3083 Background: PG545 (pixatimod, pINN) is a novel immunomodulatory agent which stimulates dendritic cells (DC) via TLR9/IL-12 pathway to activate natural killer (NK) cells. It also inhibits tumour-associated macrophages in cancer models. We report on safety, PK, PD, and antitumor activity of PG545 monotherapy. Methods: In this dose escalation (3+3 design) study, eligible pts (ECOG≤1) with advanced solid malignancies who failed standard therapies received PG545 once weekly as a 1-hour i.v. infusion until disease progression or discontinuation due to intolerability. The primary objective was determination of the maximum tolerated dose (MTD). Secondary objectives evaluated safety, antitumor activity based on RECIST (1.1) criteria, PK and PD (plasmacytoid DC & NKp46+NK cells from PBMC, and plasma cytokines/chemokines). Results: The study recruited 23 subjects across four cohorts (25, 50, 100 & 150 mg). Three dose limiting toxicities (DLTs) - hypertension (2), epistaxis (1) - occurred in the 150 mg cohort, which was identified as a non-tolerated dose level. No DLTs occurred in the 100 mg cohort, which was identified as the MTD. Six SAEs were reported to be possibly or likely related to PG545 treatment. No RECIST 1.1 objective responses were reported; best response was prolonged stable disease up to 24 weeks (mCRC), with disease control rate in evaluable subjects of 38% (6/16) at eight weeks. Exposure (AUC0-last) was proportional up to 100mg and mean half-life was 144 hours. At 50 and 100mg dose levels, two subjects in each cohort exhibited up to 4-fold increased numbers of NKp46+NK cells, IFN-α-producing pDCs, and increases (up to 25-fold) in plasma IFN-γ, TNF-α, IP-10 and MCP-1. Conclusions: PG545 is well tolerated up to 100 mg once-weekly via i.v. infusion. Human exposure data at 50mg and 100mg reach exposures consistent with those required for preclinical efficacy. Preliminary PD data support the proposed mechanism of action, which represents a promising approach to improve the efficacy of existing therapies. These data, and the absence of toxicities associated with chemo- or immunotherapies, support the development of PG545 in combination clinical trials. Clinical trial information: NCT02042781.