You would probably be best leaving that alone tbh, biro ink is pretty hard to remove.
Even if you manage to lift some of it, it's on the reverse of the cardback so any excessive rubbing (ooer!) will start to damage the card. You might have *some* success with one of those magic eraser thingies but even so, you'll still be left with the imprint of the scribbles on the card.
I've had some success removing ink from glossy card fronts - http://starwarsforum.co.uk/viewtopic.php?f=16&t=8778&p=122448#p122406 but that was marker and not biro, so again - probably best you leave it alone.