Design of biologically active, conformationally constrained GnRH antagonists

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Abstract
The introduction of conformational constraints into a flexible peptide hormone can be exploited to develop models for the conformation required for receptor binding and activity. In this review, we illustrate this approach to analog design using our work on antagonists of gonadotropin‐releasing hormone (GnRH). Design of a conformationally constrained, competitive antagonist of GnRH, cyclo[Δ3,4 Pro‐D4ClPhe‐DTrp‐Ser‐Tyr‐DTrp‐NMeLeu‐Arg‐Pro‐βAla], led to the prediction of its bioactive conformation. Template forcing experiments show that this conformation is accessible to other active GnRH analogs. Two‐dimensional NMR studies verified the predicted conformation in solution. The predicted binding conformation has recently been used to design two new analogs incorporating side chain–side chain linkages suggested by the conformational model: These analogs were synthesized and the one predicted to be most similar to the parent conformation had equivalent potency while the second, designed to refine the conformational hypothesis, was found to exhibit enhanced potency, thus confirming the original binding conformation hypothesis. These compounds and their derivatives now provide a new class of GnRH antagonists possessing both high biological potency and limited conformational flexibility, thus making them ideal for both biophysical and structure–activity studies.