Giving Merck & Co's immunotherapy drug Keytruda in combination with chemotherapy for previously untreated advanced lung cancer appears to provide durable benefits, according to an update of a closely watched clinical trial.

U.S. regulators approved the drug cocktail in May but the limited clinical trial data to date means that questions still remain as to whether it will retain its effectiveness over time.

In fact, the significant improvements first reported a year ago have been maintained for more than 18 months, with the risk of disease progression or death reduced by nearly half, researchers at the European Society for Medical Oncology congress reported on Friday.

People in the 123-patient trial who got Keytruda plus chemotherapy survived 19 months on average before their disease worsened, against 8.9 months for those on chemo alone, data from a scientific abstract at the meeting showed.

The overall survival rate was 70 percent for the combination compared with 56 percent for chemo on its own.

Merck senior vice president Roger Dansey said the latest data confirmed the combination had "the potential to have a meaningful impact in the lives of many of these patients."

Lung cancer is by far the largest oncology market and rival drugmakers are racing to find new combinations based on immunotherapy drugs, which boost the immune system's ability to fight tumors.

Chemotherapy, despite its toxic side effects, has for many years been the standard treatment for the disease.

Merck's small trial was the first proof of the advantages of using chemotherapy and immunotherapy together in lung cancer and the U.S. drugmaker is now awaiting more conclusive data from a bigger study.

Its Swiss rival Roche is also investigating a different immunotherapy and chemotherapy mix.

Competing companies such as Bristol-Myers Squibb and AstraZeneca have focused on combining two immunotherapies. However, AstraZeneca's endeavors here suffered a big setback in July when a large combination drug trial failed to help lung cancer patients as hoped.

Drugs like Keytruda and rival products from other companies work by taking the brakes off the immune system and allowing the body’s natural killer cells to home in on tumors.

Tens of billions of dollars of these drugs are expected to sell in the years ahead, with lung cancer, the biggest cancer killer globally, the largest potential market.