Enhanced stability and potency of novel growth hormone-releasing factor (GRF) analogues derived from rodent and human GRF sequences.

Native human GRF(1-44)-NH2(hGRF44) is subject to biological inactivation by both enzymatic and chemical routes. In plasma, hGRF44 is rapidly degraded via dipeptidylpeptidase IV (DPP-IV) cleavage between residues Ala2 and Asp3. The hGRF44 is also subject to chemical rearrangement (Asn8-->Asp8, beta-Asp8 via aminosuccinimide formation) and oxidation [Met27-->Met(O)27] in aqueous environments, greatly reducing its bioactivity. It is therefore advantageous to develop long-acting GRF analogues using specific amino acid replacements at the amino-terminus (to prevent enzymatic degradation): residue 8 (to reduce isomerization) and residue 27 (to prevent oxidation). Inclusion of Ala15 substitution (for Gly15), previously demonstrated to enhance receptor binding affinity, would be predicted to improve GRF analogue potency. Substitution of [His1,Val2]-(from the mouse GRF sequence) for [Tyr1,Ala2]-(human sequence) in [Ala15,Leu27]hGRF(1-32)-OH analogues completely inhibited (24-h incubation) DPP-IV cleavage and greatly increased plasma stability in vitro. Additional substitution of Thr8 (mouse GRF sequence), Ser8 (rat GRF sequence), or Gln8 (not naturally occurring) for Asn8 (human GRF sequence) resulted in analogues with enhanced aqueous stability in vitro (i.e., decreased rate of isomerization). These three highly stable and enzymatically resistant hGRF(1-32)-OH analogues, containing His1, Val2, Thr/Gln8, Ala15, and Leu27 replacements, were then bioassayed for growth hormone (GH)-releasing activity in vitro (rat pituitary cell culture) and in vivo (SC injection into pigs). Enhanced bioactivity was observed with all three hGRF(1-32)-OH analogues. In vitro, these analogues were approximately threefold more potent than hGRF44, whereas in vivo they were eleven- to thirteenfold more potent.(ABSTRACT TRUNCATED AT 250 WORDS)