Imaging oncogene expression

Positron emission tomography molecular imaging-based cancer phenotyping

During the past several decades, numerous studies have provided insights into biological characteristics of cancer cells and identified various hallmarks of cancer acquired in the tumorigenic processes. However, it is still challenging to image these distinctive traits of cancer to facilitate the management of patients in clinical settings. The rapidly evolving field of positron emission tomography (PET) imaging has provided opportunities to investigate cancer's biological characteristics in vivo. This article reviews the current status of PET imaging on characterizing hallmarks of cancer and discusses the future directions of PET imaging strategies facilitating in vivo cancer phenotyping.

Application and Evaluation of [99mTc]-Labeled Peptide Nucleic Acid Targeting MicroRNA-155 in Breast Cancer Imaging

It has been reported that dysregulation of microRNA-155 expression and function is associated with tumorigenesis, growth, tumor subtypes, invasion, and poor survival rates. Peptide nucleic acid (PNA), an artificially synthesized nucleic acid mimic, has been applied for molecular diagnosis. In this study, a PNA sequence that undergoes complementary binding to miR-155 was labeled with ^99mTc to evaluate whether the tracer could visualize the expression of miR-155 in breast cancer. Both antisense PNA (anti-PNA, fully complementary bound to human mature miR-155, referred to as “anti-PNA-155”) and mismatched PNA (referred to as “mis-PNA”) single strands containing 23-mer were synthesized. The relative expression of miR-155 in MCF-7 cells and tumors was higher than that in MDA-MB-231 cells and tumors. Single-photon emission computed tomography (SPECT) scan showed that radioactivity mainly accumulated in kidney. MCF-7 tumors, but not MDA-MB-231 tumors, were clearly visualized after [^99mTc]anti-PNA-155 injection. MCF-7 tumors were less visible when coinjected with 100-fold excess of anti-PNA-155 or injected with [^99mTc]mis-PNA, which suggested specific binding. Biodistribution study results were consistent with SPECT imaging. We successfully demonstrated that [^99mTc]anti-PNA-155 could visualize miR-155 expression in vivo, suggesting it may be a promising probe applied in breast cancer.

Oligonucleotide inhibitors of telomerase: prospects for anticancer therapy and diagnostics

The activity of telomerase allows eukaryotic cells to have unlimited division potential. On its functioning, telomerase synthesizes short DNA repeats at the 3'-end of DNA within chromosomes that ensures genome stability during cell division. Telomerase is active in the majority of cancer cell types and is virtually absent in somatic cells with rare exceptions. This difference allows us to consider inhibition of telomerase activity as a possible approach to antitumor therapy. Telomerase is a nucleoprotein composed of two main components: the reverse transcriptase (hTERT), which is a catalytic subunit, and telomerase RNA (hTR), which encodes a template for synthesis of repeats. The biogenesis and features of telomerase seem very promising for its inhibition due to complementary interactions. In this review, we analyze putative pathways of oligonucleotide influence on telomerase and consider the known native and modified oligonucleotide inhibitors of telomerase, as well as possible mechanisms of their action. We also discuss the application of telomerase-targeted oligonucleotide conjugates for in vivo imaging of tumor cells.

Targeted imaging of bacterial infections: advances, hurdles and hopes

Bacterial infections represent an increasing problem in modern health care, in particular due to ageing populations and accumulating bacterial resistance to antibiotics. Diagnosis is rarely straightforward and consequently treatment is often delayed or indefinite. Therefore, novel tools that can be clinically implemented are urgently needed to accurately and swiftly diagnose infections. Especially, the direct imaging of infections is an attractive option. The challenge of specifically imaging bacterial infections in vivo can be met by targeting bacteria with an imaging agent. Here we review the current status of targeted imaging of bacterial infections, and we discuss advantages and disadvantages of the different approaches. Indeed, significant progress has been made in this field and the clinical implementation of targeted imaging of bacterial infections seems highly feasible. This was recently highlighted by the use of so-called smart activatable probes and a fluorescently labelled derivative of the antibiotic vancomycin. A major challenge remains the selection of the best imaging probes, and we therefore present a set of target selection criteria for clinical implementation of targeted bacterial imaging. Altogether, we conclude that the spectrum of potential applications for targeted bacterial imaging is enormous, ranging from fundamental research on infectious diseases to diagnostic and therapeutic applications.

Synthesis, characterization, and evaluation of radiometal-containing peptide nucleic acids

Peptide nucleic acids (PNAs) have very attractive properties for applications in nuclear medicine. Because PNAs have high selectivity for DNA/RNA recognition, resistance to nuclease/protease degradation, and high thermal and radiolytic stabilities, PNA bioconjugates could transform the areas of diagnostic and therapeutic nuclear medicine. In this book chapter, we report on the current developments towards the preparation of radiometal-containing PNA constructs and summarize the protocols for labeling these probes with (99m)Tc, (111)In, (64)Cu, (90)Y, and (177)Lu.

Effects of hypoxanthine substitution in peptide nucleic acids targeting KRAS2 oncogenic mRNA molecules: theory and experiment

Genetic disorders can arise from single base substitutions in a single gene. A single base substitution for wild type guanine in the twelfth codon of KRAS2 mRNA occurs frequently to initiate lung, pancreatic, and colon cancer. We have observed single base mismatch specificity in radioimaging of mutant KRAS2 mRNA in tumors in mice by in vivo hybridization with radiolabeled peptide nucleic acid (PNA) dodecamers. We hypothesized that multimutant specificity could be achieved with a PNA dodecamer incorporating hypoxanthine, which can form Watson-Crick base pairs with adenine, cytosine, thymine, and uracil. Using molecular dynamics simulations and free energy calculations, we show that hypoxanthine substitutions in PNAs are tolerated in KRAS2 RNA:PNA duplexes where wild type guanine is replaced by mutant uracil or adenine in RNA. To validate our predictions, we synthesized PNA dodecamers with hypoxanthine, and then measured the thermal stability of RNA:PNA duplexes. Circular dichroism thermal melting results showed that hypoxanthine-containing PNAs are more stable in duplexes where hypoxanthine-adenine and hypoxanthine-uracil base pairs are formed than single mismatch duplexes or duplexes containing hypoxanthine-guanine opposition.

Radiolabeled oligonucleotides for antisense imaging

Oligonucleotides radiolabeled with isotopes emitting γ-rays (for SPECT imaging) or positrons (for PET imaging) can be useful for targeting messenger RNA (mRNA) thereby serving as non-invasive imaging tools for detection of gene expression in vivo (antisense imaging). Radiolabeled oligonucleotides may also be used for monitoring their in vivo fate, thereby helping us better understand the barriers to its delivery for antisense targeting. These developments have led to a new area of molecular imaging and targeting, utilizing radiolabeled antisense oligonucleotides. However, the success of antisense imaging relies heavily on overcoming the barriers for its targeted delivery in vivo. Furthermore, the low ability of the radiolabeled antisense oligonucleotide to subsequently internalize into the cell and hybridize with its target mRNA poses additional challenges in realizing its potentials. This review covers the advances in the antisense imaging probe development for PET and SPECT, with an emphasis on radiolabeling strategies, stability, delivery and in vivo targeting.

Metal-containing peptide nucleic acid conjugates

Peptide Nucleic Acids (PNAs) are non-natural DNA/RNA analogues with favourable physico-chemical properties and promising applications. Discovered nearly 20 years ago, PNAs have recently re-gained quite a lot of attention. In this Perspective article, we discuss the latest advances on the preparation and utilisation of PNA monomers and oligomers containing metal complexes. These metal- conjugates have found applications in various research fields such as in the sequence-specific detection of nucleic acids, in the hydrolysis of nucleic acids and peptides, as radioactive probes or as modulators of PNA·DNA hybrid stability, and last but not least as probes for molecular and cell biology.

Preparation, 99mTc-labeling and biodistribution studies of a PNA oligomer containing a new ligand derivative of 2,2'-dipicolylamine

A new azido derivative of 2,2'-dipicolylamine (Dpa), 2-azido-N,N-bis((pyridin-2-yl)methyl)ethanamine, (Dpa-N(3)) was readily prepared from the known 2-(bis(pyridin-2-ylmethyl)amino)ethanol (Dpa-OH). It was demonstrated that Dpa-N(3) could be efficiently labeled with both [Re(CO)(3)(H(2)O)(3)]Br and [(99m)Tc(H(2)O)(3)(CO)(3)](+) to give [Re(CO)(3)(Dpa-N(3))]Br and [(99m)Tc(CO)(3)(Dpa-N(3))](+), respectively. Furthermore, Dpa-N(3) was successfully coupled, on the solid phase, to a Peptide Nucleic Acid (PNA) oligomer (H-4-pentynoic acid-spacer-spacer-tgca-tgca-tgca-Lys-NH(2); spacer= -NH-(CH(2))(2)-O-(CH(2))(2)-O-CH(2)-CO-) using the Cu(I)-catalyzed [2+3] azide/alkyne cycloaddition (Cu-AAC, often referred to as the prototypical "click" reaction) to give the Dpa-PNA oligomer. Subsequent labeling of Dpa-PNA with [(99m)Tc(H(2)O)(3)(CO)(3)](+) afforded [(99m)Tc(CO)(3)(Dpa-PNA)] in radiochemical yields >90%. Partitioning experiments in a 1-octanol/water system were carried out to get more insight on the lipophilicity of [(99m)Tc(CO)(3)(Dpa-N(3))](+) and [(99m)Tc(CO)(3)(Dpa-PNA)]. Both compounds were found rather hydrophilic (log D(o/w) values at pH=7.4 are -0.50: [(99m)Tc(CO)(3)(Dpa-N(3))](+) and -0.85: [(99m)Tc(CO)(3)(Dpa-PNA)]. Biodistribution studies of [(99m)Tc(CO)(3)(Dpa-PNA)] in Wistar rats showed a very fast blood clearance (0.26 ± 0.1 SUV, 1h p.i.) and modest accumulation in the kidneys (5.45 ± 0.45 SUV, 1h p.i.). There was no significant activity in the thyroid and the stomach, demonstrating a high in vivo stability of the (99m)Tc-labeled Dpa-PNA conjugate.

Native mRNA antisense-accessible sites library for the selection of antisense oligonucleotides, PNAs, and siRNAs

A procedure for rapidly generating a library of antisense-accessible sites on native mRNAs (mRNA antisense-accessible sites library [MASL]) is described that involves reverse transcription of whole cell mRNA extracts with a random oligodeoxynucleotide primer followed by mRNA-specific polymerase chain reaction (PCR). Antisense phosphorothioate oligodeoxynucleotides (ODNs), peptide nucleic acids (PNAs), and small interfering RNAs (siRNAs) can then be identified by screening against the antisense-accessible sites. The utility of this methodology is demonstrated for the identification of more effective inhibitors of inducible nitric oxide synthase (iNOS) induction than have previously been reported. This method may also be useful for constraining folding calculations of native mRNAs and for designing mRNA imaging probes.

Genomic biomarkers for molecular imaging: predicting the future

Over the past few decades, great strides have been made in anatomical imaging of disease that has led to their diagnosis with minimal invasion. Despite these advances, diseases such as cancer continue to take one human life every minute in the United States. Complimentary approaches that pertain directly to the genesis of the disease might contribute to its early diagnosis and subsequent management. In cancer, an array of molecular abnormalities leading to the modulations in expression of key proteins important in the cellular signaling pathways and cell proliferation has been identified. These specific disease fingerprints, biomarkers, are overexpressed on malignant cell surfaces or within the cytoplasm, and they provide unique targets that are promising for improving cancer diagnosis and therapy. We and others have designed, synthesized, and evaluated some novel probes specific for those oncogenes and oncogene product biomarkers for PET and SPECT molecular imaging of certain types of cancers. This article briefly describes this approach and gives specific examples that depict the ability of molecular imaging to detect occult lesions not detectable by current scintigraphic approaches. The article also outlines a few examples predicting other possible applications of targeting such specific probes not yet used.