SYNTHESIS AND BIOLOGICAL-ACTIVITIES OF HIGHLY POTENT ANTAGONISTS OF GROWTH HORMONE-RELEASING HORMONE

In the search for antagonists of human growth hormone-releasing hormon e (hGHRH) with high activity, 22 analogs were synthesized by solid-pha se methods, purified, and tested biologically. Within the N-terminal s equence of 28 or 29 amino acids of hGHRH, all the analogs contained D- Arg(2), Phe(4-Cl)(6) (para-chlorophenylalanine), Abu(15)(alpha-aminobu tyric acid), and Nle(27) and most of them had Agm(29) (agmatine) subst ituents. All the peptides, except one, were acylated at the N terminus with different hydrophobic acids-e.g., isobutyric acid (Ibu) or 1-nap hthylacetic acid (Nac) in order to study the effect of N-terminal acyl ation on the antagonistic activity. In the superfused rat pituitary ce ll system, all the analogs inhibited more powerfully the GHRH-induced growth hormone (GH) release than the standard GHRH antagonist [Ac-Tyr( 1),D-Arg(2)]hGHRH-(1-29)NH2. Antagonists [Ibu(0),D-Arg(2),Phe(4-Cl)(6) , Abu(15),Nle(27)]hGHRH-(1-28)Agm (MZ-4-71), [Nac(0),D-Arg(2),Phe(4-Cl )(6), Abu(15),Nle(27)]hGHRH-(1-28)Agm (M2-4-243), g(2),Phe(4-Cl)(6),Ab u(15),Nle(27)]-hGHRH-(1-29)NH2 (MZ-4-169), (0)-His(1),D-Arg(2),Phe(4-C l)(6),Abu(15),Nle(27)]- hGHRH-(1-29)NH2 (MZ-4-181), and e(4-Cl)(6),Abu (15),Nle(27),Asp(28)]hGHRH-(1-28)Agm (MZ-4-209) inhibited GH release a t 3 x 10(-9) M. Among these peptides, MZ-4-243, MZ-4-169, and MZ-4-181 were also long acting in vitro, Antagonist MZ-4-243 inhibited GH rele ase 100 times more powerfully than the standard antagonist and was the most potent in vitro among GHRH antagonists synthesized. Analogs with high inhibitory effects in vitro were also found to have high affinit ies to rat pituitary GHRH receptors, In experiments in vivo, antagonis ts g(2),Phe(4-Cl)(6),Abu(15),Nle(27)]-hGHRH-(1-28)Agm (MZ-4-71), rg(2) ,Phe(4-Cl)(6),Abu(15),Nle(27)]hGHRH-(1-29)NH2 (MZ-4-169), and [Nac(0)H is(1),D-Arg(2),Phe(4-Cl)(6), Abu(15),Nle(27)]hGHRH-(1-29)NH2 (MZ-4-181 ) induced a significantly greater inhibition of GH release than the st andard antagonist. In view of their high antagonistic activity and pro longed duration of action, some of these antagonists of GHRH may find clinical applications, including treatment of certain endocrine disord ers and insulin-like growth factor I-dependent tumors.