We report the synthesis and biological activity of a series of analogues of the vasopressin antagonists [Pmp1,D-Tyr(Et)2,Val4]arginine-vasopressin (1) and [Pmp1,D-Tyr(Et)2,Val4,desGly9]arginine-vasopressin (2), where part or all of the tripeptide tail has been replaced by a simple alkyldiamine [NH(CH2)nNH2] or (aminoalkyl)guanidine [NH(CH2)nNHC(= NH)NH2] in order to examine the effects that variation of the length and orientation of the tripeptide tail have on renal vasopressin (V2) receptor antagonist activity. The results show that the entire tripeptide tail (Pro-Arg-Gly-NH2) can be replaced by an alkyldiamine or an (aminoalkyl)guanidine, compounds 15 and 16, respectively, indicating that there is no orientational requirement for the basic functional group coming off the cyclic hexapeptide ring. Also, there seems to be an "optimal" distance between the basic functional group and the hexapeptide ring since receptor affinity of the antagonists begins to fall off when the basic functional group is too close (compound 13) or extends too far (compounds 8-10) from the hexapeptide ring. These results suggest all that is necessary for retention of antagonist affinity and potency is a basic functional group, amine or guanidine, extended an optimal distance from the hexapeptide ring.
In a continuing effort to design more potent renal vasopressin (V2 receptor) antagonists, we have focused our attention on the carboxy-terminal tripeptide tail (Pro-Arg-Gly-NH2), a fragment common to both agonists and antagonists. Vasopressin antagonist analogues having a dibasic dipeptide tail, e.g., Arg-Arg-NH2 or Arg-Lys-NH2, attached directly to the cyclic hexapeptide ring are potent V2-receptor antagonists. Similar modification of a representative agonist drastically reduces its potency. We report the synthesis and pharmacological properties of a series of potent V2-receptor antagonists 3-9 where a combination of D or L dibasic dipeptide has been utilized to replace the common tripeptide fragment. Our results suggest a difference in the way agonists and antagonists bind to vasopressin receptor and further support the difference in the structure-activity relationships of agonists and antagonists. These results provide potentially useful insights for the design of novel V2-receptor antagonists.
Vasopressin antagonist analogs having alanine or glycine at position 7 were essentially equipotent with analogs with proline, N-methylalanine or sarcosine at position 7. This demonstrates that the conformational constraint imposed by an N-alkyl residue at position 7 is not necessary for binding of antagonist to the receptor, the exact opposite of what is seen in agonists. This suggests that antagonists bind t o the receptor in a manner which is very different from that of agonists.
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