Why Are Mersana Therapeutics Shares Nosediving Today

The FDA has issued a partial clinical hold pausing new patient enrollment in Mersana Therapeutics Inc's MRSN UP-NEXT and UPGRADE-A, the company's ongoing clinical trials of UpRi in platinum-sensitive ovarian cancer. 

UPLIFT, Mersana's ongoing clinical trial of UpRi in platinum-resistant ovarian cancer, completed enrollment in October 2022. Patients who are already enrolled in these trials may continue receiving UpRi. Mersana expects to lock its UPLIFT clinical trial database and disclose UPLIFT top-line data by early August.

The partial clinical hold follows a submission by Mersana of a recent aggregate safety report of all patients dosed with UpRi (approximately 560 patients) evaluating bleeding events. 

Although data on the background rate of bleeding in platinum-resistant ovarian cancer are limited, Mersana's recent assessment determined that serious bleeding events appear to occur at a higher rate than the background. 

While most bleeding cases in this aggregate safety analysis were low-grade, five (<1%) Grade 5 (fatal) bleeding events were observed. The causes of bleeding events remain under investigation.

In March, the FDA placed a clinical hold on Mersana's Phase 1 trial of XMT-2056 soon after it voluntarily suspended the trial due to a recent Grade 5 (fatal) serious adverse event deemed related to XMT-2056. 

Price Action: MRSN shares are down 64.8% at $3.36 during the premarket session on the last check Thursday.