Background: ATOR-1017 is a human agonistic IgG4 antibody targeting the co-stimulatory receptor 4-1BB (CD137). It is developed to activate T cells and natural killer cells in the tumor environment, leading to immune-mediated tumor cell killing. This is a first-in-human, multicenter, phase 1 study (NCT04144842). Methods: In this study, ATOR-1017 is administered intravenously every 21 days as a single agent to patients with solid malignancies. ATOR-1017 is administered until confirmed progressive disease, unacceptable toxicity or withdrawal of consent. The primary objective of the study is to determine the maximum tolerated dose, assessed by adverse events (AEs) and dose limiting toxicities (DLTs), and the recommended phase 2 dose. Secondary objectives include pharmacokinetics, immunogenicity and clinical efficacy, assessed with CT scans using response criteria for use in studies testing immunotherapeutics (iRECIST). The study uses a single cohort design for doses up to 40 mg, and thereafter a modified 3+3 design. Results: The first patient was dosed in December 2019; by 22 Jan 2021, twelve patients have been exposed to ATOR-1017. The following dose levels have been evaluated; 0.38 mg; 1.5 mg; 5 mg; 15 mg; 40 mg and 100 mg. Dose escalation is ongoing, and the 200 mg dose level is under evaluation. The maximum tolerated dose has not been reached. The following cancer types are included; ovarian cancer (n = 1), choroidal melanoma (n = 3), anal cancer (n = 1), cholangiocarcinoma (n = 1), gastrointestinal stromal tumor (n = 1), breast cancer (n = 1), pancreatic cancer (n = 1), adenoid cystic cancer (n = 1), malignant melanoma (n = 1), colorectal cancer (n = 1). Drug-related AEs were reported in 5 out of 12 patients; one patient experienced a grade 3, all others were grade 1 or 2. There have been two episodes each of chest pain (grades 2 and 3) and headache (grades 1 and 2). Single cases of pyrexia, upper abdominal pain, mouth ulceration, nausea, leukopenia, neutropenia, cytokine release syndrome (CRS), arthralgia, neck pain, and rash were also reported. No DLTs have been observed in the study to date. The median age of the patients were 48.5 years (range 34-76). Patients received a median of 2 prior lines of therapy (range 1-4). The median time on study were 15 weeks (range 0.14-51). Six patients are on study, and six patients have discontinued treatment. Reasons for discontinuation include; investigator decision (n = 1), confirmed disease progression (n = 1), withdrawal of consent (n = 1), death due to disease progression (n = 1) and other reason (n = 2). Preliminary PK data show dose-proportional kinetics up to 100 mg. Best response has been stable disease. Conclusions: ATOR-1017 is safe and well-tolerated up to 100 mg. Dose escalation continues and the current dose level is 200 mg. Clinical trial information: NCT04144842