What is the reaction mechanism for the hydrolysis of Amifostine? I have to do research on a drug called Amifostine: 2-(3-aminopropylamino)ethylsulfanyl
phosphonic acid.  



I'm supposed to say how the organic reaction works inside the body.  I understand that it's hydrolyzed by an enzyme into its active form; 2-((aminopropyl)amino)ethanethiol.  



I don't get what happens to the phosphonic acid functional group. Does the water break apart the $\ce{S-P}$ bond because it is a weak bond? And what does the acid group become? $\ce{H3PO4}$?
You are expelling a thiol rather than an alcohol via alkaline phosphatase-catalyzed hydrolysis.  One can also view it as a free electron pair on "X" attacking the phosphorus to pop out its =O to an anion, then the charge poking back in to expel thiolate.