This is a Phase 1, open-label, dose-escalation study to assess the safety, tolerability, pharmacokinetics (PK), and preliminary efficacy of APX-343A monotherapy (Part A) and in combination with pembrolizumab (Part B) in subjects with advanced solid tumors.

Inclusion Criteria:

1. Male or female subjects aged ≥19 years, inclusive, at the time of informed consent.
2. Willing and able to give informed consent and to comply with the requirements of the study.
3. Histologically or cytologically confirmed advanced solid tumor.
4. Life expectancy of at least 3 months in the judgment of the investigator.
5. ECOG performance status of 0 to 1 during the screening period.
6. Measurable disease per RECIST v1.1 as assessed by the site investigator/radiology.
7. Archival tumor tissue sample or newly obtained biopsy of a tumor lesion not previously irradiated has been provided.
8. Subjects who have AEs due to previous anticancer therapies must have recovered to ≤ Grade 1 or baseline.
9. Adequate organ function as defined in the following table.
10. Subjects who are hepatitis B surface antigen positive are eligible if they have received HBV antiviral therapy for at least 4 weeks and have undetectable HBV viral load prior to the first dose of study drug.
11. Subjects with history of HCV infection are eligible if HCV viral load is undetectable at screening.
12. Male subjects must agree to use an adequate method of contraception.
13. Female subjects of childbearing potential must agree to use highly effective contraceptive methods, and to abstain from breastfeeding.

Exclusion Criteria:

1. Received any prior immunotherapy.
2. Received prior systemic anticancer therapy including investigational agents.
3. Received prior radiotherapy.
4. Received a live or live-attenuated vaccine .
5. Has received an investigational agent.
6. Diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy.
7. Known additional malignancy.
8. Known active CNS metastases and/or carcinomatous meningitis.
9. Active autoimmune disease.
10. History of (noninfectious) pneumonitis/interstitial lung disease.
11. Active, uncontrolled infection requiring systemic therapy.
12. Unable to swallow study drug or disease that interfere with proper absorption of study drug.
13. Significant cardiovascular disease.
14. Cerebrovascular accident.
15. History or current evidence of any condition, therapy, laboratory abnormality, or other circumstance that might confound the results of the study or interfere with the subject's ability to cooperate with the requirements of the study.
16. History of severe hypersensitivity to pembrolizumab and/or any of its excipients.
17. History of hypersensitivity to APX-343A and/or any of its excipients.
18. History of allogeneic tissue/solid organ transplant.
19. History of HIV infection or HIV positive at screening.
20. Subjects who have not adequately recovered from major surgery or have ongoing surgical complications.