A series of triterpene compounds characterized by a stringent structure-activity relationship were identified as potent and selective inhibitors of human immunodeficiency virus type 1 (HIV-1) replication. Currently studied betulinic derivatives have 50% inhibitory concentrations (ICss) agaeHIV-1 strain III/LAI in the 10 nM range in several cellular infection assays but are inactive against HIV-2. These compounds did not significantly inhibit the in vitro activities of several purified HIV-l enzymes. Rather, they ap ed to block virus infection at a postbinding, envelope-dependent step involved in the fusion of the virus to the cell membrane. CCID5o) and cultured for 5 days. Cell viability was assessed by the spectrophotometric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-based assay (9). Mock-infected cultures were carried out in parallel to determine the degree of cytotoxicity of the compounds. p24 core antigen in culture supernatants was determined by ELISA (NEK060B kit from New England Nuclear). RT activity was measured by a poly(rA) 3H SPA scintillation proximity assay (NK9020 from Amersham). The antiviral assay using MT4 cells was as described (10). The effect of compounds on the kinetics of viral production in human peripheral blood mononuclear cells (PBMCs) was also assayed. PBMCs from seronegative donors were isolated by Ficoll/Hypaque (Pharmacia) and stimulated for 3 days with phytohemagglutinin (2.5 pg/ml; Difco). Cells were then washed three times with medium, pelleted, and incubated with 50-100 CCID50 of virus stock per ml. After 1 hr at room temperature, cells were resuspended in RPMI 1640 medium containing 10%6 fetal bovine serum, 10%6 T-cell growth factors (Lymphocult, Biotest Diagnostics, Danville, NJ), and anti-interferon a neutralizing antibody (Bayer, Wuppertal, F.R.G.) at 80 units/ml. Cells were plated at 106 per ml in six-well culture plates (Costar) in the presence of the compound to be tested. At 3, 7, 10, and 14 days after infection, cultures were harvested and viable cells were examined by trypan blue exclusion. Aliquots of supernatants were frozen at -700C until tested for p24 antigen, and cells were resuspended at the same density in complete fresh medium with the same concentration of the tested drug.Determination of Cytoplasmic Proviral DNA. Shortly after infection, the proviral DNA obtained from a selective extraction was quantified by PCR amplification. H9 cells were preincubated in the presence or absence of the drug to be tested for 1 hr, infected with HIV-1 (LAI) at 500 CCIDso/ml for 1 hr, and grown in culture medium for another hour. Cells were then washed intensively, and cytoplasmic proviral DNA was extracted by a digitonin/proteinase K/RNase Abbreviations: AZT, 3'-azido-3'-deoxythymidine; HIV, human immunodeficiency virus; PBMC, peripheral blood mononuclear cell; sCD4, soluble CD4; RT, reverse transcriptase. tTo whom reprint requests should be addressed.
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