Synthesis,Biological Activity,and NMR‐Based Structural Studies of Deltorphin I Analogs Modified in Message Domain with a New α,α‐Disubstituted Glycines

This article describes new deltorphin I analogs in which phenylalanine residues were replaced by the corresponding (R) or (S)‐α‐benzyl‐β‐azidoalanine, α‐benzyl‐β‐(1‐pyrrolidinyl)alanine, α‐benzyl‐β‐(1‐piperidinyl)alanine, and α‐benzyl‐β‐(4‐morpholinyl)‐alanine residues. The potency and selectivity of the new analogs were evaluated by a competitive receptor binding assay in the rat brain using [³H]DAMGO (a μ ligand) and [³H]DELT (a δ ligand). The affinity of analogs containing (R) or (S)‐α‐benzyl‐β‐azidoalanine in position 3 to δ‐receptors strongly depended on the chirality of the α,α‐disubstituted residue. The conformational behavior of peptides modified with (R) or (S)‐α‐benzyl‐β‐(1‐piperidinyl)Ala, which displays the opposite selectivity, was analyzed by ¹H and ¹³C NMR. The μ‐selective Tyr‐d ‐Ala‐(R)‐α‐benzyl‐β‐(1‐piperidinyl)Ala‐Asp‐Val‐Val‐Gly‐NH₂ lacks the helical conformation observed in the δ‐selective Tyr‐d ‐Ala‐(S)‐α‐benzyl‐β‐(1‐piperidinyl)Ala‐Asp‐Val‐Val‐Gly‐NH₂. Our results support the proposal that differences between δ‐ and μ‐selective opioid peptides are attributable to the presence or absence of a spatial overlap between the N‐terminal message domain and the C‐terminal address domain.     

Synthesis of a non‐peptidic PET tracer designed for α5β1 integrin receptor

Arginine–glycine–aspartic acid (RGD)‐containing peptides have been traditionally used as PET probes to noninvasively image angiogenesis, but recently, small selective molecules for α₅β₁ integrin receptor have been developed with promising results. Sixty‐one antagonists were screened, and tert‐butyl (S)‐3‐(2‐((3R,5S)‐1‐(3‐(1‐(2‐fluoroethyl)‐1H‐1,2,3‐triazol‐4‐yl)propanoyl)‐5‐((pyridin‐2‐ylamino)methyl)pyrrolidin‐3‐yloxy)acetamido)‐2‐(2,4,6‐trimethylbenzamido)propanoate (FPMt) was selected for the development of a PET tracer to image the expression of α₅β₁ integrin receptors. An alkynyl precursor (PMt) was initially synthesized in six steps, and its radiolabeling was performed according to the azide–alkyne copper(II)‐catalyzed Huisgen's cycloaddition by using 1‐azido‐2‐[¹⁸F]fluoroethane ([¹⁸F]12). Different reaction conditions between PMt and [¹⁸F]12 were investigated, but all of them afforded [¹⁸F]FPMt in 15 min with similar radiochemical yields (80–83%, decay corrected). Overall, the final radiopharmaceutical ([¹⁸F]FPMt) was obtained after a synthesis time of 60–70 min in 42–44% decay‐corrected radiochemical yield. Copyright © 2014 John Wiley & Sons, Ltd.     

Conformational properties of hybrid peptides containing α‐ and ω‐amino acids*

Abstract: This review briefly surveys the conformational properties of guest ω‐amino acid residues when incorporated into host α‐peptide sequences. The results presented focus primarily on the use of β‐ and γ‐residues in αω sequences. The insertion of additional methylene groups into peptide backbones enhances the range of accessible conformations, introducing additional torsional variables. A nomenclature system, which permits ready comparisons between α‐peptides and hybrid sequences, is defined. Crystal structure determination of hybrid peptides, which adopt helical and β‐hairpin conformations permits the characterization of backbone conformational parameters for β‐ and γ‐residues inserted into regular α‐polypeptide structures. Substituted β‐ and γ‐residues are more limited in the range of accessible conformation than their unsubstituted counterparts. The achiral β,β‐disubstituted γ‐amino acid, gabapentin, is an example of a stereochemically constrained residue in which the torsion angles about the C^β–C^γ (θ₁) and C^α–C^β (θ₂) bonds are restricted to the gauche conformation. Hybrid sequences permit the design of novel hydrogen bonded rings in peptide structures.     

18F labeled RGD‐A7R peptide for dual integrin and VEGF‐targeted tumor imaging in mice bearing U87MG tumors

The aim of this study is to develop a novel Arg‐Gly‐Asp acid (RGD) and Ala‐Thr‐Trp‐Leu‐Pro‐Pro‐Arg (ATWLPPR A7R) peptide‐containing ligand for ¹⁸F labeling as αvβ3 and vascular endothelial growth factor receptor‐targeted imaging agent. ¹⁸F‐RGD‐A7R was prepared by conjugation with ¹⁸F‐SFB. The final product was purified by high‐performance liquid chromatography and tested in vitro and in vivo. Cell‐binding assays of RGD‐A7R, RGD and RGD‐A7R, A7R were tested in U87MG cells (¹²⁵I‐RGDyK and ¹²⁵I‐A7RY as radioligand, respectively). Preliminary biodistribution of the ¹⁸F‐RGD‐A7R was also evaluated. The RGD‐A7R had good integrin binding affinity (50% inhibitory concentration (IC₅₀) = 21.67 and 23.68 nM, slightly lower than unmodified RGD (40.02 nM) and A7R (50.18 nM)). The radiotracer had receptor‐mediated activity accumulation in U87MG tumor (1.90 ± 0.34 percentage of injected dose per gram (%ID/g) at 0.5 h postinjection), which is known to be integrin positive. After blocking with RGD‐A7R, the tumor uptake was reduced to 0.47 ± 0.06 %ID/g at 0.5 h postinjection. ¹⁸F‐RGD‐A7R exhibited dual receptor targeting properties both in vitro and in vivo. The favorable characterizations of ¹⁸F‐RGD‐A7RY, such as convenient synthesis, high specific activity, and high tumor uptake, warrant its further investigation for clinical cancer imaging.     

Evaluation of a 188Re‐radiolabeled Arg‐Gly‐Asp peptide for tumor overexpressed αvβ3 receptors

To develop a peptide‐based radiopharmaceutical for the therapy of α_vβ₃ receptors overexpressed tumors, we have prepared a novel Arg‐Gly‐Asp (RGD) peptide (HCRGDCF(D)CRGDC, P12) radiolabeled with ¹⁸⁸Re. With His acid at the end of the peptide containing RGD, the label efficiency was more than 95% within 30 min. The peptide binds to human glioblastoma U87MG cells with high affinity [IC₅₀ = 86.3 nm]. The stability of ¹⁸⁸Re‐P12 in vitro was also investigated. More than 80% of radioactivity was kept in the peptide after 4 h incubation in phosphate buffer solution (pH = 7.4) or calf serum under physiological conditions. Biodistribution of this radiocompound was carried out in mice bearing S180 tumor. Fast clearance of ¹⁸⁸Re‐peptide from blood and specific uptakes by tumors realized higher tumor‐to‐blood ratio (1.80) 4 h post‐injection. Obvious difference was observed between the blocking and unblocking experiments in whole body autoradiography imaging. These results have demonstrated the potential of ¹⁸⁸Re‐labeling RGD as a radiotherapeutic agent.     

Synthesis,crystal structure and molecular conformation of Nα‐Boc‐l‐leucyl‐(Z)‐β‐(3‐pyridyl)‐α,β‐ dehydroalanyl‐l‐leucine methyl ester*

Abstract: A protected tridehydropeptide containing (Z)‐β‐(3‐pyridyl)‐α,β‐dehydroalanine (Δ^Z3Pal) residue, Boc‐Leu‐Δ^Z3Pal‐Leu‐OMe ( 1 ), was synthesized via Erlenmeyer azlactone method. X‐ray crystallographic analysis revealed that the peptide 1 adopts an extended conformation, which is similar to that of a Δ^ZPhe analog, Boc‐Leu‐Δ^ZPhe‐Leu‐OMe ( 2 ).     

Cerenkov luminescence imaging of αvβ6 integrin expressing tumors using 90Y‐labeled peptides

Cerenkov luminescence imaging (CLI) is an emerging preclinical molecular imaging modality that tracks the radiation emitted in the visible spectrum by fast moving charged decay products of radionuclides. The aim of this study was in vitro and in vivo evaluation of the two radiotracers, ⁹⁰Y‐DOTA‐PEG₂₈‐A20FMDV2 (⁹⁰Y‐1) and ⁹⁰Y‐DOTA‐Ahx‐A20FMDV2 (⁹⁰Y‐2) (>99% radiochemical purity, 3.7 GBq/µmol specific activity) for noninvasive assessment of tumors expressing the integrin α_vβ₆ and their future use in tumor targeted radiotherapy. Cell binding and internalization in α_vβ₆‐positive cells was ⁹⁰Y‐1: 10.1 ± 0.8%, 50.3 ± 2.1%; ⁹⁰Y‐2: 22.4 ± 1.7%, 44.7 ± 1.5% with <5% binding to α_vβ₆‐negative control cells. Biodistribution studies showed maximum α_vβ₆‐positive tumor uptake of the radiotracers at 1‐h post injection (p.i.) (⁹⁰Y‐1: 0.64 ± 0.15% ID/g; ⁹⁰Y‐2: 0.34 ± 0.11% ID/g) with high renal uptake (>25% ID/g at 24 h). Because of the lower tumor uptake and high radioactivity accumulation in kidneys (that could not be reduced by pre‐administration of either lysine or furosemide), the luminescence signal from the α_vβ₆‐positive tumor was not clearly detectable in CLI images. The studies suggest that CLI is useful for indicating major organ uptake for both radiotracers; however, it reaches its limitation when there is low signal‐to‐noise ratio.     

Agonist function of the recombinant α4β3δ GABAA receptor is dependent on the human and rat variants of the α4‐subunit

1. It is known that the α₄‐subunit is likely to occur in the brain predominantly in α₄β₃δ receptors at extrasynaptic sites. Recent studies have revealed that the α₁‐, α₄‐, γ₂‐ and δ‐subunits may colocalize extrasynaptically in dentate granule cells of the hippocampus. In the present study, we characterized a series of recombinant GABA_A receptors containing human (H) and rat (R) α₁/α₄‐, β₂/β₃‐ and γ_2S/δ‐subunits in Xenopus oocytes using the two‐electrode voltage‐clamp technique. 2. Both Hα₁β₃δ and Hα₄β₃γ_2S receptors were sensitive to activation by GABA and pentobarbital. Contrary to earlier findings that the α₄β₃δ combination was more sensitive to agonist action than the α₄β₃γ_2S receptor, we observed extremely small GABA‐ and pentobarbital‐activated currents at the wild‐type Hα₄β₃δ receptor. However, GABA and pentobarbital activated the wild‐type Rα₄β₃δ receptor with high potency (EC₅₀ = 0.5 ± 0.7 and 294 ± 5 μmol/L, respectively). 3. Substituting the Hα₄ subunit with Rα₄ conferred a significant increase in activation on the GABA and pentobarbital site in terms of reduced EC₅₀ and increased I_max. When the Hα₄ subunit was combined with the Rβ₃ and Rδ subunit in a heteropentameric form, the amplitude of GABA‐ and pentobarbital‐activated currents increased significantly compared with the wild‐type Hα₄β₃δ receptor. 4. Thus, the results indicate that the Rα₄β₃δ, Hα₁β₃δ and Hα₄β₃γ_2S combinations may contribute to functions of extrasynaptic GABA_A receptors. The presence of the Rα₄ subunit at recombinant GABA_A receptors containing the δ‐subunit is a strong determinant of agonist action. The recombinant Hα₄β₃δ receptor is a less sensitive subunit composition in terms of agonist activation.     

Conformational investigation of α, β-dehydropeptides

The crystal structure of Ac-Pro-ΔVal-NHCH₃ was examined to determine the influence of the α,β-dehydrovaline residue on the nature of peptide conformation. The peptide crystallizes from methanol-diethyl ether solution at 4° in needle-shaped form in orthorhombic space group P2₁2₁2₁ with a= 11.384(2) Å, b = 13.277(2) Å, c = 9.942(1) Å. V = 1502.7(4) ų Z = 4, D_m= 1.17 g cm^?3 and D_c=1.18 g cm^?3 The structure was solved by direct methods using SHELXS-86 and refined to an R value of 0.057 for 1922 observed reflections. The peptide is found to adopt a β-bend between the type I and the type III conformation with φ₁=?68.3(4)°, ψ₁=? 20.1(4)°, φ₂=?73.5(4)°= and Ψ₂=?14.1(4)°=. An intramolecular hydrogen bond between the carbonyl oxygen of ith residue and the NH of (i+ 3)th residue stabilizes the β-bend. An additional intermolecular N.,.O hydrogen bond joins molecules into infinite chains. In the literature described crystal structures of peptides having a single α,β-dehydroamino acid residue in the (i+ 2) position and forming a β-bend reveal a type II conformation.     

Synthesis of 68Ga‐labeled NOTA‐RGD‐GE11 heterodimeric peptide for dual integrin and epidermal growth factor receptor‐targeted tumor imaging

Radiolabeled Arg‐Gly‐Asp (RGD) peptide analogs have been extensively studied for α_vβ₃ integrin‐targeted angiogenesis imaging. According to recently presented evidence, the dodecapeptide GE11 has high affinity to the epidermal growth factor receptor (EGFR), which is overexpressed in many types of cancer. Dual‐receptor molecular imaging probes with two different heterodimeric peptides exhibit improved cancer targeting efficacy. In the present study, the design and synthesis of a new RGD‐GE11 peptide heterodimer for dual α_vβ₃ integrin/EGFR‐targeted cancer imaging are described. The RGD‐GE11 heterodimer was linked with 6‐aminohexanoic acid (6‐Ahx) and cysteine and conjugated with 1,4,7‐triazacyclononane‐N,N′,N″‐triacetic acid (NOTA) to form NOTA‐RGD‐cys‐6‐Ahx‐GE11. The monomeric peptides, NOTA‐cys‐6‐Ahx‐GE11 and c(RGDyK), were formed by a peptide synthesizer. The peptide heterodimer NOTA‐RGD‐GE11 was obtained by NOTA‐cys‐6‐Ahx‐GE11 and maleimidopropyl‐c(RGDyK) conjugation with a thioether linkage. The NOTA peptide conjugate was labeled with freshly eluted ⁶⁸Ga and purified using reversed‐phase high‐performance liquid chromatography. The ⁶⁸Ga‐NOTA‐RGD‐cys‐6‐Ahx‐GE11 was successfully prepared, in this study, with a radiochemical yield of 85% and a radiochemical purity of >98%. These results warrant further investigation of this heterodimeric peptide's binding affinity to the receptors.     

Synthesis of [18F]3‐[1‐(3‐fluoropropyl)‐(S)‐pyrrolidin‐2‐ylmethoxy]pyridine ([18F]NicFP): a potential α4β2 nicotinic acetylcholine receptor radioligand for PET

Nicotinic acetylcholine receptors are widely distributed throughout the human brain and are believed to play a role in several neurological and psychiatric disorders. In order to identify an effective PET radioligand for in vivo assessment of the α4β2 subtype of nicotinic receptor, we synthesized [¹⁸F]3‐[1‐(3‐fluoropropyl)‐(S)‐pyrrolidin‐2‐ylmethoxy]pyridine (NicFP). The in vitro K_D of NicFP was determined to be 1.1 nM, and the log P value obtained by HPLC analysis of the unlabelled standard was found to be 2.2. The radiosynthesis of [¹⁸F]NicFP was carried out by a nucleophilic substitution reaction of anhydrous [¹⁸F]fluoride and the corresponding mesylate precursor. After purification by HPLC, the radiochemical yield was determined to be 11.3±2.1% and the specific activity was 0.47±0.18 Ci/μmol (EOS, n = 3). The time of synthesis and purification was 99±2 min. The final product was prepared as a sterile saline solution suitable for in vivo use. Copyright © 2003 John Wiley & Sons, Ltd.     

A chimeric opioid peptide with mixed μ agonist/δ antagonist properties

Abstract: There is evidence to indicate that opioid compounds with mixed μ agonist/δ antagonist properties are analgesics with low propensity to produce tolerance and physical dependence. A chimeric peptide containing the potent and selective μ agonist H‐Dmt‐D‐Arg‐Phe‐Lys‐NH₂ ([Dmt¹]DALDA) (Dmt = 2′,6′‐dimethyltyrosine) and the potent and selective δ antagonist H‐Tyr‐TicΨ[CH₂‐NH]Cha‐Phe‐OH (TICP[Ψ]) (Cha = cyclohexylalanine), connected ‘tail‐to‐tail’ via a short linker, was synthesized using a combination of solid‐phase and solution techniques. The resulting peptide, H‐Dmt→D‐Arg→Phe→Lys‐NH‐CH₂‐CH₂‐NH‐Phe←Cha[NH‐CH₂]ΨTic←Tyr‐H, showed the expected μ agonist/δ antagonist profile in the guinea‐pig ileum and mouse vas deferens assays. Its μ and δ receptor binding affinities were in the low nanomolar range, as determined in rat brain membrane binding assays.     

Preparation and preliminary biological evaluation of 177Lu‐labeled GluDTPA‐cyclo(RGDfK) for integrin ανβ3 receptor‐positive tumor targeting

Integrin α_νβ₃ is a receptor and is highly expressed on activated and proliferating endothelial cells during the growth and metastasis of solid tumors but not on resting endothelial cells and normal organs. Because RGD peptide binds to integrin α_νβ₃ receptor, a variety of radiolabeled RGD peptides have been evaluated for non‐invasive imaging of integrin α_νβ₃‐positive tumors. In an attempt to develop RGD‐based radiopharmaceuticals, a novel GluDTPA‐cyclo arginine‐glycine‐aspartic acid‐d ‐phenylalanine‐lysine (GluDTPA‐cycloRGDfK) was simply synthesized and radiolabeled with ¹⁷⁷Lu. Also, tumor targeting and retention of the radiolabeled complex were evaluated in U87MG glioma‐bearing mice. The ¹⁷⁷Lu‐labeled GluDTPA‐cyclo(RGDfK) was formulated with a high radiolabeling yield (>98%) under mild condition, and the radiochemical purity was sustained in both saline and serum for over 4 days at 37°C. The radiolabeled compounds were rapidly cleared from the blood pool and non‐target tissue. Tumor‐to‐blood ratio was 12.09 at 2 h post injection and increased to 134.67 at 24 h, while tumor to liver ratio was 2.01 at 24 h similar to that of 2 h. Though it is inappropriate for targeted therapy due to its low uptake in tumor (~ 1 %ID/g), the acceptable results on radiochemistry and biodistribution propose to take a further assessment for non‐invasive imaging and detection of integrin α_νβ₃‐positive tumors by applying diagnostic radionuclides. Copyright © 2011 John Wiley & Sons, Ltd.     

New tools for the control of peptide conformation: the helicogenic Cα‐methyl,Cα‐cyclohexylglycine*

Abstract: The novel C^α‐tetrasubstituted α‐amino acid C^α‐methyl, C^α‐cyclohexylglycine was prepared by hydrogenation of its C^α‐methyl, C^α‐phenylglycine precursor. Terminally protected homodi‐, homotri‐, and homotetrapeptides from C^α‐methyl, C^α‐cyclohexylglycine and co‐oligopeptides to the pentamer level in combination with Gly or α‐aminoisobutyric acid residues were prepared by solution methods and fully characterized. The results of a conformational analysis, performed by use of Fourier transform infrared (FT‐IR) spectrophotomet absorption, ¹H NMR, and X‐ray diffraction techniques, support the contention that this C^α‐methylated, C^β‐trisubstituted aliphatic α‐amino acid is an effective β‐turn and 3₁₀‐helix inducer in tri‐ and longer peptides as its C^α‐methyl valine parent compound, but partially divergent from the corresponding aromatic C^α‐methyl, C^α‐diphenylmethylglycine residue, known to promote folded and fully extended structures to a significant extent in these oligomers.     

111In‐ and 203Pb‐labeled cyclic arginine‐glycine‐aspartic acid peptide conjugate as an αvβ3 integrin‐binding radiotracer

Methodology for site‐specific modification and chelate conjugation of a cyclic arginine‐glycine‐aspartic acid (cRGD) peptide for the preparation of a radiotracer molecular imaging agent suitable for detecting α_vβ₃ integrin is described. The method involves functionalizing the peptide with an aldehyde moiety and conjugation to a 1,4,7,10‐tetraazacyclododecane‐N,N′,N″,N?‐tetraacetic acid derivative that possesses an aldehyde reactive aminooxy group. The binding assay of the ¹¹¹In‐labeled peptide conjugate with α_vβ₃ integrin showed 60% bound when four equivalents of the integrin was added, a reasonable binding affinity for a monovalent modified RGD peptide.     

Protective effects of β‐sheet breaker α/β‐hybrid peptide against amyloid β‐induced neuronal apoptosis in vitro

Alzheimer's disease is most common neurodegenerative disorder and is characterized by increased production of soluble amyloid‐β oligomers, the main toxic species predominantly formed from aggregation of monomeric amyloid‐β (Aβ). Increased production of Aβ invokes a cascade of oxidative damages to neurons and eventually leads to neuronal death. This study was aimed to investigate the neuroprotective effects of a β‐sheet breaker α/β‐hybrid peptide (BSBHp) and the underlying mechanisms against Aβ₄₀‐induced neurotoxicity in human neuroblastoma SH‐SY5Y cells. Cells were pretreated with the peptide Aβ₄₀ to induce neurotoxicity. Assays for cell viability, cell membrane damage, cellular apoptosis, generation of reactive oxygen species (ROS), intracellular free Ca²⁺, and key apoptotic protein levels were performed in vitro. Our results showed that pretreatment with BSBHp significantly attenuates Aβ₄₀‐induced toxicity by retaining cell viability, suppressing generation of ROS, Ca²⁺ levels, and effectively protects neuronal apoptosis by suppressing pro‐apoptotic protein Bax and up‐regulating antiapoptotic protein Bcl‐2. These results suggest that α/β‐hybrid peptide has neuroprotective effects against Aβ₄₀‐induced oxidative stress, which might be a potential therapeutic agent for treating or preventing neurodegenerative diseases.     

Radiosynthesis of a 125I analog of a highly selective alpha3beta4 nicotinic acetylcholine receptor antagonist ligand for use in autoradiography studies

The α3β4 subtype of the nicotinic acetylcholine receptors (nAChR) is present in limited but specific areas of the brain unlike the widely distributed α4β2 nAChR subtype, known to be involved in the addictive effects of nicotine. Recently, the α3β4 nAChR subtype has been linked to addiction to nicotine as well as other drugs of abuse. However, there have been no subtype‐selective α3β4 nAChR ligands available to study the role of this receptor in drug addiction. Our laboratory has discovered a series of very high affinity and highly selective ligands for the α3β4 nAChR subtype. We now report the synthesis of a radiolabeled ¹²⁵I analog of N‐(2‐iodophenyl)‐9‐methyl‐9‐azabicyclo[3.3.1]nonan‐3‐amine (AT‐1012), a subnanomolar affinity, highly selective α3β4 nAChR ligand from this series. This analog, [¹²⁵I] AT‐1012, was synthesized by a facile radio‐iodination of a tributylstannylated precursor, which gave the radiolabeled compound with high specific activity and radiochemical purity. This high‐affinity radioactive α3β4 nAChR antagonist is very useful as a pharmacological tool in autoradiography studies, to elucidate the localization of the α3β4 nAChR in the brain and study its pharmacology in the brain reward circuit. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of an 18F‐labelled high affinity β1‐adrenoceptor PET radioligand based on ICI 89,406

To date, some non‐selective β‐adrenoceptor (β‐AR) positron emission tomography (PET) radioligands are in clinical use, but no PET radioligand for the selective imaging of cardiac β₁‐ARs is clinically available. Therefore, the aim of this study was to develop a potential high‐affinity PET radioligand for the β₁‐subtype of ARs. Here, the synthesis and in vitro evaluation of (S)‐ and (R)‐N‐[2‐[3‐(2‐cyano‐phenoxy)‐2‐hydroxy‐propylamino]‐ethyl]‐N′‐[4‐(2‐fluoro‐ethoxy)‐phenyl]‐urea ( 8a–b ), derivatives of the well‐characterized β₁‐AR selective antagonist, ICI 89,406, are described. The (S)‐isomer 8a shows both higher β₁‐AR selectivity and β₁‐AR affinity than the (R)‐enantiomer 8b (selectivity: 40 800 vs 1580; affinity: K_I1=0.049 nM vs K_I1=0.297 nM). Therefore, the ¹⁸F‐labelled analogue 8e of compound 8a was synthesized. While the direct nucleophilic ¹⁸F‐fluorination of the tosylate precursor 8d produced 8e in low radiochemical yields (?2.9% decay‐corrected) and specific activities (?3.5 GBq/µmol at the end of synthesis (EOS), n=9) the alternative two‐step synthesis of 8e from ethylene glycol di‐p‐tosylate 9 , [¹⁸F]fluoride ion and phenol precursor 8f gave satisfying results (16.4±3.2% radiochemical yield (decay‐corrected), 99.7±0.5% radiochemical purity, 40±8 GBq/µmol specific activity at the EOS within 174±3 min from the end of bombardment (EOB) (n=5)). Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis,biology, NMR and conformation studies of the topographically constrained δ‐opioid selective peptide analogs of [β‐iPrPhe3]deltorphin I

Abstract: Replacement of Phe³ in the endogenous δ‐opioid selective peptide deltorphin I with four optically pure stereoisomers of the topographically constrained, highly hydrophobic novel amino acid β‐isopropylphenylalanine (β‐iPrPhe) produced four pharmacologically different deltorphin I peptidomimetics. Radiolabeled ligand‐binding assays and in vitro biological evaluation indicate that the stereoconfiguration of the iPrPhe residue plays a crucial role in determining the binding affinity, bioactivity and selectivity of [β‐iPrPhe³]deltorphin I analogs: a (2S,3R) configuration of the iPrPhe³ residue in [β‐iPrPhe³]deltorphin I provided the most desirable biological properties with binding affinity (IC₅₀ = 2 n m ), bioassay potency (IC₅₀ = 1.23 n m in MVD assay) and exceptional selectivity for the δ‐opioid receptor over the µ‐opioid receptor (30 000). Further conformational studies based on two‐dimensional NMR and computer‐assisted molecular modeling suggested a model for the possible bioactive conformation in which the Tyr¹ and (2S,3R)‐β‐iPrPhe³ residues adopt trans side‐chain conformations, and the linear peptide backbone favors a distorted β‐turn conformation.     

Synthesis of [3α‐3H] 17α‐Hydroxy pregnenolone and [3α‐3H] Pregnenolone

For the first time, [3α‐³H] 17α‐hydroxy pregnenolone (1) was synthesized through a multiple step sequence. The presence of [3β‐³H] isomer in RP‐HPLC purified product was identified by tritium NMR. The [3β‐³H] isomer was then separated from [3α‐³H] 17α‐hydroxy pregnenolone with chiralPAK AD‐H column. [3α‐³H] pregnenolone (2) was synthesized from commercial available 5‐pregnen‐3,20‐dione in one step with an improved procedure.