Originally Posted by Emissary of Justice
Kabuto had a sealing dagger in his hand at the start of the fight. That'd do quite a lot to stop Itachi from regenerating.
We're assuming it was a sealing dagger (I'm playing devil's advocate for this one part). That tag could've been an explosive tag as well. But, if it was a sealing tag, I'm betting it was the earlier tag Kabuto pulled out to rewrite Itachi's brain...again in order for Kabuto to regain control.