The effects of 5-fluoruracil treatment on 3'-fluoro-3'-deoxythymidine (FLT) transport and metabolism in proliferating and non-proliferating cultures of human tumor cells

Dexamethasone pretreatment impairs the thymidylate synthase inhibition mediated flare in thymidine salvage pathway activity in non-small cell lung cancer

INTRODUCTION

Successful inhibition of thymidylate synthase (TS) by pemetrexed, a TS inhibitor, results in a reproducible transient burst or "flare" in thymidine salvage pathway activity at 2 hrs. of therapy which can be measurable with FLT-PET ([18F]fluorothymidine-positron emission tomography) in non-small cell lung cancer (NSCLC). Routine administration of dexamethasone with pemetrexed-based therapy could potentially confound this imaging approach since dexamethasone is known to inhibit expression of thymidine kinase 1, a key enzyme in the thymidine salvage pathway. Here we examine the potential impact of dexamethasone on the TS inhibition-mediated thymidine salvage pathway "flare" in NSCLC.

MATERIALS AND METHODS

In order to determine NSCLC cell line sensitivity to dexamethasone and pemetrexed, IC50 studies were performed on NSCLC cell lines H23, H1975, H460, H1299. TS inhibition-mediated "flare" in thymidine salvage pathway activity was then measured at 2hrs. of exposure to pemetrexed and cisplatin in NSCLC cells lines following using 3H-thymidine incorporation assays under the following conditions: control (no chemotherapy or dexamethasone), or treated with pemetrexed and cisplatin without dexamethasone, with 24 hrs. pre-treatment of dexamethasone or with dexamethasone administered together with chemotherapy. These conditions were chosen to model the delivery of pemetrexed-based therapy in the clinic.

RESULTS

The IC50 of H23, H1975, H460, H1299 for dexamethasone and pemetrexed were 40, 5.9, 718, 362 μM and 0.22, 0.73, 0.14 and 0.66 μM respectively. Significant blunting of the thymidine salvage pathway "flare" is observed at 2hrs. of pemetrexed-based therapy when dexamethasone sensitive cell lines H23 and H1975 were pretreated with dexamethasone but not when dexamethasone was given together with pemetrexed therapy or in the setting of dexamethasone resistance (H460 and H1299).

CONCLUSION

24 hr. pretreatment with dexamethasone, but not same day co-administration of dexamethasone with therapy, impairs the TS inhibition-mediated "flare" in thymidine salvage pathway activity in NSCLC.

PET imaging for assessing tumor response to therapy

Positron emission tomography (PET) is a radioisotope imaging technique capable of quantifying the regional distribution of molecular imaging probes targeted to biochemical pathways and processes allowing direct measurement of biochemical changes induced by cancer therapy, including the activity of targeted growth pathways and cellular populations. In this manuscript, we review the underlying principles of PET imaging, choices for PET radiopharmaceuticals, methods for tumor analysis and PET applications for cancer therapy response assessment including potential future directions.

Early detection of pemetrexed-induced inhibition of thymidylate synthase in non-small cell lung cancer with FLT-PET imaging

Inhibition of thymidylate synthase (TS) results in a transient flare in DNA thymidine salvage pathway activity measurable with FLT ([¹⁸F]thymidine)-positron emission tomography (PET). Here we characterize this imaging strategy for potential clinical translation in non-small cell lung cancer (NSCLC). Since pemetrexed acts by inhibiting TS, we defined the kinetics of increases in thymidine salvage pathway mediated by TS inhibition following treatment with pemetrexed in vitro. Next, using a mouse model of NSCLC, we validated the kinetics of the pemetrexed-mediated flare in thymidine salvage pathway activity in vivo using FLT-PET imaging. Finally, we translated our findings into a proof-of-principle clinical trial of FLT-PET in a human NSCLC patient. In NSCLC cells in vitro, we identified a burst in pemetrexed-mediated thymidine salvage pathway activity, assessed by ³H-thymidine assays, thymidine kinase 1 (TK1) expression, and equilibrative nucleoside transporter 1 (ENT1) mobilization to the cell membrane, that peaked at 2hrs. This 2hr time-point was also optimal for FLT-PET imaging of pemetrexed-mediated TS inhibition in murine xenograft tumors and was demonstrated to be feasible in a NSCLC patient. FLT-PET imaging of pemetrexed-induced TS inhibition is optimal at 2hrs from therapy start; this timing is feasible in human clinical trials.

Early detection of thymidylate synthase resistance in non-small cell lung cancer with FLT-PET imaging

Introduction

Inhibition of thymidylate synthase (TS) results in a transient compensatory "flare" in thymidine salvage pathway activity measureable with ¹⁸F-thymidine (FLT)- positron emission tomography (PET) at 2hrs. of therapy which may predict non-small cell lung cancer (NSCLC) sensitivity to TS inhibition.

Materials and Methods

Resistance to TS inhibition by pemetrexed was induced in NSCLC cell lines H460 and H1299 through TS overexpression. TS overexpression was confirmed with RT-PCR and Western blotting and pemetrexed resistance confirmed with IC₅₀ assays. The presence of a pemetrexed-induced thymidine salvage pathway "flare" was then measured using ³H-thymidine in both pemetrexed sensitive (H460 and H1299) and resistant (H460R, H1299R, CALU-6, H522, H650, H661, H820, H1838) lines in vitro, and validated with FLT-PET in vivo using H460 and H460R xenografts.

Results

Overexpression of TS induced pemetrexed resistance with IC₅₀ for H460, H1299, H460R and H1299R measured as 0.141 μM, 0.656 μM, 22.842 μM, 213.120 μM, respectively. Thymidine salvage pathway ³H-thymidine "flare" was observed following pemetrexed in H460 and H1299 but not H460R, H1299R, CALU-6, H522, H650, H661, H820 or H1838 in vitro. Similarly, a FLT "flare" was observed in vivo following pemetrexed therapy in H460 but not H460R tumor-bearing xenografts.

Conclusions

Imaging of TS inhibition is predictive of NSCLC sensitivity to pemetrexed.

Response Monitoring with [18F]FLT PET and Diffusion-Weighted MRI After Cytotoxic 5-FU Treatment in an Experimental Rat Model for Colorectal Liver Metastases

PURPOSE

The aim of the study was to investigate the potential of diffusion-weighted magnetic resonance imaging (DW-MRI) and 3'-dexoy-3'-[18F]fluorothymidine ([18F]FLT) positron emission tomography (PET) as early biomarkers of treatment response of 5-fluorouracil (5-FU) in a syngeneic rat model of colorectal cancer liver metastases.

PROCEDURES

Wag/Rij rats with intrahepatic syngeneic CC531 tumors were treated with 5-FU (15, 30, or 60 mg/kg in weekly intervals). Before treatment and at days 1, 3, 7, and 14 after treatment rats underwent DW-MRI and [18F]FLT PET. Tumors were analyzed immunohistochemically for Ki67, TK1, and ENT1 expression.

RESULTS

5-FU inhibited the growth of CC531 tumors in a dose-dependent manner. Immunohistochemical analysis did not show significant changes in Ki67, TK1, and ENT1 expression. However, [18F]FLT SUVmean and SUVmax were significantly increased at days 4 and 7 after treatment with 5-FU (60 mg/kg) and returned to baseline at day 14 (SUVmax at days -1, 4, 7, and 14 was 1.1 ± 0.1, 2.3 ± 0.5, 2.3 ± 0.6, and 1.5 ± 0.4, respectively). No changes in [18F]FLT uptake were observed in the nontreated animals. Furthermore, the apparent diffusion coefficient (ADCmean) did not change in 5-FU-treated rats compared to untreated rats.

CONCLUSION

This study suggests that 5-FU treatment induces a flare in [18F]FLT uptake of responsive CC531 tumors in the liver, while the ADCmean did not change significantly. Future studies in larger groups are warranted to further investigate whether [18F]FLT PET can discriminate between disease progression and treatment response.

Dynamic PET evaluation of elevated FLT level after sorafenib treatment in mice bearing human renal cell carcinoma xenograft

Background

Sorafenib, an oral multikinase inhibitor, has anti-proliferative and anti-angiogenic activities and is therapeutically effective against renal cell carcinoma (RCC). Recently, we have evaluated the tumor responses to sorafenib treatment in a RCC xenograft using [Methyl-³H(N)]-3′-fluoro-3′-deoxythythymidine ([³H]FLT). Contrary to our expectation, the FLT level in the tumor significantly increased after the treatment. In this study, to clarify the reason for the elevated FLT level, dynamic 3′-[¹⁸F]fluoro-3′-deoxythymidine ([¹⁸F]FLT) positron emission tomography (PET) and kinetic studies were performed in mice bearing a RCC xenograft (A498).

The A498 xenograft was established in nude mice, and the mice were assigned to the control (n = 5) and treatment (n = 5) groups. The mice in the treatment group were orally given sorafenib (20 mg/kg/day p.o.) once daily for 3 days. Twenty-four hours after the treatment, dynamic [¹⁸F]FLT PET was performed by small-animal PET. Three-dimensional regions of interest (ROIs) were manually defined for the tumors. A three-compartment model fitting was carried out to estimate four rate constants using the time activity curve (TAC) in the tumor and the blood clearance rate of [¹⁸F]FLT.

Results

The dynamic pattern of [¹⁸F]FLT levels in the tumor significantly changed after the treatment. The rate constant of [¹⁸F]FLT phosphorylation (k₃) was significantly higher in the treatment group (0.111 ± 0.027 [1/min]) than in the control group (0.082 ± 0.009 [1/min]). No significant changes were observed in the distribution volume, the ratio of [¹⁸F]FLT forward transport (K₁) to reverse transport (k₂), between the two groups (0.556 ± 0.073 and 0.641 ± 0.052 [mL/g] in the control group).

Conclusions

Our dynamic PET studies indicated that the increase in FLT level may be caused by the phosphorylation of FLT in the tumor after the sorafenib treatment in the mice bearing a RCC xenograft. Dynamic PET studies with kinetic modeling could provide improved understanding of the biochemical processes involved in tumor responses to therapy.

Evaluation of arginine deiminase treatment in melanoma xenografts using (18)F-FLT PET

PURPOSE

This study aims to develop a molecular imaging strategy for response assessment of arginine deiminase (ADI) treatment in melanoma xenografts using 3'-[(18)F]fluoro-3'-deoxythymidine ([(18)F]-FLT) positron emission tomography (PET).

PROCEDURES

F-FLT response to ADI therapy was studied in preclinical models of melanoma in vitro and in vivo. The molecular mechanism of response to ADI therapy was investigated, with a particular emphasis on biological pathways known to regulate (18)F-FLT metabolism.

RESULTS

Proliferation of SK-MEL-28 melanoma tumors was potently inhibited by ADI treatment. However, no metabolic response was observed in FLT PET, presumably based on the known ADI-induced degradation of PTEN, followed by instability of the tumor suppressor p53 and a relative overexpression of thymidine kinase 1, the enzyme mainly responsible for intracellular FLT processing.

CONCLUSION

The specific pharmacological properties of ADI preclude using (18)F-FLT to evaluate clinical response in melanoma and argue for further studies to explore the use of other clinically applicable PET tracers in ADI treatment.

Increased intratumoral fluorothymidine uptake levels following multikinase inhibitor sorafenib treatment in a human renal cell carcinoma xenograft model

An early identification of the tumor response to sorafenib treatment is indispensable for selecting optimal personalized treatment strategies. However, at present, no reliable predictors are clinically available. ¹⁸F-fluorothymidine (¹⁸F-FLT) positron emission tomography (PET) is used to assess tumor proliferation, since the FLT uptake level reflects thymidine kinase-1 (TK-1) activity. Thus, the present study determined whether FLT was able to evaluate the early tumor response to sorafenib treatment in a human renal cell carcinoma (RCC; A498) xenograft in comparison with the tumor proliferation marker, Ki-67. Mice bearing A498 tumors were assigned to the control and sorafenib-treated groups and the tumor volume was measured every day. [Methyl-3H(N)]-3'-fluoro-3'-deoxythymidine (³H-FLT) was injected 2 h prior to the sacrifice of the mice on days three and seven following the treatment. ³H-FLT autoradiography (ARG) and Ki-67 immunohistochemistry (IHC) were performed using adjacent tumor sections. In the visual assessment, the intratumoral ³H-FLT uptake level diffusely increased following the treatment, while no significant changes were observed in Ki-67 IHC. The intratumoral ³H-FLT uptake levels significantly increased by 2.7- and 2.6-fold on days three and seven following the treatment, while the tumor volume and Ki-67 index did not significantly change. Thus, an increased FLT uptake level was demonstrated following the treatment, which may indicate the suppression of thymidylate synthase (TS) and the compensatory upregulation of TK-1 activity by sorafenib.

[18F]FDG and [18F]FLT positron emission tomography imaging following treatment with belinostat in human ovary cancer xenografts in mice

BACKGROUND

Belinostat is a histone deacetylase inhibitor with anti-tumor effect in several pre-clinical tumor models and clinical trials. The aim of the study was to evaluate changes in cell proliferation and glucose uptake by use of 3'-deoxy-3'-[(18)F]fluorothymidine ([18F]FLT) and 2-deoxy-2-[(18)F]fluoro-D-glucose ([18F]FDG) positron emission tomography (PET) following treatment with belinostat in ovarian cancer in vivo models.

METHODS

In vivo uptake of [18F]FLT and [18F]FDG in human ovary cancer xenografts in mice (A2780) were studied after treatment with belinostat. Mice were divided in 2 groups receiving either belinostat (40 mg/kg ip twice daily Day 0-4 and 6-10) or vehicle. Baseline [18F]FLT or [18F]FDG scans were made before treatment (Day 0) and repeated at Day 3, 6 and 10. Tracer uptake was quantified using small animal PET/CT.

RESULTS

Tumors in the belinostat group had volumes that were 462 ± 62% (640 mm(3)) at Day 10 relative to baseline which was significantly different (P = 0.011) from the control group 769 ± 74% (926 mm(3)). [18F]FLT SUVmax increased from baseline to Day 10 (+30 ± 9%; P = 0.048) in the control group. No increase was observed in the treatment group. [18F]FDG SUVmean was significantly different in the treatment group compared to the control group (P = 0.0023) at Day 10. Within treatment groups [18F]FDG uptake and to a lesser extent [18F]FLT uptake at Day 3 were significantly correlated with tumor growth at Day 10.

CONCLUSIONS

[18F]FDG uptake early following treatment initiation predicted tumor sizes at Day 10, suggesting that [18F]FDG may be a valuable biomarker for non-invasive assessment of anti-tumor activity of belinostat.