The Chemotherapeutic Agent, 5,6-Dimethylxanthenone-4-Acetic Acid,

Vascular disrupting agents (VDA) represent a novel approach to the treatment of cancer, resulting in collapse of tumor vasculature and tumor death. 5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a VDA currently in advanced Phase II clinical trials, yet its precise mechanism of action is unknown despite extensive preclinical and clinical investigations. The data presented herein demonstrate that DMXAA is a novel and specific activator of the TBK1-IRF-3 signaling pathway. DMXAA treatment of primary murine macrophages resulted in robust IRF-3 activation, a ~750-fold increase in IFN-beta mRNA and, in contrast to the potent Toll-like receptor 4 (TLR4) agonist, lipopolysaccharide (LPS), signaling was independent of mitogen-activated protein kinase (MAPK) activation and elicited minimal NF-kappaB-dependent gene expression. DMXAA-induced signaling was critically dependent on the IRF-3 kinase, TBK1, and IRF-3, but MyD88-, TRIF-, IPS-1/MAVS-, and IKKbeta-independent, thus excluding all known TLRs and cytosolic helicase receptors. DMXAA pretreatment of murine macrophages induced a state of tolerance to LPS and vice versa. In contrast to LPS stimulation, DMXAA-induced IRF-3 dimerization and IFN-beta expression were inhibited by salicylic acid (SA). These findings detail a novel pathway for TBK-1-mediated IRF-3 activation and provide new insights into the mechanism of this new class of chemotherapeutic drugs. Keywords: DMXAA, IRF-3, TBK1, IFN-beta, Salicylic acid, TLR Microarray analysis was carried out using Affymetrix® mouse array 430A_2 exposed to total RNA prepared from C57BL/6J or IFN-beta-/- macrophages that had been treated with medium alone or DMXAA for 3 h. Fold-induction was calculated using Affymetrix® GeneChip® Operating Software. A >3-fold increase or decrease between inducible and basal mRNA levels was set as the criteria for inclusion of a gene as modulated.