The FDA held up Mersana’s top antibody-drug conjugate (ADC), XMT-1522 over safety issues following a patient’s death in phase 1 trial, causing a descent in the company’s shares.

No new subjects are being allowed to register for the study on HER2-positive breast, lung and gastric tumors. However, those already taking the drug are permitted to continue doing so, as reported by the Cambridge, Massachusetts based Biotech Company.

Mersana will try for a release from the clinical hold, it said, by working with the FDA and the site inspector, who found the drug to be the probable cause of the death. It emphasized that the second phase 1 ADC contender, XMT-1536 for NaPi2b-expressing tumors remains unaffected. Nonetheless, Mersana’s stock price plummeted by one-third of its previous rate on Thursday and persisted to fall after few hours.

XMT-1522 is administered in an escalating dose as an intravenous infusion once every three weeks to spot the highest tolerated dose and pick the dose for phase 2 testing. Well tolerated, the ADC dose continues until the disease progresses. ADCs can cause side effects if the dosage is given before the stipulated time.

It would be too early for Mersana to speculate the probable reasons for the patient’s death. But it can bring XMT-1522 in race with Kadcyla, which sought $1 billion in revenue in last year if it manages to get the trial running by lifting the clinical hold.

Mersana CEO Anna Protopapas said, “Patient safety is our utmost concern. Based on the totality of the data we have for XMT-1522, we believe that it continues to be a promising drug candidate in the solid tumor setting and we will be initiating the proper steps with the objective of resuming enrollment.”