Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Clinical
Cancer
Research
Imaging, Diagnosis, Prognosis
Evaluation of Deuterated 18F- and 11C-Labeled Choline
Analogs for Cancer Detection by Positron Emission
Tomography
Timothy H. Witney1, Israt S. Alam1, David R. Turton2, Graham Smith1, Laurence Carroll1, Diana Brickute2,
Nguyen1, Giampaolo Tomasi1, Ramla O. Awais2, and Eric O. Aboagye1
Frazer J. Twyman1, Quang-De
Abstract
Purpose: 11C-Choline–positron emission tomography (PET) has been exploited to detect the aberrant
choline metabolism in tumors. Radiolabeled choline uptake within the imaging time is primarily a function
of transport, phosphorylation, and oxidation. Rapid choline oxidation, however, complicates interpretation of PET data. In this study, we investigated the biologic basis of the oxidation of deuterated choline
analogs and assessed their specificity in human tumor xenografts.
Experimental Design: 11C-Choline, 11C-methyl-[1,2-2H4]-choline (11C-D4-choline), and 18F-D4-choline were synthesized to permit comparison. Biodistribution, metabolism, small-animal PET studies, and
kinetic analysis of tracer uptake were carried out in human colon HCT116 xenograft–bearing mice.
Results: Oxidation of choline analogs to betaine was highest with 11C-choline, with reduced oxidation
observed with 11C-D4-choline and substantially reduced with 18F-D4-choline, suggesting that both
fluorination and deuteration were important for tracer metabolism. Although all tracers were converted
intracellularly to labeled phosphocholine (specific signal), the higher rate constants for intracellular
retention (Ki and k3) of 11C-choline and 11C-D4-choline, compared with 18F-D4-choline, were explained
by the rapid conversion of the nonfluorinated tracers to betaine within HCT116 tumors. Imaging studies
showed that the uptake of 18F-D4-choline in three tumors with similar radiotracer delivery (K1) and choline
kinase a expression—HCT116, A375, and PC3-M—were the same, suggesting that 18F-D4-choline has
utility for cancer detection irrespective of histologic type.
Conclusion: We have shown here that both deuteration and fluorination combine to provide protection
against choline oxidation in vivo. 18F-D4-choline showed the highest selectivity for phosphorylation and
warrants clinical evaluation. Clin Cancer Res; 18(4); 1063–72. 2012 AACR.
Introduction
Choline is required for the biosynthesis of phosphatidylcholine, a key component of the plasma membrane. Following transport into the cell, choline is phosphorylated by
choline kinase to phosphocholine and then is further
metabolized to phosphatidylcholine via CDP-choline,
known as the Kennedy pathway. Once phosphorylated,
phosphocholine is trapped within the cell. Diacylglycerol,
Authors' Affiliations: 1Comprehensive Cancer Imaging Centre at Imperial
College, Faculty of Medicine, Imperial College London; and 2Hammersmith
Imanet Ltd., Hammersmith Hospital, London, United Kingdom
Note: Supplementary data for this article are available at Clinical Cancer
Research Online (http://clincancerres.aacrjournals.org/).
Corresponding Author: Eric O. Aboagye, Comprehensive Cancer Imaging
Centre, Faculty of Medicine, Imperial College London, Room 240 MRC
Cyclotron Building, Hammersmith Hospital, Du Cane Road, London, W12
0NN, United Kingdom. Phone: 020-83-83-3759; Fax: 020-83-83-17-83;
E-mail: eric.aboagye@imperial.ac.uk
doi: 10.1158/1078-0432.CCR-11-2462
2012 American Association for Cancer Research.
a product of phosphatidylcholine degradation, is mitogenic, playing a role in the regulation of cell-cycle progression
from G1 to S via increased cyclin D1 and cyclin D3 expression (1). Furthermore, aberrant activation and expression of
several oncogenes results in elevated choline kinase activity
and intracellular levels of phosphocholine (2–4). Choline
kinase overexpression is a common feature of several
human cancers (5) and in early stage non–small cell lung
cancer, choline kinase has been shown to have prognostic
significance (6). The expression of choline transporters,
including CTL1 and OCT3, is also increased following
malignant transformation and may contribute to radiotracer uptake (7, 8), with choline transport closely associated
with cell growth (9). 11C-choline has become a viable
alternative to 18F-2-fluoro-2-deoxyglucose for positron
emission tomography (PET) imaging of the prostate (10–
12), in which the increased choline kinase activity in tumors
provides the basis for tumor-specific contrast in comparison
with surrounding nonneoplastic tissues. A fluorinated
analog, 18F-fluoromethylcholine, has also been developed
for PET imaging of choline metabolism (13), with the
longer half-life of fluorine-18 (109.8 vs. 20.4 minutes for
www.aacrjournals.org
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
1063
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Witney et al.
Translational Relevance
11
C-Choline–positron emission tomography (PET) is
a marker of choline kinase expression and activity, which
is upregulated during carcinogenesis. To date, 11C-choline–PET has been used for the detection of a range of
human cancers and has emerged as a viable alternative to
18
F-2-fluoro-2-deoxyglucose for the imaging of prostate
adenocarcinoma. 11C-choline, however, is rapidly oxidized to betaine in an unwanted side reaction, complicating data interpretation. Here, we designed novel
choline analogs and tested their metabolic profiles and
sensitivity for cancer detection. The doubly fluorinated
and deuterated analog 18F-D4-choline showed lowest
betaine oxidation. This radiotracer could be used for
cancer detection, irrespective of histologic type. Therefore, the development of new choline radiotracers with
an improved metabolic profile should provide a means
to simplify interpretation of clinical PET data, while
increasing selectivity for phosphorylation.
carbon-11), potentially enabling more widespread adoption of choline imaging in the clinic and the ability to image
at later time points posttracer injection.
Within the imaging time window (60 minutes), tumor
radiolabeled choline uptake is a function of perfusion,
transport of the radiotracer from the extracellular space
into cells, where it is either converted into phosphocholine by the action of choline kinase or oxidized by choline
oxidase to betaine. Further incorporation of phosphocholine to membrane phosphatidylcholine is negligible within this time window (7, 8, 14, 15). Chromatographic
analysis also indicates that further betaine metabolism or
conversion to acetylcholine is negligible (16, 17). Hence,
radiotracer uptake broadly represents transport and phosphorylation on the one hand, and transport and oxidation on the other. One key limitation of choline–PET is
the rapid oxidation to radiolabeled betaine, making it
difficult to assess choline kinase–specific trapping of
activity (as phosphocholine) within tumors without plasma metabolite evaluation using complex kinetic analysis.
We have recently developed a novel tracer, 18F-fluoromethyl-[1,2-2H4]-choline (18F-D4-choline), with reduced
in vivo oxidation to betaine and improved sensitivity, for
the detection of choline metabolism in comparison with
the nondeuterated 18F-fluoromethylcholine (16). This
improved metabolic profile was shown (16, 17) to be
based on the deuterium isotope effect (18–21). Here, we
sought to further evaluate the structural determinants of
deuteration on substrate metabolism, as well as the effect
of deuteration on tumor-specific uptake. To this end, we
developed a novel choline tracer, 11C-[1,2-2H4]-choline
(11C-D4-choline) and compared its in vivo tumor uptake,
kinetics, and metabolic profile to 11C-choline and 18F-D4choline PET tracers.
1064
Clin Cancer Res; 18(4) February 15, 2012
Materials and Methods
Cell lines
HCT116 colorectal carcinoma (LGC Standards) and PC3M prostate adenocarcinoma cells (kind donation from Dr
Matthew Caley, Prostate Cancer Metastasis Team, Imperial
College London, United Kingdom) were grown in RPMI1640 media, supplemented with 10% fetal calf serum
(FCS), 2 mmol/L L-glutamine, 100 U/mL penicillin, and
100 mg/mL streptomycin (Invitrogen). A375 malignant
melanoma cells were a kind donation from Professor Eyal
Gottlieb, Beatson Institute for Cancer Research, Glasgow,
United Kingdom, and were grown in high glucose (4.5 g/L)
Dulbecco’s modified Eagle’s medium media, supplemented
with 10% FCS, 2 mmol/L L-glutamine, 100 U/mL penicillin,
and 100 mg/mL streptomycin (Invitrogen). All cells were
maintained at 37 C in a humidified atmosphere containing
5% CO2.
Western blots
Western blotting was done using standard techniques
(22, 23). For detailed methodology, see Supplementary
Materials.
In vivo tumor models
All animal experiments were conducted by licensed investigators in accordance with the United Kingdom Home
Office Guidance on the Operation of the Animal (Scientific
Procedures) Act 1986 and within the newly published
guidelines for the welfare and use of animals in cancer
research (24). Male BALB/c nude mice (aged 6–8 weeks;
Charles River) were used. Tumor cells (2 106) were
injected subcutaneously on the back of mice and animals
were used when the xenografts reached approximately 100
mm3. Tumor dimensions were measured continuously
using a caliper and tumor volumes were calculated by the
equation: volume ¼ (p/6) a b c, in which a, b, and c
represent 3 orthogonal axes of the tumor.
In vivo tracer metabolism
Radiolabeled metabolites from plasma and tissues were
quantified using a method adapted from Smith and colleagues (17). Briefly, tumor-bearing mice under general anesthesia (2.5% isofluorane; nonrecovery anesthesia) were
administered a bolus intravenous injection of one of the
following radiotracers: 11C-choline, 11C-D4-choline
(18.5 MBq), or 18F-D4-choline (3.7 MBq), and sacrificed by exsanguination via cardiac puncture at 2, 15, 30, or
60 minutes postradiotracer injection. For automated radiosynthesis methodology see Supplementary Materials.
Tumor, kidney, and liver samples were immediately
snap-frozen in liquid nitrogen. Aliquots of heparinized
blood were rapidly centrifuged (14,000 g, 5 minutes,
4 C) to obtain plasma. Plasma samples were subsequently
snap-frozen in liquid nitrogen and kept on dry ice prior to
analysis.
For analysis, samples were thawed and kept at 4 C
immediately before use. To ice-cold plasma (200 mL) was
Clinical Cancer Research
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Deuterated Choline–PET Radiotracers for Cancer Detection
added ice-cold methanol (1.5 mL) and the resulting suspension centrifuged (14,000 g; 4 C; 3 minutes). The
supernatant was then decanted and evaporated to dryness
on a rotary evaporator (bath temperature, 40 C), then
resuspended in high-performance liquid chromatography
(HPLC) mobile phase [solvent A: acetonitrile/water/ethanol/acetic acid/1.0 mol/L ammonium acetate/0.1 mol/L
sodium phosphate (800/127/68/2/3/10); 1.1 mL]. Samples
were filtered through a hydrophilic syringe filter (0.2-mm
filter, Millex PTFE filter, Millipore) and the sample (1 mL)
then injected via a 1-mL sample loop onto the HPLC for
analysis. Tissues were homogenized in ice-cold methanol
(1.5 mL) using an Ultra-Turrax T-25 homogenizer (IKA
Werke GmbH and Co. KG) and subsequently treated as
per plasma samples.
Samples were analyzed on an Agilent 1100 series HPLC
system (Agilent Technologies), configured as described
above, using the method of Leyton and colleagues (16).
A mBondapak C18 HPLC column (7.8 3,000 mm; Waters),
stationary phase and a mobile phase comprising of solvent
A (vide supra) and solvent B [acetonitrile/water/ethanol/
acetic acid/1.0 mol/L ammonium acetate/0.1 mol/L sodium phosphate (400/400/68/44/88/10)], delivered at a flow
rate of 3 mL/min were used for analyte separation. The
gradient was set as follows: 0% B for 5 minutes; 0% to 100%
B in 10 minutes; 100% B for 0.5 minutes; 100% to 0% B in 2
minutes; 0% B for 2.5 minutes.
PET imaging studies
Dynamic 11C-choline, 11C-D4-choline, and 18F-D4choline imaging scans were carried out on a dedicated
small animal PET scanner (Siemens Inveon PET module,
Siemens Medical Solutions USA, Inc.) following a bolus
intravenous injection in tumor-bearing mice of either
approximately 3.7 MBq for 18F studies or approximately
18.5 MBq for 11C, accommodating for substantially
shorter half-life of 11C (20.38 minutes for 11C vs.
109.77 minutes for 18F). Dynamic scans were acquired
in list mode format over 60 minutes. The acquired data
were then sorted into 0.5-mm sinogram bins and 19 time
frames for image reconstruction (4 15, 4 60, and 11
300 seconds), which was done by filtered back projection.
For input function analysis, data were sorted into 25 time
frames for image reconstruction (8 5, 1 20, 4 40,
1 80, and 11 300 seconds). The Siemens Inveon
Research Workplace software was used for visualization
of radiotracer uptake in the tumor; 30- to 60-minute
cumulative images of the dynamic data were employed
to define 3-dimensional (3D) regions of interest (ROI).
Arterial input function was estimated as follows: a single
voxel 3D ROI was manually drawn in the center of the
heart cavity using 2 to 5 minutes of cumulative images.
Care was taken to minimize ROI overlap with the myocardium. The count densities were averaged for all ROIs at
each time point to obtain a time versus radioactivity curve
(TAC). Tumor TACs were normalized to injected dose,
measured by a VDC-304 dose calibrator (Veenstra Instruments) and expressed as percentage injected dose per mL
www.aacrjournals.org
tissue. The area under the TAC, calculated as the integral
of %ID/mL from 0 to 60 minutes, and the normalized
uptake of radiotracer at 60 minutes (%ID/mL60) were also
used for comparisons.
Kinetic analysis in HCT116 tumors
A 2-tissue irreversible compartmental model was
employed to fit the TACs, as has been previously established
for 11C-choline (25, 26), described extensively in Supplementary Data. Here, both a Single Input 3k model (irreversible binding of the parent) and Double Input [3þ2]k
model (irreversible binding of the parent, reversible binding of the metabolite) were used to describe radiotracer
kinetics. K1 (radiotracer delivery; mL/mL/min) and k2 (1/
min) are the rate constants of transfer from plasma to tissue
and from tissue to plasma, respectively. k3 (1/min) represents the rate at which the parent tracer is phosphorylated.
In this model the irreversible uptake rate constant Ki (mL/
mL/min) can be expressed as a function of the microparameters as K1k3/(k2 þ k3). K10 (mL/mL/min) and k20 (1/min)
are the rate constants of transfer from plasma to tissue and
from tissue to plasma of labeled betaine. A schematic
describing the kinetic models used here is described in
Supplementary Fig. S1.
Statistics
Data were expressed as mean SEM, unless otherwise
shown. The significance of comparison between 2 data sets
was determined using Student t test. ANOVA was used for
multiparametric analysis (Prism v5.0 software for windows,
GraphPad Software). Differences between groups were considered significant if P 0.05.
Results
Deuteration leads to enhanced renal radiotracer
uptake
Time course biodistribution was done in nontumor–
bearing male nude mice with 11C-choline, 11C-D4-choline, and 18F-D4-choline tracers. Supplementary Fig. S2
shows tissue distribution at 2, 15, 30, and 60 minutes.
There were minimal differences in tissue uptake between
the 3 tracers over 60 minutes, with uptake values in
broad agreement with data previously published for 18Fcholine and 18F-D4-choline (13, 17). In all tracers there
was rapid extraction from blood, with the majority of
radioactivity retained within the kidneys, evident as
early as 2 minutes postinjection. Deuteration of 11Ccholine led to a significant 1.8-fold increase in kidney
retention over 60 minutes (P < 0.05; Supplementary Fig.
S2A and B), with a 3.3-fold increase in kidney retention
observed for 18F-D4-choline when compared with 11Ccholine at this time point (P < 0.01; Supplementary Fig.
S2A and C, respectively). There was a trend toward
increased urinary excretion for 11C-D4-choline and
18
F-D4-choline, in comparison with the nature identical
tracer, 11C-choline, although this increase did not reach
statistical significance.
Clin Cancer Res; 18(4) February 15, 2012
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
1065
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Witney et al.
Figure 1. Metabolic profile of
11
C-choline (A and D), 11C-D4choline (B and E), and 18F-D4choline (C and F) in the liver (A–C)
and kidney (D–F) of BALB/c nude
mice. Radiolabeled metabolite
profile was assessed at 2, 15, 30,
and 60 minutes after intravenous
injection of parent radiotracers
using radio-HPLC. Mean values
(n ¼ 3) and SEM are shown.
a, P < 0.05 when 11C-D4-choline is
compared with 11C-choline;
b, P < 0.05 when 18F-D4-choline is
compared with 11C-choline;
c, P < 0.05 when 18F-D4-choline is
compared with 11C-D4-choline.
Bet-ald, betaine aldehyde;
p-Choline, phosphocholine.
Deuteration of 11C-choline results in modest resistance
to oxidation in vivo
Tracer metabolism in tissues and plasma was done by
radio-HPLC (Fig. 1). Peaks were assigned as choline, betaine, betaine aldehyde, and phosphocholine, using enzymatic (alkaline phosphatase and choline oxidase) methods
to determine their identity (Supplementary Figs. S3 and S4,
respectively; ref. 16).
In the liver, both 11C-choline and 11C-D4-choline were
rapidly oxidized to betaine (Fig. 1A and B), with 49.2
7.7% of 11C-choline radioactivity already oxidized to betaine by 2 minutes. Deuteration of 11C-choline provided
significant protection against oxidation in the liver at 2
minutes postinjection, with 24.5 2.1% radioactivity as
betaine (51.2% decrease in betaine levels; P ¼ 0.037),
although this protection was lost by 15 minutes. Notably,
a high proportion of liver radioactivity (80%) was present
as 18F-D4-phosphocholine by 15 minutes with 18F-D4choline (Fig. 1C). This corresponded to a much reduced
liver-specific oxidation when compared with the 2 carbon-
1066
Clin Cancer Res; 18(4) February 15, 2012
11 tracers (15.0 3.6% of radioactivity as betaine at 60
minutes; P ¼ 0.002).
In contrast to the liver, deuteration of 11C-choline
resulted in protection against oxidation in the kidney over
the entirety of the 60-minute time course (Fig. 1D and E).
With 11C-D4-choline there was a 20% to 40% decrease in
betaine levels over 60 minutes when compared with 11Ccholine (P < 0.05), corresponding to a proportional increase
in labeled phosphocholine (P < 0.05). As shown in Fig. 1F,
18
F-D4-choline was more resistant to oxidation in the
kidney than both carbon-11–labeled choline tracers. There
was a relationship between levels of radiolabeled phosphocholine and kidney retention when data from all 3 tracers
were compared (R2 ¼ 0.504; Supplementary Fig. S5). In the
plasma, the temporal levels of betaine for both 11C-choline
and 11C-D4-choline were almost identical; it should be
noted that total radioactivity levels were low for all radiotracers. At 2 minutes, 12.1 2.6% and 8.8 3.8% of
radioactivity was in the form of betaine for 11C-choline
and 11C-D4-choline, respectively, rising to 78.6 4.4% and
Clinical Cancer Research
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Deuterated Choline–PET Radiotracers for Cancer Detection
79.5 2.9% at 15 minutes. Betaine levels were significantly
reduced with 18F-D4-choline, with 43.7 12.4% of activity
present as betaine at 15 minutes. A further increase in
plasma betaine was not observed with 18F-D4-choline over
the remainder of the time course.
Fluorination protects against choline oxidation in
tumor
11
C-choline, 11C-D4-choline, and 18F-D4-choline metabolism were measured in HCT116 tumors (Fig. 2). With all
tracers, choline oxidation was greatly reduced in the tumor
in comparison with levels in the kidney and liver. At 15
minutes, both 11C-D4-choline and 18F-D4-choline had
significantly more radioactivity corresponding to labeled
phosphocholine than 11C-choline; 43.8 1.5% and 45.1
3.2% for 11C-D4-choline and 18F-D4-choline, respectively,
in comparison with 30.5 4.0% for 11C-choline (P ¼ 0.035
and P ¼ 0.046, respectively). By 60 minutes, the majority of
radioactivity was phosphocholine for all 3 tracers, with
11
11
18
Figure 2. Metabolic profile of C-choline, C-D4-choline, and F-D4choline in HCT116 tumors. Radiolabeled metabolite profile in HCT116
tumor xenografts was assessed at 15 (A) and 60 minutes (B) after
intravenous injection of parent radiotracers using radio-HPLC. Mean
values (n ¼ 3) and SEM are shown. , P < 0.05; , P < 0.01; , P < 0.001.
p-Choline, phosphocholine.
www.aacrjournals.org
labeled phosphocholine levels increasing in the order of
11
C-choline < 11C-D4-choline < 18F-D4-choline. There was
no difference in the tumor metabolic profile for 11C-choline
and 11C-D4-choline at 60 minutes, although reduced choline oxidation was observed for 18F-D4-choline; 14.0
3.0% betaine radioactivity with 18F-D4-choline compared
with 28.1 2.9% for 11C-choline (P ¼ 0.026).
Choline tracers have similar sensitivity for imaging
tumors by PET
Despite the low systemic oxidation of 18F-D4-choline,
tumor radiotracer uptake in mice by PET was no higher than
with 11C-choline or 11C-D4-choline (Fig. 3). Figure 3A–C
shows typical (0.5 mm) transverse PET image slices showing
accumulation of all 3 tracers in HCT116 tumors. For all 3
tracers, there was heterogeneous tumor uptake, but tumor
signal-to-background levels were identical, confirmed by
normalized uptake values at 60 minutes and values for the
tumor area under the time versus radioactivity curve (data
not shown). It should be noted that the PET data represent
total radioactivity. In the case of 11C-choline or 11C-D4choline, a significant proportion of this radioactivity is
betaine (Fig. 2).
Tumor tracer kinetics
Despite there being no difference in overall tracer retention in the tumor, the kinetic profiles of tracer uptake varied
between the 3 choline tracers, detected by PET (Fig. 3D). The
kinetics for the 3 tracers were characterized by rapid tumor
influx over the initial 5 minutes, followed by stabilization of
tumor retention. Initial delivery of 18F-D4-choline over the
first 14 minutes of imaging was higher than for both 11Ccholine and 11C-D4-choline (expanded TAC for initial 14
minutes shown in Supplementary Fig. S6). Slow wash-out
of activity was observed with both 18F-D4-choline and 11CD4-choline between 30 and 60 minutes, in contrast to the
gradual accumulation detected with 11C-choline. Parameters for the irreversible trapping of radioactivity in the
tumor, Ki and k3, were calculated from a 2-tissue irreversible
model, using metabolite-corrected TAC from the heart
cavity as input function (Fig. 4A and B). A double input
model, accounting for the contribution of metabolites to
the tissue TAC, was used for kinetic analysis, described in
Supplementary Data. There was no significant difference in
flux constant measurements between deuterated and
undeuterated 11C-choline. Addition of methylfluoride,
however, resulted in 49.2% (n ¼ 3; P ¼ 0.022) and
75.2% (n ¼ 3; P ¼ 0.005) decreases in Ki and k3, respectively;
that is, when 18F-D4-choline was compared with 11C-D40
choline. K1 values were similar between all 3 tracers: 0.106
0.026, 0.114 0.019, and 0.142 0.027 for 11C-choline,
11
C-D4-choline, and 18F-D4-choline, respectively. It is possible that intracellular betaine formation (not just presence
of betaine in the extracellular space) led to a higher than
expected irreversible uptake; there was a significant 388%
and 230% increase in the ratio of betaine:phophocholine at
15 and 60 minutes, respectively (P ¼ 0.045 and 0.036) with
11
C-choline in comparison with 18F-D4-choline (Fig. 4C).
Clin Cancer Res; 18(4) February 15, 2012
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
1067
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Witney et al.
11
11
18
Figure 3. C-choline (*), C-D4-choline (~), and F-D4-choline (&) PET image analysis. HCT116 tumor uptake profiles were examined following 60-minute
dynamic PET imaging. A–C, representative axial PET-CT images of HCT116 tumor–bearing mice (30–60 minutes of summed activity) for 11C-choline (A),
11
C-D4-choline (B), and 18F-D4-choline (C). Tumor margins, indicated from CT image, are outlined in red. D, the tumor TAC. Mean SEM (n ¼ 4 mice
per group).
18
F-D4-choline shows good sensitivity for the PET
imaging of prostate adenocarcinoma and malignant
melanoma
Having confirmed that 18F-D4-choline has the most
desirable metabolic profile for in vivo studies, with good
sensitivity for the imaging of colon adenocarcinoma, we
wanted to evaluate its suitability for cancer detection in
other models of human cancer, including malignant melanoma A375 and prostate adenocarcinoma PC3-M. In vitro
uptake of 18F-D4-choline was similar in the 3 cell lines over
30 minutes (Supplementary Fig. S7), relating to near-identical levels of choline kinase expression (Supplementary Fig.
S7 insert). Retention of radioactivity was shown to be
dependent on both choline transport and choline kinase
activity, as treatment of cells with the dual choline transport
and choline kinase inhibitor, hemicholinium-3, resulted in
more than 90% decrease in intracellular tracer radioactivity
in all 3 cell lines. Similar intracellular trapping of 18F-D4choline in these cancer models were translated to their
uptake in vivo (Fig. 5A), showing similar values for flux
constant measurements, including rates of delivery (K1;
Supplementary Table S1) and other and PET imaging variables. There was a trend toward increased tumor retention
of 18F-D4-choline in the order of A375 < HCT116 < PC3-M,
reflected by the expression of choline kinase in these lines
1068
Clin Cancer Res; 18(4) February 15, 2012
(Fig. 5C). There was no discernible difference in tumor
metabolite profiles between the 3 cell cancer models at
either 15 or 60 minutes of postinjection (Fig. 5B).
Discussion
Aberrant phospholipid metabolism is a hallmark of
many cancers (5), resulting in upregulated mitotic signaling
and an increase in plasma membrane biosynthesis. One
such mediator of phospholipid metabolism, choline
kinase, has been shown to be a biomarker of malignant
transformation (2). Proton and phosphorous magnetic
resonance spectroscopic (MRS) techniques have provided
a means to measure the product of choline kinase activity,
phosphocholine, from tumor tissue biopsies ex vivo and
from noninvasive spectroscopic imaging measurements in
vivo (27). MRS, however, is hampered by inherently poor in
vivo sensitivity, making it difficult to resolve individual
choline metabolite resonances, complicating data interpretation, whereas ex vivo measurements requires invasive
sampling from a small, possibly unrepresentative, region
of interest. Given the current drawbacks of choline metabolite analysis by MRS, a more viable alternative has been the
use of radiolabeled choline for noninvasive tumor imaging.
PET-labeled choline tracers provide vastly improved
Clinical Cancer Research
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Deuterated Choline–PET Radiotracers for Cancer Detection
Figure 4. Pharmacokinetics of
11
C-choline, 11C-D4-choline, and
18
F-D4-choline in HCT116 tumors. A,
modified compartmental modeling
analysis, taking into account plasma
metabolites and their flux into the
exchangeable space in tumor, was
used to derive Ki, a measure of
irreversible retention within the
tumor. B, the kinetic parameter, k3,
an indirect measure of choline kinase
activity, was calculated using a 2-site
compartmental model as previously
described (36, 37). C, ratio of betaine
to phosphocholine in tumors.
Metabolites were quantified by
radio-HPLC at 15 and 60 minutes of
postinjection of tracer. Mean values
(n ¼ 4) and SEM are shown.
, P < 0.05; , P < 0.001. p-choline,
phosphocholine.
sensitivity, when compared with MRS, and enable dynamic
measurements of choline metabolism, but without the
chemical resolution of MRS. To date, 11C-choline has successfully been used for the clinical imaging of prostate,
brain, breast, and esophageal carcinomas (10, 25, 28–30).
Despite its relative success, 11C-choline-PET imaging has
not been widely adopted in the clinic; the short half-life of
carbon-11 requires an on-site cyclotron and rapid metabolism of the choline tracer presents complications for data
interpretation and limits the imaging time frame to early
time frames (25). We have recently developed a fluorinated
choline analog, 18F-D4-choline, labeled with a longer lived
isotope and with improved metabolic profile, required for
late tumor imaging (16). The substitution of deuterium for
Figure 5. Dynamic uptake and
18
metabolite analysis with F-D4choline in tumors of different
histologic origin. A, the tumor TAC
obtained from 60-minute dynamic
PET imaging. Mean SEM (n ¼ 3–5
mice per group). B, metabolic profile
of 18F-D4-choline in tumors.
Radiolabeled metabolite profile in
HCT116 tumor xenografts was
assessed post-PET imaging using
radio-HPLC. Mean values (n ¼ 3) and
SEM are shown. C, choline kinase
expression in malignant melanoma,
prostate adenocarcinoma, and colon
carcinoma tumors. Representative
Western blot from tumor lysates
(n ¼ 3 xenografts per tumor cell line).
Actin was used as a loading control.
CKa, choline kinase alpha; %ID,
percentage injected dose.
www.aacrjournals.org
Clin Cancer Res; 18(4) February 15, 2012
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
1069
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Witney et al.
hydrogen on the ethyl alcohol portion of choline resulted in
a large observed isotope effect in the oxidation of choline to
betaine by choline oxidase. Further studies showed that the
magnitude of the 1H/2D isotope effect was more profound
when all protons were substituted for deuterium, in comparison with partial deuteration of the fluorocholine (17).
Urinary radioactivity, however, is higher with fluorinated
choline analogs relative to 11C-choline (16, 31), potentially
limiting their use for the detection of pelvic cancers, such as
prostate adenocarcinoma. Here, we developed a novel
choline tracer, 11C-D4-choline, which, based on previous
work with fluorinated and deuterated choline tracers, was
predicted to have reduced oxidation to betaine and a
favorable urinary excretion profile.
The kidney has high choline kinase activity along the
nephron (32), shown to exhibit the greatest tissue retention
for choline-PET and, therefore, is the radiation-dose-critical
organ (13, 17). Kidney retention increased in the order of
11
C-choline < 11C-D4-choline < 18F-D4-choline over the 60minute time course (Supplementary Fig. S2), with total
kidney radioactivity shown to be proportional to the %
radioactivity retained as labeled phosphocholine (Supplementary Fig. S5; R2 ¼ 0.504). The increased conversion of
choline to phosphocholine with 11C-D4-choline relative to
11
C-choline corresponded with a significant decrease in
choline oxidation to betaine and could be ascribed to
increased substrate availability for phosphorylation. Further attenuation of choline oxidation was observed with
18
F-D4-choline, indicating that the magnitude of the 1H/2D
isotope effect is influenced by fluorination. Protection
against choline oxidation by deuteration of 11C-choline
was shown to be tissue specific, with a decrease in betaine
radioactivity measured in the liver at just 2 minutes postinjection when compared with 11C-choline (Fig. 1). This
effect is presumably due to the lower capacity of choline
oxidase in rodent kidney compared with liver. 18F-D4choline provided substantially reduced betaine oxidation
in the liver over 60 minutes postinjection, again suggesting
that fluorination, in part, drives this reduced capacity to
oxidize choline pseudosubstrates to betaine.
Despite systemic protection against choline oxidation
with 18F-D4-choline, the reduction in the rate of choline
oxidation was much more subtle in implanted HCT116
tumors (Fig. 2). At 15 minutes postinjection, there were
43.6% and 47.9% higher levels of radiolabeled phosphocholine when 11C-D4-choline and 18F-D4-choline, respectively, were injected relative to 11C-choline. By 60 minutes
there was no difference in labeled phosphocholine levels
between the 3 tracers, although there was a significant
decrease in betaine-specific radioactivity with 18F-D4-choline. This equilibration of phosphocholine-specific activity
can be explained by a saturation effect, with parent tracer
levels reduced to a minimum by 60 minutes, severely
limiting substrate levels available for choline kinase activity.
Lower betaine levels were observed in the tumor with all 3
tracers over the entire time course when compared with liver
and kidney, likely resulting from a lower capacity for choline oxidation or increased washout of betaine. It should be
1070
Clin Cancer Res; 18(4) February 15, 2012
noted that the capacity of rodents to metabolize choline is
substantially higher than that of humans (14, 33). The
slower metabolic rate in humans may, therefore, provide
a better differential between these choline tracers. Despite
this, deuteration of 11C-choline per se provided less than
expected protection against choline oxidation in the liver,
tumor, and kidney, especially in the context of improved
metabolic profile shown with deuterated fluorocholine
versus nondeuterated fluorocholine (16, 17)
Comparison of the 3 choline radiotracers by PET showed
no significant differences in overall tumor radiotracer
uptake and hence sensitivity (Fig. 3), despite large changes
observed in other organs. Initial tumor kinetics (at time
points when metabolism was lower), however, varied
between tracers, with 18F-D4-choline characterized by rapid
delivery over approximately 5 minutes, followed by slow
wash-out of activity from the tumor. This compared with
the slower uptake, but continuous tumor retention of 11Ccholine. At 60 minutes, a 2.7-fold and 4.0-fold higher
unmetabolized parent tracer was seen with 18F-D4-choline
in tumor compared with 11C-choline and 11C-D4-choline,
respectively, (Fig. 2). Deuteration did not, however, alter
total tumor radioactivity levels and the modeling approach
used did not distinguish between different intracellular
species. Although all tracers were converted intracellularly
to phosphocholine, the higher rate constants for intracellular retention (Ki and k3; Fig. 3A and B) of 11C-choline and
11
C-D4-choline, compared with 18F-D4-choline, were
explained by the rapid conversion of the nonfluorinated
tracers to betaine within HCT116 tumors, indicating greater
specificity with 18F-D4-choline. Compared with 18F-D4choline, the tumor betaine-to-phosphocholine metabolite
ratio increased by 388% (P ¼ 0.045) and 259% (P ¼ 0.061,
nonsignificant) for 11C-choline and 11C-D4-choline,
respectively (Fig. 4C). It is also important to note that the
compartmental modeling is subject to some minor experimental limitations. In humans and larger animals, a more
accurate input function can be obtained by continuous
blood sampling following radiotracer injection. Individual
plasma metabolite data can also be easily obtained and
fitted instead of the averaged data taken from a separate
cohort of animals used here.
It has been reported elsewhere that fluorination increases
urinary excretion in comparison with 11C-choline (13, 16).
However, in this study, we did not observe these undesirable
urinary excretion properties. This may be due to use of
anesthesia for immobilizing mice during imaging. There
was a trend toward increased urinary excretion in the two
deuterated tracers (suggesting a trade-off between reduced
oxidation and renal excretion) when compared with 11Ccholine, although these did not reach significance. Low
radioactivity levels in the urine prevented accurate metabolite analysis (data not shown). The low radioactivity levels
in the urine should enable accurate prostate imaging with
18
F-D4-choline, especially if patients void to reduce bladder
radioactivity prior to late time point imaging. Given the
favorable urinary excretion properties and greatly superior
systemic metabolic profile of 18F-D4-choline, PET imaging
Clinical Cancer Research
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Deuterated Choline–PET Radiotracers for Cancer Detection
was carried out in 2 further models of human cancer to
assess generic utility in tumors of different origins: A375
malignant melanoma and PC3-M prostate adenocarcinoma. PC3-M cells were chosen as a clinically relevant model
for choline imaging, whereas A375 have constitutively
active mitogen-activated protein kinase (MAPK) due to a
BRAFV600E mutation (34); the MAPK pathway is known to
regulate choline kinase activity (35). 18F-D4-choline uptake
in vitro (where betaine formation is negligible) was similar
in all 3 cell lines, reflecting near-identical levels of choline
kinase a expression. The delivery of 18F-D4-choline (K1) was
similar between the different tumor types in vivo, suggesting
that choline transporter expression was probably not deficient in any of the tumors. These in vitro findings translated
well in vivo, with comparable tumor uptake, kinetics, choline kinase a expression, and metabolism for all the tumor
types, suggesting that 18F-D4-choline may have utility for
tumor detection, irrespective of histologic type.
In conclusion, we have shown here that deuteration of
11
C-choline provides a smaller than expected protection
against choline oxidation. Despite a significant increase in
labeled phosphocholine at early time points, this did not
increase the overall sensitivity for the detection of choline
metabolism in vivo. More promising is the substantial
decrease in betaine oxidation illustrated here with 18FD4-choline, which may permit the clinical imaging of
choline without invasive blood sampling. Fluorine-18
labeling may also lead to wider clinical adoption and
permit imaging at late time points. We have further
validated 18F-D4-choline using 3 models of human cancer, including a clinically relevant model of human prostate adenocarcinoma.
Disclosure of Potential Conflicts of Interest
A patent on novel choline imaging agents has been filed.
Acknowledgments
The authors thank Dr. Magdy Khalil for his help with the PET imaging
studies and for advice about kinetic analysis, and Dr. Matthew Caley and
Professor Eyal Gottlieb for provision of cell lines.
Grant Support
This work was funded by Cancer Research UK–Engineering and Physical
Sciences Research Council grant C2536/A10337. E.O. Aboagye’s laboratory
receives core funding from the UK Medical Research Council (MC US A652
0030).
The costs of publication of this article were defrayed in part by the
payment of page charges. This article must therefore be hereby marked
advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate
this fact.
Received September 26, 2011; revised December 6, 2011; accepted
December 15, 2011; published OnlineFirst January 10, 2012.
References
1.
Ramirez de Molina A, Gallego-Ortega D, Sarmentero-Estrada J,
Lagares D, Gomez Del Pulgar T, Bandres E, et al. Choline kinase as
a link connecting phospholipid metabolism and cell cycle regulation:
implications in cancer therapy. Int J Biochem Cell Biol 2008;40:
1753–63.
2. Aboagye EO, Bhujwalla ZM. Malignant transformation alters membrane choline phospholipid metabolism of human mammary epithelial
cells. Cancer Res 1999;59:80–4.
3. Hernandez-Alcoceba R, Saniger L, Campos J, Nunez MC, Khaless F,
Gallo MA, et al. Choline kinase inhibitors as a novel approach for
antiproliferative drug design. Oncogene 1997;15:2289–301.
4. Liu D, Hutchinson OC, Osman S, Price P, Workman P, Aboagye EO.
Use of radiolabelled choline as a pharmacodynamic marker for the
signal transduction inhibitor geldanamycin. Br J Cancer 2002;87:
783–9.
5. Ramirez de Molina A, Rodriguez-Gonzalez A, Gutierrez R, MartinezPineiro L, Sanchez J, Bonilla F, et al. Overexpression of choline kinase
is a frequent feature in human tumor-derived cell lines and in lung,
prostate, and colorectal human cancers. Biochem Biophys Res Commun 2002;296:580–3.
6. Ramirez deMolina A, Sarmentero-Estrada J, Belda-Iniesta C, Taron M,
Ramirez de Molina V, Cejas P, et al. Expression of choline kinase alpha
to predict outcome in patients with early-stage non-small-cell lung
cancer: a retrospective study. Lancet Oncol 2007;8:889–97.
7. Hara T, Bansal A, DeGrado TR. Choline transporter as a novel target for
molecular imaging of cancer. Mol Imaging 2006;5:498–509.
8. Yoshimoto M, Waki A, Obata A, Furukawa T, Yonekura Y, Fujibayashi
Y. Radiolabeled choline as a proliferation marker: comparison with
radiolabeled acetate. Nucl Med Biol 2004;31:859–65.
9. Wang T, Li J, Chen F, Zhao Y, He X, Wan D, et al. Choline transporters in
human lung adenocarcinoma: expression and functional implications.
Acta Biochim Biophys Sin (Shanghai) 2007;39:668–74.
10. Hara T, Kosaka N, Kishi H. PET imaging of prostate cancer using
carbon-11-choline. J Nucl Med 1998;39:990–5.
www.aacrjournals.org
11. Kotzerke J, Prang J, Neumaier B, Volkmer B, Guhlmann A, Kleinschmidt K, et al. Experience with carbon-11 choline positron emission
tomography in prostate carcinoma. Eur J Nucl Med 2000;27:1415–9.
12. Richter JA, Rodriguez M, Rioja J, Penuelas I, Marti-Climent J, Garrastachu P, et al. Dual tracer 11C-choline and FDG-PET in the diagnosis of
biochemical prostate cancer relapse after radical treatment. Mol
Imaging Biol 2010;12:210–7.
13. DeGrado TR, Baldwin SW, Wang S, Orr MD, Liao RP, Friedman HS,
et al. Synthesis and evaluation of (18)F-labeled choline analogs as
oncologic PET tracers. J Nucl Med 2001;42:1805–14.
14. Bansal A, Shuyan W, Hara T, Harris RA, Degrado TR. Biodisposition
and metabolism of [(18)F]fluorocholine in 9L glioma cells and 9L
glioma-bearing fisher rats. Eur J Nucl Med Mol Imaging 2008;35:
1192–203.
15. Kuang Y, Salem N, Corn DJ, Erokwu B, Tian H, Wang F, et al. Transport
and metabolism of radiolabeled choline in hepatocellular carcinoma.
Mol Pharm 2010;7:2077–92.
16. Leyton J, Smith G, Zhao Y, Perumal M, Nguyen QD, Robins E, et al.
[18F]fluoromethyl-[1,2-2H4]-choline: a novel radiotracer for imaging
choline metabolism in tumors by positron emission tomography.
Cancer Res 2009;69:7721–8.
17. Smith G, Zhao Y, Leyton J, Shan B, Nguyen QD, Perumal M, et al.
Radiosynthesis and pre-clinical evaluation of [(18)F]fluoro-[1,2-(2)H(4)]
choline. Nucl Med Biol 2011;38:39–51.
18. Fan F, Gadda G. On the catalytic mechanism of choline oxidase. J Am
Chem Soc 2005;127:2067–74.
19. Fan F, Gadda G. An internal equilibrium preorganizes the enzymesubstrate complex for hydride tunneling in choline oxidase. Biochemistry 2007;46:6402–8.
20. Gadda G. pH and deuterium kinetic isotope effects studies on the
oxidation of choline to betaine-aldehyde catalyzed by choline oxidase.
Biochim Biophys Acta 2003;1650:4–9.
21. Nagel ZD, Klinman JP. Tunneling and dynamics in enzymatic hydride
transfer. Chem Rev 2006;106:3095–118.
Clin Cancer Res; 18(4) February 15, 2012
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.
1071
Published OnlineFirst January 10, 2012; DOI: 10.1158/1078-0432.CCR-11-2462
Witney et al.
22. Witney TH, Kettunen MI, Brindle KM. Kinetic modeling of hyperpolarized 13C label exchange between pyruvate and lactate in tumor cells. J
Biol Chem 2011;286:24572–80.
23. Witney TH, Kettunen MI, Hu DE, Gallagher FA, Bohndiek SE, Napolitano R, et al. Detecting treatment response in a model of human
breast adenocarcinoma using hyperpolarised [1-13C]pyruvate and
[1,4-13C2]fumarate. Br J Cancer 2010;103:1400–6.
24. Workman P, Aboagye EO, Balkwill F, Balmain A, Bruder G, Chaplin DJ,
et al. Guidelines for the welfare and use of animals in cancer research.
Br J Cancer 2010;102:1555–77.
25. Kenny LM, Contractor KB, Hinz R, Stebbing J, Palmieri C, Jiang J,
et al. Reproducibility of [11C]choline-positron emission tomography and effect of trastuzumab. Clin Cancer Res 2010;16:
4236–45.
26. Sutinen E, Nurmi M, Roivainen A, Varpula M, Tolvanen T, Lehikoinen P,
et al. Kinetics of [(11)C]choline uptake in prostate cancer: a PET study.
Eur J Nucl Med Mol Imaging 2004;31:317–24.
27. Glunde K, Bhujwalla ZM. Metabolic tumor imaging using magnetic
resonance spectroscopy. Semin Oncol 2011;38:26–41.
28. Contractor KB, Kenny LM, Stebbing J, Al-Nahhas A, Palmieri C,
Sinnett D, et al. [11C]choline positron emission tomography in estrogen receptor-positive breast cancer. Clin Cancer Res 2009;15:
5503–10.
29. Hara T, Kosaka N, Shinoura N, Kondo T. PET imaging of brain tumor
with [methyl-11C]choline. J Nucl Med 1997;38:842–7.
30. Kobori O, Kirihara Y, Kosaka N, Hara T. Positron emission tomography
of esophageal carcinoma using (11)C-choline and (18)F-fluorodeox-
1072
Clin Cancer Res; 18(4) February 15, 2012
31.
32.
33.
34.
35.
36.
37.
yglucose: a novel method of preoperative lymph node staging. Cancer
1999;86:1638–48.
DeGrado TR, Coleman RE, Wang S, Baldwin SW, Orr MD, Robertson
CN, et al. Synthesis and evaluation of 18F-labeled choline as an
oncologic tracer for positron emission tomography: initial findings in
prostate cancer. Cancer Res 2001;61:110–7.
Wirthensohn G, Vandewalle A, Guder WG. Choline kinase activity
along the rabbit nephron. Kidney Int 1982;21:877–9.
Roivainen A, Forsback S, Gronroos T, Lehikoinen P, Kahkonen M,
Sutinen E, et al. Blood metabolism of [methyl-11C]choline; implications for in vivo imaging with positron emission tomography. Eur J Nucl
Med 2000;27:25–32.
Sumimoto H, Imabayashi F, Iwata T, Kawakami Y. The BRAF-MAPK
signaling pathway is essential for cancer-immune evasion in human
melanoma cells. J Exp Med 2006;203:1651–6.
Ratnam S, Kent C. Early increase in choline kinase activity upon
induction of the H-ras oncogene in mouse fibroblast cell lines. Arch
Biochem Biophys 1995;323:313–22.
Huang SC, Yu DC, Barrio JR, Grafton S, Melega WP, Hoffman JM, et al.
Kinetics and modeling of L-6-[18F]fluoro-dopa in human positron
emission tomographic studies. J Cereb Blood Flow Metab 1991;
11:898–913.
Tomasi G, Bertoldo A, Bishu S, Unterman A, Smith CB, Schmidt KC.
Voxel-based estimation of kinetic model parameters of the L-[1-(11)
C]leucine PET method for determination of regional rates of cerebral
protein synthesis: validation and comparison with region-of-interestbased methods. J Cereb Blood Flow Metab 2009;29:1317–31.
Clinical Cancer Research
Downloaded from clincancerres.aacrjournals.org on March 3, 2014. © 2012 American Association for Cancer Research.