Key Inclusion Criteria:  Subject has a histologically- or cytologically-diagnosed solid tumor which is advanced or metastatic and which has progressed on or following at least one systemic therapy regimen administered for advanced or metastatic disease or for which no approved therapy exists. Subject's prior treatment should include all approved regimens that have demonstrated a survival advantage for the subject's disease, stage, and line of therapy. Has measurable disease per RECIST 1.1. An adequate tumor sample must be available from core needle biopsies obtained during the Screening Period and following the subject's most recent systemic therapy. Agrees to an on-treatment biopsy preferably of the same lesion from which the pre-CBX-12 treatment sample was obtained as long as the Investigator determines such biopsy can be performed with acceptable safety.  Exclusion Criteria:  Cytotoxic chemotherapy, biologic agent, investigational agent, or radiation therapy less than or equal to 3 weeks prior to the first dose of CBX-12. The interval may be reduced to 2 weeks for bone only radiation therapy or investigational agents not expected to be associated with adverse events (AEs) after 2 weeks of last administration, with Medical Monitor approval. Small-molecule kinase inhibitors or hormonal agents less than or equal to 14 days prior to the first dose of CBX-12. Subjects who are currently receiving any other anti cancer or investigational agent(s). Clinically significant intercurrent disease. Subjects with primary central nervous system (CNS) tumors or clinically active CNS metastases or carcinomatous meningitis. Subjects with stable brain metastasis may be enrolled with Medical Monitor approval.