Glypican-3-targeting F(ab')2 for 89Zr PET of hepatocellular carcinoma.

UNLABELLED: Hepatocellular carcinoma (HCC) is an increasingly lethal malignancy for which management is critically dependent on accurate imaging. Glypican-3 (GPC3) is a cell surface receptor overexpressed in most HCCs and provides a unique target for molecular diagnostics. The use of monoclonal antibodies (mAbs) that target GPC3 (αGPC3) in PET imaging has shown promise but comes with inherent limitations associated with mAbs such as long circulation times. This study used (89)Zr-conjugated F(ab')2 fragments directed against GPC3 ((89)Zr-αGPC3-F(ab')2) to evaluate the feasibility of the fragments as a diagnostic immuno-PET imaging probe. METHODS: Immobilized ficin was used to digest αGPC3, creating αGPC3-F(ab')2 fragments subsequently conjugated to (89)Zr. In vivo biodistribution and PET studies were performed on GPC3-expressing HepG2 and GPC3-nonexpressing RH7777 orthotopic xenografts. RESULTS: Reliable αGPC3-F(ab')2 production via immobilized ficin digestion was verified by high-performance liquid chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis. (89)Zr-αGPC3-F(ab')2 demonstrated F(ab')2-dependent, antigen-specific cell binding. HepG2 tumor uptake was higher than any other tissue, peaking at 100 ± 21 percentage injected dose per gram (%ID/g) 24 h after injection, a value 33- to 38-fold higher than GPC3-nonexpressing RH7777 tumors. The blood half-life of the (89)Zr-αGPC3-F(ab')2 conjugate was approximately 11 h, compared with approximately 115 h for historic mAb controls. This shorter half-life enabled clear tumor visualization on PET 4 h after administration, with a resultant peak tumor-to-liver contrast ratio of 23.3. Blocking antigen-expressing tumors with an excess of nonradiolabeled αGPC3 resulted in decreased tumor uptake similar to native liver. The kidneys exhibited high tissue uptake, peaking at 24 h with 83 ± 12 %ID/g. HepG2 tumors ranging from 1.5 to 7 mm were clearly visible on PET, whereas larger RH7777 tumors displayed signal lower than background liver tissue. CONCLUSION: This study demonstrates the feasibility of using (89)Zr-αGPC3-F(ab')2 for intrahepatic tumor localization with small-animal PET. Faster blood clearance and lower background liver uptake enable excellent signal-to-noise ratios at early time points. Increased renal uptake is similar to that as has been seen with clinical radioactive peptide imaging. (89)Zr-αGPC3-F(ab')2 addresses some of the shortcomings of whole-antibody immuno-PET probes. Further optimization is warranted to maximize probe sensitivity and specificity in the process of clinical translation.

Glypican-3-targeted 89Zr PET imaging of hepatocellular carcinoma.

UNLABELLED: Hepatocellular carcinoma (HCC) is a devastating malignancy in which imperfect imaging plays a primary role in diagnosis. Glypican-3 (GPC3) is an HCC-specific cell surface proteoglycan overexpressed in most HCCs. This paper presents the use of (89)Zr-conjugated monoclonal antibody against GPC3 ((89)Zr-αGPC3) for intrahepatic tumor localization using PET. METHODS: Polymerase chain reaction confirmed relative GPC3 expression in cell lines. In vitro binding, in vivo biodistribution, and small-animal PET studies were performed on GPC3-expressing HepG2 and non-GPC3-expressing HLF and RH7777 cells and orthotopic xenografts. RESULTS: (89)Zr-αGPC3 demonstrated antibody-dependent, antigen-specific tumor binding. HepG2 liver tumors exhibited high peak uptake (836.6 ± 86.6 percentage injected dose [%ID]/g) compared with background liver (27.5 ± 1.6 %ID/g). Tumor-to-liver contrast ratio was high and peaked at 32.5. The smallest HepG2 tumor (<1 mm) showed lower peak uptake (42.5 ± 6.4 %ID/g) and tumor-to-liver contrast (1.57) but was still clearly visible on PET. Day 7 tissue activity was still substantial in HepG2 tumors (466.4 ± 87.6 %ID/g) compared with control RH7777 tumors (3.9 ± 1.3 %ID/g, P < 0.01), indicating antigen specificity by (89)Zr-αGPC3. HepG2 tumor treated with unlabeled αGPC3 or heat-denatured (89)Zr-αGPC3 demonstrated tumor activity (2.1 %ID/g) comparable to that of control xenografts, confirming antibody dependency. CONCLUSION: This study demonstrated the feasibility of using (89)Zr-αGPC3 to image HCC in the liver, as well as the qualitative determination of GPC3 expression via small-animal PET. The ability to clarify the identity of small liver lesions with an HCC-specific PET probe would provide clinicians with vital information that could significantly alter patient management, warranting further investigation for clinical translation.

Tumor 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) uptake by PET correlates with thymidine kinase 1 expression: static and kinetic analysis of (18)F-FLT PET studies in lung tumors.

UNLABELLED: We report the first, to our knowledge, findings describing the relationships between both static and dynamic analysis parameters of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET and the expression of the proliferation marker Ki-67, and the protein expression and enzymatic activity of thymidine kinase-1 (TK1) in surgically resected lung lesions. METHODS: Static and dynamic analyses (4 rate constants and 2 compartments) of (18)F-FLT PET images were performed in a cohort of 25 prospectively accrued, clinically suspected lung cancer patients before surgical resection (1 lesion was found to be benign after surgery). The maximal and overall averaged expression of Ki-67 and TK1 were determined by semiquantitative analysis of immunohistochemical staining. TK1 enzymatic activity was determined by in vitro assay of extracts prepared from flash-frozen samples of the same tumors. RESULTS: Static (18)F-FLT uptake (partial-volume-corrected maximum-pixel standardized uptake value from 60- to 90-min summed dynamic data) was significantly correlated with the overall (ρ = 0.57, P = 0.006) and maximal (ρ = 0.69, P < 0.001) immunohistochemical expressions of Ki-67 and TK1 (overall expression: ρ = 0.65, P = 0.001; maximal expression: ρ = 0.68, P < 0.001) but not with TK1 enzymatic activity (ρ = 0.34, P = 0.146). TK1 activity was significantly correlated with TK1 protein expression only when immunohistochemistry was scored for maximal expression (ρ = 0.52, P = 0.029). Dynamic analysis of (18)F-FLT PET revealed correlations between the flux constant (K(FLT)) and both overall (ρ = 0.53, P = 0.014) and maximal (ρ = 0.50, P = 0.020) TK1 protein expression. K(FLT) was also associated with both overall (ρ = 0.59, P = 0.005) and maximal (ρ = 0.63, P = 0.002) Ki-67 expression. We observed no significant correlations between TK1 enzyme activity and K(FLT). In addition, no significant relationships were found between TK1 expression, TK1 activity, or Ki-67 expression and any of the compartmental rate constants. CONCLUSION: The absence of observable correlations of the imaging parameters with TK1 activity suggests that (18)F-FLT uptake and retention within cells may be complicated by a variety of still undetermined factors in addition to TK1 enzymatic activity.

Synthesis and in vitro evaluation of 5-fluoro-6-[(2-iminopyrrolidin-1-YL)methyl]uracil, TPI(F): an inhibitor of human thymidine phosphorylase (TP).

An investigation was conducted to determine if the 5-fluoro analog of TPI (5-chloro-6-[(2-iminopyrrolidin-1-yl)methyl]uracil), a potent inhibitor of human thymidine phosphorylase (TP), has an IC(50) in a range that might allow to use it labeled for imaging of TP expression in vivo. The previously unreported fluoro analog, TPI(F), was prepared and tested against TPI and TPI(Br) using an inhibition assay of [H-3]thymidine cleavage. An assay, performed in the presence of 0.4 mg/ml of human TP, yielded IC(50) values of 2.5 nM, 2.7 nM, and 9.0 nM for TPI, TPI(Br), and TPI(F), respectively. The results indicate that further studies to develop (18)F-labeled TPI(F) as a potential radiopharmaceutical for PET imaging of TP expression in vivo are warranted.

FLT: measuring tumor cell proliferation in vivo with positron emission tomography and 3'-deoxy-3'-[18F]fluorothymidine.

Positron emission tomography (PET) using the radiotracer 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) can image cellular proliferation in human cancers in vivo. FLT uptake has been shown to correlate with pathology-based proliferation measurements, including the Ki-67 score, in a variety of human cancers. Unlike pathology-based measurements, imaging-based methods, including FLT-PET, are noninvasive, easily repeatable, and less prone to sampling errors. FLT-PET may therefore be a useful tool for assessing tumor aggressiveness, predicting outcome, planning therapy, or monitoring response to treatment. Three recent clinical studies have reported that FLT-PET can accurately predict response very early after the initiation of chemotherapy. Especially with the advent of cytostatic chemotherapy agents, methods of biologically assessing a tumor's response will take on increasing importance, since changes in tumor size will not always be expected. To date, most studies of FLT-PET have focused on validating it as a means of quantifying cellular proliferation and testing its ability to accurately stage cancer. In some settings, FLT-PET has shown greater specificity for cancer than (18)F-fluorodeoxyglucose (FDG)-PET, which can show false-positive uptake in areas of infection or inflammation. However, because of FLT's lower overall uptake and higher background activity in liver and bone marrow, FLT-PET should not be considered a potential replacement for staging by FLT-PET. Instead, FLT-PET should be considered a powerful addition to FDG-PET, providing additional diagnostic specificity and important biological information that could be useful in predicting prognosis, planning treatment, and monitoring response.

A simple quantitative assay for the activity of thymidine kinase 1 in solid tumors.

INTRODUCTION: The activity of the pyrimidine salvage pathway enzyme thymidine kinase 1 (TK1) is tightly cell cycle regulated and has been investigated as a prognostic indicator of cancer in a variety of tissues. However, using the in vitro assay of TK1 to rank order a series of unique tumor samples by their TK1 activity can be problematic due to the complex nature of TK1 enzyme substrate kinetics. We present a refined TK1 in vitro assay and method of analysis which address these problems. METHODS: Extracts were prepared of the resected lung lesions from eight patients and assayed for TK1 activity using an in vitro assay modified to account for nonlinearities in extract protein concentration. A separate extract of exponentially growing A549 human lung carcinoma cells was used as a cross-assay control. RESULTS: In extracts prepared from eight frozen samples of resected human lung lesions, TK1 activity (mean=0.0070+/-0.0077 pmol [(3)H]-TMP/microg protein/minute) was 2 orders of magnitude below that of exponentially growing A549 human lung carcinoma cells (mean=0.1572+/-0.0218 pmol [(3)H]-TMP/microg protein/minute; n=9). TK1 activity was nonlinear with respect to extract protein concentration in both groups, with A549 cell extracts exhibiting evidence of positive cooperativity which could not be explained by the presence of detergents in the cell lysis buffer. Lung tumor extracts demonstrated evidence of negative cooperativity. CONCLUSIONS: The modified TK1 assay takes into account these nonlinearities by averaging the results of several complete time-course curves measured over a range of extract protein concentrations. An extract prepared from exponentially growing A549 cells is included in each assay for use as a cross-assay control. We demonstrate that these modifications allow for the accurate rank ordering of TK1 activity in solid tumors.

Toxicology evaluation of radiotracer doses of 3'-deoxy-3'-[18F]fluorothymidine (18F-FLT) for human PET imaging: Laboratory analysis of serial blood samples and comparison to previously investigated therapeutic FLT doses.

BACKGROUND: 18F-FLT is a novel PET radiotracer which has demonstrated a strong potential utility for imaging cellular proliferation in human tumors in vivo. To facilitate future regulatory approval of 18F-FLT for clinical use, we wished to demonstrate the safety of radiotracer doses of 18F-FLT administered to human subjects, by: 1) performing an evaluation of the toxicity of 18F-FLT administered in radiotracer amounts for PET imaging, 2) comparing a radiotracer dose of FLT to clinical trial doses of FLT. METHODS: Twenty patients gave consent to a 18F-FLT injection, subsequent PET imaging, and blood draws. For each patient, blood samples were collected at multiple times before and after 18F-FLT PET. These samples were assayed for a comprehensive metabolic panel, total bilirubin, complete blood and platelet counts. 18F-FLT doses of 2.59 MBq/Kg with a maximal dose of 185 MBq (5 mCi) were used. Blood time-activity curves were generated for each patient from dynamic PET data, providing a measure of the area under the FLT concentration curve for 12 hours (AUC12). RESULTS: No side effects were reported. Only albumin, red blood cell count, hematocrit and hemoglobin showed a statistically significant decrease over time. These changes are attributed to IV hydration during PET imaging and to subsequent blood loss at surgery. The AUC12 values estimated from imaging data are not significantly different from those found from serial measures of FLT blood concentrations (p = 0.66). The blood samples-derived AUC12 values range from 0.232 ng x h/mL to 1.339 ng x h/mL with a mean of 0.802 +/- 0.303 ng x h/mL. This corresponds to 0.46% to 2.68% of the lowest and least toxic clinical trial AUC12 of 50 ng x h/mL reported by Flexner et al (1994). This single injection also corresponds to a nearly 3,000-fold lower cumulative dose than in Flexner's twice daily trial. CONCLUSION: This study shows no evidence of toxicity or complications attributable to 18F-FLT injected intravenously.

Evaluation of 5'-deoxy-5'-[F-18]fluorothymidine as a tracer of intracellular thymidine phosphorylase activity.

Two human cell lines (A549 and U937) with cytosolic thymidine phosphorylase (TP) activity were used to evaluate the potential of 5'-deoxy-5'-[F-18]fluorothymidine ([F-18]DFT) as a tracer of intracellular TP expression. Cellular metabolism of DFT led to the production of 5-[F-18]fluoro-2,5-dideoxy-D-ribose-1alpha-phosphate ([F-18]FddR-1P), in analogy to the metabolism of thymidine, which produces 2-deoxy-D-ribose-1alpha-phosphate (dR-1P). A549 cells showed the highest production rate of FddR-1P. After A549 cells were exposed to [F-18]DFT for 40 min, the relative intracellular concentration of [F-18]FddR-1P was more than sevenfold higher in cells than its precursor in the incubating medium. For the same amount of time, a twofold concentration was seen in U937 cells. However, uptake ratios did not rank with the corresponding TP activities found in cell extracts [TP activity ratio (U937:A549)=1.6] that were independently determined with a labeled thymidine/thymine cleavage assay. The discrepancy of TP activity ratios was traced to the instability of FddR-1P in cells. This was evident from the fact that cells accumulated radioactivity by producing FddR-1P, but activity also effluxed from cells over 1 h when the medium was subsequently made tracer free. The dominant labeled molecule released by cells was characterized as a neutral and lipophilic molecule, which was presumed to be a deoxynucleoside. Our results indicate that [F-18]DFT would not be effective for imaging TP expression because its initial metabolite undergoes further conversion to a diffusible secondary metabolite, allowing activity loss from cells.

Kinetic analysis of 3'-deoxy-3'-18F-fluorothymidine in patients with gliomas.

UNLABELLED: 3'-Deoxy-3'-fluorothymidine (FLT), a thymidine analog, is under investigation for monitoring cellular proliferation in gliomas, a potential measure of disease progression and response to therapy. Uptake may result from retention in the biosynthetic pathway or leakage via the disrupted blood-tumor barrier. Visual analysis or static measures of 18F-FLT uptake are problematic as transport and retention cannot be distinguished. METHODS: Twelve patients with primary brain tumors were imaged for 90 min of dynamic 18F-FLT PET with arterial blood sampling. Total blood activity was corrected for labeled metabolites to provide an FLT input function. A 2-tissue compartment, 4-rate-constant model was used to determine blood-to-tissue transport (K1) and metabolic flux (K(FLT)). Modeling results were compared with MR images of blood-brain barrier (BBB) breakdown revealed by gadolinium (Gd) contrast enhancement. Parametric image maps of K1 and K(FLT) were produced by a mixture analysis approach. RESULTS: Similar to prior work with 11C-thymidine, identifiability analysis showed that K1 (transport) and K(FLT) (flux) could be estimated independently for sufficiently high K1 values. However, estimation of K(FLT) was less robust at low K1 values, particularly those close to normal brain. K1 was higher for MRI contrast-enhancing (CE) tumors (0.053 +/- 0.029 mL/g/min) than noncontrast-enhancing (NCE) tumors (0.005 +/- 0.002 mL/g/min; P < 0.02), and K(FLT) was higher for high-grade tumors (0.018 +/- 0.008 mL/g/min, n = 9) than low-grade tumors (0.003 +/- 0.003 mL/g/min, n = 3; P < 0.01). The flux in NCE tumors was indistinguishable from contralateral normal brain (0.002 +/- 0.001 mL/g/min). For CE tumors, K1 was higher than K(FLT). Parametric images matched region-of-interest estimates of transport and flux. However, no patient has 18F-FLT uptake outside of the volume of increased permeability defined by MRI T1+Gd enhancement. CONCLUSION: Modeling analysis of 18F-FLT PET data yielded robust estimates of K1 and K(FLT) for enhancing tumors with sufficiently high K1 and provides a clearer understanding of the relationship between transport and retention of 18F-FLT in gliomas. In tumors that show breakdown of the BBB, transport dominates 18F-FLT uptake. Transport across the BBB and modest rates of 18F-FLT phosphorylation appear to limit the assessment of cellular proliferation using 18F-FLT to highly proliferative tumors with significant BBB breakdown.

Tumor hypoxia imaging with [F-18] fluoromisonidazole positron emission tomography in head and neck cancer.

PURPOSE: Advanced head and neck cancer shows hypoxia that results in biological changes to make the tumor cells more aggressive and less responsive to treatment resulting in poor survival. [F-18] fluoromisonidazole (FMISO) positron emission tomography (PET) has the ability to noninvasively quantify regional hypoxia. We investigated the prognostic effect of pretherapy FMISO-PET on survival in head and neck cancer. EXPERIMENTAL DESIGN: Seventy-three patients with head and neck cancer had pretherapy FMISO-PET and 53 also had fluorodeoxyglucose (FDG) PET under a research protocol from April 1994 to April 2004. RESULTS: Significant hypoxia was identified in 58 patients (79%). The mean FMISO tumor/bloodmax (T/Bmax) was 1.6 and the mean hypoxic volume (HV) was 40.2 mL. There were 28 deaths in the follow-up period. Mean FDG standard uptake value (SUV)max was 10.8. The median time for follow-up was 72 weeks. In a univariate analysis, T/Bmax (P=0.002), HV (P=0.04), and the presence of nodes (P=0.01) were strong independent predictors. In a multivariate analysis, including FDG SUVmax, no variable was predictive at P<0.05. When FDG SUVmax was removed from the model (resulting in n=73 with 28 events), nodal status and T/Bmax (or HV) were both highly predictive (P=0.02, 0.006 for node and T/Bmax, respectively; P=0.02 and 0.001 for node and HV, respectively). CONCLUSIONS: Pretherapy FMISO uptake shows a strong trend to be an independent prognostic measure in head and neck cancer.

Evaluation of 18F-annexin V as a PET imaging agent in an animal model of apoptosis.

UNLABELLED: Annexin V is a 36-kDa protein that binds with high affinity to phosphatidylserine lipids in the cell membrane. Because one of the earliest measurable events in apoptosis is the eversion of phosphatidylserine from the inner membrane leaflet to the outer cell surface, annexin V has proven useful for detecting the earliest stages of apoptosis. METHODS: Annexin V was radiolabeled with 18F using N-succinimidyl-4-18F-fluorobenzoic acid chemistry, to a specific activity of 555-925 kBq/mug of protein. 18F-Annexin V (14.8-51.8 MBq) was administered intravenously to rats after pretreatment with cycloheximide (5 mg/kg) to induce liver apoptosis, and the injected rats were imaged by PET over 2 h. After imaging, rats were dissected and individual organs were weighed and counted. RESULTS: Pretreatment of rats with cycloheximide resulted in a 3- to 9-fold increase in uptake of 18F-annexin V in the liver of treated animals at 2 h, compared with controls. By morphologic analysis, treated livers showed a 3- to 6-fold higher level of apoptosis than controls, with higher levels also seen with longer exposure to cycloheximide. Terminal deoxynucleotide end-labeling (TUNEL) assays performed on liver slices showed that cycloheximide induced a 5- to 8-fold increase in the number of TUNEL-positive nuclei. These TUNEL results correlated with the uptake of 18F-annexin V in dissected liver tissue, with an r2 value of 0.89. Biodistribution analysis of normal rats showed highest uptake of 18F-annexin V in the kidneys and urinary bladder, indicating rapid renal clearance of 18F-annexin V metabolites. CONCLUSION: The PET data, the organ-specific uptake data from dissection, and the morphologic and TUNEL measures of apoptosis together indicate that 18F-annexin V binds specifically to apoptotic tissues in this model of chemically induced apoptosis in rat liver. The short physical half-life of 18F-annexin V and the rapid clearance of its metabolites to the urinary system suggest that 18F-annexin V will be useful in early assessment of the clinical response to cancer therapy in individual patients.

Kinetic modeling of 3'-deoxy-3'-fluorothymidine in somatic tumors: mathematical studies.

UNLABELLED: We present a method to measure the regional rate of cellular proliferation using a positron-emitting analog of thymidine (TdR) for human imaging studies. The method is based on the use of 3'-deoxy-3'-(18)F-fluorothymidine (FLT) to estimate the flux of TdR through the exogenous pathway. The model reflects the retention of FLT-monophosphate (FLTMP), which is generated by the phosphorylation of FLT by thymidine kinase 1 (TK1), the initial step in the exogenous pathway. METHODS: A model of FLT kinetics has been designed based on the assumptions of a steady-state synthesis and incorporation of nucleotides into DNA, an equilibration of the free nucleoside in tissue with the plasma level, and the relative rates of FLT and TdR phosphorylation from prior data using direct analysis with in vitro assays. A 2-compartment model with 4 rate constants adequately describes the kinetics of FLT uptake and retention over 120 min and leads to an estimation of the rate of cellular proliferation using the measured FLT blood clearance and the dynamic FLT uptake curve. RESULTS: Noise characteristics of kinetic parameter estimates for 3 tissues were assessed under a range of conditions representative of human cancer patient imaging. The FLT flux in these tissues can be measured with a SE of <5%, and FLT transport can be estimated with a SE of <15%. Abbreviating the data collection to 60 min or neglecting k(4), giving a 3-parameter model, results in an unsatisfactory loss of accuracy in the flux constant in tumor simulations. CONCLUSION: These analyses depict model behavior and provide expected values for the accuracy of parameter estimates from FLT imaging in human patients. Our companion paper describes the performance of the model for human data in patients with lung cancer. Further studies are necessary to determine the fidelity of K(FLT) (FLT flux) as a proxy for K(TDR) (thymidine flux), the gold standard for imaging cellular proliferation.

Kinetic analysis of 3'-deoxy-3'-fluorothymidine PET studies: validation studies in patients with lung cancer.

UNLABELLED: Assessing cellular proliferation provides a direct method to measure the in vivo growth of cancer. We evaluated the application of a model of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) kinetics described in a companion report to the analysis of FLT PET image data in lung cancer patients. Compartmental model analysis was performed to estimate the overall flux constants (K(FLT)) for FLT phosphorylation in tumor, bone marrow, and muscle. Estimates of flux were compared with an in vitro assay of proliferation (Ki-67) applied to tissue derived from surgical resection. Compartmental modeling results were compared with simple model-independent methods of estimating FLT uptake. METHODS: Seventeen patients with 18 tumor sites underwent up to 2 h of dynamic PET with blood sampling. Metabolite analysis of plasma samples corrected the total blood activity for labeled metabolites and provided the FLT model input function. A 2-compartment, 4-parameter model (4P) was tested and compared with a 2-compartment, 3-parameter (3P) model for estimating K(FLT). RESULTS: Bone marrow, a proliferative normal tissue, had the highest values of K(FLT), whereas muscle, a nonproliferating tissue, showed the lowest values. The K(FLT) for tumors estimated by compartmental analysis had a fair correlation with estimates by modified graphical analysis (r = 0.86) and a poorer correlation with the average standardized uptake value (r = 0.62) in tumor. Estimates of K(FLT) derived from 60 min of dynamic PET data using the 3P model underestimated K(FLT) compared with 90 or 120 min of dynamic data analyzed using the 4P model. Comparison of flux estimates with an independent measure of cellular proliferation showed that K(FLT) was highly correlated with Ki-67 (Spearman rho = 0.92, P < 0.001). Ignoring the metabolites of FLT in blood underestimated K(FLT) by as much as 47%. CONCLUSION: Compartmental analysis of FLT PET image data yielded robust estimates of K(FLT) that correlated with in vitro measures of tumor proliferation. This method can be applied generally to other imaging studies of different cancers after validation of parameter error. Tumor loss of phosphorylated FLT nucleotides (k(4)) is notable and leads to errors when FLT uptake is evaluated using model-independent approaches that ignore k(4), such as graphical analysis or the SUV.

Metabolism of 3'-deoxy-3'-[F-18]fluorothymidine in proliferating A549 cells: validations for positron emission tomography.

3'-Deoxy-3'-[F-18]fluorothymidine (FLT) is under clinical evaluation as a metabolic probe for imaging cell proliferation in vivo using positron emission tomography (PET). As part of our validation effort, we followed the short-term metabolism of FLT in exponentially growing tumor cells to demonstrate the enzyme activities within the DNA salvage pathway that influence retention of radioactivity. In A549 cells, thymidine kinase-1 (TK1) activity produced FLTMP, which dominated the labeled nucleotide pool. Subsequent nucleotide phosphorylations by thymidylate kinase (TMPK) and nucleotide diphosphate kinase (NDPK) led to FLTTP. After 1h, the cellular metabolic pool contained approximately 30% FLTTP. A putative deoxynucleotidase (dNT), which degrades FLTMP to FLT, provided the primary mechanism for tracer efflux from cells. In contrast, FLTTP was resistant to degradation and highly retained. The uptake and retention characteristics of FLT were also compared to those of thymidine, FMAU (2'-arabino-fluoro-TdR) and FIAU (2'-arabino-fluoro-5-iodo-2'-dexoyuridine). Despite the fact that FLT lacks the 3'-hydroxy necessary for its incorporation into DNA it out performed both FMAU and FIAU in terms of uptake and retention.

Effect of p53 activation on cell growth, thymidine kinase-1 activity, and 3'-deoxy-3'fluorothymidine uptake.

The use of thymidine (TdR) and thymidine analogs such as 3'-deoxy-3'-fluorothymidine (FLT) as positron emission tomography (PET)-based tracers of tumor proliferation rate is based on the hypothesis that measurement of uptake of these nucleosides, a function primarily of thymidine kinase-1 (TK(1)) activity, provides an accurate measure of cell proliferation in tumors. Tumor growth is influenced by many factors including the oxygen concentration within tumors and whether tumor cells have been exposed to cytotoxic therapies. The p53 gene plays an important role in regulating growth under both of these conditions. The goal of this study was to investigate the influence of p53 activation on cell growth, TK(1) activity, and FLT uptake. To accomplish this, TK(1) activity, S phase fraction, and the uptake of FLT were determined in plateau-phase and exponentially growing cultures of an isogenic pair of human tumor cell lines in which p53 expression was normal or inactivated by human papilloma virus type 16 E6 expression. Ionizing radiation exposure was used to stimulate p53 activity and to induce alterations in cell cycle progression. We found that exposure of cells to ionizing radiation induced dose-dependent changes in cell cycle progression in both cell lines. The relationship between S phase percentage, TK(1) activity, and FLT uptake were essentially unchanged in the p53-normal cell line. In contrast, TK(1) activity and FLT uptake remained high in the p53-deficient variant even when S phase percentage was low due to a p53-dependent G2 arrest. We conclude that a functional p53 response is required to maintain the normal relationship between TK1 activity and S phase percentage following radiation exposure.

Production of [F-18]fluoroannexin for imaging apoptosis with PET.

Recombinant human-annexin-V was conjugated with 4-[F-18]fluorobenzoic acid (FBA) via its reaction with the N-hydroxysuccinimidyl ester (FBA-OSu) at pH 8.5. A series of reactions using varying amounts of annexin-V, unlabeled FBA-OSu, and time produced products with different conjugation levels. Products were characterized by mass spectrometry and a cell-binding assay to assess the effect of conjugation. In each case, the conjugated protein was a mixture of proteins with a range of conjugation. Annexin-V could be conjugated with an average of two FBA mole equivalents without decreasing its affinity for red blood cells (K(d) 6-10 nM) with exposed phosphatidylserine. An average conjugation of 7.7 (range 3-13) diminished the binding 3-fold. Large-scale production and purification of [F-18]FBA-OSu from [F-18]fluoride was accomplished within 90 min and in 77% radiochemical yield (decay-corrected to the end of cyclotron bombardment). The conjugation reaction of annexin with [F-18]FBA-OSu was studied with respect to activity level, protein mass, and concentration. Under the most favorable conditions, >25 mCi [F-18]fluoroannexin (FAN) was isolated in 64% yield (decay-corrected for a 22 min conjugation process) from labeling 1.1 mg of annexin-V. A pilot PET imaging study of [F-18]fluoroannexin in normal rats showed high uptake in the renal excretory system and demonstrated sufficient clearance from most other internal organs within 1 h. [F-18]Fluoroannexin should prove useful in imaging targeted apoptosis.

Monitoring tumor cell proliferation by targeting DNA synthetic processes with thymidine and thymidine analogs.

UNLABELLED: The use of radiolabeled thymidine (TdR) and thymidine analogs as PET-based tracers of tumor growth rate is based on the assumption that measurement of uptake of these nucleosides, a function primarily of thymidine kinase-1 (TK(1)) activity, provides an accurate measure of active cell proliferation in tumors. The goal of this study was to test this hypothesis and determine how well these tracers track changes in proliferation of tumor cells. METHODS: TK(1) activity; S-phase fraction; and uptake of TdR, 3'-deoxy-3'-fluorothymidine (FLT), and 2'-fluoro-5-methyl-1-(beta-D-2-arabino-furanosyl) uracil (FMAU) were determined in plateau-phase and exponentially growing cultures of 3 human and 3 murine tumor cell lines. RESULTS: TK(1) activity and S-phase fraction increased in all cell lines as cells moved from plateau-phase conditions to exponential growth. Some cell lines had relatively large TK(1) activities and S-phase fractions under plateau-phase conditions, consistent with a loss of normal cell cycle checkpoint control in these cells. There were also 2 cell lines in which TK(1) activity changed little as cells moved from the plateau phase to exponential growth, suggesting that in these cell lines, de novo nucleotide synthesis pathways predominate over salvage pathways. Both TdR and FLT detected changes in TK(1) activity. The slope of the relationship between TdR uptake and TK(1) activity was nearly twice that for FLT and more than 40-fold that for FMAU. CONCLUSION: Although not all tumors show a strong TK(1) dependence of proliferation, in all cell lines for which proliferation is highly TK(1) dependent, phosphorylation of TdR or FLT accurately reflects changes in TK(1) enzyme activity.

18F-Fluorothymidine radiation dosimetry in human PET imaging studies.

UNLABELLED: 3'-Deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) is a PET imaging agent that shows promise for studying cellular proliferation in human cancers. FLT is a nucleoside analog that enters cells and is phosphorylated by human thymidine kinase 1, but the 3' substitution prevents further incorporation into DNA. We estimated the radiation dosimetry for this tracer from data gathered in patient studies. METHODS: Time-dependent tissue concentrations of radioactivity were determined from blood samples and PET images of 18 patients after intravenous injection of (18)F-FLT. Radiation-absorbed doses were calculated using the MIRD Committee methods, taking into account variations that were based on the distribution of activities observed in the individual patients. Effective dose equivalent (EDE) was calculated using International Commission on Radiological Protection Publication 60 tissue weighting factors for the standard man and woman. RESULTS: For a single bladder voiding at 6 h after (18)F-FLT injection, the (18)F-FLT EDE (mean +/- SD) was 0.028 +/- 0.012 mSv/MBq (103 +/- 43 mrem/mCi) for a standard male patient and 0.033 +/- 0.012 mSv/MBq (121 +/- 43 mrem/mCi) for a standard female patient. The organ that received the highest dose was the bladder (male, 0.179 mGy/MBq [662 mrad/mCi]; female, 0.174 mGy/MBq [646 mrad/mCi]), followed by the liver (male, 0.045 mGy/MBq [167 mrad/mCi]; female, 0.064 mGy/MBq [238 mrad/mCi]), the kidneys (male, 0.035 mGy/MBq [131 mrad/mCi]; female, 0.042 mGy/MBq [155 mrad/mCi]), and the bone marrow (male, 0.024 mGy/MBq [89 mrad/mCi]; female, 0.033 mGy/MBq [122 mrad/mCi]). CONCLUSION: Organ dose estimates for (18)F-FLT are comparable to those associated with other commonly performed nuclear medicine tests, and the potential radiation risks associated with (18)F-FLT PET imaging are within accepted limits.

In vivo validation of 3'deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) as a proliferation imaging tracer in humans: correlation of [(18)F]FLT uptake by positron emission tomography with Ki-67 immunohistochemistry and flow cytometry in human lung tumors.

PURPOSE: Tumor proliferation has prognostic value in resected early stage non-small cell lung cancer (NSCLC) and can, therefore, predict which NSCLCs are at high risk for recurrence after resection and would benefit from additional therapy. It may also predict which tumor will respond to cell cycle-targeted chemotherapy and help assess the tumor response, besides helping to differentiate benign from malignant lung lesions. We evaluated whether the uptake of the new positron emission tomography (PET) tracer 3'deoxy-3'-[18F]fluorothymidine (FLT) in a series of suspected NSCLCs correlated with tumor proliferation assessed by Ki-67 immunohistochemistry and flow cytometry. EXPERIMENTAL DESIGN: Ten patients with 11 biopsy-proven or clinically suspected NSCLC underwent 2-h dynamic PET imaging after i.v. injection of 0.07 mCi/kg FLT. Tumor FLT uptake was quantitated with the maximum pixel standardized uptake value (maxSUV), the partial volume corrected maxSUV (PV-corr-maxSUV), the average SUV over a small region-of-interest (aveSUV) and with Patlak analysis of FLT flux (aveFLTflux). The lesion diameter from computed tomography was used to correct the maxSUV for PV effects using recovery coefficients determined for the General Electric Advance PET scanner. Two of the 11 lesions were benign inflammatory lesions and 9 were NSCLCs. Immunohistochemistry for Ki-67 (proliferation index marker) was performed on all 11 tissue specimens (10 resections, 1 NSCLC percutaneous biopsy), and the S-phase fraction (SPF) from flow cytometry could be determined for 10. The specimens were reviewed for histology and cellular differentiation (poor, moderate, well). Lesions ranged from 1.6 to 7.7 cm. RESULTS: Excellent correlations were found between SUV measures of FLT uptake and Ki-67 scores [percentage of positive cells; maxSUV versus Ki-67: Rho = 0.78, P = 0.0043 (n = 11); PV-corr-maxSUV versus Ki-67: Rho = 0.83, P = 0.0028 (n = 10); aveSUV versus Ki-67: Rho = 0.84, P = 0.0011 (n = 11)]. Correlation between Ki-67 proliferation scores and Patlak measures of FLT uptake were also strong: aveFLTflux versus Ki-67: Rho = 0.94, P < 0.0001 (n = 11). The correlation between the SPF and all indices of FLT uptake was weaker and reached statistical significance for only two uptake indices [maxSUV versus SPF: Rho = 0.69, P = 0.03 (n = 10); PV-corr-maxSUV versus SPF: Rho = 0.36, P = 0.35 (n = 9); aveSUV versus SPF: Rho = 0.67, P = 0.03 (n = 10); aveFLTflux versus SPF: Rho = 0.46, P = 0.18 (n = 10)]. CONCLUSION: FLT PET may be used to noninvasively assess proliferation rates of lung masses in vivo. Therefore, FLT PET may play a significant role in the evaluation of indeterminate pulmonary lesions, in the prognostic assessment of resectable NSCLC, and possibly in the evaluation of NSCLC response to chemotherapy.

Validation of FLT uptake as a measure of thymidine kinase-1 activity in A549 carcinoma cells.

UNLABELLED: The thymidine analog (18)F-3'-deoxy-3' -fluorothymidine (FLT) is being used clinically for PET imaging of tumor proliferation. Appropriate use of this tracer requires validating the mechanisms by which it accumulates in dividing cells. We tested the accuracy with which FLT uptake predicted the activity of cytosolic thymidine kinase-1 (TK(1)), an enzyme that is upregulated before and during DNA synthesis. METHODS: Cultured A549 human lung carcinoma cells were manipulated to a range of proliferation rates from actively dividing to growth arrested. Uptake of radiolabeled FLT was compared with cell cycle activity, which was expressed as the percentage of cells in S phase, and with activity of cytosolic TK(1). We also compared uptake of FLT and deoxyglucose. We genetically manipulated A549 cells by transfecting them with human papillomavirus type 16 E6 (designated A549-E6) to abrogate function of the tumor suppressor gene, p53. Although radiation typically inhibits progression of mammalian cells through the cell cycle, abrogation of p53 function eliminates this inhibition. We then compared FLT uptake with the percentage of cells in S phase and TK(1) activity in irradiated A549-E6 cells and in irradiated control cells having normal p53 function and the expected radiation-induced growth delay. RESULTS: A549 cells with only 3%-5% cells in S phase took up little FLT and had low levels of TK(1) activity. When cells were stimulated to grow by being placed into fresh medium, we observed a strong correlation between increased FLT uptake and increased TK(1) activity. As expected, FLT uptake varied much more as a function of growth than did uptake of deoxyglucose. Nonproliferating A549 cells did not enter the cell cycle if they were irradiated before being placed into fresh medium, and they did not accumulate FLT or show elevated TK(1) activity. In contrast, radiation did not inhibit the cell cycle progression of A549-E6 cells. When subcultured, they began to grow and showed increased uptake of FLT commensurate with greater TK(1) activity. CONCLUSION: In cultured A549 cells FLT uptake is positively correlated with cell growth and TK(1) activity. Inhibition of cell cycle progression prevents FLT uptake and increased TK(1) activity. These results suggest that FLT images reflect TK(1) activity and the percentage of cells in S phase.