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In the course of our studies toward the development of novel analogs of the decapeptide gonadotropin releasing hormone (GnRH), we have examined a hexapeptide that is an antagonist of endothelin (ET). It was found that this peptide, Ac-D-Trp-Leu-Asp-Ile-Ile-Trp (peptide 1), binds specifically to the pituitary GnRH receptor. Moreover, peptide 1 exhibits a GnRH agonistic activity (i.e., it induces luteinizing hormone release from rat pituitary). This activity is mediated directly by the GnRH receptor and is suppressed by a GnRH antagonist. Removal of the acetyl group of peptide 1 results in a hexapeptide (peptide 2) with binding properties similar to those of GnRH but with a diminished affinity toward the ET receptor. Several other ET antagonists were screened for a potential interaction with the GnRH receptor. Two of these, the hexapeptide PD145065 and the cyclic pentapeptide BQ-123, expressed GnRH agonistic activity at micromolar concentrations in vitro. BQ-123, previously approved for trials on humans as an ET antagonist, is demonstrated to act in vivo as a GnRH agonist, in a dose that was demonstrated previously as the minimal required dose for significant ET antagonism. The GnRH agonistic activity of ET antagonists may therefore result in interference with the physiological control of the reproductive system. Such effects may be most severe when ET antagonists are used chronically. Thus, the major practical message of this study is the need to circumvent the potential side effects of ET antagonist-based drugs.

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The biological activity of a new synthetic analog of vasopressin, deamino[D-3-(3'-pyridyl)-Ala2, Arg8] vasopressin, was assessed in a number of assays. Antidiuretic (V2) and vasoconstrictor (V1), agonist and antagonist activities were assessed in rats in vivo. Corticotropin-releasing activity was assessed with cultured dissociated ovine anterior pituitary cells in vitro and in sheep in vivo. Compared to vasopressin, the analog is a weak agonist at antidiuretic receptors (1/381 compared to AVP); it is a weak antagonist of the vasoconstrictor response (pA2 = 6.22). Nonetheless, the analog is a full, relatively potent agonist at pituitary corticotrope receptors (relative potency of 1/36). These data indicate that analogs of vasopressin can be synthesized which are relatively selective for agonist activity at pituitary vasopressin receptors, and in doing so, further support the contention that the pituitary receptor is quite distinct from the classical V1 receptor.

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