A chiral template-driven synthesis of a 3'-deoxy-3'-18F-fluorothymidine precursor

An asymmetric synthesis of a 3'-deoxy-3'-¹⁸F-fluorothymidine (¹⁸F-FLT) precursor has been developed wherein the deoxysugar moiety was synthesized using a novel Ga-mediated allylation of (R)-2,3-cyclohexylideneglyceraldehyde as the key step. The synthesis deviates significantly from the previous syntheses of the ¹⁸F-FLT precursors wherein the expensive starting material, thymidine was used.

IPET study: an FLT-PET window study to assess the activity of the steroid sulfatase inhibitor irosustat in early breast cancer

BACKGROUND

Steroid sulfatase (STS) is involved in oestrogen biosynthesis and irosustat is a first generation, irreversible steroid sulfatase inhibitor. A pre-surgical window-of-opportunity study with irosustat was undertaken in estrogen receptor-positive (ER+) breast cancer to assess the effect of irosustat on tumour cell proliferation as measured by 3'-deoxy-3'-[18F] fluorothymidine uptake measured by PET scanning (FLT-PET) and Ki67.

METHODS

Postmenopausal women with untreated ER+ early breast cancer were recruited, and imaged with FLT-PET at baseline and after at least 2 weeks treatment with irosustat, 40 mg once daily orally. The primary endpoint was changed in FLT uptake; secondary endpoints included safety and tolerability of irosustat, changes in tumoral Ki67 and steroidogenic enzymes expression and circulating steroid hormone levels.

RESULTS

Thirteen women were recruited, and ten started irosustat for 2 weeks, followed by repeat FLT-PET scans in eight. Defining response as decreases of ≥20% in standardized uptake value (SUV) or ≥30% in Ki, 1 (12.5% (95% CI 2-47%, p = 0.001)) and 3 (43% (95% CI 16-75%, p = <0.001) patients, respectively, responded. 6 out of 7 patients had a Ki67 reduction (range = -19.3 to 76.4%), and median percentage difference in Ki67 was 52.3% (p = 0.028). In one patient with a low baseline STS expression, a 19.7% increase in Ki67 was recorded. STS decreases were seen in tumours with high basal STS expression, significant decreases were also noted in aromatase, and 17β-hydroxysteroid dehydrogenase type 1 and 2. Irosustat was generally well tolerated with all adverse event CTCAE Grade ≤2.

CONCLUSIONS

Irosustat resulted in a significant reduction in FLT uptake and Ki67, and is well tolerated. These data are the first demonstrating clinical activity of irosustat in early breast cancer. Baseline expression of STS may be a biomarker of sensitivity to irosustat.

3'-Deoxy-3'-¹⁸F-fluorothymidine positron emission tomography as an early predictor of disease progression in patients with advanced and metastatic pancreatic cancer

PURPOSE

3'-Deoxy-3'-(18)F-fluorothymidine (FLT) positron emission tomography (PET) has limited utility in abdominal imaging due to high physiological hepatic uptake of tracer. We evaluated FLT PET/CT combined with a temporal-intensity information-based voxel-clustering approach termed kinetic spatial filtering (FLT PET/CTKSF) for early prediction of response and survival outcomes in locally advanced and metastatic pancreatic cancer patients receiving gemcitabine-based chemotherapy.

METHODS

Dynamic FLT PET/CT data were collected before and 3 weeks after the first cycle of chemotherapy. Changes in tumour FLT PET/CT variables were determined. The primary end point was RECIST 1.1 response on contrast-enhanced CT after 3 months of therapy.

RESULTS

Twenty patients were included. Visual distinction between tumours and normal pancreas was seen in FLT PETKSF images. All target lesions (>2 cm), including all primary pancreatic tumours, were visualised. Of the 11 liver metastases, 3 (<2 cm) were not visible after kinetic filtering. Of the 20 patients, 7 progressed (35%). Maximum standardised uptake value at 60 min post-injection (SUV60,max) significantly increased in patients with disease progression (p = 0.04). Receiver-operating characteristic curve analysis indicated that a threshold of SUV60,max increase of ≥ 12% resulted in sensitivity, specificity and positive predictive value (PPV) of 71, 100 and 100%, respectively [area under the curve (AUC) 0.90, p = 0.0001], to predict patients with disease progression. Changes in SUV60,max were not predictive of survival.

CONCLUSION

FLT PET/CT detected changes in proliferation, with early increase in SUV60,max predicting progressive disease with a high specificity and PPV. Therefore, FLT PET/CT could be used as an early response biomarker for gemcitabine-based chemotherapy, to select a poor prognostic group who may benefit from novel therapeutic agents in advanced and metastatic pancreatic cancer.

Evaluation of FLT-PET-CT as an imaging biomarker of proliferation in primary breast cancer

BACKGROUND

[(18)F]fluorothymidine (FLT) has been proposed as a positron emission tomography (PET)-imaging biomarker of proliferation for breast cancer. The aim of this prospective study was to assess the feasibility of FLT-PET-CT as a technique for predicting the response to neoadjuvant chemotherapy (NAC) in primary breast cancer and to compare baseline FLT with Ki-67.

METHODS

Twenty women with primary breast cancer had a baseline FLT-PET-CT scan that was repeated before the second cycle of chemotherapy. Expression of Ki-67 in the diagnostic biopsy was quantified. From the FLT-PET-CT scans lesion maximum and mean standardised uptake values (SUVmax, SUVmean) were calculated.

RESULTS

Mean baseline SUVmax was 7.3, and 4.62 post one cycle of NAC, representing a drop of 2.68 (36.3%). There was no significant association between baseline, post chemotherapy, or change in SUVmax and pathological response to NAC. There was a significant correlation between pre-chemotherapy Ki-67 and SUVmax of 0.604 (P=0.006).

CONCLUSIONS

Baseline SUVmax measurements of FLT-PET-CT were significantly related to Ki-67 suggesting that it is a proliferation biomarker. However, in this series neither the baseline value nor the change in SUVmax after one cycle of NAC were able to predict response as most patients had a sizeable SUVmax reduction.

Parametric imaging of ¹⁸F-fluoro-3-deoxy-3-L-fluorothymidine PET data to investigate tumour heterogeneity

PURPOSE

[(18)F]Fluoro-3'-deoxy-3'-L-fluorothymidine ([(18)F]FLT) is a tissue proliferation marker which has been widely validated as a tumour-specific imaging tracer for PET. [(18)F]FLT uptake in breast cancer is generally quantified at the region level or through first-order statistical descriptors (mean or maximum value), approaches that ignore the known complexity and heterogeneity of cancer tissues. Our aims were: (1) to validate a robust and reproducible voxel-wise approach to the quantification of [(18)F]FLT PET data in breast cancer patients, and (2) to exploit the entire distribution of the [(18)F]FLT retention estimates and their variability in the tumour region for the prediction of early treatment response.

METHODS

The dataset was derived from 15 patients with stage II-IV breast cancer, scanned twice before chemotherapy and once 1 week after therapy. Using RECIST criteria (after 60 days) nine patients were categorized as responders or nonresponders to treatment. Kinetic modelling (compartmental modelling, Patlak analysis and spectral analysis with iterative filter), tissue-to-plasma ratio and standardized uptake value were applied at the voxel level. Test-retest estimates were used to assess reproducibility and reliability of the [(18)F]FLT uptake values before and after therapy for responder/nonresponder prediction.

RESULTS

All the methods provided a measure of [(18)F]FLT uptake that was reliable and reproducible with ICC >0.94. Moreover, a very strong correlation was found among the methods (R (2) > 0.81). All the methods provided a limited number of outliers (<20 % in tumour), with the exception of compartmental modelling (>25 %) which was therefore excluded from the prediction analysis. Differences between before and after therapy in mean voxel-wise uptake in tumour did not allow a complete responder/nonresponder classification. In contrast, considering the full estimate distributions within the tumour (changes in median and mode between before and after therapy) improved therapy response for all the analysed methods.

CONCLUSION

We showed that kinetic modelling (Patlak and spectral analysis with iterative filter) applied voxel-wise allows appropriate [(18)F]FLT uptake estimation in breast cancer with good reproducibility. Notably, this study indicated that a more comprehensive statistical investigation could improve tumour characterization and prediction of treatment response.

Imaging of cellular proliferation in liver metastasis by [18F]fluorothymidine positron emission tomography: effect of therapy

Although [(18)F]fluorothymidine positron emission tomography (FLT-PET) permits estimation of tumor thymidine kinase-1 expression, and thus, cell proliferation, high physiological uptake of tracer in liver tissue can limit its utility. We evaluated FLT-PET combined with a temporal-intensity information-based voxel-clustering approach termed kinetic spatial filtering (FLT-PET(KSF)) for detecting drug response in liver metastases. FLT-PET and computed tomography data were collected from patients with confirmed breast or colorectal liver metastases before, and two weeks after the first cycle of chemotherapy. Changes in tumor FLT-PET and FLT-PET(KSF) variables were determined. Visual distinction between tumor and normal liver was seen in FLT-PET(KSF) images. Of the 33 metastases from 20 patients studied, 26 were visible after kinetic filtering. The net irreversible retention of the tracer (Ki; from unfiltered data) in the tumor, correlated strongly with tracer uptake when the imaging variable was an unfiltered average or maximal standardized uptake value, 60 min post-injection (SUV(60,av): r = 0.9, SUV(60,max): r = 0.7; p < 0.0001 for both) and occurrence of high intensity voxels derived from FLT-PET(KSF) (r = 0.7, p < 0.0001). Overall, a significant reduction in the imaging variables was seen in responders compared to non-responders; however, the two week time point selected for imaging was too early to allow prediction of long term clinical benefit from chemotherapy. FLT-PET and FLT-PET(KSF) detected changes in proliferation in liver metastases.

Redistribution of nucleoside transporters to the cell membrane provides a novel approach for imaging thymidylate synthase inhibition by positron emission tomography

Thymidylate synthase (EC 2.1.1.45) is a key enzyme for the de novo synthesis of DNA and as such a target for anticancer drug development. There is a need to develop noninvasive methods for assessing thymidylate synthase inhibition in tumors. The aim of this study was to assess the potential of 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) positron emission tomography (PET) for early measurement of thymidylate synthase inhibition and to elucidate the cellular mechanisms involved. Radiation-induced fibrosarcoma-1 tumor-bearing mice were injected with a single i.p. dose of the thymidylate synthase inhibitor 5-fluorouracil (5-FU; 165 mg/kg) and imaged by [(18)F]FLT-PET at 1 to 2 hours after treatment. Deoxyuridine, thymidine kinase 1 (cytoplasmic thymidine kinase; EC2.7.1.21), and ATP levels in excised tumors were measured. Cellular assays for membrane transport were also done. There was a 1.8-fold increase in the 60-minute [(18)F]FLT tumor/heart radioactivity ratio in drug-treated mice compared with vehicle controls (P = 0.0016). Plasma and tumor deoxyuridine levels increased significantly but thymidine kinase and ATP levels were unchanged. Whole-cell assays implicated a (low level) functional role for the type-1 equilibrative nucleoside transporter (ENT). There was an increase in type-1 ENT-binding sites per cell from 49,110 in untreated cells to 73,142 (P = 0.03) in cells treated with 10 microg/mL 5-FU for 2 hours, without a change in transporter affinity (P = 0.41). We conclude that [(18)F]FLT-PET can be used to measure thymidylate synthase inhibition as early as 1 to 2 hours after treatment with 5-FU by a mechanism involving redistribution of nucleoside transporters to the plasma membrane.

Purification of [18F]-fluoro-l-thymidine ([18F]-FLT) for positron emission tomography imaging

3'-Deoxy-3'-[18F]fluorothymidine ([18F]-FLT) has recently been described as a positron emission tomography (PET) radiopharmaceutical for visualizing cellular proliferation in vivo. All published radiosyntheses of [18F]-FLT provide crude products that must be purified before injection to human. This study describes a reliable purification procedure for [18F]-FLT. It is based on HPLC. In 17.9+/-0.5 min, pure [18F]-FLT is obtained that could be injected to human after a passage through a sterile 0.22 microm filter.

Imaging early changes in proliferation at 1 week post chemotherapy: a pilot study in breast cancer patients with 3′-deoxy-3′-[18F]fluorothymidine positron emission tomography

PURPOSE

3'-deoxy-3'-[18F]fluorothymidine positron emission tomography ([18F]FLT-PET) has been developed for imaging cell proliferation and findings correlate strongly with the Ki-67 labelling index in breast cancer. The aims of this pilot study were to define objective criteria for [18F]FLT response and to examine whether [18F]FLT-PET can be used to quantify early response of breast cancer to chemotherapy.

METHODS

Seventeen discrete lesions in 13 patients with stage II-IV breast cancer were scanned prior to and at 1 week after treatment with combination 5-fluorouracil, epirubicin and cyclophosphamide (FEC) chemotherapy. The uptake at 90 min (SUV90) and irreversible trapping (Ki) of [18F]FLT were calculated for each tumour. The reproducibility of [18F]FLT-PET was determined in nine discrete lesions from eight patients who were scanned twice before chemotherapy. Clinical response was assessed at 60 days after commencing FEC.

RESULTS

All tumours showed [18F]FLT uptake and this was reproducible in serial measurements (SD of mean % difference=10.5% and 15.1%, for SUV90 and Ki, respectively; test-retest correlation coefficient>or=0.97). Six patients had a significant clinical response (complete or partial) at day 60; these patients also had a significant reduction in [18F]FLT uptake at 1 week. Decreases in Ki and SUV90 at 1 week discriminated between clinical response and stable disease (p=0.022 for both parameters). In three patients with multiple lesions there was a mixed [18F]FLT response in primary tumours and metastases. [18F]FLT response generally preceded tumour size changes.

CONCLUSION

[18F]FLT-PET can detect changes in breast cancer proliferation at 1 week after FEC chemotherapy.

Quantifying the Activity of Adenoviral E1A CR2 Deletion Mutants Using Renilla Luciferase Bioluminescence and 3′-Deoxy-3′-[18F]Fluorothymidine Positron Emission Tomography Imaging

The adenoviral E1A CR2 mutant dl922-947 has potent activity in ovarian cancer. We have used Renilla luciferase bioluminescence imaging to monitor viral E1A expression and replication and [18F]fluorothymidine positron emission tomography ([18F]FLT-PET) to quantify the activity of dl922-947 in vivo. We created dlCR2 Ren, with the same E1A CR2 deletion as dl922-947 and the luciferase gene from Renilla reniformis downstream of E1. Light emitted from s.c. and i.p. IGROV1 ovarian carcinoma xenografts was measured following treatment with dlCR2 Ren. Mice bearing s.c. IGROV1 xenografts were injected with 2.96 to 3.7 MBq of [18F]FLT 48 and 168 hours following i.t. injection of dl922-947 or control virus Ad LM-X. The presence of Renilla luciferase in dlCR2 Ren did not reduce in vitro nor in vivo potency compared with dl922-947. Light emission correlated closely with E1A expression in vitro and peaked 48 hours after dlCR2 Ren injection in both s.c. and i.p. IGROV1 xenografts. It diminished by 168 hours in s.c. tumors but persisted for at least 2 weeks in i.p. models. Normalized tumor [18F]FLT uptake at 60 minutes (NUV60), fractional retention, and area under radioactivity curve all decreased marginally 48 hours after dl922-947 treatment and significantly at 168 hours compared with controls. There was a close linear correlation between NUV60 and both tumor proliferation (Ki67 labeling index) and thymidine kinase 1 expression. Renilla luciferase bioluminescence and [18F]FLT-PET imaging are capable of quantifying the activity and effectiveness of E1A CR2-deleted adenoviral mutants in ovarian cancer.

In vivoBiological Activity of the Histone Deacetylase Inhibitor LAQ824 Is detectable with 3′-Deoxy-3′-[18F]Fluorothymidine Positron Emission Tomography

Histone deacetylase inhibitors (HDACI) are emerging as growth inhibitory compounds that modulate gene expression and inhibit tumor cell proliferation. We assessed whether 3'-deoxy-3'-[(18)F]fluorothymidine-positron emission tomography ([18F]FLT-PET) could be used to noninvasively measure the biological activity of a novel HDACI LAQ824 in vivo. We initially showed that thymidine kinase 1 (TK1; EC2.7.1.21), the enzyme responsible for [18F]FLT retention in cells, was regulated by LAQ824 in a drug concentration-dependent manner in vitro. In HCT116 colon carcinoma xenograft-bearing mice, LAQ824 significantly decreased tumor [18F]FLT uptake in a dose-dependent manner. At day 4 of treatment, [18F]FLT tumor-to-heart ratios at 60 minutes (NUV60) were 2.16 +/- 0.15, 1.86 +/- 0.13, and 1.45 +/- 0.20 in vehicle, and 5 and 25 mg/kg LAQ824 treatment groups, respectively (P < or = 0.05). LAQ825 at 5 mg/kg also significantly reduced both TK1 levels and [18F]FLT uptake at day 10 but not at day 2 (P < or = 0.05). [18F]FLT NUV60 correlated significantly with cellular proliferation (r = 0.68; P = 0.0019) and was associated with drug-induced histone H4 hyperacetylation. Of interest to [18F]FLT-PET imaging, both TK1 mRNA copy numbers and protein levels decreased in the order vehicle >5 mg/kg LAQ824 > 25 mg/kg LAQ824, providing a rationale for the use of [18F]FLT-PET in this setting. We also observed increases in Rb hypophosphorylation and p21 levels, factors that could have contributed to the alteration in TK1 transcription in vivo. In conclusion, we have shown the utility of [18F]FLT-PET for monitoring the biological activity of the HDACI, LAQ824. Drug-induced changes in tumor [18F]FLT uptake were due, at least in part, to reductions in TK1 transcription and translation.

Semiautomatic synthesis of 3′-deoxy-3′-[18F]fluorothymidine using three precursors

To facilitate clinical studies with [18F]FLT, we modified 2-vessel [18F]FDG synthesis module (manufactured by CTI) to produce [18F]FLT. Three thymidine derivatives were used as precursors for [18F]FLT synthesis. Among these precursors, 3-N-t-butoxycarbonyl-[5'-O-(4,4'-dimethoxytrityl)-2'-deoxy-3'-O-(4-nitrobenzenesulfonyl)-beta-D-threopentofuranosyl]thymine (thymidine derivative II) gave the best radiochemical yield (37.9%) when the reaction was carried out at 140 degrees C for 5 min. This semiautomatic synthesis system was not only simple and convenient, but also showed good reproducibility. The total synthesis time was 50 minutes from the end of bombardment (EOB) by the use of this modified synthesizer (including the manual process).

Quantification of Cellular Proliferation in Tumor and Normal Tissues of Patients with Breast Cancer by [18F]Fluorothymidine-Positron Emission Tomography Imaging: Evaluation of Analytical Methods

There is an unmet need to develop imaging methods for the early and objective assessment of breast tumors to therapy. 3'-Deoxy-3'-[18F]fluorothymidine ([18F]FLT)-positron emission tomography represents a new approach to imaging thymidine kinase activity, and hence, cellular proliferation. We compared graphical, spectral, and semiquantitative analytic methodologies for quantifying [18F]FLT kinetics in tumor and normal tissue of patients with locally advanced and metastatic breast cancer. The resultant kinetic parameters were correlated with the Ki-67 labeling index from tumor biopsies. [18F]FLT accumulation was detected in primary tumor, nodal disease, and lung metastasis. In large tumors, there was substantial heterogeneity in regional radiotracer uptake, reflecting heterogeneity in cellular proliferation; radiotracer uptake in primary tumors also differed from that of metastases. [18F]FLT was metabolized in patients to a single metabolite [18F]FLT-glucuronide. Unmetabolized [18F]FLT accounted for 71.54 +/- 1.50% of plasma radioactivity by 90 minutes. The rate constant for the metabolite-corrected net irreversible uptake of [18F]FLT (Ki) ranged from 0.6 to 10.4 x 10(-4) and from 0 to 0.6 x 10(-4) mL plasma cleared/s/mL tissue in tumor (29 regions, 15 patients) and normal tissues, respectively. Tumor Ki and fractional retention of radiotracer determined by spectral analysis correlated with Ki-67 labeling index (r = 0.92, P < 0.0001 and r = 0.92, P < 0.0001, respectively). These correlations were superior to those determined by semiquantitative methods. We conclude that [18F]FLT-positron emission tomography is a promising clinical tool for imaging cellular proliferation in breast cancer, and is most predictive when analyzed by graphical and spectral methods.

Early detection of tumor response to chemotherapy by 3'-deoxy-3'-[18F]fluorothymidine positron emission tomography: the effect of cisplatin on a fibrosarcoma tumor model in vivo

We have assessed the potential of [18F]fluorothymidine positron emission tomography ([18F]FLT-PET) to measure early cytostasis and cytotoxicity induced by cisplatin treatment of radiation-induced fibrosarcoma 1 (RIF-1) tumor-bearing mice. Cisplatin-mediated arrest of tumor cell growth and induction of tumor shrinkage at 24 and 48 hours, respectively, were detectable by [18F]FLT-PET. At 24 and 48 hours, the normalized uptake at 60 minutes (tumor/liver radioactivity ratio at 60 minutes after radiotracer injection; NUV60) for [18F]FLT was 0.76 +/- 0.08 (P = 0.03) and 0.51 +/- 0.08 (P = 0.03), respectively, compared with controls (1.02 +/- 0.12). The decrease in [18F]FLT uptake at 24 hours was associated with a decrease in cell proliferation assessed immunohistochemically (a decrease in proliferating cell nuclear antigen labeling index, LI(PCNA), from 14.0 +/- 2.0% to 6.2 +/- 1.0%; P = 0.001), despite the lack of a change in tumor size. There were G1-S and G2-M phase arrests after cisplatin treatment, as determined by cell cycle analysis. For the quantitative measurement of tumor cell proliferation, [18F]FLT-PET was found to be superior to [18F]fluorodeoxyglucose-PET (NUV60 versus LIPCNA: r = 0.89, P = 0.001 and r = 0.55, P = 0.06, respectively). At the biochemical level, we found that the changes in [18F]FLT and [18F]fluorodeoxyglucose uptake were due to changes in levels of thymidine kinase 1 protein, hexokinase, and ATP. This work supports the further development of [18F]FLT-PET as a generic pharmacodynamic readout for early quantitative imaging of drug-induced changes in cell proliferation in vivo.

[18F]FLT-PET in oncology: current status and opportunities

In recent years, [18F]-fluoro-3'-deoxy-3'-L: -fluorothymidine ([18F]FLT) has been developed as a proliferation tracer. Imaging and measurement of proliferation with PET could provide us with a non-invasive staging tool and a tool to monitor the response to anticancer treatment. In this review, the basis of [18F]FLT as a proliferation tracer is discussed. Furthermore, an overview of the current status of [18F]FLT-PET research is given. The results of this research show that although [18F]FLT is a tracer that visualises cellular proliferation, it also has certain limitations. In comparison with the most widely used PET tracer, [18F]FDG, [18F]FLT uptake is lower in most cases. Furthermore, [18F]FLT uptake does not always reflect the tumour cell proliferation rate, for example during or shortly after certain chemotherapy regimens. The opportunities provided by, and the limitations of, [18F]FLT as a proliferation tracer are addressed in this review, and directions are given for further research, taking into account the strong and weak points of the new tracer.

The uptake of 3?-deoxy-3?-[18F]fluorothymidine into L5178Y tumours in vivo is dependent on thymidine kinase 1 protein levels

PURPOSE

The aim of this study was to investigate the role of thymidine kinase 1 (TK1) protein in 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) positron emission tomography (PET) studies.

METHODS

We investigated the in vivo kinetics of [18F]FLT in TK1+/- and TK1-/- L5178Y mouse lymphoma tumours that express different levels of TK1 protein.

RESULTS

[18F]FLT-derived radioactivity, measured by a dedicated small animal PET scanner, increased within the tumours over 60 min. The area under the normalised tumour time-activity curve were significantly higher for the TK1+/- compared with the -/- variant (0.89+/-0.02 vs 0.79+/-0.03 MBq ml(-1) min, P=0.043; n=5 for each tumour type). Ex vivo gamma counting of tissues excised at 60 min p.i. (n=8) also revealed significantly higher tumour [18F]FLT uptake for the TK1+/- variant (6.2+/-0.6 vs 4.6+/-0.4%ID g(-1), P=0.018). The observed differences between the cell lines with respect to [18F]FLT uptake were in keeping with a 48% higher TK1 protein in the TK1+/- tumours versus the -/- variant (P=0.043). On average, there were no differences in ATP levels between the two tumour variants (P=1.00). A positive correlation between [18F]FLT accumulation and TK1 protein levels (r=0.68, P=0.046) was seen. Normalisation of the data for ATP content further improved the correlation (r=0.86, P=0.003).

CONCLUSION

This study shows that in vivo [18F]FLT kinetics depend on TK1 protein expression. ATP may be important in realising this effect. Thus, [18F]FLT-PET has the potential to yield specific information on tumour proliferation in diagnostic imaging and therapy monitoring.

Fully automated synthesis system of 3′-deoxy-3′-[18F]fluorothymidine

We developed a new fully automated method for the synthesis of 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT), by modifying a commercial FDG synthesizer and its disposable fluid pathway. Optimal labeling condition was that 40 mg of precursor in acetonitrile (2 mL) was heated at 150 degrees C for 100 sec, followed by heating at 85 degrees C for 450 sec and hydrolysis with 1 N HCl at 105 degrees C for 300 sec. Using 3.7 GBq of [18F]F- as starting activity, [18F]FLT was obtained with a yield of 50.5 +/- 5.2% (n = 28, decay corrected) within 60.0 +/- 5.4 min including HPLC purification. With 37.0 GBq, we obtained 48.7 +/- 5.6% (n = 10). The [18F]FLT showed the good stability for 6 h. This new automated synthesis procedure combines high and reproducible yields with the benefits of a disposable cassette system.

High radiochemical yield synthesis of 3'-deoxy-3'-[18F]fluorothymidine using (5'-O-dimethoxytrityl-2'-deoxy-3'-O-nosyl-beta-D-threo pentofuranosyl)thymine and its 3-N-BOC-protected analogue as a labeling precursor

We prepared 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) from 3'-O-nosyl thymidine derivative 3 or its pyrimidine ring N-BOC-protected analogue 5 and optimized [(18)F]fluorination condition for a high radiochemical yield. The optimal condition for [(18)F]fluorination with precursor 3 was 30 mg (41.1 micromol)/300 microl CH(3)CN at 130 degrees C for 5 min, while precursor 5 required 34 mg (40 micromol)/300 microl CH(3)CN at 110 degrees C for 5 min. After HPLC purification at neutral pH, we achieved high radiochemical yields of 40 +/- 5.2% and 42 +/- 5.4% (decay-corrected) within 60 min of preparation time with radiochemical purities of >97%.