14-membered cyclic opioids related to dermorphin and their partially retro-inverso modified analogues. I. Synthesis and biological activity

Peptide and protein mimetics by retro and retroinverso analogs

Retroinverso analog of a natural polypeptide can sometimes mimic the structure and function of the natural peptide. The additional advantage of using retroinverso analog is that it is resistant to proteolysis. The retroinverso analogs have peptide sequence in reverse direction with respect to natural peptide and also have chirality of amino acid inverted from L to D. The D amino acids cannot be recognized by common proteases of the body; therefore, these peptides will not be degraded easily and have a longer-lasting effect as vaccine and inhibitor drugs. There have been many contested propositions about the geometric relationship between a peptide and its retro, inverso, or retroinverso analog. A retroinverso analog sometimes fails to adopt the structure that can mimic the function of the natural peptide. In such cases, partial retroinverso analog and other modifications can help in achieving the desired structure and function. Here, we review the theory, major experimental attempts, prediction methods, and alternative strategies related to retroinverso peptidomimetics.

Antinociceptive effect of D-Lys(2), Dab(4)N-(ureidoethyl)amide, a new cyclic 1-4 dermorphin/deltorphin analog

BACKGROUND

A preliminary evaluation of antinociceptive activity of a new cyclic dermorphin/deltorphin tetrapeptide analog restricted via a urea bridge and containing C-terminal ureidoethylamid {[H-Tyr-d-Lys(&(1))-Phe-Dab(&(2))-CH2CH2NHCONH2][&(1)CO&(2)]} (cUP-1) revealed a significant and long-lasting increase of pain threshold to thermal stimulation after systemic application. The current studies were aimed at further evaluation of cUP-1 activity in animal models of somatic and visceral pain. The influence of cUP-1 on motor functions was also investigated.

METHODS

The influence of cUP-1 (0.5-2mgkg(-1), iv) on nociceptive threshold to mechanical pressure and analgesic efficacy in formalin and acetic acid-induced writhing tests were estimated. The antinociceptive effect of cUP-1 was compared to that of morphine (MF). The influence of cUP-1 (1, 4 and 8mgkg(-1), iv) on locomotor activity, motor coordination and muscle strength was estimated using open field and rota-rod tests and a grip strength measurement.

RESULTS

Administration of cUP-1 in doses of 1 and 2mgkg(-1) elicited a significant increase of nociceptive threshold to mechanical pressure. MF applied in the same doses induced an antinociceptive effect only at the higher dose (2mgkg(-1)). There were no marked differences between the effect of cUP-1 and MF at each dose, at relative time points. In the writhing test and both phases of the formalin test, cUP-1 showed a significant, dose-dependent antinociceptive effect which did not markedly differ from that of MF. cUP-1 did not significantly affect motor functions of mice.

CONCLUSIONS

Systemic application of cUP-1 elicited a dose-dependent antinociceptive effect. The analgesic efficacy of cUP-1 on mechanical nociception, visceral and formalin-induced pain was comparable to that of MF. cUP-1 did not impair motor functions of mice.

Interaction energy analysis of peptide can predict the possibilities of mimetics by its retroinverso isomer

It has been previously reported that the retroinverso analog of S peptide cannot mimic the S peptide, whereas the retroinverso analog of foot-and-mouth disease virus antigen can mimic the foot-and-mouth disease virus antigen. The structures of S peptide, foot-and-mouth disease virus antigen, and their retroinverso analogs are known. Here, we have attempted to explain the structural basis of mimetics at the level of atomic interactions by elaborating upon the Guptasarma's hypothesis. Using interaction energy analysis of S peptide and foot-and-mouth disease virus antigen, we propose that if the energy of the CO and NH backbone atoms' non-covalent interactions with all other atoms is negligible as compared with the energy of other non-covalent interactions, then the retroinverso isomer can mimic the original peptide/protein. Previous work has established that the structure of the inverso analog of a protein will be the mirror image of the protein, and it will only recognize the respective mirror image substrate/binding partner. The retro peptide conformation that can be superimposed on all side chains in any conformation of the original peptide does not exist in the conformational space of the peptides.

Dmt and opioid peptides: a potent alliance

The introduction of the Dmt (2',6'-dimethyl-L-tyrosine)-Tic pharmacophore into the design of opioid ligands produced an extraordinary family of potent delta-opioid receptor antagonists and heralded a new phase in opioid research. First reviewed extensively in 1998, the incorporation of Dmt into a diverse group of opioid molecules stimulated the opioid field leading to the development of unique analogues with remarkable properties. This overview will document the crucial role played by this residue in the proliferation of opioid peptides with high receptor affinity (K(i) equal to or less than 1 nM) and potent bioactivity. The discussion will include the metamorphosis between delta-opioid receptor antagonists to delta-agonists based solely on subtle structural changes at the C-terminal region of the Dmt-Tic pharmacophore as well as their behavior in vivo. Dmt may be considered promiscuous due to the acquisition of potent mu-agonism by dermorphin and endomorphin derivatives as well as by a unique class of opioidmimetics containing two Dmt residues separated by alkyl or pyrazinone linkers. Structural studies on the Dmt-Tic compounds were enhanced tremendously by x-ray diffraction data for three potent and biologically diverse Dmt-Tic opioidmimetics that led to the development of pharmacophores for both delta-opioid receptor agonists and antagonists. Molecular modeling studies of other unique Dmt opioid analogues illuminated structural differences between delta- and mu-receptor ligand interactions. The future of these compounds as therapeutic applications for various medical syndromes including the control of cancer-associated pain is only a matter of time and perseverance.

Studies on the synthesis of cyclic pentapeptides as LHRH antagonists and the factors that influence cyclization yield

Six cyclic pentapeptides containing two or three non-protein amino acids have been synthesized by cyclization of linear precursors in dilute solution and characterized by TLC. HPLC, NMR, melting point. specific rotation etc. A total of 72 cyclization reactions were carried out to study the factors that influence head-to-tail cyclization: linear precursor sequence, coupling reagent, residue configuration, the proportion of DMAP additive, concentration, reaction temperature and reaction time. The cyclic pentapeptides will be modified by active moieties and evaluated as LHRH antagonists.

A bioactive somatostatin analog without a type II' beta-turn: synthesis and conformational analysis in solution

A cyclic somatostatin analog [structure: see text] (1) has been synthesized. Biological assays show that this compound has strong binding affinities to somatostatin hsst2 and hsst5 receptor subtypes (5.2 and 1.2 nM, respectively, and modest affinity to hsst4 (41.1 nM)). Our conformational analysis carried out in DMSO-d6 indicates that this compound exists as two structures arising from the trans and cis configurations of the peptide bond between Phe7 and N-alkylated Gly8. However, neither conformer exhibits a type II' beta-turn. This is the first report of a potent bioactive somatostatin analog that does not exhibit a type II' beta-turn in solution. Molecular dynamics simulations (500 ps) carried out at 300 K indicate that the backbone of compound 1 is more flexible than other cyclic somatostatin analogs formed by disulfide bonds.

Comparison of the solution conformations of a human immunodeficiency virus peptidomimetic and its retro-inverso isomer using 1H NMR spectroscopy

The solution conformations of the all L-alpha-peptide 1 and the corresponding retro-all D-alpha-peptide 2, two 20-metric peptides which generate antibodies that cross-react with the gp 120 envelop protein of human immunodeficiency virus-1 (HIV-1), have been investigated by high-field 1H NMR spectroscopy. Complete sequential and inter-residue interaction assignments were made from the 2D NMR spectra acquired at 500 MHz and 600 MHz in 40% deuterotrifluoroethanol (d3-TFE)/H2O at pH 2.3, and in 300 mM sodium dodecyl sulphate (SDS) in 100% D2O or 90% H2O/10% D2O at pH 2.6. Based on analysis of the nuclear Overhauser effect (NOE) and amide exchange data, peptide 1 and its retro-inverso isomer 2 in the polar solvent environment of 40% d3-TFE/H2O at pH 2.3 show very similar topological features. However, in the relatively non-polar 300 mM SDS micellar environment, peptides 1 and 2 exhibit differences in their solution structures in terms of the amide backbone and side-chain orientations. In particular, under the SDS micellar condition, peptide 1 maintains much of the secondary structure observed for this 20-mer peptide in 40% d3-TFE/H2O, pH 2.3, whereas peptide 2 adopts a more extended structure. These NMR results provide the first confirmation that the secondary structure of the all L-a-peptide 1 is maintained in both polar and non-polar environments, whereas the secondary structure and topology of the notionally equivalent retro-inverso isomer depends more on the solvent conditions. These results with the all L-a-peptide 1 and its retro-inverso isomer 2 provide important insight into the conformational influences of the C- and N-end group with L-alpha- and retro-D-alpha-isomer pairs in non-polar environments, and thus have general relevance to the design of bioactive retro-inverso peptidomimetic analogues related to immunogenic or hormonal peptides.

Synthesis of a Trisubstituted 1,4-Diazepin-3-one-Based Dipeptidomimetic as a Novel Molecular Scaffold

We describe two routes for the synthesis of a trisubstituted 1,2,5-hexahydro-3-oxo-1H-1,4-diazepine ring (DAP), a novel, conformationally constrained, seven-membered dipeptidomimetic ring system. The linear precursor for the model DAPs, targeted for conformational analysis studies, was obtained by reductive alkylation of tert-butyl alaninate or phenylalaninate by N-Boc-alpha-amino-gamma-oxo-N,N-dimethylbutyramide. Acetylation of the newly formed secondary amine followed by acidolytic deprotection of the amino and carboxyl terminal protecting groups and subsequent diphenylphosphorazidate-mediated ring formation yielded the blocked model DAPs. The synthesis of the DAP synthon started with 1-tert-butyl hydrogen N-(benzyloxycarbonyl)aspartate. The aldehyde obtained from the beta-carboxyl was used to reductively alkylate benzyl phenylalaninate, generating a secondary amine. Hydrogenolytic deprotection of the end-groups yielded the linear precursor which was cyclized via lactam formation mediated by 1-hydroxy-7-azabenzotriazolyl-N,N,N',N'-tetramethyluronium hexafluorophosphate. This route yielded the reversibly protected hexahydro-1H-3-oxo-2(S)-benzyl-5(S)-(tert-butyloxycarbonyl)-1,4-diazepine. This synthon unit can be subsequently elaborated by substituting the functional groups (secondary amine and carboxyl). Therefore, the DAPs may serve as novel molecular scaffolds to reproduce a biologically relevant topology or as a dipeptido-conformation-mimetic that can be incorporated into bioactive peptides. In addition, these synthetic routes will allow the introduction of different chiralities at positions 2 and 5 as well as the diversification of the side chains at position 2. Furthermore, the synthetic routes described here can be easily modified to obtain larger ring systems with variable degrees of conformational flexibility.