Biased Agonism In GPCR Drug Discovery: Application To Somatostatin Agonists

DESCRIPTION (provided by applicant): Analogs of the neuropeptide somatostatin are important therapeutics for the treatment of hormone secreting tumors with annual sales in excess of 1.3B. However, currently available peptide depots are only effective in approximately half the patients with growth hormone secreting tumors and patients with carcinoid tumors rapidly become resistant to the drug. These agents act by stimulating a G protein coupled receptor (GPCR) sst2A to activate Gi, but they also cause desensitization and internalization of the receptor resulting in reduced responsiveness. We hypothesize that biased agonists of somatostatin receptor that maintain strong Gi activation but do not induce internalization or desensitization would normalize hormonelevels in a greater percentage of patients and in patients not adequately controlled by currently available agents. Here we propose to test this hypothesis by using assays for receptor internalization and site-specific phosphorylation to guide medicinal chemistry optimization of nonpeptide orally active somatostatin biased agonists with the goal of providing improved therapeutic options for many patients with these tumors. This approach is premised on our recent observations that the nonpeptide L-779,976 is a biased somatostatin receptor agonist with strong Gi activation, but more rapid release of recruited 2-arrestin and reduced loss of cell surface receptor compared to peptide agonists. In Phase I we propose to extend this pharmacologic characterization to a diverse panel of nonpeptide somatostatin agonists with the goal of demonstrating feasibility of the assays to support medicinal chemistry and prioritizing lead chemical series for subsequent optimization of both pharmacologic and pharmaceutical properties in Phase II. This will include rigorous measurement of their intrinsic efficacy and ability to induce receptor desensitization-- fundamental pharmacologic data that is surprisingly lacking in the literature for this important class of therapeutics. Theproduct resulting from the Phase II efforts will be a novel orally available compound (or compounds) ready for preclinical toxicology studies in preparation to begin clinical development. In addition to improved clinical efficacy such oral agents would also reduce the need for physician office visits, eliminate the pain and discomfort of depot injections, and lower manufacturing costs compared to expensive peptide depot formulations. Beyond the creation of a novel therapeutic agent, this approach is innovative in the prospective use of receptor regulatory assays to guide early medicinal chemistry efforts, rather than the retrospective analysis of one or two compounds that have already succeeded in the clinic. GPCRs share many common regulatory and signalingmechanisms and are both the largest gene family in the human genome and a rich source of proven targets for drug discovery. Therefore, if successful, the impact of this work not only be to provide improved agents for patients with hormone secreting tumors, but it will also exemplify a novel and general strategy for optimizing agonist drugs targeting other GPCRs. PUBLIC HEALTH RELEVANCE: This project uses assays for receptor internalization, site specific phosphorylation, desensitization and intrinsic efficacy to guide design and synthesis of novel orally available biased agonists of the somatostatin receptor sst2A with improved efficacy and reduced desensitization for the treatment of hormone secreting tumors. If successful, this work would provide ageneral strategy for agonist optimization of many additional GPCR drug targets.