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

  1. Caius G. Radua,b,2
  1. aDepartment of Molecular and Medical Pharmacology, University of California, Los Angeles, CA 90095;
  2. bAhmanson Translational Imaging Division, University of California, Los Angeles, CA 90095;
  3. cAbcam, Cambridge, MA 02139-1517;
  4. dDepartment of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095;
  5. eCellSight Technologies, Inc., San Francisco, CA 94107;
  6. fCrump Institute for Molecular Imaging, University of California, Los Angeles, CA 90095;
  7. gDepartment of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, IL 60607;
  8. hThe Pasarow Mass Spectrometry Laboratory, Semel Institute for Neuroscience and Human Behavior, University of California, Los Angeles, CA 90095;
  9. iDepartment of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, CA 90095;
  10. jDepartment of Microbiology, Immunology, & Molecular Genetics, University of California, Los Angeles, CA 90095;
  11. kHoward Hughes Medical Institute, University of California, Los Angeles, CA 90095;
  12. lEli & Edythe Broad Center of Regenerative Medicine and Stem Cell Research, University of California, Los Angeles, CA 90095;
  13. mDepartment of Surgery, David Geffen School of Medicine, University of California, Los Angeles, CA 90095;
  14. nDepartment of Biological Chemistry, David Geffen School of Medicine, University of California, Los Angeles, CA 90095

  1. Contributed by Michael E. Phelps, December 9, 2015 (sent for review September 3, 2015; reviewed by Bernd J. Pichler, Orian S. Shirihai, and Wolfgang Weber)

Significance

Deoxycytidine kinase (dCK) is required for the activation of multiple nucleoside analog prodrugs used in cancer therapy and is a potential new therapeutic target in hematological malignancies. Here, we identify [18F]Clofarabine; 2-chloro-2′-deoxy-2′-[18F]fluoro-9-β-d-arabinofuranosyl-adenine ([18F]CFA) as a new candidate PET probe for dCK, with superior specificity and biodistribution in humans compared with existing probes. [18F]CFA PET may provide a useful companion biomarker for therapeutic interventions against cancer that include nucleoside analog prodrugs, dCK inhibitors, and immunotherapies.

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.

Footnotes

  • ↵1W.K. and T.M.L. contributed equally to this work.

  • ↵2To whom correspondence should be addressed. Email: cradu{at}mednet.ucla.edu or mphelps{at}mednet.ucla.edu.

  • Author contributions: W.K., T.M.L., S.P., W.R.A., J.S.V.V., D.S., R.M.G., R.S., M.E.P., K.H., J.C., and C.G.R. designed research; W.K., T.M.L., L.W., S.P., J.B., X.W., N.T.U., W.R.A., J.S.V.V., D.S., R.M.G., R.S., A.E.C., and T.T. performed research; J.T.L., S.S., K.F.F., and M.E.P. contributed new reagents/analytic tools; W.K., T.M.L., L.W., J.B., X.W., N.T.U., E.R.A., J.R.C., W.R.A., S.S.Y., J.T.L., S.S., A.L., T.R.D., H.R.H., K.H., J.C., and C.G.R. analyzed data; and W.K., T.M.L., E.R.A., J.R.C., K.F.F., O.N.W., H.R.H., K.H., J.C., and C.G.R. wrote the paper.

  • Reviewers: B.J.P., University of Tübingen; O.S.S., Boston University School of Medicine; and W.W., Sloan-Kettering Cancer Center.

  • Conflict of interest statement: C.G.R., O.N.W., M.E.P., and J.C. are cofounders of Sofie Biosciences (SB). They and the University of California (UC) hold equity in SB. C.G.R., O.N.W., and J.C. are among the inventors of [18F]CFA and analogs, which were patented by the UC and have been licensed to SB. UC has patented additional intellectual property for small molecule dCK inhibitors invented by C.G.R., J.C., A.L., S.P., and T.M.L. This intellectual property has been licensed by Trethera Corporation, in which C.G.R., J.C., O.N.W., and the UC hold equity.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1524212113/-/DCSupplemental.

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