Merck (NYSE:MRK) on Tuesday received a complete response letter (CRL) from the Food and Drug Administration for a new dosing regimen for its cancer blockbuster Keytruda. A CRL is the FDA's euphemism for a rejection letter, although companies can reapply for approval if they generate more data.

The big pharma didn't disclose what information the agency is looking for regarding Keytruda, and only noted that it's "reviewing the letter and will discuss next steps with the FDA."

Rejected stamp

Image source: Getty Images.

Merck had applied for approval to offer Keytruda in a regimen of 400 mg doses every six weeks for patients with six different types of cancer: melanoma, classical Hodgkin lymphoma, primary mediastinal large B-cell lymphoma, gastric cancer, hepatocellular carcinoma, and Merkel cell carcinoma. The immunotherapy is currently approved for those cancers at a dose of 200 mg every three weeks. The new dosing regimen would have given patients more flexibility and reduced the number of trips they have to make to get their medication infused.

The applications were based on pharmacokinetic modeling and simulation data, which estimates the amount of a drug in the patient over time based on absorption, distribution, metabolism and excretion rates. European regulators approved the 400 mg dosing regimen based on the same data last year.

PD-1 is a protein found on the surface of immune cells that helps them identify which unhealthy cells to attack, but cancer cells can use that mechanism to trick the immune cells into ignoring tumors. Keytruda  blocks the interaction that allows those cancer cells to hide, enabling the immune system to detect and fight them.