Removing ink stains from leather is not generally a simple cleaning problem

Most ink removal products do not work simply because of the length of time the ink has been on the leather but these are always the best place to start.
Most regular leather cleaners will not remove ink unless it is very fresh and a very good leather protector has been used on a regular basis.
Ink is a dye and it has re-coloured the leather (this is what it is meant to do – think of a pen on paper).
The longer the ink has been left on the leather the harder it is to remove because over time the ink soaks into the finish. Once it has soaked deep into the finish much stronger products are needed to remove the ink.  These will damage the finish and may remove the colour which will then need to be repaired - this is best left to a leather specialist as DIY repair products rarely work effectively.