Targeting Deoxycytidine Kinase Improves Symptoms in Mouse Models of Multiple Sclerosis


Multiple sclerosis (MS) is an autoimmune disease driven by lymphocyte activation against myelin autoantigens in the central nervous system leading to demyelination and neurodegeneration. The deoxyribonucleoside salvage pathway with the rate-limiting enzyme deoxycytidine kinase (dCK) captures extracellular deoxyribonucleosides for use in intracellular deoxyribonucleotide metabolism. Previous studies have shown that deoxyribonucleoside salvage activity is enriched in lymphocytes and required for early lymphocyte development. However, specific roles for the deoxyribonucleoside salvage pathway and dCK in autoimmune diseases such as MS are unknown. Here we demonstrate that dCK activity is necessary for the development of clinical symptoms in the MOG35-55 and MOG1-125 experimental autoimmune encephalomyelitis (EAE) mouse models of MS. During EAE disease, deoxyribonucleoside salvage activity is elevated in the spleen and lymph nodes. Targeting dCK with the small molecule dCK inhibitor TRE-515 limits disease severity when treatments are started at disease induction or when symptoms first appear. EAE mice treated with TRE-515 have significantly fewer infiltrating leukocytes in the spinal cord, and TRE-515 blocks activation-induced B and T cell proliferation and MOG35-55 -specific T cell expansion without affecting innate immune cells or naïve T and B cell populations. Our results demonstrate that targeting dCK limits symptoms in EAE mice and suggest that dCK activity is required for MOG35-55 -specific lymphocyte activation-induced proliferation.

Immunology. 2023 Jan;168(1):152-169. doi: 10.1111/imm.13569.

PMID: 35986643

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18 F-FAC PET Visualizes Brain-Infiltrating Leukocytes in a Mouse Model of Multiple Sclerosis


Brain-infiltrating leukocytes contribute to multiple sclerosis (MS) and autoimmune encephalomyelitis and likely play a role in traumatic brain injury, seizure, and stroke. Brain-infiltrating leukocytes are also primary targets for MS disease-modifying therapies. However, no method exists for noninvasively visualizing these cells in a living organism. 1-(2′-deoxy-2′-18F-fluoroarabinofuranosyl) cytosine (18F-FAC) is a PET radiotracer that measures deoxyribonucleoside salvage and accumulates preferentially in immune cells. We hypothesized that 18F-FAC PET could noninvasively image brain-infiltrating leukocytes.

Methods: Healthy mice were imaged with 18F-FAC PET to quantify if this radiotracer crosses the blood-brain barrier (BBB). Experimental autoimmune encephalomyelitis (EAE) is a mouse disease model with brain-infiltrating leukocytes. To determine whether 18F-FAC accumulates in brain-infiltrating leukocytes, EAE mice were analyzed with 18F-FAC PET, digital autoradiography, and immunohistochemistry, and deoxyribonucleoside salvage activity in brain-infiltrating leukocytes was analyzed ex vivo. Fingolimod-treated EAE mice were imaged with 18F-FAC PET to assess if this approach can monitor the effect of an immunomodulatory drug on brain-infiltrating leukocytes. PET scans of individuals injected with 2-chloro-2′-deoxy-2′-18F-fluoro-9-β-d-arabinofuranosyl-adenine (18F-CFA), a PET radiotracer that measures deoxyribonucleoside salvage in humans, were analyzed to evaluate whether 18F-CFA crosses the human BBB.

Results: 18F-FAC accumulates in the healthy mouse brain at levels similar to 18F-FAC in the blood (2.54 ± 0.2 and 3.04 ± 0.3 percentage injected dose per gram, respectively) indicating that 18F-FAC crosses the BBB. EAE mice accumulate 18F-FAC in the brain at 180% of the levels of control mice. Brain 18F-FAC accumulation localizes to periventricular regions with significant leukocyte infiltration, and deoxyribonucleoside salvage activity is present at similar levels in brain-infiltrating T and innate immune cells. These data suggest that 18F-FAC accumulates in brain-infiltrating leukocytes in this model. Fingolimod-treated EAE mice accumulate 18F-FAC in the brain at 37% lower levels than control-treated EAE mice, demonstrating that 18F-FAC PET can monitor therapeutic interventions in this mouse model. 18F-CFA accumulates in the human brain at 15% of blood levels (0.08 ± 0.01 and 0.54 ± 0.07 SUV, respectively), indicating that 18F-CFA does not cross the BBB in humans.

Conclusion: 18F-FAC PET can visualize brain-infiltrating leukocytes in a mouse MS model and can monitor the response of these cells to an immunomodulatory drug. Translating this strategy into humans will require exploring additional radiotracers.

Published May 1, 2020 // Journal of Nuclear Medicine 61(5):757-763.

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Structure-Guided Development of Deoxycytidine Kinase Inhibitors with Nanomolar Affinity and Improved Metabolic Stability



Recently, we have shown that small molecule dCK inhibitors in combination with pharmacological perturbations of de novo dNTP biosynthetic pathways could eliminate acute lymphoblastic leukemia cells in animal models. However, our previous lead compound had a short half-life in vivo. Therefore, we set out to develop dCK inhibitors with favorable pharmacokinetic properties. We delineated the sites of the inhibitor for modification, guided by crystal structures of dCK in complex with the lead compound and with derivatives. Crystal structure of the complex between dCK and the racemic mixture of our new lead compound indicated that the R-isomer is responsible for kinase inhibition. This was corroborated by kinetic analysis of the purified enantiomers, which showed that the R-isomer has >60-fold higher affinity than the S-isomer for dCK. This new lead compound has significantly improved metabolic stability, making it a prime candidate for dCK-inhibitor based therapies against hematological malignancies and, potentially, other cancers.

J Med Chem. 2014 Nov 26; 57(22): 9480–9494.
Published online 2014 Oct 23. doi: 10.1021/jm501124j

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Deoxycytidine Kinase Augments ATM-Mediated DNA Repair and Contributes to Radiation Resistance


Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [18F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

PLoS One. 2014; 9(8): e104125.
Published online 2014 Aug 7. doi: 10.1371/journal.pone.0104125

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Nucleoside salvage pathway kinases regulate hematopoiesis by linking nucleotide metabolism with replication stress


Nucleotide deficiency causes replication stress (RS) and DNA damage in dividing cells. How nucleotide metabolism is regulated in vivo to prevent these deleterious effects remains unknown. In this study, we investigate a functional link between nucleotide deficiency, RS, and the nucleoside salvage pathway (NSP) enzymes deoxycytidine kinase (dCK) and thymidine kinase (TK1). We show that inactivation of dCK in mice depletes deoxycytidine triphosphate (dCTP) pools and induces RS, early S-phase arrest, and DNA damage in erythroid, B lymphoid, and T lymphoid lineages. TK1(-/-) erythroid and B lymphoid lineages also experience nucleotide deficiency but, unlike their dCK(-/-) counterparts, they still sustain DNA replication. Intriguingly, dCTP pool depletion, RS, and hematopoietic defects induced by dCK inactivation are almost completely reversed in a newly generated dCK/TK1 double-knockout (DKO) mouse model. Using NSP-deficient DKO hematopoietic cells, we identify a previously unrecognized biological activity of endogenous thymidine as a strong inducer of RS in vivo through TK1-mediated dCTP pool depletion. We propose a model that explains how TK1 and dCK “tune” dCTP pools to both trigger and resolve RS in vivo. This new model may be exploited therapeutically to induce synthetic sickness/lethality in hematological malignancies, and possibly in other cancers.

J Exp Med. 2012 Nov 19;209(12):2215-28. doi: 10.1084/jem.20121061. Epub 2012 Nov 12.

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Novel Positron Emission Tomography Probes Specific for Deoxycytidine Kinase


Deoxycytidine kinase (dCK) is a rate-limiting enzyme in the deoxyribonucleoside salvage pathway and a critical determinant of therapeutic activity for several nucleoside analog pro-drugs. We have previously reported the development of 18F-FAC, (1-(2′-deoxy-2′-18F-fluoro-β-D-arabinofuranosyl) cytosine), a new probe for PET imaging of dCK activity in immune disorders and certain cancers. The objective of the current study was to develop PET probes with improved metabolic stability and specificity for dCK. Towards this goal, several candidate PET probes were synthesized and evaluated in vitro and in vivo.

High pressure liquid chromatography was used to analyze the metabolic stability of 18F-FAC and of several newly-synthesized analogs with the natural D-enantiomeric sugar configuration or the corresponding unnatural L-configuration. In vitro kinase and uptake assays were used to determine the affinity of the 18F-FAC L-nucleoside analogs for dCK. The biodistribution of selected L- analogs in mice was determined by microPET/CT imaging.


Candidate PET probes were selected using the following criteria: low susceptibility to deamination, high affinity for purified recombinant dCK, high uptake in dCK expressing cell lines and biodistribution in mice reflective of the tissue expression pattern of dCK. Amongst the ten newly-developed candidate probes, 1-(2′-deoxy-2′-18F-fluoro-β-L-arabinofuranosyl) cytosine (L-18F-FAC) and 1-(2′-deoxy-2′-18F-fluoro-β-L-arabinofuranosyl)-5-methylcytosine (L-18F-FMAC) most closely matched the selection criteria. The selection of L-18F-FAC and L-18F-FMAC was validated by showing that these two PET probes can be used to image animal models of leukemia and autoimmunity.


Promising in vitro and in vivo data warrant biodistribution and dosimetry studies of L-18F-FAC and L-18F-FMAC in humans.

J Nucl Med. Author manuscript; available in PMC 2011 Jun 22.
Published in final edited form as:
J Nucl Med. 2010 Jul; 51(7): 1092–1098.
Published online 2010 Jun 16. doi: 10.2967/jnumed.109.073361

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Requirement for deoxycytidine kinase in T and B lymphocyte development


Deoxycytidine kinase (dCK) is a rate-limiting enzyme in deoxyribonucleoside salvage, a metabolic pathway that recycles products of DNA degradation. dCK phosphorylates and therefore activates nucleoside analog prodrugs frequently used in cancer, autoimmunity, and viral infections. In contrast to its well established therapeutic relevance, the biological function of dCK remains enigmatic. Highest levels of dCK expression are found in thymus and bone marrow, indicating a possible role in lymphopoiesis. To test this hypothesis we generated and analyzed dCK knockout (KO) mice. dCK inactivation selectively and profoundly affected T and B cell development. A 90-fold decrease in thymic cellularity was observed in the dCK KO mice relative to wild-type littermates. Lymphocyte numbers in the dCK KO mice were 5- to 13-fold below normal values. The severe impact of dCK inactivation on lymphopoiesis was unexpected given that nucleoside salvage has been thought to play a limited, “fine-tuning” role in regulating deoxyribonucleotide triphosphate pools produced by the de novo pathway. The dCK KO phenotype challenges this view and indicates that, in contrast to the great majority of other somatic cells, normal lymphocyte development critically requires the deoxyribonucleoside salvage pathway.
Keywords: combined immunodeficiency, deoxyribonucleoside salvage, dNTP metabolism, immune development, T and B lymphocytes

Proc Natl Acad Sci U S A. 2010 Mar 23; 107(12): 5551–5556.
Published online 2009 Dec 31. doi: 10.1073/pnas.0913900107

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Molecular imaging of lymphoid organs and immune activation using positron emission tomography with a new 18F-labeled 2′-deoxycytidine analog


Monitoring immune function using molecular imaging could significantly impact the diagnosis and treatment evaluation of immunological disorders and therapeutic immune responses. Positron Emission Tomography (PET) is a molecular imaging modality with applications in cancer and other diseases. PET studies of immune function have been limited by a lack of specialized probes. We identified [18F]FAC (1-(2′-deoxy-2′-[18F]fluoroarabinofuranosyl) cytosine) by differential screening as a new PET probe for the deoxyribonucleotide salvage pathway. [18F]FAC enabled visualization of lymphoid organs and was sensitive to localized immune activation in a mouse model of anti-tumor immunity. [18F]FAC microPET also detected early changes in lymphoid mass in systemic autoimmunity and allowed evaluation of immunosuppressive therapy. These data support the use of [18F]FAC PET for immune monitoring and suggest a wide range of clinical applications in immune disorders and in certain types of cancer.

Nat Med. Author manuscript; available in PMC 2009 Aug 3.
Published in final edited form as:
Nat Med. 2008 Jul; 14(7): 783–788.
Published online 2008 Jun 8. doi: 10.1038/nm1724

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