By Mariam Sunny and Christy Santhosh

Aug 25 (Reuters) - The personalized cancer vaccine being developed by Moderna (MRNA.O) and Merck (MRK.N) has boosted hopes that mRNA technology could open up a new avenue of treatment, but the available evidence is still limited.

Interim data from a large trial showed the experimental vaccine, when combined with Merck’s blockbuster immunotherapy Keytruda, helped reduce the risk of melanoma recurrence and spread in patients whose tumors have been surgically removed.

Here’s what to know about the treatment:

HOW DOES THE VACCINE WORK?

Known as intismeran autogene, the vaccine is designed to target mutations unique to an individual patient’s tumor.

Once surgeons remove a tumor or cancerous lesion, the companies sequence it, looking for unique cancer mutations. The companies use those to create a customized vaccine that targets up to 34 mutations unique to that specific patient’s cancer.

With up to nine doses of Keytruda, infused every six weeks, plus up to nine doses of the customized mRNA vaccine, the treatment trains the immune system to recognize and attack cancer cells.

WHAT ELSE IS IT BEING TESTED FOR?

Experts believe the treatment may work best in highly mutated cancers such as melanoma, where abundant tumor-specific mutations provide more targets for the vaccine to attack.

Cancers with high mutation rates, such as lung and colon cancer, could be promising targets because they may be more readily recognized by the immune system, Dr. Michael Atkins, deputy director of the Georgetown Lombardi Comprehensive Cancer Center, said.

It is not clear that the vaccine technology will work against other cancers, he added. Merck and Moderna have several large trials underway in patients with non-small cell lung cancer whose tumors have been surgically removed.

Moderna is also studying the intismeran/Keytruda combination in mid-stage trials of patients with bladder and kidney cancers as well as in early-stage trials in pancreatic and stomach cancers. Data readouts from numerous trials are expected in the next year or two, according to the company.

WHAT ARE THE MANUFACTURING HURDLES?

Each patient’s vaccine must be designed and manufactured from their own tumor data, making production far more complex than for traditional vaccines produced in large batches.

The tumor samples need to be tracked through sequencing, vaccine design, manufacturing and through delivery to make sure the correct vaccine goes to the right patient, said Dr. Ryan Sullivan, director of the Center for Melanoma at Mass General Brigham Cancer Institute.

That process needs to be scaled up by 10 to 100 times over once it is a commercial product, he said.

Moderna President Stephen Hoge said in an interview that the company can scale production to meet demand over the next few years.

WAITING FOR MORE DATA The interim results of the ongoing study released on August 19 found the combination of Keytruda and intismeran met both its primary goal of reducing cancer recurrence and its secondary goal of preventing tumors from spreading to other parts of the body, compared with Keytruda alone.

However, the extent of the benefit remains unclear pending release of detailed results. Overall survival data, which will inform whether the combination treatment allows patients to live longer, were also not yet available. Five-year data from a previous mid-stage trial presented earlier this year showed the vaccine cut the risk of cancer relapse in half, and reduced the risk that it would spread to new locations by 59% compared with Keytruda alone.