Positron Emission Tomography Imaging Probes Research Articles

Synthesis and Evaluation of Novel 18F Labeled 2-Pyridinylbenzoxazole and 2-Pyridinylbenzothiazole Derivatives as Ligands for Positron Emission Tomography (PET) Imaging of β-Amyloid Plaques

A series of fluoro-pegylated (FPEG) 2-pyridinylbenzoxazole and 2-pyridinylbenzothiazole derivatives were synthesized and evaluated as novel β-amyloid (Aβ) imaging probes for PET. They displayed binding affinities for Aβ(1-42) aggregates that varied from 2.7 to 101.6 nM. Seven ligands with high affinity were selected for (18)F labeling. In vitro autoradiography results confirmed the high affinity of these radiotracers. In vivo biodistribution experiments in normal mice indicated that the radiotracers with a short FPEG chain (n = 1) displayed high initial uptake into and rapid washout from the brain. One of the 2-pyridinylbenzoxazole derivatives, [(18)F]-5-(5-(2-fluoroethoxy)benzo[d]oxazol-2-yl)-N-methylpyridin-2-amine ([(18)F]32) (K(i) = 8.0 ± 3.2 nM) displayed a brain(2min)/brain(60min) ratio of 4.66, which is highly desirable for Aβ imaging agents. Target specific binding of [(18)F]32 to Aβ plaques was validated by ex vivo autoradiographic experiment with transgenic model mouse. Overall, [(18)F]32 is a promising Aβ imaging agent for PET and merits further evaluation in human subjects.

Amino Acid Transporter-Targeted Radiotracers for Molecular Imaging in Oncology

Amino acids are required for sustenance of continuous uncontrolled growth of tumor cells, and upregulation of amino acid transporter expression has often been observed in tumor cells to facilitate their accelerated rates of amino acid uptake. Therefore, amino acid transporters have promise as ideal targets for tumor imaging. In fact, many natural and artificial amino acids have been radiolabeled for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging of tumor. In this article, we review the classification, molecular biology, and pharmacological relevance of amino acid transport systems. In addition, we discuss the chemistry, radiochemistry, current clinical applications, and future prospects for the use of radiolabeled amino acid-based probes for PET and SPECT imaging in oncology for each category of radionuclide.

In Vivo Imaging of Folate Receptor Positive Tumor Xenografts Using Novel 68Ga-NODAGA-Folate Conjugates

The overexpression of the folate receptor (FR) in a variety of malignant tumors, along with its limited expression in healthy tissues, makes it an attractive tumor-specific molecular target. Noninvasive imaging of FR using radiolabeled folate derivatives is therefore highly desirable. Given the advantages of positron emission tomography (PET) and the convenience of (68)Ga production, the aim of our study was to develop a new (68)Ga-folate-based radiotracer for clinical application. The chelator 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA) was conjugated to folic acid and to 5,8-dideazafolic acid using 1,2-diaminoethane as a spacer, resulting in two novel conjugates, namely, P3246 and P3238, respectively. Both conjugates were labeled with (68/67)Ga. In vitro internalization, efflux, and saturation binding studies were performed using the FR-positive KB cell line. Biodistribution and small-animal PET imaging studies were performed in nude mice bearing subcutaneous KB xenografts. Both conjugates were labeled with (68)Ga at room temperature within 10 min in labeling yields >95% and specific activity ~30 GBq/μmol. The K(d) values of (68/67)Ga-P3246 (5.61 ± 0.96 nM) and (68/67)Ga-P3238 (7.21 ± 2.46 nM) showed high affinity for the FR. (68/67)Ga-P3246 showed higher cell-associated uptake in vitro than (68/67)Ga-P3238 (approximately 72 and 60% at 4 h, respectively, P < 0.01), while both radiotracers exhibited similar cellular retention up to 4 h (approximately 76 and 71%, respectively). Their biodistribution profile is characterized by high tumor uptake, fast blood clearance, low hepatobiliary excretion, and almost negligible background. Tumor uptake was already high at 1 h for both (68)Ga-P3246 and (68)Ga-P3238 (16.56 ± 3.67 and 10.95 ± 2.12% IA/g, respectively, P > 0.05) and remained at about the same level up to 4 h. Radioactivity also accumulated in the FR-positive organs, such as kidneys (91.52 ± 21.05 and 62.26 ± 14.32% IA/g, respectively, 1 h pi) and salivary glands (9.05 ± 2.03 and 10.39 ± 1.19% IA/g, respectively, 1 h pi). The specificity of the radiotracers for the FR was confirmed by blocking experiments where tumor uptake was reduced by more than 85%, while the uptake in the kidneys and the salivary glands was reduced by more than 90%. Reduction of the kidney uptake was achieved by administration of the antifolate pemetrexed 1 h prior to the injection of the radiotracers, which resulted in an improvement of tumor-to-kidney ratios by more than a factor of 3. In line with the biodistribution results, small-animal PET images showed high uptake in the kidneys, clear visualization of the tumor, accumulation of radioactivity in the salivary glands, and no uptake in the gastrointestinal tract. (68)Ga-P3246 and (68)Ga-P3238 showed very high tumor-to-background contrast in PET images; however, the tumor-to-kidney ratio remained low. The new radiotracers, especially (68)Ga-P3246, are promising as PET imaging probes for clinical application due to their facile preparation and improved in vivo profile as compared to the other folate-based PET radiotracers.

Abstract 356: Photoacoustic imaging of pancreatic cancer proliferation via sigma-2 receptor/PGRMC1-eYFP

Abstract Sigma-2 receptor binding sites, recently identified as progesterone receptor membrane component 1 (PGRMC1) (Nature Communications, 2011; 2:380), are highly expressed in tumor cells. We have previously reported that measuring the sigma-2 receptor expression is a novel strategy for evaluating the proliferative status of tumors in vivo using the non-invasive imaging technique, positron emission tomography (PET). Our group has developed a series of sigma-2 receptor radioligands as PET imaging probes, one sigma-2 receptor radiotracer [18F]ISO-1 is under clinical evaluation for imaging the proliferation status in the patients with different types of cancers. PGRMC1 depleted human ovarian SKOV-3 cancer cells have been reported to lack tumor formation in mice and PGRMC1 depleted tumors displayed poor microvasculature system (Endocrinology, 2009; 150:4846-4854). To further understand the regulation of PGRMC1 in cell cycle and cancer proliferation, we have stably transfected PGRMC1-eYFP, enhanced yellow fluorescent protein conjugated PGRMC1, into human pancreatic BxPC3 cancer cells and those cells were transplanted into immunodeficient nude mice to grow into solid tumors; subsequently, a traditional fluorescence imaging system and an optical-resolution photoacoustic microscopy (OR-PAM) were utilized for in vivo monitoring of BxPC3 cancer cells growth and observing the surrounding microvasculature development for those tumors under conditions of sigma-2 receptor/PGRMC1 overexpression. Tumor tissue sections were prepared and in vitro fluorescence microscopy analysis of PGRMC1-eYFP cells revealed that PGRMC1-eYFP overexpressed cells form cluster patterns which may represent the proliferation priority created by PGRMC1 overexpression. The subcellular localization of PGRMC1-eYFP appeared to be similar to that of sigma-2 receptors reported previously by our group using the sigma-2 receptor fluorescent probes (Cancer Res 2007; 67, 6708-6716). PGRMC1-eYFP BxPC3 cells were surrounded by fully developed microvasculature system. Further application of this in vivo imaging platform for evaluating the sigma-2 receptor pharmacology and imaging proliferation is ongoing in our group. (Supported by CA 102869 and CA136398). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 356. doi:1538-7445.AM2012-356

Efficient Construction of PET/Fluorescence Probe Based on Sarcophagine Cage: An Opportunity to Integrate Diagnosis with Treatment

Due to the shortage of established platforms/methods for multimodality probe construction, in this study, we developed a heterofunctional chelator, BaAn(Boc)Sar, from sarcophagine cage as a general platform for dual-modality probe construction. A dual-modality probe for positron-emission tomography (PET) and fluorescence imaging was synthesized using the developed BaAn(Boc)Sar chelator. The c(RGDyK)(2) peptide (denoted as RGD(2)) and fluorescence dye Cy5.5 were conjugated with BaAn(Boc)Sar to form BaAnSar-RGD(2)-Cy5.5. Then, BaAnSar-RGD(2)-Cy5.5 was labeled with (64)Cu in ammonium acetate buffer. PET and fluorescent imaging were carried out to evaluate (64)Cu-BaAnSar-RGD(2)-Cy5.5 in nude mice bearing U87MG glioblastoma xenograft. The BaAnSar-RGD(2)-Cy5.5 was labeled with (64)Cu very efficiently in 0.1 M NH(4)OAc buffer within 10 min at 37 °C in the yield of 86.7 ± 4.4 % (n = 3). The specific activity of (64)Cu-BaBaSar-RGD(2) was controlled at 50-200 mCi/μmol for the consideration of both PET and optical imaging. MicroPET quantification analysis shows that the U87MG tumor uptake is 6.41 ± 0.28, 6.51 ± 1.45, and 5.92 ± 1.57 %ID/g at 1, 4, and 20 h postinjection, respectively. Good correlation was obtained between the tumor to muscle ratios measured by the radioactivity and fluorescence intensity. As a proof of concept, an animal surgery study demonstrated that this dual-modality probe would greatly benefit the patients because the PET moiety could be used for tumor detection, and the fluorescent moiety would allow image-guided surgery. Our findings demonstrated the effectiveness and feasibility of preparing dual-modality imaging probes based on the sarcophagine scaffold. The resulting PET and fluorescent imaging probe also holds a great potential for clinical translation.

Labeling Strategies with F-18 for Positron Emission Tomography Imaging

The labeling of probes with fluorine-18 [(18)F, β(+); 96.7%] continues to play a considerably important role in the development of positron emission tomography (PET) as a modality for both clinical research and clinical diagnoses. This review summarizes the strategies and recent developments in the fluorine-18 labeling of probes for PET imaging. Problems and issues relating to the practical production of fluorine-18 currently in widespread use are also discussed.

Multivalent Bifunctional Chelator Scaffolds for Gallium-68 Based Positron Emission Tomography Imaging Probe Design: Signal Amplification via Multivalency

The role of the multivalent effect has been well recognized in the design of molecular imaging probes toward the desired imaging signal amplification. Recently, we reported a bifunctional chelator (BFC) scaffold design, which provides a simple and versatile approach to impart multivalency to radiometal based nuclear imaging probes. In this work, we report a series of BFC scaffolds ((t)Bu(3)-1-COOH, (t)Bu(3)-2-(COOH)(2), and (t)Bu(3)-3-(COOH)(3)) constructed on the framework of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) for (68)Ga-based PET probe design and signal amplification via the multivalent effect. For proof of principle, a known integrin α(v)β(3) specific ligand (c(RGDyK)) was used to build the corresponding NOTA conjugates (H(3)1, H(3)2, and H(3)3), which present 1-3 copies of c(RGDyK) peptide, respectively, in a systematic manner. Using the integrin α(v)β(3) binding affinities (IC(50) values), enhanced specific binding was observed for multivalent conjugates (H(3)2: 43.9 ± 16.1 nM; H(3)3: 14.7 ± 5.0 nM) as compared to their monovalent counterpart (H(3)1: 171 ± 60 nM) and the intact c(RGDyK) peptide (204 ± 76 nM). The obtained conjugates were efficiently labeled with (68)Ga(3+) within 30 min at room temperature in high radiochemical yields (>95%). The in vivo evaluation of the labeled conjugates, (68)Ga-1, (68)Ga-2, and (68)Ga-3, was performed using male severe combined immunodeficiency (SCID) mice bearing integrin α(v)β(3) positive PC-3 tumor xenografts (n = 3). All (68)Ga-labeled conjugates showed high in vivo stability with no detectable metabolites found by radio-HPLC within 2 h postinjection (p.i.). The PET signal amplification in PC-3 tumor by the multivalent effect was clearly displayed by the tumor uptake of the (68)Ga-labeled conjugates ((68)Ga-3: 2.55 ± 0.50%ID/g; (68)Ga-2: 1.90 ± 0.10%ID/g; (68)Ga-1: 1.66 ± 0.15%ID/g) at 2 h p.i. In summary, we have designed and synthesized a series of NOTA-based BFC scaffolds with signal amplification properties, which may find potential applications as diagnostic gallium radiopharmaceuticals.

Development of radiotracers for oncology – the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.

Aromatic radiofluorination and biological evaluation of 2-aryl-6-[ 18F]fluorobenzothiazoles as a potential positron emission tomography imaging probe for β-amyloid plaques

To develop agents for radionuclide imaging Aβ plaques in vivo, we prepared three fluorine-substituted analogs of arylbenzothiazole class; compound 2 has a high affinity for Aβ ( K i = 5.5 nM) and the specific binding to Aβ in fluorescent staining. In preparation for the synthesis of these arylbenzothiazole analogs in radiolabeled form as an Aβ plaques-specific positron emission tomography (PET) imaging probe, we investigated synthetic route suitable for its labeling with the short-lived PET radionuclide fluorine-18 ( t 1/2 = 110 min) and diaryliodonium tosylate precursors ( 12, 13a– e and 14). 2-Aryl-6-[ 18F]fluorobenzothiazoles ([ 18F] 1– 3) were synthesized in efficiently short reaction times (40–60 min) with high radiochemical yields (19–40%), purities (>95%) and specific activities (85–118 GBq/μmol). Tissue distribution studies showed that high radioactivity of [ 18F] 2 accumulated in the brain with rapid clearance in healthy mice. Radioactive metabolites were analyzed in brain samples of mice and corresponded to 81% of parent remained by 30 min after a tail-vein injection. These results suggest that [ 18F] 2 is a promising probe for evaluation of Aβ plaques imaging in brain using PET.

Transforming a Targeted Porphyrin Theranostic Agent into a PET Imaging Probe for Cancer

Porphyrin based photosensitizers are useful agents for photodynamic therapy (PDT) and fluorescence imaging of cancer. Porphyrins are also excellent metal chelators forming highly stable metallo-complexes making them efficient delivery vehicles for radioisotopes. Here we investigated the possibility of incorporating (64)Cu into a porphyrin-peptide-folate (PPF) probe developed previously as folate receptor (FR) targeted fluorescent/PDT agent, and evaluated the potential of turning the resulting (64)Cu-PPF into a positron emission tomography (PET) probe for cancer imaging. Noninvasive PET imaging followed by radioassay evaluated the tumor accumulation, pharmacokinetics and biodistribution of (64)Cu-PPF. (64)Cu-PPF uptake in FR-positive tumors was visible on small-animal PET images with high tumor-to-muscle ratio (8.88 ± 3.60) observed after 24 h. Competitive blocking studies confirmed the FR-mediated tracer uptake by the tumor. The ease of efficient (64)Cu-radiolabeling of PPF while retaining its favorable biodistribution, pharmacokinetics and selective tumor uptake, provides a robust strategy to transform tumor-targeted porphyrin-based photosensitizers into PET imaging probes.

Fluorine-18 Chemistry for PET: A Concise Introduction

Fluorine-18 is the most important radionuclide used in positron emission tomography (PET) today, largely due to its attractive physical and nuclear characteristics. Agents such as the clinical oncology tracer 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), the most widely used PET-radiopharmaceutical, are driving an increasing interest in the chemistry of radiopharmaceuticals utilizing fluorine-18. This review outlines the methods for production of fluorine-18, and the development of agents for performing radiofluorination reactions. With a few exceptions, radiofluorinations can be classified as either electrophilic or nucleophilic. The electrophilic reactions mainly use molecular [18F]fluorine of moderately low specific radioactivity, or reagents prepared from it, and include additions to alkenes, reactions with carbanions and especially fluorodehydrogenation and fluorodemetallation. The nucleophilic reactions usually involve no-carrier-added (high-specific- radioactivity) [18F]fluoride as its K[18F]F-K222 complex and include SN2-type substitutions in the aliphatic series and SNAr-type substitutions in the aromatic and heteroaromatic series. Key examples from each class of radiofluorination reaction will be described, highlighting the potential of this radioisotope in the design and preparation of fluorine-18- labeled probes for PET imaging.

A cell permeable peptide analog as a potential-specific PET imaging probe for prostate cancer detection

Non-invasive detection of prostate cancer or metastases still remains a challenge in the field of molecular imaging. In our recent work of screening arginine- or lysine-rich peptides for intracellular delivery of a therapeutic agent into prostate cancer cells, an arginine-rich cell permeable peptide (NH(2)GR(11)) was found with an unexpectedly preferential uptake in prostate cancer cell lines. The goal of this work was to develop this peptide as a positron emission tomography (PET) imaging probe for specific detection of distant prostate cancer metastases. The optimal length of arginine-rich peptides was evaluated by the cell uptake efficiency of three fluorescein isothiocyanate (FITC)-tagged oligoarginines (NHGR(9), NHGR(11), and NHGR(13)) in four human prostate cell lines (LNCaP, PZ-HPV-7, DU145, and PC3). Of the three oligoarginines, NH(2)GR(11) showed the highest cell uptake and internalization efficiency with its subcellular localization in cytosol. The biodistribution of FITC-NHGR(9), FITC-NHGR(11), and FITC-NHGR(13) performed in control nude mice displayed the unique preferential accumulation of FITC-NHGR(11) in the prostate tissue. Further in vivo evaluation of FITC-NHGR(11) in PC3 tumor-bearing nude mice revealed elevated uptake of this peptide in tumors as compared to other organs. In vivo pharmacokinetics evaluated with (64)Cu-labeled NH(2)GR(11) showed that the peptide was rapidly cleared from the blood (t(1/2) = 10.7 min) and its elimination half-life was 17.2 h. The PET imaging specificity of (64)Cu-labled NH(2)GR(11) was demonstrated for the detection of prostate cancer in a comparative imaging experiment using two different human cancer xenograft models.

Imparting Multivalency to a Bifunctional Chelator: A Scaffold Design for Targeted PET Imaging Probes

Positron Emission Tomography (PET) has become a standard clinical practice in the diagnostic or prognostic imaging of cancer mainly due to the great success of [18F]FDG (2-deoxy-2-18F-fluoro-D-glucose) for non-invasive detection of glucose uptake in tumors.[1] Currently 11C and 18F are the most commonly used PET nuclides for the development of PET imaging probes. However, the short half-lives of these two radioisotopes (11C: t1/2 = 20.3 m; 18F: t1/2 = 109 m) limit their applications to biomolecules with relatively fast in vivo biodistribution kinetics and the chemical procedures to incorporated these isotopes must be carried out in the proximity of a biomedical cyclotron. Among the non-standard PET nuclides, 64Cu (t1/2 = 12.7 h; β+: 0.653 MeV, 17.4%) has drawn considerable interest in PET research due to its low positron range, commercial availability, and reasonably long decay half life. Such characteristics could enable a variety of imaging applications involving peptides, antibodies/fragments, and nanoparticles.[2, 3]

Phase I Study of Noninvasive Imaging of Adenovirus-mediated Gene Expression in the Human Prostate

To monitor noninvasively potentially therapeutic adenoviruses for cancer, we have developed a methodology based on the sodium iodide symporter (NIS). Men with clinically localized prostate cancer were administered an intraprostatic injection of a replication-competent adenovirus, Ad5-yCD/utTK(SR39)rep-hNIS, armed with two suicide genes and the NIS gene. NIS gene expression (GE) was imaged noninvasively by uptake of Na(99 m)TcO(4) in infected cells using single photon emission-computed tomography (SPECT). The investigational therapy was safe with 98% of the adverse events being grade 1 or 2. GE was detected in the prostate in seven of nine (78%) patients at 1 x 10(12) virus particles (vp) but not at 1 x 10(11) vp. Volume and total amount of GE was quantified by SPECT. Following injection of 1 x 10(12) vp in 1 cm(3), GE volume (GEV) increased to a mean of 6.6 cm(3), representing, on average, 18% of the total prostate volume. GEV and intensity peaked 1-2 days after the adenovirus injection and was detectable in the prostate up to 7 days. Whole-body imaging demonstrated intraprostatic gene expression, and there was no evidence of extraprostatic dissemination of the adenovirus by SPECT imaging. The results demonstrate that noninvasive imaging of adenovirus-mediated gene therapy in humans is feasible and safe.

Synthesis of [ 11C]FEDAA1106 as a new PET imaging probe of peripheral benzodiazepine receptor expression

Peripheral benzodiazepine receptor (PBR) is associated with neuroinflammation and tumor progression. [ 11C]DAA1106 and [ 18F]FEDAA1106 are two promising radioligands for positron emission tomography (PET) imaging of PBR. This study was designed to develop a new radiolabeled analog of [ 11C]DAA1106 and [ 18F]FEDAA1106, [ 11C]FEDAA1106, for PET imaging of PBR expression in brain and cancer. Precursor N-(5-fluoro-2-phenoxyphenyl)- N-(2-(2-fluoroethoxy)-5-hydroxybenzyl)acetamide ( 9) was synthesized in multiple steps with moderate to high chemical yields. Precursor 9 was labeled by [ 11C]CH 3OTf and isolated by high pressure liquid chromatography (HPLC) purification to provide target radioligand N-(5-fluoro-2-phenoxyphenyl)- N-(2-(2-fluoroethoxy)-5-[ 11C]methoxybenzyl)acetamide ([ 11C]FEDAA1106, [ 11C] 10) in 60–70% radiochemical yields, decay corrected to end of bombardment (EOB), based on [ 11C]CO 2. The specific activity of the target radiotracer [ 11C] 10 was in a range of 111–185 GBq/μmol at the end of synthesis (EOS).

A rational regulatory approach for positron emission tomography imaging probes: from “first in man” to NDA approval and reimbursement

We propose a new regulatory approach for positron emission tomography (PET) molecular imaging probes, essential tools in today's medicine. Even though the focus of this paper is on positron-emitting labeled probes, it is also justified to extend this proposed regulatory approach to other diagnostic nuclear medicine radiopharmaceuticals. Key aspects of this proposal include: (1) PET molecular imaging probes would be placed in a "no significant risk" category, similar to that category for devices in current Food and Drug Administration (FDA) regulations, based on overwhelming scientific evidence that demonstrates their faultless safety profile; (2) the FDA-sanctioned Radioactive Drug Research Committee (RDRC) will oversee all diagnostic research with these probes. The newly defined RDRC should approve "first in man" use; supervise a broader spectrum of diagnostic research protocols, including those looking to demonstrate initial efficacy, as well as multicenter clinical trials and the use of molecular imaging probes as a screening tool in drug discovery. The current investigational new drug (IND) mechanism is thus eliminated for these diagnostic probes; (3) when a molecular imaging probe has demonstrated diagnostic efficacy, FDA approval (i.e., NDA) will be sought. The review will be done by a newly constituted Radioactive Drug Advisory Committee (RDAC) composed of experts chosen by the professional societies, who would provide a binding assessment of the adequacy of the safety and efficacy data. When the RDAC recommends its diagnostic use on scientific and medical grounds, the molecular imaging probe becomes FDA approved. After a molecular imaging probe is approved for a diagnostic indication, the existing mechanism to seek reimbursement will be utilized; and (4) the FDA would retain its direct oversight function for traditional manufacturers engaged in commercial distribution of the approved diagnostic molecular imaging probes (i.e., under NDA) to monitor compliance with existing US Pharmacopeia (USP) requirements. With abbreviated and more appropriate regulations, new PET molecular imaging probes for diagnostic use would be then rapidly incorporated into the mainstream diagnostic medicine. Equally importantly, this approach would facilitate the use of molecular imaging in drug discovery and development, which would substantially reduce the costs and time required to bring new therapeutic drugs to market.

Assessment of cancer-associated biomarkers by positron emission tomography: advances and challenges.

Positron emission tomography (PET) provides a powerful means to non-invasively image and quantify protein expression and biochemical changes in living subjects at nano- and picomolar levels. As the field of molecular imaging develops, and as advances in the biochemistry, pharmacology, therapeutics, and molecular biology of disease are made, there is a corresponding increase in the number of clinically relevant, novel disease-associated biomarkers that are brought to the attention of those developing imaging probes for PET. In addition, due to the high specificity of the PET radiotracers being developed, there is a demand for PET cameras with higher sensitivity and resolution. This manuscript reviews advances over the past five years in clinical and pre-clinical PET instrumentation and in new PET probes and imaging methods associated with the latest trends in the molecular imaging of cancer. Included in the PET tracer review is a description of new radioligands for steroid receptors, growth factor receptors, receptor tyrosine kinases, sigma receptors, tumor-associated enzymes, gene reporter probes, markers for tumor hypoxia and metabolism, and sites associated with angiogenesis and cellular proliferation. The use of PET imaging in drug development, including the monitoring of cancer chemotherapy, also is discussed.

Binding Characteristics of Radiofluorinated 6-Dialkylamino-2-Naphthylethylidene Derivatives as Positron Emission Tomography Imaging Probes for β-Amyloid Plaques in Alzheimer's Disease

Senile plaques (SPs) and neurofibrillary tangles (NFTs) are hallmark pathologies accompanying the neurodegeneration involved in Alzheimer's disease (AD), for which beta-amyloid (Abeta) peptide is a major constituent of SPs. Our laboratories previously developed the hydrophobic, fluorescent molecular-imaging probe 2-(1-(6-[(2-[(18)F]fluoroethyl)(methyl)amino]-2-naphthyl)ethylidene)malononitrile ([(18)F]FDDNP), which crosses the blood-brain barrier and determines the localization and load of SPs and NFTs in vivo in AD patients. In this report, we used fluorimetric and radioactive binding assays to determine the binding affinities of FDDNP and its analog, 1-(6-[(2-[(18)F]fluoroethyl)(methyl)amino]naphthalen-2-yl)ethanone ([(18)F]FENE), to synthetic fibrils of Abeta(1-40). FDDNP and FENE both appeared to bind to two kinetically distinguishable binding sites on Abeta(1-40) fibrils. Fluorescence titrations yielded apparent K(d) values of 0.12 and 0.16 nm for high-affinity binding sites for FDDNP and FENE, respectively, and apparent K(d) values of 1.86 and 71.2 nm for the low-affinity binding sites. The traditional radioactive binding assays also produced apparent K(d) values in the low nanomolar range. The presence of two kinetically distinguishable binding sites for FDDNP and FENE suggests multiple binding sites for SPs and identifies the parameters that allow for the structural optimization of this family of probes for in vivo use. The high-affinity binding of the probes to multiple binding sites on fibrils are consistent with results obtained with digital autoradiography, immunohistochemistry, and confocal fluorescence microscopy using human brain specimens of AD patients.

Evaluation of 9-[(3- 18F-fluoro-1-hydroxy-2-propoxy)methyl]guanine ([ 18F]-FHPG) in vitro and in vivo as a probe for PET imaging of gene incorporation and expression in tumors

Preparation of 9-[(3- 18F-fluoro-1-hydroxy-2-propoxy)methyl]-guanine ([ 18F]-FHPG) for clinical use, and its evaluation as a positron emission tomography (PET) imaging agent for gene incorporation and expression in tumors are reported. In vitro studies in human colon cancer cells, HT-29, transduced with the retroviral vector G1Tk1SvNa and nontransduced (wild type) showed 4, 8, 12, and 15 times higher uptake of the probe in 1, 3, 5, and 7 h, respectively, in transduced cells compared with the controls. In vivo studies in tumor-bearing nude mice demonstrated that the tumor uptake of the radiotracer is three and six-fold higher in 2 and 5 h, respectively, in transduced cells compared with the control cells. These results suggest that [ 18F]-FHPG is a potential in vivo PET imaging agent for monitoring gene incorporation and expression in gene therapy of cancer.