Human biodistribution and radiation dosimetry of novel PET probes targeting the deoxyribonucleoside salvage pathway.
OBJECTIVE
Deoxycytidine kinase (dCK) is a rate-limiting enzyme in deoxyribonucleoside salvage, a metabolic pathway involved in the production and maintenance of a balanced pool of deoxyribonucleoside triphosphates (dNTPs) for DNA synthesis. dCK phosphorylates and therefore activates nucleoside analogs such as cytarabine, gemcitabine, decitabine, cladribine, and clofarabine that are used routinely in cancer therapy. Imaging probes that target dCK might allow stratifying patients into likely responders and nonresponders with dCK-dependent prodrugs. Here we present the biodistribution and radiation dosimetry of three fluorinated dCK substrates, (18)F-FAC, L: -(18)F-FAC, and L: -(18)F-FMAC, developed for positron emission tomography (PET) imaging of dCK activity in vivo.


METHODS
PET studies were performed in nine healthy human volunteers, three for each probe. After a transmission scan, the radiopharmaceutical was injected intravenously and three sequential emission scans acquired from the base of the skull to mid-thigh. Regions of interest encompassing visible organs were drawn on the first PET scan and copied to the subsequent scans. Activity in target organs was determined and absorbed dose estimated with OLINDA/EXM. The standardized uptake value was calculated for various organs at different times.


RESULTS
Renal excretion was common to all three probes. Bone marrow had higher uptake for L: -(18)F-FAC and L: -(18)F-FMAC than (18)F-FAC. Prominent liver uptake was seen in L: -(18)F-FMAC and L: -(18)F-FAC, whereas splenic activity was highest for (18)F-FAC. Muscle uptake was also highest for (18)F-FAC. The critical organ was the bladder wall for all three probes. The effective dose was 0.00524, 0.00755, and 0.00910 mSv/MBq for (18)F-FAC, L: -(18)F-FAC, and L: -(18)F-FMAC, respectively.


CONCLUSIONS
The biodistribution of (18)F-FAC, L: -(18)F-FAC, and L: -(18)F-FMAC in humans reveals similarities and differences. Differences may be explained by different probe affinities for nucleoside transporters, dCK, and catabolic enzymes such as cytidine deaminase (CDA). Dosimetry demonstrates that all three probes can be used safely to image the deoxyribonucleoside salvage pathway in humans.
Department of Molecular and Medical Pharmacology, Ahmanson Biological Imaging Division, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.
Johannes
J
Schwarzenberg
Caius G
CG
Radu
Matthias
M
Benz
Barbara
B
Fueger
Andrew Q
AQ
Tran
Michael E
ME
Phelps
Owen N
ON
Witte
Nagichettiar
N
Satyamurthy
Johannes
J
Czernin
Christiaan
C
Schiepers
D000328:Adult; D003841:Deoxycytidine / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics*; D003853:Deoxyribonucleosides / Q000378:metabolism*; D005260:Female; D006801:Humans; D008223:Lymphoma / Q000000981:diagnostic imaging / Q000378:metabolism; D008297:Male; D053858:Metabolic Networks and Pathways; D010051:Ovarian Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism / Q000556:secondary; D049268:Positron-Emission Tomography / Q000379:methods*; D011874:Radiometry; D055815:Young Adult
D003853:Deoxyribonucleosides; D003841:Deoxycytidine