The efficacy of most chemotherapeutic agents relies upon the efficient induction of apoptotic cell death. In the case of leukemias expressing activated tyrosine kinases (e.g., chronic myelogenous leukemias expressing Bcr-Abl, chronic myelomoncytic leukemias expressing Tel-PDGFR2, and acute myeloid leukemias expressing activated FLT3), the utility of conventional chemotherapeutics is limited by the fact that activated tyrosine kinases are potent inhibitors of apoptotic cell death. In addition, current targeted kinase inhibitors are plagued by problems of acquired resistance and, even when efficacious, they don't entirely eliminate the reservoir of leukemic cells and thus patients must remain on therapy indefinitely. It would be highly advantageous to develop treatments that could selectively and forcibly induce the death of leukemic cells expressing activated tyrosine kinases, while sparing normal cells. Many cellular signaling pathways impinge upon the cellular decision to die by apoptosis. However, under a variety of different circumstances, execution of the cell death program is carried out by a family of cysteine proteases known as caspases. Although caspases (such as caspase 8) can be activated in a fairly direct manner by receptor binding of extracellular "death ligands" such as Fas and TNF, caspase activation in response to damaging agents (radiation, chemotherapeutics) typically proceeds through the cell's mitochondria. By a mechanism that is still not entirely clear, apoptotic stimuli induce transit of the respiratory chain enzyme cytochrome c from the intermembrane space of the mitochondria to the cytoplasm. Engagement of a cytosolic protein, Apaf-1, by cytochrome c then nucleates the formation of a structure known as the apoptosome, in which caspase 9 is recruited to and activated by Apaf1/cytochrome c. In previous reports, Bcr-Abl has been shown to inhibit mitochondrial cytochrome c release, a function that renders cells remarkably resistant to chemotherapy-induced cell death. We have recently shown that Tel-PDGFR2 and FLT3/D835Y are similarly resistant. Moreover, even in the presence of cytosolic cytochrome c, these activated tyrosine kinases prevent activation of the apoptosome. We hypothesize that novel strategies are required to induce selective death of leukemic cells as the anti-apoptotic activity of these activated tyrosine kinases are likely to limit the utility of most chemotherapeutic agents. Towards this end, we have formulated an innovative strategy to promote selective caspase activation by a unique means in cells expressing activated tyrosine kinases.
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