Nucleoside-based inhibitors of reverse transcriptase were the first dr
ugs to be used in the chemotherapy of AIDS. After entering the cell, t
hese substances are activated to their triphosphate form by cellular k
inases, after which they are potent chain terminators for the growing
viral DNA (ref. 1). The two main factors limiting their efficacy are p
robably interrelated. These are the insufficient degree of reduction o
f viral load at the commencement of treatment and the emergence of res
istant variants of the virus. The reason for the relatively poor suppr
ession of viral replication appears to be inefficient metabolic activa
tion. Thus, for the most extensively used drug, 3'-azido-3'-deoxythymi
dine (AZT), whereas phosphorylation to the monophosphate is facile, th
e product is a very poor substrate for the next kinase in the cascade,
thymidylate kinase(2,3). Because of this, although high concentration
s of the monophosphate can be reached in the cell, the achievable conc
entration of the active triphosphate is several orders of magnitude lo
wer. Determination of the structure of thymidylate kinase as a complex
with AZT monophosphate (AZTMP) together with studies on the kinetics
of its phosphorylation have now led to a detailed understanding of the
reasons for and consequences of the poor substrate properties.