A Study to Test a New Immunotherapy, ABX-001, Alone and in Combination With the Marketed Drug, Pembrolizumab, for the Treatment of Tumors Previously Treated With Other Therapies

Inclusion Criteria- Participants must satisfy all of the following criteria at the screening visit unless otherwise stated: 1. Participant must be capable of giving consent and at least 18 years of age at the time of signing the informed consent. 2. Participants with advanced/recurrent solid tumors. Part A: Participants who have progressed despite standard therapy OR are intolerant of standard therapy, OR for whom no standard therapy exists. Part B: Participant has histologically confirmed locally advanced or metastatic disease, who have progressed despite standard therapy OR are intolerant of standard therapy, OR for whom no standard therapy exists. Participant must have received pembrolizumab as approved standard of care and have primary refractory or acquired secondary resistance. Participants must have a tumor lesion amenable to biopsy and must have another lesion that can be followed for response. As defined by the Society for Immunotherapy of Cancer: Primary resistance/refractory to anti-programmed cell death ligand-1 \[anti-PD-(L)1\]: minimum drug exposure of 8 to 12 weeks or 2 doses of the immunotherapy with lack of benefit, defined as progressive disease (PD) at the time of the first planned assessment of stable disease (SD) lasting less than 6 months. Secondary resistance: participants who experience disease progression following complete response (CR)/partial response (PR) or SD ≥6 months or immunotherapy treatment exposure. 3. Histological or cytological documentation of an advanced solid tumor. 4. Participants are willing to undergo tumor biopsy procedures. For tumor biopsies at screening, it is acceptable if a participant has a biopsy within 3 months from screening and/or during or after the most recent line of therapy (with no intervening anticancer therapy), if there is enough material to fulfil the requirements. 5. Measurable disease per RECIST version 1.1. Palpable lesions that are measurable by radiologic or photographic evaluations may be utilized as the only measurable lesion. 6. Eastern Cooperative Oncology Group performance status 0-1. 7. Life expectancy of at least 12 weeks. 8. Adequate organ functions: 1. absolute neutrophil count ≥1.5 × 10\^9/Liter, 2. hemoglobin ≥90 g/L, 3. platelets ≥100 × 10\^9/Liter, 4. lymphocyte count ≥0.5 × 10\^9/L (500 / mL) 5. prothrombin time/international normalized ratio and partial thromboplastin time (unless participant is receiving anticoagulant) \<1.5 times Upper limit of normal (ULN), 6. total bilirubin ≤1.5 times ULN, except for participants with Gilbert's syndrome who must have a total bilirubin \<3.0 mg/dL, 7. aspartate transaminase/ alanine aminotransferase ≤2.5 times ULN, for participants with liver metastases/tumor infiltration ≤5 times ULN, 8. renal system: creatinine clearance ≥45 mL/min (measured or calculated by Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] formula). Estimated glomerular filtration rate by CKD-EPI \>60 mL/min, 9. endocrine system: thyroid stimulating hormone within normal limits, 10. cardiac system: ejection fraction ≥50% by echocardiogram. 9. Male or female contraceptive requirements: a. Female participants are eligible to participate if they are not either pregnant or breastfeeding, and at least one of the following conditions applies: i. Is not a woman of childbearing potential (WOCBP) OR is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of \<1% per year), with low user dependency, during the study period (participants receiving monotherapy) or 180 days (participants receiving pembrolizumab) after the last dose of study treatment and agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during this period. The investigator should evaluate the effectiveness of the contraceptive method in relation to the first dose of study treatment. ii. A WOCBP must have a negative highly sensitive pregnancy test (urine or serum, as required by local regulations) within 72 hours before the first dose of study intervention. If a urine test cannot be confirmed as negative (eg, an ambiguous result), a serum pregnancy test is required. In such cases, the participant must be excluded from participation if the serum pregnancy result is positive. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a woman with an early undetected pregnancy. b. Male participants will agree to use contraception. 10. Participants must be willing and able to communicate and participate in the entire study. 11. Participants must be able and willing to comply with study assessments and study requirements. 12. Participants must be able to comprehend and willing to sign an Informed Consent Form (ICF) and to abide by the study restrictions. Exclusion Criteria - Participants will be excluded from the study if they satisfy any of the following criteria at the screening visit unless otherwise stated: 1. Malignancy other than disease under study with the exception of those from which the participant has been disease-free for more than 2 years and not expected to affect the safety of the participant or the endpoints of the study. Curatively treated non-melanoma skin cancer is permitted. 2. Symptomatic central nervous system (CNS) metastases or asymptomatic CNS metastases that have required steroids within 2 weeks prior to first dose of study treatment. 3. Participants with carcinomatous meningitis or leptomeningeal spread are excluded regardless of clinical stability. Note: Participants with controlled brain metastases will be allowed but must have completed radiotherapy or surgery for CNS metastases \>2 weeks prior to study entry. Participants must be neurologically stable, having no new neurologic deficits on clinical examination, and without evidence of progression for at least 4 weeks by repeat imaging (which sh