Tirapazamine Cytotoxicity for Neuroblastoma Is p53 Dependent

Relapse of neuroblastoma commonly occurs in hypoxic tissues, and is associated with an acquired and sustained high-level drug resistance, often due to p53 loss of function. Abrogating p53 function with HPV 16 E6 transduction in drug-sensitive neuroblastoma cell lines caused high-level drug resistance. Tirapazamine (TPZ) is a bioreductive agent that forms a toxic free radical in hypoxia. We determined in six neuroblastoma cell lines the cytotoxicity of TPZ using DIMSCAN, a digital imaging fluorescence assay, apoptosis and mitochondrial membrane potential (ΔΨ m ) by flow cytometry, and protein expression by immunoblotting. TPZ exhibited high cytotoxicity, especially in hypoxia (2% O 2 ), for all four p53-functional neuroblastoma cell lines, achieving >3 logs of cell kill (LC 99 ≤ 0.7 μg/mL). In p53-nonfunctional neuroblastoma cell lines, all TPZ LC 99 values were >3.0 μg/mL (average clinically achievable level). TPZ (24 hours) induced apoptosis in >46% of cells in p53-functional cell lines but failed to cause apoptosis in p53 nonfunctional cell lines. Induction of p53 and p21 expression by TPZ was observed in a p53-functional cell line (SMS-SAN) but not in a p53-nonfunctional cell line (CHLA-90). Significant ΔΨ m loss and glutathione (GSH) depletion in response to TPZ was observed in p53-functional cell lines (SMS-SAN, SMS-SAN EV, and CHLA-15) but not in p53-nonfunctional cell lines (SMS-SAN E6 and CHLA-90). N -Acetylcysteine inhibited TPZ-mediated ΔΨ m loss and GSH depletion, but neither N -acetylcysteine nor Boc-d-fmk inhibited apoptosis caused by TPZ. In response to TPZ, ΔΨ m loss preceded apoptosis. Thus, TPZ cytotoxicity for neuroblastoma cell lines in hypoxia occurred via a p53-dependent mitochondrial pathway that caused induction of p53 and p21, ΔΨ m decrease, GSH depletion, and apoptosis. These data further define the mechanism of action of TPZ and suggest that as a single agent, TPZ would only have clinical activity against p53-functional neuroblastomas.