Why Mersana Therapeutics Stock Is Plunging Today?

Mersana Therapeutics Inc MRSN shares are sinking after the company announced that the UPLIFT clinical trial of upifitamab rilsodotin (UpRi) did not meet its primary endpoint. 

UpRi is an ADC targeting the sodium-dependent phosphate transport protein NaPi2b and was developed utilizing the company's Dolaflexin platform.

UPLIFT was a single-arm clinical trial that enrolled platinum-resistant ovarian cancer patients with one to four prior treatment regimens. 

The primary endpoint for UPLIFT was the investigator-assessed objective response rate (ORR) in the NaPi2b-positive population. 

Secondary endpoints for the trial included the investigator-assessed ORR among all patients, duration of response (DOR), and safety and tolerability. 

The trial also included an assessment of ORR and DOR by independent radiology review (IRR).

UPLIFT enrolled 268 patients, 141 of whom were determined to be NaPi2b positive.

Safety and tolerability data in UPLIFT were generally consistent with prior disclosures. 

Mersana Therapeutics will now focus on next-generation ADC platforms.

The company plans to complete the dose escalation portion of the Phase 1 clinical trial of XMT-1660 in 2023 and initiate the dose expansion portion in 2024. 

Mersana's restructuring plan includes a wind-down of UpRi-related development activities, including its UP-NEXT and UPGRADE-A clinical trials and the company's regulatory and commercial readiness efforts. 

If data analyses enable the identification of a path forward for UpRi, the company will consider strategic alternatives for the asset, including partnering.

Mersana estimates that its cash balance of $286.6 million will be sufficient to support its current operating plan commitments into 2026.

Price Action: MRSN shares are down 76% at $0.94 on the last check Thursday.