(Adds comment from head of Praluent)

By Ransdell Pierson

July 29 (Reuters) - Express Scripts Holding Co, the largest pharmacy benefit manager in the United States, on Wednesday said the cost of potent, newly approved cholesterol drugs could "wreak financial havoc" among its clients.

U.S. regulators last Friday approved the first of the new medicines, Praluent, from Regeneron Pharmaceuticals Inc and Sanofi SA. Given every other week by injection, Regeneron said Praluent will have a wholesale price of $1,120 for a 28-day supply. That comes to almost $15,000 a year.

Standard older cholesterol fighters such as statins, including generic forms of Pfizer Inc's Lipitor and Merck & Co's Zocor, can cost less than $50 a month.

A rival treatment from Amgen Inc, called Repatha, is expected to gain U.S. approval next month. The new drugs work by blocking the protein PCSK9, whose natural function is to prevent "bad" LDL cholesterol from being removed from the bloodstream.

Express Scripts, which administers drug benefits for employers and health plans and also runs large mail-order pharmacies, has been challenging the rising cost of new medications.

"While these drugs are being viewed as breakthroughs, they also have the potential to wreak financial havoc on clients who do not proactively manage" their drug costs, Express Scripts President Tim Wentworth said during a conference call to discuss the company's quarterly earnings.

Glen Stettin, a senior Express Scripts executive, said an estimated 70 million Americans have high cholesterol.

He estimates fewer than 10 percent of them should qualify for Praluent, based on restrictions from the Food and Drug Administration, which limited the drug's use mostly to patients with a hereditary form of high cholesterol and people with cardiovascular disease.

"The big worry for our clients, given the cost of these drugs, is whether they will be used beyond ways they were tested," Stettin said.

Sanofi's global head of Praluent, Jay Edelberg, defended the price of the drug.

"We believe it is critical to help the right patients access the right treatment at the right time, if they need to reduce their bad cholesterol when standard of care is not enough," Edelberg, who is a cardiologist, said.

"This price reflects the potential medical benefits that Praluent can provide and its overall total value to the healthcare system," he added.

Before getting their prescriptions filled, patients in Express Scripts plans will be asked for documentation of their diagnosis, their cholesterol levels, diet and maximum tolerated statin therapy, Stettin said.

Express Scripts has become a vocal critic of highly priced medicines, including Sovaldi, an $84,000 hepatitis C treatment from Gilead Sciences Inc. A rival treatment from AbbVie Inc was approved late last year, and Express Scripts successfully pressured both companies to lower prices.

Similarly, Stettin said the expected approval of Amgen's cholesterol fighter, Repatha, should help Express Scripts push for lower prices on both drugs.

"We love competition because we know we can leverage that," Stettin said. He declined to estimate how big the discounts might be.

Officials at Regeneron were not available to comment. Amgen, in an emailed statement, said it believes Repatha will offer value for those who need to further reduce their LDL. "We look forward to working with payers to ensure that these patients can access Repatha, and that it fits within the needs of the overall healthcare system," the company said.

(Reporting by Ransdell Pierson; Editing by Dan Grebler, Bernard Orr and Leslie Adler)