Hypoxia-activated prodrugs: paths forward in the era of personalised medicine

Abstract

Tumour hypoxia has been pursued as a cancer drug target for over 30 years, most notably using bioreductive (hypoxia-activated) prodrugs that target antineoplastic agents to low-oxygen tumour compartments. Despite compelling evidence linking hypoxia with treatment resistance and adverse prognosis, a number of such prodrugs have recently failed to demonstrate efficacy in pivotal clinical trials; an outcome that demands reflection on the discovery and development of these compounds. In this review, we discuss a clear disconnect between the pathobiology of tumour hypoxia, the pharmacology of hypoxia-activated prodrugs and the manner in which they have been taken into clinical development. Hypoxia-activated prodrugs have been evaluated in the manner of broad-spectrum cytotoxic agents, yet a growing body of evidence suggests that their activity is likely to be dependent on the coincidence of tumour hypoxia, expression of specific prodrug-activating reductases and intrinsic sensitivity of malignant clones to the cytotoxic effector. Hypoxia itself is highly variable between and within individual tumours and is not treatment-limiting in all cancer subtypes. Defining predictive biomarkers for hypoxia-activated prodrugs and overcoming the technical challenges of assaying them in clinical settings will be essential to deploying these agents in the era of personalised cancer medicine.

Hypoxia-activated cytotoxicity of benznidazole against clonogenic tumor cells

“…As a hallmark of their cell-killing effects, hypoxia-activated cytotoxins cause DNA doublestranded breaks likely via free radicals generated by the oneelectron reduction, which can lead to cell death. 22,23 Here, we have found that Benznidazole can also induce DNA double-stranded breaks specifically upon hypoxia-dependent activation, as shown by the formation of 53BP1-positive nuclear foci. Consistent with our findings, it has recently been reported that Benznidazole can cause potentially lethal double-stranded breaks in Trypanosoma cruzi DNA.…”

“…22,23 Its chemically active metabolites can react with many cellular macromolecules and potentially affect multiple intracellular pathways. We have found that Benznidazole does not appear to activate apoptosis based on PARP1 cleavage, which is also consistent with the literature.…”

“…As a hallmark of their cytotoxicity, hypoxia-activated cytotoxins induce DNA damages and double-stranded breaks. 22,23 DNA double-stranded breaks lead to the formation of punctate aggregates of the nuclear protein 53BP1. 36,37 Using the nuclear 53BP1 foci assay, we found that, specifically under severe hypoxia, Benznidazole significantly induced 53BP1 foci formation in both SK-N-ER and MDA-MB-231 cells compared to DMSO-treated cells (Fig.…”

“…The idea of developing drugs that specifically target hypoxic tumor cells was largely inspired by the early studies on the bioreductive compound Mitomycin C. 21 In the ensuing years, many studies including both experimental and clinical studies have been devoted to the development of hypoxia-activated prodrugs. 22,23 However, the struggle continues to find an effective drug against hypoxic tumor cells.…”

A Tumor Vascular‐Targeted Interlocking Trimodal Nanosystem That Induces and Exploits Hypoxia

“…Besides hypoxia, the activation of AQ4N also depends on cytochrome P450 (CYP450) activating reductases, which are also dominantly responsible for the activation of most other HAPs 13. Two main CYP450 enzymes, CYP1A1 and 2B6, are shown to metabolize AQ4N into AQ4 (a potent inhibitor of topoisomerase II) efficiently (Figure S1, Supporting Information) 14.…”