A Phase 1/2, Open-label, Multi-center, First-in-human Study of the Safety, Tolerability, Pharmacokinetics, Pharmacodynamics and Anti-tumor Activity of STX-001 Delivered by Intratumoral Injection in Patients With Advanced Solid Tumors as a Monotherapy or in Combination With Pembrolizumab

General Inclusion Criteria: * ≥ 18 years of age at the time of screening. * Mentally competent and able to understand and sign the informed consent form (ICF). * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Life expectancy of ≥ 12 weeks per the Investigator. * Body weight ˃ 40 kg. * At least 4 weeks from any prior major surgery. * Willing and able to provide blood samples prior to the start of this study. * Has a tumor lesion amenable to injection, must be accessible for pre and post injection biopsy, and the patient must be willing to consent to biopsy, if deemed safe by the Investigator. * Laboratory values (Hematology): Absolute neutrophil count ≥ 1,000 cells/mm3; Platelet count ≥ 75,000 cells/mm3; Hemoglobin ≥ 8.0 g/dL. * Laboratory values (Renal): Serum creatinine \< 1.5 × upper limit of normal (ULN) or creatinine clearance ≥ 40 mL/min based on the Cockcroft-Gault glomerular filtration rate estimation * Laboratory values (Coagulation): Prothrombin/International Normalized Ratio (PT/INR) or prothrombin time must be \< 1.5 × ULN; * Activated partial thromboplastin time (aPTT) ≤ 1.5 × ULN unless undergoing anticoagulation therapy. * Laboratory values (Liver): Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< 2 × ULN; Bilirubin ≤ 2 × ULN or ≤ 5 × ULN for all patients. Phase 1 Inclusion Criteria: * Histologically or cytologically documented, locally advanced, or metastatic solid tumor. * Disease progression confirmed by imaging or other objective evidence after having received standard treatment or patients with refractory solid tumors. Patients must have progressed or are intolerant of at least one line of prior therapy. Phase 2 Inclusion Criteria (TNBC): * Histologically or cytologically documented findings consistent with TNBC not amenable to curative surgery, radiation, or other therapy. * Prior treatment (for advanced, metastatic or \[neo\]adjuvant) should have included a taxane and/or anthracycline-based therapy and, where appropriate, an approved checkpoint inhibitor. * Has disease other than the injected lesion that is measurable by RECIST 1.1. Phase 2 Inclusion Criteria (melanoma): * Histologically or cytologically documented findings consistent with advanced melanoma not amenable to curative surgery, radiation, or other therapy. Uveal melanoma is excluded. * Patients who are not candidates for or have refused available therapies are also eligible. * Received an anti-programmed death-1 (PD-1) / programmed death ligand-1 (PD-L1) inhibitor as monotherapy or in combination with anti-cytotoxic lymphocyte associated protein 4 (CTLA-4) inhibitor and have either primary or secondary checkpoint inhibitor resistance as per Society for Immunotherapy of Cancer (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 (mitogen-activated protein kinase \[MAPK\] kinase \[MEK\] inhibitors), unless deemed intolerable by the investigator. * Has disease other than the injected lesion that is measurable by RECIST 1.1. Phase 1 and 2 Exclusion Criteria: * History of primary immune deficiency. * History of autoimmune disease and/or requiring immunosuppression (except hypothyroidism). * History of History of Grade 3 or higher immune-related adverse events. Patient may be enrolled with Medical Monitor approval. * History of solid organ transplant and taking immunosuppressive medications. * Cardiovascular exclusions: Medical history of an arterial thrombotic event, stroke, or transient ischemic attack within the past 12 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 12 months; medical history of myocardial infarction or unstable angina within 6 months before Cycle 1 Day 1; QTcF prolongation to \> 470 ms in women and \> 450 ms in men based on a 12-lead electrocardiogram (ECG) in triplicate using the Fridericia formula: QTc = QT / RR1/3. Recent medical concerns exclusions: * Prior direct radiation therapy to the tumor lesion to be injected. * Active use of systemic anticoagulants * Evidence of active infection requiring intravenous (IV) antibiotics during screening requiring therapy within 7 days prior to Cycle 1 Day 1. * Active uncontrolled bleeding, or a bleeding diathesis within 7 days prior to Cycle 1 Day 1. * Serious or non-healing wound, fistula, skin ulcer, or non-healing bone fracture within 7 days prior to Cycle 1 Day 1. * Known human immunodeficiency virus (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 central nervous system tumor, epidural tumor or metastasis, or brain metastasis. Patients with any primary Central Nervous System (CNS) malignancy including glioma and current, active, 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 (magnetic resonance imaging \[MRI\] or computed tomography \[CT\] scan) during the screening period and off systemic steroids (for at least 2 weeks prior to first dose). * Another primary malignancy that has not been treated with curative intent, 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. * Any condition that, in the opinion of the Investigator, would interfere with evaluation of the investigational product or interpretation of the patient's safety or study results. * Prior IL-12 therapy. * Receipt of any vaccine within 30 days prior to the first dose of study treatment. * Use of another anticancer therapy within 3 weeks prior to Cycle 1 Day 1 or 5 half-lives, whichever is shorter. * 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. * Known psychiatric or substance use disorder that would interfere with the participant's ability to cooperate with the requirements of the study. * Currently pregnant (confirmed with positive pregnancy test), breast-feeding or planning to become pregnant within 60 days following treatment. For women of childbearing potential (WOCBP), a negative serum beta-human chorionic gonadotropin (β-HCG) result must be available within a 72 hour window before the first treatment dose. * Women of childbearing potential not willing to use a highly effective method of contraception. * Unwilling or unable to follow protocol requirements.