GSK has finally called time on its pact with Mersana Therapeutics despite sinking $100 million to secure an option for an antibody-drug conjugate (ADC).
That deal, signed back in 2022, centered on XMT-2056, an immunosynthen STING-agonist ADC aimed at treating a range of HER-2 cancers. GSK gained the exclusive option to co-develop and commercialize the preclinical-stage asset, with Mersana in line for up to $1.36 billion in biobucks if the British Big Pharma had followed through.
Developed using Mersana’s immunosynthen platform, XMT-2056 is designed to activate the innate immune system through STING signaling—a key pathway in tumor control.
But XMT-2056 had a bumpy ride through the clinic, with the FDA slapping a partial hold on the ADC during 2023 after the therapy was tied to a higher rate of serious bleeding events in a phase 1 study of patients with platinum-resistant ovarian cancer. The bleeding events included five deaths.
Despite this, GSK continued to show an interest in XMT-2056, handing Mersana a $15 million milestone last year without committing to fully taking on the ADC. By that time, Mersana was already in the process of being bought by Day One.
Now, GSK has come to a decision—disclosing in its first-quarter earnings results that XMT-2056 has been removed from its phase 1 pipeline. Fierce Biotech understands that the decision was made after GSK reviewed the available data.
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The move comes as GSK—which markets the multiple myeloma ADC Blenrep—continues to ramp up its own ADC strategy. The company revealed earlier in April that it is planning to launch five late-stage trials of mocertatug rezetecan—a B7-H4-directed ADC licensed from Hansoh Pharma—over “the next few months.”
The decision to call time on Mersana’s ADC wasn’t the only pipeline change disclosed today by GSK. The company also revealed that it has paused work on an mRNA vaccine for bird flu called GSK5536522. Fierce understands that this decision was made to prioritize GSK’s mRNA vaccine for seasonal flu, dubbed GSK4382276, which is currently in phase 2 development.
GSK is busy establishing that platform and ensuring manufacturing processes are ready. Once those processes have been established, GSK expects to reassess the bird flu program. The pharma already has an FDA-approved monovalent vaccine for bird flu, which is designed to be stockpiled in case of a future pandemic.
Finally, GSK has stopped work on a TLR8 agonist, dubbed GSK5251738, for chronic hepatitis B virus. The pharma has been going big in HBV in recent months, with bepirovirsen accepted for priority review by the FDA this week as a potential “functional cure” for the infection.
