MADRID, Sept 29 (Reuters) - Merck & Co's new immune system drug Keytruda has produced encouraging results in early tests against bladder cancer, according to a company-sponsored study, prompting the firm to prepare a clinical trial later this year.

Keytruda is the first in a new wave of immune-boosting medicines to be approved for treating melanomas in the United States, but it also has potential in a range of other cancers.

Bladder cancer is seen as a disease that is likely to be amenable to such drugs, which are designed to help the body's own immune system fend off cancer by blocking a protein known as Programmed Death receptor (PD-1), or a related target PD-L1.

Roche has a similar experimental drug that is currently in the lead in addressing the specific indication of bladder cancer.

In Merck's study involving 29 people with PD-L1 positive, advanced bladder cancer, seven patients -- or 24 percent -- saw their tumours shrink after being given Keytruda, Elizabeth Plimack of Philadelphia's Fox Chase Cancer Center told the European Society of Medical Oncology on Monday.

Based on this data, Merck said it would initiate a pivotal Phase III study this year to further explore the use of Keytruda in advanced bladder cancer.

Promising results using Keytruda in stomach cancer were also reported on Sunday.

(Reporting by Ben Hirschler; Editing by Crispian Balmer)