April 7 (Reuters) - Nearly a third of patients with

advanced liver cancer who received a personalized vaccine

developed by Geneos Therapeutics along with an immunotherapy

drug in a small, early trial saw their tumors shrink, U.S.

researchers reported on Sunday.

The result was roughly twice the response typically seen

with the immunotherapy alone, the researchers said.

Findings from the preliminary study, presented at the

American Association for Cancer Research in San Diego and

published in Nature Medicine, suggests that vaccines based on

mutations only present in a patient's tumor may boost the immune

system's ability to recognize and attack hard-to-treat cancers.

The findings, which must be confirmed in a larger trial,

moves the industry another step closer to effective cancer

vaccines, after many past failures, and may expand the types of

cancers that such therapies can treat.

Partners Moderna ( MRNA ) and Merck and Co ( MRK ) and

others have had promising results combining customized vaccines

with immunotherapy to prevent skin cancer from returning in

patients following surgery.

For the study, researchers used samples from patients'

tumors to build vaccines based on neoantigens - new mutations

only present on an individual patient's tumor. The goal was to

train the immune system to attack and kill only these unique

proteins, leaving healthy tissue unscathed.

Unlike skin cancer, which has many mutations for the body to

recognize, liver cancer is considered a cold cancer because it

contains fewer mutations, which has rendered immunotherapies

less effective.

"This vaccine essentially educates the immune system to

recognize antigens that it's ignored," said study leader Dr.

Mark Yarchoan of Johns Hopkins Kimmel Cancer Center.

The study involved 36 patients with hepatocellular

carcinoma, the most common form of liver cancer. Patients were

given custom-made vaccines on top of Merck's ( MRK ) widely used

immunotherapy Keytruda, the standard of care at the time.

Nearly a third of the patients treated with the combination

therapy (30.1%) experienced tumor shrinkage, with three of those

subjects having a complete response, meaning no detectable signs

of the tumor remaining after a median follow-up of 21.5 months.

That compares with the typical response of about 12% to 18%

in patients with liver cancer who receive immunotherapy alone.

"This certainly suggested that the vaccine actually added

clinical efficacy," Yarchoan said.

The most common adverse effect was mild injection site

reactions. There were no serious adverse events.

Unlike many vaccine candidates based on messenger RNA (mRNA)

technology, the Geneos treatment is a DNA vaccine in which the

genetic code of the mutated proteins is injected into cells

using a small electrical impulse. Each vaccine can target up to

40 mutated genes.

Yarchoan said larger trials are being planned, but declined

to provide details.