Shares of Amgen Inc. fell more than 3 percent in premarket trading Monday after the Thousand Oaks biotech released disappointing data about a pipeline cancer drug.

Data show AMG 510, an inhibitor aimed to target and block cancer at the DNA, RNA or protein level, helped 54 percent of patients who took the drug in a Phase 1 study. Seven of 13 patients with lung cancer experienced a “partial response” to the oral drug, Amgen said in a statement, while the remaining six achieved “stable disease,” meaning the cancer didn’t progress further.

The 13 patients were part of a larger study consisting of 34 patients with lung cancer, but not all patients were deemed fit to take the drug.

Although stocks reacted negatively to the study findings, Dr. David Reese, vice president of R&D at Amgen, stood behind the drug’s promise. “These new data reinforce the earlier positive response rate we shared at ASCO in more non-small cell lung cancer patients receiving AMG 510. We remain enthusiastic about the promise of AMG 510 and continue to rapidly advance its development program,” he said in a statement.

Shares of Amgen (AMGN) closed Monday down $5.39, or 2.6 percent, to $202.34 on the Nasdaq.