The Food and Drug Administration has again denied Teva Pharmaceutical Industries Ltd.'s (TEVA) citizen's petition asking the agency to block approval of a generic version of its Copaxone multiple sclerosis treatment.

The Israeli drug giant filed the letter in November after its initial attempt failed earlier in 2009. The FDA rejected the second petition for the same reason as the first, because it would be "premature and inappropriate" to grant Teva's requests, but gave a detailed response that rejects many of the company's arguments against the generic approval.

Momenta Pharmaceuticals Inc. (MNTA) and Mylan Inc. (MYL) have both filed to produce generic versions of Copaxone, a process that is likely to span years. The drug, one of the world's best selling MS treatments and a key component of Teva's business, was approved in 1996. In 2009, it racked up sales of $2.8 billion.

Teva has sued both companies. That triggered an automatic 30-month stay on FDA approval, which is required by a generics-related law, meaning no generic version of Copaxone can enter the U.S. market until early 2011.

The rejection letter is dated Tuesday, but was posted on a government website Wednesday.

The Copaxone battle is notable because it pits the world's largest generics company, which has benefited from challenging the patents of other pharmaceutical companies, against generic competitors to protect the exclusivity of its own brand.

Drug makers can ask the FDA to use special criteria when considering approval of generic copies of branded drugs, citing health or safety reasons. The petitions can slow the approval process and prolong market exclusivity for branded drugs, but recent rules require the FDA to respond to such requests within six months.

Officials from Teva, Mylan and Momenta weren't immediately available for comment.

In its original letter, Teva claims generic drug makers can't show their drug is a precise copy of Copaxone--a fact that should preclude market approval. It claimed that current analytical methods aren't capable of characterizing Copaxone's make-up and reverse engineering won't create an identical product.

In the latest FDA response, which is eight pages longer than the previous four-page rejection, the agency repeatedly asserts its "broad discretion" in determining whether the submitted information is enough to conclude that the generic product is the same as the original.

The agency said it will continue to take the specific drug into account when making its determination, and may require a generic company to prove sameness "although we cannot state with any certainty what these criteria may be."

"A finding of sameness does not, however, necessitate a finding of 'complete chemical identity.' Thus, the Agency may consider other criteria to determine sameness, taking into account the complexity of the active ingredient," it said.

Teva had also asserted that tests of a generic version sold in India and Ukraine produced by Natco Pharma Ltd. (524816.BY) "differed greatly" from branded Copaxone. Mylan signed a worldwide marketing and distribution license with Natco for its generic version of Copaxone in 2008.

The agency called Teva's contentions "unpersuasive" because those products weren't approved by the FDA and their existence doesn't preclude the possibility that a generic could meet "rigorous standards for demonstrating sameness" to Copaxone.

Despite the rejection, the FDA said the information in Teva's letter will help the agency's assessment of the types of information needed to support generic Copaxone.

-By Thomas Gryta, Dow Jones Newswires; 212-416-2169; thomas.gryta@dowjones.com