Background: IL-2 and IL-15 signal through the shared IL-2/15 βγ receptor, but unlike IL-2, IL-15 does not expand regulatory T cells (Tregs), does not mediate activation-induced cell death and may have an improved therapeutic index. KD033 is a fusion antibody combining a fully human, high affinity anti-human Programmed Death Ligand 1 (PD-L1) IgG1 antibody with the human IL-15 receptor alpha (IL15Rα) sushi domain and human IL-15 (IL-15). KD033 (or its mouse cross reactive surrogate molecule, srKD033) has been extensively characterized in multiple invitro and in vivo nonclinical studies. The fusion of anti-PD-L1 antibody to IL-15 significantly increases the maximal-tolerated dose (MTD) of srKD033 in mice compared to free IL-15. In addition, srKD033 has exhibited increased efficacy in rejecting tumors in mice as compared to the combination of its individual components, anti-PD-L1 antibody and IL-15. Methods: This is a phase 1, open-label, multiple ascending dose, multi-center clinical trial being conducted in patients with metastatic or locally advanced solid tumors (NCT04242147). The primary objective is to determine the safety and tolerability and the MTD of KD033. Secondary objectives include characterization of PK and immunogenicity, evaluation of CD8 T and NK cell activation and assessment of best overall response and duration of response. KD033 is administered by IV infusion over 30 minutes every 14 days. Accelerated intra-patient dose escalation across the initial three dose levels, followed by 3+3 escalation thereafter, is investigating dose ranges from 3 µg/kg to 600 µg/kg. Efficacy evaluation is planned in an expansion cohort of patients with PD-1/L1 refractory tumors. Results: A total of 7 patients have received treatment. Three patients were dosed in Cohort 1 and four patients were dosed in Cohort 2. Through two dose escalation cohorts (3 µg/kg – 25 µg/kg), no dose-limiting toxicities have been reported. Grade 1-2 treatment-related toxicities, when observed, resolved within 24 hours with supportive management. 6 patients are evaluable for treatment response with one patient (adenoid cystic carcinoma) in the first cohort having stable disease for more than 6 months. Conclusions: KD033 has been well tolerated early in dose escalation with on-mechanism pharmacodynamics consistent with IL-15 agonism. Clinical trial information: NCT04242147