Mode of action of 5'-linked cholesteryl phosphorothioate oligodeoxynucleotides in inhibiting syncytia formation and infection by HIV-1 and HIV-2 in vitro.

A phosphorothioate homocytidine 10-mer containing a cholesteryl moiety covalently linked to the 5'-end (Chol-SdC10) inhibited syncytium formation in susceptible T cells induced by HIV-1 and HIV-2. The syncytium inhibition effect was minimal with unmodified cytidine homopolymer of the same net charge. Chol-SdC10 was shown to protect CEM cells against infection by cell-free HIV-1 particles without any apparent toxicity to the growth of CD4+ T cells. The DNA polymerase activity of the purified reverse transcriptase (RT) of HIV-1 was markedly inhibited by Chol-SdC10 but the effect on the RNase H activity of RT was minimal. Analysis of the kinetics of reverse transcriptase inhibition mediated by the drug revealed that the inhibition at a higher concentration was competitive with respect to template primer binding and noncompetitive at lower concentrations. Chol-SdC10 also partially blocked the binding of gp120 to CD4 in a solid-phase ELISA. These results confirm that the anti-HIV activity of phosphorothioate cytidine homopolymers increases markedly by covalent modification with the cholesteryl moiety at the 5'-end and demonstrates that the cytoprotective effect is manifested at multiple steps in the virus life cycle. These steps include inhibition of retroviral replication activity as well as the binding and fusion of HIV with CD4+ T cells.