After paying $100 million for an option on an antibody-drug conjugate (ADC) from Mersana Therapeutics, GSK has finally declared its deal with the biotech company dead in the water.
The agreement, inked in 2022, was for XMT-2056, an immunosynthen STING-agonist ADC for HER-2 cancers. The agreement gave the British Big Pharma the exclusive option to co-develop and co-market the preclinical candidate, and Mersana would have scored $1.36 billion in biobucks had the British Big Pharma gone ahead with the deal.
XMT-2056, based on Mersana’s immunosynthen technology, aims to activate the body’s immune system via STING, a key pathway in the fight against tumors.
However, XMT-2056 has experienced a rocky clinical journey, with the FDA placing a partial clinical hold on the ADC in 2023 due to the ADC being associated with a higher incidence of serious bleeding events in a phase 1 trial of a platinum-resistant ovarian cancer patient population. Five of the patients died from the bleeding.
Yet GSK remained interested in XMT-2056 and last year paid Mersana a $15 million milestone payment, without agreeing to develop the ADC fully. At that point, Mersana was in the midst of an acquisition by Day One. Now, GSK has decided, announcing in its first-quarter financial report, that XMT-2056 is no longer in its phase 1 pipeline.
The decision comes as GSK, which has the multiple myeloma ADC Blenrep on the market, is gearing up its ADC portfolio. Earlier this month, the company announced it plans to start five phase 3 studies of its mocertatug rezetecan (a B7-H4-targeted ADC that it has in-licensed from Hansoh Pharma) over “the next few months”.
Calling off the ADC wasn’t the only pipeline move GSK announced today. GSK also announced it has put development of its mRNA bird flu vaccine GSK5536522 on hold. This move was to focus on its mRNA seasonal flu vaccine (GSK4382276) in phase 2 development.
GSK is now putting that in place and making sure it has processes. Once this is done, it will review the bird flu program. GSK already has an FDA-approved monovalent bird flu vaccine, which is to be stockpiled in the event of a pandemic.
Lastly, GSK has halted development of a TLR8 agonist (GSK5251738) for the hepatitis B virus. The pharma has been ramping up in HBV in recent weeks, with this week’s FDA acceptance for priority review of bepirovirsen for a “functional cure” for the virus.
GSK Suspends mRNA Bird Flu Vaccine Program – Mersana Context
In a surprising move, GSK announced the suspension of its mRNA bird flu vaccine program, reflecting a broader reassessment of its mRNA strategy. While mRNA gained massive traction during the COVID-19 pandemic, companies are now becoming more selective in their pipeline investments.
The decision indirectly impacts Mersana, as capital allocation and R&D priorities shift across the industry. With competitors like Moderna and BioNTech continuing to invest heavily in mRNA platforms, GSK’s retreat suggests a more cautious approach moving forward.
Mersana Deal Impact: GSK Withdraws $100M STING ADC Wager
A more direct impact on Mersana comes from GSK’s withdrawal of its $100 million investment tied to Mersana’s STING ADC program. This partnership had been seen as a promising step in advancing innovative cancer therapies.
Mersana’s STING (Stimulator of Interferon Genes) ADC approach aims to activate the immune system directly within tumors. Despite its potential, GSK’s decision signals uncertainty in early-stage oncology assets and a desire to streamline portfolios.
For Mersana, this move raises questions about future funding, partnerships, and the timeline for clinical development.
These dual decisions by GSK reflect a broader trend across the pharmaceutical industry:
- Increased focus on late-stage and commercially viable assets
- Reduced risk tolerance for early-stage or experimental platforms
- Strategic portfolio optimization
For Mersana, this environment presents both challenges and opportunities. While losing a major partner is a setback, it also opens doors for new collaborations or independent development strategies.
