Corticotropin releasing factor (CRF) is a 41-peptide amide which stimulates the release of ACTH (Vale et al. Science 1981, 213, 1394). CRF has been postulated to assume an alpha-helical conformation upon binding to its pituitary receptor (Hernandez et al. J. Med. Chem. 1993, 36, 2860). We have exploited this hypothesis in the design of a limited series of cyclic analogues and have taken into consideration the effects of side-chain deletion (Alanine scan, Kornreich et al. J. Med. Chem. 1992, 35, 1870) as well as of changes in chirality (Rivier et al. J. Med. Chem. 1993, 36, 2851), with the rationale that side chains necessary for binding could also be replaced by side-chain bridges. In particular, we have used computer modeling to predict likely side chain bridging opportunities and evaluated the effects of such replacements by correlating biological results with those derived from CD spectroscopy. We have synthesized 38 monocyclic peptide amides, competitive antagonists of human/rat CRF, using solid-phase methodology on MBHA resin. After purification by preparative RP-HPLC, the peptides were analyzed by RP-HPLC and capillary zone electrophoresis and characterized by mass spectroscopy and amino acid analysis. CRF antagonists were tested for their ability to interfere with CRF-induced release of ACTH by rat anterior pituitary cells. In most cases, one of the bridge heads was located at a position where substitution by a D-residue was tolerated (i.e., positions 12 and 20). It has become clear that careful optimization of bridge length and chirality is critical. This is best exemplified by the fact that out of the 38 analogues that were synthesized and tested, only two, [cyclo(20-23)[DPhe12,Glu20,Lys23, Nle21,38]h/rCRF12-41 and cyclo(20-23)[DPhe12,Glu20,Orn23,Nle21,38] h/rCRF12-41], were found to be more potent (3 and 2 times, respectively) than [DPhe12,Nle21,38]h/rCRF12-41, the parent compound. Six analogues belonging to two different families were found to be half as potent as the standard, 18 had 2-20% of the potency of the standard, and the others were significantly less potent. CD results of all analogues in 50% TFE (a concentration of TFE that induced nearly maximum helicity of [DPhe12,Nle21,38]h/rCRF12-41) suggest that while helicity may be an important factor for CRF analogue recognition, little correlation is found between percent helicity as determined by spectral deconvolution and biological activity in vitro.