LOUISVILLE, Ky. — Big Brown backed up his trainer’s boasts with an explosive finishing kick and won the Kentucky Derby on Saturday, a commanding victory turned somber by the fatal breakdown of the filly Eight Belles on horse racing’s biggest day.

The cheers for the winner’s 4¾-length victory were cut short when Eight Belles, the runner-up, collapsed while galloping out near the second turn. She broke her two front ankles and was euthanized on the track minutes later.

The unbeaten Big Brown’s start from the outside post did little to hamper his charge when the 20-horse field turned for home at Churchill Downs. Under the urging of jockey Kent Desormeaux, the 2-1 favorite cruised to an easy victory to become the seventh unbeaten Derby winner with his fourth consecutive win. The last one was Barbaro in 2006.

Big Brown’s victory sends him to the Preakness in two weeks as the only 3-year-old with a chance to become the first Triple Crown champion since Affirmed in 1978.

“We’re ready to roll,” Big Brown’s confident trainer Rick Dutrow Jr. said.

All week, Dutrow told the world he had the best horse in the field — and the big bay colt justified every accolade tossed his way.

“I can’t describe the feeling that all of us have right now,” he said.

The colt became the first Derby winner since Regret in 1915 to have raced only three times previously. He is only the third in 60 years to win after racing in just two Derby preps — Sunny’s Halo in 1983 and Street Sense last year were the others.

In addition, Big Brown became the second winner to start from the No. 20 post. The gelding Clyde Van Dusen did it in 1929.

Big Brown covered the 1¼ miles in 2:01.82 in front of the second-largest crowd in Derby history at 157,770. He paid $6.80, $5 and $4.80.

Eight Belles paid $10.60 and $6.40, and Denis of Cork, at odds of 27-1, returned $11.60.

Tale of Ekati was fourth, followed by Recapturetheglory, Colonel John, Anak Nakal, Pyro, Cowboy Cal, Z Fortune, Smooth Air, Visionaire, Court Vision, Z Humor, Cool Coal Man, Bob Black Jack, Gayego, Big Truck, Adriano and Monba.

The colt earned $1,451,800 for the win and boosted his earnings to $2,114,500 for owners IEAH Stables and Paul Pompa Jr. Pompa, who named Big Brown in honor of United Parcel Service, a client of his trucking business, sold a 75 percent interest in the colt to IEAH for about $3 million after his first race.

Desormeaux won the Derby for the third time, having won aboard Real Quiet in 1998 and Fusaichi Pegasus in 2000. Only three other riders have won more — Eddie Arcaro, Bill Hartack and Bill Shoemaker.

Big Brown was the third favorite to win in the past five years. Smarty Jones won in 2004 and Street Sense won last year.