N‐n‐alkylnicotinium and N‐n‐alkylpyridinium analogs inhibit the dopamine transporter: Selectivity as nicotinic receptor antagonists

N‐n‐Alkylnicotinium and N‐n‐alkylpyridinium analogs act as antagonists at nicotinic acetylcholine receptors (nAChRs) mediating nicotine‐evoked [³H]dopamine (DA) overflow from superfused rat striatal slices in the presence of a DA transporter (DAT) inhibitor. However, the potential interaction of these nAChR antagonists with DAT has not been evaluated. In the present study, analog inhibition of [³H]DA uptake into striatal synaptosomes and inhibition of [³H]GBR 12935 binding to striatal membranes was determined. N‐n‐Alkylnicotinium analogs with n‐alkyl chains of C_6–12 and N‐n‐alkylpyridinium analogs with n‐alkyl chains of C_7–20 inhibited [³H]DA uptake with a wide affinity range. With the exception of the C₂₀ N‐n‐alkylpyridinium, a linear relationship between chain length and inhibition of [³H]DA uptake was found in both analog series. Similarly, these analogs inhibited [³H]GBR 12935 binding (K_i=5.7–250 μM), and a linear relationship with chain length was observed, with the exception of the C₈ N‐n‐alkylnicotinium analog. Kinetic analyses of inhibition of [³H]DA uptake and [³H]GBR 12935 binding using representative C₁₂ analogs from each series revealed decreases in maximal [³H]DA transport velocity and maximal [³H]GBR 12935 binding without alterations in affinity, indicating noncompetitive interactions with DAT. In comparison, classical nAChR antagonists (mecamylamine, dihydro‐β‐erythroidine and methyllycaconitine) did not inhibit [³H]DA uptake or [³H]GBR 12935 binding. Moreover, inhibition of DAT function occurred at analog concentrations 10–120‐fold higher than those inhibiting nAChR function. Taken together with the inability of these analogs to inhibit field‐stimulation‐evoked [³H]DA overflow, the results indicate that these analogs act selectively as antagonists at nAChRs mediating nicotine‐evoked [³H]DA overflow. Drug Dev. Res. 60:270–284, 2003. © 2003 Wiley‐Liss, Inc.     

Structure–activity relationships for nicotine analogs comparing competition for [3H]nicotine binding and psychotropic potency

The structure–activity relationships were established for nicotine analogs and related agents comparing their competition for [³H]nicotine binding to rat brain membranes, Torpedo membranes, and transfected insect cells with their ability to produce prostration in rats following administration into the rat lateral ventricles. A total of 59 compounds were investigated, consisting of pyridine- and pyrrolidine-substituted analogs of nicotine, other tobacco alkaloids and related molecules, various aminoalkylpyridines, and ring-shifted analogs of nicotine. Some of the compounds were also evaluated for [³H]nicotine binding to Torpedo electroplax membranes and to Sf9 cells expressing an α4β2 nicotinic cholinergic receptor subtype. Linear regression plots of Ki vs ED₅₀ values for prostration of the various classes of the compounds yielded correlation of determination (R²) of 0.923 for the pyridine-substituted analogs, 0.725 for the pyrrolidine-substituted analogs, 0.947 for other tobacco alkaloids and related compounds, and 0.537 for all 59 compounds. An excellent correlation was observed comparing Torpedo Ki values with both prostration ED₅₀ values and rat brain Ki values. Within the pyridine-substituted series, methyl substiuents in the 6-position resulted in over three-fold greater potency compared to nicotine, whereas potency decreased markedly with bulkier alkyl and cycloalkyl substituents. Within the pyrrolidine-substituted series, methyl groups in the 2′-position only slightly reduced potency compared to nicotine, whereas 3′- and 5′-addition markedly reduces potency. A linear regression plot of the Ki values of brain vs. those of Sf9 cells expressing the α4β2 nicotinic cholinergic receptor subtype yielded a coefficient of correlation of 0.981; a finding which is consistent with the notion that the α4β2 subtype comprises over 90% of total rat brain receptor. Drug Dev. Res. 45:10–16, 1998. © 1998 Wiley-Liss, Inc.     

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.     

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.     

Radiosynthesis of the α4β2 nicotinic acetylcholine receptor ligand: 5‐((1‐[11C]‐methyl‐2‐(S)‐pyrrolidinyl)methoxy)‐2‐chloro‐3‐((E)‐2‐(2‐fluoropyridin‐4‐yl)vinyl)pyridine

5‐((1‐[¹¹C]‐methyl‐2‐(S)‐pyrrolidinyl)methoxy)‐2‐chloro‐3‐((E)‐2‐(2‐fluoropyridin‐4‐yl)‐vinyl)pyridine ([¹¹C]‐FPVC) was synthesized from [¹¹C]‐methyl iodide and the corresponding normethyl precursor. The average time of synthesis, purification, and formulation was 42 min with an average non‐decay‐corrected radiochemical yield of 19%. The average specific radioactivity was 359 GBq/µmol (9691 mCi/µmole) at end of synthesis (EOS). Copyright © 2006 John Wiley & Sons, Ltd.     

Specific α7 nicotinic acetylcholine receptor agonist ameliorates isoproterenol‐induced cardiac remodelling in mice through TGF‐β1/Smad3 pathway

It is well‐accepted that inflammation plays an important role in the development of cardiac remodelling and that therapeutic approaches targeting inflammation can inhibit cardiac remodelling. Although a large amount of evidence indicates that activation of α7 nicotinic acetylcholine receptor (α7nAChR) causes an anti‐inflammatory effect, the role of α7nAChR in cardiac remodelling and the underlying mechanism have not been established. To investigate the effect of the specific α7nAChR agonist, PNU282987, on cardiac remodelling induced by isoproterenol (ISO 60 mg/kg per day) in mice, the cardiomyocyte cross‐sectional area (CSA) and collagen volume fraction were evaluated by hematoxylin and eosin (HE) and Masson staining, respectively. Cardiac function and ventricular wall thickness were measured by echocardiography. The protein expressions of collagen I, matrix metalloproteinase 9 (MMP‐9), transforming growth factor β1 (TGF‐β1), and Smad3 were analyzed by Western blot. ISO‐induced cardiac hypertrophy, characterized by an increase in the heart weight/body weight ratio, CSA and ventricular wall thickness. Moreover, cardiac fibrosis indices, such as collagen volume fraction, MMP‐9 and collagen I protein expression, were also increased by ISO. PNU282987 not only attenuated cardiac hypertrophy but also decreased the cardiac fibrosis induced by ISO. Furthermore, PNU282987 suppressed TGF‐β1 protein expression and the phosphorylation of Smad3 induced by ISO. In conclusion, PNU282987 ameliorated the cardiac remodelling induced by ISO, which may be related to the TGF‐β1/Smad3 pathway. These data imply that the α7nAChR may represent a novel therapeutic target for cardiac remodelling in many cardiovascular diseases.     

Introduction of unsaturation into theN-n-alkyl chain of the nicotinic receptor antagonists, NONI and NDNI: Effect on affinity and selectivity

N-n-octylnicotinium iodide (NONI) and N-n-decylnicotinium iodide (NDNI) are selective nicotinic receptor (nAChR) antagonists mediating nicotine-evoked striatal dopamine (DA) release, and inhibiting [3H]nicotine binding, respectively. This study evaluated effects of introducing unsaturation into the N-n-alkyl chains of NONI and NDNI on inhibition of [3H]nicotine and [3H]methyllycaconitine binding (alpha4beta2* and alpha7* nAChRs, respectively), (86)Rb+ efflux and [3H]DA release (agonist or antagonist effects at alpha4beta2* and alpha6beta2*-containing nAChRs, respectively). In the NONI series, introduction of a C3-cis- (NONB3c), C3-trans- (NONB3t), C7-double-bond (NONB7e), or C3-triple-bond (NONB3y) afforded a 4-fold to 250-fold increased affinity for [3H]nicotine binding sites compared with NONI. NONB7e and NONB3y inhibited nicotine-evoked 86Rb+ efflux, indicating alpha4beta2* antagonism. NONI analogs exhibited a 3-fold to 8-fold greater potency inhibiting nicotine-evoked [3H]DA overflow compared with NONI (IC50 = 0.62 microM; Imax = 89%), with no change in Imax, except for NONB3y (Imax = 50%). In the NDNI series, introduction of a C4-cis- (NDNB4c), C4-trans-double-bond (NDNB4t), or C3-triple-bond (NDNB3y) afforded a 4-fold to 80-fold decreased affinity for [3H]nicotine binding sites compared with NDNI, whereas introduction of a C9 double-bond (NDNB9e) did not alter affinity. NDNB3y and NDNB4t inhibited nicotine-evoked 86Rb+ efflux, indicating antagonism at alpha4beta2* nAChRs. Although NDNI had no effect, NDNB4t and NDNB9e potently inhibited nicotine-evoked [3H]DA overflow (IC50 = 0.02-0.14 microM, Imax = 90%), as did NDNB4c (IC50 = 0.08 microM; Imax = 50%), whereas NDNB3y showed no inhibition. None of the analogs had significant affinity for alpha7* nAChRs. Thus, unsaturated NONI analogs had enhanced affinity at alpha4beta2*- and alpha6beta2*-containing nAChRs, however a general reduction of affinity at alpha4beta2* and an uncovering of antagonist effects at alpha6beta2*-containing nAChRs were observed with unsaturated NDNI analogs.     

Alpha9 nicotinic acetylcholine receptors and the treatment of pain

Chronic pain is a vexing worldwide problem that causes substantial disability and consumes significant medical resources. Although there are numerous analgesic medications, these work through a small set of molecular mechanisms. Even when these medications are used in combination, substantial amounts of pain often remain. It is therefore highly desirable to develop treatments that work through distinct mechanisms of action. While agonists of nicotinic acetylcholine receptors (nAChRs) have been intensively studied, new data suggest a role for selective antagonists of nAChRs. α-Conotoxins are small peptides used offensively by carnivorous marine snails known as Conus. A subset of these peptides known as α-conotoxins RgIA and Vc1.1 produces both acute and long lasting analgesia. In addition, these peptides appear to accelerate the recovery of function after nerve injury, possibly through immune mediated mechanisms. Pharmacological analysis indicates that RgIA and Vc1.1 are selective antagonists of α9α10 nAChRs. A recent study also reported that these α9α10 antagonists are also potent GABA-B agonists. In the current study, we were unable to detect RgIA or Vc1.1 binding to or action on cloned GABA-B receptors expressed in HEK cells or Xenopus oocytes. We review the background, findings and implications of use of compounds that act on α9* nAChRs.¹     

Selective α7 nicotinic receptor activation by AZD0328 enhances cortical dopamine release and improves learning and attentional processes

AZD0328, a novel spirofuropyridine neuronal nicotinic receptor partial agonist, was used to investigate the role of α7 neuronal nicotinic receptor (NNR) activation in the modulation of midbrain dopamine neuron function, cortical dopamine release and on two behavioral tasks known to be dependent on optimal levels of cortical dopamine. In vivo recordings from area 10 (ventral tegmental area) in rat brain showed an increased firing of putative dopamine neurons in response to low (0.00138 mg/kg) doses of AZD0328. Bursting patterns of dopamine neuron activity remained largely unchanged by application of AZD0328. In vivo microdialysis in awake rats showed an increase in extracellular prefrontal cortical dopamine in response to low doses of AZD0328. Compound-stimulated dopamine release showed an inverted dose effect relation that was maximal at the lowest dose tested (0.00178 mg/kg). Peak extracellular dopamine levels were reached 2 h after dosing with AZD0328. Acquisition of operant responding with delayed reinforcement in rats was dose dependently enhanced by AZD0328 with a plateau effect measured at 0.003 mg/kg. This effect was blocked by pre-treatment of animals with the selective α7 antagonist methyllycaconitine. AZD0328 improved novel object recognition in mice over a broad range of doses (0.00178–1.78 mg/kg) and the compound effect was found to be absent in homozygous α7 KO animals. Together, these data indicate that selective interaction with α7 NNRs by AZD0328 selectively enhances midbrain dopaminergic neuronal activity causing an enhancement of cortical dopamine levels; these neurochemical changes likely, underlie the positive behavioral responses observed in two different animal models. Our results suggest selective α7 NNR agonists may have significant therapeutic utility in neurologic and psychiatric indications where cognitive deficits and dopamine neuron dysfunction co-exist.     

Pharmacokinetics of cytisine,an α4β2 nicotinic receptor partial agonist,in healthy smokers following a single dose

Cytisine, an α₄β₂ nicotinic receptor partial agonist, is a plant alkaloid that is commercially extracted for use as a smoking cessation medication. Despite its long history of use, there is very little understanding of the pharmacokinetics of cytisine. To date, no previous studies have reported cytisine concentrations in humans following its use as a smoking cessation agent. A high performance liquid chromatography‐ultraviolet (HPLC‐UV) method was developed and validated for analysis of Tabex® and nicotine‐free oral strips, two commercial products containing cytisine. A sensitive liquid chromatography‐mass spectrometry (LC‐MS) method was developed and validated for the quantification of cytisine in human plasma and for the detection of cytisine in urine. Single‐dose pharmacokinetics of cytisine was studied in healthy smokers. Subjects received a single 3 mg oral dose administration of cytisine. Cytisine was detected in all plasma samples collected after administration, including 15 min post‐dose and at 24 h. Cytisine was renally excreted and detected as an unchanged drug. No metabolites were detected in plasma or urine collected in the study. No adverse reactions were reported. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis and evaluation of a 18F‐labeled 4‐phenylpiperidine‐4‐carbonitrile radioligand for σ1 receptor imaging

We report the design and synthesis of several 4‐phenylpiperidine‐4‐carbonitrile derivatives as σ₁ receptor ligands. In vitro radioligand competition binding assays showed that all the ligands exhibited low nanomolar affinity for σ₁ receptors (K_i(σ₁) = 1.22–2.14 nM) and extremely high subtype selectivity (K_i(σ₂) = 830–1710 nM; K_i(σ₂)/K_i(σ₁) = 680–887). [¹⁸F]9 was prepared in 42–46% isolated radiochemical yield, with a radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic ¹⁸F^‐ substitution of the corresponding tosylate precursor. Biodistribution studies in mice demonstrated high initial brain uptakes and high brain‐to‐blood ratios. Administration of SA4503 or haloperidol 5 min prior to injection of [¹⁸F]9 significantly reduced the accumulation of radiotracers in organs known to contain σ₁ receptors. Two radioactive metabolites were observed in the brain at 30 min after radiotracer injection. [¹⁸F]9 may serve as a lead compound to develop suitable radiotracers for σ₁ receptor imaging with positron emission tomography.     

alpha7 nicotinic acetylcholine receptor agonist properties of tilorone and related tricyclic analogues

Background and purpose:

The α7 nicotinic acetylcholine receptor (nAChR) has attracted considerable interest as a target for cognitive enhancement in schizophrenia and Alzheimer''s Disease. However, most recently described α7 agonists are derived from the quinuclidine structural class. Alternatively, the present study identifies tilorone as a novel α7-selective agonist and characterizes analogues developed from this lead.

Experimental approach:

Activity and selectivity were determined from rat brain α7 and α4β2 nAChR binding, recombinant nAChR activation, and native α7 nAChR mediated stimulation of ERK1/2 phosphorylation in PC12 cells.

Key results:

Tilorone bound α7 nAChR (IC₅₀ 110 nM) with high selectivity relative to α4β2 (IC₅₀ 70 000 nM), activated human α7 nAChR with an EC₅₀ value of 2.5 μM and maximal response of 67% relative to acetylcholine, and showed little agonist effect at human α3β4 or α4β2 nAChRs. However, the rat α7 nAChR maximal response was only 34%. Lead optimization led to 2-(5-methyl-hexahydro-pyrrolo[3,4-c]pyrrol-2-yl)-xanthen-9-one (A-844606) with improved binding (α7 IC₅₀ 11 nM, α4β2 IC₅₀>30 000 nM) and activity at both human and rat α7 nAChR (EC₅₀s 1.4 and 2.2 μM and apparent efficacies 61 and 63%, respectively). These compounds also activated native α7 nAChR, stimulating ERK1/2 phosphorylation in PC12 cells.

Conclusions and implications:

Tilorone, known as an interferon inducer, is a selective α7 nAChR agonist, suggesting utility of the fluorenone pharmacophore for the development of α7 nAChR selective agonists. Whether α7 stimulation mediates interferon induction, or whether interferon induction may influence the potential anti-inflammatory properties of α7 nAChR agonists remains to be elucidated.     

Identification of small molecule estrogen‐related receptor α–specific antagonists and homology modeling to predict the molecular determinants as the basis for selectivity over ERRβ and ERRγ

The estrogen‐related receptor α (ERRα) was one of the first orphan receptors identified through a search for genes encoding proteins related to the steroid nuclear receptor, Estrogen Receptor α (ERα). The physiological role of ERRα has not yet been established nor has a natural ligand been elucidated. Importantly, research indicates that ERRα may be a novel drug target to treat breast cancer and/or metabolic disorders. A homogeneous time‐resolved fluorescence (HTRF) assay has been developed to screen for ERRα‐specific antagonists. This assay uses the ERR ligand binding domain and the coactivator interaction domain of Proliferator‐activated Receptor γ Coactivator‐1α (PGC‐1α) to examine the ability of compounds to antagonize the constitutive interaction between ERRα and the coactivator. A dissociation‐enhanced lanthanide fluorescence immunoassay (DELFIA) was also created to counter screen compounds identified in the HTRF screen. Here we report the discovery of high‐affinity ERRα subtype selective antagonists. Additionally, a homology model of ERRα in an antagonist conformation has been developed and after subsequent docking studies, we offer a model showing the molecular determinants that suggest why our novel tri‐cyclic antagonist, N‐[(2Z)‐3‐(4,5‐dihydro‐1,3‐thiazol‐2‐yl)‐1,3‐thiazolidin‐2‐yl idene]‐5 H dibenzo[a,d][7]annulen‐5‐amine, binds to ERRα with high affinity but does not bind to either ERRβ or ERRγ. Drug Dev Res 69:203–218, 2008. © 2008 Wiley‐Liss, Inc.     

Synthesis and biologic activity of conformationally constrained analogs of L‐363,301

Abstract: We report the synthesis, biological activity and conformational analysis of analogs of the cyclic hexapeptide L‐363,301, c[Pro⁶‐Phe⁷‐d ‐Trp⁸‐Lys⁹‐Thr¹⁰‐Phe¹¹] (numbering as in the native hormone somatostatin‐14). The d ‐Trp in position 8 was replaced with (2R,3S)‐ and (2R,3R)‐β‐MeTrp respectively, with an added methyl group in the beta position of Trp. The objective of our study was to determine the potency and selectivity generated by the added constraint in the beta position of the d ‐Trp upon binding to human somatostatin receptors hsst1‐5. We synthesized the building blocks enantioselectively and incorporated them into the peptides by SPPS. Competition binding assays revealed that both compounds 2 and 3 were selective for hsst2 over hsst5. The (2R,3S) analog 2 was approximately 30 times more potent at hsst2 than the (2R,3R) analog 3 . Interestingly, the (2R,3R) compound showed no binding affinity at hsst5.     

Design and Synthesis of Nicotinic Acetylcholine Receptor Antagonists and their Effect on Cognitive Impairment

Structure modification of a lead compound (NSC13378) was accomplished in the present work by an in silico target‐based design aimed at ligands acting on the nicotinic acetylcholine receptor (nAChR) for neurodegenerative diseases. A 187‐compound focused library derived from the scaffold of the lead compound was screened against acetylcholine‐binding proteins (AChBPs). Six compounds were identified and synthesized for binding and biological evaluations. Five compounds were found to bind with AChBPs. Among these compounds, QN 1 and BZ 1 showed the highest affinity binding with AChBP, with K_d values of 260 and 10 nm , respectively. Functional assays on isolated cell lines containing ligand‐gated ion channels revealed that QN 1 and BZ 1 are α 4 β 2‐nAChR antagonists. QN 1 and BZ 1 significantly alleviated the memory impairment caused by the muscarinic cholinergic antagonist scopolamine (p < 0.05) in mice. Our findings demonstrate the potential of nAChR antagonists in drug development for cognitive impairments.     

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.     

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 ).     

Proline residue‐modified polycationic analogs of gramicidin S with high antibacterial activity against both Gram‐positive and Gram‐negative bacteria and low hemolytic activity*

Abstract: Novel polycationic analogs of the cyclic decapeptide antibiotic, gramicidin S, possessing NH₂, d /l ‐Phe‐NH or l ‐Lys‐NH groups at the 4α‐ or 4β‐positions of the l ‐Pro residues, were synthesized. While l ‐Pro(4α/β‐NH₂)‐containing analogs exhibited much weaker antibacterial activity, the d /l ‐Phe and l ‐Lys‐substituted analogs exhibited higher antibacterial activity against Gram‐negative bacteria than the parent gramicidin S. All of these additional amino group‐containing analogs showed substantially reduced toxicity against human blood cells.