BACKGROUND: MCLA-128 is a novel humanized full length IgG1 bispecific antibody with enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) activity that targets the transmembrane tyrosine kinase receptors, HER2 and HER3 present on a variety of epithelial tumor cells. In preclinical studies, MCLA-128 demonstrated potent anti-tumor activity. The primary objectives of this First-in-Human (FIH) study are to establish the MTD and Phase 2 recommended dose (RP2D) of MCLA-128 and to assess its safety, tolerability, PK, PD, and anti-tumor activity in selected patient (pt) groups.

METHOD: The trial consists of 2 parts. In Part 1 (dose-escalation) patients (pts) with advanced epithelial tumors with no standard management options were enrolled with an initial accelerated 1 pt per cohort design shifting to a 3 + 3 design. The DLT evaluation period was 21 days ( = 1 cycle). MCLA-128 was administered every 3 weeks (q3w) as an intravenous infusion over 60-120 minutes. Toxicities were assessed throughout dose escalation (40 mg up to 900 mg flat dose). Part 2 consists of several expansion cohorts of pts at the RP2D (expansion phase). Here we present interim results for Part 1 of the study.

RESULTS: Twenty-eight pts received MCLA-128 in 9 dose-escalation cohorts (40-160 mg in cohorts of 1 evaluable pt, 240-900 mg in cohorts of 3 to 6 evaluable pts). The most frequent drug related adverse events (AEs) observed in Part 1 were mild to moderate (G1-G2) with prompt recovery in general. AEs included infusion-related reactions (IRR), diarrhea, nausea, vomiting, fatigue, maculopapular rash, oral mucositis and G2 neutropenia in 1 pt. IRR occurred in 50% of pts with a trend for increased incidence with dose; modest and transient dose dependent cytokine release was also observed. Increasing the infusion duration to 120 min and premedication with corticosteroids was successfully implemented to mitigate IRRs and cytokine levels were no longer correlative with MCLA-128 dose levels. One drug related serious AE (SAE) has been reported: IRR G3, due to prolonged hospitalization (SUSAR). No DLTs were observed. In serum samples from 12 pts, 1 pt was positive for MCLA-128 anti-drug antibodies 36 days after starting treatment at a dose of 160 mg.

A MTD was not reached in the study; PK analysis was used to support a RP2D of 750 mg q3w. Exposure to MCLA-128 increased in proportion to dose. The RP2D was calculated to maintain trough concentrations at the end of each treatment cycle significantly above the Kd of MCLA-128 as determined on HER2 amplified cell lines in vitro and above the Km in a population PK model.

Preliminary signs of anti-tumor activity assessed by RECIST are encouraging. Three pts remain on study to date including 1 non-small-cell lung cancer pt who is experiencing an ongoing partial response for more than 10 months, 1 gastroesophageal junction pt and 1 colorectal cancer pt with stable disease for 5 months. In addition, 1 metastatic breast cancer pt received 5 months of MCLA-128 treatment until disease progression.

CONCLUSION: MCLA-128 was well-tolerated with a favorable safety profile in pts with advanced tumors treated at doses up to 900 mg q3w. PK results and preliminary signs of anti-tumor activity support the further clinical evaluation of MCLA-128 at the RP2D of 750 mg q3w. The expansion phase of the study is ongoing at this dose level.

Citation Format: Emiliano Calvo, Maria Alsina, Jan H.M. Schellens, Alwin DR Huitema, Josep Tabernero, Aurelia de Vries-Schultink, Valentina Boni, Bernard Doger, Cecile Geuijen, Robert Doornbos, Kees Bol, Martine Westendorp, Mark Throsby, Lex Bakker, Setareh Shamsili. A Phase I/II Study of MCLA-128, a full length IgG1 Bispecific Antibody Targeting HER2 and HER3, in Patients with Solid Tumors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr CT050.