Patient Death: Mersana Shares Plunge

Shares of Mersana Therapeutics have plunged nearly 40 percent this morning after the company announced the U.S. Food and Drug Administration (FDA) placed a partial clinical hold on its Phase I antibody drug conjugate XMT-1522 following the death of a patient.

Mersana disclosed that the partial clinical hold was initiated following a report of a patient death. The patient had received a “dose level 7” of XMT-1522, the company said. The death has been classified by the trial investigator as “possibly drug-related,” the Cambridge, Mass.-based company said.

XMT-1522 is a Dolaflexin ADC targeting HER2-expressing tumors. The company is exploring XMT-1522 as a potential treatment for HER2-expressing breast cancer, non-small cell lung cancer (NSCLC), and gastric cancer. According to company data, XMT-1522 includes a proprietary HER2 antibody which is conjugated with Mersana’s Dolaflexin platform – a Fleximer polymer linked with a proprietary auristatin payload. The platform allows for the attachment of an extensive range of anti-tumor payloads to Fleximer. The ADC is designed to remain stable in the bloodstream and to release the drug payloads once inside the targeted cell.

Despite the patient death, Anna Protopapas, Mersana’s chief executive officer, said the company believes XMT-1522 “continues to be a promising drug candidate in the solid tumor setting.” Protopapas added that patient safety is of “utmost concern” for the company. She said Mersana will be initiating the proper steps to address the safety of the medication and patients, with an objective or resuming enrollment in the trial.

Mersana believes that XMT-1522 has the potential to extend HER2-targeted therapy beyond the current “HER2-positive” populations into patients with lower levels of HER2 expression. That’s due, in part, to its ability to provide a drug load of approximately 12 molecules per antibody – a way of potentially increasing efficacy and tolerance. In June Mersana presented interim-dose escalation data at the American Society of Clinical Oncology meeting in Chicago. The poster presentation at ASCO showed the drug had a favorable tolerability profile and demonstrated early signs of efficacy. The early data was gathered from the Phase I trial of heavily pre-treated patients who have exhausted available therapies.

Under the partial clinical hold, Mersana said it will not be allowed to enroll new patients in the trial, although current study patients will continue to receive their treatments and participate in the trial consistent with its protocol. Mersana said it will be working closely with the FDA and the site investigators to review the death and to seek to resolve this clinical hold. The partial clinical hold does not affect the ongoing clinical development of Mersana’s product candidate XMT-1536, currently in Phase 1 clinical trials for NaPi2b-expressing cancers, including ovarian cancer, non-small cell lung cancer and other cancers.