The present invention relates to disulfide masked prodrug compounds of formula I, compositions and methods that are amenable to bioactivation by a reducing agent such as glutathione: W—P(V)LG  (Formula I), wherein P(V)LG is a pentavalent phosphorus leaving group; and W is: wherein R1, R2, R3, R4, R5, R6, R7 and R8 are as defined in the specification. Such disulfide based compounds, compositions, and methods can be useful, for example, in providing novel prodrugs for use as therapeutics. Once administered, the prodrug is metabolized in vivo into an active metabolite in a process termed bioactivation.