Fluorothymidine Research Articles

N-Myristoylglutamic acid derivative of 3′-fluoro-3′-deoxythymidine as an organogel

Designing microbicidal gels of anti-HIV drugs for local application to prevent HIV infection is a subject of major interest. 3′-Fluoro-3′-deoxythymidine (FLT), a nucleoside reverse transcriptase inhibitor (NRTI), was conjugated with a N-myristoylglutamate scaffold. The conjugate showed gelation at 1% (w/w) in different organic solvents, such as toluene, dichloromethane, and chloroform. The gels were opaque and stable at room temperature. The results indicate that myristoyl glutamate derivative of FLT can form an organogel. The gel could have potential application as a topical anti-HIV microbicidal agent.

Positron Emission Tomography with [18F]-3′-Deoxy-3′fluorothymidine (FLT) as a Predictor of Outcome in Patients with Locally Advanced Resectable Rectal Cancer: a Pilot Study

This pilot study was performed to evaluate whether tumor uptake of (18)F-labeled 3'-deoxy-3'fluorothymidine (FLT), a proliferative radiotracer, at baseline and early during therapy, is predictive of outcome in locally advanced rectal cancer. Fourteen patients underwent positron emission tomography (PET) with 2-deoxy-2-[(18)F]fluoro-D-glucose (FDG) and FLT before therapy and PET with FLT approximately 2weeks after initiating neoadjuvant chemoradiotherapy. FLT and FDG uptake were evaluated qualitatively and by maximum standardized uptake value (SUV(max)). Tumor FLT and FDG uptake were correlated with disease-free survival (DFS). Thirteen patients underwent surgery after therapy, one died before surgery with progressive disease. FDG-PET/computed tomography detected regional lymph node metastases in five and FLT-PET was positive in one. High pretherapy FDG uptake (SUV(max) ≥ 14.3), low during-therapy FLT uptake (SUV(max) < 2.2), and high percentage change in FLT uptake (≥60%) were predictive of improved DFS (p < 0.05 for all three values). Pretherapy FDG uptake, during-therapy FLT uptake, and percentage change in FLT uptake were equally predictive of DFS.

Selective CDK4/6 inhibition with tumor responses by PD0332991 in patients with mantle cell lymphoma

AbstractMantle cell lymphoma (MCL) carries an unfavorable prognosis and requires new treatment strategies. The associated t(11:14) translocation results in enhanced cyclin D1 expression and cyclin D1–dependent kinase activity to promote cell-cycle progression. A pharmacodynamic study of the selective CDK4/6 inhibitor PD0332991 was conducted in 17 patients with relapsed disease, using 2-deoxy-2-[18F]fluoro-D-glucose (FDG) and 3-deoxy-3[18F]fluorothymidine (FLT) positron emission tomography (PET) to study tumor metabolism and proliferation, respectively, in concert with pre- and on-treatment lymph node biopsies to assess retinoblastoma protein (Rb) phosphorylation and markers of proliferation and apoptosis. Substantial reductions in the summed FLT-PET maximal standard uptake value (SUVmax), as well as in Rb phosphorylation and Ki-67 expression, occurred after 3 weeks in most patients, with significant correlations among these end points. Five patients achieved progression-free survival time of > 1 year (range, 14.9-30.1+ months), with 1 complete and 2 partial responses (18% objective response rate; 90% confidence interval, 5%-40%). These patients demonstrated > 70%, > 90%, and ≥ 87.5% reductions in summed FLT SUVmax and expression of phospho-Rb and Ki67, respectively, parameters necessary but not sufficient for long-term disease control. The results of the present study confirm CDK4/6 inhibition by PD0332991 at a well-tolerated dose and schedule and suggest clinical benefit in a subset of MCL patients. This study is registered at www.clinicaltrials.gov under identifier NCT00420056.

The effects of 5-fluoruracil treatment on 3′-fluoro-3′-deoxythymidine (FLT) transport and metabolism in proliferating and non-proliferating cultures of human tumor cells

3'-Fluoro-3'-deoxythymidine (FLT) positron emission tomography (PET) has been proposed for imaging thymidylate synthase (TS) inhibition. Agents that target TS and shut down de novo synthesis of thymidine monophosphate increase the uptake and retention of FLT in vitro and in vivo because of a compensating increase in the salvage pathway. Increases in both thymidine kinase-1 (TK1) and the equilibrative nucleoside transporter hENT1 have been reported to underlie this effect. We examined whether the effects of one TS inhibitor, 5-fluorouracil (5FU), on FLT uptake require proliferating cells and whether the effects are limited to increasing TK1 activity. The effects of 5FU on FLT transport and metabolism, TK1 activity, and cell cycle progression were evaluated in the human tumor cell line, A549, maintained as either a proliferating or non-proliferating culture. There were dose-dependent increases in FLT uptake that peaked after a 10 μM 5FU exposure and then declined to baseline levels or below at higher doses in both proliferating and non-proliferating cultures. The dose-dependence for FLT uptake was mirrored by changes in TK1 activity. S phase fraction did not correlate with FLT uptake in proliferating cultures. Chemical inhibition of hENT1 reduced overall levels of FLT uptake but did not affect the low dose increase in FLT uptake. 5FU only affects FLT uptake in proliferating A549 cells and increases in FLT uptake are directly related to increased TK1 activity. Our studies did not support a role for hENT1 in the increased uptake of FLT after exposure to 5FU. Our studies with A549 cells support the suggestion that FLT-PET could provide a measure of TS inhibition in vivo.

Balancing Antiviral Potency and Host Toxicity: Identifying a Nucleotide Inhibitor with an Optimal Kinetic Phenotype for HIV-1 Reverse Transcriptase

Two novel thymidine analogs, 3'-fluoro-3'-deoxythymidine (FLT) and 2',3'-didehydro-3'-deoxy-4'-ethynylthymidine (Ed4T), have been investigated as nucleoside reverse transcriptase inhibitors (NRTIs) for treatment of HIV infection. Ed4T seems very promising in phase II clinical trials, whereas toxicity halted FLT development during this phase. To understand these different molecular mechanisms of toxicity, pre-steady-state kinetic studies were used to examine the interactions of FLT and Ed4T with wild-type (WT) human mitochondrial DNA polymerase γ (pol γ), which is often associated with NRTI toxicity, as well as the viral target protein, WT HIV-1 reverse transcriptase (RT). We report that Ed4T-triphosphate (TP) is the first analog to be preferred over native nucleotides by RT but to experience negligible incorporation by WT pol γ, with an ideal balance between high antiretroviral efficacy and minimal host toxicity. WT pol γ could discriminate Ed4T-TP from dTTP 12,000-fold better than RT, with only an 8.3-fold difference in discrimination being seen for FLT-TP. A structurally related NRTI, 2',3'-didehydro-2',3'-dideoxythymidine, is the only other analog favored by RT over native nucleotides, but it exhibits only a 13-fold difference (compared with 12,000-fold for Ed4T) in discrimination between the two enzymes. We propose that the 4'-ethynyl group of Ed4T serves as an enzyme selectivity moiety, critical for discernment between RT and WT pol γ. We also show that the pol γ mutation R964C, which predisposes patients to mitochondrial toxicity when receiving 2',3'-didehydro-2',3'-dideoxythymidine to treat HIV, produced some loss of discrimination for FLT-TP and Ed4T-TP. These molecular mechanisms of analog incorporation, which are critical for understanding pol γ-related toxicity, shed light on the unique toxicity profiles observed during clinical trials.

Abstract SY42-02: Novel PET imaging in the clinic: Selecting patient cohorts and measuring early response

Abstract SY42-02: Novel PET imaging in the clinic: Selecting patient cohorts and measuring early response

Abstract 5586: Phase I study of PARP inhibitor ABT-888 and carboplatin with novel imaging in metastatic breast cancer (MBC) (NCI-8609)

Abstract Purpose: ABT-888 is an oral inhibitor of PARP 1 and 2 and potentiates activity of platinums in preclinical models. We are conducting a phase I study of ABT-888 on 2 different schedules (7 and 14 day) plus q 3 week carboplatin (C) to identify the recommended phase II dose schedules (RPTD-7and 14 day) for this regimen in patients (pts) with MBC who are: (1) triple negative (TN); or (2) estrogen receptor positive (ER+) with defective Fanconi Anemia (FA) pathway (no FANCD2 foci in tumor). Study design: Eligible pts received C-AUC 5 Q 3weeks (except dose level 1-AUC 6) plus escalating doses of ABT-888 BID on a 7 or 14-day schedule using a 3+3 dose escalation design. Blood samples were collected during cycles 1 and 2 for PAR assay and CTCs isolated to measure gamma H2Ax. We performed 3′-[F-18] Fluoro-3′-deoxythymidine(FLT) PET scans to assess cellular tumor proliferation at baseline, cycle 1 day 7 and 14 and after cycle 3. FDG-PET-CT scans were used to assess response. Results: 22 pts (20-TN, 2-ER+ w/FA defect) with median age of 56.5 yrs (range 31-69) were enrolled on 5 dose levels. Dose level 1 with C at AUC 6 was too toxic with 3 DLTs (Table 1). Further dose escalations were performed with C at AUC-5. Dose reductions in C were made in subsequent cycles for thrombocytopenia, anemia and fatigue. Maximum reduction in thymidine uptake in tumors was seen on day 7 FLT scans. CTCs were isolated using an immunomagnetic negative depletion method on 8 pt samples to date (median-59, range 0-684 CTCs). Gamma H2Ax analysis on sequential samples is ongoing. Four pts have had an unconfirmed partial response (PR) with &amp;gt; 50% tumor shrinkage; 2 of the 4 pts had a defective FA pathway. Conclusions: Thrombocytopenia is the major DLT when ABT-888 is given in combination with C, where lower AUC of C is better tolerated and shows promising activity of this combination. FA deficiency (5/14=27%) seen in this group is consistent with our previous reports. The study is supported U01 CA076576. 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 5586. doi:1538-7445.AM2012-5586

Stellenwert der PET/CT in der Lymphomdiagnostik

Staging or re-staging of lymphomas using conventional imaging modalities is based on morphological changes, usually on the diameter of lesions. However, vitality of tumors cannot be evaluated. In this context computed tomography (CT) has been used as a standard modality. Since the introduction of positron emission tomography (PET), evaluation of tumor vitality has become possible. Moreover PET/CT hybrid scanners were brought onto the market one decade ago. The fluorodeoxyglucose (FDG) PET/CT technique is now accepted as one of the most accurate modalities in the diagnosis of aggressive lymphomas due to a high FDG uptake (overall accuracy > 90%, sensitivity >90%). However, indolent lymphomas suffer from lower FDG uptake due to a moderate metabolic activity. After the introduction of PET/CT hybrid imaging the specificity of this diagnostic technique increased significantly compared to PET alone (from > 80% to > 90%). With the utilization of PET approximately 20% more lesions are detected when comparing to CT alone and in up to 15% of the patients this also results in a change of the therapeutic regime. As post-chemotherapy scar tissue usually persists for months, evaluation of vitality within residual bulks using FDG-PET can predict therapy response much earlier than CT, enabling therapy stratification. Other PET tracers apart from FDG have low impact in imaging of lymphomas and only the thymidine analogue fluorothymidine (FLT) is used in some cases for non-invasive measurement of proliferation. Despite the capability of FDG-PET/CT there is no evidence that the improvement in diagnostics is translated into a better patient outcome and therefore warrants the high costs. False positive findings in PET can result in unnecessary treatment escalation with subsequent higher therapy-associated toxicity and costs. Some pitfalls can be avoided by scheduling PET scans carefully. As treatment-induced inflammation early after therapy can be misinterpreted as vital tumor tissue, it is recommended to wait at least 3 weeks between the last treatment cycle and the subsequent FDG-PET follow-up. Until the results of the prospective multicenter trials "PETAL" and "HD-18" become available, in Germany FDG-PET is only recommended generally for restaging Hodgkin's disease with a known rest bulk of > 2.5cm in justifiable individual cases or in clinical trials.

Evaluation of limited blood sampling population input approaches for kinetic quantification of [18F]fluorothymidine PET data

BackgroundQuantification of kinetic parameters of positron emission tomography (PET) imaging agents normally requires collecting arterial blood samples which is inconvenient for patients and difficult to implement in routine clinical practice. The aim of this study was to investigate whether a population-based input function (POP-IF) reliant on only a few individual discrete samples allows accurate estimates of tumour proliferation using [18F]fluorothymidine (FLT).MethodsThirty-six historical FLT-PET data with concurrent arterial sampling were available for this study. A population average of baseline scans blood data was constructed using leave-one-out cross-validation for each scan and used in conjunction with individual blood samples. Three limited sampling protocols were investigated including, respectively, only seven (POP-IF7), five (POP-IF5) and three (POP-IF3) discrete samples of the historical dataset. Additionally, using the three-point protocol, we derived a POP-IF3M, the only input function which was not corrected for the fraction of radiolabelled metabolites present in blood. The kinetic parameter for net FLT retention at steady state, Ki, was derived using the modified Patlak plot and compared with the original full arterial set for validation.ResultsSmall percentage differences in the area under the curve between all the POP-IFs and full arterial sampling IF was found over 60 min (4.2%-5.7%), while there were, as expected, larger differences in the peak position and peak height.A high correlation between Ki values calculated using the original arterial input function and all the population-derived IFs was observed (R2 = 0.85-0.98). The population-based input showed good intra-subject reproducibility of Ki values (R2 = 0.81-0.94) and good correlation (R2 = 0.60-0.85) with Ki-67.ConclusionsInput functions generated using these simplified protocols over scan duration of 60 min estimate net PET-FLT retention with reasonable accuracy.

18F]FLT–PET Imaging Does Not Always “Light Up” Proliferating Tumor Cells

[(18)F]FLT (3'-Fluoro-3' deoxythymidine)-PET imaging was proposed as a tool for measuring in vivo tumor cell proliferation. The aim of this article was to validate the use of [(18)F]FLT-PET imaging for measuring xenograft proliferation and subsequent monitoring of targeted therapy. In exponentially growing xenografts, factors that could impact the outcome of [(18)F]FLT-PET imaging, such as nucleoside transporters, thymidine kinase 1, the relative contribution of DNA salvage pathway, and the ratio of FLT to thymidine, were evaluated. The [(18)F]FLT tracer avidity was compared with other proliferation markers. In a panel of proliferating xenografts, [(18)F]FLT or [(3)H]thymidine tracer avidity failed to reflect the tumor growth rate across different tumor types, despite the high expressions of Ki67 and TK1. When FLT was injected at the same dose level as used in the preclinical [(18)F]FLT-PET imaging, the plasma exposure ratio of FLT to thymidine was approximately 1:200. Thymidine levels in different tumor types seemed to be variable and exhibited an inverse relationship with the FLT tracer avidity. In contrast, high-dose administration of bromdeoxyuridine (BrdUrd; 50 mg/kg) yielded a plasma exposure of more than 4-fold higher than thymidine and leads to a strong correlation between the BrdUrd uptake and the tumor proliferation rate. In FLT tracer-avid models, [(18)F]FLT-PET imaging as a surrogate biomarker predicted the therapeutic response of CDK4/6 inhibitor PD-0332991. Tumor thymidine level is one of the factors that impact the correlation between [(18)F]FLT uptake and tumor cell proliferation. With careful validation, [(18)F]FLT-PET imaging can be used to monitor antiproliferative therapies in tracer-avid malignancies.

Diagnostic Value of 11C-Methionine (MET) and 18F-Fluorothymidine (FLT) Positron Emission Tomography in Recurrent High-Grade Gliomas; Differentiation from Treatment-Induced Tissue Necrosis

We retrospectively evaluated the usefulness of combined measurement of L-methyl-[11C]methionine (MET) and 3'-deoxy-3'-[18F]fluorothymidine (FLT) positron emission tomography (PET) in the differential diagnosis between recurrent gliomas and necrotic lesions. Twenty-one patients with high-grade glioma, previously treated with surgery and radiotherapy with chemotherapy and first radiological suspicion of recurrence were enrolled. The uptake was assessed by the maximum standardized uptake value (SUVmax) and lesion-to-normal tissue count density ratio (L/N ratio). Of the 21 lesions, 15 were diagnosed recurrent gliomas and six were necrotic lesions. The average SUVmax was not significantly different between recurrent gliomas and necrotic lesions on either MET-PET or FLT-PET. The average L/N ratio of recurrent gliomas (3.36 ± 1.06) was significantly higher than that of necrotic lesions (2.18 ± 0.66) on MET-PET (p < 0.01) and the average L/N ratio of recurrent gliomas (7.01 ± 2.26) was also significantly higher than that of necrotic lesions (4.60 ± 1.23) on FLT-PET (p < 0.01). ROC curve analysis showed that the areas under the curves were high but not different between MET- and FLT-PET. PET studies using MET and FLT are useful in the differentiation of recurrent glioma from treatment-induced necrotic lesion. However, there is no complementary information in the differentiation with simultaneous measurements of MET- and FLT-PET.

Highlights of This Issue

[18F]FLT (3′-Fluoro-3′ deoxythymidine)–positron emission tomographic imaging ([18F]FLT–PET) has been used to measure in vivo tumor cell proliferation; however, its use in clinical tumor staging is limited due to the lower overall uptake of FLT. Zhang and colleagues evaluated the factors other than tumor cell proliferation that could affect the outcome of imaging to understand the translational aspects of the [18F]FLT–PET imaging modality. The preclinical use of [18F]FLT–PET imaging for tracking therapeutic response was also assessed. The results of this study strongly agreed with the clinical outcome. This work highlights the preclinical understanding of [18F]FLT–PET imaging and provides insights into the technologic applications in the oncology arena.Glioma stem cells (GSC) are a critical therapeutic target of glioblastoma multiforme (GBM). Miyazaki and colleagues addressed the hypothesis that a G-quadruplex ligand, telomestatin (TMS), eradicates GSCs. TMS treatment induced apoptosis of GSCs in vitro and in vivo through both telomeric and nontelomeric DNA damage. A proto-oncogene, c-Myb, was identified as a novel target of TMS by the cDNA microarray and pharmacodynamic analysis. c-Myb knockdown significantly reduced the growth of GSCs both in vitro and in vivo, and c-Myb expression in surgical specimens of GBM patients was statistically significantly elevated. These data suggest a novel GSC-directed therapeutic strategy for GBM through telomere disruption and c-Myb inhibition.The prostate cancer field is critically lacking a metastasis xenograft model that respects the entire physiologic metastatic cascade and provides evidence for its clinical relevance. Lange and colleagues established subcutaneous xenograft models using 4 prostate cancer cell lines and quantified tumor development and spontaneous metastasis formation. Metastatic xenografts revealed an increased activity of Mgat5b, a β(1,6)- branching glycosyltransferase previously found in neurons and testes, as detected by Phaseolus vulgaris leukoagglutinin (PHA-L) binding. According to a clinical part of the study (n = 2,085), PHA-L intensity was correlated with serum prostate-specific antigen (PSA) levels, and a cytoplasmic localization of PHA-L binding adversely affected PSA recurrence-free survival.Few studies have explored the connection between tumor imaging results and gene expression. To identify signaling pathways associated with treatment-related changes in tumor 2[18F]fluoro-2-deoxy-D-glucose–positron emission tomographic (FDG–PET) imaging, Schwarz and colleagues performed gene expression profiling using 62 pretreatment cervical cancer biopsies from patients treated with definitive chemoradiation. All patients underwent pre- and post-treatment FDG–PET. Alterations in expression of genes from the phosphoinositide 3-kinase (PI3K)/Akt pathway were associated with abnormal FDG uptake on the posttreatment PET. These results were validated using immunohistochemistry for p-Akt and comparing results with patient survival outcome data after treatment (n = 174). These results suggest that PI3K/Akt inhibition may improve response to chemoradiation.

Comparison of 3′-deoxy-3′-[18F]fluorothymidine positron emission tomography (FLT PET) and FDG PET/CT for the detection and characterization of pancreatic tumours

Despite recent advances in clinical imaging modalities, differentiation of pancreatic masses remains difficult. Here, we tested the diagnostic accuracy of molecular-based imaging including 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) positron emission tomography (PET) and [(18)F]fluorodeoxyglucose (FDG) PET/CT in patients with suspected pancreatic masses scheduled to undergo surgery. A total of 46 patients with pancreatic tumours suspicious for malignancy and scheduled for resective surgery were recruited prospectively. In 41 patients, FLT PET and FDG PET/CT scans were performed. A diagnostic CT performed on a routine basis was available in 31 patients. FLT PET and FDG PET/CT emission images were acquired according to standard protocols. Tracer uptake in the tumour [FDG and FLT standardized uptake value (SUV)] was quantified by the region of interest (ROI) technique. For FDG PET/CT analysis, correct ROI placement was ensured via side-by-side reading of corresponding CT images. Of 41 patients, 33 had malignancy, whereas 8 patients had benign disease. Visual analysis of FDG and FLT PET resulted in sensitivity values of 91% (30/33) and 70% (23/33), respectively. Corresponding specificities were 50% (4/8) for FDG PET and 75% (6/8) for FLT PET. In the subgroup of patients with contrast-enhanced CT (n = 31), sensitivities were 96% (PET/CT), 88% (CT alone), 92% (FDG PET) and 72% (FLT PET), respectively. Mean FLT uptake in all malignant tumours was 3.0 (range SUV(max) 1.1-6.5; mean FDG SUV(max) 7.9, range 3.3-17.8; p < 0.001). For differentiation of pancreatic tumours, FDG PET and FDG PET/CT showed a higher sensitivity but lower specificity than FLT PET. Interestingly, visual analysis of FLT PET led to two false-positive findings by misinterpreting physiological bowel uptake as pathological FLT uptake in the pancreas. Due to the limited number of patients, the clinical value of adding FLT PET to the diagnostic workup of pancreatic tumours remains to be determined.

Biochemistry and Biology of 2'-Fluoro-2'-Deoxythymidine (FT), A Putative Highly Selective Substrate for Thymidine Kinase Type 2 (TK2)

2'-Deoxy-2'-fluorothymidine (FT) is a bioisostere of both thymidine (TdR), in which F replaces H at C-2' in the ribosyl configuration, and methyluridine, in which F replaces OH at C-2' in the ribosyl configuration. Fluorine is bioisosteric with H with respect to atomic radius and is bioisosteric with OH with respect to polarity and H-bonding as an H acceptor. The consequences of this C-2' F for H substitution on cytotoxicity, nucleoside transporter affinity, phosphorylation by thymidine kinases (TK1, TK2), cell uptake and biodistribution of FT in a murine tumor model are now reported. FT toxicity against a bank of murine and human cells was seen only at very high (˜1 mM) concentrations, although the cellular uptake of [3H]FT in these cells was comparable to that of [3H]TdR over a 24 h period. Human equilibrative nucleoside transporters (hENT1, hENT2) displayed weaker affinity for FT than for TdR, but the concentrative transporters (hCNT1, hCNT2, hCNT3) had much higher affinities for FT. FT was phosphorylated by both mitochondrial thymidine kinase (TK2) (58 % of TdR) and cytosolic thymidine kinase (TK1) (39 % of TdR). Preliminary in vivo imaging with [18F]FT in mice bearing implanted KBALB and contralateral KBALB-STK tumors showed highly selective uptake, with a tumor:blood ratio of 33 in a small herpes simplex type 1 (HSV-1 TK) expressing tumor. In conclusion, [18F]FT appears to be a strong candidate for PET imaging of viral TK transgene imaging, based on its TK1:TK2 phosphorylation differential, its selective uptake by an HSV-TK expressing murine tumor model, its interaction with nucleoside transporters and its low toxicity.

Pharmacodynamic Study Using FLT PET/CT in Patients with Renal Cell Cancer and Other Solid Malignancies Treated with Sunitinib Malate

To characterize proliferative changes in tumors during the sunitinib malate exposure/withdrawal using 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) positron emission tomography (PET)/computed tomography (CT) imaging. Patients with advanced solid malignancies and no prior anti-VEGF exposure were enrolled. All patients had metastatic lesions amenable to FLT PET/CT imaging. Sunitinib was initiated at the standard dose of 50 mg p.o. daily either on a 4/2 or 2/1 schedule. FLT PET/CT scans were obtained at baseline, during sunitinib exposure, and after sunitinib withdrawal within cycle #1 of therapy. VEGF levels and sunitinib pharmacokinetic (PK) data were assessed at the same time points. Sixteen patients (8 patients on 4/2 schedule and 8 patients on 2/1 schedule) completed all three planned FLT PET/CT scans and were evaluable for pharmacodynamic imaging evaluation. During sunitinib withdrawal (change from scans 2 to 3), median FLT PET standardized uptake value (SUV(mean)) increased +15% (range: -14% to 277%; P = 0.047) for the 4/2 schedule and +19% (range: -5.3% to 200%; P = 0.047) for the 2/1 schedule. Sunitinib PK and VEGF ligand levels increased during sunitinib exposure and returned toward baseline during the treatment withdrawal. The increase of cellular proliferation during sunitinib withdrawal in patients with renal cell carcinoma and other solid malignancies is consistent with a VEGF receptor (VEGFR) tyrosine kinase inhibitor (TKI) withdrawal flare. Univariate and multivariate analysis suggest that plasma VEGF is associated with this flare, with an exploratory analysis implying that patients who experience less clinical benefit have a larger withdrawal flare. This might suggest that patients with a robust compensatory response to VEGFR TKI therapy experience early "angiogenic escape."

Pharmacodynamic Evaluation of Irinotecan Therapy by FDG and FLT PET/CT Imaging in a Colorectal Cancer Xenograft Model

Longitudinal changes of 3'-[(18) F]fluoro-3'-deoxythymidine (FLT) and 2-deoxy-2-[(18) F]fluoro-D-glucose (FDG) in response to irinotecan therapy in an animal model of colorectal cancer were compared. SCID/CB-17 mice with HCT116 tumors were treated with 50mg/kg irinotecan by intraperitoneal injection weekly for 3weeks. FLT and FDG-positron emission tomography (PET) were performed at baseline, the day after each treatment, and 5days after the first treatment. Proliferation and apoptosis were evaluated by immunohistochemistry (IHC) after day 15 of imaging. Irinotecan treatment resulted in a suppression of tumor growth. Tumor FLT uptake was decreased the day after each treatment but to a lesser extent 5days after the first treatment. FDG uptake increased the day after each treatment with a continuous increase throughout the experiment. IHC analysis of phospho-H3 and Ki67 confirmed FLT-PET results, indicating a decrease in proliferation the day after the final irinotecan treatment. Increased apoptosis monitored by caspase-3 was observed after day 15 with irinotecan treatment. FLT-PET may be a better method than FDG-PET for assessing treatment response to irinotecan. Changes in imaging occur before changes in tumor volume.

In vivo detection of immune responses to vaccines with 18F-labeled 3'-fluoro-3'-deoxythymidine (FLT)-PET imaging

In vivo detection of immune responses to vaccines with 18F-labeled 3'-fluoro-3'-deoxythymidine (FLT)-PET imaging

Biological basis of [11C]choline-positron emission tomography in patients with breast cancer

The biological significance of [¹¹C]choline (CHO) uptake in human tumours is unclear and probably linked to choline kinase-α (CHKα) expression and cell proliferation. We directly compared CHO with [¹⁸F]fluorothymidine (FLT), an imaging biomarker of proliferation, by positron emission tomography (PET) in patients with breast cancer to investigate whether cell proliferation is an important determinant of CHO uptake. Furthermore, we evaluated CHKα and the Ki67-labelling index (LIKi67) in tumour biopsies. Sequential CHO-PET and FLT-PET within the same imaging session were performed in 21 patients with oestrogen receptor (ER)-positive breast cancer (28 lesions). Average and maximum CHO standardized uptake values (SUV) at 60 min: SUV60,av, and SUV60,max, and the rate constant of net irreversible uptake, Ki, were compared with FLT uptake at 60 min: SUV60,av and SUV60,max. Biopsies were stained for CHKα and LIKi67 in eight cases. Tumours were equally visible on CHO-PET and FLT-PET imaging. Tumour CHO-PET strongly correlated with FLT imaging variables (Pearson's r=0.83; P<0.0001 for CHO-SUV60,max vs. FLT-SUV60,max). A statistically significant association was found between CHO-PET variables and categorical scores of cytoplasmic CHKα intensity and between FLT-PET and LIKi67 (P<0.05, one-way analysis of variance). Choline metabolism and proliferation as assessed by PET were correlated in ER-positive breast cancer, indicating that high CHO uptake is a measure of cellular proliferation in this setting. CHO uptake was also found to be related to cytoplasmic CHKα expression.

3′‐deoxy‐3′‐[18F]fluorothymidine positron emission tomography for response assessment in soft tissue sarcoma

This study sought to determine whether [(18)F]fluorothymidine (FLT) positron emission tomography (PET)/computed tomography (CT) imaging allows assessment of tumor viability and proliferation in patients with soft tissue sarcomas who are treated with neoadjuvant therapy. Twenty patients with biopsy-proven, resectable, high-grade soft tissue sarcoma underwent [(18)F]FLT PET/CT imaging before and after neoadjuvant therapy. Histologic subtypes included sarcomas not otherwise specified (n = 5), malignant peripheral nerve sheath tumors (n = 3), gastrointestinal stromal tumors (n = 3), leiomyosarcomas (n = 3), angiosarcomas (n = 2), and others (n = 4). Changes in [(18)F]FLT peak standardized uptake value (SUVpeak) were correlated with percent necrosis in excised tissue, whereas posttreatment [(18)F]FLT tumor uptake was correlated with thymidine kinase 1 (TK1) expression and Ki-67 staining indices in excised tumor tissue. Tumor FLT SUVpeak averaged 7.1 ± 3.7 g/mL (range, 1.9-16.1 g/mL) at baseline and decreased significantly to 2.7 ± 1.6 g/mL (range, 0.8-6.0 g/mL) at follow-up (P < .001); however, marked reductions in SUV were not specific for histopathological response. The posttreatment SUVpeak did not correlate with TK1 (P = .27) or Ki-67 expression (P = .21). Marked reductions in [(18)F]FLT tumor uptake in response to neoadjuvant treatment were observed in most patients with sarcoma. However, these reductions were not specific for histopathologic response to neoadjuvant therapy. Furthermore, posttreatment [(18)F]FLT tumor uptake was unrelated to tumor proliferation by Ki-67 and TK1 staining. These results question the value of [(18)F]FLT PET imaging for treatment response assessments in patients with soft tissue sarcoma.

Highlights of This Issue

The unique antigenic nature of chemically induced tumors is well established. In murine models, vaccination with 1 tumor provides protection from challenge with the same, but not other histologically identical tumors. Twitty and colleagues report that tumor-derived autophagosome vaccines, in contrast with whole-tumor vaccines, prime T cells with broader tumor-specific reactivity and provide significant cross-protection to challenge with similar tumors. Further, they show that ubiquitinated short-lived proteins, sequestered in autophagosomes in a p62-dependent fashion, contain the key antigenic components required for cross-protection. This strategy to broaden anticancer immunity is being explored in a clinical trial.Several recent clinical trials have shown successful results in the treatment of advanced thyroid cancer. All of the agents being evaluated in these trials are multikinase inhibitors targeting VEGF receptor 2 (VEGFR2) plus other receptor tyrosine kinases. Jin and colleagues report here on a preclinical study of a RAF inhibitor and a dual phosphoinositide 3-kinase (PI3) kinase/mTOR inhibitor that successfully inhibits 2 of the major downstream pathways of ras. Addition of the PI3 kinase inhibitor was effective even in BRAF mutant settings. This strategy may be a viable alternative to multikinase inhibitors in thyroid cancer.X-ray repair complementing defective repair in Chinese hamster cell 1 (XRCC1) protein is involved in DNA base excision and single strand break repair and may modulate chemoradiation outcomes by affecting efficient DNA damage repair. Ang and colleagues explored the relationship of XRCC1 protein expression with clinical outcomes in head and neck squamous cell carcinoma by immunohistochemistry. High XRCC1 expression was associated with poorer survival, particularly in patients undergoing chemoradiation. Furthermore, this association was independent of human papilloma virus status, clinical stage, and tumor primary site. Thus, patients with high XRCC1 expression before treatment may have poor outcomes from chemoradiation and may benefit from alternative management strategies.In vivo imaging of cellular proliferation with the biomarker [18F]fluorothymidine (FLT) and positron emission tomography has tremendous potential for monitoring treatment response in oncology. Wardak and colleagues investigated whether changes in tumor FLT kinetic parameters, assessed early after the start of therapy, could predict overall survival and progression-free survival in patients with recurrent high-grade glioma undergoing treatment with bevacizumab and irinotecan. They found that discriminant analysis of kinetic parameter changes could accurately classify patients into their known survival groups. This method could help physicians provide a personalized treatment regimen and thus improve patient outcomes.