Inhibition of HIV-1 by DpT-Based Iron Chelators

HIV-1 transcription is activated by HIV-1 Tat protein, which recruits transcriptional co-activators to the HIV-1 promoter. Elongation of HIV-1 transcription is mediated by the interaction of Tat with host cell cycle-dependent kinase 9 (CDK9)/cyclin T1, which phosphorylates the C-terminal domain of RNA polymerase II. Tat itself is phosphorylated by host cell cycle-dependent kinase 2 (CDK2) [1] and inhibition of CDK2 by tridentate iron chelators such as 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone, (311) or ICL670 (deferasirox) inhibits HIV-1 transcription [2]. In addition to the inhibition of CDK2, 311 and ICL670 also prevent association of CDK9 with cyclin T1 [2], which could lead to inhibition of CDK9 activity and also to inhibition of HIV-1 transcription. Recently, a group of novel di-2-pyridylketone thiosemicarbazone (DpT) based tridentate iron chelators were shown to exhibit marked antiproliferative activity in vivo [3]. Here we screened DpT-based and also 2-benzoylpyridine thiosemicarbazone (BpT)-based tridentate iron chelators and identified three chelators, Dp44mT, Bp4eT and Bp4aT, that inhibited HIV-1 transcription but were not cytotoxic as determined by propidium iodide uptake, LDH release and calcein-AM uptake. The inhibition of HIV-1 transcription was observed in CEM HIV-1 LTR-GFP cells infected with Adeno-Tat and in 293T cells transiently transfected with HIV-1-LTR LacZ and Tat-expressing vectors with IC50s in the mid-nanomolar range. These new iron chelators also inhibited HIV-1 replication in CEM and THP-1 cells at 10 mM concentration. Analysis of the molecular mechanism of HIV-1 inhibition revealed that the DpT- and BpT-based iron chelators inhibited the activities of both CDK9/cyclin T1 and CDK2. The CDK9/cyclin T1 complex was disrupted in the cells treated with iron chelators, suggesting a possible mechanism for the inhibition of CDK9. In conclusion, our findings provide further evidence that iron chelators may inhibit HIV-1 transcription by deregulating CDK2 and CDK9. The projected therapeutic index of the selected DpT-based iron chelators was over 10³ suggesting their potential usefulness as future anti-retroviral therapeutics.