Mechanism of action and in vitro activity of 1 ',3 '-dioxolanylpurine nucleoside analogues against sensitive and drug-resistant human immunodeficiency virus type 1 variants

(-)-beta-D-1',3'-Dioxolane guanosine (DXG) and 2,6-diaminopurine (DAPD) dio xolanyl nucleoside analogues have been reported to be potent inhibitors of human immunodeficiency virus type 1 (HIV-1). We have recently conducted exp eriments to more fully characterize their in vitro anti-HIV-1 profiles, Ant iviral assays performed in cell culture systems determined that DXG had 50% effective concentrations of 0.046 and 0.085 mu M when evaluated against HI V-1(IIIB) in cord blood mononuclear cells and MT-2 cells, respectively. The se values indicate that DXG is approximately equipotent to 2',3'-dideoxy-3' -thiacytidine (3TC) but 5- to 10-fold Less potent than 3'-azido-2',3'-dideo xythymidine (AZT) in the two cell systems tested. At the same time, DAPD wa s approximately 5- to 20-fold less active than DXG in the anti-HIV-1 assays , When recombinant or clinical variants of HIV-I were used to assess the ef ficacy of the purine nucleoside analogues against drug-resistant HIV-1, it was observed that AZT-resistant virus remained sensitive to DXG and DAPD. V irus harboring a mutation(s) which conferred decreased sensitivity to 3TC, 2',3'-dideoxyinosine, and 2',3'-dideoxycytidine, such as a 65R, 74V, or 184 V mutation in the viral reverse transcriptase (RT), exhibited a two- to fiv efold-decreased susceptibility to DXG or DAPD. When nonnucleoside RT inhibi tor-resistant and protease inhibitor-resistant viruses were tested, no chan ge in virus sensitivity to DXG or DAPD was observed. In vitro drug combinat ion assays indicated that DXG had synergistic antiviral effects when used i n combination with AZT, 3TC, or nevirapine. In cellular toxicity analyses, DXG and DAPD had 50% cytotoxic concentrations of greater than 500 mu M when tested in peripheral blood mononuclear cells and a variety of human tumor and normal cell lines. The triphosphate form of DXG competed with the natur al nucleotide substrates and acted as a chain terminator of the nascent DNA . These data suggest that DXG triphosphate may be the active intracellular metabolite, consistent with the mechanism by which other nucleoside analogu es inhibit HIV-1 replication. Our results suggest that the use of DXG and D APD as therapeutic agents for HIV-1 infection should be explored.