Nov 17 (Reuters) - Drugmaker Alkermes said on Tuesday the U.S. Food and Drug Administration declined to approve its treatment for schizophrenia and bipolar disorder, citing concerns related to a tablet coating process at its manufacturing site.

The company, which has FDA-approved antipsychotic and alcohol dependence treatments in the market, is preparing to resubmit data and plans to work with the agency to resolve the issue.

Alkermes Chief Executive Officer Richard Pops said on an investor conference call the company was preparing in anticipation of the drug's launch in the first quarter. "We want to be ready for that."

The company said the FDA's observations were specific to certain batches of the treatment, ALKS 3831, and that the issue has been resolved, with sufficient data available to address the agency's concerns.

The health regulator has not asked for any new clinical trials to support approval, Alkermes said.

"Given that no issues were raised with the clinical studies, we believe that ALKS 3831 will ultimately be approved," Cantor Fitzgerald analyst Brandon Folkes said.

Folkes, however, believes that the FDA's decision does provide additional uncertainty for investors and expects some weakness in the company's stock until there's further clarity on the approval pathway.

ALKS 3831 is expected to generate sales of more than $600 million in 2030 and will probably be approved by the first half of 2021, Mizuho analyst Vamil Divan said.

The FDA has designated the resubmission as a complete, class 1 response, which typically means that the agency will review the drug within a 60-day period.

Alkermes' shares were up about 1% at $17.99. (Reporting by Dania Nadeem in Bengaluru; Editing by Shinjini Ganguli and Shounak Dasgupta)