CHICAGO, June 3 (Reuters) - Britain's AstraZeneca Plc on Tuesday said data from early-stage trials of its experimental cancer drug, MEDI4736, are encouraging and support moving the immunotherapy into pivotal-stage testing.

The findings were part of a series of presentations by AstraZeneca at the annual American Society of Clinical Oncology (ASCO) meeting in Chicago. AstraZeneca had leaned on the promise of its cancer drug pipeline to fend off a $118 billion takeover bid by U.S. rival Pfizer Inc.

MEDI4736 is part of a closely watched class of drugs known as anti-PDL1 therapies, which work by blocking a tumor's ability to evade the immune system's defenses. AstraZeneca acquired the drug in its 2007 takeover of U.S.-based MedImmune.

AstraZeneca said data from the expanded enrollment phase of the trial show early evidence of patient response in multiple tumor types, including non-small lung cancer, squamous cell carcinoma of the head and neck, pancreatic cancer, gastroesophageal cancer, and cutaneous melanoma.

Phase I results for MEDI4736 in 27 patients with advanced solid tumors, including non-small cell lung cancer, melanoma and colorectal cancer, showed 19 percent experienced tumor shrinkage and 39 percent saw their cancer stabilize after at least a year of treatment, the company said.

"The data presented at ASCO reinforce the importance MEDI4736 plays in our immuno-oncology portfolio and, more broadly, the potential role it has to help in transforming cancer therapy, alone and in combination with other treatments," Dr. Edward Bradley, MedImmune senior vice president, research and development and oncology said in a statement.

AstraZeneca has two ongoing studies exploring different doses and treatment schedules for combinations of MEDI4736 and tremelimumab, an experimental drug that works by unlocking a different brake on the immune system.

The company said the lung cancer trial involving both drugs is continuing, with no maximum tolerated dose yet defined.

"Early, preliminary clinical activity has been observed with this combination in patients with advanced NSCLC (non-small cell lung cancer)," AstraZeneca said in a statement.

(Reporting By Deena Beasley; Editing by Jonathan Oatis)