Is there anyway to get AoO on spellcasters casting defensively?

[RPGaddict28]

I'm working on an anti-caster build, and if he can hit enemies who are casting defensively, he'd be amazing.

[Roog]

The Mage Slayer feat stops spellcasters from casting defensively.

[Bickerstaff]

Mage Slayer kind of does that; it make casters unable to cast spells defensively in your threatened area, so they have to cast normally (i.e. you get an AoO). Then get a reach weapon so their 5-foot step leaves them in your reach.

edit: Yarr, there be ninjas about.

[ksbsnowowl]

There is also the Spellcasting Harrier epic feat.