Glycoprotein Non‐Metastatic Melanoma Protein B ( <scp>GPNMB</scp> ): A Translational Pharmacodynamic Biomarker for <scp>PIKfyve</scp> Inhibition With <scp>VRG50635</scp>

Glycoprotein non-metastatic melanoma protein B (GPNMB) was investigated as a pharmacodynamic (PD) biomarker for PIKfyve inhibition across translational studies ex vivo, in vitro, in animals, and in the clinic, demonstrating significant response to VRG50468 in cells, in vivo in the central nervous system (CNS) and in peripheral fluids and tissues. VRG50468 is the active metabolite of VRG50635, a small molecule PIKfyve inhibitor pro-drug in development for treating amyotrophic lateral sclerosis (ALS). Peripheral pharmacology was evaluated in peripheral blood mononuclear cells (PBMCs) from healthy volunteers ex vivo and in vitro and in PBMCs from mice given oral VRG50635. Central pharmacology was evaluated in vitro using C9orf72 ALS patient-derived induced pluripotent stem cell motor neurons and mouse primary neurons, and in vivo in brains of mice given oral VRG50635. Two clinical studies in healthy adults examined plasma, PBMCs, and cerebrospinal fluid following oral VRG50635 for peripheral and central pharmacologic activity via GPNMB induction. PD GPNMB upregulation with VRG50468 was demonstrated across preclinical translational and clinical studies. A PD response to VRG50468 was observed ex vivo in rodent and human PBMCs and in primary rodent neurons and motor neurons induced from stem cells of people with ALS. Repeated administration of VRG50635 to rodents and healthy human volunteers robustly induced GPNMB peripherally and in the CNS, which was concentration and time dependent in vitro and dose and treatment duration dependent in vivo, peripherally, and in CNS. GPNMB is a robust translatable PD biomarker for clinical trials with the PIKfyve inhibitor VRG50635. TRIAL REGISTRATION: Clinical trial number: VGCS-50635-001 and VGCS-50635-003; identifier: NL81735.056.22 and NCT06286475.