(Adds Merck also presenting breast cancer data in December)

LONDON, Oct 17 (Reuters) - Swiss drugmaker Roche and U.S. group Merck & Co are to present data on their rival immunotherapy drugs in breast cancer in December, extending the novel approach to fighting tumours to another cancer type.

Early clinical results with Roche's drug, known as MPDL3280A, in so-called triple negative breast cancer (TNBC) will be revealed at the Dec. 9-13 San Antonio Breast Cancer Symposium, the firm said after announcing third-quarter results.

TNBC does not respond to either of two kinds of hormonal therapy or drugs that target HER2 receptors, such as Roche's own Herceptin.

Merck later confirmed that it, too, would present data on TNBC with its competitor drug Keytruda at the same breast cancer meeting.

Both drugs belong to a class 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, used by tumours to evade disease-fighting cells.

Developed first for melanoma, these medicines are also showing promise in a range of other tumour types.

Roche's MPDL3280A, which is not yet approved for any type of cancer, is already being tested in melanoma, as well as lung, bladder, kidney, bowel and blood cancers.

Merck's Keytruda became the first in the new wave of immune-boosting medicines to be approved for treating melanoma in the United States last month and is also being tested in a range of other tumour types.

Some analysts believe the new immunotherapy drug class could generate more than $30 billion in annual sales for the industry as a whole by 2025, reflecting both the wide range of patients who could benefit and the high cost of the medicines.

The two other main rivals in the space are Bristol-Myers Squibb and AstraZeneca.

(Reporting by Ben Hirschler in LONDON; editing by Keiron Henderson and Susan Thomas)