Inclusion Criteria:

Signed informed consent
Age ≥ 18 years
ECOG Performance Status ≤ 2
Histologically- or cytologically-confirmed solid tumor
Patient has relapsed, is refractory to, or intolerant of standard of care therapy
No available approved therapy that may provide clinical benefit (per Investigator)
Measurable or evaluable disease by RECISTv1.14
Life expectancy of > 12 weeks (per Investigator)

Adequate laboratory parameters including:

Absolute neutrophil count (ANC) ≥ 1500/mm3
Platelets ≥ 100,000/mm3
Hemoglobin ≥ 12 g/dL (in the absence of transfusion over the prior 2 weeks)
AST/SGOT ≤ 2.5 x ULN (≤ 5 x ULN if known liver involvement)
ALT/SGPT ≤ 2.5 x ULN (≤ 5 x ULN if known liver involvement)
Total bilirubin ≤ 2.0 x ULN (unless diagnosis of Gilbert’s syndrome in which case < 3.0 times ULN) Serum creatinine ≤ 2.0 x ULN or estimated GFR ≥ 45 mL/min (per Cockcroft- Gault equation) If residual treatment related toxicity from prior therapy: Treatment related toxicity resolved to ≤ Grade 1 (alopecia excepted), or Treatment related toxicity resolved to ≤ Grade 2 with prior approval of the Medical Monitor Willingness to comply with the study schedule and all study requirements [Females] Must be postmenopausal, surgically sterile, or agree to use adequate contraception (per Investigator) throughout the study and for a least 30 days following the last dose [Males] Must be surgically sterile or must agree to use adequate contraception (per Investigator) throughout the study and for at least 30 days following the last dose [Males] Willingness to refrain from donating sperm throughout the study and for at least 30 days following the last dose [Females] If of child-bearing potential, must have a negative serum pregnancy test Exclusion

Exclusion Criteria:

Participating in any other interventional clinical study Previous exposure to an anti-CD47 or SIRPα antibody ≤ 28 days (or 5 half-lives if shorter) between of systemic anti-tumor treatment (e.g., chemotherapy, endocrine therapy, immunotherapy, cellular therapy) and the 1st dose of STI-6643 ≤ 28 days from prior irradiation (≤ 7 days from limited field irradiation for control of symptoms) and the 1st dose of STI-6643 ≤ 28 days between major surgery (≤ 7 days from minor surgical procedures, no waiting period following central catheter placement) ≤ 7 days between administration of G-CSF, GM-CSF, erythropoietin, thrombopoietin or IL11 and the 1st dose of STI-6643 ≤ 7 days between systemic immunosuppressive therapy in excess of 10 mg/day prednisone equivalent and the 1st dose of STI-6643 (topical or inhaled corticosteroids not restricted) ≤ 28 days between a live attenuated vaccine and the 1st dose of STI-6643 Known central nervous system (CNS) involvement with tumor (e.g., metastases, meningeal carcinomatosis) Active second malignancy requiring ongoing systemic treatment History of primary immunodeficiency disorders History of active pulmonary tuberculosis History of COVID-19 symptoms unless COVID-19 test negative ≤ 72 hours of the 1st dose of STI-6643 ≤ 12 weeks from an allogeneic hematopoietic stem cell transplant and C1D1 or active graft-versus-host disease (GvHD) Active infection (e.g., bacterial, viral, fungal) requiring systemic treatment ≤ 72 hours of the 1st dose of STI-6643 Known HIV-positive with CD4+ cell counts < 350 cells/uL or a history of an AIDS defining opportunistic infection Known T-cell leukemia virus type 1 (HTLV1) infection, hepatitis B virus (HBV) or hepatitis C virus (HCV) viremia Significant risk for HBV reactivation (defined as HbsAg positive, HbcAb positive or HBV DNA positive) Detectable HCV RNA Pregnant or breast feeding History of clinically significant cardiovascular abnormalities including: Congestive heart failure (NYHA classification ≥ 3) within 6 months of the 1st dose of STI-6643 Unstable angina pectoris ≤ 6 months from myocardial infarction and the 1st dose of STI-6643 Arrhythmias (other than atrial fibrillation) requiring ongoing treatment QTcF interval > 480 msec (using Fridericia’s formula)
Uncontrolled hypertension (i.e., systolic BP > 180 mmHg or diastolic BP > 100
Any condition, including the presence of laboratory abnormalities, that places the subject at an unacceptable risk if the subject was to participate in the study.