Mersana Therapeutics, Inc. announced the initiation of patient dosing in its Phase 1 clinical trial of XMT-2056, the company's lead Immunosynthen product candidate. XMT-2056 is a systemically administered Immunosynthen STING agonist ADC that is designed to target a novel HER2 epitope and locally activate STING signaling in both tumor-resident immune cells and in tumor cells to provide the potential to treat patients with HER2-high or -low tumors as monotherapy or in combination with standard-of-care agents. The U.S. Food and Drug Administration (FDA) has granted orphan drug designation to XMT-2056 for the treatment of gastric cancer.

The multicenter Phase 1 open-label trial will investigate XMT-2056 in previously treated patients with advanced/recurrent solid tumors expressing HER2, including breast, gastric, colorectal and non-small-cell lung cancers. The dose escalation and dose expansion portions of the trial will evaluate and characterize the relationship of safety, tolerability and exposure of XMT-2056 and the ADC's effect on patient responses by overall response rate, duration of response and disease control rate. Mersana recently entered into an agreement with GSK that provides GSK with an exclusive option for a global license to co-develop and commercialize XMT-2056.

Under the terms of the agreement, Mersana received an upfront option purchase fee of $100 million and is eligible to receive up to $1.36 billion in the form of an option exercise payment and development, regulatory and commercial milestone payments if GSK exercises its option. Mersana has retained options to profit-share and to co-promote in the United States. If it exercises its profit-share option, Mersana will be eligible to receive tiered royalties on net sales of licensed products outside of the United States.

If Mersana does not elect to profit-share, it is eligible to receive double-digit tiered royalties on global net sales. If GSK opts into the license, the effectiveness of the license grant may be subject to customary closing conditions, including review under the Hart-Scott-Rodino Act.