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Purpose: The NHS-IL12 immunocytokine is composed of two IL12 heterodimers fused to the NHS76 antibody. Preclinical studies have shown that this antibody targets IL12 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. Patients and Methods: 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 (TRAE) 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 μg/kg. A time-dependent rise in IFNγ and an associated rise in IL10 were observed following NHS-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 T-cell receptor sequencing analysis, increases in T-cell receptor diversity and tumor-infiltrating lymphocyte density were observed after 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 μg/kg, which is the recommended phase II dose. Early clinical immune-related activity warrants further studies, including combination with immune checkpoint inhibitors. See related commentary by Lyerly et al., p. 9