In Vitro Characterization of the Bacteria-derived Hypoxia-selective Cytotoxin BE-43547

Abstract

Background/aim: Hypoxia-activated pro-drugs, such as TH-302, may kill hypoxic treatment-resistant tumor cells, but have failed in clinical trials. This may be related to variable levels of drug-activating reductases. Compounds such as bacteria-derived BE-43547, which target hypoxic cells independently of reductases, may be beneficial. This study characterized the in vitro potency and hypoxia selectivity of BE-43547 and TH-302.

Materials and methods: Tumor cells were exposed to different oxygenation levels in the presence/absence of drug, and survival was quantified using total cell number (BE-43547) or clonogenic survival (BE-43547 and TH-302) assays. Half-maximal inhibitory concentration (IC₅₀) values and the hypoxia-cytotoxicity-ratio (HCR: normoxic IC₅₀/hypoxic IC₅₀) were determined from dose-response curves. Finally, both drugs were tested in spheroids exposed to 20% or 0% O₂ for 24 h followed by assessment of clonogenic survival.

Results: BE-43547 was highly potent and displayed little inter-cell line variability. Strongly enhanced cytotoxicity was observed under oxygen-restricted conditions with HCR's of ~100 and ~20 after 24 h of treatment with 0 or 0.5% O₂, respectively. Reducing treatment time somewhat reduced hypoxia selectivity. Hypoxia selectivity was observed regardless of whether the drug was added before or during the hypoxic challenge. TH-302 IC₅₀ values varied 10-fold under oxic conditions, whereas those of the anoxic-to-normoxic HCR varied from 15 to 88. Both BE-43547 and TH-302 were unable to completely sterilize anoxic incubated spheroids.

Conclusion: BE-43547 is highly hypoxia-selective, and unlike TH-302, displayed minimal variability between cell lines, suggesting that BE-43547 targets a fundamental feature/target that is only present, or of survival importance, during hypoxia. Spheroid experiments suggested inadequate tissue penetrability, which may be overcome by designing novel drug analogs.

Keywords: Hypoxia; hypoxia-selective drugs; natural drugs; radiotherapy; treatment resistance.