Using Pharmacokinetic and Pharmacodynamic Analysis to Optimize the Dosing Regimens of Fanastomig (<scp>EMB</scp>‐02) in Patients With Advanced Solid Tumors

Fanastomig (also known as EMB-02) is a bispecific antibody targeting programmed cell death protein-1(PD-1) and lymphocyte activation gene-3 (LAG-3), developed for the treatment of advanced solid tumors. A first-in-human (FIH) Phase I study (NCT04618393) evaluated safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity, and clinical efficacy of Fanastomig in patients with advanced solid tumors. To determine the recommended Phase II dose (RP2D), population pharmacokinetics (PopPK), and exposure and response analysis (E-R) were conducted. The PopPK model, demonstrating good performance, showed no clinically meaningful relationship between areas under the concentration-time curve (AUC) or maximum concentration (C_max) of Fanastomig and selected covariates of interest. A nonlinear E_max model was fitted to Fanastomig PD-1 receptor occupancy (RO) in the peripheral blood compartment. The estimated half-maximal effective concentration (EC₅₀) was 0.084 μg/mL (95% confidence interval [CI]: 0.0369-0.131). Assuming a threefold lower exposure in tumor tissue compared to that in serum, a target trough concentration of Fanastomig at ~2.27 μg/mL would be needed for 90% PD-1 RO in the tumor. Modeling and simulation indicated that a weekly dosing (QW) of 360 mg would achieve full peripheral blood RO in approximately 90% of patients. The incidence of anti-drug antibodies (ADAs) for Fanastomig was high (95.7%, 44/46), with a negative correlation between the ADA titer and dose levels; meanwhile, ADA minimally impacted PK exposure and efficacy. An inverse trend was observed between anaphylaxis and PK exposure. Fanastomig was well tolerated and had acceptable safety profiles up to 900 mg QW. Based on these findings, two dosing regimens have been selected for further clinical development. Trial Registration: ClinicalTrials.gov identifier: NCT04618393.