Background: APRIL (“a proliferation-inducing ligand”) levels are elevated in the serum of patients diagnosed with multiple myeloma (MM) and is correlated to promotion of malignancy, chemo- and immune-resistance. BION-1301 (BION) is a recombinant, humanized monoclonal antibody against APRIL. We report on the initial pharmacokinetic/ pharmacodynamic (PK-PD) profile, safety, and tolerability of BION in adults with relapsed or refractory MM. Methods: Adults with MM and disease progression after ≥3 systemic therapies were recruited for the study. BION was administered every 14 days through intravenous infusion. This ongoing Phase 1/2, open-label, multicenter study is evaluating 6 cohorts with increasing BION dose levels of 50, 150, 450, 1350, and 2700 mg administered Q2W intravenously (cohort 6 - 1350 mg dose given QW and Q2W). Serum was analyzed for BION, anti-drug antibodies (ADA), and free APRIL (fAPRIL) at baseline and upon treatment, and evaluated by PK-PD modeling. Results: As of 7Dec2018 reporting through the first 4 cohorts, 15 patients were enrolled in the study (N = 3-4 per cohort). BION has been well-tolerated to date. While exposure increased dose-proportionally from 50 to 1350 mg, half-life and clearance did not significantly differ between 50 and 1350 mg. APRIL serum levels decreased with increasing BION doses. To date no DLT was observed. Non-neutralizing ADA were detected in 1 of the 15 patients. BION transiently reduced fAPRIL levels starting at a dose of 50 mg. A prolonged reduction was seen at higher doses, and at 450 mg, reduction was maintained in 2 patients on treatment for 6 cycles (5.5 months). The area under the normalized fAPRIL curve (Days 1-15) decreased 5-fold from 50 to 1350 mg. Data fit well in an exploratory PK-PD model, with kinetic binding of BION and fAPRIL according to in vitro parameters, and peripheral compartments for both entities. While at 450 mg, 95% target engagement (TG) was achieved around peak exposure levels, at 1350 mg this 95% TG was maintained throughout the dosing interval of 3 doses. Conclusions: BION dose-dependently inhibits serum levels of fAPRIL between 50 to 1350 mg dose levels. Exposure was approximately dose-linear over the dose range evaluated, with a low incidence of ADA. Promising TG was obtained at prolonged dosing of 450 mg Q2W. A favorable safety profile supports continued dose escalation and more frequent dosing regimens based on PK-PD modeling. The study is ongoing with subjects exposed to higher and/or more frequent doses anticipated to result in accelerated and sustained APRIL TG. Clinical trial information: NCT03340883