LEXINGTON, Ky. -- The superstar mare Beholder, who was considered one of the main threats to American Pharoah in Saturday's $5-million Breeders' Cup Classic, will not run because of a lung problem. Trainer Richard Mandella made the announcement at about 11 a.m. yesterday.

"After she galloped this morning on the training track, we scoped her and found blood in her lungs," a somber Mandella said. "We found there was irritation in her lungs, and we decided that we can't run her.

"She'll be fine, but if we put her in a race, she could do herself some real damage, so I can't take that chance."

Her withdrawal dramatically changes the pace scenario of the 11/2-mile Classic, in which Beholder, the 3-1 second favorite in the morning line, and jockey Gary Stevens were expected to pressure Pharoah. Now the Triple Crown champion is expected to be loose on the lead, which is how he dominated the Preakness and the Belmont Stakes.

"It's a sad thing," Mandella said, "because we were looking forward to the challenge of running in the Classic."

Bob Baffert, American Pharoah's trainer, empathized with Mandella, who like him is based in Southern California. Their barns are near each other at Santa Anita, and each has followed the other's horse.

"That mare means as much to them as Pharoah means to us," Baffert said minutes after learning Beholder would not run. "We still have 48 hours [until the race], and anything can happen. This is the unfortunate part, and that's why we're all on pins and needles."

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American Pharoah's owner-breeder, Ahmed Zayat, expressed his condolences on Twitter to Mandella and her owner, the locally based Spendthrift Farm. Zayat tweeted: "Shocked and heartbroken. I am a Beholder fan and lover. Wish her the best. I commend her connections for doing right by her."

Beholder spiked a fever the night after she arrived at Keeneland last week. After being treated with antibiotics the next morning, her temperature returned to normal, and she had been training impressively. "This was obviously due to the fever," Mandella said. "She's an aggressive mare and she got herself worked up shipping and made herself sick."

Travel always has been an issue for the two-time Eclipse Award winner, who is 15-for-18 in California, including wins at Santa Anita in the 2012 Juvenile Fillies and the 2013 Distaff. This trip was only her third out of state, where she's 0-for-2. In the 2013 Kentucky Oaks, she got upset in the post parade and unseated Garrett Gomez before running second. She suffered a leg injury last year in a stakes at Belmont Park.

"You could say we just haven't had any luck shipping," Mandella said, "but it's been something different every time. It's sad. The Classic is still a great race. I just wish I was a part of it."

Notes & quotes: Breeders' Cup weekend begins on Friday with four races, the mile Juvenile Turf, the Dirt Mile, the mile Juvenile Fillies Turf and the 11/8-mile Distaff.