The purpose of this study is to evaluate the safety and tolerability of ATB-320 in patients with progressive or metastatic solid tumors

1. Primary objective: to evaluate the maximum tolerable dose (MTD), recommended phase 2 dose (RP2D), and dose limiting toxicities (DLTs) of ATB-320 in patients with progressive or metastatic solid tumors.
2. Secondary objective: to evaluate safety and tolerability of ATB-320 in patients with progressive or metastatic solid tumors.
3. Exploratory objective: to evaluate pharmacokinetics, pharmacodynamics, and efficacy of ATB-320 in patients with progressive or metastatic solid tumors.

Inclusion Criteria:

1. Adult (18 years or older) at the time of screening
2. Histologically confirmed solid tumor
3. Participants must have failed standard therapy or cannot tolerate standard therapy.
4. Participants must be able to provide tissue samples, either stored at the institution or obtained from a biopsy performed prior to administration of the investigational product, and have tumor that can be safely biopsied throughout the study.
5. Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 or 1
6. Life expectancy of ≥ 12 weeks

Exclusion Criteria:

1. Has received chemotherapy within 21 days prior to the first planned dose of the investigational product, or if less than 5 times the half-life of the chemotherapeutic has passed (whichever is shorter)
2. Has received immunotherapy within 2 weeks of the first planned dose of investigational product
3. Major surgery within 4 weeks prior to the first planned dose of the investigational product
4. Previous malignant tumors, other than the target indication to be investigated in this study, within 3 years prior to screening (participants with locally malignant tumors such as basal cell carcinoma/squamous cell carcinoma of the skin, carcinoma in situ (cervix, breast), thyroid papillary carcinoma that have undergone potentially curative therapy are eligible to participate in the study.
5. Current enrollment in any other clinical trial or has received another investigational product or medical device treatment within 28 days of first planned dose (participants who are undergoing safety follow-up phase of a clinical trial may participate if more than 28 days have passed since the last dose of the previous investigational product)
6. Has received prior radiotherapy within 2 weeks of the first planned dose. Participants who have received palliative radiotherapy for non-central nervous system (CNS) disease within 2 weeks of first planned dose are allowed to participate after 1-week washout (participants must have recovered from any radiation-related toxicities, not require corticosteroid treatment, and without radiation pneumonitis)
7. History of hypersensitivity to the active ingredients or excipients of the investigational product or other drugs