Prototyping kinase inhibitor-cytotoxin anticancer mutual prodrugs activated by tumour hypoxia: A chemical proof of concept study

Nitroaromatic Hypoxia-Activated Prodrugs for Cancer Therapy

The presence of “hypoxic” tissue (with O₂ levels of <0.1 mmHg) in solid tumours, resulting in quiescent tumour cells distant from blood vessels, but capable of being reactivated by reoxygenation following conventional therapy (radiation or drugs), have long been known as a limitation to successful cancer chemotherapy. This has resulted in a sustained effort to develop nitroaromatic “hypoxia-activated prodrugs” designed to undergo enzyme-based nitro group reduction selectively in these hypoxic regions, to generate active drugs. Such nitro-based prodrugs can be classified into two major groups; those activated either by electron redistribution or by fragmentation following nitro group reduction, relying on the extraordinary difference in electron demand between an aromatic nitro group and its reduction products. The vast majority of hypoxia-activated fall into the latter category and are discussed here classed by the nature of their nitroaromatic trigger units.

Therapeutic targeting of the hypoxic tumour microenvironment

Hypoxia is prevalent in human tumours and contributes to microenvironments that shape cancer evolution and adversely affect therapeutic outcomes. Historically, two different tumour microenvironment (TME) research communities have been discernible. One has focused on physicochemical gradients of oxygen, pH and nutrients in the tumour interstitium, motivated in part by the barrier that hypoxia poses to effective radiotherapy. The other has focused on cellular interactions involving tumour and non-tumour cells within the TME. Over the past decade, strong links have been established between these two themes, providing new insights into fundamental aspects of tumour biology and presenting new strategies for addressing the effects of hypoxia and other microenvironmental features that arise from the inefficient microvascular system in solid tumours. This Review provides a perspective on advances at the interface between these two aspects of the TME, with a focus on translational therapeutic opportunities relating to the elimination and/or exploitation of tumour hypoxia.

Floxuridine Oligomers Activated under Hypoxic Environment

Floxuridine oligomers are anticancer oligonucleotide drugs composed of a number of floxuridine residues. They show enhanced cytotoxicity per floxuridine monomer because the nuclease degradation of floxuridine oligomers directly releases highly active floxuridine monophosphate in cells. However, their clinical use is limited by the low selectivity against cancer cells. To address this limitation, we herein report floxuridine oligomer prodrugs that are active under hypoxia conditions, which is one of the distinguishing features of the microenvironment of all solid tumors. We designed and synthesized two types of floxuridine oligomer prodrugs that possess hypoxia-responsive moieties on nucleobases. The floxuridine oligomer prodrugs showed lower cytotoxicity under normoxia conditions (O2 = 20%), while the parent floxuridine oligomer showed similar anticancer effects under hypoxia conditions (O2 = 1%). The floxuridine oligomer prodrug enabled tumor growth suppression in live mice. This would be the first example demonstrating the conditional control of the medicinal efficacy of oligomerized nucleoside anticancer drugs.

Hypoxia-activated nanomedicines for effective cancer therapy

Hypoxia, a common characteristic in solid tumors, is found in phenotypically aggressive cancers that display resistance to typical cancer interventions. Due to its important role in tumor progression, tumor hypoxia has been considered as a primary target for cancer diagnosis and treatment. An advantage of hypoxia-activated nanomedicines is that they are inactive in normoxic cells. In hypoxic tumor tissues and cells, these nanomedicines undergo reduction by activated enzymes (usually through 1 or 2 electron oxidoreductases) to produce cytotoxic substances. In this review, we will focus on approaches to design nanomedicines that take advantage of tumor hypoxia. These approaches include: i) inhibitors of hypoxia-associated signaling pathways; ii) prodrugs activated by hypoxia; iii) nanocarriers responsive to hypoxia, and iv) bacteria mediated hypoxia targeting therapy. These strategies have guided and will continue to guide nanoparticle design in the near future. These strategies have the potential to overcome tumor heterogeneity to improve the efficiency of radiotherapy, chemotherapy and diagnosis.