Induction of thymidine kinase 1 after 5-fluorouracil as a mechanism for 3'-deoxy-3'-[18F]fluorothymidine flare

Exploration of Imaging Biomarkers for Metabolically-Targeted Osteosarcoma Therapy in a Murine Xenograft Model

Background: Osteosarcoma (OS) is an aggressive pediatric cancer with unmet therapeutic needs. Glutaminase 1 (GLS1) inhibition, alone and in combination with metformin, disrupts the bioenergetic demands of tumor progression and metastasis, showing promise for clinical translation. Materials and Methods: Three positron emission tomography (PET) clinical imaging agents, [18F]fluoro-2-deoxy-2-D-glucose ([18F]FDG), 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT), and (2S, 4R)-4-[18F]fluoroglutamine ([18F]GLN), were evaluated in the MG63.3 human OS xenograft mouse model, as companion imaging biomarkers after treatment for 7 d with a selective GLS1 inhibitor (CB-839, telaglenastat) and metformin, alone and in combination. Imaging and biodistribution data were collected from tumors and reference tissues before and after treatment. Results: Drug treatment altered tumor uptake of all three PET agents. Relative [18F]FDG uptake decreased significantly after telaglenastat treatment, but not within control and metformin-only groups. [18F]FLT tumor uptake appears to be negatively affected by tumor size. Evidence of a flare effect was seen with [18F]FLT imaging after treatment. Telaglenastat had a broad influence on [18F]GLN uptake in tumor and normal tissues. Conclusions: Image-based tumor volume quantification is recommended for this paratibial tumor model. The performance of [18F]FLT and [18F]GLN was affected by tumor size. [18F]FDG may be useful in detecting telaglenastat's impact on glycolysis. Exploration of kinetic tracer uptake protocols is needed to define clinically relevant patterns of [18F]GLN uptake in patients receiving telaglenastat.

Establishment, functional and genetic characterization of three novel patient-derived rectal cancer cell lines

AIM

To establish patient-individual tumor models of rectal cancer for analyses of novel biomarkers, individual response prediction and individual therapy regimens.

METHODS

Establishment of cell lines was conducted by direct in vitro culturing and in vivo xenografting with subsequent in vitro culturing. Cell lines were in-depth characterized concerning morphological features, invasive and migratory behavior, phenotype, molecular profile including mutational analysis, protein expression, and confirmation of origin by DNA fingerprint. Assessment of chemosensitivity towards an extensive range of current chemotherapeutic drugs and of radiosensitivity was performed including analysis of a combined radio- and chemotherapeutic treatment. In addition, glucose metabolism was assessed with ¹⁸F-fluorodeoxyglucose (FDG) and proliferation with ¹⁸F-fluorothymidine.

RESULTS

We describe the establishment of ultra-low passage rectal cancer cell lines of three patients suffering from rectal cancer. Two cell lines (HROC126, HROC284Met) were established directly from tumor specimens while HROC239 T0 M1 was established subsequent to xenografting of the tumor. Molecular analysis classified all three cell lines as CIMP-0/ non-MSI-H (sporadic standard) type. Mutational analysis revealed following mutational profiles: HROC126: APC^wt, TP53^wt, KRAS^wt, BRAF^wt, PTEN^wt; HROC239 T0 M1: APC^mut, P53^wt, KRAS^mut, BRAF^wt, PTEN^mut and HROC284Met: APC^wt, P53^mut, KRAS^mut, BRAF^wt, PTEN^mut. All cell lines could be characterized as epithelial (EpCAM⁺) tumor cells with equivalent morphologic features and comparable growth kinetics. The cell lines displayed a heterogeneous response toward chemotherapy, radiotherapy and their combined application. HROC126 showed a highly radio-resistant phenotype and HROC284Met was more susceptible to a combined radiochemotherapy than HROC126 and HROC239 T0 M1. Analysis of ¹⁸F-FDG uptake displayed a markedly reduced FDG uptake of all three cell lines after combined radiochemotherapy.

CONCLUSION

These newly established and in-depth characterized ultra-low passage rectal cancer cell lines provide a useful instrument for analysis of biological characteristics of rectal cancer.

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.

Thymidine Metabolism as a Confounding Factor for 3'-Deoxy-3'-18F-Fluorothymidine Uptake After Therapy in a Colorectal Cancer Model

Noninvasive monitoring of tumor therapy response helps in developing personalized treatment strategies. Here, we performed sequential PET and diffusion-weighted MRI to evaluate changes induced by a FOLFOX-like combination chemotherapy in colorectal cancer xenografts, to identify the cellular and molecular determinants of these imaging biomarkers. Methods: Tumor-bearing CD1 nude mice, engrafted with FOLFOX-sensitive Colo205 colorectal cancer xenografts, were treated with FOLFOX (5-fluorouracil, leucovorin, and oxaliplatin) weekly. On days 1, 2, 6, 9, and 13 of therapy, tumors were assessed by in vivo imaging and ex vivo analyses. In addition, HCT116 xenografts, which did not respond to the FOLFOX treatment, were imaged on day 1 of therapy. Results: In Colo205 xenografts, FOLFOX induced a profound increase in uptake of the proliferation PET tracer 3'-deoxy-3'-18F-fluorothymidine (18F-FLT) accompanied by increases in markers for proliferation (Ki-67, thymidine kinase 1) and for activated DNA damage response (γH2AX), whereas the effect on cell death was minimal. Because tracer uptake was unaltered in the HCT116 model, these changes appear to be specific for tumor response. Conclusion: We demonstrated that 18F-FLT PET can noninvasively monitor cancer treatment-induced molecular alterations, including thymidine metabolism and DNA damage response. The cellular or imaging changes may not, however, be directly related to therapy response as assessed by volumetric measurements.

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.

Using Radiolabeled 3'-Deoxy-3'-18F-Fluorothymidine with PET to Monitor the Effect of Dexamethasone on Non-Small Cell Lung Cancer

Non-small cell lung cancer (NSCLC) is a leading cause of cancer mortality in the United States, and pemetrexed-based therapies are regularly used to treat nonsquamous NSCLC. Despite widespread use, pemetrexed has a modest effect on progression-free survival, with varying efficacy between individuals. Recent work has indicated that dexamethasone, given to prevent pemetrexed toxicity, is able to protect a subset of NSCLC cells from pemetrexed cytotoxicity by temporarily suppressing the S phase of the cell cycle. Therefore, dexamethasone might block treatment efficacy in a subpopulation of patients and might be contributing to the variable response to pemetrexed. Methods: Differences in retention of the experimental PET tracer 3'-deoxy-3'-fluorothymidine (FLT) were used to monitor S-phase suppression by dexamethasone in NSCLC cell models, animals with tumor xenografts, and patients with advanced cancer. Results: Significant reductions in tracer uptake were observed after 24 h of dexamethasone treatment in NSCLC cell lines and xenograft models expressing high levels of glucocorticoid receptor α, coincident with pemetrexed resistance visualized by attenuation of the flare effect associated with pemetrexed activity. Two of 4 patients imaged in a pilot study with ¹⁸F-FLT PET after dexamethasone treatment demonstrated reductions in tracer uptake from baseline, with a variable response between individual tumor lesions. Conclusion: ¹⁸F-FLT PET represents a useful method for the noninvasive monitoring of dexamethasone-mediated S-phase suppression in NSCLC and might provide a way to individualize chemotherapy in patients receiving pemetrexed-based regimens.

Novel 99mTc-labelled complexes with thymidine isocyanide: radiosynthesis and evaluation as potential tumor imaging tracers

A novel thymidine isocyanide (CN-TdR) functionalized at the N3 position of thymidine was synthesized and then radiolabelled with 99mTc(i) and [99mTc(i)(CO)3]+ cores to produce [99mTc(CN-TdR)6]+ and [99mTc(CO)3(CN-TdR)3]+, respectively. Both of them were prepared with high radiochemical purity and were stable over 6 h in saline at ambient temperature and in serum at 37 °C. The partition coefficient results demonstrated that they were hydrophilic. The cell internalization studies showed that their uptake might be mediated by nucleoside transporters. Biodistribution of these complexes in mice bearing the S180 tumor showed that they accumulated in the tumor with high uptake and cleared rapidly from blood and muscles, producing high tumor/blood and tumor/muscle ratios. Between them, [99mTc(CN-TdR)6]+ exhibited advantages concerning a higher tumor uptake, tumor/blood ratio and tumor/muscle ratio at 60 min post-injection. Single photon emission computed tomography imaging studies showed that there was a clear accumulation in tumor sites, suggesting that [99mTc(CN-TdR)6]+ could be a promising candidate for tumor imaging.

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.

[18F]fluorothymidine PET Informs the Synergistic Efficacy of Capecitabine and Trifluridine/Tipiracil in Colon Cancer

In cancer therapy, enhanced thymidine uptake by the salvage pathway can bypass dTMP depletion, thereby conferring resistance to thymidylate synthase inhibition. We investigated whether sequential combination therapy of capecitabine and trifluridine/tipiracil (TAS-102) could synergistically enhance antitumor efficacy in colon cancer xenograft models. We also examined 3'-deoxy-3'-[¹⁸F]fluorothymidine ([¹⁸F]FLT) PET as a means to predict therapeutic response to a sequential combination of capecitabine and trifluridine/tipiracil. [³H]FLT uptake after 5-fluorouracil treatment in vitro and [¹⁸F]FLT uptake after capecitabine (360 mg/kg/day) in athymic nude mice (Balb/c-nu) with xenografts (n = 10-12 per group) were measured using eight human colon cancer cell lines. We determined the synergistic effects of sequential combinations of 5-fluorouracil and trifluridine in vitro as well as the sequential combination of oral capecitabine (30-360 mg/kg) and trifluridine/tipiracil (trifluridine 75 or 150 mg/kg with tipiracil) in six xenograft models (n = 6-10 per group). We observed significant increases in [³H]FLT uptake in all cell lines and [¹⁸F]FLT uptake in five xenograft models after 5-fluorouracil and capecitabine treatment, respectively. Increased [¹⁸F]FLT uptake after capecitabine followed by extinction of uptake correlated strongly with tumor growth inhibition (ρ = -0.81, P = 0.02). The effects of these combinations were synergistic in vitro A synergy for sequential capecitabine and trifluridine/tipiracil was found only in mouse xenograft models showing increased [¹⁸F]FLT uptake after capecitabine. Our results suggest that the sequential combination of capecitabine and trifluridine/tipiracil is synergistic in tumors with an activated salvage pathway after capecitabine treatment in mice, and [¹⁸F]FLT PET imaging may predict the response to capecitabine and the synergistic antitumor efficacy of a sequential combination of capecitabine and trifluridine/tipiracil. Cancer Res; 77(24); 7120-30. ©2017 AACR.

Evaluation of [18F]Fluorothymidine as a Biomarker for Early Therapy Response in a Mouse Model of Colorectal Cancer

PURPOSE

In oncology, positron emission tomography imaging using dedicated tracers as biomarkers may assist in early evaluation of therapy efficacy. Using 3'-deoxy-3'-[¹⁸F]fluorothymidine ([¹⁸F]FLT), we investigated the early effects of chemotherapeutic treatment on cancer cell proliferation in a BRAF-mutated colorectal cancer xenograft model.

PROCEDURES

Colo205 subcutaneously inoculated animals underwent 90-min dynamic imaging before and 24 h after treatment with vehicle (control), cetuximab (resistant) or irinotecan (sensitive). Total distribution volume was quantified from dynamic data, and standardized uptake values as well as tumor-to-blood ratios were calculated from static images averaged over the last 20 min. In vivo imaging data was correlated with ex vivo proliferation and thymidine metabolism proteins.

RESULTS

All imaging parameters showed a significant post-treatment decrease from [¹⁸F]FLT baseline uptake for the irinotecan group (p ≤ 0.001) as compared with the cetuximab and vehicle group and correlated strongly with each other (p ≤ 0.0001). In vivo data were in agreement with Ki67 staining, showing a significantly lower percentage of Ki67-positive cells in the irinotecan group as compared with other groups (p ≤ 0.0001). Tumor expression of thymidine kinase 1 phosphorylated on serine 13, thymidylate synthase, and thymidine phosphorylase remained unaffected, while thymidine kinase 1 expression was, surprisingly, significantly higher in irinotecan-treated animals (p ≤ 0.01). In contrast, tumor ATP levels were lowest in this group.

CONCLUSIONS

[¹⁸F]FLT positron emission tomography was found to be a suitable biomarker of early tumor response to anti-proliferative treatment, with static imaging not being inferior to full compartmental analysis in our xenograft model. The dynamics of thymidine kinase 1 protein expression and protein activity in low ATP environments merits further investigation.

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.

Cetuximab Immunoliposomes Enhance Delivery of 5-FU to Skin Squamous Carcinoma Cells

BACKGROUND

Topical chemotherapy of skin cancers is a promising strategy for reduction of side effects and for improvement of patient compliance. The combination of the chemotherapeutic 5-fluouracil (5-FU) and the anti- EGFR antibody cetuximab is a strategy to inhibit tumor growth. Their skin penetration, however, is hampered by their high hydrophilicity, which could be improved by encapsulation in delivery systems. Furthermore, it is a challenge to encapsulate hydrophilic drugs. The conjugation of an antibody to a liposome, maintaining its activity, is also a difficult task.

OBJECTIVE

Thus, we aimed to develop 5-FU liposomes and cetuximab-conjugated liposomes (immunoliposomes) of 5- FU to improve drug cytotoxicity against skin cancer cells.

METHOD

We characterized them by particle size, zeta potential, loading efficiency and antibody integrity. To optimize the loading efficiency of 5-FU, a series of liposomes were prepared, using different methods and drug-to-lipid ratios.

RESULTS

Liposomes containing DSPC and Chol at drug-to-lipid ratio 0.1 prepared by the thin lipid hydration method resulted in the best 5-FU encapsulation and were chosen to conjugate with cetuximab. Cetuximab was directly coupled to preformed liposomes using DSPE-mPEG2000-Mal as an anchor. In A431 skin carcinoma cells, at 72 h, 5-FU liposomes showed a 5-fold lower IC50 than 5-FU solution. Additionally, 5-FU immunoliposomes resulted in a 4-fold lower cetuximab IC50 than cetuximab solution, demonstrating synergism with a combination index lower than 1 and potential to improve 5-FU and cetuximab cytotoxicity.

CONCLUSION

Liposomes and immunoliposomes containing 5-FU were developed and cetuximab remained active as demonstrated in cell culture studies.

Gemcitabine Mechanism of Action Confounds Early Assessment of Treatment Response by 3'-Deoxy-3'-[18F]Fluorothymidine in Preclinical Models of Lung Cancer

3'-Deoxy-3'-[18F]fluorothymidine positron emission tomography ([18F]FLT-PET) and diffusion-weighted MRI (DW-MRI) are promising approaches to monitor tumor therapy response. Here, we employed these two imaging modalities to evaluate the response of lung carcinoma xenografts in mice after gemcitabine therapy. Caliper measurements revealed that H1975 xenografts responded to gemcitabine treatment, whereas A549 growth was not affected. In both tumor models, uptake of [18F]FLT was significantly reduced 6 hours after drug administration. On the basis of the gemcitabine concentration and [18F]FLT excretion measured, this was presumably related to a direct competition of gemcitabine with the radiotracer for cellular uptake. On day 1 after therapy, [18F]FLT uptake was increased in both models, which was correlated with thymidine kinase 1 (TK1) expression. Two and 3 days after drug administration, [18F]FLT uptake as well as TK1 and Ki67 expression were unchanged. A reduction in [18F]FLT in the responsive H1975 xenografts could only be noted on day 5 of therapy. Changes in ADCmean in A549 xenografts 1 or 2 days after gemcitabine did not seem to be of therapy-related biological relevance as they were not related to cell death (assessed by caspase-3 IHC and cellular density) or tumor therapy response. Taken together, in these models, early changes of [18F]FLT uptake in tumors reflected mechanisms, such as competing gemcitabine uptake or gemcitabine-induced thymidylate synthase inhibition, and only reflected growth-inhibitory effects at a later time point. Hence, the time point for [18F]FLT-PET imaging of tumor response to gemcitabine is of crucial importance. Cancer Res; 76(24); 7096-105. ©2016 AACR.

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.

Effects of capecitabine treatment on the uptake of thymidine analogs using exploratory PET imaging agents: 18F-FAU, 18F-FMAU, and 18F-FLT

Background

A principal goal for the use of positron emission tomography (PET) in oncology is for real-time evaluation of tumor response to chemotherapy. Given that many contemporary anti-neoplastic agents function by impairing cellular proliferation, it is of interest to develop imaging modalities to monitor these pathways. Here we examined the effect of capecitabine on the uptake of thymidine analogs used with PET: 3’-deoxy-3’-[¹⁸F]fluorothymidine (¹⁸F-FLT), 1-(2’-deoxy-2’-[¹⁸F]fluoro-β-D-arabinofuranosyl) thymidine (¹⁸F-FMAU), and 1-(2’-deoxy-2’-[¹⁸F]fluoro-β-D-arabinofuranosyl) uracil (¹⁸F-FAU) in patients with advanced cancer.

Methods

Fifteen patients were imaged, five with each imaging agent. Patients had been previously diagnosed with breast, colorectal, gastric, and esophageal cancers and had not received therapy for at least 4 weeks prior to the first scan, and had not been treated with any prior fluoropyrimidines. Subjects were imaged within a week before the start of capecitabine and on the second day of treatment, after the third dose of capecitabine. Tracer uptake was quantified by mean standard uptake value (SUV_mean) and using kinetic analysis.

Results

Patients imaged with ¹⁸F-FLT showed variable changes in retention and two patients exhibited an increase in SUV_mean of 172.3 and 89.9 %, while the other patients had changes ranging from +19.4 to -25.4 %. The average change in ¹⁸F-FMAU retention was 0.2 % (range -24.4 to 23.1) and ¹⁸F-FAU was -10.2 % (range -40.3 to 19.2). Observed changes correlated strongly with SUV_max but not kinetic measurements.

Conclusions

This pilot study demonstrates that patients treated with capecitabine can produce a marked increase in ¹⁸F-FLT retention in some patients, which will require further study to determine if this flare is predictive of therapeutic response. ¹⁸F-FAU and ¹⁸F-FMAU showed little change, on average, after treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s40644-016-0092-2) contains supplementary material, which is available to authorized users.

18F-FLT Positron Emission Tomography (PET) is a Pharmacodynamic Marker for EWS-FLI1 Activity and Ewing Sarcoma

Ewing sarcoma is a bone and soft-tissue tumor that depends on the activity of the EWS-FLI1 transcription factor for cell survival. Although a number of compounds have been shown to inhibit EWS-FLI1 in vitro, a clinical EWS-FLI1-directed therapy has not been achieved. One problem plaguing drug development efforts is the lack of a suitable, non-invasive, pharmacodynamic marker of EWS-FLI1 activity. Here we show that ¹⁸F-FLT PET (¹⁸F- 3′-deoxy-3′-fluorothymidine positron emission tomography) reflects EWS-FLI1 activity in Ewing sarcoma cells both in vitro and in vivo. ¹⁸F-FLT is transported into the cell by ENT1 and ENT2, where it is phosphorylated by TK1 and trapped intracellularly. In this report, we show that silencing of EWS-FLI1 with either siRNA or small-molecule EWS-FLI1 inhibitors suppressed the expression of ENT1, ENT2, and TK1 and thus decreased ¹⁸F-FLT PET activity. This effect was not through a generalized loss in viability or metabolic suppression, as there was no suppression of ¹⁸F-FDG PET activity and no suppression with chemotherapy. These results provide the basis for the clinical translation of ¹⁸F-FLT as a companion biomarker of EWS-FLI1 activity and a novel diagnostic imaging approach for Ewing sarcoma.

Positron emission tomography imaging of human colon cancer xenografts in mice with [18F]fluorothymidine after TAS-102 treatment

PURPOSE

TAS-102 is an orally administered anticancer agent composed of α,α,α-trifluorothymidine (FTD) and thymidine phosphorylase inhibitor (TPI). This study assessed 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) uptake after TAS-102 administration.

METHODS

The human colorectal carcinoma cell lines HCT116, HT29, HCT8 and SW620 were exposed to FTD for 2 h, further incubated for 0, 2 and 24 h, and assayed for [(3)H]FLT uptake, nucleoside transport, thymidine kinase 1 (TK1) expression and TK1 activity. Static and 2-h dynamic [(18)F]FLT positron emission tomography (PET) was performed in mice bearing HT29 or SW620 tumours orally administered with vehicle or TAS-102.

RESULTS

FTD decreased the viability of all cell lines, whereas increased [(3)H]FLT uptake (P < 0.05). Increased nucleoside transport and/or TK1 expression were observed 24 h after FTD, but not in 0-2 h. Static [(18)F]FLT PET in mice bearing HT29 tumours showed accumulation of [(18)F]FLT in tumours 1 h (day 1) after TAS-102. Two-hour dynamic PET in mice bearing SW620 tumours showed increased influx constant and volume of distribution of phosphorylated [(18)F]FLT on days 1 and 8 (P < 0.05) after TAS-102 with decreased dephosphorylation on day 1 (P < 0.001). Ex vivo studies showed that SW620 tumours after TAS-102 had higher TK1 expression than those with vehicle on days 8 and 15.

CONCLUSION

TAS-102 administration induces an increase in [(18)F]FLT uptake. Mechanisms may involve decreased dephosphorylation of [(18)F]FLT phosphate early after TAS-102 administration. Increased TK1 expression and/or nucleoside transporter may be related to increased [(18)F]FLT uptake at a later time. [(18)F]FLT PET has a potential to assess the pharmacodynamics of TAS-102 in cancer patients.

Imaging biomarkers in primary brain tumours

We are getting used to referring to instrumentally detectable biological features in medical language as "imaging biomarkers". These two terms combined reflect the evolution of medical imaging during recent decades, and conceptually comprise the principle of noninvasive detection of internal processes that can become targets for supplementary therapeutic strategies. These targets in oncology include those biological pathways that are associated with several tumour features including independence from growth and growth-inhibitory signals, avoidance of apoptosis and immune system control, unlimited potential for replication, self-sufficiency in vascular supply and neoangiogenesis, acquired tissue invasiveness and metastatic diffusion. Concerning brain tumours, there have been major improvements in neurosurgical techniques and radiotherapy planning, and developments of novel target drugs, thus increasing the need for reproducible, noninvasive, quantitative imaging biomarkers. However, in this context, conventional radiological criteria may be inappropriate to determine the best therapeutic option and subsequently to assess response to therapy. Integration of molecular imaging for the evaluation of brain tumours has for this reason become necessary, and an important role in this setting is played by imaging biomarkers in PET and MRI. In the current review, we describe most relevant techniques and biomarkers used for imaging primary brain tumours in clinical practice, and discuss potential future developments from the experimental context.

Assessment of tumor response to chemotherapy in patients with breast cancer using (18)F-FLT: a meta-analysis

PURPOSE

To determine the diagnostic performance of 3'-deoxy-3'-(18)F-fluorothymidine positron emission tomography/computed tomography (FLT PET/CT) and FLT PET for evaluating response to chemotherapy in patients with breast cancer.

METHODS

Databases such as PubMed (MEDLINE included) and excerpta medica database (EMBASE), were searched for relevant original articles. The included studies were assessed for methodological quality with quality assessment of diagnosis accuracy studies (QUADAS) score tool. Histopathological analysis and/or clinical and/or radiological follow-up for at least 6 months were used as the reference standard. The data were extracted by two reviewers independently to analyze the sensitivity, specificity, summary receiver operating characteristic (SROC) curve, area under the curve (AUC), and heterogeneity.

RESULTS

The present study analyzed a total of 4 selected articles. The pool sensitivity was 0.773 [95% confidence interval (CI): 0.594-0.900]. The pooled specificity was 0.685 (95% CI: 0.479-0.849) on basis of FEM. The pooled LR+, LR-, and DOR were 2.874 (1.492-5.538), 0.293 (0.146-0.589), and 14.891 (3.238-68.475), respectively. The AUC was 0.8636 (±0.0655), and the Q* index was 0.7942 (±0.0636).

CONCLUSIONS

Our results indicate that (18)F-FLT PET/CT or PET is useful to predict chemotherapy response in breast cancer with reasonable sensitivity, specificity and DOR. However, future larger scale clinical trials will be needed to assess the regimen of (18)F-FLT PET/CT or PET in monitoring the response to chemotherapy in breast cancer patients.

Serum thymidine kinase 1 activity in solid tumor (breast and colorectal cancer) patients treated with adjuvant chemotherapy

BACKGROUND

The aim of this study was to evaluate the changing of TK1 (where TK is thymidine kinase) activity before and after adjuvant chemotherapy in patients with breast and colorectal cancer.

METHODS

The study included 16 breast cancer, 25 colorectal cancer, and 38 healthy volunteers as the control group. Blood samples were taken twice from each patient; first at the beginning of the chemotherapy and second after six cycles of chemotherapy. TK1 activity was measured enzyme immunoassay method.

RESULTS

The mean TK1 activity in the breast and colorectal cancer was significantly higher than the controls. TK1 activity in the colorectal cancer was higher than the breast cancer but this difference was not significant. TK1 activity after six doses of chemotherapy was lower than baseline TK1 activity before the start of chemotherapy in breast and colorectal cancer. TK1 activity was positively correlated with CA15-3, before and after chemotherapy in patients with breast cancer. TK1 activity in the colorectal cancer was also positively correlated with CA19-9, before and after chemotherapy. The values for the cutoff point, sensitivity, specificity, and the area under curve were determined for TK1 as >44.36 Du/L, 68.29%, 100% and 0.819, respectively in all subjects.

CONCLUSION

Our results showed that serum TK1 activity in patients with breast and colorectal cancer was significantly higher than that of the healthy controls. Moreover, after the completion of chemotherapy the values were lower than baseline. Pretreatment TK1 activity should be considered as a useful marker for assessment tumor cell proliferation in breast and colorectal cancer. Further work is needed to understand TK1 activity better in large populations of patients with solid tumor.

Imaging of treatment response to the combination of carboplatin and paclitaxel in human ovarian cancer xenograft tumors in mice using FDG and FLT PET

Introduction

A combination of carboplatin and paclitaxel is often used as first line chemotherapy for treatment of ovarian cancer. Therefore the use of imaging biomarkers early after initiation of treatment to determine treatment sensitivity would be valuable in order to identify responders from non-responders. In this study we describe the non-invasive PET imaging of glucose uptake and cell proliferation using 2-deoxy-2-[¹⁸F]fluoro-D-glucose (FDG) and 3’-deoxy-3’-[¹⁸F]fluorothymidine (FLT) for early assessment of treatment response in a pre-clinical mouse model of human ovarian cancer treated with carboplatin and paclitaxel.

Methods

Invivo uptake of FLT and FDG in human ovarian cancer xenografts in mice (A2780) was determined before treatment with carboplatin and paclitaxel (CaP) and repeatedday 1, 4 and 8 after treatment start. Tracer uptake was quantified using small animal PET/CT. Tracer uptake was compared with gene expression of Ki67, TK1, GLUT1, HK1 and HK2.

Results

Tumors in the CaP group was significantly smaller than in the control group (p=0.03) on day 8. On day 4 FDG SUVmax ratio was significantly lower in the CaP group compared to the control group (105±4% vs 138±9%; p=0.002) and on day 8 the FDG SUVmax ratio was lower in the CaP compared to the control group (125±13% vs 167±13%; p=0.05). On day 1 the uptake of FLT SUVmax ratio was 89±9% in the CaP group and 109±6% in the control group; however the difference was not statistically significant (p=0.08).

Conclusions

Our data suggest that both FDG and FLT PET may be used for the assessment of anti-tumor effects of a combination of carboplatin and paclitaxel in the treatment of ovarian cancer. FLT provides an early and transient signal and FDG a later and more prolonged response. This underscores the importance of optimal timing between treatment and FLT or FDG imaging since treatment response may otherwise be overlooked.

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.

3'-Deoxy-3'-18F-fluorothymidine PET for the early prediction of response to leucovorin, 5-fluorouracil, and oxaliplatin therapy in patients with metastatic colorectal cancer

The aim of this study was to evaluate 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET for early prediction of the standard anatomic response and survival outcomes in patients with metastatic colorectal cancer (mCRC) receiving leucovorin, 5-fluorouracil (5-FU), and oxaliplatin (FOLFOX).

METHODS

The main eligibility criteria included histologically confirmed mCRC, ≥ 1 extrahepatic measurable lesions, and no prior chemotherapy in a metastatic setting. Chemotherapy consisted of leucovorin on day 1, followed by the continuous infusion of 5-FU on days 1 and 2, and oxaliplatin on day 3. In the second and subsequent cycles of chemotherapy, oxaliplatin was administered simultaneously with leucovorin on day 1. (18)F-FLT PET scans were obtained 3 times during the first cycle of chemotherapy: before chemotherapy, 24 h after infusion of 5-FU (day 2), and 48 h after completion of chemotherapy (day 5). The maximum standardized uptake value (SUVMAX) of (18)F-FLT was measured. Treatment responses were assessed by CT after 3 cycles of FOLFOX.

RESULTS

Eighteen patients were included in the study. The response rate after 3 cycles of FOLFOX was 27.8% (5/18). The SUVMAX was increased in responders (P = 0.043) and nonresponders (P < 0.001) on day 2 and was decreased, compared with baseline values, on day 5 in responders only (P = 0.043). Receiver-operating-characteristic curve analysis indicated that the use of a threshold of an SUVMAX increase on day 2 of ≤ 45.8% resulted in a sensitivity of 100%, specificity of 69.2%, and relative risk of 2.250 (P = 0.029) for the diagnosis of responders. Use of a threshold of an SUVMAX decrease on day 5 of ≥ 10.6% resulted in a sensitivity of 100%, specificity of 76.9%, and relative risk of 2.667 (P = 0.007). Patients with low (18)F-FLT flare tended to have longer survivals than patients with high flare (2-y overall survival rate, 77.8% vs. 44.4%; P = 0.051).

CONCLUSION

The (18)F-FLT flare observed during 5-FU infusion was associated with poor treatment response in patients with mCRC. The degree of (18)F-FLT flare might be used to predict the outcome of patients who receive infusional 5-FU-based chemotherapy.

Wheat germ agglutinin-functionalised crosslinked polyelectrolyte microparticles for local colon delivery of 5-FU: in vitro efficacy and in vivo gastrointestinal distribution

We have previously reported the development and characterisation of wheat germ agglutinin (WGA)-functionalised chitosan-Ca-alginate (CTS-Ca-ALG) microparticles (MPs) loaded with acid-resistant particles of 5-fluorouracil (5-FU). In the present work, our goal was to evaluate the potential of these carriers for efficient treatment of colon cancer by studying in vitro permeability and cell association of 5-FU and [methyl-³H]thymidine uptake in Caco-2 cells, as well as in vivo gastrointestinal distribution. The amount of 5-FU permeated through Caco-2 cells was 15.1, 7.7 and 6.5% for 5-FU solution, CTS-Ca-ALG MPs and WGA conjugates. The concentration of 5-FU associated with Caco-2 cells was significantly greater when delivered from MPs. By incorporation of 5-FU into MPs and further decoration with WGA, an increased [methyl-³H]thymidine uptake was observed few hours after continuous drug treatment followed by significantly reduced uptake after 6 h. Gastrointestinal distribution was in favour of increased localisation and concentration of the particles in colon region.

Molecular imaging for monitoring treatment response in breast cancer patients

Currently, tumour response following drug treatment is based on measurement of anatomical size changes. This is often done according to Response Evaluation Criteria in Solid Tumours (RECIST) and is generally performed every 2-3 cycles. Bone metastases, being the most common site of distant metastases in breast cancer, are not measurable by RECIST. The standard response measurement provides no insight in changes of molecular characteristics. In the era of targeted medicine, knowledge of specific molecular tumour characteristics becomes more important. A potential way to assess this is by means of molecular imaging. Molecular imaging can visualise general tumour processes, such as glucose metabolism with (18)F-fluorodeoxyglucose ((18)F-FDG) and DNA synthesis with (18)F-fluorodeoxythymidine ((18)F-FLT). In addition, an increasing number of more specific targets, such as hormone receptors, growth factor receptors, and growth factors can be visualised. In the future molecular imaging may thus be of value for personalised treatment-selection by providing insight in the expression of these drug targets. Additionally, when molecular changes can be detected early during therapy, this may serve as early predictor of response. However, in order to define clinical utility of this approach results from (ongoing) clinical trials is required. In this review we summarise the potential role of molecular imaging of general tumour processes as well as hormone receptors, growth factor receptors, and tumour micro-environment for predicting and monitoring treatment response in breast cancer patients.

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

3'-Fluoro-3'-deoxythymidine (FLT) positron emission tomography (PET) has been proposed for imaging thymidylate synthase (TS) inhibition. Agents that target TS and shut down de novo synthesis of thymidine monophosphate increase the uptake and retention of FLT in vitro and in vivo because of a compensating increase in the salvage pathway. Increases in both thymidine kinase-1 (TK1) and the equilibrative nucleoside transporter hENT1 have been reported to underlie this effect. We examined whether the effects of one TS inhibitor, 5-fluorouracil (5FU), on FLT uptake require proliferating cells and whether the effects are limited to increasing TK1 activity.

METHODS

The effects of 5FU on FLT transport and metabolism, TK1 activity, and cell cycle progression were evaluated in the human tumor cell line, A549, maintained as either a proliferating or non-proliferating culture.

RESULTS

There were dose-dependent increases in FLT uptake that peaked after a 10 μM 5FU exposure and then declined to baseline levels or below at higher doses in both proliferating and non-proliferating cultures. The dose-dependence for FLT uptake was mirrored by changes in TK1 activity. S phase fraction did not correlate with FLT uptake in proliferating cultures. Chemical inhibition of hENT1 reduced overall levels of FLT uptake but did not affect the low dose increase in FLT uptake.

CONCLUSIONS

5FU only affects FLT uptake in proliferating A549 cells and increases in FLT uptake are directly related to increased TK1 activity. Our studies did not support a role for hENT1 in the increased uptake of FLT after exposure to 5FU. Our studies with A549 cells support the suggestion that FLT-PET could provide a measure of TS inhibition in vivo.

Combining small interfering RNAs targeting thymidylate synthase and thymidine kinase 1 or 2 sensitizes human tumor cells to 5-fluorodeoxyuridine and pemetrexed

Thymidylate synthase (TS) is the only de novo source of thymidylate (dTMP) for DNA synthesis and repair. Drugs targeting TS protein are a mainstay in cancer treatment, but off-target effects and toxicity limit their use. Cytosolic thymidine kinase (TK1) and mitochondrial thymidine kinase (TK2) contribute to an alternative dTMP-producing pathway, by salvaging thymidine from the tumor milieu, and may modulate resistance to TS-targeting drugs. Combined down-regulation of these enzymes is an attractive strategy to enhance cancer therapy. We have shown previously that antisense-targeting TS enhanced tumor cell sensitivity to TS-targeting drugs in vitro and in vivo. Because both TS and TKs contribute to increased cellular dTMP, we hypothesized that TKs mediate resistance to the capacity of TS small interfering RNA (siRNA) to sensitize tumor cells to TS-targeting anticancer drugs. We assessed the effects of targeting TK1 or TK2 with siRNA alone and in combination with siRNA targeting TS and/or TS-protein targeting drugs on tumor cell proliferation. Down-regulation of TK with siRNA enhanced the capacity of TS siRNA to sensitize tumor cells to traditional TS protein-targeting drugs [5-fluorodeoxyuridine (5FUdR) and pemetrexed]. The sensitization was greater than that observed in response to any siRNA used alone and was specific to drugs targeting TS. Up-regulation of TK1 in response to combined 5FUdR and TS siRNA suggests that TK knockdown may be therapeutically useful in combination with these agents. TKs may be useful targets for cancer therapy when combined with molecules targeting TS mRNA and TS protein.