Choline kinase alpha (ChoKα) is regarded as an attractive cancer target. The enzyme catalyses the formation of phosphocholine (PCho), an important precursor in the generation of phospholipids essential for cell growth. ChoKα has oncogenic properties and is critical for the survival of cancer cells. Overexpression of the ChoKα protein can transform noncancer cells into cells with a cancerous phenotype, and depletion of the ChoKα protein can result in cancer cell death. However, the mechanisms underlying the tumourigenic properties of ChoKα are not fully understood. ChoKα was recently demonstrated to associate with other oncogenic proteins, raising the possibility that a non-catalytic protein scaffolding function drives the tumourigenic properties of ChoKα rather than a catalytic function. In order to differentiate these two roles, we compared the impact on cancer cell survival using two tools specific for ChoKα: (1) small interfering RNA (siRNA) to knockdown the ChoKα protein levels; and (2) compound V-11-0711, a novel potent and selective ChoKα inhibitor (ChoKα IC50 20 nℳ), to impede the catalytic activity. Both treatments targeted the endogenous ChoKα protein in HeLa cells, as demonstrated by a substantial reduction in the PCho levels. siRNA knockdown of the ChoKα protein in HeLa cells resulted in significant cell death through apoptosis. In contrast, compound V-11-0711 caused a reversible growth arrest. This suggests that inhibition of ChoKα catalytic activity alone is not sufficient to kill cancer cells, and leads us to conclude that there is a role for the ChoKα protein in promoting cancer cell survival that is independent of its catalytic activity.
Background. MK-0457 (VX-680) is a small-molecule inhibitor of Aurora kinases A, B, and C, FLT3, and JAK-2 with nanomolar level broad spectrum pre-clinical anti-tumor activity. The T315I BCR-ABL mutation mediates high level resistance to imatinib, dasatinib and nilotinib. A Phase I study of MK-0457 is being conducted in patients with refractory hematologic malignancies. The study regime is a 5 day continuous IV regimen given every 2 to 3 weeks. Methods. Fifteen CML patients received MK-0457 dosed at 8, 12, 16, 20, 24, 28, and 32 mg/m2/hr (11/15 had detectable BCR-ABL T315I). Peripheral blood for biomarker studies was available from patients at the 12 mg/m2/hr dose and above. Specimens were drawn prior to initiation of the infusion and at the end of the infusion (Day 5) during Cycle 1. Patient specimens have been analyzed using flow cytometry assays to measure BCR-ABL inhibition (phospho-CRKL), FLT3 and JAK-2 inhibition (phospho-STAT5) and Aurora kinase inhibition (phospho-histone H3). Results. MK-0457 has in vitro activity against cells expressing wild-type or mutated BCR-ABL, including the T315I BCR-ABL mutation. In CML and ALL patients with the T315I BCR-ABL mutation, MK-0457 is capable of inhibiting BCR-ABL as measured by the phospho-CRKL assay. The observed decrease in BCR-ABL activity trends with plasma exposure to MK-0457. At dose levels less than 20 mg/m2/hr, plasma exposure of MK-0457 typically does not exceed 1 micromolar and inhibition of BCR-ABL has not been observed. At exposures that exceed 1 micromolar, MK-0457 inhibits BCR-ABL, with pCRKL abundance falling below the lower limit of detection in one patient dosed at 24 mg/m2/hr and decreasing compared to baseline in several other patients. There is also a trend correlating degree of BCR-ABL inhibition as measured by the pCRKL biomarker and clinical response to MK-0457. Conclusions. MK-0457 inhibits BCR-ABL activity in CML and ALL patients with the BCR-ABL T315I mutation. Objective responses have been observed in patients without demonstrable BCR-ABL inhibition, suggesting that inhibition of Aurora kinase may also contribute to the clinical efficacy of MK-0457 in CML and ALL patients with the BCR-ABL T315I mutation.
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