NOVEL-APPROACH TO THE DESIGN OF SYNTHETIC RADIOIODINATED LINEAR V1A RECEPTOR ANTAGONISTS OF VASOPRESSIN

We report the solid phase synthesis of six analogs of the potent and selective linear AVP vasopressor (V1a receptor) antagonist: Phaa1-D-Tyr(Et)2-Phe3-Gln4-Asn5-Lys6-Pro7-Arg-NH28 (A) (where Phaa = phenylacetyl) in which the Phaa1 residue is replaced by hydroxyphenylacetyl (HO-Phaa), hydroxyphenylpropionyl (HO-Phpa) and phenylpropionyl (Phpa) and the D-Tyr(Et)2 and Lys6 residues by D-Tyr(Me)2 and Arg6 substituents. The phenolic-containing peptides were synthesized to test the feasibility of using this approach for the design of high affinity selective ligands for AVP V1a receptors. The following analogs of A were synthesized: 1. [(HO)Phaa1]; 2. [(HO)Phaa1,D-Tyr(Me)2]; 3. [(HO)Phaa1,D-Tyr(Me)2,Arg6]; 4. [(HO)Phaa1,Arg6]; 5. [Phpa1]; 6. [(HO)Phpa1]. All six peptides were examined for agonistic and antagonistic potencies in vasopressor (V1a-receptor) and antidiuretic (V2-receptor) and in vitro oxytocic assays in rats. The affinities of the phenolic-containing peptides for hepatic V1a and uterine receptors were also determined. The phenolic-containing peptides all exhibit potent V1a antagonism. Their anti-V1a pA2 Values range from 8.23 to 8.63 (the anti-V1a pA2 value of A = 8.69). Their inhibition constants (Ki in nM) range from 0.4 to 1.0. They are weak antidiuretic agonists with activities ranging from 0.022 U/mg to 0.13 U/mg (A = 0.033 U/mg). They all exhibit OT antagonism in vitro. Their anti-OT pA2 values range from 7.28 to 7.71 (A = 7.62). All five phenolic compounds were iodinated using iodine chloride and tested in the same in vivo and in vitro assay systems. The iodinated derivatives all exhibited potent V1a antagonism and OT antagonism. With Ki values in the liver of 0.1 nm the affinities of the iodinated derivatives of peptides 3 and 6 are particularly impressive. These two peptides are very promising candidates for radioiodination as potential high affinity, selective ligands for V1a receptors. Thus this approach which complements the traditional approach of utilizing tyrosine substitutions, offers promise for the design of high affinity radioligands for AVP V1a receptors, AVP pituitary (V1b) receptors, AVP V2 receptors and OT uterine receptors. It may also have application for the design of radioligands for other biologically active peptides.