Amgen Inc. (AMGN) - Get Report shares traded lower Monday after the drugmaker presented details of a trial of its key lung cancer treatment that fell short of Wall Street forecasts.

Amgen told the World Conference on Lung Cancer in Barcelona Sunday that its AMG 510 treatment was able to reduce tumor sizes in just over half of the patients with advanced lung cancer tested in a small trial of the drug. Amgen said just under half of the 23 patients tests with various does of AMG 510 saw notable reductions in tumor sizes of around 30%.

The overall response rate for the high-dose patients, who were treated with 960mg of AMG 510, was 54%, Amgen said. Analysts at JPMorgan had expected the trial to reveal a success rate "well in excess of 50%" of the trial participants in the high-dose category, a rate that reflects the broader consensus on Wall Street.

"These new data reinforce the earlier positive response rate we shared at ASCO in more non-small cell lung cancer patients receiving AMG 510," said Amgen's R&D vice president David  Reese. "We remain enthusiastic about the promise of AMG 510 and continue to rapidly advance its development program both as monotherapy and in combination."

Amgen shares were marked 3.2% lower at the start of trading Monday to change hands at $201.20 each, a move that would trim gains from its recent two-month rally to around 15%. 

Amgen touted a 50% response rate for its AMG 510 treatment, which targets so-called KRAS mutations in patients with non-small lunch cancer (NSCLC) -- the most common form of the deadly disease -- during its second quarter earnings call earlier this summer. 

"We remain extraordinarily enthusiastic about the promise of AMG 510 and continue to aggressively prosecute the development program," Reese told investors at the time.

Amgen said Sunday that the U.S. Food & Drug Administration has given AMG 501 "fast track" approval status, and plans a fuller Phase 1 trial update later this month at an event hosted by the European Society for Medical Oncology.