ANALYSIS OF ANTIRETROVIRAL NUCLEOSIDES BY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY AND COLLISION-INDUCED DISSOCIATION

Antiretroviral nucleoside drugs used against the human immunodeficienc y virus (HIV) infection have been analyzed using negative ion electros pray ionization (ESI) mass spectrometry and collision-induced dissocia tion (CID-MS/MS). Mass fragmentation of azidothymidine (AZT), didanosi ne (ddI), dideoxycytidine (ddC) and dideoxythiacytidine (3TC) were obt ained at different cone voltages and collision energies. Fragmentation of purines and pyrimidines occurred by different pathways. For purine s (ddI), the fragmentation was similar to those found in endogenous nu cleosides; mainly the pseudo molecular ion is present (M-H)(-) and a c leavage through the glycosidic bond forming (B)(-) was observed. For p yrimidines (AZT, ddC, 3TC), the fragmentation pathways were different from endogenous nucleosides; for AZT, the fragmentation occurred prima rily through the elimination of the azido group in the 3'-position (M- H-2-N-3)(-), whereas ddC and 3TC presented more complex fragmentation patterns. For ddC, fragmentation appeared to be dominated by a retro D iels-Alder mechanism (M-CONH)(-). For 3TC, the sulfur atom in the suga r moiety provided greater stability to the charge, producing fragments where the charge resided initially in the dideoxyribose (M-C2O2H6)(-) .