Agonistic 4-1BB monoclonal antibodies were preclinically validated as promising cancer immunotherapies, both as monotherapy and as potentiators of the activity of PD-(L) 1–blocking agents. However, toxicity and a narrow therapeutic window have hampered their clinical development. DuoBodyPD-L1×4-1BB, a first-in-class, bispecific, next-generation checkpoint immunotherapy, was designed to overcome these limitations by activating T cells through conditional 4-1BB costimulation, while simultaneously blocking the PD-L1 axis. We present preliminary data from the ongoing, first-in-human, open-label, phase I/IIa trial of DuoBody-PD-L1×4-1BB in advanced solid tumors (NCT03917381).