Pharmacological studies on novel neurotensin mimetics: Discovery of a pharmacologically unique agent exhibiting concentration-dependent dual effects as antagonist and agonist

We report the development of two novel neurotensin mimetics (mimics 1 and 2). These compounds were rationally designed and synthesized according to the multiple template approach. We present results of experiments designed to define their pharmacological profiles. In radioligand binding assays with murine neuroblastoma clone N1E-115, we determined the equilibrium dissociation constants for these compounds at the neurotensin receptor. The K_d values for mimic 1 and mimic 2 were 3.3 μM and 1.9 μM, respectively. Functionally, both mimetics antagonized the neurotensin-stimulated production of cGMP, with K_d values in the low micromolar range. Interestingly, mimic 2 displayed a dualistic pharmacological profile, which was concentration dependent. At doses in the 10-100 μM range, mimic 2 became a full agonist, stimulating cGMP production in N1E-115 cells with an EC₅₀ value of 19 μM. Furthermore, mimic 1 did not antagonize the cGMP response elicited by mimic 2. When the neurotensin receptor was desensitized with a neurotensin receptor agonist, mimic 2 failed to stimulate significant cGMP production. We propose that mimic 2 binds to a higher affinity site when acting as an antagonist and binds to a lower affinity and different site when acting as an agonist. Thus, mimic 2 would appear to represent a unique pharmacological tool to characterize the neurotensin receptor and its diverse binding sites in N1E-115 cells.