriate treatment and monitoring.
PMCID: PMC3629977  PMID: 20956475
T140 peptide; CXCR4; PET; 18F-fluoride
8.  Role of 2′,6′-Dimethyl-L-Tyrosine (Dmt) in Some Opioid Lead Compounds 
Bioorganic & medicinal chemistry  2010;18(16):6024-6030.
Here we evaluated how the interchange of the amino acids 2′,6′-dimethyl-L-tyrosine (Dmt), 2′,6′-difluoro-L-tyrosine (Dft), and tyrosine in position 1 can affect the pharmacological characterization of some reference opioid peptides and pseudopeptides. Generally, Dft and Tyr provide analogues with a similar pharmacological profile, despite different pKa values. Dmt/Tyr(Dft) replacement gives activity changes depending on the reference opioid in which the modification was made. Whereas, H-Dmt-Tic-Asp*-Bid is a potent and selective δ agonist (MVD, IC50 = 0.12 nM); H-Dft-Tic-Asp*-Bid and H-Tyr-Tic-Asp*-Bid are potent and selective δ antagonists (pA2 = 8.95 and 8.85, respectively). When these amino acids are employed in the synthesis of deltorphin B and its Dmt1 and Dft1 analogues, the three compounds maintain a very similar δ agonism (MVD, IC50 0.32–0.53 nM) with a decrease in selectivity relative to the Dmt1 analogue. In the less selective H-Dmt-Tic-Gly*-Bid the replacement of Dmt with Dft and Tyr retains the δ agonism but with a decrease in potency. Antagonists containing the Dmt-Tic pharmacophore do not support the exchange of Dmt with Dft or Tyr.
PMCID: PMC2918654  PMID: 20637637
Dmt-Tic pharmacophore; opioid peptides; opioid receptors; δ opioid agonists; UFP-512; δ opioid antagonists
9.  microPET of Tumor Integrin αvβ3 Expression Using 18F-Labeled PEGylated Tetrameric RGD Peptide (18F-FPRGD4) 
In vivo imaging of αvβ3 expression has important diagnostic and therapeutic applications. Multimeric cyclic RGD peptides are capable of improving the integrin αvβ3–binding affinity due to the polyvalency effect. Here we report an example of 18F-labeled tetrameric RGD peptide for PET of αvβ3 expression in both xenograft and spontaneous tumor models.
The tetrameric RGD peptide E{E[c(RGDyK)]2}2 was derived with amino-3,6,9-trioxaundecanoic acid (mini-PEG; PEG is poly(ethylene glycol)) linker through the glutamate α-amino group. NH2-mini-PEG-E{E[c(RGDyK)]2}2 (PRGD4) was labeled with 18F via the N-succinimidyl-4-18F-fluorobenzoate (18F-SFB) prosthetic group. The receptor-binding characteristics of the tetrameric RGD peptide tracer 18F-FPRGD4 were evaluated in vitro by a cell-binding assay and in vivo by quantitative microPET imaging studies.
The decay-corrected radiochemical yield for 18F-FPRGD4 was about 15%, with a total reaction time of 180 min starting from 18F-F−. The PEGylation had minimal effect on integrin-binding affinity of the RGD peptide. 18F-FPRGD4 has significantly higher tumor uptake compared with monomeric and dimeric RGD peptide tracer analogs. The receptor specificity of 18F-FPRGD4 in vivo was confirmed by effective blocking of the uptake in both tumors and normal organs or tissues with excess c(RGDyK).
The tetrameric RGD peptide tracer 18F-FPRGD4 possessing high integrin-binding affinity and favorable biokinetics is a promising tracer for PET of integrin αvβ3 expression in cancer and other angiogenesis related diseases.
PMCID: PMC4183663  PMID: 17704249
microPET; integrin αvβ3; tetrameric RGD peptide; PEGylation; 18F
10.  18F-labeled mini-PEG spacered RGD dimer (18F-FPRGD2): synthesis and microPET imaging of αvβ3 integrin expression 
We have previously reported that 18F-FB-E[c(RGDyK)]2 (18F-FRGD2) allows quantitative PET imaging of integrin αvβ3 expression. However, the potential clinical translation was hampered by the relatively low radiochemical yield. The goal of this study was to improve the radiolabeling yield, without compromising the tumor targeting efficiency and in vivo kinetics, by incorporating a hydrophilic bifunctional mini-PEG spacer.
18F-FB-mini-PEG-E[c(RGDyK)]2 (18F-FPRGD2) was synthesized by coupling N-succinimidyl-4-18F-fluorobenzoate (18F-SFB) with NH2-mini-PEG-E[c(RGDyK)]2 (denoted as PRGD2). In vitro receptor binding affinity, metabolic stability, and integrin αvβ3 specificity of the new tracer 18F-FPRGD2 were assessed. The diagnostic value of 18F-FPRGD2 was evaluated in subcutaneous U87MG glioblastoma xenografted mice and in c-neu transgenic mice by quantitative microPET imaging studies.
The decay-corrected radiochemical yield based on 18F-SFB was more than 60% with radiochemical purity of >99%. 18F-FPRGD2 had high receptor binding affinity, metabolic stability, and integrin αvβ3-specific tumor uptake in the U87MG glioma xenograft model comparable to those of 18F-FRGD2. The kidney uptake was appreciably lower for 18F-FPRGD2 compared with 18F-FRGD2 [2.0±0.2%ID/ g for 18F-FPRGD2 vs 3.0±0.2%ID/g for 18F-FRGD2 at 1 h post injection (p.i.)]. The uptake in all the other organs except the urinary bladder was at background level. 18F-FPRGD2 also exhibited excellent tumor uptake in c-neu oncomice (3.6±0.1%ID/g at 30 min p.i.).
Incorporation of a mini-PEG spacer significantly improved the overall radiolabeling yield of 18F-FPRGD2. 18F-FPRGD2 also had reduced renal uptake and similar tumor targeting efficacy as compared with 18F-FRGD2. Further testing and clinical translation of 18F-FRGD2 are warranted.
PMCID: PMC4167588  PMID: 17492285
Integrin αvβ3; Dimeric RGD peptide; Mini-PEG spacer; MicroPET; Fluorine-18
11.  Effect of Lysine at C-Terminus of the Dmt-Tic Opioid Pharmacophore 
Journal of medicinal chemistry  2006;49(18):5610-5617.
Substitution of Gly with side-chain protected or unprotected Lys in lead compounds containing the opioid pharmacophore Dmt-Tic [H-Dmt-Tic-Gly-NH-CH2-Ph, μ agonist / δ antagonist; H-Dmt-Tic-Gly-NH-Ph, μ agonist / δ agonist and H-Dmt-Tic-NH-CH2-Bid, δ agonist (Bid = 1H-benzimidazole-2-yl)] yielded a new series of compounds endowed with distinct pharmacological activities. Compounds (1-10) included high δ- (Kiδ = 0.068-0.64 nM) and μ-opioid affinities (Kiδ = 0.13-5.50 nM) with a bioactivity that ranged from μ-opioid agonism {10, H-Dmt-Tic-NH-CH[(CH2)4-NH2]-Bid (IC50 GPI = 39.7 nM)} to a selective μ-opioid antagonist [3, H-Dmt-Tic-Lys-NH-CH2-Ph (pA2μ = 7.96)] and a selective δ-opioid antagonist [5, H-Dmt-Tic-Lys(Ac)-NH-Ph (pA2δ = 12.0)]. The presence of a Lys linker provides new lead compounds in the formation of opioid peptidomimetics containing the Dmt-Tic pharmacophore with distinct agonist and / or antagonist properties.
PMCID: PMC2533050  PMID: 16942034
12.  Biodistribution and Elimination Study of Fluorine-18 Labeled Nε-Carboxymethyl-Lysine following Intragastric and Intravenous Administration 
PLoS ONE  2013;8(3):e57897.
Nε-carboxymethyl-lysine (CML) is a major advanced glycation end-product (AGEs) widely found in foods. The aim of our study was to evaluate how exogenous CML-peptide is dynamically absorbed from the gastrointestinal tract and eliminated by renal tubular secretion using microPET imaging.
The present study consisted of three investigations. In study I, we synthesized the imaging tracer 18F-CML by reacting N-succinimidyl 4-18F-fluorobenzoate (18F-SFB) with CML. In study II, the biological activity of 18F-CML was evaluated in RAW264.7 cells and HepG2 cells. In study III, the biodistribution and elimination of AGEs in ICR mice were studied in vivo following tail vein injection and intragastric administration of 18F-CML.
The formation of 18F-CML was confirmed by comparing its retention time with the corresponding reference compound 19F-CML. The radiochemical purity (RCP) of 18F-CML was >95%, and it showed a stable character in vitro and in vivo. Uptake of 18F-CML by RAW264.7 cells and HepG2 cells could be inhibited by unmodified CML. 18F-CML was quickly distributed via the blood, and it was rapidly excreted through the kidneys 20 min after tail vein injection. However, 18F-CML was only slightly absorbed following intragastric administration. After administration of 18F-CML via a stomach tube, the radioactivity was completely localized in the stomach for the first 15 min. At 150 min post intragastric administration, intense accumulation of radioactivity in the intestines was still observed.
PET technology is a powerful tool for the in vivo analysis of the gastrointestinal absorption of orally administered drugs. 18F-CML is hardly absorbed by the gastrointestinal tract. It is rapidly distributed and eliminated from blood following intravenous administration. Thus, it may not be harmful to healthy bodies. Our study showed the feasibility of noninvasively imaging 18F-labeled AGEs and was the first to describe CML-peptide gastrointestinal absorption by means of PET.
PMCID: PMC3591457  PMID: 23505446
13.  Synthesis and evaluation of [11C]PyrATP-1, a novel radiotracer for PET imaging of glycogen synthase kinase-3β (GSK-3β) 
Nuclear medicine and biology  2014;41(6):507-512.
The dysfunction of glycogen synthase kinase-3β(GSK-3β) has been implicated in a number of diseases, including Alzheimer’s disease. The ability to non-invasively quantify GSK-3βactivity in vivo is therefore of critical importance, and this work is focused upon development of inhibitors of GSK-3βradiolabeled with carbon-11 to examine quantification of the enzyme using positron emission tomography (PET) imaging.
[11C]PyrATP-1 was prepared from the corresponding desmethyl-piperazine precursor in an automated synthesis module. In vivo rodent and primate imaging studies were conducted on a Concorde MicroPET P4 scanner to evaluate imaging properties, and in vitro autoradiography studies with rat brain samples were carried out to examine specific binding.
2035 ± 518 MBq (55 ± 14 mCi) of [11C]PyrATP-1 were obtained (1–2% noncorrected radiochemical yield at end-of-synthesis based upon [11C]CO2) with high chemical (>95%) and radiochemical (>99%) purities, and good specific activities (143 ± 52 GBq/µmol (3874 ± 1424 Ci/mmol)), n = 5. In vivo microPET imaging studies revealed poor brain uptake in rodents and non-human primates. Pretreatment of rodents with cyclosporin A resulted in moderately increased brain uptake suggesting Pgp transporter involvement. Autoradiography demonstrated high levels of specific binding in areas of the rodent brain known to be rich in GSK-3β.
[11C]PyrATP-1 is readily synthesized using standard carbon-11 radiochemistry. However, the poor brain uptake in rodents and non-human primates indicates that the radiotracer is not suitable for the purposes of quantifying GSK-3βin neurological and psychiatric disorders.
PMCID: PMC4034144  PMID: 24768148
14.  Radiofluorinated Rhenium Cyclized α-MSH Analogs for PET Imaging of Melanocortin Receptor 1 
Bioconjugate chemistry  2010;21(12):2355-2360.
In order to accomplish in vivo molecular imaging of melanoma biomarker melanocortin 1 receptor (MC1R), several alpha-melanocyte-stimulating hormone (α-MSH) analogs have been labeled with N-succinimidyl-4-18F-fluorobenzoate (18F-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 2-18F-fluoropropionate (18F-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 19F-NFP or 18F-NFP. The resulting cold or radiofluorinated metallopeptides, 18/19F-FP-RMSH-1 and 18/19F-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 the 19F-FP-RMSH-1 and 19F-FP-RMSH-2) were determined to be within low nM range. In vivo studies revealed that both 18F-labeled metallopeptides possessed good tumor uptake in B16F10 murine model with high MC1R expression, while much lower uptake in A375M human melanoma xenografts. Moreover, 18F-FP-RMSH-1 displayed more favorable in vivo performance in terms of higher tumor uptake and much lower accumulation in kidney and liver, when compared to 18F-FP-RMSH-2 at 2 h post-injection (p.i.). 18F-FP-RMSH-1 also displayed lower liver and lung uptake when compared with the same peptide labeled with 18F-SFB (named as 18F-FB-RMSH-1). Small animal PET imaging of 18F-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 organs contrast were observed for A375M model than that of B16F10 model. 18F-FP-RMSH-1 also exhibited higher tumor uptake and better tumor retention when compared with 18F-FB-RMSH-1.
18F-FP-RMSH-1 demonstrates significant advantages over 18F-FB-RMSH-1 and 18F-FP-RMSH-2. It is a promising PET probe for imaging MC1R positive melanoma and MC1R expression in vivo.
PMCID: PMC3046310  PMID: 21073170
α-MSH; MC1R; PET; Malignant Melanoma; Metallopeptide, 18F
15.  Development of a new thiol site-specific prosthetic group and its conjugation with [cys40]-exendin-4 for in vivo targeting of insulinomas 
Bioconjugate chemistry  2013;24(7):1191-1200.
A new tracer, N-5-[18F]fluoropentylmaleimide ([18F]FPenM), for site-specific labeling of free thiol group in proteins and peptides was developed. The tracer was synthesized in three steps (18F displacement of the aliphatic tosylate, di-Boc removal by TFA to expose free amine and incorporation of the free amine into a maleimide). The radiosynthesis was completed in 110 min with 11–17% radiochemical yield (uncorrected), and specific activity of 20–49 GBq/µmol. [18F]FPenM showed comparable labeling efficiency with N-[2-(4-[18F]fluorobenzamido)ethyl]maleimide ([18F]FBEM). Its application was demonstrated by conjugation with glucagon-like peptide type 1 (GLP-1) analogue [cys40]-exendin-4. The cell uptake, binding affinity, imaging properties, biodistribution and metabolic stability of the radiolabeled [18F]FPenM-[cys40]-exendin-4 were studied using INS-1 tumor cells and INS-1 xenograft model. Positron emission tomography (PET) results showed that the new thiol-specific tracer, [18F]FPenM-[cys40]-exendin-4, had high tumor uptake (20.32 ± 4.36 %ID/g at 60 min post-injection) and rapid liver and kidney clearance, which was comparable to the imaging results with [18F]FBEM-[cys40]-exendin-4 reported by our group.
PMCID: PMC3779517  PMID: 23750453
16.  A Thiol-Reactive 18F-Labeling Agent, N-[2-(4-18F-Fluorobenzamido)Ethyl]Maleimide, and Synthesis of RGD Peptide-Based Tracer for PET Imaging of αvβ3 Integrin Expression 
The cell adhesion molecule integrin αvβ3 plays a key role in tumor angiogenesis and metastasis. A series of 18F-labeled RGD peptides have been developed for PET of integrin expression based on primary amine-reactive prosthetic groups. In this study we introduced a new method of labeling RGD peptides through a thiol-reactive synthon, N-[2-(4-18F-fluorobenzamido)ethyl]maleimide (18F-FBEM).
18F-FBEM was synthesized by coupling N-succinimidyl 4-18F-fluorobenzoate (18F-SFB) with N-(2-aminoethyl)maleimide. After high-pressure liquid chromatography purification, it was allowed to react with thiolated RGD peptides, and the resulting tracers were subjected to receptor-binding assay, in vivo metabolic stability assessment, biodistribution, and microPET studies in murine xenograft models.
Conjugation of monomeric and dimeric sulfhydryl-RGD peptides with 18F-FBEM was achieved in high yields (85% ± 5% nondecay-corrected on the basis of 18F-FBEM). The radiochemical purity of the 18F-labeled peptides was >98% and the specific activity was 100~ 150 TBq/mmol. Noninvasive microPET and direct tissue sampling experiments demonstrated that both 18F-FBEM-SRGD (RGD monomer) and 18F-FBEM-SRGD2 (RGD dimer) had integrin-specific tumor uptake in subcutaneous U87MG glioma and orthotopic MDA-MB-435 breast cancer xenografts.
The new tracer 18F-FBEM-SRGD2 was synthesized with high specific activity via 18F-FBEM and the tracer exhibited high receptor-binding affinity, tumor-targeting efficacy, metabolic stability, as well as favorable in vivo pharmacokinetics. The new synthon 18F-FBEM developed in this study will also be useful for radiolabeling of other thiolated biomolecules.
PMCID: PMC1704081  PMID: 16818952
thiol-reactive synthon; 18F-FBEM; microPET; 18F labeling; integrin αvβ3
17.  Opioid and Melanocortin Receptors: Do They Have Opioid Overlapping Pharmacophores? 
Biopolymers  2008;90(3):433-438.
We have identified compound 1 as a novel ligand for opioid and melanocortin (MC) receptors, which is derived from the overlapping of a well known structure for the δ opioid receptor, 2,6-dimethyltyrosine (Dmt)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic), and a small molecule for the MC receptor, Tic-DPhe(p-Cl)-piperidin-4-yl-N-phenyl-propionamide. Ligand 1 showed that there is an overlapping pharmacophore between opioid and MC receptors through the Tic residue. The ligand displayed high biological activities at the δ opioid receptor (Ki = 0.38 nM in binding assay, EC50 = 0.48 nM in GTP-γ-S binding assay, IC50 = 74 nM in MVD) as an agonist instead of an antagonist and showed selective binding affinity (IC50 = 2.3 μM) at the MC-3 receptor rather than at the MC-5 receptor. A study of the structure-activity relationships demonstrated that the residues in positions 2, 3, and the C-terminus act as a pharmacophore for the MC receptors, and the residues in positions 1 and 2 act as a pharmacophore for the opioid receptors. Thus, this structural construct can be used to prepare chimeric structures with adjacent or overlapping pharmacophores for opioid and MC receptors.
PMCID: PMC2693099  PMID: 17657709
opioid receptor; melanocortin receptor; anti-opioid effect; multi-target drug; overlapping pharmacophores; antinociception; side effect; Dmt-Tic; fentanyl
18.  Efficient radiosynthesis of 3′-deoxy-3′-[18F]fluorothymidine using electrowetting-on-dielectric digital microfluidic chip 
Access to diverse PET tracers for preclinical and clinical research remains a major obstacle to research in cancer and other diseases research. The prohibitive cost and limited availability of tracers could be alleviated by microfluidic radiosynthesis technologies combined with high-yield microscale radiosynthetic method. In this report, we demonstrate the multistep synthesis of 3′-deoxy-3′-[18F]fluorothymidine ([18F]FLT) with high yield on an electrowetting on dielectric (EWOD) microfluidic radiosynthesizer, previously developed in our group. We have identified and established several parameters that are most critical in the microscale radiosynthesis such as the reaction time, reagent concentration, and molar ratios, to successfully synthesize [18F]FLT in this compact platform.
[18F]FLT was synthesized from the 3-N-Boc-1-[5-O-(4,4′-dimethoxytrityl)-3-O-nosyl-2-deoxy-β-d-lyxofuranosyl] thymine precursor on the EWOD chip starting from the first solvent exchange and [18F]fluoride ion activation step to the final deprotection step. The fluorination reaction was performed in a mixture of thexyl alcohol and DMSO. The crude product after deprotection was collected from the chip and purified on a custom-made solid phase extraction (SPE) cartridge and subjected to quality control testing. The purified [18F]FLT was suitable for microPET studies in multiple nude mice xenografted with the A431 carcinoma cell line.
[18F]FLT was successfully synthesized on the EWOD microdevice coupled with an off-chip SPE purification with a decayed-corrected radiochemical yield of 63±5% (n=5) and passed all of the quality control test required by the United States Pharmacopeia for radiotracers to be injected into humans. We have successfully demonstrated the synthesis of several batches of [18F]FLT on EWOD starting with ∼ 333 MBq of radioactivity and obtained up to 52 MBq (non-decay corrected) of [18F]FLT upon cartridge purification. The specific activity of two representative preparations of [18F]FLT synthesized on the EWOD chip were measured to be 1800 and 2400 GBq/μmol.
The EWOD microchip and optimized synthesis method in combination represent an effective platform for synthesizing [18F]FLT with high yield and of good quality for imaging. This compact platform, with configurable synthesis steps, could potentially form the basis of a stand-alone system that decouples PET probe production from the cyclotron and specialized radiochemistry facilities and increases diversity and flexibility in probe production.
PMCID: PMC4494735  PMID: 24365651
[18F]FLT; microfluidic chip; radiosynthesis; high specific activity
19.  PET of Malignant Melanoma Using 18F-Labeled Metallopeptides 
Melanocortin type 1 receptor (MC1R), also known as α-melanocyte–stimulating hormone (α-MSH) receptor, is an attractive molecular target for melanoma imaging and therapy. An 18F-labeled linear α-MSH peptide (18F-FB-Ac-Nle-Asp-His-D-Phe-Arg-Trp-Gly-Lys-NH2 [NAPamide]) shows promising melanoma imaging properties but with only moderate tumor uptake and retention. A transition metal rhenium-cyclized α-MSH peptide, ReO[Cys3,4,10,D-Phe7,Arg11] α-MSH3–13 (ReCCMSH(Arg11)), has shown high in vitro binding affinity to MC1R and excellent in vivo melanoma-targeting pro-files when labeled with radiometals. Therefore, we hypothesized that ReCCMSH(Arg11) could be a good platform for the further development of an 18F-labeled probe for PET of MC1R-positive malignant melanoma.
In this study, the metallopeptide Ac-D-Lys-ReCCMSH(Arg11) was synthesized using conventional solid-phase peptide synthesis chemistry and a rhenium cyclization reaction. The resulting peptides were then labeled with N-succinimidyl-4-18F-fluorobenzoate (18F-SFB). The 18F-labeled metallopeptides were further tested for their in vitro receptor binding affinities, in vivo biodistribution, and PET imaging properties.
Both isomers of Ac-D-Lys-ReCCMSH(Arg11), named as RMSH-1 and RMSH-2, were purified and identified by high-performance liquid chromatography. The binding affinities of RMSH-1 and RMSH-2 and their respective 19F-SFB–conjugated peptides (19F-FB-RMSH-1 and 19F-FB-RMSH-2) were all determined to be within nanomolar range. Both 18F-labeled metallopeptides showed good tumor uptake in the B16F10 murine model, with high MC1R expression, but much lower uptake in the A375M human melanoma xenografted in mice, indicating low MC1R expression. 18F-FB-RMSH-1, when compared with 18F-FB-RMSH-2, displayed more favorable in vivo performance in terms of slightly higher tumor uptakes and much lower accumulations in the kidney and liver at 2 h after injection. Small-animal PET of 18F-FB-RMSH-1 and -2 in mice bearing B16F10 tumors at 1 and 2 h showed good tumor imaging quality. As expected, much lower tumor uptakes and poorer tumor–to–normal organ contrasts were observed for the A375M model than for the B16F10 model. 18F-FB-RMSH-1 and -2 showed higher tumor uptake and better tumor retention than did 18F-FB-NAPamide.
Specific in vivo targeting of 18F-FB-RMSH-1 to malignant melanoma was successfully achieved in preclinical models with high MC1R expression. Thus, the radiofluorinated metallopeptide 18F-FB-RMSH-1 is a promising molecular probe for PET of MC1R-positive tumors.
PMCID: PMC4136965  PMID: 19837749
malignant melanoma; α-MSH; PET; imaging; 18F
20.  Design of novel neurokinin 1 receptor antagonists based on conformationally constrained aromatic amino acids and discovery of a potent chimeric opioid agonist-neurokinin 1 receptor antagonist 
Journal of medicinal chemistry  2011;54(7):2467-2476.
A screening of conformationally constrained aromatic amino acids as base cores for the preparation of new NK1 receptor antagonists resulted in the discovery of three new NK1 receptor antagonists, 19 [Ac-Aba-Gly-NH-3′,5′-(CF3)2-Bn], 20 [Ac-Aba-Gly-NMe-3′,5′-(CF3)2-Bn] and 23 [Ac-Tic-NMe-3′,5′-(CF3)2-Bn], which were able to counteract the agonist effect of substance P, the endogenous ligand of NK1R. The most active NK1 antagonist of the series, 20 [Ac-Aba-Gly-NMe-3′,5′-(CF3)2-Bn], was then used in the design of a novel, potent chimeric opioid agonist-NK1 receptor antagonist, 35 [Dmt-D-Arg-Aba-Gly-NMe-3′,5′-(CF3)2-Bn], which combines the N-terminus of the established Dmt1-DALDA agonist opioid pharmacophore (H-Dmt-D-Arg-Phe-Lys-NH2) and 20, the NK1R ligand. The opioid component of the chimeric compound 35, i.e. Dmt-D-Arg-Aba-Gly-NH2 36, also proved to be an extremely potent and balanced μ- and δ opioid receptor agonist with subnanomolar binding and in vitro functional activity.
PMCID: PMC3096782  PMID: 21413804
NK1 receptor antagonists; opioids; multitarget drug design; designed multiple ligands
21.  A novel facile method of labeling octreotide with 18F-fluorine 
Several methods have been developed to label peptides with fluorine-18. However, in general these are laborious and require a multistep synthesis. We present a facile method based on the chelation of [18F]aluminum fluoride (“Al18F”) by NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid). The method is characterized by labeling NOTA-octreotide (IMP466) with 18F.
Octreotide was conjugated with the NOTA chelate and was labeled with 18F in a two-step, one-pot method. The labeling procedure was optimized with regard to the labeling buffer, peptide, and aluminum concentration. Radiochemical yield, specific activity, in vitro stability, and receptor affinity were determined. Biodistribution of 18F-IMP466 was studied in AR42J tumor-bearing mice and compared to that of 68Ga-labeled IMP466. In addition, microPET/CT images were acquired.
IMP466 was labeled with “Al18F” in a single step with 50% yield. The labeled product was purified by HPLC to remove unbound “Al18F” and unlabeled peptide. The radiolabeling, including purification, was performed in 45 min. The specific activity was 45,000 GBq/mmol and the peptide was stable in serum for 4 h at 37° C. Labeling was performed at pH 4.1 in sodium citrate, sodium acetate, HEPES and MES buffer and was optimal in sodium acetate buffer. The apparent IC50 of the 19F-labeled IMP466 determined on AR42J cells was 3.6 nM. Biodistribution studies at 2 h p.i. showed a high tumor uptake of 18F-IMP466 (28.3 ± 5.2 %ID/g, tumor-to-blood ratio: 300 ± 90), which could be blocked by an excess of unlabeled peptide (8.6 ± 0.7%ID/g), indicating that the accumulation in the tumor was receptor-mediated. Biodistribution of 68Ga-IMP466 was similar to that of 18F-IMP466. 18F-IMP466 was stable in vivo, since bone uptake was only 0.4 ± 0.2 %ID/g, whereas free “Al18F” accumulated rapidly in the bone (36.9 ± 5.0 %ID/g at 2 h p.i.). MicroPET/CT scans showed excellent tumor delineation and high preferential accumulation in the tumor.
NOTA-octreotide could be labeled rapidly and efficiently with 18F using a two-step, one-pot, method. The compound was stable in vivo and showed rapid accretion in SSTR2-receptor expressing AR42J tumors in nude mice. This method can be used to label other NOTA-conjugated compounds with 18F.
PMCID: PMC2908260  PMID: 20150268
octreotide; radiofluorination; NOTA; peptide; PET; aluminum fluoride
22.  18F-Fluorobenzoate–Labeled Cystine Knot Peptides for PET Imaging of Integrin αvβ6 
Integrin αvβ6 is a cell surface receptor minimally expressed by healthy tissue but elevated in lung, colon, skin, ovarian, cervical, and pancreatic cancers. A molecular PET agent for integrin αvβ6 could provide significant clinical utility by facilitating both cancer staging and treatment monitoring to more rapidly identify an effective therapeutic approach.
Here, we evaluated 2 cystine knot peptides, R01 and S02, previously engineered with a 3–6 nM affinity for integrin αvβ6, for 18F radiolabeling and PET imaging of BxPC3 pancreatic adenocarcinoma xenografts in mice. Cystine knot peptides were labeled with N-succinimidyl-4-18F-fluorobenzoate and evaluated for binding affinity and serum stability. Peptides conjugated with 18F-fluorobenzoate (2–3 MBq) were injected via the tail vein into nude mice xenografted with BxPC3 (integrin αvβ6–positive) or 293 (integrin αvβ6–negative) tumors. Small-animal PET scans were acquired at 0.5, 1, and 2 h after injection. Ex vivo γ-counting of dissected tissues was performed at 0.5 and 2 h.
18F-fluorobenzoate peptides were produced in 93% (18F-fluorobenzoate-R01) and 99% (18F-fluorobenzoate-S02) purity. 18F-fluorobenzoate-R01 and 18F-fluorobenzoate-S02 had affinities of 1.1 ± 0.2 and 0.7 ± 0.4 nM, respectively, and were 87% and 94%, respectively, stable in human serum at 37°C for 2 h. 18F-fluorobenzoate-R01 and 18F-fluorobenzoate-S02 exhibited 2.3 ± 0.6 and 1.3 ± 0.4 percentage injected dose per gram (%ID/g), respectively, in BxPC3 xenografted tumors at 0.5 h (n = 4–5). Target specificity was confirmed by low tumor uptake in integrin αvβ6–negative 293 tumors (1.4 ± 0.6 and 0.5 ± 0.2 %ID/g, respectively, for 18F-fluorobenzoate-R01 and 18F-fluorobenzoate-S02; both P < 0.05; n = 3–4) and low muscle uptake (3.1 ± 1.0 and 2.7 ± 0.4 tumor to muscle for 18F-fluorobenzoate-R01 and 18F-fluorobenzoate-S02, respectively). Small-animal PET data were corroborated by ex vivo γ-counting of dissected tissues, which demonstrated low uptake in nontarget tissues with only modest kidney uptake (9.2 ± 3.3 and 1.9 ± 1.2 %ID/g, respectively, at 2 h for 18F-fluorobenzoate-R01 and 18F-fluorobenzoate-S02; n = 8). Uptake in healthy pancreas was low (0.3% ± 0.1% for 18F-fluorobenzoate-R01 and 0.03% ± 0.01% for 18F-fluorobenzoate-S02; n = 8).
These cystine knot peptide tracers, in particular 18F-fluorobenzoate-R01, show translational promise for molecular imaging of integrin αvβ6 overexpression in pancreatic and other cancers.
PMCID: PMC4130341  PMID: 23670900
integrin αvβ6; positron emission tomography; cystine knot
23.  In vitro and in vivo pharmacological profile of UFP-512, a novel selective δ-opioid receptor agonist; correlations between desensitization and tolerance 
British Journal of Pharmacology  2007;152(8):1312-1324.
Background and purpose:
δ-Opioid receptors (DOP receptors) could represent a novel target in the treatment of depressive disorders. To explore this new field of interest, the development of highly selective DOP receptor agonists is essential. UFP-512 [H-Dmt-Tic-NH-CH(CH2-COOH)-Bid], was recently shown to behave in vitro as a selective and potent DOP receptor agonist and to promote antidepressant- and anxiolytic-like effects in vivo (Vergura et al., 2007). Here, we have characterized the pharmacological properties of UFP-512 and established a link between desensitization and tolerance.
Experimental approach:
Studies were performed in the human neuroblastoma SK-N-BE cells to establish i) binding parameters for UFP-512 ii) signalling pathways activated after acute and chronic treatment iii) regulation (phosphorylation and trafficking) of human DOP (hDOP) receptors after sustained activation by UFP-512. In vivo, we studied UFP-512-induced antidepressant-like effects after acute or chronic treatment in the mouse forced swimming test.
Key results:
In vitro, UFP-512 was a high affinity agonist for DOP receptors. While UFP-512 induced marked phosphorylation of DOP receptors on Ser363, we observed a low desensitization of the cAMP pathway, associated with receptor endocytosis and recycling without any reduction on extracellular signal-regulated protein kinase 1/2 activation. In vivo, acute administration of UFP-512 produced an antidepressant-like effect, without any sign of tolerance after chronic administration.
Conclusions and implications:
There was a correlation between weak desensitization, significant internalization and recycling of the human DOP receptors and lack of tolerance to UFP-512. This suggests that this compound would be a promising drug prototype for exploring innovative treatments for mood disorders.
PMCID: PMC2189997  PMID: 17982482
UFP-512; morphine; human δ-opioid receptor (hDOP receptor); phosphorylation; desensitization; endocytosis; trafficking; antidepressant-like effect; opioid tolerance
24.  Radiosynthesis of N-(4-chloro-3-[11C]methoxyphenyl)-2-picolinamide ([11C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4) 
Bioorganic & medicinal chemistry  2013;21(19):10.1016/j.bmc.2013.07.046.
N-(Chloro-3-methoxyphenyl)-2-picolinamide (3, ML128, VU0361737) is an mGlu4 positive allosteric modulator (PAM), which is potent and centrally penetrating. 3 is also the first mGlu4 PAM to show efficacy in a preclinical Parkinson disease model upon systemic dosing. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers a possibility to investigate mGlu4 expression in vivo under physiologic and pathological conditions. We synthesized a carbon-11 labeled ML128 ([11C]3) as a PET radiotracer for mGlu4, and characterized its biological properties in Sprague Dawley rats. [11C]3 was synthesized from N-(4-chloro-3-hydroxyphenyl)-2-picolinamide (2) using [11C]CH3I. Total synthesis time was 38±2.2 min (n = 7) from the end of bombardment to the formulation. The radioligand [11C]3 was obtained in 27.7±5.3% (n = 5) decay corrected radiochemical yield based on the radioactivity of [11C]CO2. The radiochemical purity of [11C]3 was >99%. Specific activity was 188.7±88.8 GBq/μmol (n = 4) at the end of synthesis (EOS).
PET images were conducted in 20 normal male Sprague Dawley rats including 11 control studies, 6 studies blocking with an mGlu4 modulator (4) to investigate specificity and 3 studies blocking with an mGlu5 modulator (MTEP) to investigate selectivity. These studies showed fast accumulation of [11C]3 (peak activity between 1-3 min) in several brain areas including striatum, thalamus, hippocampus, cerebellum, and olfactory bulb following with fast washout. Blocking studies with the mGlu4 modulator 4 showed 22-28 % decrease of [11C]3 accumulation while studies of selectivity showed only minor decrease supporting good selectivity over mGlu5. Biodistribution studies and blood analyses support fast metabolism. Altogether this is the first PET imaging ligand for mGlu4, in which the labeled ML128 was used for imaging its in vivo distribution and pharmacokinetics in brain.
PMCID: PMC3811911  PMID: 23978356
[11C]ML128; PET; Metabotropic glutamate receptor subtype 4 (mGlu4); Positive allosteric modulator
25.  18F-Labeled Galacto and PEGylated RGD Dimers for PET Imaging of αvβ3 Integrin Expression 
In vivo imaging of αvβ3 has important diagnostic and therapeutic applications. 18F-Galacto-arginine–glycine–aspartic acid (RGD) has been developed for positron emission tomography (PET) imaging of integrin αvβ3 expression and is now being tested on humans. Dimerization and multimerization of cyclic RGD peptides have been reported to improve the integrin αvβ3-binding affinity due to the polyvalency effect. Here, we compared a number of new dimeric RGD peptide tracers with the clinically used 18F-galacto-RGD.
RGD monomers and dimers were coupled with galacto or PEG3 linkers, and labeled with 18F using 4-nitrophenyl 2-18F-fluoropropionate (18F-NFP) or N-succinimidyl 4-18F-fluorobenzoate as a prosthetic group. The newly developed tracers were evaluated by cell-based receptor-binding assay, biodistribution, and small-animal PET studies in a subcutaneous U87MG glioblastoma xenograft model.
Starting with 18F-F−, the total reaction time for 18F-FP-SRGD2 and 18F-FP-PRGD2 is about 120 min. The decay-corrected radiochemical yields for 18F-FP-SRGD2 and 18F-FP-PRGD2 are 52±9% and 80±7% calculated from 18F-NFP. Noninvasive small-animal PET and direct tissue sampling experiments demonstrated that the dimeric RGD peptides had significantly higher tumor uptake as compared to 18F-galacto-RGD.
Dimeric RGD peptide tracers with relatively high tumor integrin-specific accumulation and favorable in vivo kinetics may have the potential to be translated into clinic for integrin αvβ3 imaging.
PMCID: PMC2999579  PMID: 19949981
RGD dimer; Integrin αvβ3; Small-animal PET; Polyvalency

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