Distinct genomic landscapes of gastroesophageal adenocarcinoma depending on PD-L1 expression identify mutations in RAS-MAPK pathway and TP53 as potential predictors of immunotherapy efficacy.

BACKGROUND: The impact of molecular alterations on programmed death-ligand 1 (PD-L1) combined positive score (CPS) is not well studied in gastroesophageal adenocarcinomas (GEAs). We aimed to characterize genomic features of tumors with different CPSs in GEAs. PATIENTS AND METHODS: Genomic alterations of 2518 GEAs were compared in three groups (PD-L1 CPS ≥ 10, high; CPS = 1-9, intermediate; CPS < 1, low) using next-generation sequencing. We assessed the impact of gene mutations on the efficacy of immune checkpoint inhibitors (ICIs) and tumor immune environment based on the Memorial Sloan Kettering Cancer Center and The Cancer Genome Atlas databases. RESULTS: High, intermediate, and low CPSs were seen in 18%, 54% and 28% of GEAs, respectively. PD-L1 positivity was less prevalent in women and in tissues derived from metastatic sites. PD-L1 CPS was positively associated with mismatch repair deficiency/microsatellite instability-high, but independent of tumor mutation burden distribution. Tumors with mutations in KRAS, TP53, and RAS-mitogen-activated protein kinase (MAPK) pathway were associated with higher PD-L1 CPSs in the mismatch repair proficiency and microsatellite stability (pMMR&MSS) subgroup. Patients with RAS-MAPK pathway alterations had longer overall survival (OS) from ICIs compared to wildtype (WT) patients [27 versus 13 months, hazard ratio (HR) = 0.36, 95% confidence interval (CI): 0.19-0.7, P = 0.016] and a similar trend was observed in the MSS subgroup (P = 0.11). In contrast, patients with TP53 mutations had worse OS from ICIs compared to TP53-WT patients in the MSS subgroup (5 versus 21 months, HR = 2.39, 95% CI: 1.24-4.61, P = 0.016). CONCLUSIONS: This is the largest study to investigate the distinct genomic landscapes of GEAs with different PD-L1 CPSs. Our data may provide novel insights for patient selection using mutations in TP53 and RAS-MAPK pathway and for the development of rational combination immunotherapies in GEAs.

The hard road to data interpretation: 3 or 6 months of adjuvant chemotherapy for patients with stage III colon cancer?

Background: Six months of adjuvant oxaliplatin-based chemotherapy is standard for patients with stage III colon cancer following surgery. However, oxaliplatin is associated with peripheral neurotoxicity which worsens over treatment duration. Consequently, a shorter treatment duration, if equally effective, would be extremely beneficial. A pooled analysis of data for 12 834 stage III colon cancer patients, from six randomised phase III trials of adjuvant therapy, the International Duration Evaluation of Adjuvant chemotherapy study, was carried out and the results presented at the ASCO Annual Meeting 2017. To clarify the potential impact of these results on clinical practice, ESMO decided to sponsor a special session at their 2017 Annual Meeting dedicated to achieving a more meaningful interpretation of the results. Methods: Medical oncologists from Europe, the United States and Asia selected for their involvement in the trials, together with an independent statistician and an independent clinician, were invited to provide their independent interpretations of the results and contribute to a moderated panel discussion. The pooled analysis evaluated the non-inferiority of 3 versus 6 months of adjuvant FOLFOX/CAPOX therapy but not the non-inferiority of 3 months CAPOX versus 6 months FOLFOX therapy. Results: There was strong evidence of an interaction between the choice of regimen (CAPOX or FOLFOX) and duration of treatment. Patients were classified as either 'fighters' or 'fatalists', and 3-month CAPOX was considered standard for patients classified as fatalists even if they had high-risk disease. However, patients classified as 'fighters' would only receive 3 months of CAPOX if they had low-risk disease but would always receive 6 months of CAPOX/FOLFOX if they had T4 disease. The panel was split on whether they would advocate 3 or 6 months CAPOX therapy based on high-risk N2 disease. Conclusions: The main drivers of the duration of treatment were choice of regimen and patient attitude, with risk, based mainly on T4 stage, having less influence.

Higher capecitabine AUC in elderly patients with advanced colorectal cancer (SWOGS0030).

BACKGROUND: The aging process is accompanied by physiological changes including reduced glomerular filtration and hepatic function, as well as changes in gastric secretions. To investigate what effect would aging have on the disposition of capecitabine and its metabolites, the pharmacokinetics between patients ≥70 years and <60 years were compared in SWOG0030. METHODS: Twenty-nine unresectable colorectal cancer patients were stratified to either ≥70 or <60 years of age, where the disposition of capecitabine and its metabolites were compared. RESULTS: Notable increase in capecitabine area under the curve (AUC) was accompanied by reduction in capecitabine clearance in ≥70 years patients (P<0.05). No difference in 5'-deoxy-5-fluorocytidine, 5'-deoxy-5-fluorouridine (DFUR), and 5-fluorouracil (5FU) AUCs between the two age groups, suggesting that carboxylesterase and cytidine deaminase (CDA) activity was similar between the two age groups. These results suggest that metabolic enzymes involved in converting capecitabine metabolites are not altered by age. An elevation in capecitabine Cmax and reduction in clearance was seen in females, where capecitabine AUC was 40.3% higher in women. Elevation of DFUR Cmax (45%) and AUC (46%) (P<0.05) was also noted, suggesting that CDA activity may be higher in females. CONCLUSION: Increases in capecitabine Cmax and AUC was observed in patients ≥70 years when compared with younger patients who were >60 years.

Imaging proliferation in vivo with [F-18]FLT and positron emission tomography.

Positron emission tomography (PET) is now regularly used in the diagnosis and staging of cancer. These uses and its ability to monitor treatment response would be aided by the development of imaging agents that can be used to measure tissue and tumor proliferation. We have developed and tested [F-18]FLT (3'-deoxy-3'-fluorothymidine); it is resistant to degradation, is retained in proliferating tissues by the action of thymidine kinase 1 (TK), and produces high-contrast images of normal marrow and tumors in canine and human subjects.

A phase II study of bevacizumab, oxaliplatin, and docetaxel in locally advanced and metastatic gastric and gastroesophageal junction cancers.

BACKGROUND: Bevacizumab has demonstrated antitumor activity in multiple diseases. This phase II study was undertaken to determine the effects of adding bevacizumab to a regimen of docetaxel and oxaliplatin in patients with advanced adenocarcinoma of the stomach or gastroesophageal junction. PATIENTS AND METHODS: Previously untreated patients with locally advanced or metastatic disease and a performance status (PS) of 0-1 were eligible for this study. Patients received bevacizumab at 7.5 mg/kg, docetaxel at 70 mg/m(2), and oxaliplatin at 75 mg/m(2) administered on day 1 of a 21-day cycle. The primary end point of the study was progression-free survival (PFS). RESULTS: A total of 38 eligible patients (median age 57 years, 45% gastric, 55% PS 0) were enrolled on to the study. Median PFS was 6.6 months [95% confidence interval (CI) 4.4-10.5] and median survival 11.1 months (95% CI 8.2-15.3). Complete responses were documented in 2 (5%) patients, partial responses in 14 (37%), and stable disease in 14 (37%). No treatment-related deaths were observed. The most commonly reported grade 3-4 toxicity was neutropenia (34%), and gastrointestinal perforation occurred in three patients (8%). CONCLUSION: The combination of bevacizumab, docetaxel, and oxaliplatin has promising activity for further evaluation in randomized trials.

The detection of bone marrow involvement by lymphoma using magnetic resonance imaging.

We used magnetic resonance (MR) to image the bone marrow of 31 patients with lymphoma. Images were obtained of the femoral, pelvic, and vertebral marrow with a 0.15 tesla imaging system using a T1-weighted spin echo sequence (TR600/TE 40). With this pulse sequence, normal marrow produces a high intensity signal that reflects the presence of marrow fat (short T1 relaxation time). We previously reported MR imaging of patients with leukemia in relapse and found a diffusely and symmetrically decreased marrow signal intensity due to the replacement of normal marrow fat by cellular material with a long T1. Unlike leukemia, patients with lymphomatous marrow involvement often had patchy, often discrete, areas of low signal intensity, representing focal marrow infiltration. Five of six patients in this study with lymphoma detected by histologic examination also had marrow lesions seen on MR. An additional four patients had marrow lesions detected by MR that were not detected on initial marrow biopsies; two of these had marrow involvement proven on subsequent biopsies, one had disease isolated to the vertebrae that was never pathologically documented, and one had progression of disease in the marrow documented by MR without biopsy confirmation. These results indicate that marrow involvement with lymphoma can be detected by MR imaging and that MR can complement bone marrow biopsy.

Phase II study of gemcitabine, cisplatin, and infusional fluorouracil in advanced pancreatic cancer.

PURPOSE: This phase II study was undertaken to determine the efficacy of adding infusional fluorouracil (FU) to the chemotherapy doublet of gemcitabine and cisplatin in patients with advanced pancreatic cancer. PATIENTS AND METHODS: The eligibility criteria included histologically or cytologically confirmed adenocarcinoma of the pancreas that was either unresectable or metastatic. No prior gemcitabine therapy was allowed. Patients received a combination of gemcitabine 1000 mg/m2 intravenously (IV) on days 1, 8, and 15; cisplatin 50 mg/m2 IV on days 1 and 15; and FU 175 mg/m2/d from days 1 to 15 by continuous IV infusion. Cycles were repeated every 28 days. Objective tumor response and toxicity were evaluated according to the World Health Organization criteria. RESULTS: A total of 47 patients (median age, 57 years; males, 59%) were enrolled. Sixteen patients had locally advanced (LA) disease, and 31 patients had metastatic disease. A total of 183 cycles of chemotherapy were administered. In patients with metastatic disease (n = 31), the probability of survival at 6 and 12 months was 66% and 34%, respectively. Objective partial response or stable disease was observed in 26% (90% confidence interval [CI], 0.14 to 0.41) and 61% (90% CI, 0.45 to 0.74) of patients, respectively. In patients with LA disease (n = 16), there were three partial responses (19%; 90 CI, 0.07 to 0.39). One patient in this group was successfully resected after FU-based radiotherapy. The most common grade 3 to 4 toxicities were neutropenia (60%), thrombocytopenia (42%), and anemia (26%). Thirteen patients were hospitalized for treatment-related complications. CONCLUSION: The combination of gemcitabine, cisplatin, and infusional FU has significant activity in patients with advanced pancreatic cancer.

Magnetic resonance imaging of bone marrow in hairy cell leukemia: correlation with clinical response to alpha-interferon.

Five patients with progressive hairy cell leukemia were treated with recombinant alpha 2-interferon and examined prospectively at 3-month intervals using serial bone marrow biopsies and magnetic resonance (MR) imaging. Pretreatment iliac crest marrow biopsies demonstrated hairy cell infiltration involving 30-90% of marrow cellularity. Concurrent pretreatment coronal images of the proximal femurs and pelvis using MR revealed extensive marrow involvement that varied in distribution from patchy to diffuse. At 6 months, all patients had responded to alpha 2-interferon with improvement in peripheral blood counts and decrease in lymphoid infiltration on marrow biopsy to 15-40%. Six-month follow-up MR scans demonstrated decreasing marrow leukemia infiltration and increasing marrow fat. MR bone marrow imaging appears useful in the initial diagnosis of hairy cell leukemia, for monitoring the response to treatment, and possibly for predicting relapse.

A phase II trial of bryostatin 1 in the treatment of metastatic colorectal cancer.

Current chemotherapy for patients with advanced colorectal cancer is relatively ineffective and may be associated with significant toxicity. Bryostatin 1 (bryo 1) influences cell proliferation, intracellular metabolism and signaling, differentiation, and apoptosis in human cancer cell lines via modulation of protein kinase C (PKC) activity. This trial investigates the efficacy and toxicity of bryo 1 as a novel therapeutic agent for patients with advanced colorectal cancer who have had previous 5-fluorouracil therapy. The primary end point was tumor response to bryo 1. Toxicity was also assessed. Twenty-eight patients with advanced colorectal cancer were enrolled. The mean age was 59 years (range, 38-76), with 16 men and 12 women, and good minority representation (11 African-Americans). The first 10 patients initially received 25 microg/m2 of bryo 1 weekly as a 24-h infusion for 3 weeks of every 4-week cycle, with dose escalation to 35 microg/m2 starting with the second cycle. The remaining patients were started at 35 microg/m2 and escalated to 40 microg/m2, if toxicity was minimal. Twenty-five patients were evaluable for objective tumor response, and complete data on toxicity were collected on 26 patients. No partial or complete tumor responses were observed. All 25 patients had disease progression within four cycles. Myalgia was the most common toxicity. Myelosuppression was not seen. bryo 1 as a weekly 24-h continuous infusion lacks single-agent antitumor activity in advanced colorectal cancer. Toxicity differs from that of traditional chemotherapeutic drugs.

Characterizing tumors using metabolic imaging: PET imaging of cellular proliferation and steroid receptors.

Treatment decisions in oncology are increasingly guided by information on the biologic characteristics of tumors. Currently, patient-specific information on tumor biology is obtained from the analysis of biopsy material. Positron emission tomography (PET) provides quantitative estimates of regional biochemistry and receptor status and can overcome the sampling error and difficulty in performing serial studies inherent with biopsy. Imaging using the glucose metabolism tracer, 2 -deoxy-2- fluoro-D-glucose (FDG), has demonstrated PET's ability to guide therapy in clinical oncology. In this review, we highlight PET approaches to imaging two other aspects of tumor biology: cellular proliferation and tumor steroid receptors. We review the biochemical and biologic processes underlying the imaging, positron-emitting radiopharmaceuticals that have been developed, quantitative image-analysis considerations, and clinical studies to date. This provides a basis for evaluating future developments in these promising applications of PET metabolic imaging.

A single-institution experience with concurrent capecitabine and radiation therapy in gastrointestinal malignancies.

PURPOSE: We report our clinical experience with 32 patients receiving concurrent irradiation and capecitabine. METHODS AND MATERIALS: Medical records of patients with gastrointestinal malignancies treated with radiation and capecitabine therapy were reviewed. RESULTS: The population consisted of 20 males and 12 females, with a median age of 67.5 years (45-84 years) and adequate hepatic and bone marrow function. Histology was adenocarcinoma in all patients, except two with esophageal squamous carcinoma. Twenty-one patients received the regimen as adjuvant therapy, three received preoperative therapy, and 8 patients received therapy for palliation. The median dose of capecitabine was 1600 mg/m(2)/day (1200-2500 mg/m(2)/day) orally for 5 days per week for the duration of radiation therapy. Thirty patients received a total dose ranging from 45 Gy to 64 Gy over 4-6 weeks. Two previously radiated patients received total doses of 29.9 Gy and 46 Gy. Grade 3/4 toxicities observed were neutropenia in 3 patients and diarrhea, thrombocytopenia, fatigue, and myocardial infarction in 1 patient each. No treatment-related mortality was observed. Twenty of 21 patients (95.2%) who received adjuvant therapy continue to be in complete remission. Four of 11 (36%) evaluable patients demonstrated a response. CONCLUSION: Concurrent capecitabine and radiation were very well tolerated and warrant further investigation in prospective trials.

The use of ranitidine in bone marrow transplantation. A review of 223 cases.

To determine the incidence of ranitidine-induced myelosuppression in the bone marrow transplant setting, we reviewed the records of all patients at our institution (223) who received ranitidine while undergoing bone marrow transplantation. We identified 37 cases in which a myelosuppressive episode was temporally associated with ranitidine use. In-depth analysis of the 37 cases showed 12 in which no alternative cause could be found and in which strong evidence for a ranitidine effect existed. Three of these cases are included herein as case reports. Based on this series, the overall incidence of ranitidine-induced myelosuppression in bone marrow transplant patients was approximately 5% (12/223).

Short-term thymidine uptake in normal and neoplastic tissues: studies for PET.

Uptake of H-3 thymidine was studied in mice, both normal and with spontaneous lymphoma, and in the organs and tumors of dogs with spontaneous tumors. Uptake was compared with relative blood flow as measured by the distribution of C-14 iodoantipyrine. Initial distribution of thymidine in normal mice measured 20 sec after injection, correlated with the relative perfusion measurements; however, all measurements of thymidine uptake made between 1 and 60 min after injection showed no correlation with perfusion. This indicates that the distribution more than 1 min after injection is primarily dependent on subsequent redistribution and/or metabolism of thymidine. A time-course study demonstrated that normal mouse organs with high rates of proliferation retained all the labeled thymidine initially taken up. Organs with low rates of proliferation lost their label in a nearly exponential washout. These studies provide further evidence of the feasibility of using C-11 thymidine for positron emission tomography (PET).

Cellular sources of thymidine nucleotides: studies for PET.

The relative utilization of endogenously synthesized thymidine nucleotides and exogenously supplied thymidine analog was compared in a number of mammalian cell lines, tissues, and tumors. To measure the relative utilization, cells were incubated in tissue culture media containing the thymidine analog [3H]-5-bromo-2'-deoxyuridine (BUDR). After extraction of the DNA, the degree of substitution of the thymidine by BUDR was determined by density gradient centrifugation. All the cell lines and tissues tested utilized both exogenous BUDR and endogenous thymidine sources to a similar extent. The relative utilization of the exogenous pathway could be manipulated by varying the exogenous concentration of BUDR. Our results demonstrate that one can predict the relative utilization of these two pathways and can calculate the effective specific activity of the intracellular thymidine nucleotide pool. Such information is needed in interpreting 11C-labeled thymidine uptake as measured by positron emission tomography.

A graphical analysis method to estimate blood-to-tissue transfer constants for tracers with labeled metabolites.

UNLABELLED: The Patlak graphical analysis technique is a popular tool for estimating blood-to-tissue transfer constants from multiple-time uptake data. Our objective was to extend this technique to tracers with labeled metabolites, the presence of which can cause errors in the standard Patlak analysis. METHODS: Based on previously described formulations, we generalized the graphical technique for use under specific conditions. To test the extended graphical approach, we applied the method to both simulated and patient data using a preliminary compartmental model for the PET tumor proliferation marker, 2-[11C]-thymidine. RESULTS: When given conditions are met, a linear relationship exists between the normalized tissue activity (tissue activity/blood activity) and a new set of graphical analysis basis functions, including a new definition of normalized time, which takes the presence of labeled metabolites into account. Graphical estimations of the tumor thymidine incorporation rate for simulated data were accurate and showed close agreement to the results of detailed compartmental analysis. In patient studies, the graphical and compartmental estimates showed good agreement but a somewhat poorer correlation than in the simulations. CONCLUSION: The extended graphical analysis approach provides an efficient method for estimating blood-tissue transfer constants for tracers with labeled metabolites.

5-Fluoro-1-(2'-deoxy-2'-fluoro-beta-D-ribofuranosyl) uracil trapping in Morris hepatoma cells expressing the herpes simplex virus thymidine kinase gene.

UNLABELLED: The planning and individualization of gene therapy with suicide genes such as herpes simplex virus thymidine kinase (HSV-tk) necessitates the assessment of the enzyme activity expressed in the tumor. This can be done by uptake measurements of specific substrates for HSV-tk. Due to the molecular structure of 5-fluoro-1-(2'-deoxy-fluoro-beta-D-ribofuranosyl)uracil (FFUdR), it may be a substrate for both the mammalian thymidine kinase and HSV-tk. METHODS: Using a HSV-tk-expressing rat hepatoma cell line and a control cell line (bearing the empty vector) the uptake of 3H-FFUdR was determined with increasing incubation periods. Furthermore, measurements with graded mixtures of HSV-tk-expressing cells and control cells were made. To elucidate the mechanism of FFUdR transport into cells, a series of inhibition/competition experiments was performed with challenge inhibitors of the nucleoside and the nucleobase transport systems. RESULTS: The uptake studies with tritiated FFUdR revealed a 14- to 19-fold higher accumulation in the HSV-tk-expressing cell line compared to the control cell line. While the 3H-FFUdR uptake was 3- to 4-fold higher than the 3H-ganciclovir uptake in the HSV-tk-expressing cells, it was also higher in control cells (5-fold). Furthermore, FFUdR accumulation was linearly correlated with the amount of HSV-tk-expressing cells. FFUdR uptake and growth inhibition by therapeutic doses of ganciclovir were highly correlated, with r = 0.96. Inhibition/competition experiments showed that FFUdR is transported mainly by the equilibrative and the concentrative nucleoside transporter but not by the nucleobase transport systems. CONCLUSION: The FFUdR uptake is an indicator of the HSV-tk activity in tumor cells and can be used as a prognostic marker during gene therapy with HSV-tk. The relative merits of ganciclovir and FFUdR as specific substrates for HSV-tk will need to be further explored in vivo.

Utilization of labeled thymidine in DNA synthesis: studies for PET.

Carbon-111-thymidine and positron emission tomography (PET) have the potential for noninvasively measuring DNA synthesis in tumors and tissues. In order to validate this potential, one needs to construct accurate biochemical models that reflect the metabolism of thymidine, including its uptake and degradation as well as its incorporation into DNA. Our studies employed thymidine labeled in the methyl position using 11C, 14C, and 3H. Studies with rapidly proliferating tissues of mice demonstrated that most of the activity, 60 min after injection of labeled thymidine, was present in DNA, with smaller amounts found in metabolites. Studies in dogs, however, reveal that more activity was present in metabolites, rather than in the DNA of tumors and spleen tissue. HPLC analysis of canine blood after the injection of thymidine revealed rapid conversion of thymidine to thymine and other metabolites. We have found significant differences in the metabolism of [3H]thymidine versus thymidine labeled with radioactive carbon. These differences, which were not found when comparing 14C and 11C, indicate that [3H]thymidine is not an appropriate standard for comparison with PET studies employing [11C] thymidine. To accurately interpret images of [11C]thymidine as representations of DNA synthesis we are developing kinetic models that take into account the metabolism of thymidine and the contribution of degradation products to the 11C images.

Analysis of 2-carbon-11-thymidine blood metabolites in PET imaging.

UNLABELLED: Carbon-11-thymidine labeled in the ring-2 position was used with PET to image tumor and tissue proliferation. Since thymidine is rapidly degraded in the body, one must consider the generation of metabolites to fully interpret the PET data. METHODS: We have measured the blood time-activity curves of thymidine and its metabolites in arterial blood samples. Blood was processed to obtain three input curves, including the total activity, the activity with CO2 removed and the fraction of CO2-free activity in intact thymidine (% Tdr). RESULTS: We found that CO2 reached a plateau of 65% (+/- 12%) of total blood activity by 11 min after injection. When a 1-min infusion of labeled thymidine is used, the time to 50% degradation to thymine and metabolites other than CO2 (measured in acidified samples by HPLC) was 2.9 +/- 0.6 min. We fit the results of the blood metabolism with a compartmental model. We found that we could accurately determine the % Tdr curve with as few as three measured points with an root mean square (RMS) error of 2% in the integrated curve, compared to the curve using all blood samples (mean of seven samples per patient). The integral of thymidine blood activity serves as the input to thymidine models, so similar errors could be expected in calculations of DNA synthetic rates. We found that the determination of CO2 could be accomplished with as few as five samples, with an RMS error of 4% in plateau %CO2 value. CONCLUSION: While it is essential to take metabolites into account when interpreting results obtained with 11C-thymidine, the reproducibility of these degradation curves may allow the use of a limited number of samples to measure the catabolic products of thymidine. These data from the blood, along with tissue kinetic models, are needed to calculate DNA synthetic rates.

Kinetic analysis of 2-[11C]thymidine PET imaging studies: validation studies.

UNLABELLED: 2-[11C]thymidine has been tested as a PET tracer of cellular proliferation. We have previously described a model of thymidine and labeled metabolite kinetics for use in quantifying the flux of thymidine into DNA as a measure of tumor proliferation. We describe here the results of studies to validate some of the model's assumptions and to test the model's ability to predict the time course of tracer incorporation into DNA in tumors. METHODS: Three sets of studies were conducted: (a) The uptake of tracers in proliferative tissues of normal mice was measured early after injection to assess the relative delivery of thymidine and metabolites of thymidine catabolism (thymine and CO2) and calculate relative blood-tissue transfer rates (relative K1s). (b) By using sequential injections of [11C]thymidine and [11C]thymine in normal human volunteers, the kinetics of the first labeled metabolite were measured to determine whether it was trapped in proliferating tissue such as the bone marrow. (c) In a multitumor rat model, 2-[14C]thymidine injection, tumor sampling and quantitative DNA extraction were performed to measure the time course of label uptake into DNA for comparison with model predictions. RESULTS: Studies in mice showed consistent relative delivery of thymidine and metabolites in somatic tissue but, as expected, showed reduced delivery of thymidine and thymine in the normal brain compared to CO2. Thymine studies in volunteers showed only minimal trapping of label in bone marrow in comparison to thymidine. This quantity of trapping could be explained by a small amount of fixation of labeled CO2 in tissue, a process that is included as part of the model. Uptake experiments in rats showed early incorporation of label into DNA, and the model was able to fit the time course of uptake. CONCLUSION: These initial studies support the assumptions of the compartmental model and demonstrate its ability to quantify thymidine flux into DNA by using 2-[11C]thymidine and PET. Results suggest that further work will be necessary to investigate the effects of tumor heterogeneity and to compare PET measures of tumor proliferation to in vitro measures of proliferation and to clinical tumor behavior in patients undergoing therapy.

Kinetic analysis of 2-[carbon-11]thymidine PET imaging studies: compartmental model and mathematical analysis.

UNLABELLED: Carbon-11-thymidine is a PET tracer of DNA synthesis and cellular proliferation. Quantitative analysis of [11C]thymidine images is complicated by the presence of significant quantities of labeled metabolites. Estimation of the rate of thymidine incorporation into DNA using [11C]thymidine requires a kinetic model that is capable of describing the behavior of thymidine and labeled metabolites. METHODS: Based on previous studies with labeled thymidine, we constructed a five-compartment model describing the kinetic behavior of 2-[11C]thymidine and its labeled metabolites. In addition, we have performed a series of calculations and simulations to calculate the sensitivity and identifiability of model parameters to estimate the extent to which individual parameters can be estimated; to determine appropriate model constraints necessary for reproducible estimates of the constant describing flux of thymidine from the blood into DNA, i.e., thymidine flux constant; and to determine the potential accuracy of model parameter and thymidine flux constant estimates from PET imaging data. RESULTS: The underlying assumptions in the thymidine compartmental model lead to a description of the thymidine flux constant for DNA incorporation in terms of model parameters. Sensitivity and identifiability analyses suggest that the model parameters pertaining to labeled metabolites will be difficult to estimate independently of the thymidine parameters. Exact evaluation of the kinetic parameters of the labeled metabolites is not the principal goal of this model. Simulations were performed that suggest that it is preferable to tightly constrain these parameters to preset values near the center of their expected ranges. Although it is difficult to estimate individual thymidine model parameters, the flux constant for incorporation into DNA can be accurately estimated (r > 0.9 for estimated versus true simulated flux constant). Flux constant estimates are not affected by modest levels of local degradation of thymidine that may occur in proliferating tissue. CONCLUSION: By using a kinetic model for thymidine and labeled metabolites, it is possible to estimate the flux of thymidine uptake and incorporation into DNA and, thereby, noninvasively estimate regional cellular proliferation using [11C]thymidine and PET.