Kiran Kumar Solingapuram Sai

Synthesis and structure-activity relationship studies of conformationally flexible tetrahydroisoquinolinyl triazole carboxamide and triazole substituted benzamide analogues as σ2 receptor ligands.

Two novel classes of compounds targeting the sigma-2 (σ2) receptor were synthesized, and their bioactivities to binding σ1 and σ2 receptors were measured. Four novel triazole carboxamide analogues, 24d, 24e, 24f, and 39c, demonstrated high affinity and selectivity for the σ2 receptor. These data suggest (11)C-labeled versions of these compounds may be potential σ2-selective radiotracers for imaging the proliferative status of solid tumors.

18F-AFETP, 18F-FET, and 18F-FDG Imaging of Mouse DBT Gliomas

The goal of this study was to evaluate the 18F-labeled nonnatural amino acid (S)-2-amino-3-[1-(2-18F-fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid (18F-AFETP) as a PET imaging agent for brain tumors and to compare its effectiveness with the more-established tracers O-(2-18F-fluoroethyl)-l-tyrosine (18F-FET) and 18F-FDG in a murine model of glioblastoma. The tracer 18F-AFETP is a structural analog of histidine and is a lead compound for imaging cationic amino acid transport, a relatively unexplored target for oncologic imaging. Methods: 18F-AFETP was prepared using the click reaction. BALB/c mice with intracranially implanted delayed brain tumor (DBT) gliomas (n = 4) underwent biodistribution and dynamic small-animal PET imaging for 60 min after intravenous injection of 18F-AFETP. Tumor and brain uptake of 18F-AFETP were compared with those of 18F-FDG and 18F-FET through small-animal PET analyses. Results: 18F-AFETP demonstrated focally increased uptake in tumors with good visualization. Peak tumor uptake occurred within 10 min of injection, with stable or gradual decrease over time. All 3 tracers demonstrated relatively high uptake in the DBTs throughout the study. At late time points (47.5–57.5 min after injection), the average standardized uptake value with 18F-FDG (1.9 ± 0.1) was significantly greater than with 18F-FET (1.1 ± 0.1) and 18F-AFETP (0.7 ± 0.2). The uptake also differed substantially in normal brain, with significant differences in the standardized uptake values at late times among 18F-FDG (1.5 ± 0.2), 18F-FET (0.5 ± 0.05), and 18F-AFETP (0.1 ± 0.04). The resulting average tumor-to-brain ratio at the late time points was significantly higher for 18F-AFETP (7.5 ± 0.1) than for 18F-FDG (1.3 ± 0.1) and 18F-FET (2.0 ± 0.3). Conclusion: 18F-AFETP is a promising brain tumor imaging agent, providing rapid and persistent tumor visualization, with good tumor–to–normal-brain ratios in the DBT glioma model. High tumor-to-brain, tumor-to-muscle, and tumor-to-blood ratios were observed at 30 and 60 min after injection, with higher tumor-to-brain ratios than obtained with 18F-FET or 18F-FDG. These results support further development and evaluation of 18F-AFETP and its derivatives for tumor imaging.

Development of 18 F-Labeled PET Probes for Imaging Cell Proliferation

In recent years, two different methods have been developed to image cell proliferation with the functional imaging technique, Positron emission Tomography (PET), proliferation rate and proliferative status. Proliferation rate is a measure of the tumor doubling time and uses radiolabeled analogs of the DNA precursor thymidine. This approach measures the activity of the enzyme thymidine kinase 1 (TK1) and provides a pulse label of the S phase fraction of a tumor. Proliferative status provides a measure of the ratio of proliferating (P) and quiescent (Q) cells in a tumor. This imaging approach for measuring proliferative status involves measuring the sigma-2 (σ(2)) receptor status of a tumor, the only protein which has been validated for making this measurement in vivo with PET. This article provides an overview of the biological information obtained from these different imaging strategies, and the development of radiotracers for imaging proliferation rate and proliferative status.

Synthesis, radiolabeling and initial in vivo evaluation of [11C]KSM-01 for imaging PPAR-α receptors

Peroxisome proliferator-activated receptor alpha (PPAR-α) is a ligand-activated nuclear receptor transcription factor that regulates the fatty acid β-oxidation. An in vitro assay identified the p-methoxy phenyl ureido thiobutyric acid derivative KSM-01 (IC(50)=0.28±0.09nM) having a higher affinity to activate PPAR-α than the PPAR-α agonist GW7647 (IC(50)=0.46±0.19nM). In this study, we report the synthesis and initial in vivo evaluation of [(11)C]KSM-01. The radiosynthesis was carried out by first alkylating the corresponding p-phenol precursor with [(11)C]MeI in DMF using NaOH, followed by deprotection of the t-butyl ester group by TFA, yielding [(11)C]KSM-01. SUV analysis of dynamic micro PET/CT imaging data showed that [(11)C]KSM-01 accumulation was ∼2.0-fold greater in cardiac-specific PPAR-α overexpressing transgenic mice compared to wild-type littermates. The post-PET biodistribution studies were consistent with these results and demonstrated 2.5-fold greater radiotracer uptake in the heart of transgenic mice compared to the wild-type littermates. These results demonstrate the potential utility of PPAR-α agonists as PET radiopharmaceuticals.

Metabolic PET Imaging in Oncology

OBJECTIVE In this article, we provide a general overview of how cancer cells subvert critical metabolic pathways to support their growth and unchecked division. Furthermore, we outline how molecular imaging can diagnostically exploit the resulting differences between cancer and normal cells. CONCLUSION Molecular PET can provide valuable information about the metabolic dysregulation in cancer.

Radiosynthesis and In Vivo Evaluation of [11C]A1070722, a High Affinity GSK-3 PET Tracer in Primate Brain

Dysfunction of glycogen synthase kinase 3 (GSK-3) is implicated in the etiology of Alzheimers disease, Parkinsons disease, diabetes, pain, and cancer. A radiotracer for functional positron emission tomography (PET) imaging could be used to study the kinase in brain disorders and to facilitate the development of small molecule inhibitors of GSK-3 for treatment. At present, there is no target-specific or validated PET tracer available for the in vivo monitoring of GSK-3. We radiolabeled the small molecule inhibitor [11C]1-(7-methoxy- quinolin-4-yl)-3-(6-(trifluoromethyl)pyridin-2-yl)urea ([11C]A1070722) with high affinity to GSK-3 (Ki = 0.6 nM) in excellent radiochemical yield. PET imaging experiments in anesthetized vervet/African green monkey exhibited that [11C]A1070722 penetrated the blood-brain barrier (BBB) and accumulated in brain regions, with highest radioactivity binding in frontal cortex followed by parietal cortex and anterior cingulate, and with the lowest bindings found in caudate, putamen, and thalamus, similarly to the known distribution of GSK-3 in human brain. Our studies suggest that [11C]A1070722 can be a potential PET radiotracer for the in vivo quantification of GSK-3 in brain.

Amino Acid Uptake Measured by [18F]AFETP Increases in Response to Arginine Starvation in ASS1-Deficient Sarcomas

Rational: In a subset of cancers, arginine auxotrophy occurs due to the loss of expression of argininosuccinate synthetase 1 (ASS1). This loss of ASS1 expression makes cancers sensitive to arginine starvation that is induced by PEGylated arginine deiminase (ADI-PEG20). Although ADI-PEG20 treatment is effective, it does have important limitations. Arginine starvation is only beneficial in patients with cancers that are ASS1-deficient. Also, these tumors may metabolically reprogram to express ASS1, transforming them from an auxotrophic phenotype to a prototrophic phenotype and thus rendering ADI-PEG20 ineffective. Due to these limitations of ADI-PEG20 treatment and the potential for developing resistance, non-invasive tools to monitor sensitivity to arginine starvation are needed. Methods: Within this study, we assess the utility of a novel positron emission tomography (PET) tracer to determine sarcomas reliant on extracellular arginine for survival by measuring changes in amino acid transport in arginine auxotrophic sarcoma cells treated with ADI-PEG20. The uptake of the 18F-labeled histidine analogue, (S)-2-amino-3-[1-(2-[18F]fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid (AFETP), was assessed in vitro and in vivo using human-derived sarcoma cell lines. In addition, we examined the expression and localization of cationic amino acid transporters in response to arginine starvation with ADI-PEG20. Results: In vitro studies revealed that in response to ADI-PEG20 treatment, arginine auxotrophs increase the uptake of L-[3H]arginine and [18F]AFETP due to an increase in the expression and localization to the plasma membrane of the cationic amino acid transporter CAT-1. Furthermore, in vivo PET imaging studies in mice with arginine-dependent osteosarcoma xenografts showed increased [18F]AFETP uptake in tumors 4 days after ADI-PEG20 treatment compared to baseline. Conclusion: CAT-1 transporters localizes to the plasma membrane as a result of arginine starvation with ADI-PEG20 in ASS1-deficient tumor cells and provides a mechanism for using cationic amino acid transport substrates such as [18F]AFETP for identifying tumors susceptible to ADI-PEG20 treatment though non-invasive PET imaging techniques. These findings indicate that [18F]AFETP-PET may be suitable for the early detection of tumor response to arginine depletion due to ADI-PEG20 treatment.

Radiosynthesis and in Vivo Evaluation of [11C]MPC-6827, the First Brain Penetrant Microtubule PET Ligand

Abnormalities of microtubules (MTs) are implicated in the pathogenesis of many CNS diseases. Despite the potential of an MT imaging agents, no PET ligand is currently available for in vivo imaging of MTs in the brain. We radiolabeled [11C]MPC-6827, a high affinity MTA, and demonstrated its specific binding in rat and mice brain using PET imaging. Our experiments show that [11C]MPC-6827 has specific binding to MT in brain, and it is the first MT-binding PET ligand.