By Ben Hirschler

LONDON, July 31 (Reuters) - AstraZeneca is accelerating its push into cancer immunotherapy, with plans to test a key experimental drug in new tumour types.

The company is seen as No. 4 in a race to develop the first drug in a new class that fights cancer by unleashing the body's immune system, behind rivals Roche, Merck & Co and Bristol-Myers Squibb.

But its MEDI4736 medicine is still viewed as a highly promising agent, both on its own and in combination with other treatments.

AstraZeneca said on Thursday it would launch a pivotal clinical trial programme with MEDI4736 in head and neck cancer this year, in addition to ongoing tests in lung cancer, and was also looking at expanding tests into other cancer types.

"There is the potential we would announce an additional tumour type beyond head and neck and non-small cell lung cancer by the end of this year," head of medicines development Briggs Morrison told analysts in a post-results conference call.

MEDI4736 is part of a closely watched class of drugs known as anti-PD-L1 therapies, which block a tumour's ability to evade the immune system's defences.

AstraZeneca said it would present further data looking at MEDI4736 in lung cancer, as well as early results for head and neck cancer, in September at the annual meeting of the European Society for Medical Oncology.

Analysts said the decision to study MEDI4736 in head and neck cancer made sense, given encouraging results with a similar Merck drug for the condition reported in June.

The commercial promise of MEDI4736 was flagged by AstraZeneca during its recent $118 billion takeover battle with Pfizer, when it argued that the drug - used on its own and in combination with other therapies - could potentially generate annual sales of $6.5 billion.

In recent years, the British drugmaker has become best known for its cholesterol drug Crestor and its ulcer and heartburn pill Nexium. But it has a long history in oncology, having broken new ground decades ago in developing tamoxifen and other hormonal therapies. It also produced one of the first "targeted" cancer drugs, Iressa.

AstraZeneca earlier raised its sales and earnings forecasts for 2014 after second-quarter sales and earnings both beat expectations, helped by several one-off factors.

(Editing by David Holmes)