Background: Immunotherapy with the anti-PD-(L)1 checkpoint inhibitors (CPIs) has been largely ineffective in so-called non-immunogenic “cold tumors”. Facilitating T cell infiltration is necessary to invoke an immune response which may be augmented or complemented by the activity of CPIs like durvalumab. Selective T cell Redirection Proteins (STRs) are fusion proteins that consist of genetically engineered Superantigen (Sag) linked to Fragment antigen binding (Fab) moieties directed to tumor-associated antigens. Nap is a first in class STR compound, recognizing the tumor-associated oncofetal antigen 5T4, whereas the SAg moiety selectively engages the T cell receptor β variable (TRBV) 7-9. Nap has been shown to induce specific T cells expansion, activation and infiltration into the tumor in pre-clinical and clinical studies. Pre-clinical data demonstrated that the combination of STR with CPI may lead to long term durable responses not possible in most patients receiving single agent CPI therapy and suggests that combining CPIs with STR may be a promising therapeutic strategy for patients with solid tumors. Methods: Patients will be treated with the combination of Nap and durvalumab using a flat dose of durvalumab (1120 mg) and the 3+3 design for Nap dose escalation (2, 5, 10, 15 and 20 mcg/kg). The MTD of Nap for the combination treatment will be established based on DLTs occurring during the first treatment cycle. The dose escalation part will be followed by MTD expansion cohort in which 10–15 patients will be treated with MTD of Nap and 1120 mg durvalumab (Clinical trial registry number NCT03983954). Major eligibility criteria include patients with pretreated advanced or metastatic, 5T4 expressing solid tumors, including patients previously progressed on CPI therapy. As of January 2020, enrollment to dose levels 2, 5 and 10 mcg/kg has been completed without DLT, enrollment to dose level 15 mcg/kg will start on February 2020. Clinical trial information: NCT03983954.