Exploring 6-Azaindole and 7-Azaindole Rings for Developing Cannabinoid Receptor 1 Allosteric Modulators.
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Introduction and Objective: Org27569 is a prototypical allosteric modulator of the cannabinoid receptor 1 (CB1). It belongs to the indole-2-carboxamide scaffold and has been intensively investigated in pharmacology and in structure-activity relationship (SAR) studies. Although azaindoles are rare in natural products and differ only by the presence of an extra ring nitrogen, they were demonstrated as valuable bioisosteres in many pharmacologically important molecules. To extend the SAR investigation of the indole-2-carboxamide class of CB1 allosteric modulators, azaindole (pyrrolopyridine) rings were used to replace the indole ring of Org27569 analogs to explore the potential of azaindole-2-carboxamides as CB1 allosteric modulators. Using 6- and 7-azaindole in lieu of the indole moiety within this class of CB1 allosteric modulators indeed improved the aqueous solubility. Materials and Methods: We synthesized 6- and 7-azaindole-2-carboxamides and their indole-2-carboxamide counterparts. The molecules were evaluated by [3H]CP55,940 binding and [35S]GTPS binding assays for their allosteric modulation of the CB1 receptor. Results: The 7-azaindole-2-carboxamides lost the ability to bind to the CB1 receptor. The 6-azaindole-2-carboxamides (e.g., 3c and 3d) showed markedly reduced binding affinities to the CB1 receptor in comparison with their indole-2-carboxamide counterparts. However, they behaved similarly as indole-2-carboxamides in potentiating the orthosteric agonist binding and inhibiting the orthosteric agonist-induced G-protein coupling. The results indicated that some azaindole scaffolds (e.g., 6-azaindole) are worth further exploration, whereas the 7-azaindole ring is not a viable bioisostere of the indole ring in the Org27569 class of CB1 allosteric modulators.
