Abstract 3796: Targeting cancer stem cells using ALDH-dependent 5-nitrofuran prodrugs

We hypothesise that cancer stem cells with high aldehyde dehydrogenase (ALDHhigh) activity present a new therapeutic target and will be selectively sensitive to 5-nitrofuran pro-drugs. Cancers are heterogeneous and contain subpopulations of ALDHhigh cells with tumour initiating potential. ALDH enzymes metabolize toxic aldehydes, and are highly expressed in somatic and cancer stem cells (CSCs), although their function in stem cells is not fully understood. In a small molecule screen coupled with target ID, we recently discovered that clinically active 5-nitrofurans (5-NFNs) are substrates of ALDH2 (Zhou et al., 2012). 5-NFNs are a class of pro-drug widely used to treat bacterial and parasitic infections where their relative specificity is driven by nitroreductases, but little is known about the enzymes that bio-activate 5-NFNs in humans. Recent clinical cancer research has found that the 5-NFN nifurtimox has anti-cancer properties and it is currently in Phase 2 clinical trials for neuroblastoma and medulloblastoma (ClinicalTrials.gov Identifier: NCT00601003), however the mechanism underlying this anti-cancer activity is unknown. In melanoma and other cancers, ALDH1A1 and ALDH1A3 are highly expressed in CSCs. We find that cancer cell lines are highly sensitive to 5-NFNs in cell viability assays, where we use a logarithmic drug dose range and assess cell viability by PrestoBlue™ (e.g. A375 melanoma cells EC50 = 86nM). To test if ALDH1 isoforms are substrates of 5-NFNs, we preformed in vitro activity assays by monitoring NADH production (λ = 340nm). We find that the clinically active 5-NFNs nifuroxazide and nifurtimox, in addition to our own newly synthesised 5-NFNs, are competitive substrates for human ALDH1A3 activity in vitro (p