Inclusion Criteria:

          -  Histologically and/or cytologically confirmed diagnosis of any progressive locally
             advanced (unresectable) or metastatic solid tumors that have relapsed or are
             refractory following the last line of treatment, for which prior standard therapy has
             been ineffective, standard therapy does not exist, or is not considered appropriate.

          -  With AE(s) excluding alopecia or Grade 2 toxicities that are deemed stable or
             irreversible (eg, peripheral neuropathy) from prior therapy that have improved to
             Grade 1 or the baseline grade more than 14 days prior to the first administration of
             the study drug

          -  Adequate hematologic, hepatic, and renal functions confirmed based on the screening
             laboratory tests and reconfirmed with additional safety laboratory tests performed
             within 72 hours prior to the first administration of ABL503

        Exclusion Criteria:

          -  Prior anticancer monoclonal antibody treatment or investigational therapy within 28
             days prior to the first administration of study drug or has not recovered (ie, ≤ Grade
             1 or at baseline grade) from AEs due to previously administered agent more than 14
             days prior to ABL503 administration

          -  Prior chemotherapy or radiation therapy within 2 weeks or targeted small molecule
             therapy within 5 half-lives prior to the first administration of study drug or has not
             recovered (ie, ≤ Grade 1 or at baseline grade) from AEs due to previously administered
             agent more than 14 days prior to ABL503 administration

          -  Requiring or received systemic steroid therapy or any other immunosuppressive therapy
             within 14 days prior to study drug administration.

          -  Risk factors for bowel obstruction or bowel perforation (including but not limited to
             a history of acute diverticulitis, intra-abdominal abscess, and abdominal
             carcinomatosis.

          -  Discontinued from prior immunomodulatory therapy due to any intolerable immune-related
             adverse events (IrAEs) requiring systemic steroid treatment

          -  History of drug-induced pneumonitis (interstitial lung disease) or currently has
             pneumonitis

          -  Received prior treatment with an anti-4-1BB antibody