Background: MCLA-128 is a novel IgG1 bispecific antibody with enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) activity targeting HER2 and HER3 receptors. We report final phase 1 single agent escalation data, and safety and preliminary activity at the recommended phase 2 dose (RP2D). Methods: In the phase 1 part, patients (pts) with advanced solid tumors received MCLA-128 every 3 weeks (q3w) IV over 1-2 hr from 40 to 900 mg. In the phase 2 part, pts with selected metastatic indications were treated at the RP2D. Antitumor activity was assessed as per RECIST 1.1. Clinical benefit rate (CBR) was defined as CR + PR + SD ≥12 weeks. Results: As of January 2017, 28 advanced solid tumor pts were treated in the escalation part. No dose limiting toxicities were seen. The RP2D was established as 750 mg q3w (flat dose, corticosteroid premedication) based on safety and PK data. Fifteen pts with HER2 amplified tumors were treated at the RP2D (8 MBC, 4 gastric, 2 ovarian, 1 colorectal). Median age was 52 years (range 33-71), ECOG PS 0/1: 3/12, all ≥2 metastatic sites. The safety profile at the RP2D confirmed dose escalation data; the most common AEs were infusion related reactions in 6 pts (40%; G1-2 in 5 pts, G4 in 1 pt), and G1-2 diarrhea, rash, fatigue in 2 pts each (13%). No congestive heart failure or significant LVEF decreases occurred. The 8 MBC pts had a median 5.5 prior lines of metastatic therapy (range 4-14), all had progressed on 3 prior Her2 inhibitor therapies and received a median of 4.5 MCLA-128 cycles (range 2-12); 1 had a confirmed PR, 5 had SD (including 2 sustained, 11 and 12 cycles). SD was also seen in 2 evaluable MBC pts treated at 480 mg in the phase 1 part (7 and 4 cycles). Overall, CBR in these 10 MBC pts was 70%. Evaluation of other indications is ongoing. Conclusions: MCLA-128 showed a well tolerated safety profile. Consistent antitumor activity was seen in heavily pretreated MBC patients progressing on HER2 therapies. Further exploration of MCLA-128 based combinations with chemotherapy or trastuzumab in less pretreated MBC patients progressing after ≥2 prior Her2 inhibitors including TDM-1 is planned. Clinical trial information: NCT02912949