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

A Subset of Non-Small Cell Lung Cancer Patients Treated with Pemetrexed Show 18F-Fluorothymidine "Flare" on Positron Emission Tomography

Thymidylate synthase (TS) remains a major target for cancer therapy. TS inhibition elicits increases in DNA salvage pathway activity, detected as a transient compensatory "flare" in 3'-deoxy-3'-[18F]fluorothymidine positron emission tomography (18F-FLT PET). We determined the magnitude of the 18F-FLT flare in non-small cell lung cancer (NSCLC) patients treated with the antifolate pemetrexed in relation to clinical outcome.

METHOD

Twenty-one patients with advanced/metastatic non-small cell lung cancer (NSCLC) scheduled to receive palliative pemetrexed ± platinum-based chemotherapy underwent 18F-FLT PET at baseline and 4 h after initiating single-agent pemetrexed. Plasma deoxyuridine (dUrd) levels and thymidine kinase 1 (TK1) activity were measured before each scan. Patients were then treated with the combination therapy. The 18F-FLT PET variables were compared to RECIST 1.1 and overall survival (OS).

RESULTS

Nineteen patients had evaluable PET scans at both time points. A total of 32% (6/19) of patients showed 18F-FLT flares (>20% change in SUVmax-wsum). At the lesion level, only one patient had an FLT flare in all the lesions above (test-retest borders). The remaining had varied uptake. An 18F-FLT flare occurred in all lesions in 1 patient, while another patient had an 18F-FLT reduction in all lesions; 17 patients showed varied lesion uptake. All patients showed global TS inhibition reflected in plasma dUrd levels (p < 0.001) and 18F-FLT flares of TS-responsive normal tissues including small bowel and bone marrow (p = 0.004 each). Notably, 83% (5/6) of patients who exhibited 18F-FLT flares were also RECIST responders with a median OS of 31 m, unlike patients who did not exhibit 18F-FLT flares (15 m). Baseline plasma TK1 was prognostic of survival but its activity remained unchanged following treatment.

CONCLUSIONS

The better radiological response and longer survival observed in patients with an 18F-FLT flare suggest the efficacy of the tracer as an indicator of the early therapeutic response to pemetrexed in NSCLC.

Association of expression and genotypes of thymidylate synthase in non-small cell lung cancer patients with different clinicopathological characteristics

Objective

To explore the expression and genotypes of thymidylate synthase (TS) in patients of non-small cell lung cancer (NSCLC) with different clinicopathological characteristics.

Methods

The expression profiles of TS were examined by immunohistochemical staining and quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) in 160 patients with NSCLC. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was used to detect TS-5′UTR tandem repeats, G/C nucleotide polymorphisms, and 3′UTR 6 bp deletion/insertion polymorphisms. The relationships between clinicopathological characteristics and TS expression or genotypes were investigated through χ 2 test. Kaplan–Meier survival analysis was used to analyze the association between TS expression and overall survival (OS) and disease-free survival (DFS) of NSCLC patients.

Results

The expression levels of TS protein and TS gene in NSCLC tissues were significantly higher than that in paracancerous tissues (P < 0.05). Furthermore, high expression of TS protein and 5′UTR polymorphism of TS gene showed significant correlation with differentiation, TNM stage, and lymph node metastases. The frequency of −6 bp/−6 bp genotypes in patients with NSCLC was 43.13% (69/160), which was higher than others. In addition, the rate of TS protein overexpression in NSCLC patients with 3R/3R was 79.79%, which was higher than others. Interestingly, high expression of TS protein predicted shorter DFS and OS and lower 3-year DFS rate and 3-year OS rate.

Conclusions

The expression levels of TS in NSCLC were significantly increased and may help to predict the prognosis of NSCLC, and high expression of TS protein and 5′UTR polymorphism of TS gene were significantly related to differentiation, TNM stage, and lymph node metastases.

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 kinase 1 as a tumor biomarker: technical advances offer new potential to an old biomarker

Thymidine kinase 1 (TK1) is a key enzyme in DNA precursor synthesis. It is upregulated during the S phase of the cell cycle and its presence in cells is an indicator of active cell proliferation. In studies since the 1980s, TK1 has been shown as a clinically valuable biomarker for the management of hematological malignancies. However, TK1 activity assays may underestimate serum TK1 in subjects with solid tumors limiting its sensitivity. The development of TK1 immunoassays has made the assay of TK1 more widely available and increased its applicability to solid tumor diseases. This paper will review TK1 as a tumor biomarker with emphasis on recent studies and technologies plus highlight its potential in drug discovery and as a therapeutic target.

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 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.

Approaches and techniques to characterize cancer metabolism in vitro and in vivo

Cancer metabolism is wired to sustain uncontrollable cell proliferation and ensure cell survival. Given the multitude of available approaches to study metabolic alterations it remains a challenging task to select the most appropriate method. In this mini-review we describe how cancer metabolism can be studied in vitro and in vivo providing an overview of available approaches and techniques, discussing their advantages and drawbacks and guiding through selection of an appropriate method to address particular research needs. This work is particularly intended to those cancer researchers who are new in the field but want to investigate metabolic alterations in their cancer model systems.