Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics

We recently reviewed the status of peptide and nonpeptide agonists and antagonists for the V(1a), V(1b) and V(2) receptors for arginine vasopressin (AVP) and the oxytocin receptor for oxytocin (OT). In the present review, we update the status of peptides and nonpeptides as: (i) research tools and (ii) therapeutic agents. We also present our recent findings on the design of fluorescent ligands for V(1b) receptor localisation and for OT receptor dimerisation. We note the exciting discoveries regarding two novel naturally occurring analogues of OT. Recent reports of a selective VP V(1a) agonist and a selective OT agonist point to the continued therapeutic potential of peptides in this field. To date, only two nonpeptides, the V(2) /V(1a) antagonist, conivaptan and the V(2) antagonist tolvaptan have received Food and Drug Administration approval for clinical use. The development of nonpeptide AVP V(1a), V(1b) and V(2) antagonists and OT agonists and antagonists has recently been abandoned by Merck, Sanofi and Pfizer. A promising OT antagonist, Retosiban, developed at Glaxo SmithKline is currently in a Phase II clinical trial for the prevention of premature labour. A number of the nonpeptide ligands that were not successful in clinical trials are proving to be valuable as research tools. Peptide agonists and antagonists continue to be very widely used as research tools in this field. In this regard, we present receptor data on some of the most widely used peptide and nonpeptide ligands, as a guide for their use, especially with regard to receptor selectivity and species differences.

Time-resolved FRET between GPCR ligands reveals oligomers in native tissues

G protein-coupled receptor (GPCR) oligomers have been proposed to play critical roles in cell signaling, but confirmation of their existence in a native context remains elusive, as no direct interactions between receptors have been reported. To demonstrate their presence in native tissues, we developed a time-resolved FRET strategy that is based on receptor labeling with selective fluorescent ligands. Specific FRET signals were observed with four different receptors expressed in cell lines, consistent with their dimeric or oligomeric nature in these transfected cells. More notably, the comparison between FRET signals measured with sets of fluorescent agonists and antagonists was consistent with an asymmetric relationship of the two protomers in an activated GPCR dimer. Finally, we applied the strategy to native tissues and succeeded in demonstrating the presence of oxytocin receptor dimers and/or oligomers in mammary gland.

Peptide and non-peptide agonists and antagonists for the vasopressin and oxytocin V1a, V1b, V2 and OT receptors: research tools and potential therapeutic agents

Oxytocin (OT) and vasopressin (AVP) mediate their biological actions by acting on four known receptors: The OT (uterine) and the AVP V(1a) (vasopressor), V(1b) (pituitary), V(2) (renal) receptors and a fifth putative AVP V(1c)? (vasodilating) receptor. This presentation will summarize some highlights of the recent progress, in the design and synthesis of selective peptide agonists, antagonists, radioiodinated ligands, fluorescent ligands and bivalent ligands for these receptors. Here we present published and unpublished pharmacological data on the most widely used agonists, antagonists and labelled ligands. The pharmacological properties of promising new selective OT antagonists and V(1b) agonists are also presented. This review should serve as a useful guide for the selection of the most appropriate ligand for a given study. The current status of non-peptide OT and AVP antagonists and agonists is also summarized. The relative merits of peptide and non-peptide AVP and OT agonists and antagonists as: (1) research tools and (2) therapeutic agents will be evaluated. Many of the receptor selective peptide agonists and antagonists from this and other laboratories are far more widely used as pharmacological tools for studies on the peripheral and central effects of OT and AVP than their non-peptide counterparts. In addition to OT and to a lesser extent AVP (pitressin), a number of OT and AVP analogues; such as carbetocin (OT agonist) dDAVP (desmopressin, V(2) agonist), terlipressin (V(1a) agonist), felypressin (V(1a) agonist) and atosiban (Tractocile OT antagonist) are also in clinical use. Despite much early promise, no non-peptide V(1a) or OT antagonists are currently in clinical trials. While a number of orally active non-peptide V(2) antagonists (Vaptans); notably, Tolvaptan, Lixivaptan and Satavaptan, are currently in Phase III clinical trials; to date, only the mixed V(2)/V(1a), antagonist Conivaptan (Vaprisol), has been approved by the US FDA for clinical use (by i.v. administration), for the treatment of euvolemic and hypervolemic hyponatremia in hospitalized patients. Promising new non-peptide V(1b) and OT antagonists, as well as non-peptide V(2) and OT agonists are now in pre-clinical development.

Design and synthesis of highly selective in vitro and in vivo uterine receptor antagonists of oxytocin: comparisons with Atosiban

We report the solid phase synthesis and some pharmacological properties of seven position two analogues (peptides 1-7) of one of our lead oxytocin antagonists, des-9-glycinamide[1-(beta-mercapto-beta,beta-pentamethylenepropionic+ ++ acid),2-O-methyltyrosine,4-threonine]ornithinevasotocin(desGly+ ++-NH2, d(CH2)5-[Tyr(Me)2,Thr4]OVT) (A). Peptides 1-7 have the following substituents at position two (1) D-Tyr(Me); (2) L-Tyr(Et); (3) D-Tyr(Et); (4) L-Tyr; (5) D-Tyr; (6) D-Phe and (7) D-Trp. These were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in vivo vasopressor (V1a-receptor) assays and in vivo antidiuretic (V2-receptor) assays. None of the seven peptides exhibits oxytocic or vasopressor agonism. Peptides 1, 2, 4, 6 and 7 are extremely weak V2 agonists (V2 activities range from 0.001 to 0.02 U/mg). Peptides 3 and 5 exhibit weak V2 antagonism (pA2 < 6.0 and < 5.5, respectively). Peptides 1-7 exhibit potent in vitro (no Mg2+) OT antagonism (anti-OT pA2 values range from 7.66 to 8.03). Peptides 1 and 4-7 exhibit potent in vivo OT antagonism. Estimated in vivo anti-OT pA2 values range from 7.06 to 7.79 (peptides 2 and 3 were not tested). With anti-V1a pA2 values of 5.17-6.25 all seven peptides exhibit reduced anti-V1a potencies relative to the parent peptide (A) (anti-V1a pA2 = 6.46). Four of these peptides (4-7) exhibit striking gains in in vitro and in vivo anti-OT/anti-V1a selectivities compared to (A) which has an in vitro selectivity of 30 and an in vivo selectivity of 18. The D-Tyr2 (5), D-Trp2 (7), D-Phe2 (6) and L-Tyr2 (4) analogues of (A) exhibit anti-OT (in vitro)/anti-V1a selectivities = 240, 390, 404 and 540, respectively. The L-Tyr2 (4), D-Trp2 (7), D-Phe2 (6) and D-Tyr2 (5) analogues exhibited anti-OT (in vivo)/anti-V1a selectivities of 72, 80, 88 and 95, respectively. Peptides 4-7 appear to be the most selective peptide OT antagonists reported to date. In this regard it may be noted that they appear to be as or more potent and much more selective than the closely related OT antagonist 1-deamino[D-Tyr(Et)2,Thr4]OVT (Atosiban) which is currently undergoing clinical trial as a potential therapeutic agent for the prevention of premature labor. Atosiban (peptide 8) was resynthesized and pharmacologically evaluated in our laboratories. Atosiban exhibits the following antagonistic potencies. Anti-OT (in vitro, no Mg2+) pA2 = 7.71; anti-OT in vivo pA2 = 7.05; anti-V1a pA2 = 6.14 and anti-V2 pA2 approximately 5.9. Its anti-OT (in vivo)/anti-V1a selectivity is 8. Some of these antagonists may be suitable candidates for evaluation as potential tocolytic agents for use in the treatment of pre-term labor. They could also serve as useful new pharmacological tools for studies on the physiological roles of oxytocin. Finally, the findings presented here provide useful clues for the design of more potent and more selective OT antagonists.

Design and Characterization of Superpotent Bivalent Ligands Targeting Oxytocin Receptor Dimers via a Channel-Like Structure

Dimeric/oligomeric states of G-protein coupled receptors have been difficult to target. We report here bivalent ligands consisting of two identical oxytocin-mimetics that induce a three order magnitude boost in G-protein signaling of oxytocin receptors (OTRs) in vitro and a 100- and 40-fold gain in potency in vivo in the social behavior of mice and zebrafish. Through receptor mutagenesis and interference experiments with synthetic peptides mimicking transmembrane helices (TMH), we show that such superpotent behavior follows from the binding of the bivalent ligands to dimeric receptors based on a TMH1-TMH2 interface. Moreover, in this arrangement, only the analogues with a well-defined spacer length (∼25 Å) precisely fit inside a channel-like passage between the two protomers of the dimer. The newly discovered oxytocin bivalent ligands represent a powerful tool for targeting dimeric OTR in neurodevelopmental and psychiatric disorders and, in general, provide a framework to untangle specific arrangements of G-protein coupled receptor dimers.

[1-deamino-4-cyclohexylalanine] arginine vasopressin: a potent and specific agonist for vasopressin V1b receptors

To date, there are no vasopressin (VP) agonists that exhibit a high affinity and selectivity for the VP V1b receptor with respect to the V1a, V2, and oxytocin receptors. In this study, we describe the synthesis and pharmacological properties of [1-deamino-4-cyclohexylalanine] arginine vasopressin (d[Cha4]AVP). Binding experiments performed on various membrane preparations revealed that d[Cha(4)]AVP exhibits a nanomolar affinity for V1b receptors from various mammalian species (rat, bovine, human). It exhibits high V1b/V1a and V1b/oxytocin selectivity for rat, human, and bovine receptors. Furthermore, it exhibits high V1b/V2 specificity for both bovine and human vasopressin receptors. Functional studies performed on biological models that naturally express V1b receptors indicate that d[Cha4]AVP is an agonist. Like VP, it stimulated basal and corticotropin-releasing factor-stimulated ACTH secretion and basal catecholamine release from rat anterior pituitary and bovine chromaffin cells, respectively. In vivo experiments performed in rat revealed that d[Cha4]AVP was able to stimulate both ACTH and corticosterone secretion and exhibits negligible vasopressor activity. It retains about 30% of the antidiuretic activity of VP. This long-sought selective VP V1b receptor ligand with nanomolar affinity will allow a better understanding of V1b-mediated VP physiological effects and is a promising new tool for V1b receptor structure-function studies.

Design of potent and selective linear antagonists of vasopressor (V1-receptor) responses to vasopressin

We report the solid-phase synthesis of 21 linear analogues of A and D, two nonselective antagonists of the vasopressor (V1) and antidiuretic (V2) responses to arginine vasopressin (AVP). A is Aaa-D-Tyr(Et)-Phe-Val-Asn-Abu-Pro-Arg-Arg-NH2 (where Aaa = adamantylacetyl at position 1). D is the des-Arg9 analogue of A. Nine new analogues of A (1-9) and 12 new analogues of D (10-21) were obtained. The following substitutions either alone or in combination were incorporated in A and/or in D: phenylacetic acid (Phaa) and tert-butylacetic acid (t-Baa) at position 1; D-Tyr2, D-Tyr(Me)2; Gln4; Arg6, Lys6, Orn6, MeAla7. The nine new analogues of A are (1) [Arg6], (2) [Lys6], (3) [Orn6], (4) [Phaa1,Lys6], (5) [Phaa1,Orn6], (6) [D-Tyr2], (7) [D-Tyr2,Arg6], (8) [Phaa1,D-Tyr2], (9) [Phaa1,D-Tyr2,Arg6]. The 12 new analogues of D are (10) [Arg6], (11) [Lys6], (12) [Orn6], (13) [Phaa1,Lys6], (14) [Phaa1,Gln4,Lys6], (15) [Phaa,D-Tyr(Me)2,Lys6], (16) [Phaa,D-Tyr(Me)2,Gln4,Lys6], (17) [Phaa1,D-Tyr2,Gln4,Lys6], (18) [t-Baa1,Lys6], (19) [t-Baa1,Gln4,Lys6], (20) [Arg6,MeAla7], (21) [t-Baa1,Arg6,MeAla7]. All 21 peptides were examined for agonistic and antagonistic potencies in AVP V2 and V1 assays in rats. With the exception of 6, the eight remaining new analogues of A are equipotent or more potent than A as V1 antagonists. Peptides 2-9 are less potent than A as V2 antagonists. Three, 4, 5, and 9, exhibit significant gains in anti-V1/anti-V2 selectivities (selectivity ratio = 41, 14, and infinite, respectively), compared to A (anti-V1, pA2 = 7.75 +/- 0.07; selectivity ratio = 0.44). Peptide 9 is unique in both series. It is a highly potent V1 antagonist (anti-V1 pA2 = 8.62 +/- 0.11 and is the first linear peptide to exhibit substantial antidiuretic agonism (4.1 +/- 0.2 units/mg). With the exception of 12, the remaining 11 analogues of D are 8-40 times more potent than D as V1 antagonists. Eight of these peptides exhibit significant gains in anti-V1/anti-V2 selectivities compared to D (anti-V1 pA2 = 7.43 +/- 0.06; selectivity ratio = 1.6).(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and characterization of fluorescent antagonists and agonists for human oxytocin and vasopressin V(1)(a) receptors

The fluoresceinyl (Flu) group has been linked by an amide bond to the side chain amino group at position 8 of (a) two oxytocin (OT) antagonists, to give d(CH(2))(5)[Tyr(Me)(2),Thr(4),Orn(8)(5/6C-Flu),Tyr-NH(2)(9)]VT (Orn(8)(5/6C-Flu)OTA) (1) and desGly-NH(2),d(CH(2))(5)[D- Tyr(2),Thr(4),Orn(8)(5/6C-Flu)]VT (2), and (b) eight Lys(8) and Orn(8) analogues of potent OT agonists, to give d[Lys(8)(5/6C-Flu)]VT (3), d[Thr(4),Lys(8)(5/6C-Flu)]VT (4), [HO(1)][Lys(8)(5/6C-Flu)]VT (5), [HO(1)][Thr(4),Lys(8)(5/6C-Flu)]VT (6), d[Orn(8)(5/6C-Flu)]VT (7), d[Thr(4),Orn(8)(5/6C-Flu)]VT (8), [HO(1)][Orn(8)(5/6C-Flu)]VT (9), and [HO(1)][Thr(4),Orn(8)(5/6C-Flu)]VT (10). The tetramethylrhodamyl (Rhm) group was attached to the precursor peptide of 9 to give [HO(1)][Orn(8)(5/6C-Rhm)]VT (11). All 11 fluorescent peptides were evaluated in human OT and vasopressin V(1a) (vasoconstrictor), V(1b) (pituitary), and V(2) (antidiuretic) receptor binding and functional assays. With K(d) = 6.24, 217, >10000, and >10000 nM for the OT, V(1a), V(1b), and V(2) receptors, peptide 1 is a potent and selective fluorescent OT antagonist and may be useful for specifically labeling OT receptors while peptide 2 exhibits low affinities for all the receptors. The fluorescent peptides 3-10 are all very potent agonists for the human OT receptor. They exhibit the following K(d) values (nM) for the human OT, V(1a), V(1b), and V(2) receptors, respectively: (3) 0.29, 57, 124, >10000; (4) 1.8, 25.5, 150, >10000; (5) 0.34, 13.7, 66, nd (not determined); (6) 0.32, 17.3, 53, >10000; (7) 0.25, 107, 393, >10000; (8) 0.40, 30, 282, >10000; (9) 0.18, 12.2, 126, nd; (10) 0.17, 11.8, 87, >1000; (11) 0.092, 7.36, nd, nd. Peptide 7 exhibits both a high affinity and a high selectivity for human OT receptors. Peptides 7 and 11 were utilized to study the internalization of the OT receptor-ligand complex. Preliminary studies indicate that this process is similar to that observed for the vasopressin V(1a) receptor and differs from that observed for vasopressin V(2) receptors. Some or all of the fluorescent OT antagonists and agonists reported here are very promising new fluorescent ligands for labeling cells which express the human OT receptor and are also useful tools to follow endocytosis of the receptor-ligand complex.

Design of oxytocin antagonists, which are more selective than atosiban

We report the solid phase synthesis of four pairs of L- and D-thienylalanine (Thi/D-Thi) position two modified analogues of the following four oxytocin (OT) antagonists: des-9-glycinamide [1-(beta-mercapto-beta,beta-pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly(NH2)9,d (CH2)5[Tyr(Me)2,Thr4]OVT) (A); the Tyr-(NH2)9 analogue of (A), d(CH2)5[Tyr(Me)2,Thr4,Tyr-(NH2)9]OVT (B); the Eda9 analogue (where Eda = ethylenediamine) of (A), d(CH2)5[Tyr(Me)2, Thr4, Eda9]OVT (C); and the retro Tyr10 modified analogue of (C), d(CH2)5[Tyr(Me)2, Thr4, Eda9<--Tyr10]OVT (D). The eight new analogues of A-D are (1) desGly(NH2),d(CH2)5[Thi2,Thr4]OVT, (2) desGly(NH2),d(CH2)5[D-Thi2,Thr4]OVT, (3) d(CH2)5[Thi2, Thr4,Tyr-(NH2)9]OVT, (4) d(CH2)5[D-Thi2,Thr4,Tyr-(NH2)9]OVT (5) d(CH2)5[Thi2,Thr4Eda9]OVT, (6) d(CH2)5[D-Thi2,Thr4,Eda9]OVT, (7) d(CH2) [Thi2,Thr4,Eda9<--Tyr10]OVT, (8) d(CH2),[D-Thi2,Thr4,Eda9<--Tyr10]OVT. We also report the synthesis of (C). Peptides 1-8 and C were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in in vivo vasopressor (V1a receptor) assays and in in vivo antidiuretic (V2 receptor) assays. None of the eight peptides nor C exhibit oxytocic or vasopressor agonism. Peptides 1-8 are extremely weak V2 agonists (antidiuretic activities range from < 0.0005 to 0.20 U/mg). Peptide C is a weak mixed V2 agonist/antagonist. Peptides 1-8 and C exhibit potent in intro (no Mg2+) OT antagonism (anti-OT pA2 values range from 7.76 to 8.05). Peptides 1-8 are all OT antagonists in vivo (estimated in vivo anti-OT pA2 values range from 6.54-7.19). With anti-V1a pA2 values of approximately 5-5.80, peptides 1-8 exhibit marked reductions in anti-V1a potencies relative to those of the parent peptides A-D (anti-V1a pA2 range from 6.48 to 7.10) and to l-deamino[D-Tyr(Et)2, Thr4]OVT (Atosiban, trade name Tractocile) (anti-V1a pA2-6.14). Atosiban has recently been approved in Europe for clinical use for the prevention of premature labour (Pharm. J. 264(7-100): 871). Peptides 1-8 exhibit striking gains in in vitro anti-OT/anti-V1a selectivities with respect to the parent peptides A, B, C and D and to Atosiban. Peptides 1-8 exhibit anti-OT (in vitro)/anti-V1a selectivities of 450, 525, 550, 450, approximately 1080, 116, 355, 227 respectively. The corresponding values for A-D and Atosiban are 30, 4.2, 4.3, 2.6 and 37. With the exception of peptide 6, the remaining seven peptides exhibit 3-18-fold gains in anti-OT (in vivo)/anti-V1a selectivity with respect to Atosiban, peptides 1-8 exhibit anti-OT (in vivo)/anti-V1a selectivities of 22, approximately 82, approximately 82, 147, approximately 83, 11, 31 and 42. By comparison, Atosiban exhibits an anti-OT (in vivo)/anti-V1a selectivity = 8. With an estimated in vivo anti-OT pA2 value = 7.19+/-0.06, peptide 4 is equipotent with Atosiban (pA2 = 7.05+/-0.05). However, with its significantly reduced anti-vasopressor potency, pA2 = approximately 5, it is approximately 18 times more selective for OT receptors with respect to VP V1a receptors than Atosiban. Since we have shown that V1a antagonism could be an unwanted side-effect in tocolytics, peptide 4 and some of the OT antagonists reported here have advantages over Atosiban and thus may be suitable candidates for evaluation as potential tocolytic agents for the treatment of preterm labour.

Experimental coccidiosis provoked by Eimeria acervulina in chicks simultaneously fed on ochratoxin A contaminated diet

The progression of coccidiosis provoked by Eimeria acervulina was followed in chicks fed on OTA-contaminated as well as on OTA-free diets. More heavy progress of duodenal coccidiosis, including mortality, occurred in OTA-treated chicks as can be seen from the higher value of lesion (3.50) and oocyst (31.65) indices. A stronger decrease of serum total protein was found in OTA-treated chicks (22.80 g/l) than in chicks infected with E. acervulina(24.20 g/l), but that decrease was strongest in chicks treated with OTA and simultaneously infected with E. acervulina (19.71 g/l). The serum concentration of uric acid was significantly increased in all chicks exposed to OTA, most notably in those additionally infected with E. acervulina (1020.6 (micro mol/L), whereas the serum enzyme activity of AST was increased only in chicks infected with E. acervulina and highest in those fed OTA contaminated diet (122.2 U/L). OTA induced degenerative changes in kidneys, liver and heart as well as a depletion of lymphoid tissue in the lymphoid organs and a decrease of body weight. Coccidiosis induced only a slight growth depression and duodenal hemorrhages in addition to characteristic duodenal damages. The impairment of kidney function, histopathological changes and general growth depression were stronger when chicks infected with E. acervulina were also given OTA.

Synthesis and some pharmacological properties of potent and selective antagonists of the vasopressor (V1-receptor) response to arginine-vasopressin

We report the solid-phase synthesis of eight position-9-modified analogues of the potent V1-receptor antagonist of arginine-vasopressin, [1-(beta-mercapto-beta,beta-pentamethylenepropionic acid),2-O-methyltyrosine]arginine-vasopressin (d(CH2)5Tyr(Me)AVP) (1-8) and five position-9-modified analogues of the closely related beta,beta-dimethyl less potent V1 antagonist, [1-deaminopenicillamine,2-O-methyltyrosine]arginine-vasopressin (dPTyr(Me)AVP) (9-13). In d(CH2)5Tyr(Me)AVP the C-terminal Gly-NH2 was replaced by (1) ethylenediamine (Eda), (2) methylamine (NHMe), (3) Ala-NH2, (4) Val-NH2, (5) Arg-NH2, (6) Thr-NH2, (7) Gly-Eda, (8) Gly-N-butylamide (Gly-NH-Bu); in dPTyr(Me)AVP the C-terminal Gly-NH2 was replaced by (9) Ala-NH2, (10) Val-NH2, (11) Thr-NH2, (12) Arg-NH2, and (13) Tyr-NH2. All 13 analogues were tested for agonistic and antagonistic activities in in vivo rat vasopressor (V1-receptor) and rat antidiuretic (V2-receptor) assays. They exhibit no evident vasopressor agonism. All modifications in both antagonists were well-tolerated with excellent retention of V1 antagonism and striking enhancements in anti-V1/anti-V2 selectivity. With anti-V1 pA2 values of 8.75, 8.73, 8.86, and 8.78, four of the analogues of d-(CH2)5Tyr(Me)AVP (1-3 and 6) are equipotent with d(CH2)5Tyr(Me)AVP (anti-V1 pA2 = 8.62) but retain virtually none of the V2 agonism of d(CH2)5Tyr(Me)AVP. They are in fact weak V2 antagonists and strong V1 antagonists with greatly enhanced selectivity for V1 receptors relative to that of d(CH2)5Tyr(Me)AVP. With anti-V1 pA2 values respectively of 8.16, 8.05, 8.04, 8.52, and 8.25, all five analogues (9-13) of dPTyr(Me)AVP are at least as potent V1 antagonists as dPTyr(Me)AVP (pA2 = 7.96) and three of these (9, 12, 13) actually show enhanced V1 antagonism over that of dPTyr(Me)AVP. In fact, the Arg-NH2(9) analogue (12) is almost equipotent with d(CH2)5Tyr(Me)AVP. These new V1 antagonists are potentially useful as pharmacological tools for studies on the cardiovascular roles of AVP. Furthermore the analogues of dPTyr(Me)AVP may be useful in studies on the role(s) of AVP in the V1b-receptor-mediated release of ACTH from corticotrophs.

A unique case of naturally occurring mummification of human brain tissue

When skulls and bones were exhumed from a mass grave in Bulgaria and subjected to medicolegal examination they were found to originate from 39 humans aged 36-60 years old who had been buried approximately 45-50 years ago. Solid structures which strongly resembled shrunken human brain tissue were found inside 2 intact skulls. Among other bones 5 similar structures were found one of which was an almost entirely preserved human brain, and the others were fragments from different regions of the human brain. Samples of these structures were immersed in 15% aqueous glycerol solution to soften and were examined by light and electron microscopy. Samples of this material and of fresh human brain were subjected to elementary atomic spectral analysis. These complex studies indicated the samples to be naturally mummified human brain tissue and that this process had occurred due to specific conditions within the cranial cavities after burial.

Pharmacological and physiological characterization of d[Leu4, Lys8]vasopressin, the first V1b-selective agonist for rat vasopressin/oxytocin receptors

Recently, we synthesized and characterized the first selective V(1b) vasopressin (VP)/oxytocin receptor agonist, d[Cha(4)]arginine vasopressin. However, this agonist was only selective for the human receptors. We thus decided to design a selective V(1b) agonist for the rodent species. We started from previous observations showing that modifying [deamino(1),Arg(8)]VP in positions 4 and 8 altered the rat VP/oxytocin receptor selectivity. We synthesized a series of 13 [deamino(1),Arg(8)]VP analogs modified in positions 4 and 8. Among them, one seemed very promising, d[Leu(4), Lys(8)]VP. In this paper, we describe its pharmacological and physiological properties. This analog exhibited a nanomolar affinity for the rat, human, and mouse V(1b) VP receptors and a strong V(1b) selectivity for the rat species. On AtT20 cells stably transfected with the rat V(1b) receptor, d[Leu(4), Lys(8)]VP behaved as a full agonist on both phospholipase C and MAPK assays. Additional experiments revealed its ability to induce the internalization of enhanced green fluorescent protein-tagged human and mouse V(1b) receptors as expected for a full agonist. Additional physiological experiments were performed to further confirm the selectivity of this peptide. Its antidiuretic, vasopressor, and in vitro oxytocic activities were weak compared with those of VP. In contrast, used at low doses, its efficiency to stimulate adrenocorticotropin or insulin release from mouse pituitary or perfused rat pancreas, respectively, was similar to that obtained with VP. In conclusion, d[Leu(4), Lys(8)]VP is the first selective agonist available for the rat V(1b) VP receptor. It will allow a better understanding of V(1b) receptor-mediated effects in rodents.

The discovery of novel vasopressin V1b receptor ligands for pharmacological, functional and structural investigations

Until recently, pharmacological studies dealing with vasopressin receptor isoforms were severely hampered by the lack of selective agonists or antagonists that recognize the pituitary V(1b) vasopressin receptor. By contrast, many selective vasopressin-related compounds are available for characterization of the vasopressor (V(1a)) or antidiuretic (V(2)) vasopressin receptor subtypes. Recently, SSR149415, a selective nonpeptide molecule, was discovered with nanomolar affinity for mammalian V(1b) receptors and good selectivity for the other vasopressin and oxytocin receptor isoforms. This molecule exhibits potent antagonist properties both in vitro and in vivo. We also designed synthetic peptides derived from [deaminocysteine(1),arginine(8)]vasopressin (dAVP), modified in position 4 by various amino acid residues. Some of these, d[cyclohexylalanine(4)]AVP or d[lysine(4)]AVP, have a high affinity and an excellent selectivity for the human V(1b) receptor subtype. However, they exhibit a mixed V(1b)/V(2) pharmacological profile for the rat vasopressin receptor isoforms. Whatever the species considered, these peptides behave as agonists both in bioassays performed in vitro and in vivo. The d[cyclohexylalanine(4)]AVP was tritiated and represents the first selective radiolabelled ligand available for studying the human V(1b) receptors. The discovery of these new selective V(1b) agonists and V(1b) antagonist allows an accurate pharmacological characterization of all the vasopressin receptor isoforms. As emphasized in this review, attention to the vasopressin and oxytocin receptor species differences is of critical importance in studies with all vasopressin and oxytocin ligands.

Potent V2 vasopressin antagonists with structural changes at their C-terminals

A variety of structural changes were made in the C-terminals of four potent antidiuretic (V2) antagonists. The parent analogs were all derivatives of [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)]arginine-vasopressin, d(CH2)5AVP, namely d(CH2)5[D-Phe2,Ile4]AVP, d(CH2)5[D-Ile2,Ile4]AVP, d(CH2)5[D-Tyr(Et)2, Val4]AVP and d(CH2)5[D-Tyr(Et)2,Ile4]AVP. A number of amino acid amides were substituted for the C-terminal 9-glycinamide without reducing their V2-antagonistic potencies in rats. Many non-amino acid structures were also tolerated at the C-terminals of these antagonists and this end of these peptides can be prolonged without interfering with antagonistic potencies. Such altered V2-antagonists may be useful for the development of radioactive ligands, affinity labels and in affinity columns for studies on antidiuretic receptors. These C-terminal modifications also provide useful information for the further development of potent and specific V2-antagonists which can be valuable pharmacological tools and also promise to become useful clinically for the treatment of excessive water retention.

Design of peptide oxytocin antagonists with strikingly higher affinities and selectivities for the human oxytocin receptor than atosiban

The peptide oxytocin (OT) antagonist atosiban, approved for tocolytic use in Europe (under the tradename Tractocile), represents an important new therapeutic advance for the treatment of premature labor. This paper presents some new peptide OT antagonists which offer promise as superior tocolytics. The solid phase synthesis is reported of four pairs of L and D-2-naphthylalanine (L/D-2Nal) position-2 modified analogs of the following four oxytocin (OT) antagonists: des-9-glycinamide [1-(beta-mercapto-beta,beta-pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly-NH(2),d(CH(2))(5)[Tyr(Me)(2),Thr(4)]OVT) (A); the Tyr-NH(2) (9) analog of (A), d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2) (9)]OVT (B); the Eda(9) analog of (A), d(CH(2))(5)[Tyr(Me)(2),Thr(4),Eda(9)]OVT (C); and the retro COCH(2)Ph(4-0H)(10) modified analog of (C), d(CH(2))(5)[Tyr(Me)(2),Thr(4),Eda(9)<-- COCH(2)Ph(4-0H)(10)]OVT (D). The eight new analogs of A-D are (1) desGly-NH(2),d(CH(2))(5)[D-2Nal(2),Thr(4)]OVT, (2) desGly-NH(2),d(CH(2))(5)[2-Nal(2),Thr(4)]OVT, (3) d(CH(2))(5)[D-2Nal(2),Thr(4),Tyr-NH(2) (9)]OVT, (4) d(CH(2))(5)[2Nal(2),Thr(4),Tyr-NH(2) (9)]OVT, (5) d(CH(2))(5)[D-2Nal(2),Thr(4),Eda(9)]OVT, (6) d(CH(2))(5)[2Nal(2),Thr(4),Eda(9)]OVT, (7) d(CH(2))(5)[D-2Nal(2),Thr(4),Eda(9)<-- COCH(2)Ph(4-0H)(10)]OVT, (8) d(CH(2))(5)[2Nal(2),Thr(4),Eda(9)<-- COCH(2)Ph(4-OH)(10)]OVT. Peptides 1-8 were evaluated for agonistic and antagonistic activities in in vitro and in vivo rat bioassays, in rat OT receptor (rOTR) binding assays and in human OT receptor (hOTR) and human vasopressin (VP) vasopressor (V(1a)) receptor (hV(1a)R) binding assays. Also reported are the hOTR and hV(1a)R affinity data for atosiban and for B. None of the eight peptides exhibit oxytocic or vasopressor agonism. Peptides 1-8 exhibit weak antidiuretic agonism (activities in the range 0.014-0.21 U/mg). Peptides 1-6 exhibit potent in vitro (no Mg(2+)) OT antagonism (anti-OT pA(2) values range from 7.63 to 8.08). Peptides 7 and 8 are weaker OT antagonists. Peptides 1-6 are all OT antagonists in vivo (estimated in vivo anti-OT pA(2) values in the range 6.94-7.23). Peptides 1-8 exhibit vasopressor antagonism, anti-V(1a) pA(2) values in the range 5.1-7.65. Peptides 1-8 exhibit high affinities for the rOTR (K(i) values = 0.3-7.8 nM). Peptides 1-4 and B exhibit surprisingly very high affinities for the hOTR; their K(i) values are 0.17, 0.29, 0.07, 0.14 and 0.59 nM, respectively. Peptides 1-4 and B exhibit respectively 449, 263, 1091, 546 and 129 times greater affinity for the hOTR than atosiban (K(i) = 76.4 nM). Peptides 1-4 exhibit high affinities for the hV(1a)R (K(i)s = 1.1 nM, 1.3 nM, 0.19 nM and 0.54 nM, all higher than the hV1(a)R affinities exhibited by atosiban (K(i) = 5.1 nM) and by B (K(i) = 5.26 nM). Because of their strikingly higher affinities for the hOTR than atosiban, peptides 1-4 and B exhibit gains in anti hOT/anti hV(1a) receptor selectivity compared with atosiban of 93, 64, 39, 56 and 127, respectively. These OT antagonists are thus promising candidates for development as potential new tocolytic agents.

Design and Synthesis of the First Selective Agonists for the Rat Vasopressin V1bReceptor: Based on Modifications of Deamino-[Cys]arginine Vasopressin at Positions 4 and 8

The neurohypophyseal peptides arginine vasopressin (AVP) and oxytocin (OT) mediate a wide variety of peripheral and central physiological and behavioral effects by acting on four different G-protein coupled receptors, termed V1a (vascular), V1b (pituitary), V2 (renal), and OT (uterine). We recently reported that d[Cha4]AVP (A), d[Leu4]AVP (B), d[Orn4]AVP (C), and d[Arg4]AVP (D) have high affinity and are selective agonists for the human V1b receptor. However, peptides A-D were subsequently shown to be potent antidiuretic agonists in the rat and are, thus, not selective V1b agonists in the rat. Peptides A-D served as leads for the studies reported here. They were modified at position 8 by Lys, ornithine (Orn), diaminobutyric acid (Dab), and diaminopropionic acid (Dap) to give d[Cha4,Lys8]VP (1), d[Cha4,Orn8]VP (2), d[Cha4,Dab8]VP (3), d[Cha4,Dap8]VP (4), d[Leu4,Lys8]VP (5), d[Leu4,Orn8]VP (6), d[Leu4,Dab8]VP (7), d[Leu4,Dap8]VP (8), d[Orn4,Lys8]VP (9), d[Orn4,Orn8]VP (10), d[Arg4,Lys8]VP (11), d[Arg4,Orn8]VP (12), and d[Arg4,Dab8]VP (13). All peptides were synthesized by the Merrifield solid-phase method. Their binding and functional properties were evaluated in rat AVP V1a, V1b, and V2 receptors and on the rat OT receptor expressed either in native tissues or in stably transfected cells. They were also examined in rat vasopressor, antidiuretic, and in in vitro (no Mg++) oxytocic assays. Functional studies performed on chinese hamster ovary cells expressing the different AVP/OT receptors confirm that d[Cha4,Lys8]VP (1), d[Cha4,Dab8]VP (3), d[Leu4,Lys8]VP (5), and d[Leu4,Dap8]VP (8) are the first selective agonists for the rat V1b receptor. These selective V1b agonists are promising new tools for studies of the role of the V1b receptor in the rat.

Design of potent and selective agonists for the human vasopressin V1b receptor based on modifications of [deamino-cys1]arginine vasopressin at position 4

The glutamine(4) residue in [deamino-Cys(1)]arginine vasopressin (dAVP) was replaced by a broad series of aliphatic, aromatic, polar, and charged amino acids to give the following peptides: d[Gly(4)]AVP (1), d[Ala(4)]AVP (2), d[Abu(4)]AVP (3), d[Nva(4)]AVP (4), d[Nle(4)]AVP (5), d[Leu(4)]AVP (6), d[Ile(4)]AVP (7), d[Thi(4)]AVP (8), d[Phe(4)]AVP (9), d[Tyr(4)]AVP (10), d[Trp(4)]AVP (11), d[Asn(4)]AVP (12), d[Ser(4)]AVP (13), d[Thr(4)]AVP (14), d[Dap(4)]AVP (15), d[Dab(4)]AVP (16), d[Orn(4)]AVP (17), d[Lys(4)]AVP (18), d[Arg(4)]AVP (19), d[Har(4)]AVP (20), and d[Glu(4)]AVP (21). All peptides were synthesized by solid-phase methods using BOC chemistry for all but one peptide (8), which required the use of Fmoc chemistry. The binding and functional properties of these position 4 substituted analogues of dAVP (d[X(4)]AVP) and the previously reported d[Cha(4)]AVP (Derick et al. Endocrinology 2002, 143, 4655-4664) were evaluated on human arginine vasopressin (AVP) V(1a), V(1b), and V(2) receptors and on the human oxytocin (OT) receptor expressed in living Chinese hamster ovary (CHO) cells. Binding studies revealed that broad modifications of the fourth residue of dAVP do not significantly alter affinity for the human V(1b) receptor. Only aromatic (Phe, Tyr, Trp) or negatively charged (Glu) residues reduce V(1b) affinity. By contrast, the human V(1a) and more particularly the human V(2) and the OT receptors are more sensitive to many of these modifications. Thus, the replacement of the Gln(4) residue of dAVP by aliphatic (Leu, Cha) or positively charged (Orn, Lys, Arg, Har) amino acids led to analogues exhibiting drastic reductions of their affinity for the human V(1a), V(2), and OT receptors. Consequently, in addition to the previously reported d[Cha(4)]AVP, peptides 6 and 17-20 display excellent selectivities for the human V(1b) receptor. The key structural requirement responsible for optimal V(1b) selectivity appears to be the length and branching of the aliphatic side chain of the fourth residue of dAVP. Functional studies performed on CHO cells expressing the different human AVP/OT receptors confirm the V(1b) selectivity of peptides 6, 17, 18, 20, and d[Cha(4)]AVP. However, d[Arg(4)]AVP (19), which triggers an excellent coupling between the human V(2) receptor and adenylyl cyclase, was found to exhibit both V(1b) and V(2) agonism in functional tests. More interestingly, these functional experiments revealed that, depending on the AVP/OT receptor, a given d[X(4)]AVP analogue may behave as a full agonist or as a partial agonist. This strongly suggests that the fourth residue of dAVP plays an important role in the coupling between the hormone-receptor complex, the heterotrimeric G protein, and the effectors. In conclusion, the synthesis of these d[X(4)]AVP analogues led to the discovery of new V(1b) agonists with high affinity and greatly enhanced selectivities. Thus, in addition to d[Cha(4)]AVP, d[Leu(4)]AVP (6), d[Orn(4)]AVP (17), d[Lys(4)]AVP (18), and d[Har(4)]AVP (20) are useful new tools for studying the structure and the function of the human V(1b) receptor.

Position three in vasopressin antagonist tolerates conformationally restricted and aromatic amino acid substitutions: a striking contrast with vasopressin agonists

We report the solid-phase synthesis and some pharmacological properties of 12 position three modified analogues (peptides 1-12) of the potent non-selective antagonist of the antidiuretic (V2-receptor), vasopressor (V1a-receptor) responses to arginine vasopressin (AVP) and of the uterine contracting (OT-receptor) responses to oxytocin (OT), [1(-beta mercapto-beta,beta-pentamethylenepropionic acid)-2-O-ethyl-D-tyrosine 4-valine] arginine vasopressin [d(CH2)5D-Tyr(Et)2VAVP] (A) and two analogues of (B) (peptides 13,14), the 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid3 (Tic3) analogue of (A). Peptides 1-12 have the following substituents at position three in (A): (1) Pro; (2) Oic; (3) Atc; (4) D-Atc; (6) D-Phe; (7) Ile; (8) Leu; (9) Tyr; (10) Trp; (11) Hphe; (12) [HO]Tic; Peptide (13) is the Tyr-NH2(9) analogue of (B): Peptide (14) is the D-Cys(6) analogue of (B). All 14 new peptides were evaluated for agonistic and antagonistic activities in in vivo V2 and V1a assays and in vitro (no Mg2+)n oxytocic assays. With the exception of the D-Phe3 peptide (No. 6), which exhibits very weak V2 agonism (approximately 0.0017 U/mg), none of the remaining 13 peptides exhibit any agonistic activities in these assays. In striking contrast to their deleterious effects on agonistic activities in AVP, the Pro3, Oic3, Tyr3 and Hphe3 substitutions in (A) are very well tolerated, leading to excellent retention of V2, V1a and OT antagonistic potencies. All are more potent as V2 antagonists than the Ile3 and Leu3 analogues of (A). The Tyr-NH2(9) and D-Cys(6) substitutions in (B) are also well tolerated. The anti-V2 pA2 values of peptides 1-5 and 7-14 are as follows (1) 7.77 +/- 0.03; (2) 7.41 +/- 0.05; (3) 6.86 +/- 0.02; (4) 5.66 +/- 0.09; (5) approximately 5.2; (7) 7.25 +/- 0.08; (8) 6.82 +/- 0.06; (9) 7.58 +/- 0.05; (10) 7.61 +/- 0.08; (11) 7.59 +/- 0.07; (12) 7.20 +/- 0.05; (13) 7.57 +/- 0.1; (14) 7.52 +/- 0.06. All analogues antagonize the vasopressor responses to AVP, with anti-V1a pA2 values ranging from 5.62 to 7.64, and the in vitro responses to OT, with anti-OT pA2 values ranging from 5.79 to 7.94. With an anti-V2 potency of 7.77 +/- 0.03, the Pro3 analogue of (A) is surprisingly equipotent with (A), (anti-V2 pA2 = 7.81 +/- 0.07). These findings clearly indicate that position three in AVP V2/V1a antagonists, in contrast to position three in AVP agonists, is much more amenable to structural modification than had heretofore been anticipated. Furthermore, the surprising retention of V2 antagonism exhibited by the Pro3, Oic3, Tyr3, Trp3 and Hphe3 analogues of (A), together with the excellent retention of V2 antagonism by the Tyr-NH2(9) and D-Cys6 analogues of (B) are promising new leads to the design of potent and possibly orally active V2 antagonists for use as pharmacological tools and/or as radioiodinatable ligands and for development as potential therapeutic agents for the treatment of the hyponatremia caused by the syndrome of the inappropriate secretion of the antidiuretic hormone (SIADH).

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).

Design, synthesis, and pharmacological characterization of fluorescent peptides for imaging human V1b vasopressin or oxytocin receptors

Among the four known vasopressin and oxytocin receptors, the specific localization of the V1b isoform is poorly described because of the lack of selective pharmacological tools. In an attempt to address this need, we decided to design, synthesize, and characterize fluorescent selective V1b analogues. Starting with the selective V1b agonist [deamino-Cys(1),Leu(4),Lys(8)]vasopressin (d[Leu(4),Lys(8)]VP) synthesized earlier, we added blue, green, or red fluorophores to the lysine residue at position 8 either directly or by the use of linkers of different lengths. Among the nine analogues synthesized, two exhibited very promising properties. These are d[Leu(4),Lys(Alexa 647)(8)]VP (3) and d[Leu(4),Lys(11-aminoundecanoyl-Alexa 647)(8)]VP (9). They remained full V1b agonists with nanomolar affinity and specifically decorated the plasma membrane of CHO cells stably transfected with the human V1b receptor. These new selective fluorescent peptides will allow the cellular localization of V1b or OT receptor isoforms in native tissues.

Discovery of novel selective hypotensive vasopressin peptides that exhibit little or no functional interactions with known oxytocin/vasopressin receptors

1. Arginine-vasopressin (VP) has both vasoconstricting and vasodilating action. We report here the discovery of four novel selective hypotensive VP analogues: d(CH2)5[D-Tyr(Et)2,Arg3,Val4]AVP; d(CH2)5[D-Tyr(Et)2,Lys3,Val4]AVP and their iodinatable Tyr-NH2(9) analogues. 2. Bioassays in rats for activities characteristic of neurohypophysial peptides showed that the four VP peptides possessed little or no V1a, V2 or oxytocin (OT) receptor agonistic or antagonistic activities. 3. In anaesthetized rats, these peptides (0.05-0.10 mg kg(-1) i.v.) elicited a marked fall in arterial blood pressure. 4. Blockade of cholinoceptors, adrenoceptors and bradykinin B2 receptors, and inhibition of prostaglandin synthesis had little effect on their vasodepressor action. 5. Classical V1a, V2 and OT receptor antagonists did not block the vasodepressor response. 6. L-NAME, 0.2 mg kg(-1) min(-1), markedly suppressed the hypotensive response to ACh but not the vasodepressor response to the hypotensive VP peptides. However, the duration of the vasodepressor response was shortened. Very high doses of L-NAME attenuated both the vasodepressor response and the duration of action. 7. These findings indicate that the vasodepressor action of these VP peptides is independent of the peripheral autonomic, bradykinin and PG systems and is not mediated by the known classical OT/VP receptors. NO does not appear to have an important role in their vasodepressor action. 8. The discovery of these novel VP peptides could lead to the development of new tools for the investigation of the complex cardiovascular actions of VP and the introduction of a new class of hypotensive agents. The two iodinatable hypotensive VP peptides could be radiolabelled as potential markers for the localization of the receptor system involved.

An investigation of position 3 in arginine vasopressin with aliphatic, aromatic, conformationally-restricted, polar and charged amino acids

We report the solid-phase synthesis and some pharmacological properties of 23 new analogs of arginine vasopressin (AVP) which have the Phe3 residue replaced by a broad variety of amino acids. Peptides 1-9 have at position 3: (1) the mixed aromatic/aliphatic amino acid thienylalanine (Thi) and the aliphatic amino acids; (2) cyclohexylalanine (Cha); (3) norleucine (Nle); (4) Leu; (5) norvaline (Nva); (6) Val; (7) alpha-aminobutyric acid (Abu); (8) Ala; (9) Gly. Peptides 10-23 have at position 3: the aromatic amino acids, (10) homophenylalanine (Hphe): (11) Tyr; (12) Trp; (13) 2-naphthylalanine (2-Nal); the conformationally-restricted amino acids (14) Pro; (15) 2-aminotetraline-2-carboxylic acid (Atc); the polar amino acids (16) Ser; (17) Thr; (18) Gln; and the charged amino acids (19) Asp; (20) Glu; (21) Arg; (22) Lys; (23) Orn. All 23 new peptides were evaluated for agonistic and, where appropriate, antagonistic activities in in vivo antidiuretic (V2-receptor) and vasopressor (V1a-receptor) assays and in in vitro (no Mg2+) oxytocic assays. The corresponding potencies (units/mg) in these assays for AVP are: 323+/-16; 369+/-6 and 13.9+/-0.5. Peptides 1-9 exhibit the following potencies (units/mg) in these three assays: (1) 379+/-14; 360+/-9; 36.2+/-1.9; (2) 294+/-21: 73.4+/-2.7; 0.33+/-0.02; (3) 249+/-28; 84.6+/-4.3; 4.72+/-0.16; (4) 229+19; 21.4+/-0.6; 2.1+/-0.2; (5) 134+/-5; 31.2+/-0.9; 28.4+/-0.2; (6) 114+/-9; 45.3+2.3; 11.3+/-1.6; (7) 86.7+/-2.5; 4.29+/-0.13; 0.45+/-0.03; (8) 15.5+/-1.5; 0.16+/-0.01; approximately 0.02: (9) 3.76+/-0.03; < 0.02; in vitro oxytocic agonism was not detected. These data show that the aliphatic amino acids Cha, Nle, Leu, Nva and Val are well-tolerated at position 3 in AVP with retention of surprisingly high levels of antidiuretic activity. Peptides 2-9 exhibit significant gains in both antidiuretic/vasopressor (A/P) and antidiuretic/oxytocic (A/O) selectivities relative to AVP. [Thi3]AVP appears to be a more potent antidiuretic and oxytocic agonist than AVP and is equipotent with AVP as a vasopressor agonist. The antidiuretic potencies of peptides 10-23 exhibit drastic losses relative to AVP. They range from a low of 0.018+/-0.001 units/mg for the Lys3 analog (peptide 22) to a high of 24.6+/-4.6 units,mg for the Hphe3 analog (peptide 10). Their vasopressor potencies are also drastically reduced. These range from a low of < 0.002 units/mg for peptide 22 to a high of 8.99+0.44 units/mg for the Atc3 analog (peptide 15). Peptides 10-23 exhibit negligible or undetectable in vitro oxytocic agonism. The findings on peptides 10-23 show that position 3 in AVP is highly intolerant of changes with aromatic, conformationally-restricted, polar and charged amino acids. Furthermore, these findings are in striking contrast to our recent discovery that position 3 in the potent V2/V1a/OT antagonist d(CH2)5D-Tyr(Et)2VAVP tolerates a broad latitude of structural change at position 3 with many of the same amino acids, to give excellent retention of antagonistic potencies. The data on peptides 1-4 offer promising clues to the design of more potent and selective AVP V2 agonists.