Quantitation of plasma thymidine by high-performance liquid chromatography—atmospheric pressure chemical ionization mass spectrometry and its application to pharmacodynamic studies in cancer patients

STRATEGIES AND CHALLENGES IN METHOD DEVELOPMENT AND VALIDATION FOR THE ABSOLUTE QUANTIFICATION OF ENDOGENOUS BIOMARKER METABOLITES USING LIQUID CHROMATOGRAPHY-TANDEM MASS SPECTROMETRY

Metabolomics is a dynamically evolving field, with a major application in identifying biomarkers for drug development and personalized medicine. Numerous metabolomic studies have identified endogenous metabolites that, in principle, are eligible for translation to clinical practice. However, few metabolomic-derived biomarker candidates have been qualified by regulatory bodies for clinical applications. Such interruption in the biomarker qualification process can be largely attributed to various reasons including inappropriate study design and inadequate data to support the clinical utility of the biomarkers. In addition, the lack of robust assays for the routine quantification of candidate biomarkers has been suggested as a potential bottleneck in the biomarker qualification process. In fact, the nature of the endogenous metabolites precludes the application of the current validation guidelines for bioanalytical methods. As a result, there have been individual efforts in modifying existing guidelines and/or developing alternative approaches to facilitate method validation. In this review, three main challenges for method development and validation for endogenous metabolites are discussed, namely matrix effects evaluation, alternative analyte-free matrices, and the choice of internal standards (ISs). Some studies have modified the equations described by the European Medicines Agency for the evaluation of matrix effects. However, alternative strategies were also described; for instance, calibration curves can be generated in solvents and in biological samples and the slopes can be compared through ratios, relative standard deviation, or a modified Stufour suggested approaches while quantifying mainly endogenous metabolitesdent t-test. ISs, on the contrary, are diverse; in which seven different possible types, used in metabolomics-based studies, were identified in the literature. Each type has its advantages and limitations; however, isotope-labeled ISs and ISs created through isotope derivatization show superior performance. Finally, alternative matrices have been described and tested during method development and validation for the quantification of endogenous entities. These alternatives are discussed in detail, highlighting their advantages and shortcomings. The goal of this review is to compare, apprise, and debate current knowledge and practices in order to aid researchers and clinical scientists in developing robust assays needed during the qualification process of candidate metabolite biomarkers. © 2019 John Wiley & Sons Ltd. Mass Spec Rev.

3'-Deoxy-3'-[18F]Fluorothymidine Uptake Is Related to Thymidine Phosphorylase Expression in Various Experimental Tumor Models

PURPOSE

We recently reported that high thymidine phosphorylase (TP) expression is accompanied by low tumor thymidine concentration and high 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) uptake in four untreated lung cancer xenografts. Here, we investigated whether this relationship also holds true for a broader range of tumor models.

PROCEDURES

Lysates from n = 15 different tumor models originating from n = 6 institutions were tested for TP and thymidylate synthase (TS) expression using western blots. Results were correlated to [18F]FLT accumulation in the tumors as determined by positron emission tomography (PET) measurements in the different institutions and to previously published thymidine concentrations.

RESULTS

Expression of TP correlated positively with [18F]FLT SUVmax (ρ = 0.549, P < 0.05). Furthermore, tumors with high TP levels possessed lower levels of thymidine (ρ = - 0.939, P < 0.001).

CONCLUSIONS

In a broad range of tumors, [18F]FLT uptake as measured by PET is substantially influenced by TP expression and tumor thymidine concentrations. These data strengthen the role of TP as factor confounding [18F]FLT uptake.

Sex as a Biologic Variable in Preclinical Imaging Research: Initial Observations with 18F-FLT

The study objective was to investigate whether sex influences 3'-deoxy-3'-¹⁸F-fluorothymidine (¹⁸F-FLT) uptake and tissue distribution in mouse models of cancer. Methods:¹⁸F-FLT biodistribution was measured in 3 strains of male and female mice (129S6/SvEv, athymic nude, and BALB/c). ¹⁸F-FDG biodistribution was measured for comparison. ¹⁸F-FLT uptake was also measured in female 129S6/SvEv mice bearing estrogen-dependent SSM3 mouse mammary tumors, male athymic nude mice bearing androgen-dependent CWR22 prostate cancer xenografts, and male and female athymic nude mice bearing estrogen-independent MDA-MB-231 human breast cancer xenografts. Ki-67 expression was assayed by immunohistochemistry. PET/CT imaging was performed to visualize ¹⁸F-FLT biodistribution and to determine pharmacokinetics. Results: Greater ¹⁸F-FLT activity was observed in blood, liver, muscle, heart, kidney, and bone in female than male mice. Pharmacokinetic analysis demonstrated higher early renal ¹⁸F-FLT activity and greater accumulation of ¹⁸F-FLT in the urinary bladder in male than female mice. The differential pattern of ¹⁸F-FLT biodistribution between the sexes seen with ¹⁸F-FLT was not observed with ¹⁸F-FDG. Increased tumoral ¹⁸F-FLT uptake compared with muscle was observed in both the SSM3 mammary tumors (2.4 ± 0.17 vs. 1.6 ± 0.14 percentage injected dose [%ID]/g at 2 h after injection, P = 0.006) and the CWR22 prostate cancer xenografts (0.34 ± 0.08 vs. 0.098 ± 0.033 %ID/g at 2 h after injection, P = 0.03). However, because of higher nonspecific muscle uptake in female mice, tumor-to-muscle uptake ratios were greater for CWR22 tumors than for SSM3 tumors (4.2 ± 0.78 vs. 1.5 ± 0.049 at 2 h after injection, P = 0.008). Sex-dependent differences in ¹⁸F-FLT uptake were also observed for MDA-MB-231 xenografts (tumor-to-muscle ratio, 7.2 ± 0.9 for female vs. 16.9 ± 8.6 for male, P = 0.039). Conversely, greater tumoral Ki-67 staining was observed in female mice (71% ± 3% for female vs. 54% ± 2% for male, P = 0.009), and this finding more closely matched the relative differences in absolute ¹⁸F-FLT tumor uptake values (4.5 ± 0.99 %ID/g for female vs. 1.9 ± 0.30 %ID/g for male, P = 0.03). Conclusion: Depending on whether female or male mice are used, differences in biodistribution and nonspecific tissue uptake can adversely affect quantitative measures of ¹⁸F-FLT uptake. Thus, sex is a potential variable to consider in defining quantitative imaging metrics using ¹⁸F-FLT to assess tumor proliferation.

The relationship between endogenous thymidine concentrations and [(18)F]FLT uptake in a range of preclinical tumour models

BACKGROUND

Recent studies have shown that 3'-deoxy-3'-[(18)F] fluorothymidine ([(18)F]FLT)) uptake depends on endogenous tumour thymidine concentration. The purpose of this study was to investigate tumour thymidine concentrations and whether they correlated with [(18)F]FLT uptake across a broad spectrum of murine cancer models. A modified liquid chromatography-mass spectrometry (LC-MS/MS) method was used to determine endogenous thymidine concentrations in plasma and tissues of tumour-bearing and non-tumour bearing mice and rats. Thymidine concentrations were determined in 22 tumour models, including xenografts, syngeneic and spontaneous tumours, from six research centres, and a subset was compared for [(18)F]FLT uptake, described by the maximum and mean tumour-to-liver uptake ratio (TTL) and SUV.

RESULTS

The LC-MS/MS method used to measure thymidine in plasma and tissue was modified to improve sensitivity and reproducibility. Thymidine concentrations determined in the plasma of 7 murine strains and one rat strain were between 0.61 ± 0.12 μM and 2.04 ± 0.64 μM, while the concentrations in 22 tumour models ranged from 0.54 ± 0.17 μM to 20.65 ± 3.65 μM. TTL at 60 min after [(18)F]FLT injection, determined in 14 of the 22 tumour models, ranged from 1.07 ± 0.16 to 5.22 ± 0.83 for the maximum and 0.67 ± 0.17 to 2.10 ± 0.18 for the mean uptake. TTL did not correlate with tumour thymidine concentrations.

CONCLUSIONS

Endogenous tumour thymidine concentrations alone are not predictive of [(18)F]FLT uptake in murine cancer models.

PET imaging of tumor growth: not as easy as it looks

Positron emission tomography can be used to image tumor proliferation when combined with appropriate labeled tracers, such as the thymidine analog [(18)F]-3'-deoxy-3'-fluorothymidine. Although thymidine kinase 1 is the principal mechanism of cell trapping, other variables, such as the cellular level of native thymidine, may need to be considered.

[(18)F]FLT-PET imaging does not always "light up" proliferating tumor cells

PURPOSE

[(18)F]FLT (3'-Fluoro-3' deoxythymidine)-PET imaging was proposed as a tool for measuring in vivo tumor cell proliferation. The aim of this article was to validate the use of [(18)F]FLT-PET imaging for measuring xenograft proliferation and subsequent monitoring of targeted therapy.

EXPERIMENTAL DESIGN

In exponentially growing xenografts, factors that could impact the outcome of [(18)F]FLT-PET imaging, such as nucleoside transporters, thymidine kinase 1, the relative contribution of DNA salvage pathway, and the ratio of FLT to thymidine, were evaluated. The [(18)F]FLT tracer avidity was compared with other proliferation markers.

RESULTS

In a panel of proliferating xenografts, [(18)F]FLT or [(3)H]thymidine tracer avidity failed to reflect the tumor growth rate across different tumor types, despite the high expressions of Ki67 and TK1. When FLT was injected at the same dose level as used in the preclinical [(18)F]FLT-PET imaging, the plasma exposure ratio of FLT to thymidine was approximately 1:200. Thymidine levels in different tumor types seemed to be variable and exhibited an inverse relationship with the FLT tracer avidity. In contrast, high-dose administration of bromdeoxyuridine (BrdUrd; 50 mg/kg) yielded a plasma exposure of more than 4-fold higher than thymidine and leads to a strong correlation between the BrdUrd uptake and the tumor proliferation rate. In FLT tracer-avid models, [(18)F]FLT-PET imaging as a surrogate biomarker predicted the therapeutic response of CDK4/6 inhibitor PD-0332991.

CONCLUSIONS

Tumor thymidine level is one of the factors that impact the correlation between [(18)F]FLT uptake and tumor cell proliferation. With careful validation, [(18)F]FLT-PET imaging can be used to monitor antiproliferative therapies in tracer-avid malignancies.

Pemetrexed pharmacokinetics and pharmacodynamics in a phase I/II study of doublet chemotherapy with vinorelbine: implications for further optimisation of pemetrexed schedules

The purpose of this study was to investigate the utility of plasma pharmacokinetic and pharmacodynamic measures including plasma deoxynucleosides, homocysteine and methylmalonic acid concentrations in understanding the time course and extent of the inhibition of thymidylate synthase (TS) by pemetrexed in the context of a phase I/II combination study with vinorelbine. Eighteen patients received supplementation with folic acid and Vitamin B₁₂ 1 week before beginning treatment with pemetrexed and vinorelbine administered in a dose-escalating manner on a 21-day cycle. Heparinised blood samples were collected from consenting patients in the first cycle for pharmacokinetic analyses and in the first two cycles for determination of plasma thymidine, deoxyuridine, homocysteine and methylmalonic acid concentrations. These values were correlated with response and toxicity. Plasma deoxyuridine was used as a measure of TS inhibition, and concentrations of deoxyuridine were significantly elevated relative to baseline on days 1 (P<0.01), 2 (P<0.001) and 3 (P<0.05) after treatment at all pemetrexed dose levels (400–700 mg m⁻²). The magnitude of deoxyuridine elevation correlated with pemetrexed area under the plasma concentration–time curve (AUC) (r²=0.23, P<0.05). However, deoxyuridine concentrations returned to baseline between 8 and 15 days after treatment with pemetrexed, suggesting that inhibition of TS was not durable. Pemetrexed AUC correlated with the percentage decline (relative to baseline) in both platelets (r²=0.58, P<0.001) and leucocytes (r²=0.26, P<0.05) at day 8. Baseline homocysteine was also significantly correlated with these measures of haematological toxicity (r²=0.37, P<0.01 and r²=0.39, P<0.01, respectively). In addition, there was a significant reduction of plasma homocysteine on days 8 (P<0.005) and 15 (P<0.05) in cycle 1 compared to baseline values. The results suggest that the TS inhibitory effects of pemetrexed are short-lived and make the case for a more frequent schedule of administration such as every 2 weeks. The lack of protracted TS inhibition may be due to concomitant vitamin administration, and this may be the mechanism by which vitamins prevent life-threatening toxicity from pemetrexed. Baseline homocysteine concentration remains a predictive marker for haematological toxicity even following folate supplementation.

Altered deoxyuridine and thymidine in plasma following capecitabine treatment in colorectal cancer patients

AIMS

To investigate the relationship between changes in plasma deoxynucleoside concentrations and response and toxicity in patients treated with capecitabine.

METHODS

Twenty-six patients received 2 g capecitabine twice daily orally for 2 weeks of a 3-week cycle. Blood samples were collected on day 0 (baseline), day 8, day 15 and day 22 of the first cycle for the determination of plasma thymidine (TdR) and deoxyuridine (UdR) concentrations. Patients were reviewed weekly during the first cycle, then 3-weekly for toxicity assessment. Response was assessed according to Response Evaluation Criteria in Solid Tumours (RECIST) criteria.

RESULTS

The plasma UdR and UdR/TdR ratios were significantly elevated (P < 0.001) compared with baseline (49.3 +/- 20.8 nmol l(-1)) for the entire 3-week treatment period. In contrast, the plasma TdR concentrations of these patients were significantly reduced only on day 8 (P < 0.01) compared with baseline (12.1 +/- 3.83 nmol l(-1)), but returned gradually to basal levels by day 15. There were no significant correlations demonstrated between pretreatment or maximal post-treatment plasma nucleoside ratio and either toxicity or response. The TSER genotype frequencies of homozygous TSER*2, TSER*3 and heterozygous TSER*2/*3 were 7.7%, 42.3% and 50%, respectively. These preliminary data also indicate no direct relationship between thymidylate synthase (TS) genotype and plasma nucleoside levels.

CONCLUSIONS

Capecitabine mimics continuous infusion of 5-FU to achieve sustained cellular TS inhibitory effects and suggests the antiproliferative mechanism of capectabine is at least partly due to TS inhibition through its active metabolite FdUMP. Although plasma UdR and TdR concentrations and the UdR/TdR ratio can provide some pharmacodynamic indication of TS inhibition, they are unlikely to predict therapeutic response or toxicity accurately following capecitabine treatment in cancer patients.

Rapid quantitation of fluoxetine and norfluoxetine in serum by micro-disc solid-phase extraction with high-performance liquid chromatography–ultraviolet absorbance detection

A rapid, robust and sensitive method for the extraction and quantitative analysis of serum fluoxetine (FLX) and norfluoxetine (N-FLX) using a solid-phase extraction (SPE) column and high-performance liquid chromatography (HPLC) with ultraviolet (UV) detection was developed and validated. The sample clean-up step was performed by simple micro-disc mixed-mode (non-polar and strong cation exchange (SCX)) SPE cartridges. Separation of analytes and internal standard (IS) clomipramine (CLO) from endogenous matrix interference was achieved using a Waters Symmetry C(8) (150 mm x 2.1 mm i.d., 5 microm) reversed-phase narrow bore column. The relative retention times were 8.5, 9.6 and 10.5 min for FLX, N-FLX and CLO, respectively with a low isocratic flow rate of 0.3 ml/min. Chromatographic run time was completed in 15 min and peak area ratios of analytes to IS were used for regression analysis of the calibration curve. The latter was linear from 10 to 4000 nmol/l using 0.5 ml sample volume of serum. The average recovery was 95.5% for FLX and 96.9% for N-FLX. The lowest limit of quantitation (LLOQ) for serum FLX and N-FLX was 10 nmol/l (on-column amount of 200 fmol). The method described was used to analyse serum samples obtained from rats given chronic FLX treatment and to examine the relationship between steady state serum drug concentrations and neurochemical changes in several brain regions.