Purpose: We aim to characterize VTX-2337, a novel Toll-like receptor (TLR) 8 agonist in clinical development, and investigate its potential to improve monoclonal antibody-based immunotherapy that includes the activation of natural killer (NK) cells. Experimental Design: HEK-TLR transfectants were used to compare the selectivity and potency of VTX-2337, imiquimod, CpG ODN2006, and CL075. The ability of VTX-2337 to induce cytokine and chemokine production from human peripheral blood mononuclear cells (PBMC) and activation of specific immune cell subsets was examined. The potential for VTX-2337 to activate NK cell activity through direct and indirect mechanisms was also investigated. Finally, we tested the potential for VTX-2337 to augment antibody-dependent cell-mediated cytotoxicity (ADCC), especially in individuals with low-affinity Fc gamma R3A single-nucleotide polymorphism (SNP). Results: VTX-2337 selectively activates TLR8 with an EC(50) of about 100 nmol/L and stimulates production of TNF alpha and interleukin (IL)-12 from monocytes and myeloid dendritic cells (mDC). VTX-2337 stimulates IFN gamma production from NK cells and increases the cytotoxicity of NK cells against K562 and ADCC by rituximab and trastuzumab. Effects of VTX-2337 on NK cells were, in part, from direct activation as increased IFN gamma production and cytotoxic activity were seen with purified NK cells. Finally, VTX-2337 augments ADCC by rituximab in PBMCs with different Fc gamma R3A genotypes (V/V, V/F, and F/F at position 158). Conclusions: VTX-2337 is a novel small-molecule TLR8 agonist that activates monocytes, DCs, and NK cells. Through the activation of NK cells, it has the potential to augment the effectiveness of monoclonal antibody treatments where a polymorphism in Fc gamma R3A limits clinical efficacy. Clin Cancer Res; 18(2); 499-509. (C) 2011 AACR.
