ARTICLE

2-[11C]Thymidine Positron Emission Tomography as an Indicator of Thymidylate Synthase Inhibition in Patients Treated With AG337

Paula Wells, Eric Aboagye, Roger N. Gunn, Safiye Osman, Alan V. Boddy, Gordon A. Taylor, Imran Rafi, Andrew N. Hughes, A. Hilary Calvert, Pat M. Price, David R. Newell

Affiliations of authors: P. Wells, E. Aboagye, R. N. Gunn, S. Osman, P. M. Price, Imperial College School of Medicine, Hammersmith Hospital, London, U.K.; A. V. Boddy, G. A. Taylor, I. Rafi, A. N. Hughes, A. H. Calvert, D. R. Newell, Northern Institute for Cancer Research, University of Newcastle, Newcastle, U.K.

Correspondence to: Pat Price, M.D., Molecular Imaging Centre, Academic Department of Radiation Oncology, Christie Hospital NHS Trust, Wilmslow Rd., Manchester, M20 4BX, U.K. (e-mail: anne.mason{at}man.ac.uk).


    ABSTRACT
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Background: Some anticancer drugs inhibit thymidylate synthase (TS), a key enzyme for thymidine nucleotide biosynthesis. Cells can compensate for depleted thymidine levels by taking up extracellular thymidine via a salvage pathway. We investigated the use of 2-[11C]thymidine positron emission tomography (PET) to measure thymidine salvage kinetics in vivo in humans. Methods: Five patients with advanced gastrointestinal cancer were PET scanned both before and 1 hour after oral administration of the TS inhibitor AG337 (THYMITAQ [nolatrexed]); seven control patients were scanned twice but not treated with AG337. Thymidine salvage kinetics were measured in vivo using 2-[11C]thymidine PET and spectral analysis to obtain the standardized uptake values (SUV), the area under the time–activity curve (AUC), and the fractional retention of thymidine (FRT). Changes in PET parameters between scans in the AG337-treated and control groups were compared using the Mann–Whitney U test. The relationship between AG337 exposure and AG337-induced changes in tumor FRT and in plasma deoxyuridine levels (a conventional pharmacodynamic systemic measure of TS inhibition) was examined using Spearman’s regression analysis. Statistical tests were two-sided. Results: The between-scan change in FRT in patients treated with AG337 (38% increase, 95% confidence interval [CI] = 8% to 68%) was higher than that in control patients (3% increase, 95% CI = –11% to 17%) (P = .028). The level of AG337-induced increase in both 2-[11C]thymidine FRT and plasma deoxyuridine levels was statistically significantly correlated with AG337 exposure (r = 1.00, P = .01 for both). Conclusions: AG337 administration was associated with increased tumor tracer retention that was consistent with tumor cell uptake of exogenous 2-[11C]thymidine as a result of TS inhibition. 2-[11C]Thymidine PET can be used to measure thymidine salvage kinetics directly in the tissue of interest.



    INTRODUCTION
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Thymidylate synthase (TS) is an important target for antiproliferative chemotherapy because it is a key enzyme in the de novo biosynthetic pathway of thymidine nucleotides used in the synthesis of DNA. TS catalyzes the methylation of 2'-deoxyuridine-5'-monophosphate (dUMP) to 2'-deoxythymidine-5'-monophosphate (dTMP), and inhibition of the enzyme results in the depletion of intracellular thymidine nucleotide pools. The clinical activity seen with TS inhibitors that are based on both pyrimidine (e.g., 5-fluorouracil [5-FU], 5-fluorodeoxyuridine) and folate cosubstrates (e.g., CB3717, Tomudex [raltitrexed], LY231514 [pemetrexed]) highlights the importance of TS as a cancer chemotherapeutic target (1,2).

Inherent or acquired resistance to TS inhibitors, however, is a substantial problem. Resistance can arise from several mechanisms. For example, cells can overcome thymidine nucleotide pool depletion by importing extracellular thymidine, which is phosphorylated by thymidine kinase (TK) to produce dTMP. The existence of this salvage pathway is supported by the reduced cellular activity of clinically evaluated TS inhibitors in the presence of excess extracellular thymidine (37) and their increased activity following inhibition of thymidine transport (8,9). Furthermore, TS inhibitors are more active against TK-deficient tumors (3,7,10) and in the presence of elevated levels of thymidine phosphorylase, which catalyses the reversible phosphorolytic cleavage of thymidine (11). Another resistance mechanism is impaired drug transport into cells. Classical folate antagonists possess a terminal glutamate residue that gives these compounds a negative charge that impedes their transport into cells. Consequently, such compounds can enter cells only by means of transport mechanisms that facilitate drug uptake, e.g., the reduced folate carrier and the membrane folate-binding protein (12). The efficiency of cellular uptake is determined by the polarity of the inhibitor, the presence of extracellular folates or antifolates (which may compete for membrane transporters), intracellular folate concentration, and the levels and functional status of the carriers (12). Decreased intracellular polyglutamation can also result in cellular resistance to classical antifolate TS inhibitors. Polyglutamation occurs when an antifolate has been transported into the cell, involves the addition of glutamate residues by folylpolyglutamate synthetase, and results in reduced efflux from the cell. In addition, polyglutamation markedly increases the affinity of some antifolates for certain folate-dependent enzymes, in some cases increasing TS inhibition by more than 100-fold relative to the monoglutamated form (4,6).

AG337 (THYMITAQ [nolatrexed dihydrochloride]) is a nonclassical antifolate TS inhibitor that was developed using protein structure-based drug design to overcome the resistance mechanisms that limit the activity of classical TS inhibitors (13). AG337 lacks a terminal glutamate side chain, is uncharged at physiological pH, does not require a specific transport mechanism to gain entry into cells, and is not a substrate for folylpolyglutamate synthetase (7). The drug is cytotoxic in vitro and has antitumor activity in vivo toward TK-deficient murine and human tumors (7). A phase I study of AG337 demonstrated substantial TS inhibition during a 24-hour intravenous infusion period, as reflected by increased plasma deoxyuridine levels that normalized rapidly at the end of the infusion (14). These observations led to studies of continuous 5-day infusions to optimize TS inhibition and therefore the antiproliferative effects of AG337. From these latter studies, the dose-limiting toxicities were identified as myelosuppression and mucositis, and a phase II dose of 800 mg m-2 day-1 was recommended (15). Several phase II studies have been undertaken in different tumor types using the continuous 5-day intravenous infusion, and promising results have been seen for squamous cell carcinoma of the head and neck and for hepatoma (16,17). AG337 is currently under investigation in phase III trials (16). An oral preparation of the drug has also been developed and investigated in the phase I setting in 5- and 10-day protocols (18,19). The recommended dose for oral AG337 in phase II studies was again 800 mg m-2 day-1, and the dose-limiting toxicities were nausea and vomiting, with neutropenia and stomatitis observed at higher dose levels (19).

Pharmacokinetic and pharmacodynamic analyses showed no relationship between toxicity and drug dose following oral administration of AG337, a weak relationship between area under the AG337 plasma concentration versus time curve (AUCplasma) and peak plasma concentration, and a more statistically significant relationship between AG337 AUCplasma and the trough concentration (19). The pharmacodynamics of AG337 have also been assessed in the clinical setting by monitoring plasma levels of deoxyuridine, which increase following TS inhibition (14). Measurement of plasma deoxyuridine by high-pressure liquid chromatography (HPLC) has potential for routine clinical use; however, it does not provide a direct assessment of TS inhibition in either tumor or normal tissue.

The fact that plasma deoxyuridine measurements do not reflect TS inhibition in specific tissues and limitations of other methods of measuring TS function in biopsy samples (e.g., the need for taking a biopsy or the inability to take multiple samples to obtain a time course) have led to interest in the development of a noninvasive method of monitoring TS inhibition in tumors. Noninvasive imaging with positron emission tomography (PET), which can provide quantitative information on the concentration of a radiolabeled tracer in tissues, is being developed as a pharmacokinetic and pharmacodynamic tool in oncology. PET imaging has potential for predicting the activity and toxicity of both conventional and investigational anticancer agents under clinical development (2022). For example, PET imaging using 2-[11C]thymidine is being developed as a measure of tumor proliferation and cancer response to treatment (23). Because TS inhibition results in the depletion of intracellular thymidine nucleotide pools and because cells can overcome the resulting cell growth inhibition by incorporating thymidine from the extracellular pool during DNA synthesis (3,57,24,25), we hypothesized that 2-[11C]thymidine PET imaging could also be used as a pharmacodynamic measure of tumor thymidine uptake and retention in response to TS inhibition. To test this hypothesis, we carried out PET using 2-[11C]thymidine before and after oral administration of AG337 in patients with advanced gastrointestinal malignancies. We also examined associations between the AG337 pharmacokinetic data (i.e., drug exposure) and AG337-induced changes in 2-[11C]thymidine incorporation into tumors and in plasma deoxyuridine and thymidine levels as conventional pharmacodynamic systemic measures of TS inhibition.


    PATIENTS AND METHODS
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Patients

The study was not randomized. Two 2-[11C]thymidine PET scans were performed on five patients treated as part of a phase I study of AG337 administered orally for 5 days. Patients received test doses of the drug for 2 days for estimation of pharmacokinetics and bioavailability, followed by 5 days of treatment, making a 7-day trial protocol. The phase I study was conducted under the auspices of the Cancer Research UK Phase I/II Committee, and a detailed description can be found elsewhere (19). All five patients had advanced gastrointestinal cancer and a performance status of 0 (Table 1Go), and in all five patients, the cancer had metastasized to the liver. AG337 was supplied as the dihydrochloride salt (Agouron Pharmaceuticals, San Diego, CA), and all patients described here were treated at the highest dose level in the phase I trial (800 mg m-2 day-1, with all doses referring to AG337 free base). A control group of seven patients with advanced gastrointestinal malignancies and a performance status of 0 or 1 were also scanned twice. These control patients met the same eligibility requirements as the phase I trial patients, and these requirements are described elsewhere (15). In addition, to be eligible for PET, patients had to have tumors greater than 3 cm in diameter, twice the limit of the spatial resolution of the PET camera. The local ethics committees of the Newcastle General and Hammersmith Hospitals approved the studies, and all patients gave written informed consent. Permission to administer a radioactive tracer was obtained from the Administration of Radioactive Substances Advisory Committee of the United Kingdom.


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Table 1. Patient characteristics
 
Plasma Pharmacokinetic and Pharmacodynamic Studies

The plasma pharmacokinetic and pharmacodynamic studies (15) and the methods used in those studies (14) have been described in detail elsewhere. AG337 pharmacokinetic and pharmacodynamic (i.e., plasma deoxyuridine and thymidine) measurements were conducted on days 1 and 2 of the trial protocol, when patients received test doses of 400–450 mg to estimate AG337 bioavailability and to confirm that the plasma concentrations would be consistent with those seen previously (19). Beginning on day 3 of the 7-day trial protocol, patients started the 5-day course of treatment at 800 mg m-2 day-1. Previous work demonstrated the predictability of the 5-day pharmacokinetic data from the 2-day pharmacokinetic data (19). Therefore, to minimize intravenous sampling from patients, pharmacokinetic and PET data were not obtained in parallel. AG337 concentrations were determined in plasma prepared by HPLC from heparinized blood samples (14). AG337 AUC values were calculated using a non-compartmental analysis employing the trapezoidal rule. The systemic pharmacodynamics of AG337 were studied by measuring the effects of drug administration on plasma concentrations of deoxyuridine and thymidine (14).

PET Scanning

AG337-treated patients were PET scanned 4 days before the commencement of the AG337 phase I trial and on day 3 of the 7-day trial protocol, 1 hour after commencing oral administration of the drug (i.e., 1 week after the first PET scan). Each scan lasted for 1 hour. The timing of the second scan was chosen to coincide with the known peak plasma concentration of AG337. The control patients were scanned twice at an interval of 1 week and with no intervening treatment. To examine any changes in normal tissue, normal liver regions were also scanned, and normal liver at a distance from the tumor was analyzed. PET scans were performed using an ECAT 931–08/12 scanner (CTI/Siemens, Knoxville, TN) in 2-D mode at the Hammersmith Hospital after insertion of arterial and intravenous lines for blood sampling and injection of radiotracer, respectively. A 68Ge phantom was used for calibration. 2-[11C]Thymidine was produced as described elsewhere (26) and administered intravenously as a bolus over 30 seconds, beginning 30 seconds after the start of scanning. The radioactivity of blood collected via a radial artery catheter was measured at 1-second intervals using an online bismuth germanate counting system. Discrete blood samples were also taken to calibrate the radioactive counts and to measure, using HPLC, the relative amounts of 2-[11C]thymidine and metabolites in the plasma (27). A time course of the release of [11C]CO2 from the body was also obtained using nasal sampling (28). Metabolite data from blood samples and exhaled [11C]CO2 were used to correct the total plasma input function for the contribution from labeled metabolites and to produce the parent thymidine, intermediate metabolite (thymine, dihydrothymine, {beta}-aminobutyric acid), and CO2 input functions required for spectral analysis (28,29).

Data Acquisition and Processing

Voxel dimensions were 2.1 x 2.1 x 6.4 mm, and the spatial resolution of reconstructed tomographic images was 8 mm at the center of the field of view. The time courses of radioactivity accumulation in plasma and in the tumor were used to derive physiologic variables. For the calculation of standardized uptake value (SUV), the 2-[11C]thymidine data were normalized to patient body weight and injected dose (SUV = measured tissue activity/[injected activity/patient weight]). An SUV was calculated for each time frame from the start (t = 0) to the end (t = 3600 seconds) of the scan. The data were analyzed to give SUV3000–3600 (kg/mL), which was calculated as the midpoint between the SUVs obtained at 3000 and 3600 seconds. The integral of the SUV time curve to the end of the scan was used to calculate the area under the time–activity curve (AUC) parameter (kg {bullet} min {bullet} mL-1). Spectral analysis was used to calculate the tumor impulse response function (IRF) using the metabolite-corrected plasma input function and the tumor time–activity curve (30). IRF values were obtained for both the delivery (IRF1min) and the uptake (IRF60min) of tracer in the tumor. The fractional retention at 1 hour (FRT) was calculated as IRF60min/IRF1min. FRT values can range from 0 to 1, corresponding to 0% to 100% of the delivered tracer being retained in a tissue at 60 minutes.

Statistical Methods

The mean and 95% confidence intervals (CIs) of the difference between data obtained in repeat scans was calculated for all of the PET parameters. The statistical significance of the differences between repeat scan data for control patients versus drug-treated patients was examined using the nonparametric Mann–Whitney U test. Correlations between the PET and AG337 AUCplasma data were investigated using Spearman’s regression analysis.


    RESULTS
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Patients

The characteristics of patients in both the AG337-treated and control groups are summarized in Table 1Go. There were no statistically significant differences in the paired data for the level of 2-[11C]thymidine in the blood, for the rate of 2-[11C]thymidine metabolism, or for the proportion of labeled metabolites in blood for either the control or drug-treated patients (data not shown).

Effect of AG337 Treatment on PET Parameters in Tumor and Liver

Time–activity curves from the tumor and normal liver segments, such as the curves shown in Fig. 1Go, were used to obtain 2-[11C]thymidine SUV, AUC, and FRT data for all patients (Tables 2Go–4Go). The differences between repeat scans for each PET parameter in AG337-treated and control patients are also given in Fig. 2Go. All patients treated with AG337 showed an increase in 2-[11C]thymidine SUV (which represents the amount of tracer in the tissue at the end of the scan corrected for the activity of the injected tracer and patient weight) in the tumor (Table 2Go). Tumor SUV increased by an average of 43% (95% CI = 24% to 62%) in AG337-treated patients and decreased by 7% (95% CI = –14% to 0%) in control patients (Table 5Go). Tumor FRT, which represents tracer retention, also increased in most patients treated with AG337 (by an average of 38%, 95% CI = 8% to 68%) and increased (by an average of 3%, 95% CI = –11% to 17%) in control patients (Table 5Go). By contrast, both decreases and increases in tumor AUC were seen in AG337-treated patients (Table 3Go).



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Fig. 1. Tumor time–activity curves for 2-[11C]thymidine PET for two patients. Standardized uptake value curves were generated from the time–activity curves by correcting the tissue activity, in ECAT counts (the calibration between the picture element response and the absolute radioactive concentration), for the contribution of body weight and injected dose.

 

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Table 2. 2-[11C]Thymidine standardized uptake value (SUV) data for tumor and normal liver tissue from AG337-treated and control patients*
 

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Table 4. 2-[11C]Thymidine fractional retention of thymidine (FRT) data for tumor and normal liver tissue from AG337-treated and control patients*
 


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Fig. 2. Absolute differences between repeat scans in 2-[11C]thymidine PET data for all patients in the study. Each point represents either an AG337-treated patient or a control patient not treated with AG337. The two sets of points at left show data for tumor tissue; the two sets of points at right show data for liver tissue. Standardized uptake value (SUV), area under the time–activity curve (AUC), and fractional retention of thymidine (FRT) were calculated as described in the "Patients and Methods" section. All data come from Tables 2Go–4Go.

 

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Table 5. 2-[11C]Thymidine positron emission tomography (PET) parameter summary statistics*
 

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Table 3. 2-[11C]Thymidine area under the time–activity curve (AUC) data for tumor and normal liver tissue from AG337-treated and control patients*
 
Comparison of the absolute differences in SUV between repeat scans (Table 5Go) shows that the AG337-treated and control patient groups were statistically significantly different (P = .004). Treated and control patients also showed statistically significant differences in absolute between-scan differences in FRT (P = .028). However, no statistically significant difference between patient groups was seen for AUC. In addition, in normal liver, absolute between-scan differences in SUV, FRT, and AUC were not statistically significantly different between AG337-treated and control patients (Table 5Go). There was also no statistically significant difference between IRF1min (i.e., tracer delivery to tumor) data obtained before and after AG337 administration (P = .57) (data not shown).

Relationship of AG337 Pharmacokinetic Measurements of Drug Exposure with PET 2-[11C]Thymidine Retention and With Systemic Pharmacodynamic Parameters

Data on the levels of deoxyuridine and thymidine in plasma were available for three of the five AG337-treated patients and for two of the seven control patients (Table 6Go). Plasma deoxyuridine and thymidine levels increased 1 hour after AG337 administration in all three AG337-treated patients, indicating TS inhibition, but stayed the same or decreased in the control patients. An analysis was made of the relationship between drug exposure (i.e., AG337 AUC) and the different pharmacodynamic endpoints of TS inhibition (i.e., AG337-induced changes in tumor 2-[11C]thymidine retention and in plasma deoxyuridine and thymidine levels). A statistically significant relationship was found between the AG337 plasma AUC and both the drug-induced changes in tumor 2-[11C]thymidine FRT (r =1.00, P = .01) and plasma deoxyuridine concentration (r =1.00, P = .01). Fig. 3Go illustrates the relationship between the level of AG337 exposure and drug-induced changes in FRT and plasma deoxyuridine.


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Table 6. AG337 area under the curve (AUC) data and plasma deoxyuridine (UdR) and thymidine (TdR) concentrations in AG337-treated and control patients*
 


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Fig. 3. Relationship between drug exposure, as assayed by AG337 plasma area under the curve (AUC, in mg {bullet} min {bullet} mL-1), and thymidylate synthase inhibition, as determined by PET analysis and changes in plasma deoxyuridine (UdR). Relationship between AG337 plasma AUC and interscan differences in tumor 2-[11C]thymidine fractional retention of thymidine (FRTdiff) for four of the five AG337-treated patients (top). Data (each point represents an individual patient) are from Tables 4Go and 6Go. Relationship between AG337 plasma AUC and differences in plasma UdR concentrations (µM) before and 60 minutes after AG337 treatment for three of the five AG337-treated patients (bottom). Data (each point represents an individual patient) are from Table 6Go. Relationships were analyzed using Spearman’s regression analysis.

 

    DISCUSSION
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 
In this study, we demonstrated that PET scanning with 2-[11C]thymidine could be used to show a statistically significant increase in tumor thymidine incorporation, measured as FRT, in patients with advanced gastrointestinal cancer treated with AG337 relative to a group of untreated controls. Although this study is the first, to our knowledge, that used PET to measure thymidine incorporation as an indicator of TS inhibition, a previous study using single photon emission computed tomography showed an increase in [123I]5-iodo-2'-deoxyuridine in liver metastases in response to 5-FU and folinic acid treatment (31). That study was based on the concept that cells will substitute deoxyuridine for thymidine if TS is inhibited; TS inhibition will therefore enhance tumor uptake of deoxyuridine, facilitating its use as a marker of proliferation.

The increase in the retention of the thymidine tracer in tumors seen in the current study is consistent with the hypothesis that TS inhibition enhances exogenous thymidine use by the salvage pathway. No statistically significant AG337-induced increase in tracer incorporation was measured in normal liver, an observation consistent with AG337’s effects being restricted primarily to proliferating tissues. The lack of effect on IRF1min suggests that AG337 has no effect on blood flow that might influence the delivery of 2-[11C]thymidine to the tumor.

HPLC analysis of plasma deoxyuridine concentrations can also be used as a surrogate measure of TS inhibition (14). Deoxyuridine in plasma is derived from an intracellular deoxyuridine monophosphate pool that increases after TS inhibition (1). All three AG337-treated patients for whom data on plasma deoxyuridine were available showed an increase in plasma deoxyuridine concentration that correlated with the plasma AG337 AUC. This observation suggests a relationship between drug exposure and the magnitude of TS inhibition. Furthermore, the increase in 2-[11C]thymidine FRT was correlated with the plasma AG337 AUC. Together, these data suggest that TS inhibition can be assessed either by plasma deoxyuridine measurements or by 2-[11C]thymidine PET pharmacodynamic analysis, the advantage of the latter being that data are provided for the tissue of interest.

Interest in 2-[11C]thymidine as a radiolabel for PET studies in humans lies in the well-established use of thymidine (and related analogs) as a marker of tumor proliferation. Although 2-[11C]thymidine PET is still under development as a clinical tool, it appears to have potential in the early assessment of tumor response to chemotherapy (32). Recent data from our group have shown that 2-[11C]thymidine PET FRT, but not SUV or AUC, is correlated with the level of tumor proliferation measured using Ki-67 immunohistochemistry (33). The latter finding supports the potential use of 2-[11C]thymidine PET in cancer patients.

A number of methodologic factors affect the measurement of 2-[11C]thymidine incorporation in human tumors by PET. One key issue in the analysis of 2-[11C]thymidine PET data is the need to distinguish labeled thymidine from labeled metabolites of thymidine in tissues. The SUV and AUC parameters only normalize the measured radioactivity for body weight and injected dose of the radiolabeled tracer and do not account for differences between patients in tracer delivery, tracer kinetics, or the presence of labeled metabolites in tissue. By the 60-minute time point, much of the radioactivity can be attributed to labeled metabolites that cannot be distinguished from 2-[11C]thymidine bound to DNA. Although we observed a statistically significant increase in thymidine tracer incorporation and retention using SUV, this parameter is a non–model-based, composite parameter that depends not only on the amount of thymidine extracted but also on the partitioning of cardiac output and tissue perfusion. The partitioning of cardiac output and tissue perfusion will affect tracer delivery and the subsequent perfusion washout of the non-extracted tracer, respectively. However, because the delivery parameter IRF1min, which is derived from the kinetic analysis, did not change in response to AG337 administration, it appears that there were no statistically significant drug-induced alterations in tumor perfusion. Hence, assuming that the partitioning of the cardiac output did not change, the increase in SUV is consistent with the finding of increased retention, measured as FRT. This finding may seem surprising, given that SUV did not correlate with Ki-67 expression in our earlier study (33). However, those data were assembled from single readings from a range of subjects. Consequently, the flow variation between tumors could mask the underlying correlation with an independent marker of proliferation. In the case of subjects being studied twice, and presumably in whom flow and the partitioning of the cardiac output remain constant, it is reasonable to conclude that the increase in SUV does reflect increased extraction of thymidine.

Analysis of PET data can be improved by using an appropriate model to relate the observed tissue time–activity curve to the concentration of tracer in plasma. A compartmental analysis can be used to determine the rate constants for the exchange of tracer between plasma and tissue (34,35). The latter approach involves five tissue compartments and requires the estimation of eight rate constants from three measured blood input functions (thymidine, intermediate metabolites, and CO2) and from the time–activity curve, all of which can lead to problems of parameter identifiability. In our study, spectral analysis was used to obtain pharmacokinetic parameters that required few modeling assumptions (30,36). Spectral analysis does not require a priori assumptions about the number of components required but is based on a general compartmental framework. The FRT parameter (i.e., the ratio of the IRF at 60 minutes to the IRF at 1 minute) gives a measure of the tracer retention in tissue. The increase in FRT in tumors in response to AG337 administration and the strong correlation between tumor FRT and the pharmacokinetic measure of drug exposure (AG337 AUC) supports the use of spectral analysis of 2-[11C]thymidine PET data in measuring TS inhibition in vivo.

In summary, despite the small number of patients available for this pilot study, we have shown pharmacodynamic changes consistent with TS inhibition in tumors in vivo following AG337 administration. The data illustrate the potential utility of 2-[11C]thymidine PET as a pharmacodynamic tool for cancer patients and suggests that further research in this area is warranted. A larger study has been initiated to determine whether 2-[11C]thymidine PET can be used to measure TS inhibition in vivo in patients with gastrointestinal cancer treated with 5-FU and folinic acid. The PET data will be analyzed in relation to direct measurements of TS inhibition in biopsy specimens and to changes in plasma deoxyuridine levels.


    NOTES
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 
A. V. Boddy, A. H. Calvert, and D. R. Newell have, in the past, conducted research supported by Agouron Pharmaceuticals.

Supported by the Medical Research Council of the United Kingdom; Cancer Research UK; Public Health Service grant CA83028 from the National Cancer Institute, National Institutes of Health, Department of Health and Human Services; and Agouron Pharmaceuticals, Inc. (a Pfizer company). Agouron provided the drug for this study, financial support for the clinical trial, and a grant for the plasma PK studies.

The authors thank Colin Steel for preparation of the 2-[11C]thymidine, Alex Ranicar for help with measurements of arterial metabolite data, the research nurses and data managers based at the Northern Centre for Cancer Treatment, Newcastle General Hospital, and the Drug Development Office of Cancer Research UK.


    REFERENCES
 Top
 Notes
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 

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Manuscript received June 12, 2002; revised February 25, 2003; accepted March 6, 2003.


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