The present invention provides methods for improving the pharmacokinetics of protease inhibitors and protease inhibitor precursors and pharmaceutical composition comprising protease inhibitors or protease inhibitor precursors of formula I and a cytochrome P450 monooxigenase inhibitor; when the compound of formula I comprises an amino group, pharmaceutically acceptable ammonium salts thereof, wherein R1 may be, for example, (HO)2P(O)—, (NaO)2P(O)—, alkyl-CO— or cycloalkyl-CO—, wherein X may be, for example, F, Cl, and Br, and wherein R2 and R3 are as defined herein.