This study is to characterize the safety, tolerability, pharmacokinetics (PK), immunogenicity, pharmacodynamics (PD) and anti-tumor activity of AK104 as a single agent in adult subjects with advanced solid tumor malignancies. The study consists of a dose escalation phase (Phase 1a) to determine the maximum tolerated dose (MTD), or recommended Phase 2 dose (RP2D) for AK104 as a single agent, and a dose expansion phase (Phase 1b) which will characterize treatment of AK104 as a single agent at the MTD or RP2D.