Antinociceptive profile of potent opioid peptide AM94, a fluorinated analogue of biphalin with non-hydrazine linker.

AM94 is a fluorinated analog of biphalin with non-hydrazine linker that has an in vitro affinity for µ-opioid and δ-opioid receptors tenfold higher than biphalin. Furthermore, in vivo evaluation in rats showed that AM94 has in hot plate test - after both intracerebroventricular and intravenous administrations - a greater and more durable efficacy than biphalin. Here, the antinociceptive profile of AM94 is further evaluated by following two different administration routes, intrathecal and subcutaneous, and two different animal species, rats and mice. The analgesic potency of AM94 is compared with that of both the parent peptide biphalin and morphine. Results show that in rats (tail flick test) and in mice (formalin test), AM94 has a higher and more durable analgesic effect than biphalin after intrathecal and subcutaneous administrations. Conformational properties of biphalin and AM94 were also investigated by variable-temperature (1)H NMR and energy minimization.

Structure-activity relationships of biphalin analogs and their biological evaluation on opioid receptors.

Biphalin (Tyr-D-Ala-Gly-Phe-NH-NH < -Phe < -Gly < -D-Ala < -Tyr) is an opioid octapeptide with a dimeric structure based on two identical pharmacophore portions, derived from enkephalins, joined "tail to tail" by a hydrazide bridge. This particular structure enhances the antinociceptive activity of the native enkephalins with an unknown mechanism, probably based on a cooperative binding and improved enzymatic stability. Biphalin has excellent binding affinity for µ and δ receptors and it is a highly potent analgesic, as potent as or more than ethorphine. A definitive explanation of the extraordinary in vivo potency shown by this compound, which has pronounced efficacy in pain modulation, is still not available; it has been suggested, however, that the high agonist activity may be related to its binding mode at both µ and δ opioid receptors. Biphalin has significantly higher potency than other analgesics with novel biological profiles; in particular, most recent data show that biphalin is unlikely to produce dependency in chronic use. In the past 20 years, there have been many attempts to modify its structure to obtain products unaffected by the action of enkephalinases, to enhance its antinociceptive activity and to modify the BBB penetration. In addition, structure-activity relationship studies (SAR) were performed in order to understand the elements responsible for biphalin's high activity. The aim of the studies reported in this review was to clarify: i) the role of the hydrazide bridge, ii) the role of residues in position 4, 4' and 3, 3', iii) the consequences of molecular simplifications (truncation, delection), iv) the consequences of cyclization through a disulfide bridge, v) conjugation with PEG and fluorescet residues, and vi) radiolabeling on Tyr.

cis-4-amino-L-proline residue as a scaffold for the synthesis of cyclic and linear endomorphin-2 analogues: part 2.

Recently, we reported synthesis and activity of a constrained cyclic analogue of endomorphin-2 (EM-2: Tyr-Pro-Phe-Phe-NH(2)) and related linear models containing the cis-4-amino-L-proline (cAmp) in place of native Pro(2). In the present article, the adopted rationale is the possible modulation of the receptor affinity of the cAmp containing EM-2 analogues by assigning a different stereochemistry to the Phe(3) and Phe(4) residues present in the ring. Thus, eight more analogues with different absolute configuration at the chiral center of the aromatic residues in positions 3 and 4 have been synthesized and their opioid activity examined. The stereochemical change at the α-carbon atoms leads to a meaningful enhancement of the affinity and activity toward µ opioid receptors with respect to the prototype compound 9: e.g., 9a, K(i)(µ) = 63 nM, GPI (IC(50)) = 480 nM; 9b, K(i)(µ) = 38 nM, GPI (IC(50)) = 330 nM.

Synthesis and biological evaluation of new active For-Met-Leu-Phe-OMe analogues containing para-substituted Phe residues.

In the present study, we report synthesis and biological evaluation of the N-Boc-protected tripeptides 4a-l and N-For protected tripeptides 5a-l as new For-Met-Leu-Phe-OMe (fMLF-OMe) analogues. All the new ligands are characterized by the C-terminal Phe residue variously substituted at position 4 of the aromatic ring. The agonism of 5a-l and the antagonism of 4a-l (chemotaxis, superoxide anion production, lysozyme release as well as receptor binding affinity) have been examined on human neutrophils. No synthesized compounds has higher activity than the standard fMLF-OMe tripeptide to stimulate chemotaxis, although compounds 5a and 5c with -CH(3) and -C(CH(3))(3), respectively, in position 4 on the aromatic ring, are better than the standard tripeptide to stimulate the production of superoxide anion, in higher concentration. Compounds 4f and 4i, containing -F and -I in position 4, respectively, on the aromatic ring of phenylalanine, exhibit significant chemotactic antagonism. The influence of the different substitution at the position 4 on the aromatic ring of phenylalanine is discussed.

The cis-4-amino-L-proline residue as a scaffold for the synthesis of cyclic and linear endomorphin-2 analogues.

Endomorphin-2 (EM-2: Tyr-Pro-Phe-Phe-NH(2)) is an endogenous tetrapeptide that combines potency and efficacy with high affinity and selectivity toward the µ opioid receptor, the most responsible for analgesic effects in the central nervous system. The presence of the Pro(2) represents a crucial factor for the ligand structural and conformational properties. Proline is in fact an efficient stereochemical spacer, capable of inducing favorable spatial orientation of aromatic rings, a key factor for ligand recognition and interaction with receptors. Here the Pro(2) has been replaced by 4(S)-NH(2)-2(S)-proline (cAmp), a proline/GABA cis-chimera residue. This bivalent amino acid maintains the capacity to influenc the tetrapeptide conformation and offers the opportunity to generate new linear models and unusually constrained cyclic analogues characterized by an N-terminal Tyr bearing a free α-amino group. The results indicate that the new analogues do not show affinity for both δ and κ opioid receptors and bind only poorly to the µ receptors (for cyclopeptide 9: K(i)(µ) = 660 nM; GPI (IC(50)) = 1.4% at 1 µM; for linear tetrapeptide acid 13: K(i)(µ) = 2000 nM; GPI (IC(50)) = 0% at 1 µM; for linear tetrapeptide amide 15: K(i)(µ) = 310 nM; GPI (IC(50)) = 894 nM).

Conformationally constrained histidines in the design of peptidomimetics: strategies for the χ-space control.

A successful design of peptidomimetics must come to terms with χ-space control. The incorporation of χ-space constrained amino acids into bioactive peptides renders the χ(1) and χ(2) torsional angles of pharmacophore amino acids critical for activity and selectivity as with other relevant structural features of the template. This review describes histidine analogues characterized by replacement of native α and/or ß-hydrogen atoms with alkyl substituents as well as analogues with α, ß-didehydro unsaturation or C(α)-C(ß) cyclopropane insertion (ACC derivatives). Attention is also dedicated to the relevant field of ß-aminoacid chemistry by describing the synthesis of ß(2)- and ß(3)-models (ß-hHis). Structural modifications leading to cyclic imino derivatives such as spinacine, aza-histidine and analogues with shortening or elongation of the native side chain (nor-histidine and homo-histidine, respectively) are also described. Examples of the use of the described analogues to replace native histidine in bioactive peptides are also given.

New potent biphalin analogues containing p-fluoro-L-phenylalanine at the 4,4' positions and non-hydrazine linkers.

We report the synthesis and the biological evaluation of two new analogues of the potent dimeric opioid peptide biphalin. The performed modification is based on the replacement of two key structural elements of the native biphalin, namely: the hydrazine bridge which joins the two palindromic moieties and the phenylalanine residues at the 4,4' positions of the backbone. The new analogues 9 and 10 contain 1,2-phenylenediamine and piperazine, respectively, in place of the hydrazidic linker and p-fluoro-L-phenylalanine residues at 4 and 4' positions. Binding values are: Kµ(i)=0.51 nM and Kδ(i)=12.8 nM for compound 9, Kµ(i)=0.09 nM and Kδ(i)=0.11 nM for analogue 10.

Synthesis and activity of endomorphin-2 and morphiceptin analogues with proline surrogates in position 2.

The opioid agonists endomorphins (Tyr-Pro-Trp-Phe-NH(2); EM1 and Tyr-Pro-Phe-Phe-NH(2); EM2) and morphiceptin (Tyr-Pro-Phe-Pro-NH(2)) exhibit an extremely high selectivity for mu-opioid receptor. Here a series of novel EM2 and morphiceptin analogues containing in place of the proline at position 2 the S and R residues of beta-homologues of proline (HPro), of 2-pyrrolidinemethanesulphonic acid (HPrs) and of 3-pyrrolidinesulphonic acid (betaPrs) have been synthesized and their binding affinity and functional activity have been investigated. The highest micro-receptor affinity is shown by [(S)betaPrs(2)]EM2 analogue (6e) which represents the first example of a beta-sulphonamido analogue in the field of opioid peptides.

Synthesis and evaluation of new endomorphin-2 analogues containing (Z)-alpha,beta-didehydrophenylalanine (Delta(Z)Phe) residues.

New endomorphin-2 (EM-2) analogues incorporating (Z)-alpha,beta-didehydrophenylalanine (Delta(Z)Phe) in place of the native phenylalanine in EM-2 are reported. Tyr-Pro-Delta(Z)Phe-Phe-NH(2) {[Delta(Z)Phe(3)]EM-2} (1), Tyr-Pro-Phe-Delta(Z)Phe-NH(2) {[Delta(Z)Phe(4)]EM-2} (2), and Tyr-Pro-Delta(Z)Phe-Delta(Z)Phe-NH(2) {[Delta(Z)Phe(3,4)]EM-2}(3) have been synthesized, their opioid receptor binding affinities and tissue bioassay activities were determined, and their conformational properties were examined. Compound 2 shows high mu opioid receptor selectivity and mu agonist activity comparable to those of the native peptide. The conformation adopted in solution and in the crystal by N-Boc-Tyr-Pro-Delta(Z)Phe-Phe-NH(2) (8) is reported.

Investigation of amino acid containing [FeFe] hydrogenase models concerning pendant base effects.

The present investigations deal with the modeling of the peptide surrounding of [FeFe] hydrogenase using amine containing disulphides to simulate possible influences of the amino acid lysine (K237) on the electrochemical and electrocatalytic properties of biomimetic compounds based on [Fe2S2] moieties. Fe(3)(CO)(12) was reacted with Boc-4-amino-1,2-dithiolane, Boc-Adt-OMe (Adt=4-amino-1,2-dithiolane-4-carboxylic acid, Boc=tert-butoxycarbonyl) and Boc-Adp tert-butyl ester (Adp=(S)-2-amino-3-(1,2-dithiolan-4-yl)propionic acid) to elongate the Fecdots, three dots, centeredN distance in comparison to the well known [Fe(2){(SCH(2))(2)NR}(CO)(6)] model complexes. Efforts to deprotect the complexes containing Boc-4-amino-1,2-dithiolane with trifluoroacetic acid result in the formation of [Fe(3)(mu(3)-O)(mu-O(2)C(2)F(3))(6)(OC(4)H(8))(2)(H(2)O)]. The novel [2Fe2S] complexes are characterized using spectroscopic, electrochemical techniques and X-ray diffraction studies.

Synthesis and evaluation of new endomorphin analogues modified at the Pro(2) residue.

Six new endomorphin analogues, incorporating constrained amino acids in place of native proline have been synthesized. Residues of (S)-azetidine-2-carboxylic acid (Aze), 3,4-dehydro-(S)-proline (Delta(3)Pro), azetidine-3-carboxylic acid (3Aze) and dehydro-alanine (DeltaAla) have been used to prepare [Delta(3)Pro(2)]EM-2 (1), [Aze(2)]EM-1 (2), [Aze(2)]EM-2 (3), [3Aze(2)]EM-1 (4), [3Aze(2)]EM-2 (5) and [DeltaAla(2)]EM-2 (6). Binding assays and functional bioactivities for mu- and delta-receptors are reported. The highest affinity, bioactivity and selectivity are shown by peptides 2 and 3 containing the Aze residue.

Synthesis and activity of fibrillogenesis peptide inhibitors related to the 17-21 beta-amyloid sequence.

Peptide derivatives 1-5, incorporating synthetic non-proteinogenic amino acids, related to the beta-amyloid 17-21 fragment of the amyloidogenic Abeta(1-40), and the N-protected decapeptide 6, corresponding to a dimeric sequence of the same fragment, have been synthesized. These compounds were designed by using Soto's pentapeptide Ac-LPFFD-NH(2) (iAbeta5p) as lead compound. Their activity as inhibitors of fibrillogenesis and stability against enzymatic degradation have been determined. Compounds 1, 5 and 6 are potent inhibitors in comparison to the lead compound. Exposure to chymotrypsin of peptide derivatives 1-5, all containing unnatural amino acids, shows increased stability as compared with iAbeta5p and 6. Conformational properties of the new compounds have been determined by CD and FT-IR spectroscopies.

Synthesis and bioactivity of chemotactic tetrapeptides: fMLF-OMe analogues incorporating spacer aminoacids at the lateral positions.

A small library of N-For and N-Boc tetrapeptidic analogues of the chemotactic tripeptide For-Met-Leu-Phe-OMe (fMLF-OMe), obtained by incorporating three different spacer aminoacids (Gly, betaAla and Pro) between the native residues of Met and Leu (N-For- and N-Boc-Met-Xaa-Leu-Phe-OMe; Xaa2 series) and Leu and Phe (N-For- and N-Boc-Met-Leu-Xaa-Phe-OMe; Xaa3 series), have been synthesized and examined for their biological activity as agonists and antagonists. Chemotaxis, lysozyme release and superoxide anion production have been measured. All the N-For analogues maintain good to moderate chemotactic activity with the betaAla3 15 model reaching the maximum value. All the N-Boc tetrapeptides are efficient chemotactic antagonists. Conversely, with the exception of the moderate antagonistic activity exhibited by the N-Boc Xaa2 models against lysozyme release, all the other N-Boc analogues do not show significant activity against both superoxide anion and lysozyme release.

Novel chemotactic For-Met-Leu-Phe-OMe (fMLF-OMe) analogues based on met residue replacement by 4-amino-proline scaffold: synthesis and bioactivity.

cis-(2S,4S) 4-amino-proline (cAmp) and trans-(2S,4R) 4-amino-proline (tAmp) residues, bearing N-For or N-Boc substituents at the two amino groups, have been incorporated into the potent chemotactic agent fMLF-OMe in place of the N-terminal native (S)-methionine to give the analogues 17a-19a and 17b-19b. The new ligands have been examined for their activity (chemotaxis, superoxide anion production and lysozyme release) on human neutrophils as agonists and antagonists. Compounds 19a and 19b, bearing two N-For groups at the proline scaffold, are active and selective chemoattractants. The ligand 18b, containing N-For at the 4-amino group of the N-Boc-tAmp residue, exhibits significant chemotactic antagonism. The influence of the different substitution at the N-terminal position of the new analogues is discussed.

Alpha-peptide/beta-sulfonamidopeptide hybrids: analogs of the chemotactic agent for-Met-Leu-Phe-OMe.

In order to gain information on the activity shown by alpha-peptide/beta-sulfonamidopeptide hybrid analogs of the potent chemotactic agent fMLF-OMe, a structure-activity study is reported on N-Boc- and N-formyl tripeptide models containing an aminoalkanesulfonic acid as central residue. Directed migration (chemotaxis), superoxide anion production, and lysozyme release have been measured. The biochemical functions and the conformational properties of the new compounds are discussed and related to previously studied models containing beta-residues.

Chemotactic peptides: fMLF-OMe analogues incorporating proline-methionine chimeras as N-terminal residue.

The new fMLF analogues 1-4, incorporating chimeric S-proline-methionine residues (namely the homochiral cis-4(S)-methylthio-(S)-proline (10) and the heterochiral trans-4(R)-methylthio-(S)-proline) (17) in place of the native S-methionine, have been prepared; their solution conformation and activity as agonists or antagonists of formylpeptide receptors have been studied. In addition to peptides 1-4, which maintain the Met gamma-thiomethyl-ether function, the analogues Boc-PLF-OMe (18) and For-PLF-OMe (19) devoid, as compared with 1-4, of position 1 side chain, have been synthesized and their activity examined.

Peptides containing 4-amino-1,2-dithiolane-4-carboxylic acid (Adt): conformation of Boc-Adt-Adt-NHMe and NH...S interactions.

To study the conformational preferences induced by the insertion of the 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) residue into a peptide backbone, the achiral N-protected dipeptide methylamide Boc-Adt-Adt-NHMe (1) was synthesized and its crystal state and solution conformation studied and compared with that exhibited by its carba-analogue Boc-Ac5c-Ac5c-NHMe containing two residues of 1-aminocyclopentane-1-carboxylic acid (Ac5c). Compound 1 in the crystal adopts a type-III beta-turn conformation and an analogous structure is that preferred in chloroform solution as established by 1H-NMR and NOE information. In the crystal packing three different Adt rings form a cavity and the involved sulphur atoms give rise to unusual multiple interactions with one NH group. The chemical nature of these intermolecular and intramolecular main-chain...side-chain NH...S interactions are discussed in terms of quantum chemical calculations.

Beta-peptido sulfonamides: for-Met-Leu-Phe-OMe analogues containing taurine and chiral beta-amino-ethanesulfonic acid residues.

A series of new beta-peptido sulfonamides, related to the chemotactic tripeptide fMLF-OMe, has been synthesized. The examined 1a,b-7a,b models contain the achiral -HN-(CH(2))(2)-SO(2)- taurine (Tau) residue or the chiral -HN-CH(nBu)-CH(2)-SO(2)- and -HN-CH(iBu)-CH(2)-SO(2)- residues, corresponding to the beta-aminocarboxylic acid counterparts beta(3)-HNle and beta(3)-HLeu, respectively. The biological activity of the new analogues has been determined on human neutrophils and compared with that of the reference ligand as well as that of the previously studied related models. The results are analyzed in terms of structure-activity relationships. The conformational preferences of the new tripeptides 1b and 2b, containing a central chiral beta-amino-ethanesulfonic acid residue, have also been discussed.

Hybrid alpha/beta3-peptides with proteinogenic side chains. Monosubstituted analogues of the chemotactic tripeptide For-Met-Leu-Phe-OMe.

The alpha/beta3-mixed tripeptides R-CO-beta3-HMet-Leu-Phe-OMe (1a,b), R-CO-Met-beta3-HLeu-Phe-OMe (2a,b) and R-CO-Met-Leu-beta3-HPhe-OMe (3a,b) (a, R = tert-butyloxy-; b, R = H-), analogues of the potent chemoattractant For-Met-Leu-Phe-OMe, have been synthesized by classical solution methods and fully characterized. The activities of the new analogues as chemoattractants, superoxide anion producers and lysozyme releasers have been determined on human neutrophils. Whereas all of the three N-formyl derivatives are significantly less active than the parent tripeptide as chemoattractants, compound 1b has been found to be highly active as a superoxide anion producer and 3b as a lysozyme releaser. The results show that the replacement of the native Leu residue at the central position is, in each of the examined cases, the least favourable modification. The three N-Boc derivatives are, as expected, devoid of activity as agonists, but they are all good inhibitors of chemotaxis. Information on the solution conformation has been obtained by examining the involvement of the NH groups in intramolecular H-bonds using 1H NMR. The conformation of the N-Boc analogue 1a has also been determined in the crystal state by x-ray diffraction analysis. The molecule is extended at the beta3-HMet residue (phi1 = -87 degrees; theta1 = 172 degrees; psi1 = 126 degrees) and no intramolecular H-bond is present.

Synthesis and activity of HCO-Met-Leu-Phe-OMe analogues containing beta-alanine or taurine at the central position.

New synthetic analogues of the chemotactic N-formyltripeptide HCO-Met-Leu-Phe-OMe have been synthesized. The reported new models, namely Boc-Met-beta-Ala-Phe-OMe (1), HCO-Met-beta-Ala-Phe-OMe (2), Boc-Met-Tau-Phe-OMe (3), HCO-Met-Tau-Phe-OMe (4) and HCl.Met-Tau-Phe-OMe (5), are characterized by the presence at the central position of a residue of beta-alanine or 2-aminoethanesulfonic acid (taurine) replacing the native L-leucine. Whereas tripeptides 1 and 2 have been found quite inactive as chemoattractants, all the three models containing the Tau residue exhibit a remarkable activity. Superoxide anion production and lysozyme release have been also evaluated and the biological results are discussed together with the conformational preferences of the examined models.