Discovery of cGAS Inhibitors for Interferon-Driven Autoimmune Diseases

SUMMARY Cytoplasmic nucleic acids alert the immune system to invading pathogens and trigger a robust type I interferonIFNresponse via activation of the STINGstimulator of interferon genesreceptorThe sensor for cytoplasmic nucleic acids was recently discovered to be cyclic GMP AMP synthasean enzyme that produces a unique cyclic dinucleotide second messengercGAMPthat serves as an agonist for the STING receptorAberrant activation of the cGAS STING pathway is rapidly emerging as an important underlying cause of debilitating and sometimes fatal autoimmune disorders including Aicardi Goutieres SyndromeAGSa monogenic encephalopathy that is usually fatal before adulthoodand systemic lupus erythematosusSLEDevelopment of cGAS inhibitors is clearly a therapeutic strategy that should to be exploredThere are no drugs approved specifically for any of the IFN driven autoimmune diseasesand current IFN targeted therapies in clinical development are mostly biologicalse gantibodies against IFNor the type IFN receptorInhibiting cGASthe molecular trigger for type I IFN inductionis likely to be more efficient than blocking downstream targetsand a small molecule cGAS drug would have obvious advantages in terms of costdosing and CNS exposureAberrant activation of cGAS by dsDNA is the trigger for a constitutive type I IFN responseresulting in autoimmune diseases such AGS and SLE with no curative therapiesWe developed an HTS compatible cGAS enzymatic assay and used it to screen ak diversity library resulting in the identification of four novel cGAS inhibitor chemotypesFollowing the primary screenwe performed iterative rounds of SAR informed by computational modeling and medicinal chemistry expertise to prioritize two chemotypes with different mechanisms of actionand to increase potency into the nanomolar rangeDuring the course of this work we uncovered interplay between cGAS sensivity to activation by nucleic acids and its modulation by small molecules that could inform the development of drugs with a favorable therapeutic windowThe goals of this Phase I proposal are to use structure based design to improve the potency and ADME PK properties of the lead chemotypes and to demonstrate target engagement and efficacy in cellsWe have made substantial progress toward these goalsincluding generation of a high resolution co crystal X ray structure for one of the lead chemotypes and demonstration of specific inhibition of cGAS driven IFN gene expression in human monocytesIn Phase Iwe will continue to improve the potency and other drug like properties of the two lead chemotypes using structure based design enabled by a more powerful computational methodIn additionwe will use stem cell derived neural cell models and primary human peripheral immune cells to test inhibitor efficacy in a disease relevant contextSuccessful completion of these aims will clearly establish feasibility for Phase II in vivo efficacy studies in a mouse model for AGSa key milestone for clinical translationBlocking cGAS with a small molecule could lead to a curative therapy for AGS and would likely spur development of other drugs targeting the cGAS STING pathway with potential impact on millions of people suffering from debilitating autoimmune diseasesThis is a multidisciplinary lead discovery effort that will combine BellBrookandapos s extensive enzymology and screening expertisemedicinal chemistry and translational research expertise from David Maloney and Matt Boxer of Nexus Discovery AdvisorsFrederickMDLOS and bios attachedstructural and biophysical expertise from XTAL BioStructuresNatickMALOS attachedand computational chemistry expertise from SilcsBioBaltimoreMDLOS attachedDrKeith ElkonCodirector of the Center for Innate Immunity and Immune Disease and Head of Rheumatology at University of Washington School of Medicine is serving as a consultantLOS and bio attachedUnder a separate SBIR grantRGMin collaboration with DrElkonBellBrook is developing methods for detecting cGAMP in cell and tissue samples to enable monitoring of cGAS inhibition in animal modelsand eventually for stratification and monitoring of patients in clinical studiese gAGS patients or SLE patients with high levels of cGAMP in PBMCs as candidates for cGAS inhibitorsThe availability of a companion diagnostic would increase the potential medical impact and value of a cGAS inhibitor drug substantially Narrative Aberrant activation of the innate immune machinery that senses DNA from microbial pathogens results in debilitating autoimmune diseases such as lupus and congenital encephalopathiesWe are proposing to develop lead molecules targeting a recently discovered enzymecyclic GAMP synthasethat serves as the trigger for autoimmunity in response to self DNA