Mersana Therapeutics Inc. has signed a potential billion-dollar agreement with Johnson & Johnson’s Janssen Biotech Inc. to research and develop antibody-drug conjugates (ADCs) for three cancer targets. Janssen will pay $40 million up front as well as more than $1 billion in potential milestone payments in a big day for ADCs, which saw Mersana’s other development partner, Synaffix BV, announce a tie-up with Macrogenics, Inc. in a deal worth up to $586 million.