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Page 69<br />

determining the mechanism(s) underlying drug resistance and, from this understanding, to devise ways<br />

for identifying combinations of drugs which might provoke drug-resistance mutations incompatible with<br />

viral survival.<br />

Several protocols have been designed for combination therapy using a variety of anti-HIV-1 drugs (see<br />

reviews [5,11]). Combinations of different drugs that interact with the same binding site of the same<br />

viral protein but lead to mutually antagonistic or suppressive resistance mutations have been studied<br />

extensively, especially for the combined uses of different but structurally related NRTIs (see for<br />

example [136–138]) or NNRTIs [139–141]. Combinations of drugs or inhibitors that target different<br />

sites of the same viral protein, primarily the combination of NRTIs and NNRTIs of HIV-1 RT, show<br />

enhanced inhibition of HIV-1 RT polymerase activity and suppression of the emergence of drugresistance<br />

mutations (for example [142–146]). Experiments have also been conducted with combinations<br />

of drugs that target different viral proteins, e.g., inhibitors of virus adsorption, virus-cell fusion, and/or<br />

uncoating proteins have been tested in combination with protease inhibitors and/or RT inhibitors.<br />

Combinations of AZT with the glycosylation inhibitor castanospermine [147], or with the Tat inhibitor<br />

Ro 24-7429 [148], or with the protease inhibitor Ro 31-8959 [149] have been shown to potently inhibit<br />

HIV-1 viral replication in vitro.<br />

Combination therapy can increase the effectiveness of inhibition and significantly impair efficiency of<br />

viral replication. However, both NRTI- and NNRTI-resistance mutations can affect the positioning of<br />

the nucleic acid and/or the overall structure of HIV-1 RT [23]. These two sets of resistance mutations<br />

can communicate with each other and can result in cross resistance. Moreover, new drug-resistance<br />

mutations that confer cross-resistance to both NRTIs and NNRTIs can be selected, which reduce the<br />

effectiveness of some drug combinations (see reviews [5,11,26]). Biochemical studies showed that both<br />

HIV-1 RT mutants [150] and viral variants [151] could be obtained that are resistant to the combination<br />

of AZT, ddI, and nevirapine. In clinical trials, treatment with AZT and ddI or AZT and ddC led to a<br />

different spectrum of NRTI-resistance mutations [152,153]. The most notable of these new mutations is<br />

Gln151Met, which is located at a position close to the dNTP-binding site. Structural analysis of the HIV-<br />

1 RT/DNA/Fab complex suggests that the side chain of Gln151 in the wild-type enzyme may interact<br />

with the first unpaired template nucleotide. The side chain of this residue may play a role in selecting the<br />

correct base for the incoming nucleotide [72]. Since the RT mutant containing only the Gln151Met<br />

mutation can confer high-level resistance to a number of NRTIs, including AZT, ddI, and ddC, it is not<br />

clear why this mutation did not emerge in monotherapy of these NRTIs. However, Gln151 is relatively<br />

well conserved and mutations at this position may have an unfavorable impact on HIV-1 RT.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_69.html [4/5/2004 4:50:49 PM]

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