Study to Investigate the Safety, Tolerability, Pharmacokinetics, Pharmocodynamics and Preliminary Antitumor Activity of AT-1965 in Patients With Advanced, Refractory or Recurrent Solid Tumors

Inclusion Criteria: 1. The patient has a histologically or cytologically confirmed unresectable or metastatic solid tumor that is refractory to standard therapy or for which in the opinion of the investigator no standard therapy is suitable. NOTE: For the backfill cohort, patient must have a histologically or cytologically confirmed unresectable or metastatic solid tumor and have received at least three prior treatments. Enrollment of patients in the backfill cohort will occur after receiving the sponsor's approval. 2. Patient should have at least 1 measurable lesion per RECIST version 1.1 as assessed by the investigator. For Part A only, patients with radiographically evaluable but non-measurable disease are allowed after discussion with the sponsor. 3. Recovered from AEs (except irAEs) of prior chemotherapy (per NCI CTCAE version 5.0) to Grade ≤ 1 or return to baseline status (except for alopecia) as per Investigator's discretion. 4. The patient has an ECOG performance status of 0 to 2. 5. The patient has adequate bone marrow, renal, and hepatic function, defined as follows: 1. Hemoglobin ≥9.5 g/dL (without transfusion in the prior 3 weeks). 2. Platelets ≥100 × 109 cells/L (may be achieved with transfusion as per PI discretion) 3. ANC ≥1.5 ×109 cells/L 4. Creatinine Clearance ≥60 mL/min (by using Cockcroft-gault equation) 5. Total bilirubin ≤1.5 × ULN, unless the patient has a prior history of Gilbert's syndrome, in which case ≤3.0 × ULN is acceptable. 6. AST and ALT ≤2.5 × ULN; or ≤5 × ULN if due to liver involvement by tumor 6. Female patients of child-bearing potential must have a negative serum pregnancy test at screening and negative urine pregnancy test before each start of a new treatment cycle. NOTE: Women are considered of childbearing potential unless they are surgically sterile (have undergone a hysterectomy, bilateral tubal ligation, or bilateral oophorectomy) or are postmenopausal (at least 12 consecutive months with no menses without an alternative medical cause) and have an elevated follicle-stimulating hormone (FSH) at screening. 7. Female patients of childbearing potential must agree to use a highly effective method of contraception during the study and for a minimum of 3 months following administration of study drug, which includes a barrier method plus 1 or more of the following: 1. Hormonal contraceptives (e.g., birth control pills, skin patches, vaginal rings, or the Depo-Provera® shot) 2. Intrauterine device (IUD) 3. Male or female condoms with spermicide 4. Diaphragm with spermicide 5. Permanent tubal occlusive birth control system 8. Male patients with female partners of childbearing potential must be vasectomized or be willing to use an acceptable method of birth control or to practice abstinence during the study and for 3 months after the last dose of IMP. 9. Must be 28 days since mRNA Covid 19 vaccine injection. 10. Willing to avoid sun exposure, wear protective clothing, and/or apply broad spectrum (ultraviolet A \[UVA\] and ultraviolet B \[UVB\] protection) sunscreen if sun exposure is unavoidable. 11. The patient is capable of understanding the written informed consent, provides signed and witnessed written informed consent and authorization permitting use of collected tissue and personal health information, and agrees to comply with protocol requirements. For Part B Dose Expansion in TNBC only: 1. Histologically or cytologically confirmed metastatic triple-negative breast cancer (mTNBC) who had received at least two prior treatments (a taxane and sacituzumab govitecan-hziy) for metastatic disease, for advanced disease. 2. In addition, patients with TNBC with either Low HER2 (IHC 1+ or IHC 2+/in situ hybridization-negative), PD-L1 (CPS\>10) or BRCA mutation need to have prior FDA approved available therapies before participation in this expansion arm. Exclusion Criteria: 1. The patient has an uncontrolled or life-threatening, symptomatic, current or recurrent disease (e.g., cardiovascular, renal, hepatic, endocrine) or other abnormality that could affect the action, absorption, or disposition of the study drug, may impact the ability of the patient to participate, may affect clinical or laboratory assessments, or otherwise has the potential to confound the study results. 2. Unstable or severe uncontrolled medical condition (e.g., unstable cardiac function, unstable pulmonary condition) or any important medical or psychiatric illness or abnormal laboratory finding that would, in the Investigator's judgment, increase the risk to the patient associated with his or her participation in the study. 3. Uncontrolled diabetes. 4. Patients with an active uncontrolled autoimmune disease. Excluded autoimmune conditions are listed in the Protocol Appendix 1. 1. Patients with history of transient autoimmune manifestations of an acute infectious disease that resolved upon treatment of the infectious agent are not excluded (e.g. acute Lyme arthritis). 2. Please contact the medical monitor regarding any uncertainty over autoimmune exclusions. 5. History of interstitial lung disease, idiopathic pulmonary fibrosis, pneumonitis (including drug induced), organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic organizing pneumonia, etc.), or evidence of active pneumonitis on chest computed tomography scan in the last 6 months; NOTE: history of radiation pneumonitis in the radiation field (fibrosis) is permitted. 6. History of hemolysis or hemolytic anemia. 7. Evidence of ongoing subclinical hemolysis (high LDH and low serum haptoglobin with increased reticulocyte count). 8. History of adrenal gland disorders such as Cushing Syndrome, Congenital adrenal hyperplasia, Addison's Disease and hyperaldosteronism 9. Recipient of an allogeneic bone marrow transplantation or solid organ transplantation. 10. Endocrinopathy, unless on stable hormone replacement therapy. 11. History of known human immunodeficiency virus (HIV);