This is a first-in-human Phase 1, single-arm, open-label, multicenter, multiple-dose, dose-escalation and dose-expansion study of ABL503 to evaluate the safety, tolerability, MTD and/or RP2D, PK, immunogenicity, preliminary antitumor activity, and the PD effect of ABL503 in subjects with any progressive locally advanced (unresectable) or metastatic solid tumors who are relapsed or refractory following the last line of treatment and have no available standard of care option. This study includes 3 parts: a dose-escalation part, a dose-expansion part and tumor-expansion part

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