* First-ever approval of immuno-oncology combination - company

* Two-drug combination to cost $256,000 a year

* Drugs separately cost $295,000 a year - cancer specialist (Recasts; adds study details, patient population size)

By Bill Berkrot and Vidya L Nathan

Oct 1 (Reuters) - The U.S. Food and Drug Administration on Thursday approved the first combination of two drugs that help the immune system fight cancer, a therapy regimen that could cost $256,000 a year.

Bristol-Myers Squibb Co said the FDA had given the green light to combine its immuno-oncology drugs Opdivo and Yervoy to treat advanced or inoperable melanoma, the deadliest form of skin cancer, in patients who do not have a mutation of a gene called BRAF.

The approval sets the scene for many more such combinations of expensive treatments currently undergoing clinical trials by numerous drugmakers at a time when the soaring cost of prescription medicines have come under intense criticism from politicians, medical societies and patient advocacy groups alike.

Merck & Co, for example, is testing its Opdivo rival, Keytruda, in combination with other cancer treatments in at least 10 studies.

The accelerated approval was based on results of a trial in which far more patients responded to the combination than to Yervoy alone, and is contingent on Bristol-Myers replicating those results in a larger ongoing Phase III study.

Four doses of the combination would cost $141,000, about 6 percent more than the cost of Yervoy alone. Those who respond to the combination would continue receiving Opdivo alone, bringing the cost for a year of treatment to $256,000, a Bristol-Myers spokeswoman told Reuters.

Opdivo and Yervoy, bought separately, would cost about $295,000 for just under a year's treatment, according to calculations by cancer specialist Leonard Saltz.

The drugs block mechanisms which cancer uses to keep the immune system from attacking tumors. Wall Street analysts forecast sales of immuno-oncology drugs exceeding $20 billion a year by 2020.

In the trial that led to approval, 60 percent of combination patients experienced significant tumor shrinkage, including 17 percent who were cancer free, compared with an 11 percent response rate for Yervoy alone.

The combination delayed worsening of the cancer for an average of 8.9 months, only four months longer than for Yervoy. Bristol does not yet have data available showing the combination helps patients live longer, likely critical to wide acceptance by doctors.

Yervoy was approved in 2011 and Opdivo in late 2014. The two drugs had sales of $783 million in the first half of this year.

There are about 11,000 U.S. patients with untreated advanced melanoma. Those without the BRAF gene mutation account for 55 percent to 60 percent of cases, Bristol said.

The company is also testing the combination in kidney cancer and the far-larger lung cancer population.

Bristol-Myers shares were last up 40 cents at $59.60 in early afternoon trading on the New York Stock Exchange.

(Reporting by Bill Berkrot in New York and Vidya L Nathan in Bengaluru; editing by Kirti Pandey, Savio D'Souza and G Crosse)