Epigenetic potentiation of somatostatin-2 by guadecitabine in neuroendocrine neoplasias as a novel method to allow delivery of peptide receptor radiotherapy.

BACKGROUND: Somatostatin receptor-2 (SSTR2) is expressed on cell surface of neuroendocrine neoplasias; its presence is exploited for the delivery of peptide receptor radionuclide therapy (PRRT). Patients with no or low expression of SSTR2 are not candidates for PRRT. SSTR2 promotor undergoes epigenetic modification, known to regulate gene expression. We investigated whether the demethylation agent, guadecitabine, could enhance the expression of SSTR2 in NET models, using radioligand uptake/PET imaging as a biomarker of epigenetic modification. METHODS: The effects of guadecitabine on the transcriptional, translational, and functional regulation of SSTR2 both in vitro and in vivo using low (QGP-1) and high (BON-1) methylated neuroendocrine neoplasia models was characterised. Promotor region methylation profiling of clinical samples (n = 61) was undertaken. Safety of combination guadecitabine and PRRT was assessed in vivo. RESULTS: Pyrosequencing of cell lines illustrated differential methylation indices - BON: 1 94%, QGP: 1 21%. Following guadecitabine treatment, a dose-dependent increase in SSTR2 in BON-1 at a transcriptional, translational, and functional levels using the SSTR2-directed radioligand, 18F-FET-ßAG-TOCA ([18F]-FETO) (150% increase [18F]-FETO uptake, p < 0.05) was observed. In vivo, guadecitabine treatment resulted in a 70% increase in [18F]-FETO uptake in BON-1 tumour models compared models with low baseline percentage methylation (p < 0.05). No additive toxicity was observed with the combination treatment of PRRT and guadecitabine in vivo. Methylation index in clinical samples was 10.5% compared to 5.2% in controls (p = 0.03) and correlated with SSTR2 expression (Wilcoxon rank sign -3.75,p < 0.01). CONCLUSION: Guadecitabine increases SSTR2 expression both in vitro and in vivo. The combination of demethylation agents with PRRT warrants further investigation.

Consideration of Metabolite Efflux in Radiolabelled Choline Kinetics.

Hypoxia is a complex microenvironmental condition known to regulate choline kinase α (CHKA) activity and choline transport through transcription factor hypoxia-inducible factor-1α (HIF-1α) and, therefore, may confound the uptake of choline radiotracer [18F]fluoromethyl-[1,2-2H4]-choline ([18F]-D4-FCH). The aim of this study was to investigate how hypoxia affects the choline radiotracer dynamics. Three underlying mechanisms by which hypoxia could potentially alter the uptake of the choline radiotracer, [18F]-D4-FCH, were investigated: 18F-D4-FCH import, CHKA phosphorylation activity, and the efflux of [18F]-D4-FCH and its phosphorylated product [18F]-D4-FCHP. The effects of hypoxia on [18F]-D4-FCH uptake were studied in CHKA-overexpressing cell lines of prostate cancer, PC-3, and breast cancer MDA-MB-231 cells. The mechanisms of radiotracer efflux were assessed by the cell uptake and immunofluorescence in vitro and examined in vivo (n = 24). The mathematical modelling methodology was further developed to verify the efflux hypothesis using [18F]-D4-FCH dynamic PET scans from non-small cell lung cancer (NSCLC) patients (n = 17). We report a novel finding involving the export of phosphorylated [18F]-D4-FCH and [18F]-D4-FCHP via HIF-1α-responsive efflux transporters, including ABCB4, when the HIF-1α level is augmented. This is supported by a graphical analysis of human data with a compartmental model (M2T6k + k5) that accounts for the efflux. Hypoxia/HIF-1α increases the efflux of phosphorylated radiolabelled choline species, thus supporting the consideration of efflux in the modelling of radiotracer dynamics.

Tracing Nutrient Flux Following Monocarboxylate Transporter-1 Inhibition with AZD3965.

The monocarboxylate transporter 1 (MCT1) is a key element in tumor cell metabolism and inhibition of MCT1 with AZD3965 is undergoing clinical trials. We aimed to investigate nutrient fluxes associated with MCT1 inhibition by AZD3965 to identify possible biomarkers of drug action. We synthesized an 18F-labeled lactate analogue, [18F]-S-fluorolactate ([18F]-S-FL), that was used alongside [18F]fluorodeoxyglucose ([18F]FDG), and 13C-labeled glucose and lactate, to investigate the modulation of metabolism with AZD3965 in diffuse large B-cell lymphoma models in NOD/SCID mice. Comparative analysis of glucose and lactate-based probes showed a preference for glycolytic metabolism in vitro, whereas in vivo, both glucose and lactate were used as metabolic fuel. While intratumoral L-[1-13C]lactate and [18F]-S-FL were unchanged or lower at early (5 or 30 min) timepoints, these variables were higher compared to vehicle controls at 4 h following treatment with AZD3965, which indicates that inhibition of MCT1-mediated lactate import is reversed over time. Nonetheless, AZD3965 treatment impaired DLBCL tumor growth in mice. This was hypothesized to be a consequence of metabolic strain, as AZD3965 treatment showed a reduction in glycolytic intermediates and inhibition of the TCA cycle likely due to downregulated PDH activity. Glucose ([18F]FDG and D-[13C6]glucose) and lactate-based probes ([18F]-S-FL and L-[1-13C]lactate) can be successfully used as biomarkers for AZD3965 treatment.

Identification of ABC Transporter Interaction of a Novel Cyanoquinoline Radiotracer and Implications for Tumour Imaging by Positron Emission Tomography.

BACKGROUND: The epidermal growth factor receptor (EGFR) is overexpressed in many cancers including lung, ovarian, breast, head and neck and brain. Mutation of this receptor has been shown to play a crucial role in the response of non-small cell lung carcinoma (NSCLC) to EGFR-targeted therapies. It is envisaged that imaging of EGFR using positron emission tomography (PET) could aid in selection of patients for treatment with novel inhibitors. We recognised multi-drug resistant phenotype as a threat to development of successful imaging agents. In this report, we describe discovery of a novel cyanoquinoline radiotracer that lacks ABC transporter activity. METHODS: Cellular retention of the prototype cyanoquinoline [18F](2E)-N-{4-[(3-chloro-4-fluorophenyl)amino]-3-cyano-7-ethoxyquinolin-6-yl}-4-({[1-(2-fluoroethyl)-1H-1,2,3-triazol-4-yl]methyl}amino)-but-2-enamide ([18F]FED6) and [18F](2E)-N-{4-[(3-chloro-4-fluorophenyl)amino]-3-cyano-7-ethoxyquinolin-6-yl}-4-[({1-[(2R,5S)-3-fluoro-4,5-dihydroxy-6-(hydroxymethyl)oxan-2-yl]-1H-1,2,3-triazol-4-yl}methyl)amino]but-2-enamide ([18F]FED20) were evaluated to establish potential for imaging specificity. The substrate specificity of a number of cyanoquinolines towards ABC transporters was investigated in cell lines proficient or deficient in ABCB1 or ABCG2. RESULTS: FED6 demonstrated substrate specificity for both ABCG2 and ABCB1, a property that was not observed for all cyanoquinolines tested, suggesting scope for designing novel probes. ABC transporter activity was confirmed by attenuating the activity of transporters with drug inhibitors or siRNA. We synthesized a more hydrophilic compound [18F]FED20 to overcome ABC transporter activity. FED20 lacked substrate specificity for both ABCB1 and ABCG2, and maintained a strong affinity for EGFR. Furthermore, FED20 showed higher inhibitory affinity for active mutant EGFR versus wild-type or resistant mutant EGFR; this property resulted in higher [18F]FED20 cellular retention in active mutant EGFR expressing NSCLC. CONCLUSION: [18F]FED20 binds EGFR but is devoid of ABC transporter activity, thus, has potential for EGFR imaging.