Foscarnet is a broad-spectrum Viral DNA polymerase inhibitor active in
vitro and in vivo against human immunodeficiency virus type 1 (HIV-1)
. Strains of HIV-1 resistant to foscarnet were selected by in vitro pa
ssage in increasing concentrations of drug. Reduced susceptibility to
foscarnet was evident at the levels of both HIV-1 replication and reve
rse transcriptase. Biologically cloned, foscarnet-resistant strains wi
th distinct genotypes were hypersensitive to zidovudine, azidodeoxyuri
dine, nevirapine, and R82913 but had unchanged susceptibility to zalci
tibine and didanosine. The reverse transcriptase of foscarnet-resistan
t strains had unique substitutions Glu89-Lys, Leu92-lle, or Ser156-Ala
, the third being associated with six polymorphic changes. Introductio
n of these mutations into wild-type HIV-1 by site-directed mutagenesis
confirmed their role in foscarnet resistance. In the three-dimensiona
l structure of the reverse transcriptase enzyme these amino acids are
located close to the template strand of the template primer and far aw
ay from the putative pyrophosphate binding site, suggesting that the m
echanism by which HIV-1 becomes resistant to foscarnet is indirect. Fo
scarnet resistance is thus likely to be mediated through an altered in
teraction of the mutant enzyme with the template strand of the templat
e primer which distorts the geometry of the polymerase active site and
thereby decreases foscarnet binding. (C) 1995 Academic Press, Inc.