Pilot Trial of Sirolimus/MEC in High Risk Acute Myelogenous Leukemia (AML)

Recent improvements in our understanding of leukemia biology have led to the introduction of highly effective, molecularly targeted therapies. This is exemplified by the development of BCR-ABL tyrosine kinase inhibitors such as imatinib as monotherapy for chronic myeloid leukemia (CML) and in combination with chemotherapy for BCR-ABL+ acute lymphoblastic leukemia (ALL). Imatinib mesylate blocks the protein made by the BCR-ABL oncogene.

The PI3K (phosphatidylinositol 3-kinases) signaling is critical to leukemia cell survival and can be targeted. Growth and survival stimulating signal transduction pathways are abnormally and universally activated in AML (Acute Myeloid Leukemia). This signal cascade is thought to contribute to survival and growth in tumor cells via downstream effects upon target proteins AKT/Protein kinase B and mammalian target of rapamycin (mTOR) a protein that helps control several cell functions.

In AML, we and others have shown that PI3K signaling is constitutively activated in over 85% of primary samples and that the small molecule PI3K inhibitor LY294002 is cytotoxic in vitro to virtually all samples tested. As LY294002 is poorly suited for drug development, we have concentrated upon other ways to inhibit signal transduction through this pathway. Mammalian target of rapamycin (mTOR) emerged as a reasonable target due to the availability of clinically available, highly specific inhibitors with favorable safety profiles. Mammalian target of rapamycin (mTOR) plays a central but complex role in cancer cells' metabolic regulation and survival. This serine/threonine kinase coordinates several important cellular functions and its activity is modulated in response to amino acid, glucose, oxygen, and ATP availability as well as extracellular growth factor ligation. Mammalian target of rapamycin (mTOR) activity regulates protein translation, nutrient and amino acid uptake, mitochondrial respiration, glycolysis, cell size regulation, cell cycle entry and progression, ribosome biogenesis, and autophagy. Constitutive mammalian target of rapamycin (mTOR) activation is commonly seen in cancer cells and is thought to promote survival in the setting of a wide variety of cellular insults. Importantly, mTOR opening may cause chemotherapy resistance. Although regulation of mTOR signaling in leukemia occurs through by several inputs, mTOR activity in AML is thought to be primarily regulated by PI3K signaling through AKT via the agent tumor suppressor tuberous sclerosis complex (TSC1& 2) and its target rheb GTPase.

Taken together, mammalian target of rapamycin mTOR is a smart target for molecularly targeted therapy in AML due to its importance in the growth and survival of AML cells, its necessity for AML cell survival in certain contexts, and its probable role in chemotherapy resistance and relapse.