(Reuters) - Merck & Co’s immunotherapy Keytruda proved effective as an initial treatment for nearly a quarter of patients with advanced bladder cancer too frail for standard cisplatin chemotherapy, according to interim results from a study presented on Saturday.

Evaluation of the first 100 patients from a 374-patient trial showed that 24 percent experienced significant tumor shrinkage, including 6 percent with complete responses, meaning no detectable cancer, researchers said.

Keytruda belongs to a class of drugs that block a mechanism tumors use to evade detection, allowing the immune system to recognize and attack cancer. It is approved to treat advanced melanoma, non-small cell lung cancer and head and neck cancers.

In May, Roche’s Tecentriq became the first of the new immuno-oncology drugs to gain U.S. approval to treat bladder cancer that has progressed following chemotherapy treatment.

Tecentriq had remarkably similar results to Keytruda in its earlier trial of advanced bladder cancer patients ineligible for cisplatin chemotherapy - a 24 percent response rate, including 7 percent complete responses.

The new data on top of the Roche data confirm that these drugs have anti-tumor activity and are safe in this patient population, Dr. Arjun Balar, the study’s lead investigator from New York University Langone Medical Center, said in a telephone interview.

“It further strengthens the argument that these drugs should be considered a potential new standard of care,” added Balar, who presented the results at the European Society of Medical Oncology meeting in Copenhagen.

The median duration of Keytruda responses had not been reached, but 83 percent have so far lasted at least six months, researchers reported.

“So we know responses are durable in these patients,” Balar said.

Typical survival with advanced bladder cancer in cisplatin-ineligible patients is nine to 10 months.

Cisplatin extends survival by a few months, but nearly half of bladder cancer patients cannot endure its toxicity. Most patients in the Keytruda trial had a grim prognosis with cancer that had spread to the lungs, liver or bones.

Biomarker testing of tumors and immune cells for the PD-L1 protein the drug targets found a greater response rate among patients whose PD-L1 expression was 10 percent or more, including four of the six complete responses.

While 5 percent of patients dropped out due to Keytruda side effects, researchers said the level of toxicity and discontinuation rate was far lower than what is typical with chemotherapy.