No abstract

Triciribine (TCN) and its 5'-monophosphate (TCN-P) are novel tricyclic compounds with known antitumor activity; TCN-P is currently in phase II human clinical trials. We now report that these compounds have potent and selective activity against HIV-1 and HIV-2. Using a syncytial plaque assay, TCN and TCN-P were active against HIV-1 at 0.01-0.02 microM and had differential selectivities of 2250 and 1900, respectively, compared to 1850 for AZT. In contrast, TCN and TCN-P had minimal selectivity against human cytomegalovirus (50 and 27, respectively). TCN and TCN-P markedly inhibited HIV-1-induced p24 core antigen production, reverse transcriptase, and infectious virus production in a dose-dependent manner using HIV-1 acutely infected CEM-SS, H9, and persistently infected H9IIIB and U1 cells. In acutely infected PBL cells, TCN and TCN-P inhibited reverse transcriptase and infectious virus production but not p24 core antigen production. Using a microtiter XTT assay, TCN and TCN-P were active against a panel of HIV-1 and HIV-2 strains at IC50 values ranging from 0.02 to 0.46 microM. Evaluation of matched pairs of predrug and postdrug therapy HIV-1 isolates established that AZT-resistant and TIBO-resistant variants of HIV-1 were sensitive to TCN or TCN-P. Furthermore, unlike AZT and other fraudulent nucleosides, neither TCN, TCN-P, nor TCN-TP inhibited the viral reverse transcriptase. Thus, even though triciribine is a nucleoside chemically, it does not act biologically by classic nucleoside modalities but rather by a unique mechanism yet to be elucidated.

The toxicity of 3′-azido-3′-deoxythymidine (AZT) and the appearance of drug-resistant mutants in patients treated with AZT emphasizes the critical importance of the development of alternative strategies for the therapy of AIDS patients. Combination antiviral chemotherapy provides an attractive therapeutic strategy since the dose of the individual agents may be lowered to reduce toxicity and the use of two potent antiviral agents may limit the development of drug resistance. Two analogues of tetrahydro-imidazo[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thione (TIBO) potently and selectively inhibit the replication of HIV-1 in cell culture. In combination with AZT, either of the two TIBO compounds, R82913 and R86183, was highly synergistic in cell culture against HIV-1. However, in biochemical enzyme inhibition assays, utilizing recombinant HIV-1 reverse transcriptase, synergy was not detected at the enzymatic level. These results suggest that one of these two known inhibitors of HIV-1 reverse transcriptase may have a secondary mechanism of action distinct from inhibition of the reverse transcriptase.

R82913 and R86183, two derivatives of tetrahydroimidazo[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thione (TIBO), were found to potently and selectively inhibit the replication and cell killing effects of a panel of biologically diverse laboratory and clinical strains of HIV-1. The two compounds exhibited significant activity in all human cell lines tested, as well as in fresh human peripheral blood lymphocytes and macrophages. One of these two compounds (R82913) was found to significantly inhibit the replication of a murine retrovirus (Rauscher murine leukemia virus) in both UV-XC plaque formation and virus yield reduction assays. R86183, despite differing from R82913 only in the positioning of a single chlorine molecule, was not active against the murine retrovirus but was 10-fold more potent in inhibiting HIV-1 replication. Combination antiviral assays with other reverse transcriptase inhibitors, including AZT, ddC, and carbovir, yielded synergistic anti-HIV activity with both TIBO derivatives. Additive to slightly synergistic results were obtained in combinations with ddI and phosphonoformic acid whereas additive to antagonistic activity was detected in combination with dextran sulfate.

A cocultivation assay system consisting of uninfected human T cells and cells chronically infected with human immunodeficiency virus type 1 has been used to investigate syncytium formation in short-term assays. Continuous treatment or short-term pretreatment of uninfected CD4-expressing human T-cell lines with 3'-azido-3'-deoxythymidine (AZT) reduces the ability of these cells to participate in syncytium formation when mixed with chronically infected cells. The effect of AZT on syncytium formation is observed both as a reduction in the number of syncytia and as a reduction in the size of the syncytia that are detected. This syncytiumreducing effect of AZT is dose and time dependent and does not result from a modulation of CD4 antigen expression on the cell surface of uninfected, treated cells. Maximum syncytium reduction is observed with the continuous presence of AZT; however, pretreatment for times as short as 15 min results in a significant reduction in syncytium formation. Since reverse transcription is not required for efficient syncytium formation, the syncytium-reducing effect of AZT on uninfected human cells may represent an antiviral property of AZT with important therapeutic potential.sections from AIDS patients (22). The ability to efficiently form syncytia may be a major contribution to the virulence of HIV and may play a role in disease progression. Several reports have suggested that an increased ability to form syncytia in cell culture is associated with increasing virulence of HIV (23)(24)(25).Few studies have addressed the effects of AZT on syncytium formation (20,26). We have determined that AZT has a significant effect on syncytium formation and cell killing in an in vitro cocultivation assay. The results of experiments presented here indicate that exposure to AZT exerts a syncytium-reducing effect on the uninfected CD4-expressing cell population that is both time and dose dependent. Pretreatment of the chronically infected cell population with AZT has no effect on the ability of these cells to participate in syncytium formation. It has been postulated that cell fusion and syncytium formation are not dependent on virus replication and reverse transcription (27), and the results of our experiments support this conclusion. Thus, we believe that the ability to reduce syncytium formation is an additional antiviral property of AZT.