Background: ATOR-1015 is a human bispecific IgG1 antibody targeting cytotoxic T-lymphocyte associated protein 4 (CTLA-4) and the tumor necrosis factor receptor superfamily member 4, OX40 (also known as CD134). Both in vitro and in vivo, ATOR-1015 induces activation of cytotoxic T cells and depletion of regulatory T cells (1). In syngeneic tumor models, using human OX40 transgenic mice cross-reacting with both targets, ATOR-1015 is demonstrated to localize to the tumor. Further, the effects of ATOR-1015 are shown to occur in the tumor area and not in the spleen (1). Treatment with ATOR-1015 also reduces tumor growth and improves survival in several tumor models in mice, including bladder, colon and pancreatic cancer (1). The non-clinical safety profile and the pharmacokinetics were established in cynomolgus monkeys and the data were used for the dosing schedule. Methods: This is a multicenter, open-label, dose escalation study enrolling patients with advanced and/or refractory solid malignancies (NCT03782467). The primary objective of the study is to determine the maximum tolerated dose (MTD) or the recommended phase 2 dose (RP2D) and to establish the safety profile of ATOR-1015. ATOR-1015 is administered intravenously biweekly as a single agent until confirmed progressive disease, unacceptable toxicity or withdrawal of consent. The study will start with single patient cohorts until grade 2 toxicities are observed, thereafter the study follows a modified 3+3 design. At the MTD/RP2D, or a lower dose, up to 20 patients are planned for additional safety and efficacy evaluation. Study enrollment was initiated in January 2019. A total of up to 53 patients are estimated to be enrolled in the study. (1) Månsson Kvarnhammar et al.Journal for ImmunoTherapy of Cancer 2018; 6(Suppl 1):115. Abstract P683. Clinical trial information: NCT03782467