Phase 1/2 Study of Intravenous Injection of STX-003 in Advanced Solid Tumors as Monotherapy or in Combination With Pembrolizumab

Inclusion Criteria: * Inclusion Criteria (Phase 1 and Phase 2) 1. Mentally competent and able to understand and sign the ICF. 2. Tumor type for which there is an FDA-approved anti-PD-1/L1 antibody therapy, specifically including biomarker labeled subsets (i.e. PD-L1+) as well as anatomical subsets in some disease (cutaneous melanoma is eligible whereas acral, mucosal and uveal are not; Phase 1) and advanced cutaneous melanoma or PD-L1+ NSCLC. 3. Histologically or cytologically documented, locally advanced, or metastatic solid tumor. 4. At least one measurable lesion per RECIST v1.1 criteria. 5. The patient lacks a curative therapy or has progressive disease despite, or refused, standard therapy. 6. ECOG performance status of 0 or 1. 7. Life expectancy of ≥ 12 weeks per the Investigator. 8. ≥ 18 years of age at the time of informed consent. 9. Body weight ≥ 40 kg. 10. WOCBP and males with female partners of child-bearing potential must agree to use adequate birth control throughout their participation and for 3 months following the last dose of STX-003. 11. Willing and able to comply with protocol required assessments. Hematology: * ANC ≥ 1,000 cells/mm3. * Platelet count ≥ 75,000 cells/mm3. * Hemoglobin ≥ 8.0 g/dL. * Renal: Serum creatinine \< 1.5 × ULN or creatinine clearance ≥ 40 mL/min based on the Cockcroft-Gault glomerular filtration rate estimation Coagulation: * PT/INR or PT must be ≤ 1.5 × ULN. * aPTT ≤ 1.5 × ULN unless undergoing anticoagulation therapy. • Liver: * Albumin ≥ 3.5 g/dL or within the normal range of the local reference laboratory. * AST and ALT \< 2 × ULN. * Bilirubin ≤ 1.5 × ULN (except participants with documented Gilbert's syndrome who may be enrolled if the conjugated bilirubin is within normal limits) or ≤ 5 × ULN with liver metastases. Phase 2 Inclusion Criteria 13\. NSCLC : * Histologically or cytologically documented findings consistent with NSCLC not amenable to curative surgery, radiation, or other therapy. * Biomarker confirmed as PD-L1+ per local institutional standard practice. * Patients with known activating EGFR, STK11 or KEAP1 mutations as well as ALK/ROS1 fusions, are not eligible. * Prior treatment (for advanced, metastatic or \[neo\]adjuvant) should have included a platinum-based therapy and a PD-1/L1 pathway checkpoint inhibitor, unless the patient is not a candidate for or has refused viable therapies due to inability to tolerate treatment (i.e. cell therapy) or potential side effects (i.e. checkpoint inhibitor toxicity). 14\. Melanoma : Histologically or cytologically documented findings consistent with advanced cutaneous melanoma not amenable to curative surgery, radiation, or other therapy. Acral, mucosal and uveal melanoma are excluded. * Patients who are not candidates for or have refused available therapies due to inability to tolerate treatment (i.e. cell therapy) or potential side effects (i.e. checkpoint inhibitor toxicity) are also eligible. * Received an anti-PD-1/PD-L1 inhibitor as monotherapy or in combination with anti-CTLA-4 inhibitor and have either primary or secondary checkpoint inhibitor resistance asper SITC consensus definition, unless deemed intolerable by the Investigator. Patients with BRAF V600E mutant melanoma should have received a BRAF inhibitor as monotherapy or in combination with other targeted agents (MAPK kinase MEK inhibitors), unless deemed intolerable by the Investigator. Exclusion Criteria: * Exclusion Criteria (Phase 1 and 2) 1. Medical Conditions * History of primary immune deficiency. * History of clinically significant autoimmune disease that has required intervention in the last 6 months. Consultation with the MM may inform the discussion of whether autoimmune disease is clinically significant. * History of Grade 3 or higher IRAEs that has not resolved to Grade ≤ 1 at the time of enrollment. Exceptions include endocrine disorders that are well-managed with stable physiological hormone replacement and Grade 3 immune-related rash. Patients meeting this criterion may be considered for enrollment following approval by the MM. * History of solid organ transplant and taking immunosuppressive medications. 2. Cardiovascular exclusions * Medical history of an arterial thrombotic event, stroke, or transient ischemic attack within the past 6 months. * Medical history of symptomatic congestive heart failure (New York Heart Association classes II-IV) or a cardiac arrhythmia that required treatment within the past 6 months. * Medical history of myocardial infarction or unstable angina within 6 months before C1D1. 3. Recent medical concerns exclusions * Evidence of active infection requiring IV antibiotics within 7 days prior to C1D1. * Active uncontrolled bleeding, or a bleeding diathesis within 7 days prior to C1D1. * Serious or non-healing wound, fistula, skin ulcer, or non-healing bone fracture within 7 days prior to C1D1. * Known HIV infection, active hepatitis B infection, or hepatitis C infection: * Virology evaluation should be conducted at Screening to include serum HIV antibody, HBc antibody, HBsAg antigen, and HCV antibody. Patients with a positive antibody evaluation for HCV and/or HBc should undergo evaluation to measure HCV RNA or HBV DNA, respectively. * Untreated CNS tumor, epidural tumor or metastasis, or brain metastasis. Patients with any primary CNS malignancy including glioma and current, active, or progressing CNS malignancy, including carcinomatosis meningitis, are excluded. * Patients with treated brain metastases are eligible if there is no evidence of progression for at least 4 weeks after CNS-directed treatment, as ascertained by clinical examination and brain imaging (MRI or CT scan) during the Screening period and are off systemic steroids (for at least 2 weeks prior to first dose). * Another primary malignancy that has not been treated with curative intent (discuss with MM), except for non-metastatic cutaneous basal cell or squamous cell carcinoma, or non-muscle invasive bladder cancer. * Serious illness considered by the Investigator as incompatible with participating in this clinical study. * Major surgery within 4 weeks of first dose of study drug. * Any condition that, in the opinion of the Investigator, would interfere with evaluation of the investigational product or interpretation of the patients' safety or study results. 4\. Prior/Concomitant Therapy * Prior treatment with STX-003 or other VEEV-based replicating RNA. * Prior IL-12 therapy. * Receipt of any live vaccine within 30 days prior to the first dose of study treatment. * Use of another systemic anticancer therapy within 3 weeks prior to C1D1 or 5 halflives, whichever is shorter or use of radiotherapy within 1 week prior to C1D1 5. Prior/Concurrent Clinical Study Experience * Previously enrolled in this study. * Actively enrolled in another clinical study unless it is an observational (noninterventional) clinical study or the follow-up component of an interventional study. * Known severe hypersensitivity (Grade ≥ 3) to study treatment or any of the excipients of the products. Other Exclusions * History of interstitial lung disease or active, non-infectious pneumonitis (combination cohorts only). * Known psychiatric or substance use disorder that would interfere with the patient's ability to cooperate with the requirements of the study. * Currently pregnant (confirmed with a positive pregnancy test), breast-feeding or planning to become pregnant within 6 months following treatment. For WOCBP, a negative serum β-HCG result must be available within a 72-hour window before the first treatment dose.