• Males or females, age 18 years or older.

• Histologically confirmed diagnosis of locally advanced or metastatic solid tumor. For Part 2, subjects must have one of the following locally advanced or metastatic tumor types: hepatocellular carcinoma (HCC), colorectal cancer (CRC) with liver metastasis, urothelial cancer, squamous cell carcinoma of the head and neck (SCCHN), adenocarcinoma of the pancreas, non-small cell lung cancer (NSCLC), dMMR/MSI-High tumor (Part 2c only).

• Subject must have exhausted all available therapy or have declined treatment or treatment is contraindicated. Subjects with tumors that have known actionable molecular alteration such as EGFR, ALK, ROS-1, BRAF, RET, MET, and KRAS must have progressed on directed molecular therapy. For Part 2c, participants with a tumor type for which a CPI has been approved must have received a CPI during one or more lines of therapy.

• Measurable disease (at least 1 measurable lesion) per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 as defined by tumor type.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

• Life expectancy of at least 3 months.

• Female subjects must be of non-childbearing potential (surgically sterile or at least 2 years postmenopausal) or agree to use a highly effective contraception method while receiving treatment with Decoy20 and for 30 days after the last dose of Decoy20.

• Male subjects must utilize reliable contraceptive precautions for the duration of Decoy20 treatment and 30 days after the last dose of Decoy20.

• Adequate organ function as demonstrated by baseline laboratory assessment.

⁃ Left ventricular ejection fraction (LVEF) ≥ 45% by echocardiogram (ECHO) or multi-gated acquisition scan (MUGA).

⁃ Recovered from toxicities due to prior therapies.

⁃ Willing and able to comply with all scheduled visits, laboratory tests, and other study procedures including mandatory pre-treatment and on- treatment biopsies for subjects enrolled to Part2.