Background and purpose: Preclinical studies of MR309, a selective sigma1 receptor (σ1R) antagonist, support a potential role in treating neuropathic pain. We report two studies that provide insight into the pharmacokinetics (PK) and brain σ1R binding of MR309. Experimental approach: Steady-state PK of MR309 (400 mg QD and 200 mg BID for 10 days; EudraCT 2015-001818-99 [PK study]) and the relationship between MR309 plasma exposure and brain σ1R occupancy (EudraCT 2017-000670-11 [PET study]) were investigated in healthy volunteers. Positron emission tomography (PET) using the σ1R ligand [11C]SA4503 was conducted at baseline, 2h and 8h after a single dose of MR309 (200–800 mg). The relationship between brain σ1R occupancy and MR309 exposure was explored using data-driven model fitting. Key results: MR309 was well tolerated, brain σ1R occupancy ranged between 30.5% and 74.9% following single-dose MR309 (n=7). MR309 BID provided a plasma PK profile with less fluctuation than QD dosing (n=16). MR309 200 mg BID yielded average steady state plasma concentrations between 2000 and 4000 ng/mL in the PK study, which corresponded to an estimated brain σ1R occupancy of 59–74%. Conclusions and implications: MR309 200 mg BID dose was below the 75% σ1R occupancy threshold expected to elicit maximal antinociceptive effect as observed in neuropathic pain models. Further investigations of MR309 for neuropathic pain will require higher brain σ1R occupancy, and establish the optimal dose by elucidating the clinical impact of a broad range of brain σ1R occupancy across different neuropathic pain indications.