The U.S. Food and Drug Administration is expected to issue a formal acceptance and filing confirmation for Daiichi Sankyo and Merck's ifinatamab deruxtecan (I-DXd) on Thursday, following the priority review designation granted to the lung cancer therapy this week. The move signals the agency's intent to complete its review within six months, potentially bringing a new treatment option to patients with extensive-stage small cell lung cancer by late 2026.

Ifinatamab deruxtecan is a HER3-directed antibody-drug conjugate being developed jointly by Tokyo-based Daiichi Sankyo and Kenilworth, New Jersey-based Merck under their multi-billion-dollar collaboration agreement. The drug works by delivering a cytotoxic payload directly to cancer cells expressing the HER3 protein, a mechanism that has already demonstrated clinical promise across multiple tumor types in the IDeate clinical trial program.

The priority review designation, announced Wednesday, reflects the FDA's assessment that I-DXd addresses an unmet medical need in small cell lung cancer, a disease with historically poor prognosis and limited second-line treatment options. The priority review pathway cuts the standard 12-month review clock to roughly six months, meaning the agency's action date could fall in the fourth quarter of 2026.

Analysts at JPMorgan and Jefferies have flagged ifinatamab deruxtecan as one of the most commercially significant pipeline assets in the antibody-drug conjugate space, projecting peak sales potentially exceeding $3 billion annually if approvals expand across multiple indications. Merck's partnership with Daiichi Sankyo, valued at up to $22 billion when announced, has already yielded the approved HER2-targeting drug patritumab deruxtecan in certain markets.

Patient advocacy groups including the Lung Cancer Research Foundation welcomed the priority review, noting that fewer than 7% of patients with extensive-stage small cell lung cancer survive five years after diagnosis. A spokesperson for the foundation said Thursday that faster regulatory pathways for drugs showing meaningful clinical benefit represent exactly the kind of urgency this disease demands. Formal label negotiations and a potential advisory committee meeting are expected to be scheduled in the coming weeks.