Moody's placed Express Scripts Holding Co.'s Baa2 senior unsecured debt under review for downgrade.

The rating agency placed the ratings under review with direction uncertain in March, after Cigna Corp. agreed to acquire Express Scripts for about $67 billion.

Moody's said the rating of the St. Louis, Mo.-based pharmacy benefit manager is unlikely to be upgraded, based on the progress to date in the rating review of Bloomfield, Conn.-based Cigna.

Express Scripts's Baa2 rating reflects its leading position in the market, moderate financial leverage and strong cash flow, according to Moody's.