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N5-Pyrazinylcarbonylornithine, an Effective Substituent for Position 5 of Antagonists of LHRH

Computer modeling was used to find an effective substituent for position 5 in antagonists of the luteinizing hormone-releasing hormone (LHRH). In particular, it was desired to replace cis-3-(4-pyrazinylcarbonylaminocyclohexyl)alanine in position 5 because this substituent is laborious to synthesize. Calculations revealed that N5-pyrazinylcarbonylornithine (PzOrn) should be a suitable substitute for cPzACAla, with N6-pyrazinylcarbonyllysine (PzLys) being a second choice. Twelve analogs were synthesized in four series with DNal or DQal in position 1 and ILys or Arg in position 8. Biological assays validated the calculations and show that antagonists with PzOrn are more potent in antiovulatory assay than antagonists with PzLys. The antagonists with PzOrn were comparable in AOA with antagonists with cPzACAla. What is also important, antagonists with PzOrn released significantly less histamine than those with cPzACAla.