Astressin analogues (corticotropin-releasing factor antagonists) with extended duration of action in the rat

In earlier reports we identified specific point substitutions (DPhe12,Nle21,38), cyclization strategies [in particular, introduction of lactam rings such as that of cyclo(Glu30,Lys33)], and deletions (residues 1-7) in the CRF molecule that led to agonists. We also noted that further deletions (residues 8-14) produced antagonists such as astressin ¿cyclo(30-33)[DPhe12,Nle21,38, Glu30, Lys33]hCRF(12-41)¿ (1). We hypothesized that the lactam ring promoted conformational stability to yield analogues with increased potency both in vitro and in vivo as compared to that of their linear counterparts. Additionally, we reported that cyclo(30-33)[DPhe12,Nle21,38, Glu30,DHis32,Lys33]hCRF(12-41) (3) and dicyclo(26-36,30-33)[Ac-Asp9,DPhe12,Nle21,38, Cys26, Glu30,Lys33, Cys36]hCRF(9-41) were ca. twice and 1/100 as potent as astressin, respectively, suggesting a putative turn that encompasses residues 30-33 (previous paper: Koerber et al. J. Med. Chem. 1998, 41). To increase the potency of 1 and/or 3 in vivo, we extended their chain length by one (5-8), two (9, 10), and three (11, 12) residues at the N-terminus and acetylated (6, 8, 10, 12). Of the compounds tested for duration of action (1, 3-6, 8), we found 6 and 8 to be slightly longer-acting than astressin or [DHis32]astressin, while their potencies in vitro were not significantly different from that of 3. Additionally, we introduced CalphaMe-leucine residues in lieu of leucine at positions 14, 15, 19, 27, and 37 in [DHis32]astressin. The analogue [CalphaMe-Leu27,DHis32]astressin (16) was more potent (although not statistically in all cases) than the other four analogues in vitro. While acetylation of the N-terminus of 16 (i.e., 18) or of [CalphaMe-Leu27]astressin (i.e., 19) did not have a significant effect on in vitro potency, elongation of the N-terminus by one or three residues in addition to acetylation resulted in cyclo(30-33)[DPhe12,Nle21,CalphaMe-Leu27,Glu3 0,DHis32,Lys33, Nle38]Ac-hCRF(11-41) (21), cyclo(30-33)[DPhe12,Nle21,CalphaMe-Leu27, Glu30,Lys33,Nle38]Ac-hCRF(9-41) (22), and cyclo(30-33)[DPhe12, Nle21, CalphaMe-Leu27,Glu30,DHis32,Lys33,Nle38 ]Ac-hCRF(9-41) (23) that were longer-acting than 6 and 8 (ca. 2 h inhibition of ACTH secretion at 25 micrograms/adrenalectomized rat). Analogues 22 and 23 were also more potent than astressin at reversing intracisternal CRF- and abdominal surgery-induced delay of gastric emptying in conscious rats.