D18 is an immune modulator. D18 acts as a TLR7/8 dual agonist (EC50=24 nM for hTLR7 and 10 nM for hTLR8, respectively). D18 increases PD-L1 expression through epigenetic regulation, thus sensitizing tumors to PD-1/PD-L1 blockade. D18 is a ADC cytotoxin uesd for the systhesis of ADC HE-S2.