Hypoxia-activated delivery of chemical probes and imaging agents
Keywords: Positron Emission Tomography
Fluorescence Imaging
One-Electron Reduction
Hypoxia
Chemical Synthesis
Fluorescence Microscopy
Nitroreductase

Regions of hypoxia are a common feature of solid tumours and are associated with an aggressive phenotype and resistance to chemotherapy and radiotherapy. The differences in redox environment between normoxic and hypoxic tissue can be exploited by hypoxia-activated prodrugs (HAPs), which enable targeted delivery of drugs and imaging agents. Several HAPs have been developed but have had limited success in the clinic, partly resulting from a lack of understanding of their distribution and activation in vivo. We therefore wished to develop hypoxia-activated prodrugs, and fluorophores for use in drug-fluorophore conjugates. These conjugates would be activated in hypoxic tissue, releasing both a ‘switch-on’ fluorophore and a biologically active cargo molecule. We proposed that these molecules could be used to study HAP behaviour in tissue.
We began by building upon our earlier work around a hypoxia-activated prodrug of a clinical candidate Chk1 inhibitor, SAR020106. The original prodrug lacked efficacy in clonogenic survival assays, which we attributed to slow fragmentation kinetics. We attempted to synthesise analogues which, we hypothesised, would have more rapid fragmentation kinetics than the original prodrug and therefore greater efficacy. This synthesis proved challenging, and no new prodrug forms could be produced.
We then developed three near-infrared fluorophore scaffolds as potential hypoxia-activated bioreductive imaging probes. The first, a very small fluorophore with an unusual mechanism of activation was synthesised in a potential hypoxia-activated form, and whilst it underwent metabolism and fragmentation in hypoxia, the modified fluorophore exhibited greatly reduced fluorescence and was deemed unsuitable for imaging applications. The second scaffold was based on an arylazide-containing hydrogen sulfide probe with the uncommon benzopyrylium core. The nitroaryl analogue was synthesised and evaluated in vitro. The molecule proved to be susceptible to biologically-relevant nucleophiles including NADPH and L-GSH, although we demonstrated that the molecule could be used to selectively image hypoxic cancer cells. Due to this molecule’s instability in a biological environment, we moved onto a third scaffold which had been explored previously in our group. Synthesis of analogues of this fluorophore with a biologically active cargo proved challenging, and we thus investigated a related fluorophore with visible-wavelength emission properties. We synthesised the nitroaryl and azidoaryl analogues an evaluated them in vitro. Biochemical reduction assays were unsuccessful, and cellular imaging experiments showed that whilst both molecules could be used as imaging agents, neither were effective as hypoxia sensors. We also synthesised an analogue of the nitro compound to which we attached a bromodomain ligand. This molecule proved to be highly unstable and could not be adequately evaluated.
Finally, we wished to incorporate 18F-PET as a second imaging modality to our drug-fluorophore conjugates. We modified the synthesis of our bromodomain ligands to enable the use of fluorination chemistry which is applicable to radiolabelling. The fluorination was demonstrated cold using 19F NMR, and a deprotection reaction compatible with radiosynthesis was also identified. This chemistry will be useful in the radiolabelling of our bromodomain ligands for use in dual-modal imaging agents.