The FDA has placed a clinical hold on Mersana Therapeutics’ phase 1 trial evaluating XMT-2056 following a fatal adverse event that was determined to be related to the investigational drug.

XMT-2056 is an antibody-drug conjugate candidate intended for use in previously treated patients with HER2-positive recurrent or metastatic solid tumors.

The fatality occurred in the dose escalation portion of the phase 1 study and affected the second clinical trial participant who had been enrolled at the initial dose level.

Under the clinical hold, no new patients will be enrolled or dosed in the trial and the company says it is analyzing the serious adverse event and considering “potential next steps for development.”