Deregulation of cell proliferation in cancer and leukemia generally occurs following mutation and/or amplification of signalling molecules termed tyrosine kinases (TK). FLT3, a tyrosine kinase receptor, is the most commonly mutated gene in acute myeloid leukemia (AML) with internal tandem duplications (FLT3-ITD) occurring in 20-25% of patients. FLT3-ITD mutations are associated with aggressive disease characteristics, and FLT3 kinase inhibition can improve the otherwise poor prognosis of these leukemia patients. Although more than 20 small molecule inhibitors with inhibitory activity against FLT3 have been reported to date, clinical success of FLT3 inhibitors is largely limited by pharmacokinetic and pharmacodynamic difficulties in achieving sustained FLT3 inhibition without significant off-target toxicity. This indicates the need to broaden the therapeutic window of the FLT3 TKIs. In this proposal, we will utilize novel hypoxia-activated 'prodrug'technologies pioneered by Drs. Jeff Smaill and Adam Patterson at the University of Auckland. It takes advantage of the pathological hypoxia which was recently discovered by us as a prevalent environment of the leukemic but not normal bone marrow. The central hypothesis is that hypoxia-induced activation of TKIs in the leukemic BM niche would result in higher local concentrations of the drug with increased efficacy against leukemic blasts, while limiting toxicity to normal cells.
In Aim 1, a hypoxia-activated prodrug strategy will be utilized to introduce increased tumour-selectivity to FLT3 inhibitors and thereby broaden their therapeutic index. As a prototype of this technology, hypoxia-activated irreversible pan-HER inhibitor PR610 was developed, and is scheduled to begin Phase I clinical testing in mid-2012. We will synthesise nitromethylaryl quaternary ammonium salt (NMQ) prodrugs of the known FLT3 inhibitors AC220, MLN-518, sunitinib and crenolanib. This class of prodrugs carry a permanent positive charge and are therefore excluded from cells, preventing binding of the kinase inhibitor with its intracellular kinase target in normal cells. In turn, the prodrug can undergo one-electron reduction and will fragment selectively under hypoxic conditions to release the cell permeable kinase inhibitor. This restricts normal tissue exposures and permits significantly greater doses to be administered (typically 50-100 fold increase in plasma AUC). We anticipate that the FLT3 pro-drugs would be the first prototype of hypoxia-activated TKI in leukemia. We will characterize the activity, selectivity and potency of FLT3 scaffolds in FLT3 mutated AML. MTD/PK/PD testing and in vivo efficacy studies in the FLT3-mutated xenograft model of human AML will be carried out in Aim 2. If successful, this approach will provide mechanism-based rationale for eliminating leukemic cells within the hypoxic BM niches and improve cure rates in AML.
Specific genetic aberration called FLT3 gene mutation is one of the most common molecular lesions in acute myelogenous leukemia (AML) that confers poor prognosis and short survival. Several inhibitors of the oncogenic FLT3 kinase have shown initial signs of efficacy which are typically transient and short-lived, thought to be due to incomplete blockade of FLT3. The goal of the proposal is to exploit pathologic hypoxia discovered by us in the leukemic bone marrow, as a trigger for activation of the prodrug with FLT3-inhibitory properties. We have already generated several such drugs using proprietary hypoxia-activated technology. If successful, this strategy will result in substantially greater loal concentrations of the drug with increased potency against leukemic blasts.
