The U.S. Food and Drug Administration declined to approve Merck and Japan-based Daiichi Sankyo's lung cancer treatment, which belongs to a lucrative class of cancer therapies that work like "guided missiles."

The FDA cited findings from an inspection of a third-party manufacturing facility in its so-called complete response letter, the companies said late on Wednesday.

The letter, which indicates the agency has reviewed the companies' application and has outstanding questions, did not identify any issues with the efficacy or safety data submitted.

The companies said they will work with the FDA and the third-party manufacturer to address the feedback.

The treatment, called patritumab deruxtecan, is one of three antibody-drug conjugates (ADCs) that were part of Merck's up to $22-billion joint development and commercialization deal with Daiichi Sankyo signed last year.

ADCs are targeted cancer therapies that involve two key components — a monoclonal antibody that binds to specific tumor cells and a toxin that kills those cells while leaving healthy ones unharmed — in a way working like a "guided missile."

The companies sought approval for the treatment to treat non-small cell lung cancer in patients who have failed two prior lines of therapy and whose tumor expresses a certain type of mutation that leads to uncontrolled growth of an EGFR protein.

Johnson & Johnson's Rybrevant and AstraZeneca's Tagrisso and Iressa are currently approved in the U.S. for EGFR-mutated non-small cell lung cancer.