Ons (IL15/ IL15R-Het-Fc) with decreased potency to enhance tolerability, slow receptor-mediated clearance, and prolong half-life. Methods We engineered IL15/IL15R-Het-Fc by fusing IL15 to one side of a heterodimeric Fc, as well as the sushi domain of IL15R to the other. Fcfusions had been tuned for optimal activity by engineering amino acid substitutions in IL15 – at the IL2R or c interface – that decreased in vitro potency. In vitro proliferation of lymphocytes in standard human PBMCs was monitored by counting Ki67+ cells following incubation with Fc-fusions for four days and by measuring signaling in a STAT5 phosphorylation assay. In vivo activity was evaluated applying a huPBMC-NSG mouse model by measuring the extent of human leukocyte engraftment by flow cytometry and IFN. Tolerability, immune stimulation, and pharmacokinetics were evaluated in nonhuman primates (NHP). A computational PK/PD model was created and trained on available data to quantify relationships in between affinity, dose, and biological activity. Outcomes IL15/IL15R-Het-Fc had been developed with good yield and purity. The Fc-fusions enhanced proliferation of CD8+ T and NK cells in vitro. Variants with substitutions at the IL2R and/or c interface reduced potency as much as 700-fold when compared with wild-type IL15/IL15R-Het-Fc. Remedy of huPBMC-NSG mice with IL15/IL15R-Het-Fc promoted enhanced T cell engraftment and elevated IFN. NHP studies indicated half-lives of numerous days for potency-reduced IL15/IL15R-HetFc, which are significantly longer than the 1 hr half-life of IL15. In each in vivo settings, a marked inverse correlation of pharmacodynamics and clearance was observed, with lowered potency variants permitting higher, more tolerated doses and enhanced lymphocyte proliferation due to more sustained exposure. Ourmechanism-based PK/PD model was used to predict optimal drug affinities, balancing potency vs. target-mediated clearance, and can be made use of to facilitate prediction of human PK/PD and regimen design. A lead candidate XmAb24306 was selected determined by combined experimental observations and modeling predictions, and has been chosen for clinical development. Conclusions Multiple IL15/IL15R heterodimeric Fc-fusions were engineered for lowered potency and evaluated in vitro and in vivo. We identified a variant, named XmAb24306, that optimally balanced potency and exposure. P412 Tumor cell-intrinsic defects in STING pathway signaling Blake Flood, BS1, Leticia Corrales2, Thomas Gajewski, MD, PhD1 1 University of Chicago, Chicago, IL, USA; 2Aduro, Berkeley, CA, USA Correspondence: Blake Flood ([email protected]) Journal for Protease Nexin I Proteins Formulation towards the nucleus where it acts as a transcription issue to induce numerous genes including IFN-. STING signaling and IFN- receptor signaling in tumor-infiltrating immune cells, in turn, are expected for optimal priming of CD8+ T cells against tumor antigens. According to this notion, STING agonists happen to be pursued as a pharmacologic approach to activate the pathway. However, irrespective of whether tumor cells themselves also can practical experience STING pathway activation via to IFN- production has been unclear. Approaches We stimulated several cell populations present within the.