Mike Chiappetta • E-mail

It was on, then off, then on, and now off again.

WEC middleweight champion Paulo Filho has officially withdrawn from his title defense against Chael Sonnen after checking himself into rehabilitation for an undisclosed substance abuse problem, according to the WEC.

The 29-year-old Brazilian is considered one of the top middleweights in the world, and has one of the sport's longest unbeaten streaks with 16 straight wins. Many in the sport believe he's the middleweight with the best chance of beating reigning UFC champ Anderson Silva, though that fight is unlikely as the two are friends and have trained together in the past.

Filho came to prominence in the PRIDE Bushido series, where he defeated notables such as Murilo "Ninja" Rua and Kazuo Misaki. He made his WEC debut in August 2007 with a TKO win over Joe Doerksen, then defeated Sonnen via armbar submission in December.

Afterward, Filho was criticized by some for what they considered an uninspired performance, and controversy soon followed after Sonnen claimed he never tapped out, leading to a quick signing of a rematch between the two.

In recent weeks, there has been speculation that Filho would back out of the match. Earlier this month, the Brazilian MMA site Tatame.com reported Filho had asked the fight be moved to June, but just days later, Filho's manager Ed Soares stated the match was still on. Now comes the official news that Filho is entering rehab in his home country.

With that fight now off the table, the main event for the March 26 card will be a light-heayvweight title fight between champion Doug "Rhino" Marshall and undefeated challenger Brian Stann.

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