Soak it in a bowl of safe bleach made especially for clothes.
There is an amazing product that I discovered. I ordered it on line and have since ordered many. It is called Motsenbocker's LIFT OFF 3 (Pen, Ink and permanent Marker Graffiti Remover)
This is really a good product. Works on Nail Polish, too!
This is really a good product. Works on Nail Polish, too!