Mersana Therapeutics Inc. is getting $100 million up front in an option deal with GSK plc for preclinical-stage antibody-drug conjugate (ADC) XMT-2056, which could bring up to $1.36 billion more in an option exercise payment, development, regulatory and commercial milestones. It’s the second potential $1 billion-plus ADC deal for Cambridge, Mass.-based Mersana in 2022 and the first for its Immunosynthen platform, which uses a STING agonist payload specifically designed for ADCs.