Synthesis of new potent agonistic analogs of growth hormone-releasing hormone (GHRH) and evaluation of their endocrine and cardiac activities

Abstract

In view of the recent findings of stimulatory effects of GHRH analogs, JI-34, JI-36 and JI-38, on cardiomyocytes, pancreatic islets and wound healing, three series of new analogs of GHRH(1-29) have been synthesized and evaluated biologically in an endeavor to produce more potent compounds. "Agmatine analogs", MR-356 (N-Me-Tyr(1)-JI-38), MR-361(N-Me-Tyr(1), D-Ala(2)-JI-38) and MR-367(N-Me-Tyr(1), D-Ala(2), Asn(8)-JI-38), in which Dat in JI-38 is replaced by N-Me-Tyr(1), showed improved relative potencies on GH release upon subcutaneous administration in vivo and binding in vitro. Modification with N-Me-Tyr(1) and Arg(29)-NHCH3 as in MR-403 (N-Me-Tyr(1), D-Ala(2), Arg(29)-NHCH3-JI-38), MR-406 (N-Me-Tyr(1), Arg(29)-NHCH3-JI-38) and MR-409 (N-Me-Tyr(1), D-Ala(2), Asn(8), Arg(29)-NHCH3-JI-38), and MR-410 (N-Me-Tyr(1), D-Ala(2), Thr(8), Arg(29)-NHCH3-JI-38) resulted in dramatically increased endocrine activities. These appear to be the most potent GHRH agonistic analogs so far developed. Analogs with Apa(30)-NH2 such as MR-326 (N-Me-Tyr(1), D-Ala(2), Arg(29), Apa(30)-NH2-JI-38), and with Gab(30)-NH2, as MR-502 (D-Ala(2), 5F-Phe(6), Ser(28), Arg(29),Gab(30)-NH2-JI-38) also exhibited much higher potency than JI-38 upon i.v. administration. The relationship between the GH-releasing potency and the analog structure is discussed. Fourteen GHRH agonists with the highest endocrine potencies were subjected to cardiologic tests. MR-409 and MR-356 exhibited higher potency than JI-38 in activating myocardial repair in rats with induced myocardial infarction. As the previous class of analogs, exemplified by JI-38, had shown promising results in multiple fields including cardiology, diabetes and wound healing, our new, more potent, GHRH agonists should manifest additional efficacy for possible medical applications.

Keywords: Cardioprotection; hGHRH agonist; hGHRH(1–29); s.c. administration.

Fig. 1

HPLC analysis of crude sample MR-409 on Agilent Zorbax 300SB C18 column (2.1 mm × 50 mm, particle size 5 μm), UV detector 220 nm, sample injection 20 μg/25 μl, Eluent A: 0.1% trifluoroacetic acid in water, Eluent B: 0.1% trifluoroacetic acid, 70% acetonitrile in water, linear gradient from 20% B to 60% B in 40 min. MR-409 peak purity: 81%.

Fig. 2

Molecular weight of crude sample MR-409 analyzed by Agilent 6210 time-of-flight LC/MS. Expected molecular weight of MR-409 is 3394.89, detected molecular weight of MR-409 is 3395.84.

Fig. 3

Impact of GHRH agonists on cardiac function. Bar graphs showing the percentage of change in ejection fraction (EF%) at 4 weeks relative to day 2 post-MI. All values are shown as mean ± SEM (*p < 0.05, ^†p < 0.01 vs. Placebo, n = 6–12).

Fig. 4

Impact of GHRH agonists on myocardial infarct burden (MI%). Bar graphs showing percentage of infarct size. Note that MI% was significantly attenuated in the groups treated with JI-38, MR-356 and MR-409. All values are shown as mean ± SEM (*p < 0.05 vs. Placebo, n = 6–12).