FTX-1821 for Treatment of FSHD at its Root Cause: Results of a Clinical Trial in a Dish (S23.006)

Objective: Assess FTX-1821 inhibition of DUX4 activity and cell death in a population of FSHD1 and FSHD2 patient-derived skeletal muscle cell lines. Background: Facioscapulohumeral dystrophy (FSHD) is caused by loss of repression at the D4Z4 locus resulting in mis-expression of the homeobox transcription factor DUX4. DUX4 expression in skeletal muscle activates its downstream transcriptional program resulting in cell death, loss of skeletal muscle and progressive motor disability. Using optimized in vitro myotube culture systems and robust assays to screen a small molecule probe library, we identified and validated a drug target that, when inhibited, reduces or prevents expression of DUX4 and cell death. FTX-1821 is a potent and highly selective small molecule inhibitor of this target resulting in the reduction of DUX4 activity and its downstream transcriptional program preventing cell death without impacting myogenesis. Design/Methods: A clinical trial in a dish was prospectively designed to investigate the efficacy of FTX-1821. Eleven primary FSHD (8 FSHD1 and 3 FSHD2) and 3 control patient-derived skeletal muscle cell lines were treated with FTX-1821. Four concentrations were tested: 30, 100, 300 and 1000 nM compared to vehicle during myoblast differentiation into myotubes. Investigators were blinded to treatment and FSHD type. DUX4 activity was assessed by measuring mRNA levels of MBD3L2 , a well characterized DUX4 target gene. Activated capsase-3 was measured to quantify activation of cell death driven by DUX4 expression. Results: FTX-1821 demonstrated a dose-dependent decrease in MBD3L2 in both FSHD1 and FSHD2 patient-derived skeletal muscle myotubes. Cell death was also strongly inhibited. Conclusions: Treatment with FTX-1821 results in potent dose-dependent reduction of DUX4 activity and prevention of cell death across the full spectrum of patient-derived skeletal muscle cells tested. These results support advancing FTX-1821 into clinical trials for treatment of FSHD at its root cause. Disclosure: Dr. Mellion has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics, Vertex Pharmaceuticals. Dr. Mellion has received compensation for serving on the Board of Directors of Fulcrum Therapeutics, Vertex Pharmaceuticals. Dr. Mellion holds stock and/or stock options in Vertex Pharmaceuticals which sponsored research in which Dr. Mellion was involved as an investigator. Dr. Mellion holds stock and/or stock options in Vertex Pharmaceuticals. Dr. Mellion has received research support from Fulcrum Therapeutics, Vertex Pharmaceuticals. Dr. Valentine has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Valentine has received research support from Fulcrum Therapeutics. Dr. Accorsi has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum therapeutics. Dr. Accorsi has received research support from Fulcrum Therapeutics. Dr. Maglio has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Maglio has received research support from Fulcrum Therapeutics. Dr. Shen has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Shen has received research support from Fulcrum Therapeutics. Dr. Robertson has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Robertson has received research support from Fulcrum Therapeutics. Dr. Barnes has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Barnes has received research support from Fulcrum Therapeutics. Dr. Kazmirski has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Kazmirski has received research support from Fulcrum Therapeutics. Dr. Chang has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Chang has received research support from Fulcrum Therapeutics. Dr. Eyerman has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Eyerman holds stock and/or stock options in Alkermes which sponsored research in which Dr. Eyerman was involved as an investigator. Dr. Eyerman has received research support from Fulcrum Therapeutics. Dr. Thompson has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Thompson has received research support from Fulcrum Therapeutics. Dr. Ronco has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Ronco has received research support from Fulcrum Therapeutics. Dr. Rahl has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Rahl has received research support from Fulcrum Therapeutics. Dr. Wallace has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Wallace has received research support from Fulcrum Therapeutics. Dr. Tawil has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics, Acceleron Pharma. Dr. Tawil has received research support from Fulcrum Therapeutics. Dr. Cacace has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics, Arvinas. Dr. Cacace has received research support from Fulcrum Therapeutics, Arvinas. Dr. Cadavid has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Cadavid has received research support from Fulcrum Therapeutics. Dr. Rojas has received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Fulcrum Therapeutics. Dr. Rojas has received research support from Fulcrum Therapeutics.