TARGETING THE NS5A PROTEIN OF HCV: AN EMERGING OPTION

Hepatitis C virus (HCV) infects more than 3% of the world's population, leading to an increased risk of cirrhosis and hepatocellular carcinoma. The current standard of care, a combination of pegylated interferon alfa and ribavirin, is poorly tolerated and often ineffective against the most prevalent genotype of the virus, genotype 1. The very recent approval of boceprevir and telaprevir, two HCV protease inhibitors, promises to significantly improve treatment options and outcomes. In addition to the viral protease NS3 and the viral polymerase NS5B, direct-acting antivirals are now in development against NS5A. A multifunctional phosphoprotein, NS5A is essential to HCV genome replication, but has no known enzymatic function. Here we report how the design of small-molecule inhibitors against NS5A has evolved from promising monomers to highly potent dimeric compounds effective against many HCV genotypes. We also highlight recent clinical data and how the inhibitors may bind to NS5A, itself capable of forming dimers.

Reduced peptide bond cyclic somatostatin based opioid octapeptides Synthesis, conformational properties and pharmacological characterization

The conformational and pharmacological properties that result from peptide bond reduction as well as the use of secondary amino acids in a series of cyclic peptides related to the mu opioid receptor selective antagonist D-Phe1-Cys2-Tyr3-D-Trp4-Orn5-Thr6-Pen7+ ++-Thr8-NH2 (IV), have been investigated. Peptide analogues that contain [CH2NH] and [CH2N] pseudo-peptide bonds (in primary and secondary amino acids, respectively) were synthesized on a solid support. Substitution of Tyr3 in IV by the cyclic, secondary amino acid 1,2,3,4-tetrahydroisoquinoline carboxylate (Tic) and of D-Trp4 with D-1,2,3,4-tetrahydro-beta-carboline(D-Tca4), gave peptides 4 and 1, respectively. Both analogues displayed reduced affinities for mu opioid receptors. Conformational analysis based on extensive NMR investigations demonstrated that the backbone conformations of 1 and 4 are similar to those of the potent and selective analogue D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (I), while the conformational properties of the side chains of Tic3 (4) and D-Tca4 (1) resulted in topographical properties that were not well recognized by the mu opioid receptor. Peptide bond modifications were made including (Tyr3-psi[CH2NH]-D-Trp4), 3; (Tyr3-psi[CH2N]-D-Tca4), 2; and (Cys2-psi[CH2N]-Tic3), 6. These analogues showed decreases in their mu opioid receptor affinities relative to the parent compounds IV, 1, and 4, respectively. 1H NMR based conformational analysis in conjunction with receptor binding data led to the conclusion that the reduced peptide bonds in 2, 3, 5, and 6 do not contribute to the process of discrimination between mu and delta opioid receptors, and in spite of their different dynamic behaviors (relative to 1 and 4), they are still capable of attaining similar receptor bound conformations, possibly due to their increased flexibility.

Novel inhibitors of HIV protease: design, synthesis and biological evaluation of picomolar inhibitors containing cyclic P1/P2 scaffolds

A novel series of HIV protease inhibitors containing cyclic P1/P2 scaffolds has been synthesized and evaluated for biological activity. The trans 3,5-dibenzyl-2-oxo pyrrolidinone ring system resulted in a 50 pM enzyme inhibitor against HIV protease in vitro when combined with an indanolamine derived P'-backbone. This compound also shows comparable activity to currently marketed drugs in the MT-4 cell-based antiviral assay.

Synthesis of Dipeptides with Ψ[CH(2)0] Amide Bond Mimetics

This chapter describes the synthetic procedures leading to ether dipeptide isosteres of the Phe-ψ[CH(2)O]-spiro-C(x) (9a-9e, Fig. 1) and Phe-ψ[CH(2)O]-Allylglycine (13, Fig. 2). Development of methods which lead to mimetics of amide bonds is central to the conversion of peptide leads into pharmaceutically viable molecules. Our strategy utilizes template 1, which provides derivatives 6b-6e upon alkylation with I, Cl-alkanes (C(3)-C(6)). Subsequent Finklestein conversion to iodides 7b-7e is then followed by cyclization to spirocyclic derivatives 5b-5e (Fig. 3). Figure 1. Synthesis of Phe ψ[CH(2)O]Spiro isosteres 9a-9e. Figure 2. Synthesis of Phe ψ[CH(2)O]Allylglycine 13 Figure 3. Synthesis of morpholinones 5b-5e.

N-(2-Benzoylphenyl)-L-tyrosine PPARgamma agonists. 2. Structure-activity relationship and optimization of the phenyl alkyl ether moiety

We previously reported the identification of (2S)-((2-benzoylphenyl)amino)-3-¿4-[2-(5-methyl-2-phenyloxazol-4-y l)e thoxy]phenyl¿propanoic acid (2) (PPARgamma pKi = 8.94, PPARgamma pEC50 = 9.47) as a potent and selective PPARgamma agonist. We now report the expanded structure-activity relationship around the phenyl alkyl ether moiety by pursuing both a classical medicinal chemistry approach and a solid-phase chemistry approach for analogue synthesis. The solution-phase strategy focused on evaluating the effects of oxazole and phenyl ring replacements of the 2-(5-methyl-2-phenyloxazol-4-yl)ethyl side chain of 2 with several replacements providing potent and selective PPARgamma agonists with improved aqueous solubility. Specifically, replacement of the phenyl ring of the phenyloxazole moiety with a 4-pyridyl group to give 2(S)-((2-benzoylphenyl)amino)-3-¿4-[2-(5-methyl-2-pyridin-4-yloxazol+ ++- 4-yl)ethoxy]phenyl¿propionic acid (16) (PPARgamma pKi = 8.85, PPARgamma pEC50 = 8.74) or a 4-methylpiperazine to give 2(S)-((2-benzoylphenyl)amino)-3-(4-¿2-[5-methyl-2-(4-methylpiperazin+ ++- 1-yl)thiazol-4-yl]ethoxy¿phenyl)propionic acid (24) (PPARgamma pKi = 8.66, PPARgamma pEC50 = 8.89) provided two potent and selective PPARgamma agonists with increased solubility in pH 7.4 phosphate buffer and simulated gastric fluid as compared to 2. The second strategy took advantage of the speed and ease of parallel solid-phase analogue synthesis to generate a more diverse set of phenyl alkyl ethers which led to the identification of a number of novel, high-affinity PPARgamma ligands (PPARgamma pKi's 6.98-8.03). The combined structure-activity data derived from the two strategies provide valuable insight on the requirements for PPARgamma binding, functional activity, selectivity, and aqueous solubility.

Iron chelates bind nitric oxide and decrease mortality in an experimental model of septic shock

The hydroxamic acid siderophore ferrioxamine B [FeIII(HDFB)+] and the iron complex of diethylenetri-aminepentaacetic acid [FeIII(DTPA)2-] protected mice against death by septic shock induced by Corynebacterium parvum + lipopolysaccharide. Although FeIII(DTPA)2- was somewhat more effective than FeIII(HDFB)+, the iron-free ligand H4DFB+ was significantly more effective than DTPA. The hydroxamic acid chelator has a much higher iron affinity than the amine carboxylate, allowing for more efficient formation of the FeIII(HDFB)+ complex upon administration of the iron-free ligand. Electrochemical studies show that FeIII(DTPA)2- binds NO stoichiometrically upon reduction to iron(II) at biologically relevant potentials to form a stable NO adduct. In contrast, FeIII(HDFB)+ is a stable and efficient electrocatalyst for the reduction of NO to N2O at biologically relevant potentials. These results suggest that the mechanism of protection against death by septic shock involves NO scavenging and that particularly effective drugs that operate a low dosages may be designed based on the principle of redox catalysis. These complexes constitute a new family of drugs that rely on the special ability of transition metals to activate small molecules. In addition, the wealth of information available on siderophore chemistry and biology provides an intellectual platform for further development.

Inhibitors of human immunodeficiency virus type 1 derived from gp41 transmembrane protein: structure--activity studies

We synthesized analogues of gp41 (553-590), 1, and evaluated them for their inhibitory activity against HIV-1 in MT4 cell assay (IC50(1) = 2.7 microM). (The numbering scheme for gp41 (e.g., gp41(553-590) for 1) adapted throughout the text is from ref 6.) Gradual truncation of either the N- or C-terminal end of gp41 (553-590) resulted in a substantial loss of inhibitory properties of resulting compounds. Unexpectedly, simultaneous truncations of both N- and C-termini of gp41(553-590) resulted in a potent heptadecamer, 13, IC50 = 10.4 microM. Coupling of a racemic alpha-aminotetradecanoic acid (Atd) to gp41 fragments afforded diastereomeric conjugates, most of which were chromatographically separable. In this series, pentadecamer 27 had an IC50 of 8.9 microM, while its Atd diastereomer 28 was much less inhibitory. This finding is consistent with relative inhibitory potencies of other Atd-containing diastereomeric pairs and could reflect a chiral sense of Atd residue interacting with the receptor. Compounds 13 and 27, which are practically equipotent to 1, represent minimalistic fragments of the leucine-zipper region of gp41 and constitute a basis for design of a second generation of gp41-based inhibitors. Circular dichroism studies suggested that compounds in this series are likely to inhibit HIV-1 replication by virtue of their alpha-helical character. The observed structure-activity relationship supports impairment of viral gp41 as a possible mechanism of action of 1.

A topographical model of mu-opioid and brain somatostatin receptor selective ligands. NMR and molecular dynamics studies

We have refined the 1H NMR-based conformations of the mu-opioid receptor selective peptides related to somatostatin of general formula Xxx-Yyy1-Cys-Zzz-D-Trp-Lys(Orn)5-Thr-Pen-Thr8- NH2, where Xxx, Yyy, Zzz are 0, D-Phe and Tyr for 1; 0, D-Tic and Tyr for 2; Gly, D-Tic and Tyr for 3; and 0, D-Phe and Tic for 4, respectively, (Kazmierski et al., J. Am. Chem. 113, 2275-2283), using a molecular-dynamics approach. We present evidence that the NMR data are compatible with beta II'-, gamma- and gamma'-turns for the central tetrapeptide Tyr-D-Trp-Lys/Orn-Thr. Based on detailed structural and topographical considerations, we suggest that the mu-opioid receptor selectivity of 2 is due to a particular spatial arrangement of aromatic side chains of D-Tic1 and Tyr3 (7.5 A), and that the opioid receptor recognition domain is located in the N-terminal part of the peptide while the somatostatin receptor recognition domain is determined by the central, turn forming part of this class of cyclic peptides. A model for a mu-opioid selective ligand has emerged from these studies that shows excellent structural similarities to rigid opioid alkaloids.

Efficient synthesis of metal binding peptides incorporating aminodiacetic acid based ligands

New N alpha-Fmoc/Bu(t) protected amino acids bearing half-EDTA side chains (CH2)nN(Ada-O-Bu(t))2 n = 1 (5), n = 2 (24), n = 3 (10), n = 4 (15) were prepared in satisfactory yields. These derivatives can be conveniently used in a solid-phase peptide synthesis as they are devoid of serious shortcomings of Boc/Bzl based syntheses of metallopeptides, such as preliminary peptide capping as well as undesired lactamization of 5 during the peptide synthesis. Munksgaard 1995.

Unexpected antinociceptive potency of cyclic [D-Tca1]CTAP: potential for a novel mechanism of action

This study tested the hypothesis that compounds which may bind simultaneously to delta and mu receptors may be more potent antinociceptive agents than would be predicted from their binding affinities at individual mu and delta opioid receptors. D-Tca-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 ([D-Tca1]CTAP) (where D-Tca is a cyclic D-tryptophan analogue) was synthesized and evaluated in radioligand competition assays, opioid bioassays, and in an antinociceptive assay (the tail-flick test in mice). Additionally, the metabolic stability of [D-Tca1]CTAP was evaluated in striatal and cerebellar tissue slices. In rat brain in vitro, [D-Tca1]CTAP competed weakly for sites labelled by [3H]D-Phe-Cys-Tyr-D-Trp-Om-Thr-Pen-Thr-NH2 ([3H]CTOP) (mu-ligand), and [3H][D-Pen2,pCl-Phe4,D-Pen5]enkephalin (delta-ligand); [D-Pen2,D-Pen5]enkephalin (DPDPE) (delta-agonist) was 6.5-fold less and 230-fold more potent, respectively, against these ligands. Additionally, in mouse isolated vas deferens and guinea pig isolated ileum smooth muscle preparations, [D-Tca1]CTAP proved to be weak as either a delta (IC50 of approximately 2 microM) or mu (IC50 > 8 microM) receptor agonist. Surprisingly, however, i.c.v. [D-Tca1]CTAP produced antinociception with potency similar to DPDPE. The antinociceptive actions of [D-Tca1]CTAP were apparently not due to a metabolite or the release of endogenous opioids, as this compound proved stable in both striatal and cerebellar tissue slices and its antinociceptive actions were not enhanced by the 'enkephalinase' inhibitor thiorphan. The suggestion that [D-Tca1]CTAP might be acting by binding simultaneously to mu and delta receptors to produce its antinociceptive effect is supported by the demonstrated antagonism resulting from mu receptor blockade with either beta-funaltrexamine (beta-FNA) or naloxonazine, or by delta receptor blockade by ICI 174,864 ([N,N-diallyl-Tyr1,Aib2,3,Leu5] enkephalin). Furthermore, the antinociceptive properties of [D-Tca1]CTAP were antagonized by (naltrindole-5'-isothiocyanate) (5'-NTII), an antagonist at the delta 2 opioid receptor subtype, but not by the delta 1 antagonist [D-Ala2,D-Leu5,Cys6]enkephalin (DALCE). Additionally, no antagonism was produced by nor-binaltorphimine (nor-BNI), a kappa antagonist. From these data, [D-Tca1]CTAP appears to bind to mu, and 5'-NTII-sensitive delta 2, opioid receptors, and may represent the first of a class of compounds which may act at an opioid receptor complex via 'self-potentiation'.

Constrained phenylalanine analogues. Preferred conformation of the 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) residue

Three Tic-containing (Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) model peptides were synthesized to assess the tendency of this constrained Phe analogue to fold into a beta-bend and a helical structure, and to adopt a preferred side-chain disposition. The results of the solution conformational analysis, performed by using Fourier transform infrared absorption and 1H nuclear magnetic resonance, indicate that in chloroform the -Aib-D-Tic-Aib-, -(Aib)2-D-Tic-(Aib)2-, and -L-Pro-D-Tic- sequences fold into intramolecularly H-bonded forms to a great extent. An X-ray diffraction analysis on p-BrBz-(Aib)2-DL-Tic-(Aib)2-OMe monohydrate and p-BrBz-L-Pro-D-Tic-NHMe allows us to conclude that, while the pentapeptide methylester forms an incipient (distorted) 3(10)-helix, the dipeptide methylamide adopts a type-II beta-bend conformation. In both cases, the D-Tic side-chain conformation is D, gauche(-). The implications for the use of the Tic residue in designing conformationally restricted analogues of bioactive peptides are briefly discussed.

A new type of synthetic peptide library for identifying ligand-binding activity

Our aim was to improve techniques for drug development by facilitating the identification of small molecules that bind with high affinity to acceptor molecules (for example, cell-surface receptors, enzymes, antibodies) and so to mimic or block their interaction with the natural ligand. Previously such small molecules have been characterized individually on a serial basis. The systematic synthesis and screening of peptide libraries of defined structure represents a new approach. For relatively small libraries, predetermined sequence variations on solid-phase supports have been used, and large libraries have been produced using a bacteriophage vector into which random oligodeoxynucleotide sequences have been introduced, but these techniques have severe limitations. Here we investigate an alternative approach to synthesis and screening of peptide libraries. Our simple methodology greatly enhances the production and rapid evaluation of random libraries of millions of peptides so that acceptor-binding ligands of high affinity can be rapidly identified and sequenced, on the basis of a 'one-bead, one-peptide' approach.