Formal:
* Concerning
* Pertaining to
* Relating to
* With respect to
* In relation to
* In reference to
* On the subject of
* With regard to
Informal:
* About
* On
* As for
* When it comes to
* In terms of
Other options:
* Considering
* Touching upon
* Addressing
* Dealing with
The best choice will depend on the specific context and the desired tone.