Targeted imaging agents for detecting tumour cell death following therapy

Cell death is an important target for imaging the early response of tumours to treatment. In this study a phosphatidylserine-binding protein (C2Am) has been derivatised with a fluorine-18 containing maleimide for imaging tumour cell death in a xenograft murine advanced colorectal cancer. A one-pot, two-step automated synthesis of N-(5-[18F]fluoropentyl)maleimide using a GE TRACERlab FXFN automated module (within 58±5.8 min (n = 12), >98% radiochemical purity and 12±3% decay corrected yield) has been developed. This was used to label the single cysteine present on C2Am within 30 min in PBS (Am=212000±30000 MBq/μmol (n = 3)). Using xenograft models of breast and colorectal cancer, and a TRAIL-R2 agonist for inducing cell death, the binding of [18F]FPenM-C2Am was tested in vitro and in vivo using biodistribution and dynamic PET imaging studies. Cell death detection was validated by cell death histology assays CC3 and TUNEL. For colorectal cancer, there was a positive correlation between [18F]FPenM-C2Am signal in tumours post treatment and the corresponding histologic markers of cell death CC3 (Pearson r = 0.82) and TUNEL (Pearson r = 0.95). [18F]FPenM-C2Am showed a favourable biodistribution profile, with predominantly renal clearance and minimal retention in spleen (0.79±0.05 %ID/g), liver (1.18±0.13 %ID/g), small intestine (0.97±0.25 %ID/g) and kidney (6.90±0.56 %ID/g) at 2 h after probe administration. In a xenograft model of colorectal cancer treated with a TRAIL-R2 agonist, for 24h, at 0.2-0.4 mg/kg, i.v. [18F]FPenM-C2Am generated tumour-to-muscle and tumour-to-blood ratios following treatment of 6.7±0.8-fold and 1.89±0.23-fold, respectively, at 2 h after administration. A statistically significant pairwise difference was obtained between the tumour-to-muscle contrast prior to and following therapy (P=0.0137, unpaired two-tailed t-test). Given the favourable biodistribution profile of [18F]FPenM-C2Am, and its ability to produce rapid and cell death-specific image contrast, this agent has potential for clinical translation. We have initiated cGMP manufacture and toxicology studies required for a Phase 1 trial. Aberrant cell surface glycosylation has been described as one of the key hallmarks of cancer. Monitoring glycosylation could provide an insight into tumour progression, proliferation and ultimately could potentially be used for monitoring treatment response. Aberrant glycosylation can be observed by harnessing the cell’s metabolism to incorporate into its glycome unnatural sugars bearing bioorthogonal chemical reporters. These reporters are targeted subsequently by fluorescent, magnetic or optoacoustic probes that allow imaging. As part of this work, we have demonstrated in vitro that a peracetylated cyclopropene mannosamine (Ac4ManNCCp)-modified sugar can be used as a tool for imaging hypersialylation in an advanced colorectal cancer model. Further optimisation of the probe to improve solubility is required to facilitate transitioning from in vitro to in vivo imaging. Nevertheless, overcoming this solubility issue would allow for facile labelling of surface glycans using PET radionuclides.