Targeting the Hypoxic Fraction of Tumours Using Hypoxia-Activated
Total Page：16
File Type：pdf, Size：1020Kb
Cancer Chemother Pharmacol (2016) 77:441–457 DOI 10.1007/s00280-015-2920-7 REVIEW ARTICLE Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs Roger M. Phillips1 Received: 8 September 2015 / Accepted: 13 November 2015 / Published online: 25 January 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract The presence of a microenvironment within Introduction most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implica- One of the characteristic features of solid tumour biology tions. Tumour hypoxia is known to promote the develop- is the presence of a poor and inadequate blood supply [1]. ment of an aggressive phenotype, resistance to both chem- This leads to the establishment of microenvironments that otherapy and radiotherapy and is strongly associated with are characterised by gradients of oxygen tension, nutri- poor clinical outcome. Paradoxically, it is recognised as a ents, extracellular pH, catabolites and reduced cell prolif- high-priority target and one of the therapeutic strategies eration, all of which vary as a function of distance from a designed to eradicate hypoxic cells in tumours is a group of supporting blood vessel (Fig. 1). These microenvironments compounds known collectively as hypoxia-activated prod- can be chronic in nature caused by poor blood supply (dif- rugs (HAPs) or bioreductive drugs. These drugs are inac- fusion limited) or acute caused by the temporal opening tive prodrugs that require enzymatic activation (typically and closing of blood vessels (perfusion limited). Hypoxia by 1 or 2 electron oxidoreductases) to generate cytotoxic in tumours has been the focus of intense research for over species with selectivity for hypoxic cells being determined 60 years, and both diffusion-limited hypoxia and perfusion- by (1) the ability of oxygen to either reverse or inhibit the limited hypoxia are established features of solid tumours activation process and (2) the presence of elevated expres- [2]. A third mechanism to explain the induction of hypoxia sion of oxidoreductases in tumours. The concepts under- in tumours has been described, namely longitudinal arteri- pinning HAP development were established over 40 years ole gradients whereby oxygen-rich inflowing blood vessels ago and have been refined over the years to produce a new branch and coalesce to form poorly oxygenated outflow- generation of HAPs that are under preclinical and clinical ing blood [3]. In this model, hypoxia would be formed development. The purpose of this article is to describe cur- along the axis of the vessel over a multimillimetre range, rent progress in the development of HAPs focusing on the which contrasts with the submillimetre distances typically mechanisms of action, preclinical properties and clinical associated with perfusion- and diffusion-limited hypoxia. progress of leading examples. The origins of tumour hypoxia are therefore linked to the abnormal vascular supply that develops within tumours, Keywords Hypoxia-activated prodrugs · TH-302 · and there is a substantial body of evidence demonstrating AQ4N · EO9 · Tirapazamine · PR-104 · TH-4000 · that hypoxia is a common feature of most if not all-solid Hypoxia · Bioreductive drugs tumours. The presence of hypoxia in tumours has significant bio- logical and therapeutic implications. Biologically, hypoxia is implicated in promoting resistance to apoptosis [4], * Roger M. Phillips suppression of DNA repair pathways and promotion of [email protected] genomic instability [5] increased invasion and metastasis 1 Department of Pharmacy, University of Huddersfield, [6], promotion of angiogenesis [7], modulation of tyros- Queensgate, Huddersfield HD1 3DH, UK ine kinase-mediated cell signalling pathways [8], evasion 1 3 442 Cancer Chemother Pharmacol (2016) 77:441–457 Oxygen Resistance Hypoxia Drug penetration Necrosis Necrosis BV 1e- reduction 1e- reduction 2e- reduction PDR PD PD PDR O2 O2 1e- reduction PD PDR T T Bystander effect O O T 2 2 Activation of HAPs by 1e- reduction Activation of HAPs by 2e- reduction Fig. 1 Cartoon of the hypoxic tumour microenvironment and a gen- icals. In the absence of oxygen, the PDR is able to undergo further eralised scheme for the mechanistic activation of HAPs by one- and reactions (fragmentation or disproportionation) to generate the active two-electron reductases under aerobic and hypoxic conditions. The toxic drug (T). Once the active drug has formed, it ideally should be cartoon describes a central blood vessel (BV) with tumour cells resid- able to diffuse back into the aerobic fraction and create a bystander ing various distances away from the vascular supply. Cells that reside effect. Even with a good bystander effect, HAPs are typically used in close to the blood vessel are ‘happy’ in that they are receiving nutri- combination with radiotherapy or chemotherapy to eradicate the aer- ents and oxygen but as you move further away from the vessel, condi- obic fraction. The right-hand side of the figure describes the activa- tions become more stressful in terms of lack of oxygen (hypoxia) and tion of HAPs by two-electron reduction pathways. In this case, two- nutrients (together with other physiological changes such as acidic electron reduction bypasses the oxygen-sensitive PDR step leading extracellular pH) until conditions can no longer support cell viabil- directly or indirectly to the formation of the active toxic drug. This ity and necrosis occurs. As distance from the supporting blood ves- pathway is typically oxygen insensitive, and both the aerobic fraction sel increases, resistance to radiotherapy and chemotherapy increases and hypoxic fraction can theoretically be targeted. These pathways and the delivery of drugs to hypoxic cells becomes increasingly prob- for HAP activation are generally applicable to most HAPs although lematical. The left-hand side of the cartoon describes the activation exceptions do exist. AQ4N, for example, is reduced by sequential of HAPs by one-electron reduction pathways. The prodrug (PD) is two-electron reduction steps that are inhibited by oxygen as described reduced to a prodrug radical (PDR) which in the presence of oxygen in the main body of the text redox cycles back to the parent compound generating superoxide rad- from immune surveillance [9], induction of autophagy [10], caution should also be extended to include targeted thera- hypoxia-driven changes in central metabolic pathways peutics following the demonstration that some (such as [11], global changes in the metabolome [12], production of dasatinib) are preferentially active against cell lines in vitro L-2-hydroxyglutarate leading to altered histone methyla- under hypoxic conditions [22]. tion [13], metabolic adaptation to hypoxia-induced reduc- Whilst the extent and severity of hypoxia in tumours tive stress [14] and the provision of a niche where cancer varies between tumour types and within individual stem cells reside [15]. The plethora of effects on tumour tumours, the combined biological and therapeutic implica- biology is mediated largely by hypoxia-inducible factors tions of hypoxia have a significant bearing on prognosis. (HIF) [16] although HIF1-independent hypoxia responses There is now an extensive body of evidence demonstrating have also been described [17]. In terms of therapeutic that hypoxia can adversely affect clinical outcome [23, 24] implications, the seminal work conducted by Gray in the and this makes hypoxia a high-priority therapeutic target. 1950s [18] provided the first evidence that hypoxia is an The importance of hypoxia as a target has been recognised underlying cause of resistance to radiotherapy. Since then, for many years, but the translation of preclinical strategies hypoxia has been strongly implicated in resistance to sev- designed to target hypoxic cells into mainstream clinical eral cytotoxic chemotherapy drugs and targeted therapeu- use has remained stubbornly difficult to achieve. Two main tics [19, 20]. Multiple mechanisms contribute to hypoxia- approaches are being used to eradicate hypoxic cells (1) induced drug resistance, but as pointed out by Wilson and the use of ‘bioreductive drugs’ or ‘hypoxia-activated prod- Hay [21], the generalisation that hypoxia causes resistance rugs (HAPs)’ and (2) molecularly targeted drugs aimed at to all cytotoxic drugs must be viewed with caution as some exploiting biochemical responses to hypoxia, particularly drugs are effective under hypoxic conditions. This note of HIF pathways. The current status of HIF-targeted strategies 1 3 Cancer Chemother Pharmacol (2016) 77:441–457 443 is beyond the scope of this article which focuses on HAPs, of oxidoreductases in tumour tissue (Fig. 1). In general their mechanism of action and recent progress in the pre- terms, one-electron reduction generates a prodrug radical clinical and clinical evaluation of leading compounds species that can be back-oxidised in the presence of oxy- in this class of drugs. This article also describes novel gen to generate the parental prodrug and reactive oxygen approaches where HAP-based approaches are being used to species. Host defence mechanisms can detoxify these radi- improve the selectivity of targeted therapeutics. cal species, thereby reducing toxic effects in oxygenated tissue, but in the absence of oxygen, the prodrug radical species undergoes further reduction/disproportionation or Hypoxia-activated prodrugs (HAPs): general fragmentation reactions to generate products