cx-5461 and Precursor-Cell-Lymphoblastic-Leukemia-Lymphoma

Ribosome biogenesis is a fundamental cellular process and is elevated in cancer cells. As one of the most energy consuming cellular processes, it is highly regulated by signaling pathways in response to changing cellular conditions. Many of the regulators of this process are aberrantly activated in various cancers. Recently two novel rRNA synthesis inhibitors, CX-5461 and BMH-21, have been shown to selectively kill cancer cells while sparing normal cells. Here, we tested the effectiveness of pre-rRNA synthesis inhibitor CX-5461 on acute lymphoblastic leukemia cells with different cytogenetic abnormalities. Acute lymphoblastic leukemia cells are more sensitive to rRNA synthesis inhibition compared to normal bone marrow cells. CX-5461 treated cells undergo caspase-dependent apoptosis independent of their p53 status. More-over, CX5461, activates checkpoint kinases and arrests cells in G2 phase of cell cycle. Finally, overcoming this G2 arrest by inhibiting ATR kinase leads to robust cell killing. These results show that CX-5461 can be even more potent in combination with ATR inhibitors.

Topics: Antineoplastic Agents; Antineoplastic Combined Chemotherapy Protocols; Apoptosis; Ataxia Telangiectasia Mutated Proteins; Benzothiazoles; Caspases; Cell Line, Tumor; Cell Proliferation; Dose-Response Relationship, Drug; G2 Phase Cell Cycle Checkpoints; Humans; Naphthyridines; Nucleic Acid Synthesis Inhibitors; Precursor Cell Lymphoblastic Leukemia-Lymphoma; Protein Kinase Inhibitors; RNA Polymerase I; RNA, Ribosomal; Signal Transduction; Tumor Suppressor Protein p53

Enhanced rRNA synthesis is a downstream effect of many of the signaling pathways that are aberrantly activated in cancer, such as the PI3K/mTOR and MAP kinase pathways. Recently, two new rRNA synthesis inhibitors have demonstrated therapeutic effects on cancer cells while sparing normal cells. One of them, CX-5461, is currently in phase 1 clinical trials for hematological malignancies. Here, we investigate the effectiveness of transient treatment with this drug on acute lymphoblastic leukemia cells. Our results show that short exposure to CX-5461 followed by drug washout is sufficient to induce persistent G2 cell-cycle arrest and irreversible commitment to cell death, in spite of rRNA synthesis returning to normal within 24 hours of drug washout. The magnitude of cell death after transient exposure is similar to continuous exposure, but the time to cell death is relatively delayed with transient exposure. In this report, we also investigate rational drug combinations that can potentiate the effect of continuous CX-5461 treatment. We show that the checkpoint abrogator UCN-01 can relieve CX-5461-induced G2 arrest and potentiate the cytotoxic effects of CX-5461. Finally, we show that ERK1/2 is activated upon CX-5461 treatment, and that pharmacological inhibition of MEK1/2 leads to enhanced cell death in combination with CX-5461. In summary, our results provide evidence for the effectiveness of CX-5461 pulse treatment, which may minimize drug related toxicity, and evidence for enhanced effectiveness of CX-5461 in combination with other targeted agents.

Topics: Antineoplastic Combined Chemotherapy Protocols; Benzothiazoles; Blotting, Western; Cell Death; Cell Proliferation; Drug Synergism; Flow Cytometry; G2 Phase Cell Cycle Checkpoints; Humans; Naphthyridines; Nucleic Acid Synthesis Inhibitors; Precursor Cell Lymphoblastic Leukemia-Lymphoma; Real-Time Polymerase Chain Reaction; RNA, Ribosomal; Tumor Cells, Cultured