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Banoxantrone dihydrochloride is a novel bioreductive agent that can be reduced to a stable, DNA-affinic compound AQ4, which is a potent topoisomerase II inhibitor.

Banoxantrone (AQ4N) can be reduced in a hypoxic environment to a stable DNA-affinic agent AQ4. AQ4, a potent topoisomerase II inhibitor, would be capable of damaging cells recruited into the cell cycle following radiation damage to the well-oxygenated cells of the tumor[1]. Banoxantrone shows more than 8-fold higher cytotoxicity under hypoxia than normoxia in cultures of 9L rat gliosarcoma and H460 human non-small-cell lung carcinoma cells but not for 11 other human cancer cell lines. DT-diaphorase protein levels and banoxantrone chemosensitivity are poorly correlated across the cancer cell line panel, and banoxantrone chemosensitivity is not affected by DT-diaphorase inhibitors[2]. Banoxantrone is a bis-N-oxide that is reduced via two sequential two-electron reductions to the tertiary amine, AQ4, which is a potent cytotoxic agent toward both aerobic and hypoxic cells. AQ4, but not AQ4N, intercalates in DNA with high affinity to generate a stable persistent complex that can inhibit topoisomerase II and cause DNA damage and cell death[3].

Banoxantrone (200 mg/kg) significantly enhances the tumor growth delay caused by radiation. This occurred when radiation is administered both as a single dose (12 Gy) and in a multifraction regimen (5x3 Gy). A study of the scheduling of Banoxantrone (AQ4N) administration shows that there is a very long time period over which a maximal effect can be elicited (drug given 4 days before to 6 h after radiation). These results suggest that Banoxantrone has significant potential as a bioreductive drug[1]. The activation of banoxantrone cytotoxicity in vivo requires tumor hypoxia that is more extensive or prolonged than can readily be achieved by vasodilation or by antiangiogenic drug treatment[2]. Incorporation of banoxantrone into conventional chemoradiation protocols therefore targets both oxygenated and hypoxic regions of tumors, and potentially will increase the effectiveness of therapy. A single dose of 60 mg/kg banoxantrone enhances the response of RT112 (bladder) and Calu-6 (lung) xenografts to treatment with cisplatin and radiation therapy. Banoxantrone will increase the efficacy of chemoradiotherapy in preclinical models[3].

[1]. Hejmadi MV, et al. DNA damage following combination of radiation with the bioreductive drug AQ4N: possible selective toxicity to oxic and hypoxic tumour cells. Br J Cancer. 1996 Feb;73(4):499-505. [2]. Manley E Jr, et al. Impact of tumor blood flow modulation on tumor sensitivity to the bioreductive drug banoxantrone. J Pharmacol Exp Ther. 2013 Feb;344(2):368-77. [3]. Williams KJ, et al. In vivo activation of the hypoxia-targeted cytotoxin AQ4N in human tumor xenografts. Mol Cancer Ther. 2009 Dec;8(12):3266-75.

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