Improved, one-pot synthesis of 6-[18 F]fluorodopamine and quality control testing for use in patients with neuroblastoma.

6-[18 F]Fluorodopamine ([18 F]F-DA) is taken into cells via the norepinephrine transporter (NET). Recent [18 F]F-DA positron emission tomography-computed tomography (PET-CT) imaging of adult neuroendocrine tumors shows a dramatic improvement in sensitivity over the standard-of-care, meta-iodobenzylguanidine (MIBG) single-photon emission computed tomography (SPECT)-CT. A new precursor (ALPdopamine™) allows no-carrier-added synthesis resulting in high-molar activity [18 F]F-DA. Automated synthesis of [18 F]F-DA was performed in a single reactor using a two-step procedure: 1) fluorination via thermolysis of a diaryliodonium salt precursor, followed by 2) acid hydrolysis. Phase transfer agents, Kryptofix 222 and two tetraalkylammonium salts, were investigated. Optimized synthesis of [18 F]F-DA was achieved in 56 to 60 minutes (26% end of synthesis [EOS], nondecay corrected). The product passed all Food and Drug Administration (FDA)-required quality control testing for human use. Accumulation of [18 F]F-DA in SK-N-BE(2)-C (high NET expression) cells was significantly higher than in SH-EP (minimal NET expression) cells (P < 0.0001). ALPdopamine provides an effective scaffold for the routine production of [18 F]F-DA for human use. Validation of uptake by neuroblastoma (NB) cell lines supports the use of [18 F]F-DA for imaging NB patients. A pediatric NB imaging trial using [18 F]F-DA PET has been approved (Investigational New Drug application (IND) no. 138638) based on the methods reported here. We expect [18 F]F-DA will be localized in NB tumors and that high-quality functional images will be obtained within minutes after injection.

Thermolysis and radiofluorination of diaryliodonium salts derived from anilines.

Aniline-derived diaryliodonium salts were synthesized and functionalized in good to excellent yields by judicious utilization of electron-withdrawing protecting groups. This simple approach opens another route to radiolabeling amino arenes in relatively complex molecules, such as flutemetamol.

Efficient automated syntheses of high specific activity 6-[18F]fluorodopamine using a diaryliodonium salt precursor.

6-[(18)F]Fluorodopamine (6-[(18) F]F-DA) is a positron emission tomography radiopharmaceutical used to image sympathetic cardiac innervation and neuroendocrine tumors. Imaging with 6-[(18)F]F-DA is constrained, in part, by the bioactivity and neurotoxicity of 6-[(19)F]fluorodopamine. Furthermore, routine access to this radiotracer is limited by the inherent difficulty of incorporation of [(18)F]fluoride into electron-rich aromatic substrates. We describe the simple and direct preparation of high specific activity (SA) 6-[(18)F]F-DA from no-carrier-added (n.c.a.) [(18)F]fluoride. Incorporation of n.c.a. [(18)F]fluoride into a diaryliodonium salt precursor was achieved in 50-75% radiochemical yields (decay corrected to end of bombardment). Synthesis of 6-[(18)F]F-DA on the IBA Synthera® and GE TRACERlab FX-FN automated platforms gave 6-[(18)F]F-DA in >99% chemical and radiochemical purities after HPLC purification. The final non-corrected yields of 6-[(18)F]F-DA were 25 ± 4% (n = 4, 65 min) and 31 ± 6% (n = 3, 75 min) using the Synthera and TRACERlab modules, respectively. Efficient access to high SA 6-[(18)F]F-DA from a diaryliodonium salt precursor and n.c.a. [(18)F]fluoride is provided by a relatively subtle change in reaction conditions - replacement of a polar aprotic solvent (acetonitrile) with a relatively nonpolar solvent (toluene) during the critical radiofluorination reaction. Implementation of this process on common radiochemistry platforms should make 6-[(18)F]F-DA readily available to the wider imaging community.

Automated radiosynthesis of [18F]DPA-714 on a commercially available IBA Synthera®.

The goal of this work was to develop a reliable method to produce the well-validated microglial activation PET tracer, [18F]DPA-714, routinely for clinical and preclinical research using an IBA Synthera®. Optimization of literature methods included reduced precursor mass and use of TBA HCO3 as the phase transfer agent in place of Kryptofix® 222 in a 65-min synthesis with an average activity yield of 24.6 ± 3.8% (n = 5). Successful quality control testing and process validation results are reported.

[11C]-methionine positron emission tomography in the evaluation of pediatric low-grade gliomas.

Background: [11C]-Methionine positron emission tomography (PET; [11C]-MET-PET) is principally used for the evaluation of brain tumors in adults. Although amino acid PET tracers are more commonly used in the evaluation of pediatric brain tumors, data on [11C]-MET-PET imaging of pediatric low-grade gliomas (pLGG) is scarce. This study aimed to investigate the roles of [11C]-MET-PET in the evaluation of pLGGs. Methods: Eighteen patients with newly diagnosed pLGG and 26 previously treated pLGG patients underwent [11C]-MET-PET met the inclusion and exclusion criteria. Tumor-to-brain uptake ratio (TBR) and metabolic tumor volumes were assessed for diagnostic performances (newly diagnosed, 15; previously treated 26), change with therapy (newly diagnosed, 9; previously treated 7), and variability among different histology (n = 12) and molecular markers (n = 7) of pLGGs. Results: The sensitivity of [11C]-MET-PET for diagnosing pLGG, newly diagnosed, and previously treated combined was 93% for both TBRmax and TBRpeak, 76% for TBRmean, and 95% for qualitative evaluation. TBRmax showed a statistically significant reduction after treatment, while other PET parameters showed a tendency to decrease. Median TBRmax, TBRpeak, and TBRmean values were slightly higher in the BRAFV600E mutated tumors compared to the BRAF fused tumors. Median TBRmax, and TBRpeak in diffuse astrocytomas were higher compared to pilocytic astrocytomas, but median TBRmean, was slightly higher in pilocytic astrocytomas. However, formal statistical analysis was not done due to the small sample size. Conclusions: Our study shows that [11C]-MET-PET reliably characterizes new and previously treated pLGGs. Our study also shows that quantitative parameters tend to decrease with treatment, and differences may exist between various pLGG types.

Positron Emission Tomography Detects In Vivo Expression of Disialoganglioside GD2 in Mouse Models of Primary and Metastatic Osteosarcoma.

The cell membrane glycolipid GD2 is expressed by multiple solid tumors, including 88% of osteosarcomas and 98% of neuroblastomas. However, osteosarcomas are highly heterogeneous, with many tumors exhibiting GD2 expression on <50% of the individual cells, while some tumors are essentially GD2-negative. Anti-GD2 immunotherapy is the current standard of care for high-risk neuroblastoma, but its application to recurrent osteosarcomas, for which no effective therapies exist, has been extremely limited. This is, in part, because the standard assays to measure GD2 expression in these heterogeneous tumors are not quantitative and are subject to tissue availability and sampling bias. To address these limitations, we evaluated a novel, sensitive radiotracer [64Cu]Cu-Bn-NOTA-hu14.18K322A to detect GD2 expression in osteosarcomas (six patient-derived xenografts and one cell line) in vivo using positron emission tomography (PET). Tumor uptake of the radiolabeled, humanized anti-GD2 antibody [64Cu]Cu-Bn-NOTA-hu14.18K322A was 7-fold higher in modestly GD2-expressing osteosarcomas (32% GD2-positive cells) than in a GD2-negative tumor (9.8% vs. 1.3% of the injected dose per cc, respectively). This radiotracer also identified lesions as small as 29 mm3 in a 34% GD2-positive model of metastatic osteosarcoma of the lung. Radiolabeled antibody accumulation in patient-derived xenografts correlated with GD2 expression as measured by flow cytometry (Pearson r = 0.88, P = 0.01), distinguishing moderately GD2-expressing osteosarcomas (32%-69% GD2-positive cells) from high GD2 expressors (>99%, P < 0.05). These results support the utility of GD2 imaging with PET to measure GD2 expression in osteosarcoma and thus maximize the clinical impact of anti-GD2 immunotherapy. SIGNIFICANCE: In situ assessment of all GD2-positive osteosarcoma sites with a novel PET radiotracer could significantly impact anti-GD2 immunotherapy patient selection and enable noninvasive probing of correlations between target expression and therapeutic response.

In vitro and in vivo evaluation of bifunctional bisthiosemicarbazone 64Cu-complexes for the positron emission tomography imaging of hypoxia.

The copper(II) bisthiosemicarbazonato complex, copper-diacetyl-bis(N4-methylthiosemicarbazonate) (Cu-ATSM), has been used clinically as a positron emission tomography (PET) tracer for the delineation of hypoxia. Six novel, asymmetric bis(thiosemicarbazones) derived from diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-amino-3-thiosemicarbazone) (H2ATSM/A), one of which contained a nitroimidazole functionality, were radiolabeled with 64Cu (t1/2=12.7 h, beta+=19.3%). In vitro studies were performed on three of the compounds using EMT6 mammary carcinoma cells under hypoxic and normoxic conditions. All compounds displayed rapid cellular association and appreciable hypoxic selectivity with increased uptake under normoxic and hypoxic conditions when compared to 64Cu-ATSM. Biodistribution and small animal PET imaging studies were then carried out in vivo using two compounds in EMT6 tumor-bearing mice. The compounds showed high tumor uptake, but also substantial accumulation in the liver. These complexes demonstrate that H 2ATSM/A represents a novel and versatile synthetic platform that can be utilized to provide hypoxic cell selectivity through functionalization of the bisthiosemicarbazonate group.

1-11C-acetate as a PET radiopharmaceutical for imaging fatty acid synthase expression in prostate cancer.

UNLABELLED: Although it is accepted that the metabolic fate of 1-(11)C-acetate is different in tumors than in myocardial tissue because of different clearance patterns, the exact pathway has not been fully elucidated. For decades, fatty acid synthesis has been quantified in vitro by the incubation of cells with (14)C-acetate. Fatty acid synthase (FAS) has been found to be overexpressed in prostate carcinomas, as well as other cancers, and it is possible that imaging with 1-(11)C-acetate could be a marker for its expression. METHODS: In vitro and in vivo uptake experiments in prostate tumor models with 1-(11)C-acetate were performed both with and without blocking of fatty acid synthesis with either C75, an inhibitor of FAS, or 5-(tetradecyloxy)-2-furoic acid (TOFA), an inhibitor of acetyl-CoA carboxylase (ACC). FAS levels were measured by Western blot and immunohistochemical techniques for comparison. RESULTS: In vitro studies in 3 different prostate tumor models (PC-3, LNCaP, and 22Rv1) demonstrated blocking of 1-(11)C-acetate accumulation after treatment with both C75 and TOFA. This was further shown in vivo in PC-3 and LNCaP tumor-bearing mice after a single treatment with C75. A positive correlation between 1-(11)C-acetate uptake into the solid tumors and FAS expression levels was found. CONCLUSION: Extensive involvement of the fatty acid synthesis pathway in 1-(11)C-acetate uptake in prostate tumors was confirmed, leading to a possible marker for FAS expression in vivo by noninvasive PET.

Examining the relationship between Cu-ATSM hypoxia selectivity and fatty acid synthase expression in human prostate cancer cell lines.

INTRODUCTION: Positron emission tomography (PET) imaging with copper (II)-diacetyl-bis(N4-Methylthiosemicarbazone)(Cu-ATSM) for delineating hypoxia has provided valuable clinical information, but investigations in animal models of prostate cancer have shown some inconsistencies. As a defense mechanism in prostate cancer cells, the fatty acid synthesis pathway harnesses its oxidizing power for improving the redox balance despite conditions of extreme hypoxia, potentially altering Cu-ATSM hypoxia selectivity. METHODS: Human prostate tumor-cultured cell lines (PC-3, 22Rv1, LNCaP and LAPC-4), were treated with a fatty acid synthase (FAS) inhibitor (C75, 100 microM) under anoxia. The 64Cu-ATSM uptake in these treated cells and nontreated anoxic cells was then examined. Fatty acid synthase expression level in each cell line was subsequently quantified by ELISA. An additional study was performed in PC-3 cells to examine the relationship between the restoration of 64Cu-ATSM hypoxia selectivity and the concentration of C75 (100, 20, 4 or 0.8 microM) administered to the cells. RESULTS: Inhibition of fatty acid synthesis with C75 resulted in a significant increase in 64Cu-ATSM retention in prostate tumor cells in vitro under anoxia over 60 min. Inhibition studies demonstrated higher uptake values of 20.9+/-3.27%, 103.0+/-32.6%, 144.2+/-32.3% and 200.1+/-79.3% at 15 min over control values for LAPC-4, PC-3, LNCaP and 22Rv1 cells, respectively. A correlation was seen (R2=.911) with FAS expression plotted against percentage change in 64Cu-ATSM uptake with C75 treatment. CONCLUSIONS: Although Cu-ATSM has clinical relevance in the PET imaging of hypoxia in many tumor types, its translation to the imaging of prostate cancer may be limited by the overexpression of FAS associated with prostatic malignancies.

Gallium-68-labeled DOTA-rhenium-cyclized alpha-melanocyte-stimulating hormone analog for imaging of malignant melanoma.

INTRODUCTION: Diagnosis of malignant melanoma is critical, since a patient's prognosis is poor. Previous studies have shown that 64Cu- and 86Y-DOTA-ReCCMSH(Arg11) have the potential for early detection of malignant melanoma by exploiting the sensitivity and high resolution of positron emission tomography (PET). This encouraged us to investigate DOTA-ReCCMSH(Arg11) labeled with another beta+-emitting radionuclide, 68Ga. METHODS: DOTA-ReCCMSH(Arg11) was successfully labeled with 68Ga at pH 3.8-4 at 85 degrees C. Acute biodistribution and small-animal PET imaging studies were performed in mice bearing B16/F1 melanoma tumor. RESULTS: Biodistribution studies showed moderate receptor-mediated tumor uptake, fast nontarget organ clearance and high tumor to nontarget tissue ratios. Preadministration of d-lysine significantly reduced kidney uptake without affecting the uptake of the agent in the tumor. Small-animal PET images showed that the tumor could be clearly visualized at all time points examined (0.5-2 h) with the standardized uptake value analysis following a similar trend as the biodistribution data. CONCLUSIONS: The preliminary data obtained suggest that 68Ga-DOTA-ReCCMSH(Arg11) is a promising PET imaging agent for early detection of malignant melanoma.

Clinical Applications of Small-molecule PET Radiotracers: Current Progress and Future Outlook.

Radiotracers, or radiopharmaceuticals, are bioactive molecules tagged with a radionuclide used for diagnostic imaging or radiotherapy and, when a positron-emitting radionuclide is chosen, the radiotracers are used for PET imaging. The development of novel PET radiotracers in many ways parallels the development of new pharmaceuticals, and small molecules dominate research and development pipelines in both disciplines. The 4 decades since the introduction of [18F]FDG have seen the development of many small molecule PET radiotracers. Ten have been approved by the US Food and Drug Administration as of 2016, whereas hundreds more are being evaluated clinically. These radiotracers are being used in personalized medicine and to support drug discovery programs where they are greatly improving our understanding of and ability to treat diseases across many areas of medicine including neuroscience, cardiovascular medicine, and oncology.

Preparation, biodistribution, and small animal PET of 45Ti-transferrin.

UNLABELLED: Investigation of 45Ti-transferrin was pursued to provide insight into the mechanism of action of titanocene dichloride, a chemotherapeutic agent currently in clinical trials. METHODS: Plasma protein-binding studies of processed 45Ti were performed by solubilizing the 45Ti residue in 0.05N HCl, of which 1.22 MBq (33 microCi) in 10 microL were added to 250 microL of dog plasma. 45Ti-Transferrin was prepared by redissolving the processed 45Ti in 25 micromol/L apotransferrin or by in vivo incorporation through preparation and introduction of 45Ti-citrate. Biodistribution studies were performed on normal Sprague-Dawley rats and EMT-6 tumor-bearing BALB/c mice with 45Ti-transferrin coinjected with 67Ga-citrate for direct comparison. microPET was performed on mice bearing EMT-6 tumors and the images were analyzed for tumor-to-muscle uptake ratios. RESULTS: Direct labeling of apotransferrin in situ with 45Ti was achieved as well as in vivo incorporation by 2 h after injection with 45Ti-citrate. The biodistribution of 45Ti-transferrin and 67Ga-citrate showed similar trends. In Sprague-Dawley rats, initial blood uptake was higher for the 45Ti-transferrin, whereas bone uptake increased more for the 67Ga-citrate. EMT-6 tumor uptake in both cases was relatively high (14.6 +/- 1.83 %ID/g for 45Ti and 8.72 +/- 0.98 %ID/g for 67Ga [%ID/g = percentage injected dose per gram]) and remained elevated even out to 24 h after injection. The tumor-to-muscle ratio of the 67Ga-citrate reached 6.7 at 24 h, whereas the ratio of the 45Ti-transferrin increased to 4.3 at this time point. Uptake of 45Ti-transferrin was visualized in the EMT-6 murine mammary carcinoma tumor with microPET. In all cases, the tumor was clearly delineated from the surrounding tissue with tumor-to-muscle ratios on the order of 1.6. CONCLUSION: 45Ti forms a complex with apotransferrin that remains intact in vivo. Results of the biodistribution in mice showed that the tumor had increased uptake compared with nontarget organs (e.g., muscle). The microPET images of tumor-bearing mice clearly delineate the tumors from the surrounding tissue. Comparison of the data suggests that tissue uptake is similar whether injecting 45Ti-transferrin directly or as 45Ti-citrate, which transchelates to transferrin before the time of imaging.

Production, processing and small animal PET imaging of titanium-45.

INTRODUCTION: Titanium-45 was prepared as a tool for elucidation of the mechanism of action of titanium anticancer drugs in vivo using microPET imaging. METHODS: Titanium-45 was produced by the 45Sc(p,n)45Ti nuclear reaction using 14.5 MeV protons. Sufficient yields of 45Ti were produced and separated from the target material with 99.8% radionuclidic purity using a simple, efficient separation procedure. RESULTS: A typical bombardment of 5 microA for 1 h produced an average of 2105+/-150 MBq (56.9+/-4.0 mCi) at the end of bombardment (EOB), well within acceptable range of the calculated theoretical yields of 2165 MBq and 433 MBq microA-1 h-1 (58.5 mCi and 11.7 mCi microA-1 h-1). This amount of activity is sufficient for the radiosynthesis of target compounds as well as imaging studies. MicroPET images of a miniature Derenzo phantom show excellent resolution where rods of 1.25 mm were separated by four times their diameter. CONCLUSIONS: Titanium-45 can be easily produced on a biomedical cyclotron with excellent yields as compared to calculated theoretical values with imaging studies demonstrating that the decay properties of titanium-45 are well suited for microPET.

A practical, automated synthesis of meta-[(18)F]fluorobenzylguanidine for clinical use.

Many neuroendocrine tumors, such as neuroblastoma (NB), arise from neural crest cells of the sympathetic nervous system. This nerve-like phenotype has been exploited for functional imaging using radioactive probes originally designed for neuronal and adrenal medullary applications. NB imaging with meta-[(123)I]iodobenzylguanidine ([(123)I]MIBG) is limited by the emissions of (123)I, which lead to poor image resolution and challenges in quantification of its accumulation in tumors. meta-[(18)F]Fluorobenzylguanidine ([(18)F]MFBG) is a promising alternative to [(123)I]MIBG that could change the standard of practice for imaging neuroendocrine tumors, but interest in this PET radiotracer has suffered due to its complex and inefficient radiosynthesis. Here we report a two-step, automated method for the routine production of [(18)F]MFBG by thermolysis of a diaryliodonium fluoride and subsequent acid deprotection. The synthesis was adapted for use on a commercially available synthesizer for routine production. Full characterization of [(18)F]MFBG produced by this route demonstrated the tracer's suitability for human use. [(18)F]MFBG was prepared in almost 3-fold higher yield than previously reported (31% corrected to end of bombardment, n = 9) in a synthesis time of 56 min with >99.9% radiochemical purity. Other than pH adjustment and dilution of the final product, no reformulation was necessary after purification. This method permits the automated production of multidose batches of clinical grade [(18)F]MFBG. Moreover, if ongoing clinical imaging trials of [(18)F]MFBG are successful, this methodology is suitable for rapid commercialization and can be easily adapted for use on most commercial automated radiosynthesis equipment.

Evaluation of the biodistribution of 11C-methionine in children and young adults.

UNLABELLED: The purpose of this study was to evaluate the biodistribution of (11)C-labeled methionine in non-tumor-involved organs in pediatric patients studied for malignant diseases. METHODS: Ninety-three children and young adults with known or suspected malignancies underwent (11)C-methionine PET and CT scans. Imaging began 5-15 min after injection of 740 MBq (20 mCi) per 1.7 m(2) of body surface area. Images were acquired from the top of the head through the mid thighs. Standardized uptake values were determined using regions of interest drawn on the CT image and transferred to the corresponding transverse PET slice. RESULTS: The highest concentrations of (11)C-methionine were found in the pancreas and liver. Less intense uptake was seen in other regions, such as the salivary glands, tonsils, and bone marrow. There was little uptake in the lungs, fat (including brown adipose tissue), and muscle. Uptake in bone marrow, parotid glands, and tonsils was slightly but statistically significantly higher in men than women. Testicular, bone marrow, and left ventricular uptake increased with age. There was little variability statistically between comparisons of uptake change and groupings of age, race, sex, and patients studied at the time of diagnosis versus previously treated patients. CONCLUSION: High uptake of (11)C-methionine is reliably found in the pancreas and liver, consistent with the anabolic functions of these organs. Low uptake in the brain, neck, chest, pelvis, and extremities will facilitate tumor localization in those areas. However, intense uptake in the upper abdomen may limit the diagnostic utility of (11)C-methionine in that area.

Molecular imaging of cancer with radiolabeled peptides and PET.

Radiolabeled peptides hold promise for diagnosis and therapy of cancer as well as for early monitoring of therapy outcomes, patient stratification, etc. This manuscript focuses on the development of peptides labeled with 18F, 64Cu, 68Ga and other positron-emitting radionuclides for PET imaging. The major techniques for radionuclide incorporation are briefly discussed. Then, examples of positron-emitting peptides targeting somatostatin receptors, integrins, gastrin-releasing peptide receptors, vasointestinal peptide receptors, melanocortin 1 receptors and others are reviewed.

64Cu-p-NH2-Bn-DOTA-hu14.18K322A, a PET radiotracer targeting neuroblastoma and melanoma.

UNLABELLED: The hu14.18K322A variant of the GD2-targeting antibody hu14.18 has been shown to elicit a level of antibody-dependent cell-mediated cytotoxicity toward human neuroblastoma cells similar to that of the parent antibody. However, hu14.18K322A exhibited a decreased complement activation and associated pain, the dose-limiting toxicity in neuroblastoma immunotherapy. PET with a radiolabeled analog of the same antibody used in treatment will provide insight into the ability of hu14.18K322A to reach its target, as well as nontarget uptake that may cause side effects. Such antibody radiotracers might also provide a method for measuring GD2 expression in tumors, thus enabling the prediction of response to anti-GD2 therapy for individual patients. METHODS: The conjugation of hu14.18K322A with p-NH(2)-Bn-DOTA was accomplished using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide with subsequent (64)Cu radiolabeling at 37°C for 30 min. Immunoreactivity of the conjugate was assessed by a dose-escalation blocking experiment measuring binding to purified GD2 versus GD1b as a negative control. Cell uptake and biodistribution studies in M21 (GD2-positive) and PC-3 (GD2-negative) tumor models were performed, as was small-animal PET/CT of M21 and PC-3 tumor-bearing mice. RESULTS: The labeling of (64)Cu-p-NH(2)-Bn-DOTA-hu14.18K322A was achieved at more than 95% radiochemical purity and a specific activity of 127-370 MBq/mg (3.4-10 mCi/mg) after chromatographic purification. Preliminary in vitro data demonstrated a greater than 6-fold selectivity of binding to GD2 versus GD1b and dose-dependent inhibition of binding by unmodified hu14.8K322A. In vivo data, including small-animal PET/CT, showed significant GD2-positive tumor-targeting ability, with a persistent 2-fold-higher uptake of radiotracer than in GD2-negative tumors. CONCLUSION: (64)Cu-p-NH(2)-Bn-DOTA-hu14.18K322A represents a novel PET radiotracer to facilitate clinical investigations of anti-GD2 immunotherapies and to complement other imaging modalities in the staging and treatment of neuroblastoma.

A novel technology for the imaging of acidic prostate tumors by positron emission tomography.

Solid tumors often develop an acidic environment due to the Warburg effect. The effectiveness of diagnosis and therapy may therefore be enhanced by the design and use of pH-sensitive agents that target acidic tumors. Recently, a novel technology was introduced to target acidic tumors using pH low insertion peptide (pHLIP), a peptide that inserts across cell membranes as an alpha-helix when the extracellular pH (pH(e)) is acidic. In this study, we expanded the application of the pHLIP technology to include positron emission tomography imaging of the acidic environment in prostate tumors using (64)Cu conjugated to the pHLIP ((64)Cu-DOTA-pHLIP). Studies showed that this construct avidly accumulated in LNCaP and PC-3 tumors, with higher uptake and retention in the LNCaP tumors. Uptake correlated with differences in the bulk pH(e) of PC-3 and LNCaP tumors measured in magnetic resonance spectroscopy experiments by the (31)P chemical shift of the pH(e) marker 3-aminopropylphosphonate. This article introduces a novel class of noninvasive pH-selective positron emission tomography imaging agents and opens new research directions in the diagnosis of acidic solid tumors.

Cu-ATSM: a radiopharmaceutical for the PET imaging of hypoxia.

Copper(II)-diacetyl-bis(N(4)-methylthiosemicarbazone), Cu-ATSM, labeled with a positron emitting isotope of copper ((60)Cu, (61)Cu, (62)Cu or (64)Cu) has been shown, in vitro and in vivo, to be selective for hypoxic tissue. In silico studies have explored the mechanism of its hypoxia selectivity, and clinical studies with this agent have shown non-invasive imaging data that is predictive of a cancer patients' response to conventional therapy. This Perspective discusses the evolution of Cu-ATSM, how its selectivity can be improved upon, and where this metal-ligand platform could be taken in the future.