Conformationally restricted TRH analogs: a probe for the pyroglutamate region.

In principle, the development of the active analog approach to computer-assisted drug design can offer a unique opportunity for dramatically increasing the efficiency of designing new drug candidates by predicting the biologically active conformation of peptide ligands.1 But how do we know that such molecular modeling predictions are accurate, and can a specific set of chemical probes be developed for answering this question? In the mid-1980s, Marshall and Font initiated an inquiry into this area by using the active analog approach to design a series of conformationally restricted thyrotropin-releasing hormone (thyroliberin, TRH) analogs represented by the general structure of III (Scheme 1).2 From the start, it was realized that TRH would make an ideal test case for probing the effectiveness of the active analog approach because of the important role of TRH in regulating functions of the anterior pituitary gland and the nervous system,3 the fact that TRH contains only six-rotatable bonds, and the existence of activity data for a large number of TRH analogs.4,5 Due to the increased difficulty associated with the synthesis of imidazole-containing peptide analogs, the initial restricted analogs were designed to mimic the [Phe2]TRH analog of TRH. Although the affinity and potency of this analog were significantly lower than TRH itself (see Table 1), at high concentrations [Phe2]TRH completely displaced TRH from its G protein-coupled receptor (TRH-R) and led to the same maximal extent of TRH-R stimulation as TRH. For this reason, [Phe2]TRH was judged to be a reasonable model for TRH, at least until it was determined if an added conformational constraint would still allow for binding of the analog to TRH-R. The constrained analogs were designed by replacing † Department of Chemistry, Washington University. ‡ Cornell University Medical College. § Department of Molecular Biology and Pharmacology, Washington University. ⊥ Center for Molecular Design, Washington University. Table 1. Binding and Activation of TRH-R Receptors by TRH Analogs