FLT Uptake Research Articles

High FDG and Low FLT Uptake in a Thyroid Papillary Carcinoma Incidentally Discovered by FDG PET/CT

We report a 58-year-old man whose incidentally discovered papillary thyroid carcinoma in the left lobe showed high FDG and low FLT uptake on PET/CT. The SUVmax was 19.7 for FDG and 3.0 for FLT. The Ki-67 labeling index of the tumor was 1.9%. Thus, the low FLT uptake might be attributed to the low proliferative activity of this cancer.

Correlation between Ki-67 immunohistochemistry and 18F-Fluorothymidine uptake in patients with cancer: A systematic review and meta-analysis

BackgroundPositron emission tomography (PET) imaging using the radiotracer 18F-Fluorothymidine (FLT) has been proposed as an imaging biomarker of tumour proliferation. If FLT-PET can be established as such it will provide a non-invasive, quantitative measurement of tumour proliferation across the entire tumour. Results from validation studies have so far been conflicting with some studies confirming a good correlation between FLT uptake and Ki-67 score and others presenting negative results. MethodsFirstly we performed a systematic review of published studies between 1998 and 2011 that explored the correlation between FLT uptake and Ki-67 score and examined possible variations in the methods used. Studies were eligible if they: (a) included patients with cancer, (b) investigated the correlation between Ki-67 measured by immunohistochemistry and FLT uptake measured with PET scanning, and (c) were published as a full paper in a peer-reviewed scientific journal.Secondly a meta-analysis of the correlation coefficient values reported from each study was performed. Correlation coefficient (r) values were extracted from each study and 95% confidence intervals (CIs) were calculated after applying Fisher’s z transformation. For subgroup analysis, studies were classified by the index used to characterise Ki-67 expression (average or maximum expression), the nature of the sample (whole specimen or biopsy) and the cancer type. FindingsTwenty-seven studies were identified as eligible for the meta-analysis. In the studies we examined there were variations in aspects of the methods and reporting. The meta-analysis showed that given an appropriate study design the FLT/Ki-67 correlation is significant and independent of cancer type. Specifically subgroup analysis showed that FLT/Ki-67 correlation was high in studies measuring the Ki-67 average expression regardless of use of surgery or biopsy samples (r=0.70, 95% CI=0.43–0.86, p<0.001). Of the studies that measured Ki-67 maximum expression, only those that used the whole surgical specimen provided a significant r value (r=0.72, 95% CI=0.54–0.84, p<0.001). Studies that used biopsy samples for Ki-67 maximum measurements did not produce a significant r value (r=0.04, 95% CI=−0.18–0.26, p=0.71). In terms of the cancer type subgroup analysis there is sufficient data to support a strong FLT/Ki-67 correlation for brain, lung and breast cancer. No publication bias was detected. InterpretationThis systematic review and meta-analysis highlights the importance of the methods used in validation studies comparing FLT-PET imaging with the biomarker Ki-67. The correlation is significant and independent of cancer type provided a study design that uses Ki-67 average measurements, regardless of nature of sample, or whole surgical samples when measuring Ki-67 maximum expression. Sufficient data to support a strong correlation for brain, lung and breast cancer exist. However, larger, prospective studies with improved study design are warranted to validate these findings for the rest of the cancer types.

PD-0293 REPRODUCIBILITY AND VOLUME SEGMENTATION METHODS ON F-MISO AND FLT PET-CT IMAGES IN PATIENTS WITH LUNG CANCER

Purpose/Objective: Our objectives were (i) to investigate whether different Nuclear Medicine centres would rate similarly 18 Fmisonidasole (F-miso hypoxia) or 18fluorothymidine (FLT proliferation) uptake and (ii) to compare different segmentation methods, using F-miso and FLT. Materials and Methods: Twenty PET-CT (10 F-miso and 10 FLT) were performed before and during curative-intent radiotherapy in 5 patients with NSCLC. The images were electronically sent to 18 centres of Nuclear Medicine connected through the French Society of Nuclear Medicine network (SFMNnet). In a first phase, each centre had to rank the F-miso and FLT uptake intensity on a 5-level scale, and secondly into a 2-level scale (0=normal, 1=pathologic). The concordance between readers was quantified by weighted kappa tests (K). The presence or absence of F-miso and FLT uptake defined by experts was used as gold standard. In a second phase, potential target F-miso and FLT volumes were delineated with 22 different methods of segmentation: a threshold of 40% and 50% of the maximum standardized uptake value (SUVmax), a threshold fixed at 1.2 and 1.4 of the SUVmax, and 3 methods using an adaptive thresholding related to the SUVmax or SUVmean of a reference tissue (muscle, aorta and lung). Results: As for uptake intensity, the K values between the centres were 0.44 for F-miso and 0.43 for FLT with a 5-level scale, and 0.77 for F-miso and FLT with a 2-level scale. The various segmentation methods yielded significantly different volumes for F-miso and FLT (p max of aortic uptake (sensitivity: 0.87 and 0.71, specificity: 0.72 and 0.96 respectively). For FLT, 6 methods (fixed threshold at 1.4, adaptive threshold based on 1.5 or 1.6 of aortic SUVmax, and adaptive threshold on based 1.3, 1.5 or 1.6 muscle SUVmax) gave the best agreement. Conclusions: The 18 centres were able to reproducibly grade F-miso and FLT uptake on a 2-level (positive/negative) scale. Our data support a fixed threshold (1.4 of aortic SUVmax) to delineate increased F-miso uptake and an adaptive threshold using the aorta and muscle SUVmax background for FLT.

Abstract 376: Limits of [18F]-FLT PET as a predictive imaging biomarker of proliferation in oncology

Abstract Background: Surrogate, non-invasive imaging biomarkers of cellular proliferation hold great promise for quantifying response to personalized medicine in oncology. An emerging imaging approach to assess tumor proliferation utilizes the positron emission tomography (PET) tracer 3′-deoxy-3′[18F]-fluorothymidine, [18F]-FLT. Though several studies have associated serial changes in [18F]-FLT-PET with elements of therapeutic response, the degree to which [18F]-FLT uptake quantitatively reflects proliferative index has been continuously debated for more that a decade. The goal of this preclinical study was to elucidate quantitative relationships between [18F]-FLT-PET and cellular, proteomic, and genomic metrics proliferation in treatment naive tumors. Methods and Findings: [18F]-FLT-PET imaging was conducted in mice bearing human colorectal or breast cancer cell line xenografts. Quantitative relationships between PET imaging, thymidine kinase 1 (TK1) protein and RNA levels, and immunostaining for proliferation markers (Ki67, TK1) were evaluated using imaging-matched tumor specimens. Overall, we found that [18F]-FLT-PET reflects TK1 protein levels in tumors. However, the cell cycle specificity of TK1 expression and the variable degree to which tumors utilize thymidine salvage for DNA synthesis tend to decouple [18F]-FLT-PET data from estimates of proliferation by standard, clinically employed means. Conclusions: Our findings illustrate that [18F]-FLT-PET reflects tumor proliferation as a function of thymidine salvage pathway utilization. Therefore, unlike more generalizable proliferation markers, such as Ki67 immunostaining, tumor uptake of [18F]-FLT reflects proliferative indices to variable and potentially unreliable extents. [18F]-FLT-PET can not discriminate moderately proliferative, thymidine salvage driven disease from highly proliferative tumors that rely primarily upon de novo thymidine synthesis. For those reasons, the magnitude of [18F]-FLT uptake in treatment naive tumors should not be considered a surrogate of proliferative index. These data provide rationale for the diversity of correlative results reported, both positive and discordant, comparing [18F]-FLT-PET and tumor proliferation across numerous single-center clinical studies and provide a basis for future clinical best-practices when this imaging biomarker is employed in oncology. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 376. doi:1538-7445.AM2012-376

Abstract SY24-02: Imaging immune responses in cancer patients

Abstract Antigen-specific immunotherapy has recently progressed through the development of effective therapeutic vaccinations in advanced melanoma (1,2) and prostate cancer (3). Antigen-specific immune responses in cancer patients can also be induced by exploiting autologous dendritic cells (DCs) that are “educated” ex vivo; i.e., DCs that are appropriately activated and loaded with tumor antigens. DCs are the most potent antigen-presenting cells of the immune system and play a central role in the induction and maintenance of antigen-specific immunity (4). These cells capture and process antigen and migrate to the lymph nodes (LNs), where they present the antigen to the adaptive arm of the immune system, inducing antigen-specific T- and B-cell responses. The detection of vaccine-induced immune responses in vivo using a clinically applicable means is critical for the optimization of novel immunotherapies. Positron emission tomography (PET) is a widely available, highly sensitive imaging modality for the in vivo visualization and quantification of molecular processes at a cellular level. Furthermore, whole body imaging allows localization in a longitudinal fashion. These features are necessary to measure immune responses by quantification of the low numbers of proliferating T and B cells early after vaccination in relevant LNs. Thus far, investigators have mainly exploited [18F]-labeled fluoro-2-deoxy-2-d-glucose ([18F]FDG) for PET imaging of proliferating cells, based on the increased glucose metabolism of these cells. Recently, novel tracers have been developed that facilitate imaging of other cellular processes: [18F]-labeled 3α-fluoro-3α-deoxy-thymidine ([18F]FLT) was designed as a tracer for cell proliferation (5) and is increasingly being applied in oncology. However, it has been recognized that enhanced nucleoside demand is not restricted to tumor cells (6). A successful vaccination results in the proliferation of activated lymphocytes in a highly controlled manner within LNs. This proliferation is accompanied by a large metabolic switch in lymphocytes and could serve as a marker of immune responsiveness. We demonstrate that [18F]FLT PET can be used to directly monitor antigen-specific immune responses in vivo shortly after therapeutic vaccination, because it offers a sensitive tool to study the kinetics and localization of induction of antigen-specific lymphocyte activation upon vaccination with antigen-loaded autologous DCs (7). Melanoma patients with lymph node (LN) metastases received DC vaccine therapy, injected intranodally, followed by [18F]FLT PET at varying time points after vaccination. Control LNs received saline or DCs without antigen. De novo immune responses were readily visualized in treated LNs early after the prime vaccination, and these signals persisted for up to 3 weeks. This selective [18F]FLT uptake was markedly absent in control LNs, although tracer uptake in treated LNs increased profoundly with as little as 4.5 × 105 DCs. Immunohistochemical staining confirmed injected DC dispersion to T-cell areas and resultant activation of CD4+ and CD8+ T cells. The level of LN tracer uptake significantly correlates to the level of circulating antigen-specific IgG antibodies and antigen-specific proliferation of T cells in peripheral blood. Furthermore, this correlation was not observed with [18F]FDG. Therefore, [18F]FLT PET offers a sensitive tool to study the kinetics, localization, and involvement of lymphocyte subsets in response to vaccination. Recently published large phase III trials, demonstrate that immunotherapies can be effective even in advanced cancer patients. Both antigen-specific (2,3) and nonantigen specific agents (2) have been approved by the Food and Drug Administration. Considering the amount of effort poured into the development of these novel agents, PET-based monitoring will advance our knowledge of the immunological processes that precede the failure or success of novel immunotherapies. Furthermore, it should vastly improve efficient application by aiding in individualized decision making.

Imaging of a localized bacterial infection with endogenous thymidine kinase using radioisotope-labeled nucleosides

The importance of noninvasive imaging methods to bacterial infections is widely recognized. To obtain bacterial infection imaging with radioisotope-labeled nucleosides, bacterial thymidine kinase (tk) activities of Salmonella typhimurium with [125I]5-iodo-1-(2′-fluoro-2′-deoxy-β-d-arabinofuranosyl)uracil ([125I]FIAU) or 3′-deoxy-3′-[18F]fluorothymidine ([18F]FLT) were measured. The infection model in BALB/c mice was imaged with [125I]FIAU or [18F]FLT using small-animal Single Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET), respectively.The accumulated radioactivity of [125I]FIAU or [18F]FLT in the two strains showed a linearly increased pattern with increasing incubation time or bacterial numbers. The image clearly demonstrated a high uptake of [125I]FIAU and [18F]FLT in the bacterial infection site. [18F]FLT uptake in the infection site of was 7.286±2.405, whereas that in the uninfected site was 0.519±0.561. The relative activity ratio of the infected region in relation to the uninfected region was 2.98 at 4h after an injection with [125I]FIAU determined by biodistribution data.In conclusion, the bacterial tk activity was confirmed by the cellular uptake and imaging with [125I]FIAU or [18F]FLT. Therefore, a localized bacterial infection in living mice can be monitored using radioisotope-labeled nucleosides with a nuclear medicine imaging modality.

A Pilot Study Comparing FLT-PET and FDG-PET in the Evaluation of Response to Cetuximab and Radiation Therapy in Advanced Head and Neck Malignancies

Background: We prospectively compared FLT-PET and FDG-PET in evaluating response to cetuximab and chemoradiotherapy for HNSCC. Methods: Six patients with HNSCC received cetuximab followed by chemoradiotherapy. Patients had FLTand FDG-PET scans at baseline, after cetuximab, and 2 weeks into chemoradiotherapy. Changes in SUVmax on successive scans were compared to baseline. Results: After induction therapy, changes in SUVmax ranged from -2 to 32% for FLT-PET and -24 to 0% for FDGPET. After two weeks of chemoradiotherapy, changes in SUVmax ranged from -71 to 9% for FLT-PET and -80 to -7% for FDG-PET. One patient experienced consecutive increases in FLT uptake not detected by FDG-PET. No patient recurred at a median 14.6 months. Conclusions: Functional imaging early during definitive therapy for HNSCC is feasible. Similar changes in FLT and FDG uptake are detected during chemoradiotherapy; however, distinct differences were seen after induction cetuximab therapy. Further follow-up will facilitate correlation of radiotracer uptake with clinical outcome.

OT2-05-03: ACRIN 6688 Phase II Study of Fluorine-18 3′-Deoxy-3′ Fluorothymidine (FLT) in Invasive Breast Cancer.

Abstract Background Neoadjuvant chemotherapy (NAC) prior to surgery provides enhanced options for locoregional management and has become an integral component of primary breast cancer management. Initial tumor response in patients receiving NAC is generally determined at therapy completion. This evaluation is determined by either the presence/absence of palpable tumor as a clinical response and/or presence/absence of invasive tumor cells in the breast and nodes as a pathological response. The ability to evaluate the effectiveness of neoadjuvant therapy early during treatment would be of significant importance. FDG PET imaging has been shown to be predictive of subsequent tumor response, but the tendency of FDG to accumulate in inflammatory tissues can complicate image interpretation. MRI changes have also been touted as predictors of response. Preliminary data suggest that early FLT PET is better able to predict response to therapy, as FLT uptake has been shown to correlate with cellular proliferation, and does not significantly accumulate in inflammatory tissue (Kenny et al, Eur J Nucl Med Mol Imaging 2007:1339–1347). The analysis of these data may provide a better understanding of early treatment response and improve the clinical management of breast cancer in the future. Trial design and eligibility: In this phase II multi-institutional study, breast cancer patients with locally advanced disease with a tumor size ≥2cm (measured on imaging or estimated by physical exam) are eligible. Participants will receive standard of care NAC at their respective institutions. Participants will have 3 FLT imaging sessions to evaluate therapy response: at baseline, early-treatment (5-10 days after initiating treatment), and post-treatment prior to surgery. Specific aims: The primary objective is to correlate the percentage change in the standardized uptake value between baseline and early therapy FLT in the primary tumor with pathologic response. Correlatively, FLT PET parameters will be compared with proliferative indices from the initial biopsy and residual tumor surgical samples using Ki-67 staining, mitotic index, and residual cancer burden. Potential safety issues and the physiologic effects associated with FLT administration will also be evaluated. Statistical methods: To evaluate the relationship between an uptake parameter and pathologic complete response, a ROC curve will be estimated and the area under the curve, along with its 95% confidence interval, will be determined. Accrual: Currently, 45/67 patients have accrued to the study. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr OT2-05-03.

18F]-3′Deoxy-3′-Fluorothymidine Positron Emission Tomography and Breast Cancer Response to Docetaxel

To establish biomarkers indicating clinical response to taxanes, we determined whether early changes in [(18)F]-3'deoxy-3'-fluorothymidine positron emission tomography (FLT-PET) can predict benefit from docetaxel therapy in breast cancer. This was a prospective unblinded study in 20 patients with American Joint Committee on Cancer (AJCC) stage II-IV breast cancer unresponsive to first-line chemotherapy or progressing on previous therapy. Individuals underwent a baseline dynamic FLT-PET scan followed by a scan 2 weeks after initiating the first or second cycle of docetaxel. PET variables were compared with anatomic response midtherapy (after 3 cycles). Average and maximum tumor standardized uptake values at 60 minutes (SUV(60,av) and SUV(60,max)) normalized to body surface area ranged between 1.7 and 17.0 and 5.6 and 26.9 × 10(-5) m(2)/mL, respectively. Docetaxel treatment resulted in a significant decrease in FLT uptake (P = 0.0003 for SUV(60,av) and P = 0.0002 for SUV(60,max)). Reduction in tumor SUV(60,av) was associated with target lesion size changes midtherapy (Pearson R for SUV(60,av) = 0.64; P = 0.004) and predicted midtherapy target lesion response (0.85 sensitivity and 0.80 specificity). Decreases in SUV(60,av) in responders were due, at least in part, to reduced net intracellular trapping of FLT (rate constant, K(i)). Docetaxel significantly reduced K(i) by 51.1% (±28.4%, P = 0.0009). Changes in tumor proliferation assessed by FLT-PET early after initiating docetaxel chemotherapy can predict lesion response midtherapy with good sensitivity warranting prospective trials to assess the ability to stop therapy in the event of non-FLT-PET response.

Characterization of Alisertib (MLN8237), an Investigational Small-Molecule Inhibitor of Aurora A Kinase Using Novel In Vivo Pharmacodynamic Assays

Small-molecule inhibitors of Aurora A (AAK) and B (ABK) kinases, which play important roles in mitosis, are currently being pursued in oncology clinical trials. We developed three novel assays to quantitatively measure biomarkers of AAK inhibition in vivo. Here, we describe preclinical characterization of alisertib (MLN8237), a selective AAK inhibitor, incorporating these novel pharmacodynamic assays. We investigated the selectivity of alisertib for AAK and ABK and studied the antitumor and antiproliferative activity of alisertib in vitro and in vivo. Novel assays were used to assess chromosome alignment and mitotic spindle bipolarity in human tumor xenografts using immunofluorescent detection of DNA and alpha-tubulin, respectively. In addition, 18F-3'-fluoro-3'-deoxy-l-thymidine positron emission tomography (FLT-PET) was used to noninvasively measure effects of alisertib on in vivo tumor cell proliferation. Alisertib inhibited AAK over ABK with a selectivity of more than 200-fold in cells and produced a dose-dependent decrease in bipolar and aligned chromosomes in the HCT-116 xenograft model, a phenotype consistent with AAK inhibition. Alisertib inhibited proliferation of human tumor cell lines in vitro and produced tumor growth inhibition in solid tumor xenograft models and regressions in in vivo lymphoma models. In addition, a dose of alisertib that caused tumor stasis, as measured by volume, resulted in a decrease in FLT uptake, suggesting that noninvasive imaging could provide value over traditional measurements of response. Alisertib is a selective and potent inhibitor of AAK. The novel methods of measuring Aurora A pathway inhibition and application of tumor imaging described here may be valuable for clinical evaluation of small-molecule inhibitors.

Does 18F-Fluorodeoxyglucose Outperform 18F-Fluorothymidine When Using Positron Emission Tomography in Predicting Transformation of Indolent Non-Hodgkin's Lymphoma,

Abstract 3658Introduction Patients (pts) with transformed NHL have an extremely poor prognosis. However when diagnosed at an early stage outcome may be better, making early diagnosis crucial. Nowadays, transformation is diagnosed using biopsy of a lymph node when it starts growing rapidly or when the patient develops B symptoms or hypercalcemia. These pts often have advanced disease and diagnosis may be delayed because the lymph node biopsied is not representative. Positron emission tomography (PET) might be of value here: the commonly used tracer 18F-fluorodeoxyclucose (FDG)-PET can distinguish between indolent and aggressive lymphoma, but with considerable overlap in Standard Uptake Values (SUV). However, 18F-fluorothymidine (FLT) is thought to more directly reflect proliferation. We performed a head-to-head comparison of FDG and FLT PET in pts with indolent and transformed lymphoma to explore which tracer qualifies best for future research in timely diagnosis of transformation.Materials and methods Pts were selected based on histology: either follicular lymphoma or transformed lymphoma (defined as diffuse large B cell lymphoma diagnosed in a patient with former or simultaneous diagnosis of follicular lymphoma in a lymph node).In each patient two PET scans were made, one with FDG and one with FLT, maximum one week apart, before any treatment.Scans were made on the Philips Gemini TF PET-CT camera, 1 hour after injection of 185 MBq of FDG or FLT.Uptake (we present data as SUVmax normalized to body weight) was measured in all lymph nodes ≥ 2 cm (minimizing partial volume effects).Results 17 pts with indolent lymphoma and 10 with transformed lymphoma were included. Median age was 59 years (range 35–81) and a median of 9 lymph nodes were measured per patient (range 2–23). Between patients, SUVmax in the lymph node with the highest uptake for both FDG (p=0.01) as well as FLT (p=0.04) uptake was significantly higher in transformed lymphoma. For FDG values ranged from 4,9 to 19,6 in indolent and from 9,6 to 29,9 in transformed lymphoma, for FLT values were respectively 3,6 to 16,6 and 5,45 to 16,3. Accordingly, between patients, we found considerable overlap between highest values for indolent and transformed lymphoma with either tracer, making the determination of a cut off level for transformation difficult. The data suggest that, at least for FDG, the range between the lymph node with the highest and the lowest uptake within one patient was the best criterium to identify the transformed lymphomas, with significantly higher ranges for transformed lymphoma (p<0,001; ranges from 0,34 to 11,3 for indolent and from 5,1 to 20,2 for transformed lymphoma). A cut off value of 7 (highest SUVmax minus lowest SUVmax) has a positive predictive value of 90% for transformation.Conclusion Uptake of both FDG and FLT are significantly higher in transformed lymphoma than in indolent lymphoma. However, FDG-PET may better distinguish between indolent and transformed lymphoma. For further research, intrapatient heterogeneity of FDG uptake qualifies as a potential marker of transformation, ultimately to facilitate timely diagnosis of transformation and improved patient outcome. Disclosures:No relevant conflicts of interest to declare.

Abstract A44: 18F-FLT, a potential biomarker for predicting the synergistic combination of PI3K inhibitor BAY 80-6946* and MEK inhibitor BAY 86-9766.

Abstract Signaling pathways of particular interest in the cancer field are the RAS-RAF-MEK and PI3K-AKT pathways that act as parallel conduits to transduce signals involved in tumor cell proliferation, survival, and cell migration. Compared to targeting either pathway separately, simultaneous inhibition of these two key pathways has been considered to have greater therapeutic efficacy and broader applicability for the treatment of cancers. One of the challenges is the discovery of biomarkers that correlate with efficacy and synergy of PI3K and MEK combination therapy in early-stage clinical trials. To identify the best imaging tracer for monitoring the synergistic anti-tumor activity of BAY 80-6946* and BAY 86-9766, we first compared FDG and FLT uptake in vitro in 2 colorectal (CRC) and 2 non-small cell lung cancer (NSCLC) cell lines bearing either KRAS or KRAS plus PIK3CA mutations. Partial inhibition of FDG- and FLT-uptake (&amp;lt;50%) was observed in tumor cells treated with each agent, while synergistic and complete inhibition of FLT-uptake (&amp;gt;94%) was demonstrated in the combination group. Interestingly, the maximum inhibition of FDG-uptake was much weaker compared to FLT-uptake, indicating that FLT might be a better tracer for PI3K and MEK combination therapy. Further investigation iwas conducted in HCT116 CRC xenografts in mice and H460 NSCLC xenografts in rats. BAY 80-6946* was administered intraveneously (Q2D) at 14 mg/kg in mice and at 3 mg/kg in rats whereas BAY 86-9766 was dosed orally (QD) at 25 mg/kg in mice and at 15 mg/kg in rats. Tumor uptake of 18F-FLT was investigated by PET imaging as well as by quantitative biodistribution 1 day before and 1 day after treatment start. In both models, 18F-FLT tumor uptake was not significantly altered in animals treated with each single agent. However, one day after the treatment in the combination group, 18F-FLT tumor uptake was reduced from 8.50±0.99 %ID/g to 3.91±0.66 %ID/g (n=5, vs vehicle, p&amp;lt;0.05) in HCT116 bearing mice and from 0.91±0.27 %ID/g to 0.31±0.04 %ID/g (n=5, vs vehicle, p&amp;lt;0.05) in H460 bearing rats. Inhibition of 18F-FLT uptake in tumors ocurred before any significant reduction in tumor volume and correlates with tumor growth inhibition assessed by the tumor size at the later time points. Further detailed molecular mechanism on the synergistic inhibition of 18F-FLT uptake by combination of PI3K and MEK inhibitors will be presented. In conclusion, 18F-FLT might be a non-invasive PET imaging tracer that, when used serially ealy in treatment, may predict clinical efficacy and combination synergy of PI3K and MEK inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A44.

Early identification of antigen-specific immune responses in vivo by [ 18 F]-labeled 3′-fluoro-3′-deoxy-thymidine ([ 18 F]FLT) PET imaging

Current biomarkers are unable to adequately predict vaccine-induced immune protection in humans with infectious disease or cancer. However, timely and adequate assessment of antigen-specific immune responses is critical for successful vaccine development. Therefore, we have developed a method for the direct assessment of immune responses in vivo in a clinical setting. Melanoma patients with lymph node (LN) metastases received dendritic cell (DC) vaccine therapy, injected intranodally, followed by [(18)F]-labeled 3'-fluoro-3'-deoxy-thymidine ([(18)F]FLT) PET at varying time points after vaccination. Control LNs received saline or DCs without antigen. De novo immune responses were readily visualized in treated LNs early after the prime vaccination, and these signals persisted for up to 3 wk. This selective [(18)F]FLT uptake was markedly absent in control LNs, although tracer uptake in treated LNs increased profoundly with as little as 4.5 × 10(5) DCs. Immunohistochemical staining confirmed injected DC dispersion to T-cell areas and resultant activation of CD4(+) and CD8(+) T cells. The level of LN tracer uptake significantly correlates to the level of circulating antigen-specific IgG antibodies and antigen-specific proliferation of T cells in peripheral blood. Furthermore, this correlation was not observed with [(18)F]-labeled fluoro-2-deoxy-2-D-glucose. Therefore, [(18)F]FLT PET offers a sensitive tool to study the kinetics, localization, and involvement of lymphocyte subsets in response to vaccination. This technique allows for early discrimination of responding from nonresponding patients in anti-cancer vaccination and aid physicians in individualized decisionmaking.

Comparison of the diagnostic value of 3-deoxy-3-18F-fluorothymidine and 18F-fluorodeoxyglucose positron emission tomography/computed tomography in the assessment of regional lymph node in thoracic esophageal squamous cell carcinoma: a pilot study

We used pathological examination as golden standard to determine whether 3-deoxy-3-(18)F-fluorothymidine positron emission tomography/computed tomography (FLT PET/CT) can detect regional lymph node metastasis in untreated thoracic esophageal squamous cell carcinoma and additionally performed (18)F-fluorodeoxyglucose (FDG) PET/CT for direct comparison with that of FLT. Twenty-two patients with thoracic esophageal squamous cell carcinoma underwent dual-tracer PET/CT examinations before surgery. The results of reviewing CT images and side-by-side FDG PET and FLT PET images for the diagnosis of locoregional lymph node metastasis were compared prospectively in relation to pathologic findings. All patients underwent esophagectomy and lymphadenectomy. Pathologic examination confirmed nodes positive for metastasis in 16 patients and 47 of 424 excised nodes. The uptake of FDG (median SUVmax, 5.4; range, 2.4-10.6) in locoregional lymph nodes metastases was significantly higher than that of FLT (median SUVmax, 2.8; range, 1.3-4.6). There were 14 false-positive nodes in FDG PET/CT and only 3 in FLT PET/CT; 8 false-negative nodes in FDG PET/CT, while there were 12 false negative nodes in FLT PET/CT. The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of FLT PET/CT were 74.47%, 99.20%, 96.46%, 92.11%, and 96.89%, respectively, whereas those of FDG PET/CT were 82.98%, 96.29%, 94.81%, 82.98%, and 96.29%, respectively. P-values were 0.450, 0.014, 0.313, 0.050, and 0.555, respectively. FLT uptake in regional lymph nodes of esophageal carcinoma is significantly lower compared with FDG uptake. FLT PET/CT has fewer false-positive findings and higher specificity compared with FDG PET/CT.

Gastric Cancer Growth Control by BEZ235In VivoDoes Not Correlate with PI3K/mTOR Target Inhibition but with [18F]FLT Uptake

In this study, we tested the antitumor activity of the dual phosphoinositide 3-kinase (PI3K)/mTOR inhibitor BEZ235 against gastric cancer in vitro and in vivo. Gastric cancer cell lines (N87, MKN45, and MKN28) were incubated with BEZ235 and assessed for cell viability, cell cycle, and PI3K/mTOR target inhibition. In vivo, athymic nude mice were inoculated with N87, MKN28, or MKN45 cells and treated daily with BEZ235. 3'-Deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) uptake was measured via small animal positron emission tomography (PET). In vitro, BEZ235 dose dependently decreased the cell viability of gastric cancer cell lines. The antiproliferative activity of BEZ235 was linked to a G(1) cell-cycle arrest. In vivo, BEZ235 treatment resulted in PI3K/mTOR target inhibition as shown by dephosphorylation of AKT and S6 protein in all xenograft models. However, BEZ235 treatment only inhibited tumor growth of N87 xenografts, whereas no antitumor effect was observed in the MKN28 and MKN45 xenograft models. Sensitivity to BEZ235 in vivo correlated with downregulation of the proliferation marker thymidine kinase 1. Accordingly, [(18)F]FLT uptake was only significantly reduced in the BEZ235-sensitive N87 xenograft model as measured by PET. In conclusion, in vivo sensitivity of gastric cancer xenografts to BEZ235 did not correlate with in vitro antiproliferative activity or in vivo PI3K/mTOR target inhibition by BEZ235. In contrast, [(18)F]FLT uptake was linked to BEZ235 in vivo sensitivity. Noninvasive [(18)F]FLT PET imaging might qualify as a novel marker for optimizing future clinical testing of dual PI3K/mTOR inhibitors.

An Evaluation of 2-deoxy-2-[18F]Fluoro-D-Glucose and 3′-deoxy-3′-[18F]-Fluorothymidine Uptake in Human Tumor Xenograft Models

The aim of this study is to assess the variability of 2-deoxy-2-[(18)F]fluoro-D: -glucose ([(18)F]-FDG) and 3'-deoxy-3'-[(18)F]-fluorothymidine ([(18)F]-FLT) uptake in pre-clinical tumor models and examine the relationship between imaging data and related histological biomarkers. [(18)F]-FDG and [(18)F]-FLT studies were carried out in nine human tumor xenograft models in mice. A selection of the models underwent histological analysis for endpoints relevant to radiotracer uptake. Comparisons were made between in vitro uptake, in vivo imaging, and ex vivo histopathology data using quantitative and semi-quantitative analysis. In vitro data revealed that [1-(14)C]-2-deoxy-D: -glucose ([(14)C]-2DG) uptake in the tumor cell lines was variable. In vivo, [(18)F]-FDG and [(18)F]-FLT uptake was highly variable across tumor types and uptake of one tracer was not predictive for the other. [(14)C]-2DG uptake in vitro did not predict for [(18)F]-FDG uptake in vivo. [(18)F]-FDG SUV was inversely proportional to Ki67 and necrosis levels and positively correlated with HKI. [(18)F]-FLT uptake positively correlated with Ki67 and TK1. When evaluating imaging biomarkers in response to therapy, the choice of tumor model should take into account in vivo baseline radiotracer uptake, which can vary significantly between models.

SU‐E‐J‐94: FLT PET Imaging for Evaluating Bone Marrow Recovery after Chemoradiation Therapy in Cervical Cancer Patients

Purpose: To evaluate the ability of FLT PET imaging to measure bone marrow recovery from chemoradiation therapy in cervical cancer patients. Methods: Two cervical cancer patients were enrolled in an IRB approved protocol to obtain FLT PET images at simulation, during chemoradiation, and ∼30 days post chemoradiation therapy. WBC counts and bone marrow FLT uptake change inside and outside the radiation therapy (RT) field were measured to correlate with recovery measured with FLT uptake 30 days post‐therapy. Bone marrow response and recovery was measured within the RT field in 1 Gy/week dose volumes and outside the field within the lumbar vertebral bodies. Results: Each subject was unique in her bone marrow recovery due to chemoradiation therapy when compared to pre‐therapy values. Inside the RT field, subject 1 bone marrow that received &lt; 3 Gy/week recovered ∼85% while bone marrow that received &gt; 6 Gy/week recovered at least 50% of pre‐therapy FLT uptake 30 days post‐therapy. However, subject 2 bone marrow that received &lt; 3 Gy/week recovered ∼45% and bone marrow that received &gt; 6 Gy/week recovered ∼35% of pre‐therapy FLT uptake. Outside the RT field, subject 1 L1 FLT mean SUV increased over initial values 60.6% after 3 weeks of RT and 59.1% 30 days post‐therapy. Subject 2 L1 FLT mean SUV increased over initial values only 18.5% after 3 weeks of RT and 10.6% 30 days post‐therapy. Pre‐therapy bone marrow activity may also influence bone marrow recovery. Subject 1 had lower initial WBCs and FLT uptake than subject 2 (5.3 vs. 12.2 K/mm3 and 10.4 vs. 13.6 max SUV). Conclusion: This preliminary data shows that bone marrow recovery, pre‐therapy FLT uptake, and compensatory response can be quantified with FLT PET. These metrics may be valuable in correlating pre‐, during, and post‐therapy response to radiation dose and long‐term systemic toxicity.

SU-F-BRA-01: Object-Based Analysis of Spatial Concordance Between Coregistered Images of Intratumoral Uptake of [18F]FLT and Distribution of Cell Proliferation Markers

Purpose: To propose a new objective method for deformable coregistration of multimodality images acquired with digital autoradiography (DAR) and microscopy in the context of PET tracer histopathological validation. To analyze the spatial concordance between the uptake pattern of 18F-fluorothymidine (FLT) as imaged with DAR and the distribution of cell proliferation as revealed by immunofluorescence microscopy imaging.Methods: Tumor-bearing mice were injected with FLT and other markers including bromodeoxyuridine (cell proliferation). After sacrifice, tumors were excised, frozen and sectioned. Multiple stacks of sequential 8μm sections were collected from each tumor. Selected sections were used for DAR to image FLT uptake distribution. Adjacent sections were used to acquire histopathological data. To correct for imperfections of the tissue cutting and collection, all images were deformably coregistered to the FLT DAR image based on biological images of tumor blood flow (Hoechst) that was acquired from each tissue section used. For each FLT DAR — cell proliferation microscopy image pair, object-based analysis was conducted, including overlap and relative operating characteristics (ROC) analysis. Results: Total registration error of proposed coregistration method was 44.86μm. This supersedes current rigid registration methods with reported errors of 100–200μm. In tumors with well-compartmentalized functional aspects, area under the ROC curve (AUCroc= 0.7) indicated FLT DAR image thresholding as an accurate method of detecting cell proliferation. For these tumors, Dice overlap index indicated maximum detection rates at thresholds between 20% and 40% of the maximum DAR intensity. For the tumors characterized by more heterogeneous distribution of cell proliferation across the tumor section, FLT DAR image thresholding could not predict cell proliferation beyond random chance. Conclusions: We developed a comprehensive method of obtaining and analyzing coregistered images of cell proliferation markers and intratumoral uptake of FLT. Tumor microenvironment heterogeneity is a significant factor affecting the utility of FLT for imaging cell proliferation.

SU‐E‐I‐167: Impact of Peak SUV Definition on Quantification of Treatment Response

Purpose: PET‐based treatment response assessment typically measures the change in SUVmax, which is adversely affected by noise. SUVpeak has been recommended as a more robust alternative, but currently its associated region‐of‐interest (ROIpeak) is not uniquely defined. We investigated the impact of different ROIpeak definitions on quantification of SUVpeak and tumor response. Methods: Seventeen patients with solid malignancies were treated with an anti‐proliferative, molecular targeted agent. Using the cellular proliferation marker [F‐18]FLT, whole‐body PET/CT scans were acquired at baseline and during treatment. Lesions with highest FLT uptake (∼2/patient) were segmented on PET images and tumor response was assessed via the relative change in SUVpeak. For each tumor, 24 different SUVpeak and response values were determined by changing ROIpeak shape, size, and location. Within each tumor, variation of the 24 values was measured using range, coefficient of variation (CV) for SUVpeak, and standard deviation (SD) for response. For each ROIpeak definition, population average SUVpeak and response were determined over all tumors. Results: Substantial variation in both SUVpeak and response resulted from changing the ROIpeak definition. Intra‐tumor SUVpeak variation (CV: 17%) and response variation (SD: 9%) ranged as far as 50% from the mean. Population average SUVpeak variation (CV: 14%) ranged as far as 30% from the mean but population average tumor response variation (SD: 2%) ranged only 3% from the mean. Size of ROIpeak caused more variation in SUVpeak and response than location or shape of ROIpeak. Conclusion: Quantification of individual tumor response using SUVpeak is highly sensitive to ROIpeak definition, which can significantly impact the use of SUVpeak for treatment response assessment. However, population average response is robust to ROIpeak definition. Standardization of SUVpeak is crucial for consistent assessment of treatment response. Clinical trials are necessary to compare the efficacy of SUVpeak and SUVmax for quantification of response to therapy.

TH-C-220-03: Extraction of Radiosensitivity Coefficients from Multiparametric Molecular Imaging Data through Computational Tumor Modeling

Purpose: Patient‐specific differences in tumor radiosensitivity are generally disregarded in radiation therapy (RT), an approach that overlooks subgroups with different therapeutic responses. In this study, we extracted voxel‐based radiosensitivity coefficients from multiparametric molecular imaging data using a computational tumor growth and response model. Methods: Thirteen canine patients with sinonasal tumors underwent pre‐therapy PET/CT imaging of metabolic activity ([18F]FDG), proliferation ([18F]FLT), and hypoxia ([61Cu]Cu‐ATSM) using bite‐block immobilization. Following two RT fractions and two off‐treatment days, proliferative response was assessed via [18F]FLT‐PET/CT. Using a previously developed tumor simulation model, voxel‐based ‘effective’ linear radiosensitivity coefficients (alpha) were determined by simulating treatment response based upon pre‐therapy PET data and iteratively minimizing differences between simulated and imaged proliferative response. Resulting alpha values were investigated for inter‐ and intra‐patient heterogeneity, histology dependence, and spatial correlations with pre‐therapy PET data. Results: Alpha values in carcinomas (mean=0.061/Gy, SD=0.051/Gy) were higher than those found in sarcomas (mean=0.042/Gy, SD=0.024/Gy). Carcinomas were more heterogeneous in response (range=0.554/Gy) than sarcomas (range=0.110/Gy), which correlated with a more heterogeneous pre‐therapy uptake of all three PET imaging surrogate markers. Increased [61Cu]Cu‐ATSM uptake (SUV>4) alone indicated radioresistance, while simultaneous [18F]FLT uptake lowered effective alpha values. Using Akaike's information criterion, five carcinomas were identified to exhibit bimodal or trimodal alpha distributions, while all sarcomas were found to be unimodal. Conclusion: Within this patient population, carcinomas were more radiosensitive and heterogeneous in alpha and PET tracer uptake. Extracted alpha values and their correlations with pre‐therapy PET data differed between tumor histologies. Results suggest that imaged hypoxia might not necessarily indicate increased levels of radioresistance as high proliferation potentially offsets this effect. After correlating early treatment response to late outcome, this model could be used to identify potential biological targets within tumors, stratify candidate dose painting patients, and to transform voxel‐ based information into biological treatment planning objectives.