PURPOSE: The NHS-IL12 immunocytokine is composed of two IL-12 heterodimers fused to the NHS76 antibody. Preclinical studies have shown that this antibody targets IL-12 to regions of tumor necrosis by binding histones on free DNA fragments in these areas, resulting in enhanced antitumor activity. The objectives of this phase I study were to determine the maximum tolerated dose (MTD) and pharmacokinetics of NHS-IL12 in subjects with advanced solid tumors. EXPERIMENTAL DESIGN: Subjects (n=59) were treated subcutaneously with NHS-IL12 in a single ascending dose cohort followed by a multiple ascending dose cohort (n=37 with every 4-week dosing). RESULTS: The most frequently observed treatment-related adverse events (TRAEs) included decreased circulating lymphocytes, increased liver transaminases and flu-like symptoms. Of the grade >/= 3 TRAEs, all were transient and only one was symptomatic (hyperhidrosis). The MTD is 16.8 microg/kg. A time-dependent rise in IFN-gamma and an associated rise in IL-10 were observed postNHS-IL12. Of peripheral immune cell subsets evaluated, most noticeable were increases in frequencies of activated and mature natural killer (NK) cells and NKT cells. Based on TCR sequencing analysis, increases in T-cell receptor diversity and tumor-infiltrating lymphocyte density were observed post-treatment where both biopsies and peripheral blood mononuclear cells were available. Although no objective tumor responses were observed, 5 subjects had durable stable disease (range 6-30+ months). CONCLUSIONS: NHS-IL12 was well tolerated up to a dose of 16.8 microg/kg, which is the recommended phase II dose. Early clinical immune-related activity warrants further studies, including combination with immune checkpoint inhibitors.