Analogues of arginine vasopressin modified in the N-terminal part of the molecule with enantiomers of N-methylphenylalanine

Modeling the human oxytocin receptor for drug discovery efforts

The oxytocin receptor belongs to class A receptors within the great family of G protein-coupled receptors. The endogenous ligand oxytocin is a nonapeptide hormone that induces the uterine contractions at parturition and is used to induce the labor. The peptide oxytocin and, even more, its non-peptide antagonist, could be valuable tools in tocolysis. The knowledge of the three-dimensional structure of the oxytocin receptor and the determination of the main interaction points between the receptor and the ligands may help to develop selective oxytocin agonists and antagonist. This review summarizes the knowledge about the mapping of the binding domain of the oxytocin receptor and the efforts in the field of molecular modeling studies related to oxytocin receptor-ligand interactions.

Molecular docking-based study of vasopressin analogues modified at positions 2 and 3 with N-methylphenylalanine: influence on receptor-bound conformations and interactions with vasopressin and oxytocin receptors

In this study, four cyclic vasopressin (CYFQNCPRG-NH(2), AVP) analogues substituted at positions 2 and 3 with four combinations of enantiomers of N-methylphenylalanine have been investigated. Three-dimensional structures of analogues have been formerly determined using NMR spectroscopy in dimethyl sulfoxide. Three-dimensional models of the vasopressin and oxytocin receptors were constructed by combining the multiple sequence alignment and the RD crystal structure as a template. The analogues have been docked into the receptor using the AutoDock program. The relaxation of the receptor-ligand complexes using energy minimization, followed by the constrained simulated annealing protocols (CSA), has been performed. The receptor-bound conformations of the investigated analogues have been proposed. We concluded that the N-methylated residues at positions 2 and 3 act as a structural restraint, determining the conformation of analogues, their location inside the receptor cavity, and mutual arrangement of the aromatic side chains. The conserved polar residues constitute the handles keeping the biologically active analogues inside the binding cavity. The Arg(8)-D(2.65) salt bridge might be responsible for analogue-selective binding in OTR and V1aR versus V2R, where the positively charged K(2.65) 100 is present at the equivalent position.

Conformational studies of vasopressin analogues modified with N-methylphenylalanine enantiomers in dimethyl sulfoxide solution

The conformations of [Arg8]vasopressin (AVP) analogues substituted at positions 2 and 3 with N-methylphenylalanine (MePhe) enantiomers were earlier investigated by using nuclear magnetic resonance (NMR) spectroscopy in aqueous solution. A comparison of the results obtained in H2O/D2O (9:1) and DMSO-d6 has shown the structures in the first solution to be more flexible than those in DMSO-d6. This is manifested by a higher percentage of minor conformations in H2O/D2O. The largest differences between the NMR spectra in both solvents were noticed for [MePhe2, D-MePhe3]AVP (II) and [D-Cys1,MePhe2,D-MePhe3]AVP (III). Namely, in the ROESY spectra in aqueous solution, the cis/trans isomerization between MePhe2-DMePhe3 and D-Cys1-MePhe2 for II and III, respectively, is observed, while in DMSO-d6, the appropriate cross peaks indicate isomerization across the Cys6-Pro7 peptide bond. In the case of the remaining peptides, the position of cis/trans isomerization is the same in aqueous solution and in dimethyl sulfoxide. [D-MePhe2,MePhe3]AVP (V) displays low antiuterotonic and antipressor activities, while [D-MePhe2,)]AVP (IV) is a weak but selective blocker of oxytocin (OT) receptors in the uterus. The former shows similar conformational preferences as another antagonist of V1a and OT receptors-namely, [Acc2,D-Arg8]VP (Acc: 1-aminocyclohexane-1-carboxylic acid)-investigated by us. In the case of IV, the cis peptide bond between residues at positions 2 and 3 might be the reason for selectivity.

Position 4 analogues of [deamino-Cys1] arginine vasopressin exhibit striking species differences for human and rat V2/V1b receptor selectivity

Arginine vasopressin (AVP) mediates a wide variety of biological actions by acting on three distinct G-protein coupled receptors, termed V(1a) (vascular), V(1b) (pituitary) and V(2) (renal). It also binds to the oxytocin (OT) receptor. As part of a program aimed at the design of selective agonists for the human V(1b) receptor, we recently reported the human V(1b), V(1a), V(2) and OT receptor affinities of the following position 4 substituted analogues of [deamino-Cys(1)] arginine vasopressin (dAVP)-(1) d[Leu(4)]AVP, (2) d[Orn(4)]AVP, (3) d[Lys(4)]AVP, (4) d[Har(4)]AVP, (5) d[Arg(4)]AVP, (6) d[Val(4)]AVP, (7) d[Ala(4)]AVP, (8) d[Abu(4)]AVP, (9) d[Nva(4)]AVP, (10) d[Nle(4)]AVP, (11) d[Ile(4)]AVP, (12) d[Phe(4)]AVP, (13) d[Asn(4)]AVP, (14) d[Thr(4)]AVP: (15) d[Dap(4)]AVP. With the exception of Nos. 7 and 12, all peptides exhibit very high affinities for the human V(1b) receptor. Furthermore, peptides 1-4 exhibit high selectivities for the human V(1b) receptor with respect to the V(1a), V(2) and OT receptors and, with d[Cha(4)]AVP, in functional tests, are the first high affinity selective agonists for the human V(1b) receptor (Cheng LL et al., J. Med. Chem. 47: 2375-2388, 2004). We report here the pharmacological properties of peptides 1-4, 5 (from a resynthesis), 7, 9-13, 15 in rat bioassays (antidiuretic, vasopressor and oxytocic) (in vitro: no Mg(++)) with those previously reported for peptides 5, 6, 8, 14. We also report the rat V(1b), V(1a), V(2) and OT receptor affinities of peptides 1-5 and the rat V(2) receptor affinities for peptides: 7-15.The antidiuretic activities in units/mg of peptides 1-15, are: 1=378; 2=260; 3=35; 4=505; 5=748; 6=1150; 7=841; 8=1020; 9=877; 10=1141; 11=819, 12=110; 13=996; 14=758; 15=1053. Peptides 1-4 exhibit respectively the following rat and human (in brackets) V(2) receptor affinities: 1=3.1 nm (245 nm); 2=3.4 nm (1125 nm); 3=24.6 nm (11,170 nm); 4=0.6 nm (1386 nm). Their rat V(1b) receptor affinities are 1=0.02 nm; 2=0.45 nm; 3=9.8 nm; 4=0.32 nm. Their rat V(1a) receptor affinities are 1=1252 nm; 2=900 nm; 3=1478 nm; 4=32 nm. Their rat oxytocin (OT) receptor affinities are 1=481 nm; 2=997 nm; 3=5042 nm; 4=2996 nm. All four peptides have high affinities and selectivities for the rat V(1b) receptor with respect to the rat V(1a) and OT receptors. However, in contrast to their high selectivity for the human V(1b) receptor with respect to the human V(2) receptor, they are not selective for the V(1b) receptor with respect to the V(2) receptor in the rat. These findings confirm previous observations of profound species differences between the rat and human V(2) receptors. Peptides 1-4 are promising leads to the design of the first high affinity selective agonists for the rat V(1b) receptor.

Investigation ofcis/trans ratios of peptide bonds in AVP analogues containingN-methylphenylalanine enantiomers

The solution conformation of vasopressin analogues, modified at positions 2 and 3 with N-methylphenylalanine or its enantiomer, [D-MePhe2,MePhe3]AVP and [MePhe2,D-MePhe3]AVP, were studied by 2D NMR spectroscopy in H2O/D2O and theoretical calculations (EDMC/ANALYZE). In the case of [MePhe2,D-MePhe3]AVP, the synthesis afforded two products, A and B, which had identical molecular ions and similar retention times on HPLC. This finding was explained by racemization of Cys1, which gave an additional analogue, [D-Cys1,MePhe2,D-MePhe3]AVP (B). The possibility is not excluded of racemization of Cys1 in the remaining analogues of this series. However, only in the case of [MePhe2,D-MePhe3]AVP was this process so distinct that two strong peaks in the HPLC chromatogram were observed. The NMR spectra of all the analogues showed several distinct sets of residual proton resonances. This suggests that the peptides adopt more than two groups of conformations in H2O/D2O. This fact is due to cis/trans isomerization. Two more populated isomers arise from the cis/trans isomerization across the 2-3 peptide bond in [D-MePhe2,MePhe3]AVP and [MePhe2,D-MePhe3]AVP (A) and across the 1-2 peptide bond in [D-Cys1,MePhe2,D-MePhe3]AVP (B).