Well it depends on the pen and just how far you 
have gotten in the setup of the cel. If you are 
at the start of a new cel, well almost any type 
of ascetate will remove the pen, though you will 
want to be careful as ascetate will also disolve 
some plastics so test it first (and don't sniff 
the stuff). 

However if you are working on a cel already half 
finished definately don't do this as it also 
removes paint and anything else on the cel. 
Actually almost anything that will remove the pen 
will probably remove the paint too so you are 
probably caught in a bit of a bind unless you are 
just trying to get back to a blank cel. 

Another small hint is that filling in an area in 
back with a pen is probably not a great approach 
as the pen stroked will show after a while as the 
pen will fade, and fade differently in the middle 
of the stroke in comparison to the edge which 
will make the are look streaky. 

Hope this helps....