Pharmacokinetic evaluation of superactive analogues of growth hormone-releasing factor (1-29)-amide.

D-amino acid-substituted analogues of growth hormone-releasing factor 1-29)-amide with superagonist activities in the rat were examined for increased plasma half-life and resistance to degradation in vivo. After IV injection, half-lives of the analogues were in the range 4.7-7.4 min, none of which was significantly different from that of the parent compound (6.2 min). Following SC injection, 4.6-7.2% of the dose of the analogues reached the circulation compared with 5.1% of the parent compound. Conformational restraints introduced into the N-terminal region of the molecule, which gave enhanced potency, did not alter the susceptibility of the peptide to proteolytic degradation.