No horse has captured the American triple crown since Affirmed in 1978, but 20 hopefuls led by favourites Orb, Verrazano and Goldencents run for the dream in Saturday's Kentucky Derby.

The 139th edition of the famed 1 1/4-mile (2000m) race at Churchill Downs will be a $US2 million launch of the US treble for three-years-olds, which continues with the Preakness on May 18 and concludes June 8 with the Belmont Stakes.

In the past 11 years, five horses have won the first two races only to falter at the last and longest event, Smarty Jones the nearest miss after a runner-up Belmont finish to Birdstone in 2004.

Last year, I'll Have Another won the first two races but pulled out of the Belmont Stakes with a left leg injury, ending the dream of the colt being the 12th triple crown winner and stretching the drought to 35 years.

Florida Derby winner Orb was made the morning-line Kentucky Derby favourite at 7-2 with Wood Memorial winner Verrazano next at 4-1 and Santa Anita Derby winner Goldencents third at 5-1. No other horse has single-digit odds.

"I'm fine with that," Orb's trainer Shug McGaughey said of the favoured role. "Verrazano is a very, very talented horse. We will see what the (betting) public does on Saturday but I think (Orb) will probably still be the favorite."

Orb, with jockey Joel Rosario, will start from gate 16 with Verrazano, ridden by John Velazquez, starting two posts inside of that and Goldencents going out of the eighth post with jockey Kevin Krigger aboard.

"From where he is, we will try to hold our position and maybe try to creep in a little bit around the first turn," McGaughey said. (Rosario) can watch what Johnny is doing on Verrazano.

Orb, owned by Stuart Janney III and Phipps Stable, has won his past four races, including the Fountain of Youth Stakes, and earned $US921,050 in seven starts.

Verrazano trainer Todd Pletcher, who has five Derby starters in all, said Orb being favoured might help his horse, which is unbeaten in four starts, all this year.

"As far as Orb being the favourite over Verrazano, that's not an issue," said Pletcher. "(Orb) deserves to be the favourite. And it might even be an advantage. There's usually more pressure on the favourite."

Verrazano, whose wins include the Tampa Bay Derby, could become the first Derby winner since Apollo in 1882 not to run as a two-year-old.

Goldencents, partly owned by US college champion University of Louisville coach Rick Pitino, is trained by Doug O'Neill, who won last year with I'll Have Another. Goldencents has four wins in six starts.

The field features one European horse in the Aidan O'Brien-trained Lines Of Battle. The 30-1 winner of the UAE Derby last month arrived in Kentucky on Wednesday.