High efficacy of particle beam therapies against tumors under hypoxia and prediction of the early stage treatment effect using 3'-deoxy-3'-[18F]fluorothymidine positron emission tomography.

OBJECTIVE: Compared with radiation therapy using photon beams, particle therapies, especially those using carbons, show a high relative biological effectiveness and low oxygen enhancement ratio. Using cells cultured under normoxic conditions, our group reported a greater suppressive effect on cell growth by carbon beams than X-rays, and the subsequent therapeutic effect can be predicted by the cell uptake amount of 3'-deoxy-3'-[18F]fluorothymidine (18F-FLT) the day after treatment. On the other hand, a hypoxic environment forms locally around solid tumors, influencing the therapeutic effect of radiotherapy. In this study, the influence of tumor hypoxia on particle therapies and the ability to predict the therapeutic effect using 18F-FLT were evaluated. METHODS: Using a murine colon carcinoma cell line (colon 26) cultured under hypoxic conditions (1.0% O2 and 5.0% CO2), the suppressive effect on cell growth by X-ray, proton, and carbon irradiation was evaluated. In addition, the correlation between decreased 18F-FLT uptake after irradiation and subsequent suppression of cell proliferation was investigated. RESULTS: Tumor cell growth was suppressed most efficiently by carbon-beam irradiation. 18F-FLT uptake temporarily increased the day after irradiation, especially in the low-dose irradiation groups, but then decreased from 50 h after irradiation, which is well correlated with the subsequent suppression on tumor cell growth. CONCLUSIONS: Carbon beam treatment shows a strong therapeutic effect against cells under hypoxia. Unlike normoxic tumors, it is desirable to perform 18F-FLT positron emission tomography 2-3 days after irradiation for early prediction of the treatment effect.

Visualization research on ENT1/NIS dual-function gene therapy to reverse drug resistance mediated by MUC1 in GEM-resistant pancreatic cancer.

PURPOSE: To use bifunctional target genes to increase the intracellular transport of gemcitabine (GEM) to reverse chemotherapy resistance and to simultaneously use reporter gene imaging to localize therapeutic genes. The therapeutic effect was evaluated by [18F]FLT PET/CT to visualize the effect of gene therapy. METHODS: A viral gene vector containing the pancreatic cancer-targeting promoter MUC1 for specific transcription of equilibrative nucleoside transporter 1 (ENT1) and NIS (nuclide transport channel) was employed. [125I]NaI uptake tests and [131I]NaI SPECT imaging were performed to verify the function of NIS and the target function of MUC1. The correlation between [18F]FLT uptake and GEM resistance were assessed, and the influence ENT1 and thymidine kinase 1 (TK1) expression on [18F]FLT micro-PET/CT was measured, which provides a theoretical basis for the use of [18F]FLT micro-PET/CT to evaluate the efficacy of gene therapy. RESULTS: First, functions of gene therapy were confirmed: ENT1 reversed the drug resistance of GEM-resistant pancreatic cancer cells by increasing GEM intracellular transport; MUC1 drove NIS target gene expression in pancreatic cancer; and therapeutic genes could be localized using [131I]NaI SPECT reporter gene imaging. Second, the [18F]FLT uptake ratio was affected by drug resistance and GEM treatment. The mechanism underlying this effect was related to ENT1 and TK1. Increased expression of ENT1 inhibited the expression of TK1 after GEM chemotherapy to reduce the uptake of [18F]FLT. Finally, micro-PET/CT indicated that the SUVmax of [18F]FLT could predict survival time. SUVmax exhibited an increasing trend in resistant pancreatic cancer but a trend of inhibition after upregulation of ENT1, which was more significant after GEM treatment. CONCLUSIONS: Bifunctional targeted genes can localize therapeutic genes through reporter gene imaging, reverse the drug resistance of GEM-resistant pancreatic cancer and be visually evaluated through [18F]FLT micro-PET/CT.

PD1 blockade alters cell-cycle distribution and affects 3'-deoxy-3'-[18F]fluorothymidine uptake in a mouse CT26 tumor model.

OBJECTIVE: We previously reported that alterations of the tumor microenvironment (TME) by programmed death receptor-1 (PD1) blockade affected tumor glucose metabolism and tumor 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) uptake. In cancer cells, high glycolysis allows cells to sustain rapid proliferation since glycolysis is closely related to the proliferation of cancer cells. Therefore, imaging of cellular proliferation may provide more detail of TME alterations. In this study, we investigated how TME alterations by PD1 blockade affects the uptake of 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT), which is a 18F-radiolabeled thymidine derivative and is taken up by proliferating cells. METHODS: Mice inoculated with murine colon carcinoma CT26 cells were intraperitoneally administered an anti-PD1 antibody on Day 0, when the tumor volume exceeded 50 mm3, and Day 5. [18F]FLT-PET imaging was performed pre-treatment (Day 0) and post treatment (Day 7). Tumor infiltrating lymphocytes (TILs) were identified by flow cytometry. [18F]FLT accumulation and localization in tumor tissue was evaluated by autoradiography and immunohistochemistry. The cell-cycle distribution of tumors and CT26 cells exposed to cytokines (interleukin-2, interferon [INF]-γ, and tumor necrosis factor [TNF]-α) was analyzed by flow cytometry. RESULTS: PD1 blockade increased CD8+ and CD4+ T cells in tumor tissue and significantly suppressed tumor proliferation; however, tumor [18F]FLT uptake remained unchanged. Autoradiography and immunohistochemistry showed that [18F]FLT was mainly taken up by cancer cells, but not TILs. Flow cytometric analysis demonstrated that the population of cells in G2/M phase increased after PD1 blockade. Moreover, INF-γ and TNF-α significantly increased cells in G2/M phase in vitro. CONCLUSION: PD1 blockade-induced alteration of the TME increased CT26 tumor cells in the G2/M phase, which have high thymidine kinase 1 activity. Therefore, [18F]FLT is taken up by tumor cells even if tumor proliferation is suppressed. This observation may be useful for evaluating the response to immunotherapy.

3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT) Positron Emission Tomography as an In Vivo Biomarker of inhibition of CDK 4/6-Rb pathway by Palbociclib in a patient derived bladder tumor.

BACKGROUND: Several new generation CDK4/6 inhibitors have been developed and approved for breast cancer therapy in combination with endocrine therapeutics. Application of these inhibitors either alone or in combination in other solid tumors has been proposed, but no imaging biomarkers of response have been reported in non-breast cancer animal models. The purpose of this study was to evaluate 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT) Positron Emission Tomography (PET) as in vivo biomarker of response to palbociclib in a non-breast cancer model. METHODS: Twenty-four NSG mice bearing patient derived xenografts (PDX) of a well-characterized bladder tumor were randomized into 4 treatment groups: vehicle (n = 6); palbociclib (n = 6); temozolomide (n = 6); and palbociclib plus temozolomide (n = 6) and treated with two cycles of therapy or vehicle. Tumor uptake of [18F]FLT was determined by micro-PET/CT at baseline, 3 days, and 9 days post initiation of therapy. Following the second cycle of therapy, the mice were maintained until their tumors reached a size requiring humane termination. RESULTS: [18F]FLT uptake decreased significantly in the palbociclib and combination arms (p = 0.0423 and 0.0106 respectively at day 3 and 0.0012 and 0.0031 at day 9) with stable tumor volume. In the temozolomide arm [18F]FLT uptake increased with day 9 uptake significantly different than baseline (p = 0.0418) and progressive tumor growth was observed during the treatment phase. All groups exhibited progressive disease after day 22, 10 days following cessation of therapy. CONCLUSION: Significant decreases in [18F]FLT uptake as early as three days post initiation of therapy with palbociclib, alone or in combination with temozolomide, in this bladder cancer model correlates with an absence of tumor growth during therapy that persists until day 18 for the palbociclib group and day 22 for the combination group (6 days and 10 days) following cessation of therapy. These results support early modulation of [18F]FLT as an in vivo biomarker predictive of palbociclib therapy response in a non-breast cancer model.

Liver Abscess With High 18F-FDG Uptake and No 18F-Fluorothymidine Uptake.

ABSTRACT: 18F-FDG accumulates not only in malignant lesions but also in infectious and inflammatory ones. 3'-deoxy-3'-18F-fluorothymidine (FLT) has been investigated as a promising PET tracer for evaluating tumor proliferating activity. We report a case of liver abscess during therapy of pancreatic cancer that underwent FDG PET/CT and FLT PET/CT studies. Although FDG PET/CT demonstrated several regions of increased uptake in the liver, FLT PET/CT showed no increased uptake in the liver.

Assessment of tumour hypoxia, proliferation and glucose metabolism in head and neck cancer before and during treatment.

OBJECTIVE: The aim of the study was to assess the feasibility of multitracer positron emission tomography (PET) imaging before and during chemoradiation and to evaluate the predictive value of image-based factors for outcome in locally advanced head and neck cancers treated with chemoradiation. METHODS: In the week prior to the treatment [18F]-2-flu-2-deoxy-D-glucose (FDG), [18F]-3'-flu-3'deoxythymidine (FLT) and [18F]-flumisonidazole (FMISO) imaging was performed. FLT scans were repeated at 14 and 28 Gy and FMISO at 36 Gy. Overall survival, disease-free survival and local control were correlated with subvolume parameters, and with tumour-to-muscle ratio for FMISO. For every tracer, total metabolic tumour volume was calculated. RESULTS: 33 patients were included. No correlation was found between pre-treatment maximum standardised uptake value for FDG, FLT, FMISO and outcomes. Tumour volume measured on initial CT scans and initial FLT volume correlated with disease-free survivall (p = 0.007 and 0.04 respectively). FDG and FLT metabolic tumour volumes correlated significantly with local control (p = 0.005 and 0.02 respectively). In multivariate Cox analysis only individual initial TMRmax correlated with overall survival. CONCLUSION: PET/CT imaging is a promising tool. However, various aspects of image analysis need further clinical validation in larger multicentre study employing uniform imaging protocol and standardisation, especially for hypoxia tracer. ADVANCES IN KNOWLEDGE: Monitoring of biological features of the tumour using multitracer PET modality seems to be a feasible option in daily clinical practice.Evaluation of hypoxic subvolumes is more patient dependent; thus, exploration of individual parameters of hypoxia is needed. tumour-to-muscle ratio seems to be the most promising so far.

False-Positive 18F-FDG and 18F-Fluorothymidine Uptake in a Patient With Round Atelectasis.

A 64-year-old man with no significant medical history demonstrated a right lower lobe opacity on routine chest radiography. Transaxial CT showed a round, well-circumscribed, pleural mass with comet-tail sign in the right lower lobe and pleural thickening with pleural effusion. F-FDG PET/CT showed hypermetabolic activity in the mass (SUVmax 9.61). F-fluorothymidine PET/CT also showed mild increased uptake in the mass (SUVmax 3.26). Lung biopsy and follow-up CT scan revealed round atelectasis.

Combining 3'-Deoxy-3'-[18F] fluorothymidine and MRI increases the sensitivity of glioma volume detection.

OBJECTIVE: 3'-Deoxy-3'-[18F] fluorothymidine (18F-FLT) is a marker of cell proliferation and displays a high tumor-to-background ratio in brain tumor lesions. We determined whether combining 18F-FLT PET and MRI study improves the detection of tumoral tissue compared to MRI alone and whether 18F-FLT uptake has a prognostic value by studying its association with histopathological features. METHODS: Thirteen patients with a supratentorial malignant glioma were recruited and scheduled for surgery. The tumor volume was defined in all patients on both 18F-FLT PET and MRI images. The images were coregistered and uploaded onto a neuronavigation system. During surgery, an average of 11 biopsies per patient were taken in regions of the brain that were positive to one or both imaging modalities, as well as from control peritumoral regions. The standardized uptake values (SUVs) of each biopsy region were correlated to histopathological data (i.e., proliferation index and number of mitoses) and the SUV values of high and low-grade samples were compared. RESULTS: Out of a total of 149 biopsies, 109 contained tumoral tissue at histopathological analysis. The positive predictive value was 93.1% for MRI alone and 78.3% for MRI and PET combined. In addition, 40% of the biopsy samples taken from areas of the brain that were negative at both PET and MRI had evidence of malignancy at pathology. The SUV values were not significantly correlated to either the proliferation index or the number of mitoses, and could not differentiate between high- and low-grade samples. CONCLUSION: In patients with newly diagnosed glioma, a combination of MRI and 18F-FLT-PET detects additional tumoral tissue and this may lead to a more complete surgical resection. Also, the addition of a negative PET to a negative MRI increases the negative predictive value. However, 18F-FLT still underestimated the margins of the lesion and did not correlate with histopathological features.

Evaluation of [18F]FDG/[18F]FLT/[18F]FMISO-based micro-positron emission tomography in detection of liver metastasis in human colorectal cancer.

INTRODUCTION: Positron emission tomography (PET) is extensively used in clinical oncology for tumor detection. This study aimed to explore the application of the radiotracers [18F]fluorodeoxyglucose ([18F]FDG), 3'-deoxy-3'- [18F]fluorothymidine ([18F]FLT), and [18F]fluoromisonidazole ([18F]FMISO) in the diagnosis and monitoring of hepatic metastasis in human colorectal cancer (CRC). METHODS: A mouse model of human CRC with hepatic metastasis was established by intrasplenic implantation of human CRC cell lines LoVo or HCT8. Metastatic potential of these two cell lines was evaluated by wound healing assay in vitro and survival analysis. Uptake of radiotracers between LoVo and HCT8 cells and uptake of radiotracers in the resulting mouse tumor models were examined by in vivo and in vitro experiments. Uptake of each radiotracer in hepatic metastatic lesions was quantified and expressed as standard uptake value (SUV). Protein expression of multiple tumor biomarkers was determined in metastatic lesions. The correlation between tracer uptake and tumor marker expression was evaluated using linear regression. RESULTS: LoVo cells exhibited a stronger metastatic potential and a higher radiotracer uptake ability than HCT8 cells, as evidenced by significantly greater wound closure percentage, shorter survival, higher incidence of liver metastases, and higher cellular radiotracer levels in LoVo cells or LoVo cell-xenografted mice. SUV values of [18F]FLT and [18F]FMISO, but not [18F]FDG, in LoVo cell-derived metastatic lesions were significantly greater than those in HCT8 lesions. Mechanistically, the expression of MACC1, HIF-1α, and GLUT-1(metastasis associated in colon cancer 1, MACC1; hypoxia-inducible factor 1-alpha, HIF-1α; and glucose transporter 1, GLUT-1, respectively) in LoVo cell-derived metastatic lesions was more effectively induced than in HCT8-derived ones. A linear regression analysis demonstrated significant positive correlations between [18F]FLT/[18F]FMISO uptake and tumor biomarker expression in metastatic tissues. CONCLUSIONS: [18F]FLT and [18F]FMISO-based PET imaging may serve as a promising method for early detection and monitoring of hepatic metastasis in patients with CRC.

Equilibrative Nucleoside Transporter 1 (ENT1, SLC29A1) Facilitates Transfer of the Antiretroviral Drug Abacavir across the Placenta.

Abacavir is a preferred antiretroviral drug for preventing mother-to-child human immunodeficiency virus transmission; however, mechanisms of its placental transfer have not been satisfactorily described to date. Because abacavir is a nucleoside-derived drug, we hypothesized that the nucleoside transporters, equilibrative nucleoside transporters (ENTs, SLC29A) and/or Na+-dependent concentrative nucleoside transporters (CNTs, SLC28A), may play a role in its passage across the placenta. To test this hypothesis, we performed uptake experiments using the choriocarcinoma-derived BeWo cell line, human fresh villous fragments, and microvillous plasma membrane (MVM) vesicles. Using endogenous substrates of nucleoside transporters, [3H]-adenosine (ENTs, CNT2, and CNT3) and [3H]-thymidine (ENTs, CNT1, and CNT3), we showed significant activity of ENT1 and CNT2 in BeWo cells, whereas experiments in the villous fragments and MVM vesicles, representing a model of the apical membrane of a syncytiotrophoblast, revealed only ENT1 activity. When testing [3H]-abacavir uptakes, we showed that of the nucleoside transporters, ENT1 plays the dominant role in abacavir uptake into placental tissues, whereas contribution of Na+-dependent transport, most likely mediated by CNTs, was observed only in BeWo cells. Subsequent experiments with dually perfused rat term placentas showed that Ent1 contributes significantly to overall [3H]-abacavir placental transport. Finally, we quantified the expression of SLC29A in first- and third-trimester placentas, revealing that SLC29A1 is the dominant isoform. Neither SLC29A1 nor SLC29A2 expression changed over the course of placental development, but there was considerable interindividual variability in their expression. Therefore, drug-drug interactions and the effect of interindividual variability in placental ENT1 expression on abacavir disposition into fetal circulation should be further investigated to guarantee safe and effective abacavir-based combination therapies in pregnancy.

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.

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.

Correlations of 18F-FDG and 18F-FLT uptake on PET with Ki-67 expression in patients with lung cancer: a meta-analysis.

Background Positron emission tomography (PET) imaging using the radiotracers 18F-fluorodeoxyglucose (FDG) or 18F-fluorothymidine (FLT) has been proposed as imaging biomarkers of cell proliferation. Purpose To explore the correlations of FDG and FLT uptake with the Ki-67 labeling index in patients with lung cancer. Material and Methods Major databases were systematically searched for all relevant literature published in English. The correlation coefficient (rho) and its 95% confidence interval (CI) of individual studies were meta-analyzed using a random-effects model. The sources of heterogeneity were explored by subgroup analyses. Results Twenty-seven articles involving 1213 patients were included in this meta-analysis, comprising 22 studies for FDG uptake/Ki-67 expression correlation and eight for FLT uptake/Ki-67 expression correlation. The pooled rho values for 18F-FDG/Ki-67 correlation and 18F-FLT/Ki-67 correlation were 0.45 (95% CI, 0.41-0.50) and 0.65 (95% CI, 0.56-0.73), respectively, which indicated a moderate correlation for the former and a significant one for the latter. Although the subgroup analyses based on study design, scanner, sample method, and Ki-67 labeling method did not significantly explain the heterogeneity, these factors were potential sources of heterogeneity. In lung cancer, the pooled SUVmax of FDG uptake was significantly higher than that of FLT uptake (7.59 versus 3.86, P < 0.05). In addition, compared to FDG, FLT showed higher specificity yet lower sensitivity for the diagnosis of pulmonary lesions. Conclusion Both 18F-FDG and 18F-FLT correlate significantly with the Ki-67 labeling index in pulmonary lesions, and the latter, with a stronger correlation, may be more reliable for assessing tumor cell proliferation in lung cancer.

Preliminary 19F-MRS Study of Tumor Cell Proliferation with 3'-deoxy-3'-fluorothymidine and Its Metabolite (FLT-MP).

The thymidine analogue 3'-deoxy-3'-[18F]fluorothymidine, or [18F]fluorothymidine ([18F]FLT), is used to measure tumor cell proliferation with positron emission tomography (PET) imaging technology in nuclear medicine. FLT is phosphorylated by thymidine kinase 1 (TK1) and then trapped inside cells; it is not incorporated into DNA. Imaging with 18F-radiolabeled FLT is a noninvasive technique to visualize cellular proliferation in tumors. However, it is difficult to distinguish between [18F]FLT and its metabolites by PET imaging, and quantification has not been attempted using current imaging methods. In this study, we successfully acquired in vivo19F spectra of natural or nonradioactive 3'-deoxy-3'-fluorothymidine ([19F]FLT) and its monophosphate metabolite (FLT-MP) in a tumor xenograft mouse model using 9.4T magnetic resonance imaging (MRI). This preliminary result demonstrates that 19F magnetic resonance spectroscopy (MRS) with FLT is suitable for the in vivo assessment of tumor aggressiveness and for early prediction of treatment response.

MDR1 and BCRP Transporter-Mediated Drug-Drug Interaction between Rilpivirine and Abacavir and Effect on Intestinal Absorption.

Rilpivirine (TMC278) is a highly potent nonnucleoside reverse transcriptase inhibitor (NNRTI) representing an effective component of combination antiretroviral therapy (cART) in the treatment of HIV-positive patients. Many antiretroviral drugs commonly used in cART are substrates of ATP-binding cassette (ABC) and/or solute carrier (SLC) drug transporters and, therefore, are prone to pharmacokinetic drug-drug interactions (DDIs). The aim of our study was to evaluate rilpivirine interactions with abacavir and lamivudine on selected ABC and SLC transporters in vitro and assess its importance for pharmacokinetics in vivo Using accumulation assays in MDCK cells overexpressing selected ABC or SLC drug transporters, we revealed rilpivirine as a potent inhibitor of MDR1 and BCRP, but not MRP2, OCT1, OCT2, or MATE1. Subsequent transport experiments across monolayers of MDCKII-MDR1, MDCKII-BCRP, and Caco-2 cells demonstrated that rilpivirine inhibits MDR1- and BCRP-mediated efflux of abacavir and increases its transmembrane transport. In vivo experiments in male Wistar rats confirmed inhibition of MDR1/BCRP in the small intestine, leading to a significant increase in oral bioavailability of abacavir. In conclusion, rilpivirine inhibits MDR1 and BCRP transporters and may affect pharmacokinetic behavior of concomitantly administered substrates of these transporters, such as abacavir.

Baseline and longitudinal variability of normal tissue uptake values of [18F]-fluorothymidine-PET images.

PURPOSE: [18F]-fluorothymidine ([18F]-FLT) is a PET-tracer enabling in-vivo visualization and quantification of tumor cell proliferation. For qualitative and quantitative analysis, adequate knowledge of normal tissue uptake is indispensable. This study aimed to quantitatively investigate baseline tracer uptake of blood pool, lung, liver and bone marrow and their precision, and to assess the longitudinal effect of systemic treatment on biodistribution. METHODS: 18F-FLT-PET(/CT) scans (dynamic or static) of 90 treatment-naïve oncological patients were retrospectively evaluated. Twenty-three patients received double baseline scans, and another 39 patients were also scanned early and late during systemic treatment with a tyrosine kinase inhibitor. Reproducible volume of interest were placed in blood pool, lung, liver, and bone marrow. For semi-quantitative analysis, SUVmean, SUVmax, and SUVpeak with several normalizations were derived. RESULTS: SUVs of basal lung, liver, and bone marrow were not significantly different between averaged dynamic and static images, in contrast with blood pool and apical lung. Highest repeatability was seen for liver and bone marrow, with repeatability coefficients of 18.6% and 20.4% when using SUVpeak. Systemic treatment with TKIs both increased and decreased normal tissue tracer uptake at early and late time points during treatment. CONCLUSION: Simultaneous evaluation of liver and bone marrow uptake in longitudinal response studies may be used to assess image quality, where changes in uptake outside repeatability limits should trigger investigators to perform additional quality control on individual PET images. ADVANCES IN KNOWLEDGE: For [18F]-FLT PET images, liver and bone marrow have low intra-patient variability when quantified with SUVpeak, but may be affected by systemic treatment. IMPLICATIONS FOR PATIENT CARE: In [18F]-FLT-PET response monitoring trials, liver and bone marrow uptake may be used for quality control of [18F]-FLT PET images.

Intratumoral heterogeneity of 18F-FLT uptake predicts proliferation and survival in patients with newly diagnosed gliomas.

BACKGROUND: The nucleoside analog 3'-deoxy-3'-18F-fluorothymidine (FLT) has been investigated for evaluating tumor proliferating activity in brain tumors. We evaluated FLT uptake heterogeneity using textural features from the histogram analysis in patients with newly diagnosed gliomas and examined correlation of the results with proliferative activity and patient prognosis, in comparison with the conventional PET parameters. METHODS: FLT PET was investigated in 37 patients with newly diagnosed gliomas. The conventional parameters [tumor-to-contralateral normal brain tissue (T/N) ratio and metabolic tumor volume (MTV)] and textural parameters (standard deviation, skewness, kurtosis, entropy, and uniformity) were derived from FLT PET images. Linear regression analysis was used to compare PET parameters and the proliferative activity as indicated by the Ki-67 index. The associations between parameters and overall survival (OS) were tested by Cox regression analysis. RESULTS: Median OS was 662 days. For the conventional parameters, linear regression analysis indicated a significant correlation between T/N ratio and Ki-67 index (p = 0.02) and MTV and Ki-67 index (p = 0.02). Among textural parameters, linear regression analysis indicated a significant correlation for kurtosis (p = 0.003), entropy (p < 0.001), and uniformity (p < 0.001) as compared to Ki-67 index, exceeding those of the conventional parameters. The results of univariate analysis suggested that skewness and kurtosis were associated with OS (p = 0.03 and 0.02, respectively). Mean survival for patients with skewness values less than 0.65 was 1462 days, compared with 917 days for those with values greater than 0.65 (p = 0.02). Mean survival for patients with kurtosis values less than 6.16 was 1616 days, compared with 882 days for those with values greater than 6.16 (p = 0.006). CONCLUSIONS: Based on the results of this preliminary study in a small patient population, textural features reflecting heterogeneity on FLT PET images seem to be useful for the assessment of proliferation and for the potential prediction of survival in newly diagnosed gliomas.

The Synergistic Effect of Selumetinib/Docetaxel Combination Therapy Monitored by [(18)F]FDG/[(18)F]FLT PET and Diffusion-Weighted Magnetic Resonance Imaging in a Colorectal Tumor Xenograft Model.

PURPOSE: Positron emission tomography (PET) and diffusion-weighted MRI (DW-MRI) were used to characterize the treatment effects of the MEK1/2 inhibitor selumetinib (AZD6244), docetaxel, and their combination in HCT116 tumor-bearing mice on the molecular level. PROCEDURES: Mice were treated with vehicle, selumetinib (25 mg/kg), docetaxel (15 mg/kg), or a combination of both drugs for 7 days and imaged at four time points with 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) or 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) followed by DW-MRI to calculate the apparent diffusion coefficient (ADC). Data was cross-validated using the Pearson correlation coefficient (PCC) and compared to histology (IHC). RESULTS: Each drug led to tumor growth inhibition but their combination resulted in regression. Separate analysis of PET or ADC could not provide significant differences between groups. Only PCC combined with IHC analysis revealed the highest therapeutic impact for combination therapy. CONCLUSION: Combination treatment of selumetinib/docetaxel was superior to the respective mono-therapies shown by PCC of PET and ADC in conjunction with histology.

A boronate-caged [¹8F]FLT probe for hydrogen peroxide detection using positron emission tomography.

Reactive oxygen species (ROS) play important roles in the development and progression of cancer and other diseases, motivating the development of translatable technologies for biological ROS imaging. Here we report Peroxy-Caged-[(18)F]Fluorodeoxy thymidine-1 (PC-FLT-1), an oxidatively immolative positron emission tomography (PET) probe for H2O2 detection. PC-FLT-1 reacts with H2O2 to generate [(18)F]FLT, allowing its peroxide-dependent uptake and retention in proliferating cells. The relative uptake of PC-FLT-1 was evaluated using H2O2-treated UOK262 renal carcinoma cells and a paraquat-induced oxidative stress cell model, demonstrating ROS-dependent tracer accumulation. The data suggest that PC-FLT-1 possesses promising characteristics for translatable ROS detection and provide a general approach to PET imaging that can be expanded to the in vivo study of other biologically relevant analytes.

Monitoring the early biologic response of esophageal carcinoma after irradiation with 18F-FLT: an in-vitro and in-vivo study.

OBJECTIVE: The aim of our study was to explore the value of 3'-deoxy-3'-[F]fluorothymidine (F-FLT) and F-FLT PET in monitoring the early biologic response of esophageal carcinoma after irradiation in vitro and in vivo. METHODS: After 2, 4, and 8 h of irradiation at different doses (0, 5, 10, and 15 Gy) of esophageal carcinoma cells in vitro, the uptake ratio of F-FLT, the relative cell survival rate, and ATP levels were measured. The tumor uptake ratio of F-FLT [tumor-to-nontumor (T/NT)] was measured through PET scans before and on the first, seventh, and 15th day after irradiation. The expression of proliferating cell nuclear antigen and Ki-67 was determined in both untreated and treated tumors. RESULTS: Compared with the control group, the uptake ratio changes of F-FLT after 2 h of irradiation with 5 Gy showed no statistical significance (3.65±0.17 vs. 4.00±0.17%, P>0.05), whereas the uptake ratios of the other groups decreased notably (F=33.93, P<0.01). The differences in the relative survival rates were not statistically significant (F=4.02, P>0.05). Linear regression analysis indicated a significant correlation between F-FLT and ATP levels (r=0.89, P<0.01). On F-FLT PET scan images of the xenografts, the baseline uptake ratio (T/NT) was 2.24±0.06. It decreased to 1.99±0.09, 1.85±0.04, and 1.15±0.10 at 1, 7, and 15 days after irradiation with 10 Gy. Tumor uptake of F-FLT was closely correlated with proliferating cell nuclear antigen and Ki-67 expressions (r=0.83, P<0.001, and r=0.88, P<0.001). CONCLUSION: The uptake changes of F-FLT in esophageal carcinoma cells and tumor xenografts may reflect the early biological response of esophageal carcinoma after irradiation. Thus, F-FLT PET may be potentially used to monitor the early response of esophageal carcinoma after radiotherapy.