Background: DF1001 is a first in class, Tri-specific, NK cell Engager Therapy (TriNKET), that uses HER2 as an anchor to modulate innate and adaptive immunity. DF1001 redirects both NK and CD8 T cells to reprogram the tumor microenvironment, turning cold tumors hot. DF1001-001 (NCT04143711) is an ongoing phase I/II, first in human study investigating the safety, tolerability, clinical and biologic activity of DF1001. The safety, pharmacokinetic (PK), pharmacodynamic (PD) and efficacy data from the phase 1 monotherapy escalation and safety/PK/PD expansion is presented in this report. Methods: The Phase 1 dose escalation of DF1001-001 evaluated doses ranging from 0.000052-15mg/kg in patients with advanced and/or refractory solid tumors. Safety/PK/PD expansions, mandating paired biopsies, enrolled at doses 0.52-15mg/kg for biomarker assessment in patients with at least HER2 1+ expressing tumors. The primary objective of the Phase 1 study was to evaluate the safety of DF1001. The secondary objective was to assess efficacy. Results: As of 8 Dec 2022, 36 patients were treated in the dose escalation and 68 patients were treated in the safety/PK/PD expansion. 63% of the patients were female with a median age of 61 years. Most common tumor types included breast (38%), urothelial carcinoma (10%), esophageal (8%), non-small cell lung (7%) and colorectal (7%) cancer. Treatment Related Adverse Events (TRAE) were reported in 79% of patients. TRAEs were mostly low grade (86% were Grade 1-2) and rarely led to treatment discontinuation. No Grade 4 or 5 TRAEs were reported. The most common TRAEs were infusion related reactions (26%), asthenia (15%) and fatigue (12%). No dose limiting toxicities were detected, and the MTD has not been reached. Best overall response (BoR) of Partial Response (PR) by RECIST 1.1 was observed in 5 patients, with duration of up to 13 months, and 22 patients experienced stable disease (SD), for a Clinical Benefit Rate (CBR) of 39.7%. Responders included patients with refractory lung and breast cancer, whose tumors expressed HER2 1+ to 3+ by IHC. Biomarker analysis in responders demonstrated increases in inflammatory cytokines and chemokines by gene expression profiling, and consonant increases in NK and CD8 cells in the tumor microenvironment, consistent with TriNKETmechanism of action (MOA). Conclusions: In this dose escalation study, DF1001 is shown to be a safe and well-tolerated novel NK cell engager therapy targeting HER2. DF1001 monotherapy demonstrates promising anti-tumor activity in treatment refractory patients with a spectrum of HER2 expression as well as MOA consistent pharmacodynamic changes in the tumor. This promising data set supports further exploration of DF1001 alone, and in combination with other compounds, as a potential novel therapeutic option for patients with solid tumors. Clinical trial information: NCT04143711.