(New throughout, adds Neville comments, background on cholesterol drugs)

By Ransdell Pierson

NEW YORK, Sept 30 (Reuters) - Express Scripts Holding Co said on Wednesday two costly new cholesterol fighters will not be "budget busters" for its clients and that most prescriptions for the potent drugs have been rejected because patients did not meet required medical criteria.

"We're seeing a lot of patients who either don't qualify or their physicians are not providing (needed) information," said Everett Neville, a vice president of Express Scripts, the largest pharmacy benefit manager in the United States.

Neville, in an interview, said Express Scripts and insurers had rejected a surprisingly high number of prescriptions for the two injectable drugs, Praluent from Regeneron Pharmaceuticals and Sanofi, and Amgen Inc's Repatha.

They were approved during the summer and belong to a new class of medicines called PCSK9 inhibitors that can slash "bad" LDL cholesterol by more than 60 percent.

The products each cost more than $14,000 a year and are meant for patients with a hereditary form of high cholesterol and those with cardiovascular disease.

Before getting their prescriptions filled, patients in Express Scripts plans are asked for documentation of their diagnosis, their cholesterol levels, diet and whether they are already taking maximum levels of standard cholesterol treatments called statins, which include Pfizer Inc's Lipitor.

Neville predicted the number of rejected prescriptions for Praluent and Repatha will eventually drop sharply as doctors prescribe them more narrowly.

Express Scripts, which administers drug benefits for employers and health plans and also runs large mail-order pharmacies, in July said Praluent and Repatha are viewed as breakthrough medicines but could wreak financial havoc on its clients.

But Neville, speaking on the sidelines of the PSA Pharmaceutical Strategy Conference in New York, said sales of the drugs have been at the low end of its expectations.

"We feel very confident we can manage this and this won't mess up our clients' budgets in 2016." But Neville said that could change in 2017, when results from a large study will show if Praluent significantly reduces the risk of heart attack and stroke.

If it does, analysts say that could strengthen the argument for giving it to a wider patient population. Repatha is undergoing a similar large study.

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.

(Reporting by Ransdell Pierson; Editing by Chris Reese and David Gregorio)