Zarandi, M., M. Kovacs, J. E. Horvath, K. Toth, G. Halmos, K. Groot, A. Nagy, Z. Kele, and A. V. Schally, Synthesis and in vitro evaluation of new potent antagonists of growth hormone-releasing hormone (GH-RH). Peptides 18(3) 423–430, 1997.—In the search for more potent antagonists of hGH-RH, 20 new analogs were synthesized, purified and tested in vitro. All the analogs were based on the N-terminal sequence of 28 or 29 amino acid residues of hGH-RH, but contained D-Arg2 and Nle27 modifications. Most analogs had Phe (pCl)6 and Agm29 substituents. The effect of other substitutions such as Abu8 and/or Abu15 and Ala 15 and various hydrophobic and hydrophilic d or l amino acids at position 8 were also investigated. All the peptides were acylated at the N-terminus in an attempt to increase the antagonistic activity. In the superfused rat pituitary cell system, most analogs inhibited more powerfully the GH release induced by GH-RH than the standard antagonist [Ac-Tyr1, D-Arg2]hGH-RH (1-29)-NH2. Some antagonists were long acting. Among the peptides synthesized, antagonist PhAc-[D-Arg2, Phe (pCl)6, Abu15, Nle27] hGH-RH (1-28)Agm (MZ-5-156) appeared to be the most potent and inhibited GH release in vitro 63-200 times more powerfully than the standard antagonist. MZ-5-156 and other antagonists showed high binding affinities to membrane receptors for GH-RH. Some of these hGH-RH antagonists could be further developed for possible onocological applications.