Synthesis and biological evaluation of antagonists of growth hormone-releasing hormone with high and protracted in vivo activities

Some antagonists of human growth hormone-releasing hormone (hGH-RH) synthes ized previously were shown to inhibit in vivo proliferation of various huma n cancers in nude mice. However, the activity of these analogs requires an increase to assure clinical efficacy, In an attempt to prepare hGH-RH antag onists with a high and protracted activity, we synthesized and biologically tested 22 antagonistic analogs of hGH-RH(1-29)NH2. The ability of the anta gonists to inhibit hGH-RH-induced GH release was evaluated in vitro in a su perfused rat pituitary system, as well as in vivo after i.v. injection into rats. The binding affinity of the peptides to GH-RH receptors also was det ermined. All antagonistic analogs had the common core sequence [PhAc-Tyr(1) ,D-Arg(2), Phe(4-Cl)(6) (para-chlorophenylalanine), Abu(15) (alpha-aminobut yric acid),Nle(27)] hGH-RH(1-29)NH2 and contained Arg, D-Arg, homoarginine (Har), norleucine (Nle), and other substitutions. The following analogs wer e determined to have a high and/or protracted antagonistic activity: [PhAc- Tyr(1),D-Arg(2),Phe(4-Cl)(6),Arg(9),Abu(15),Nle(27),D-Arg(29)] hGH-RH (1-29 )NH2 (JV-1-10), [PhAc-Tyr(1),D-Arg(2),Phe(4-Cl)(6),Abu(15),Nle(27) D-Arg(28 ),Har(29)] hGH-RH(1-29)NH2 (MZ-6-55), [PhAc-Tyr(1),D-Arg(2),Phe(4-Cl)(6), A rg(9),Abu(15),Nle(27),D-Arg(28),Har(29)] hGH-RH(1-29)NH2 (JV-1-36), and [Ph Ac-Tyr(1),D-Arg(2),Phe(4-Cl)(6), Har(9),Tyr(Me)(10),Abu(15),Nle(27),D-Arg(2 8),Har(29)]hGH-RH (1- 29)NH2 (JV-1-38). Among the peptides tested, analog J V-1-36 showed the highest GH-RH antagonistic activity in vitro and also ind uced a strong and prolonged inhibition of GH release in vivo for at least 3 0 min. The antagonist JV-1-38 was slightly less potent than JV-1-36 both in vitro and in vivo but proved to be very long-acting in vivo, suppressing t he GH-RH-induced GH release even after 60 min. High and protracted in vivo activities of these antagonists indicate an improvement over earlier GH-RH analogs, Some of these hGH-RH antagonists could find clinical applications in the treatment of cancers dependent on insulin-like growth factors I and II.