Shares of antibody drug conjugate developer Mersana Therapeutics Inc. (NASDAQ:MRSN) fell 39.1% to $8.56 on Sept. 10 after the company disclosed a second death from pneumonitis in the ongoing phase I trial testing its antibody drug conjugate, upifitamab rilsodotin against tumor types likely to express the sodium-dependent phosphate transport protein NaPi2b, such as ovarian cancer.