Deserted by Takeda, Mersana abandons lead drug

Mersana Therapeutics’ rocky road in developing its lead drug has culminated in the abandonment of the experimental cancer treatment, days after its high-profile partner Takeda decided to walk away from their deal.

Back in 2016, Takeda and its unit Millennium expanded a prior partnership with Mersana by securing the the ex-US commercial rights to the Cambridge, Massachusetts-based company’s then-lead preclinical antibody drug conjugate — XMT-1522 — for $60 million upfront and a host of other milestone payments, which in tandem with their existing agreement added up to a $1 billion-plus deal.

Takeda backed Mersana $MRSN on the promise of its next-gen antibody-drug conjugates (ADCs), or armed antibodies.

This potent class of drugs brings together two ingredients by chemically linking antibodies to a biologically active drug or cytotoxic compound. This combines the targeting capabilities of antibodies — allowing sensitive discrimination between healthy and cancer tissues — with the cell-killing ability of chemotherapies. While existing ADCs can be limited to just a few therapies, Mersana’s tech platform is designed to produce drugs that can link up with 12 to 15, in theory vastly enhancing the therapeutic potential of their crop of ADCs for diseases previously out of reach.

Anna Protopapas

It was this additional firepower that attracted Takeda to Mersana, which is run by former Millennium chief Anna Protopapas. Mersana went public in 2017, but a year later trouble was afoot. Last July, Mersana reported a patient death in a Phase I trial testing XMT-1522 in patients with breast, gastric, and non-small cell lung cancers whose tumors express HER2. The company acknowledged the death could be related to the experimental drug, and the FDA slapped a partial clinical hold on the trial. Mersana’s other cancer drug-in-development, XMT-1536, was unaffected.

By September, the company and the FDA agreed to a revamped development program for XMT-1522, and the agency lifted its hold. However, it looks like Takeda management decided that the program, and indeed Mersana’s tech, was too risky to continue their collaboration. In an SEC filing, Mersana said that the Japanese drugmaker’s Millennium unit had elected to terminate the partnership on January 2.

In its press release on Friday, Mersana said it had abandoned the development of XMT-1522 altogether to focus on its other experimental drug, XMT-1536. The company’s shares were down about 15% in early trading.

“We have made the difficult decision to terminate the further development of XMT-1522 despite a favorable emerging profile of efficacy and tolerability due to the competitive environment for HER2-targeted therapies,” Mersana chief Protopapas said in a statement.

Leerink and Cowen analysts suggested Mersana’s decision was pertinent, considering the competitive landscape for HER2-targeting therapies. Several such therapies have already been sanctioned approval, and others are in development, including Daiichi Sankyo’s DS-8201 that carries 7-8 payloads, noted Cowen’s Boris Peaker.

“Many investors already had very low expectations for XMT-1522…especially following the brief partial clinical hold placed on XMT-1522 that has contributed to the stock trading near historic lows. The decision to focus resources on advancing XMT-1536 could also have a positive impact on the company’s cash runway,” Leerink analysts wrote in a note.

The company also provided encouraging initial data on XMT-1536 — a NaPi2b- targeting ADC — from an ongoing dose escalation study.

“Ultimately…it will be important for XMT-1536 to demonstrate durability of responses achieved given Roche’s past NaPi2b disappointments,” they added.

Mersana contacted Endpoints News to assert that the Takeda partnership was terminated by mutual consent, echoing its press release posted on Friday. 

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Research Scientist - Immunology
Recursion Pharmaceuticals Salt Lake City, UT
Director of Operations
Atlas Venture Cambridge, MA

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