We've actually shared footage of the unreleased Akira Game Boy game quite some time ago. It was only in prototype form, but there were a number of playable portions in that prototype. Obviously, the prototype is extremely rare, and anyone who can get their hands on one is the new owner of a collector's item.
Just today, another round of Game Boy prototypes for Akira have been sold on eBay. The seller managed to scoop up $3,000 for the carts, which honestly seems like a steal for such an interesting part of gaming history. To this day, the prototypes have not been leaked online. We'll see if that stays the case when this new owner has the carts in his posession.