Conformationally restricted analogs of oxytocin; stabilization of inhibitory conformation

Conformations of a model cyclic hexapeptide, CYIQNC: (1)H-NMR and molecular dynamics studies

Solution conformation of the cyclic hexapeptide sequence, [cyclo-S-Cys-Tyr-Ile-Gln-Asn-Cys-S] (CYIQNC) - a disulfide-linked fragment of a neurohypophyseal peptide hormone oxytocin (OT) - has been investigated by high-field one-dimensional (1D) and two-dimensional (2D) NMR spectroscopic methods and compared with the results obtained from computer simulation studies. (1)H-NMR results based on temperature dependence of amide proton chemical shifts and nuclear Overhauser effect indicate that peptide in solution populates different conformations, characterized by two fused β-turns. The segment Ile(3)-Gln(4)-Asn(5)-Cys(6) yields a preferred type-III β-turn at residues 4, 5 (HB, 3HN → 6CO), while the segment Cys(6), Cys(1)-Tyr(2)-Ile(3) exhibits inherently weaker, flexible β-turn either of type I/II'/III/half-turn at residues 1, 2 (HB, 6HN → 3CO). The computer simulation studies using a mixed protocol of distance geometry-simulated annealing followed by constrained minimization, restrained molecular dynamics, and energy minimization showed the possibility of existence of additional conformations with the hydrogen bonds, (a) 5HN → 3CO and (b) 2HN → 6CO. These results, therefore, indicate that the additional conformations obtained from both NMR and simulation studies can also be possible to the peptide. These additional conformations might have very small population in the solution and did not show their signatures in these conditions. These findings will be helpful in designing more analogs with modifications in the cyclic moiety of OT.

Towards molecularly imprinted polymers selective to peptides and proteins. The epitope approach

In this paper, we describe the epitope approach to molecular imprinting. The applicability of molecular imprinting, a method that allows the preparation of biomimetic compounds (artificial receptors and antibodies), is extended by this approach. Our approach makes it possible to obtain imprinted polymers selective to peptides and proteins whereas, to date, molecular imprinting has been used primarily for the preparation of polymers that selectively bind to relatively low molecular weight substances. The epitope approach is based on using (as a template) a short peptide that represents only part of a larger peptide or protein (as an epitope represents an antigen), which in turn can be recognized by the synthesized polymer. It is demonstrated that although other parts of peptides can influence the process of molecular recognition, the polymers imprinted with a short peptide efficiently recognize both the template and larger peptides (for example, oxytocin) that possess the same C-terminal part of the structure.

Synthesis of oxytocin antagonists containing conformationally constrained amino acids in position 2

Analogues of oxytocin containing D-Trp, 2-amino-1,2,3,4-tetrahydronaphthalene-1-carboxylic acid (Atc) or 1,2,3,4-tetrahydro-beta-carboline-1-carboxylic acid (Car) with R or S configurations in position 2 were synthetized, and their receptor bindings were tested on isolated guinea-pig uterus, rat liver and rat kidney inner medulla plasma membranes. The peptides were synthetized in the solid phase by using racemates of Car and Atc. The resulting diastereomeric mixtures were separated by means of RP-HPLC. The binding to the oxytocin receptor was somewhat decreased for the Atc isomers and dramatically decreased for both R- and S-Car, while the D-Trp-containing analogue displayed a relatively high receptor affinity. However, the V1 receptor affinities were almost the same as those of the parent peptide for the Car-containing analogues and dramatically decreased for the S-Atc substituted analogue, which has a relatively high OT/V1 receptor selectivity of 44.5.

In vivo effects of 2-substituted [Mpa1Sar7Arg8]-oxytocin antagonists on postpartum rat

A set of oxytocin antagonists consisting of [Mpa1Sar7Arg8]-oxytocin substituted by various conformationally restricted or bulky D amino acids at position 2 were synthetized and biologically tested. In in vivo pharmacological investigations, the effects of these peptides were examined on the spontaneous motor activity of postpartum rat. Three of the newly prepared peptides proved at least as effective in inhibiting uterine contractions as clinically investigated atosiban.

An exploration of the effects of L- and D-tetrahydroisoquinoline-3-carboxylic acid substitutions at positions 2, 3 and 7 in cyclic and linear antagonists of vasopressin and oxytocin and at position 3 in arginine vasopressin

We have investigated the effects of mono-substitutions with the conformationally restricted amino acid, 1,2,3,4 tetrahydroisoquinoline-3-carboxylic acid (Tic) at position 3 in arginine vasopressin (AVP), at positions 2, 3 and 7 in potent non-selective cyclic AVP V2/V1a antagonists, in potent and selective cyclic and linear AVP V1a antagonists, in a potent and selective oxytocin antagonist and in a new potent linear oxytocin antagonist Phaa-D-Tyr(Me)-Ile-Val-Asn-Orn-Pro-Orn-NH2 (10). We report here the solid-phase synthesis of peptide 10 together with the following Tic-substituted peptides: 1. [Tic3]AVP: 2. dICH2)5[D-TIc2]VAVP: 3, d(CH2)5[D-Tyr(Et)2Tic3]VAVP: 4, d(CH2)5[Tic2Ala-NH2(9)]AVP: 5. d(CH2)5[Tyr]Me)2.Tic3,Ala-NH2(9)]AVP: 6. d(CH2)5 [Tyr(Me)2,Tic7]AVP: 7, Phaa-D-Tyr(Me)-Phe-Gln-Asn-Lys-Tic-Arg-NH2: 8, desGly-NH2,d[CH2]5[Tic2,Thr4]OVT: 9. desGly-NH2d(CH2)5[Tyr(Me)2Thr4, Tic7[OVT; 11, Phaa-D-Tic-Ile-Val-Asn-Orn-Pro-Orn-NH2, using previously described methods. The protected precursors were synthesized by the solid-phase method, cleaved, purified and deblocked with sodium in liquid ammonia to give the free peptides 1-11 which were purified by methods previously described. Peptides 1-11 were examined for agonistic and antagonistic potency in oxytocic (in vitro, without Mg2+) and AVP antidiuretic (V2-receptor) and vasopressor (V1a-receptor) assays. Tic3 substitution in AVP led to drastic losses of V2, V1a and oxytocic agonistic activities in peptide 1, L- and D-Tic2 substitutions led to drastic losses of anti-V2/anti-V1a and anti-oxytocic potencies in peptides 2, 4, 8 and 11 (peptide 2 retained substantial anti-oxytocic potency; pA2 = 7.25 +/- 0.025). Whereas Tic3 substitution in the selective V1a antagonist d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP(C) led to a drastic reduction in anti-V1a potency (from anti-V1a pA2 8.75 to 6.37 for peptide 5, remarkably, Tic3 substitution in the V2/V1a antagonist d(CH2)5(D-Tyr(Et)2]VAVP(B) led to full retention of anti-V2 potency and a 95% reduction in anti-V1a potency. With an anti-V2 pA2 = 7.69 +/- 0.05 and anti-V1a pA2 = 6.95 +/- 0.03. d(CH2)5[D-Tyr(Et)2, Tic3]VAVP exhibits a 13-fold gain in anti-V2/anti-V1a selectivity compared to (B). Tic7 substitutions are very well tolerated in peptides 6, 7 and 9 with excellent retention of the characteristic potencies of the parent peptides. The findings on the effects of Tic3 substitutions reported here may provide promising leads to the design of more selective and possibly orally active V2 antagonists for use as pharmacological tools and as therapeutic clinical agents for the treatment of the syndrome of the inappropriate secretion of antidiuretic hormone (SIADH).

Synthesis of methylated phenylalanines via hydrogenolysis of corresponding 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acids. Synthesis and biological activity of oxytocin analogs with methylated phenylalanines in position 2

A new method of synthesizing ortho-methylated phenylalanines has been developed. Phenylalanines with at least one free ortho-position undergo a Pictet-Spengler cyclization with formaldehyde followed by hydrogenolytic splitting of the endocyclic benzylic C--N bond of 1,2,3,4-tetrahydroisoquinolines and afford corresponding ortho-methyl derivatives. Repeating this reaction sequence on the ortho-substituted phenylalanines yielded ortho,ortho-disubstituted derivatives, and para-substituted phenylalanines yielded ortho,para-disubstituted analogs. Our modified method of cyclization preserved the configuration at the chiral center: hydrogenolysis, on the other hand, led to racemization. Incorporation of the methylated phenylalanines into position 2 of oxytocin led to, in the case of the D-isomers, potent uterotonic inhibitors.

Observation of a conformational effect in peptide molecule by reversed-phase high-performance liquid chromatography

RP-HPLC of analogues of oxytocin containing a reduced peptide bond was studied using various columns and buffers. Anomalous behavior of one analogue served as a basis for the discussion of possible conformational consequences of this substitution.

Conformational and topographical considerations in the design of biologically active peptides

An outline of the basic considerations that are under development for the rational design of biologically active peptides and peptidomimetics is given. The necessary interplay of biophysical, chemical, and biological considerations is emphasized. The importance of properly designed biological assays to provide chemical information analogous to that from biophysical studies is discussed. The development of asymmetric synthesis in conjunction with conformational considerations for the preparation of specialized amino acids and amino acid mimetics is a critical aspect of the approach. The overall approach is illustrated with three examples from our laboratory: (1) the redesign of somatostatin to a highly potent and selective mu-opioid receptor antagonist using conformational and topographical considerations in design and for obtaining insights into the pharmacophor; (2) the use of topographical considerations for obtaining oxytocin antagonists; and (3) the application of designer amino acids prepared by asymmetric synthesis to obtain insight into the topographical requirements at delta-opioid receptors.

Conformational properties of oxytocin in dimethyl sulfoxide solution: NMR and restrained molecular dynamics studies

The conformation of oxytocin, the neurohypophyseal nonapeptide hormone, in solution in deuterated dimethyl sulfoxide has been determined by 1H-nmr. The structural determination is based on the experimental data set of nuclear Overhauser effect restraints. Obtained after the restrained molecular dynamics simulation on an initial structure of extended conformation, five resultant structures satisfy the experimental restraints well. These structures resemble that of the crystal structure of deamino-oxytocin, an analogue of oxytocin, in terms of a close correlation observed both at two beta-turn regions of the 20-membered tocin ring and at the tripeptide tail end. Based on this comparison and analysis of restrained molecular dynamics trajectories, we found that, although the turns are stabilized by the formation of hydrogen bonds, the oxytocin molecule possesses a slight twist in DMSO solution relative to the orientation of deamino-oxytocin in the crystalline state. Analyses of oxytocin conformation indicate that the tripeptide tail is more flexible than the tocin ring.

Synthesis and theoretical conformational studies of gonadotropin-releasing hormone analogs containing tetrahydroisoquinoline carboxylic acid

A series of analogs of gonadotropin-releasing hormone (GnRH) containing the conformationally restrictive residue tetrahydroisoquinoline carboxylic acid (Tic) or its non-restricted parent phenylalanine were synthesized and evaluated for anti-ovulatory activity in the rat. The series, based on the potent linear parent compound Ac-DNal1-DCpa2-DPal3- Ser4-Tyr5-DPal6-Leu7-Arg8-Pro9-DAla10-NH2, included L-Tic in positions 1, 2, 3, 7 and 9, D-Tic in positions 1, 2, 3, 6 and 9, or D-Phe in positions 2 and 3 for comparison. The most potent analog in this series, with D-Tic in position 6, fully inhibited ovulation at 2.5 micrograms compared to near complete inhibition at 0.5 microgram for the parent compound. A theoretical analysis of the conformational restrictions imposed on mainchain and sidechain torsional angles by the incorporation of Tic was conducted in vacuo using molecular mechanics techniques. Using cyclo(4-10)-[Ac-delta 3Pro1,DCpa2,DTrp3,Asp4,DNal6,Dpr10]- GnRH as a template conformer for which NMR conformational data is available, it was found that the potency of the different analogs correlated with the strain energy required to deform the mainchain and backbone angles of residues to values which would be expected if Tic were present and if the analog assumed the same solution structure. In particular, the effect of DTic at position 6 is to maintain the type II' beta-turn involving residues 5-8 found in active GnRH analogs.

High-performance liquid chromatography of peptides at reduced temperatures: separation of isomers

Cholecystokinin analogues containing N-methyl amino acids were studied by reversed-phase high-performance liquid chromatography at reduced temperatures. A reduction in temperature to -17 degrees C led to lower efficiency, but at the same time separations of cis and trans isomers (and some impurities) were achieved. The velocity constants for cis-trans equilibria were calculated.

Designing peptide and protein ligands for biological receptors

A clearer understanding of structure-function relationships of protein hormone-receptor systems is emerging from the increased use of molecular biology approaches. On the other hand, the introduction of rationally designed conformation constraints into peptide hormones and neurotransmitters is leading to the development of highly receptor-selective ligands that allow further investigations into the topographic modulation of their bioactivities and rational design principles for peptide antagonists.

Oxytocin antagonists with changes in the Asn5 position shed light on hormone-oxytocin receptor interactions

Since oxytocin agonists and antagonists have different structure-activity relationships, we have investigated the stereostructural and stereoelectronic requirements of the Asn5 residue in oxytocin antagonists by the synthesis of four analogues of the potent, prolonged acting oxytocin antagonist [Pen1,D-Phe2,Thr4,Orn8]-oxytocin (I) in which Asn5 was replaced respectively with Thr (II), Leu5 (III), Asp5 (IV) and Tyr5 (V). These analogues had pA2 values in the antioxytocic in vitro rat uterine assay of 7.23 (I), 7.16 (II), 6.67 (III), 7.21 (IV), and 6.76 (IV), respectively. All were also found to be weakly potent in the in vivo anti-vasopressor assay in the rat. These studies demonstrate very different structural and stereoelectronic requirements for oxytocin agonists and antagonists when they interact with the oxytocin uterine receptor.