@ Abstract published in Advance ACS Abstracts, March 15, 1995. lular membranes. To facilitate the cellular penetration of these molecules, a variety of protecting groups for the anionic oxygens at the phosphorus have been examined. Recently, the bis [(pivaloyloxy) methyll esters of PMEA and PMPA were shown to have 100-fold increased cellular uptake and considerably greater antiviral activity in vitro in comparison with the parent corn ...