By Ransdell Pierson

Aug 25 (Reuters) - Advaxis Inc on Monday said it will test an experimental immuno-oncology drug in combination with a high-profile immunotherapy from Merck & Co Inc as a treatment for patients with advanced prostate cancer.

The tiny U.S. biotechnology company, in its second cancer collaboration with a large drugmaker in the past month, said it would evaluate the use of its ADXS-PSA as a standalone treatment and also study it in combination with Merck's pembrolizumab in the Phase I/Phase II trial.

Merck's drug is a member of an exciting new class of medicines called "PD-1 inhibitors" that work by blocking the PD-1 protein, thereby taking the brakes off immune system cells and prodding them to attack tumors.

The Advaxis drug consists of a modified form of the Listeria bacterium fused to a protein called PSA that is shed by prostate tumors. It is meant to spur creation of immune system soldiers, called T-cells, that are primed to recognize and kill prostate cancer cells.

By using different approaches, the drugmakers hope the two drugs have a better chance of knocking down the prostate cancer than either could achieve on its own.

"If the combination of drugs shows an added effect, the companies would likely continue their collaboration," perhaps under a licensing deal, said Daniel O'Connor, chief executive officer of Advaxis.

Prostate cancer is the most common cancer among men, other than skin cancer. One man in six will get prostate cancer during his lifetime, and about 3 percent of men die from the disease, according to Advaxis.

O'Connor, in an interview, said his company will bear most costs of the study, which is slated to begin in early 2015 and probably be conducted by a contract research organization. Merck will provide the pembrolizumab, an injectable monoclonal antibody, to be used in the trial.

PD-1 inhibitors and other immuno-oncology drugs have already shown promise against other types of cancer, including melanoma, but have not yet been tested to any great extent against prostate cancer, according to David Mauro, executive director of oncology at Merck.

"We think this combination gives us the opportunity to do early exploration in prostate cancer and bring immuno-oncology into this space," Mauro said in an interview.

Merck previously announced collaborations with several other drugmakers on combined uses of pembrolizumab with their medicines, including British drugmaker GlaxoSmithKline, Pfizer Inc, Amgen Inc and Incyte Corp.

Advaxis last month announced a collaboration with British drugmaker AstraZeneca to test immunotherapy drugs from both companies in combination.

Under that deal, AstraZeneca's drug MEDI4736 will be evaluated in a Phase I/II clinical study together with the U.S. company's cancer vaccine ADXS-HPV. The AstraZeneca drug blocks PD-L1, a protein similar to PD-1.

The combination will be tested in patients with cervical cancer and head and neck cancer, tumors that are closely linked to infection with the human papillomavirus.

(Reporting by Ransdell Pierson; Editing by Leslie Adler)