[ ¹⁸ F]CFA as a clinically translatable probe for PET imaging of deoxycytidine kinase activity

Abstract

Deoxycytidine kinase (dCK), a rate-limiting enzyme in the cytosolic deoxyribonucleoside (dN) salvage pathway, is an important therapeutic and positron emission tomography (PET) imaging target in cancer. PET probes for dCK have been developed and are effective in mice but have suboptimal specificity and sensitivity in humans. To identify a more suitable probe for clinical dCK PET imaging, we compared the selectivity of two candidate compounds—[18F]Clofarabine; 2-chloro-2′-deoxy-2′-[18F]fluoro-9-β-d-arabinofuranosyl-adenine ([18F]CFA) and 2′-deoxy-2′-[18F]fluoro-9-β-d-arabinofuranosyl-guanine ([18F]F-AraG)—for dCK and deoxyguanosine kinase (dGK), a dCK-related mitochondrial enzyme. We demonstrate that, in the tracer concentration range used for PET imaging, [18F]CFA is primarily a substrate for dCK, with minimal cross-reactivity. In contrast, [18F]F-AraG is a better substrate for dGK than for dCK. [18F]CFA accumulation in leukemia cells correlated with dCK expression and was abrogated by treatment with a dCK inhibitor. Although [18F]CFA uptake was reduced by deoxycytidine (dC) competition, this inhibition required high dC concentrations present in murine, but not human, plasma. Expression of cytidine deaminase, a dC-catabolizing enzyme, in leukemia cells both in cell culture and in mice reduced the competition between dC and [18F]CFA, leading to increased dCK-dependent probe accumulation. First-in-human, to our knowledge, [18F]CFA PET/CT studies showed probe accumulation in tissues with high dCK expression: e.g., hematopoietic bone marrow and secondary lymphoid organs. The selectivity of [18F]CFA for dCK and its favorable biodistribution in humans justify further studies to validate [18F]CFA PET as a new cancer biomarker for treatment stratification and monitoring.

Future cancer research priorities in the USA: a Lancet Oncology Commission

“…The strategies used to target the relevant immune cells are as varied and complex as immunotherapy itself, including ex-vivo labelling of, 303–305 or introduction of, 306–309 imaging reporters to adoptively transferred cells, immunoimaging with radiolabelled antibodies 310,311 or antibody fragments, 312,313 and radiopharmaceuticals of low molecular weight. 314 All strategies are being pursued in parallel, and therapeutic clinical trials that integrate them are beginning to appear. Crucial to this effort will be collaboration between immunologists, oncologists, and imaging specialists to obtain the most informative compounds for each indication in immunooncology.…”

Human Biodistribution and Radiation Dosimetry of 18F-Clofarabine, a PET Probe Targeting the Deoxyribonucleoside Salvage Pathway

“…The synthesis of 18 F-clofarabine was performed as previously described (10,11). In brief, protection groups were first added to chloroadenosine.…”

“…This resistance can be overcome using small-molecule inhibitors of dCK (6)(7)(8)(9). dCK is also required to phosphorylate and activate inactive prodrugs such as cytarabine, gemcitabine, decitabine, and cladribine (10). Therefore, a PET probe that specifically measures tissue dCK expression could serve as a predictive biomarker for tumor responses to dCK-dependent prodrugs, stratify patients into likely responders to small-molecule dCK inhibitors, and demonstrate target engagement in patients treated with dCK inhibitors (6).…”

Multiplexed imaging for diagnosis and therapy

“…For instance, PET generally informs on tissue phenotypes or processes (such as receptor expression [11][12][13][14][15][16] , microenvironment 17,18 , metabolism 16,[19][20][21][22][23] or perfusion) with outstanding sensitivity, but it is hampered by low resolution, scarcity of anatomic detail and dosimetric stipulations 24 . In contrast, MR provides anatomic detail with exquisite resolution, but lacks the sensitivity offered by radionuclide imaging; specific aspects of individual modalities are indicated in Table 3 (refs 24-31 ).…”