An advanced model of Nebula cruiser was produced during the 2250's to streamline that cruiser hull. Additional warp nacelles were added the hull, though at the expense of the modular systems pod common to the Nebula class.
One of the first Nebula Advanced Cruisers to see battle was the U.S.S. Melbourne which was destroyed in the massacre of Starfleet forces in Wolf 359 at the hands of the Borg.
The Nebula Advanced Cruiser never caught on and is currently undergoing further development to make it a more practical addition to the fleet.