PS1322 ASTX660, A NON‐PEPTIDOMIMETIC ANTAGONIST OF CIAP1/2 AND XIAP, INDUCES APOPTOSIS IN T CELL LYMPHOMA BY ENHANCING IMMUNE MEDIATED AND DEATH RECEPTOR DEPENDENT KILLING

Abstract

Background:ASTX660 is a potent, non‐peptidomimetic antagonist of the cellular and X‐linked inhibitors of apoptosis proteins (cIAP1/2 and XIAP), which is currently being tested in a first in human phase I‐II study in patients with advanced solid tumors and lymphomas (NCT02503423). IAP antagonists enhance tumor necrosis factor (TNF) receptor superfamily mediated apoptosis and are potent anti‐tumor immune enhancers.Aims:Herein, we describe the profile of ASTX660 in a range of T cell lymphoma (TCL) cell lines and evaluate ASTX660's ability to enhance immune mediated killing of tumor cells.Methods:A panel of human and mouse T‐cell lymphoma cell lines were tested in proliferation (Alamar blue or CellTiterGlo) or apoptosis assays (activated caspase‐3 substrate assays by IncuCyte or FACS) for sensitivity to ASTX660 alone or in combination with recombinant Death Receptor ligands (TNFα, FASL or TRAIL). Additionally, we used a novel co‐culture system of tumor cell lines with anti‐CD3 activated human peripheral blood mononuclear cells (PBMC) to assess ASTX660 effects on immune mediated cell killing. Target engagement and induction of apoptosis markers were analysed by Western blotting.Results:ASTX660 antagonises IAPs in TCL cell lines, as indicated by a decrease in cIAP1 protein levels. This ASTX660‐dependent decrease in cIAP was associated with an increase in TNFα‐dependent apoptosis in the EL4 and L5178 TCL cell lines. Several T‐cell lymphoma models, including HuT‐78, HH and My‐La expressed low levels of TNFR1 and therefore did not respond to ASTX660 in the presence of TNFα. However, in these cell lines, ASTX660 conferred a significant increase in FASL or TRAIL‐dependent apoptosis, indicating that ASTX660 sensitises TCL cells to various death receptor ligands and response correlates with receptor expression levels. In addition to this direct effect on TCL cell lines, ASTX660 also enhances anti‐CD3 stimulated PBMC‐dependent killing of multiple tumour cell lines, including TCL lines, via induction of caspase activity. Additional preclinical experiments (both in vitro and in vivo) are underway to further characterise the mode of action of ASTX660 in TCL.Summary/Conclusion:The combination of both direct and indirect effects of ASTX660 on TCL lines, described here, supports the ongoing clinical testing of ASTX660 in TCL (NCT02503423). Preliminary clinical efficacy and safety data of ASTX660 in relapsed/refractory (r/r) peripheral T cell lymphoma and cutaneous T cell lymphoma is the subject of a separate abstract.