Kei - shareholders would get something in a buyout of PTX (or any other company) at ANY price.
We are not talking about a Chapter 11 liquidation where creditors would be paid first and equity holders would divvy up any remaining scraps. In a non ch-11 buyout the buyer buys the company, debts and all, and those proceeds would be shared among the equity holders.
Perhaps a better question would be "who would pay $500MM or more for PTX, knowing that they would be assuming $300MM (is that right? just a guess on my part from what I think I read) in debt? That would be assigning an enterprise value of $800MM to PTX - is it really worth that much to anyone?"