Roche Holding AG announced positive results from an experimental cancer drug which shrank lung tumors among a small group of smokers by 26 percent in a clinical trial.

The Swiss company reported the early-stage results of the drug called MPDL3280Aon Sunday, calling the response among the 53 patients as a step forward in fighting a lethal form of cancer.

"There is no discussion, this is really working," lead investigator Jean-Charles Soria told reporters at the European Cancer Congress in Amsterdam, according to a Bloomberg report. "This is the first targeted agent showing more activity in smoking patients than in never smokers." And he speculated that the larger number of genetic mutations present in the lung cancer that afflicts smokers helped explain the higher rate of success for a drug that can unleash an immune system assault.

Soria said out of the 53 patients with a form of the disease called non-small cell lung cancer (NSCLC) tumors, 23 percent saw their tumors shrink, according to results presented at the European Cancer Congress (ECC) in Amsterdam.

According to the report, the most encouraging numbers were among smokers, where the response rate was 26 percent compared with 10 percent of patients who had never smoked.

Roche's MPDL3280A is an artificially designed antibody that singles out a protein called PD-L1 - a defense mechanism that tumors use to trick the immune system's T-cells into being inactive.

By preventing PD-L1 from attacking, the Roche drug allows the T-cells to acknowledge the alien cancerous cells, and then grow and multiply to attack it more efficiently.

According to Reuters, Cora Sternberg, co-chair of the ECC's scientific committee and an oncologist at the San Camillo and Forlanini hospitals in Rome who was not involved in the study, noted the great potential of the new Roche drug, saying it was "definitely a game changer" in lung cancer.