Raunak Varshney

Comparative evaluation of F-18 FDOPA, F-18 FDG, and F-18 FLT-PET/CT for metabolic imaging of low grade gliomas.

Introduction: We undertook this prospective study to compare amino acid metabolism, glucose metabolism, and proliferation in primary and recurrent low grade gliomas using positron emission tomography (PET)/computed tomography with F-18 FDOPA, F-18 FDG, and F-18 FLT. Methods: Fifteen patients with newly diagnosed or previously treated low grade gliomas (WHO grade I or II) were subjected to F-18-FDOPA, F-18 FDG, and F-18 FLT PET/computed tomography studies on consecutive days. This included 2 patients in remission as control subjects. Uptake of all the 3 tracers were analyzed visually and quantified using standardized uptake values and tumor to normal (T/N) ratios. The accuracy of all the 3 PET tracers in the detection of newly diagnosed and recurrent low grade gliomas was compared. Results: F-18 FDOPA was positive in all cases of primary and recurrent low grade gliomas and negative in the patients in remission. Tumor was visualized on F-18 FDG in 7 of 13 cases, F-18-FLT was positive in 4 of 13 cases. Average tumor standardized uptake values max for F-18 FDOPA (5.75 ± 4.9) and F-18 FLT (1.8 ± 0.91) was lower than that of F-18 FDG (8.5 ± 4.4). T/N ratios for F-18-FDOPA (2.3 ± 0.51) and F-18 FLT (1.8 ± 0.91) were higher than F-18 FDG (1.03 ± 0.64) providing good image contrast for tumor detection in positive cases. Conclusion: F-18 FDOPA scan is superior to both F-18 FLT and F-18 FDG for visualization of primary and recurrent low grade gliomas. F-18-FLT should not be considered for evaluation of recurrent low grade gliomas.

Comparison of F-18 FDG and C-11 methionine PET/CT for the evaluation of recurrent primary brain tumors.

Purpose of Study: With the availability of multiple positron emission tomography (PET) tracers for neurooncology, there is a need to define the appropriate tracer in a given clinical setting, and it is in this regard that we undertook this study to directly compare F-18 flurodeoxyglucose (FDG) PET and C-11 methionine (MET) PET for the evaluation of recurrence in primary brain tumors. Patients and Methods: Thirty-seven patients with a history of treated primary brain tumors referred for evaluation of recurrent disease were initially included in the study. Two patients had to be excluded because of insufficient follow-up. There were 23 males and 12 females, mean age: 33.7 ± 16.4 years; range: 5 to 65 years. All patients underwent the MET and FDG study on the same day. Visual image interpretation was performed independently by 2 PET physicians for each tracer using the plain PET and fused PET/CT images; the FDG images were evaluated first. Images were analyzed semiquantitatively using tumor to normal contralateral cortex ratios (T/N). Each patient was followed up for a minimum of 18 months. Imaging results were compared with histopathology on tumor excision or biopsy in 14 patients and with clinical follow-up and MRI/MRS at the end of 18 months in 21 patients. Results: The final diagnosis was tumor recurrence in 24 patients and no recurrence/stable disease in 11 patients. On FDG, findings in 15/35 (42%) were suggestive of recurrent tumors. On MET, findings in 24/34 (70.5%) cases were suggestive of recurrent tumors. Spatially separated secondary lesions including intraventricular deposits were clearly delineated in 5 cases, 3 were glioblastoma multiforme (GBM) and 2 were anaplastic astrocytomas. One of the secondary lesions was missed on FDG PET. Using a cutoff for T/N ratio on FDG of >0.75 to differentiate recurrence from no recurrence, sensitivity of FDG was 81.2% (confidence interval [CI] = 54.4%–96%), whereas specificity was 88.9% (CI = 51.8%–99.7%). Area under the curve was 0.819 (CI = 0.615–0.943), P = 0.0003. Using a cutoff for T/N ratio of >1.9 to differentiate recurrence from no recurrence, sensitivity of MET was 94.7% (CI = 74.0%–99.9%), whereas specificity was 88.89% (CI = 51.8%–99.7%). Area under the curve was 0.942 (CI = 0.785–0.995), P < 0.0001. Interobserver agreement, &kgr; coefficient, for MET was 0.93, suggesting good interobserver agreement, whereas for FDG, it was fair (0.23). Conclusions: MET should be the radiotracer of choice in the evaluation of recurrence of primary brain tumors because the sensitivity for detection and delineation of the possible recurrent tumor, as well as secondary deposits, is higher with MET. MET-PET is an easier technique to interpret, irrespective of the glioma grade, with less interobserver variability and straightforward localization of tumorous accumulation.

Targeting Neuropeptide Receptors for Cancer Imaging and Therapy: Perspectives with Bombesin, Neurotensin, and Neuropeptide-Y Receptors

Receptors for some regulatory peptides are highly expressed in tumors. Selective radiolabeled peptides can bind with high affinity and specificity to these receptors and exhibit favorable pharmacologic and pharmacokinetic properties, making them suitable agents for imaging or targeted therapy. The success encountered with radiolabeled somatostatin analogs is probably the first of a long list, as multiple peptide receptors are now recognized as potential targets. This review focuses on 3 neuropeptide receptor systems (bombesin, neurotensin, and neuropeptide-Y) that offer high potential in the field of nuclear oncology. The underlying biology of these peptide/receptor systems, their physiologic and pathologic roles, and their differential distribution in normal and tumoral tissues are described with emphasis on breast, prostate, and lung cancers. Radiolabeled analogs that selectively target these receptors are highlighted.

Synthesis and evaluation of biodegradable PCL/PEG nanoparticles for neuroendocrine tumor targeted delivery of somatostatin analog

Purpose: Neuroendocrine tumors often present a diagnostic and therapeutic challenge. We have aimed to synthesize and develop biodegradable nanoparticles of somatostatin analogue, octreotide for targeted therapy of human neuroendocrine pancreatic tumor. Methods: Direct solid phase peptide synthesis of octreotide was done. Octreotide loaded PCL/PEG nanoparticles were prepared by solvent evaporation method and characterized for transmission electron microscopy, differential scanning calorimetery (DSC), Zeta potential measurement studies. The nanoparticles were evaluated in vitro for release studies and peptide content. For biological evaluations, receptor binding & cytotoxicity studies were done on BON-1 neuroendocrine tumor cell line. Biodistribution of radiolabeled peptide and nanoparticles, tumor regression studies were performed on tumor-bearing mouse models. Results: We have synthesized and purified octreotide with the purity of 99.96% in our laboratory. PEG/PCL nanoparticles with an average diameter of 130–195 nm having peptide loading efficiency of 66–84% with a negative surface charge were obtained with the formulation procedure. Octreotide nanoparticles have a negative action on the proliferation of BON-1 cells. In vivo biodistribution studies exhibited major accumulation of octreotide nanoparticles in tumor as compared to plain octreotide. Octreotide nanoparticles inhibited tumor growth more efficiently than free octreotide. Conclusions: Thus, it was concluded that the PCL/PEG nanoformulation of octreotide showed high tumor uptake due to the enhanced permeation and retention (EPR) effect and then peptide ligand imparts targetability to the sst2 receptor and there by showing increase tumor growth inhibition. Selective entry of nanoparticles to the tumor also give the reduce side effects both in vivo and in vitro.

Solid phase synthesis, radiolabeling and biological evaluation of a 99mTc-labeled αVβ3 tripeptide (RGD) conjugated to DOTA as a tumor imaging agent

Solid phase synthesis of peptides-radiometal chelator can facilitate the creation of radioactive peptide libraries to be utilized in high throughput in in vivo screening of targeted molecular imaging agents. An αVβ3 tripeptide derivative DOTA-NH-Arg-Gly-Asp was synthesized by Fmoc solid phase peptide synthesis and analyzed by spectroscopic techniques. In order to radiolabel this RGD peptide with 99mTc-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was incorporated as a chelator. The DOTA-peptide conjugate binds to 99mTc with high efficiency at ambient temperature. The resulting conjugate is stable under physiological conditions for at least 24 h after radiocomplexation. The receptor binding studies of 99mTc-DOTA- αVβ3-tripeptide in established human tumor cell lines U-87MG and BMG revealed Kd values in the nM and μM range respectively. U-87MG tumors in athymic mice were accumulated in the γ-images and major accumulation of the radiotracer was observed in kidneys followed by liver and lungs. High tumor uptake was shown in the U-87MG tumor bearing athymic mice; tumor to muscle ratios reached 8.13 ± 2.18 and 35.09 ± 4.78 at 1 and 4 h after post injection respectively.

Synthesis, conjugation and relaxation studies of gadolinium(III)-4-benzothiazol-2-yl-phenylamine as a potential brain specific MR contrast agent

Magnetic resonance (MR) imaging is widely used in clinical research to map the structural and functional organization of the brain. We have designed and synthesized a Gd-based specific MR contrast agent that binds to regions in the brain. The presented compound {4-[(4-benzothiazol-2-yl-phenylcarbamoyl)-methyl]-7,10-bis-carboxymethyl-1,4,7,10-tetraazacyclododec-1-yl} acetic acid (DO3A-BT) was synthesized by conjugating the chloroacetylated product of 4-benzothiazol-2-yl-phenylamine with a trisubstituted cyclen. The lanthanide complex (Ln-DO3A-BT) was evaluated in vitro for both MR (Gd-DO3A-BT) and optical (Eu-DO3A-BT) imaging applications. The complex Gd-DO3A-BT displays a relaxivity of r1 = 4.18 mM(-1) s(-1) at 4.7 T which is 1.2 times greater than Dotarem and significantly higher than the brain specific MR contrast agent Luxol Fast Blue (LFB). The protonation constant of the ligand (pKa1 = 9.91, pKa2 = 8.22, pKa3 = 5.01) and the stability constant of the complex formed between Gd(III), Eu(III) and Ca(II) and ligand DO3A-BT (log βGdL = 18.4, log βEuL = 18.3, log βZn2L = 7.1, log βCa2L = 6.3) were recorded by potentiometric titration. The constants reflect the high stability of the ligand with lanthanides compared with endogenous metal ions. The transmetalation stability of Gd-DO3A-BT toward Zn proved to be excellent with a rate constant of 3.07 × 10(-5) s(-1) which is in line with other tetraazatetraacetic acid (DOTA)-monoamide complexes. The hydration number (q) was found to be 0.92, and is calculated from the difference in the luminescence lifetime of Eu-DO3A-BT in H2O and D2O solutions to determine the coordination state of this complex. The in vivo biodistribution of (99m)Tc-DO3A-BT in BALB/c mice showed a brain uptake of 1.2% ID g(-1) at 2 min post injection when injected with mannitol which disrupts the blood-brain-barrier (BBB) due to osmotic shock. In vitro binding on the brain homogenate revealed a high uptake by the neuronal/glial cells for in vivo applications.

68 Ga-Labeled Bombesin Analogs for Receptor-Mediated Imaging

Targeted receptor-mediated imaging techniques have become crucial tools in present targeted diagnosis and radiotherapy as they provide accurate and specific diagnosis of disease information. Peptide-based pharmaceuticals are gaining popularity, and there has been vast interest in developing (68)Ga-labeled bombesin (Bn) analogs. The gastrin-releasing peptide (GRP) family and its Bn analog have been implicated in the biology of several human cancers. The three bombesin receptors GRP, NMB, and BRS-3 receptor are most frequently ectopically expressed by common, important malignancies. The low expression of Bn/GRP receptors in normal tissue and relatively high expression in a variety of human tumors can be of biological importance and form a molecular basis for Bn/GRP receptor-mediated imaging. To develop a Bn-like peptide with favorable tumor targeting and pharmacokinetic characteristics for possible clinical use, several modifications in the Bn-like peptides, such as the use of a variety of chelating agents, i.e., acyclic and macrocyclic agents with different spacer groups and with different metal ions (gallium), have been performed in recent years without significant disturbance of the vital binding scaffold. The favorable physical properties of (68)Ga, i.e., short half-life, and the fast localization of small peptides make this an ideal combination to study receptor-mediated imaging in patients.

Synthesis of [DTPA-bis(D-ser)] chelate (DBDSC): an approach for the design of SPECT radiopharmaceuticals based on technetium.

D-Serine is a physiological coagonist of the N-methyl D-aspartate (NMDA) type of glutamate receptor-a key excitatory neurotransmitter receptor in the brain. D-Serine appears to be a part of the synapse through a variety of transporters located on both neurons and astrocytes. The development of 99mTc radiolabeled amino acid based radiopharmaceuticals for imaging a variety of tumors has found to be useful in diagnostic imaging. Diethylene triamine penta acetic acid (DTPA) is one of the most well-known chelating reagent for the production of stable complexes with various heavy metal ions. We have synthesized [DTPA-bis(D-ser)] in 90% yield and analyzed the chelate by spectroscopic techniques. The DBDSC chelate binds to 99mTc with high efficiency at ambient temperature. The resulting chelate is stable under physiological conditions (37oC, pH=7.4) for at least 24 h after radiocomplexation. The receptor binding studies of 99mTc-[DTPA-bis(D-ser)] in established lung adeno carcinoma A549 exhibited Kd value to be 26nM. A549 Tumor in athymic mice was accumulated in the γ-images. The major accumulation of the radiotracer was observed in tumor, followed by kidneys. 99mTc-[DTPA-bis(D-ser)] has promising utility as SPECT-radiopharmaceutical.

Biological evaluation of avidin-based tumor pretargeting with DOTA-Triazole-Biotin constructed via versatile Cu(I) catalyzed click chemistry

Background: The biotin-avidin interaction remains a gold standard for the two-step pretargeting approach to image tumor sites. We aim to develop two-step pretargeting systems utilizing 99mTc labeled biotin functionalized macrocyclic chelating agents synthesized using the highly efficient Cu(I) catalyzed azide–alkyne cycloaddition for potential radioimaging applications. Methods: A facile synthesis of DOTA-Triazole-Biotin, radiocomplexation with 99mTc and the pretargeting protocol is described. The synthesis features Cu(I) catalyzed click conjugation between biotinylated azide and propynyl functionalized DO3A. 99mTc radiolabeling was performed to detect the accumulation of avidin as the pretargeting agent. Cytotoxicity was determined using the trypan blue exclusion assay, macrocolony, and MTT assay. Cell uptake studies were performed using radiolabeled DOTA-Triazole-Biotin and compared with avidin treated cells for 2 h. Tumor imaging was performed in U-87MG cell line implanted tumor bearing nude mice and uptake of the radiotracer was estimated. Results: All compounds have been successfully characterized by NMR and MS spectroscopy. More than 96% radiolabeling efficiency was obtained and the radioconjugate exhibited sufficient stability under physiological conditions. Conclusion: To summarize, a new candidate for avidin based two-step pretargeting of tumors has been synthesized and evaluated for potential imaging and diagnostic applications. The chelate possesses high stability under physiological conditions, exhibits effective interaction with its avidin target, and low nonspecific retention in vivo.

Metabolic assessment of intracranial tuberculomas using 11C-methionine and 18F-FDG PET/CT.

Background18F-fluorodeoxyglucose (18F-FDG) has limited specificity in the evaluation of intracranial lesions as it is taken up by inflammatory and granulomatous lesions as well. 11C-methionine is known to have a higher specificity in tumor detection, delineation, and differentiation of benign from malignant lesions. However, its uptake in granulomatous lesions remains unclarified. ObjectiveThe aim of this study was to explore the value of 11C-methionine PET/CT and 18F-FDG in the evaluation of intracranial tuberculomas. Methods11C-methionine PET/CT followed by 18F-FDG PET/CT study was performed on 12 patients with intracranial tuberculomas. The diagnosis was confirmed for all cases on histopathological evaluation and/or follow-up. Quantitative analysis was performed for all cases by measuring the lesion-to-normal gray matter uptake ratio. ResultsA high lesion-to-normal gray matter uptake ratio was observed on both 11C-methionine (1.8±0.38) and 18F-FDG scans (1.64±0.26) in all newly diagnosed cases. Lesion detection and delineation was superior on 11C-methionine PET/CT. In addition, 11C-methionine appeared to be a more sensitive indicator for assessing early therapeutic response and incomplete therapeutic response in intracranial tuberculomas. There was complete concordance in the number and sites of lesions detected on 11C-methionine PET/CT and radiological imaging modalities (namely, CT and MRI). ConclusionThis preliminary study suggests that in newly diagnosed, untreated intracranial tuberculomas, 11C-methionine, like 18F-FDG, may have limited specificity in distinguishing it from a neoplastic lesion. However, it may play an important role in assessing the response to antitubercular treatment. Further studies are warranted to explore the potential of 11C-methionine in this regard.