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 (V 1a receptor)antagonist: Phaa 1 ‐ d ‐Tyr(Et) 2 ‐Phe 3 ‐Gln 4 ‐Asn 5 ‐Lys 6 ‐Pro 7 ‐Arg‐NH 2 8 (A) (where Phaa = phenylacetyl) in which the Phaa 1 residue is replaced by hydroxyphenylacetyl (HO‐Phaa), hydroxyphenylpropionyl (HO‐Phpa) and phenylpropionyl (Phpa) and the d ‐Tyr(Et) 2 and Lys 6 residues by d ‐Tyr(Me) 2 and Arg 6 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 V 1a receptors. The following analogs of A were synthesized: 1. [(HO)Phaa 1 ]; 2. [(HOPhpa 1 , d ‐Tyr(Me) 2 ]; 3. [(HO)Phaa 1 , d ‐Tyr(Me) 2 ,Arg 6 ]; 4. [(HO)Phaa 1 ,Arg 6 ]; 5. [Phpa 1 ]; 6. [(HO)Phpa 1 ]. All six peptides were examined for agonistic and antagonistic potencies in vasopressor (V 1a ‐receptor) and antidiuretic (V 2 ‐receptor) and in vitro oxytocic assays in rats. The affinities of the phenolic‐containing peptides for hepatic V 1a and uterine receptors were also determined. The phenolic‐containing peptides all exhibit potent V 1a antagonism. Their anti‐V 1a pA 2 values range from 8.23 to 8.63 (the anti‐V 1a pA 2 value of A = 8.69). Their inhibition constants (Ki in n m ) 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 pA 2 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 V 1a 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 Vla receptors. Thus this approach which complements the traditional approach of utilizing tyrosine substitutions, offers promise for the design of high affinity radioligands for AVP V 1a receptors, AVP pituitary (V 1b ) receptors, AVP V 2 receptors and OT uterine receptors. It may also have application for the design of radioligands for other biologically active peptides.