Novel Toll-like Receptor 2 Ligands for Targeted Pancreatic Cancer Imaging and Immunotherapy

Abstract

Toll-like receptor 2 (TLR2) is a target for immune system stimulation during cancer immunotherapy and a cell-surface marker for pancreatic cancer. To develop targeted agents for cancer imaging and therapy, we designed, synthesized, and characterized 13 novel, fully synthetic high affinity TLR2 agonists. Analogue 10 had the highest agonist activity (NF-κB functional assay, EC(50) = 20 nM) and binding affinity (competitive binding assay, K(i) = 25 nM). As an immune adjuvant, compound 10 stimulated the immune system in vivo by generation and persistence of antigen-specific CD8+ T cells indicating its potential use in cancer immunotherapy. After conjugation of near-infrared dye to 10, agonist activity (EC(50) = 34 nM) and binding affinity (K(i) = 11 nM) were retained in 13. Fluorescence signal was present in TLR2 expressing pancreatic tumor xenografts 24 h after injection of 13, while an excess of unlabeled ligand blocked 13 from binding to the tumor, resulting in significantly decreased signal (p < 0.001) demonstrating in vivo selectivity.