Distribution of (76)Br-labeled antisense oligonucleotides of different length determined ex vivo in rats

Recent advances in therapeutic nucleic acids and their analytical methods

Therapeutic nucleic acids are various chemically modified RNA or DNA with different functions, which mainly play roles at the gene level. Owing to its accurately targeting at pathogenic genes, nucleic acid based therapeutics have a wide range of application prospects. Recently, the improvement on chemical synthesis and delivery materials accelerated the development of therapeutic nucleic acids rapidly. Up to now, 17 nucleic acid based therapeutics approved by Food and Drug Administration (FDA) or European Medicines Agency (EMA). The development of therapeutics raised higher requirements for analytical methods, both in quality control and in clinical research. The first part of this review introduces different classes of therapeutic nucleic acids, including antisense oligonucleotide (ASO), RNA interference (RNAi) therapy, mRNA, aptamer and other classes which are under research. The second part reviews the therapeutic nucleic acids commercialized from 2019 to now. The third part discusses the analytical methods for nucleic acid based therapeutics, including liquid chromatography-based methods, capillary gel electrophoresis (CGE), hybridization enzyme-linked immunosorbent assay (ELISA) and other infrequently used methods. Finally, the advantages and shortcomings of these methods are summarized, and the future development of analysis methods are prospected.

Noninvasive and Quantitative Monitoring of the Distributions and Kinetics of MicroRNA-Targeting Molecules in Vivo by Positron Emission Tomography

MicroRNAs (miRNAs) are endogenous, small, noncoding ribonucleic acids (RNAs) that bind to the 3' untranslated regions of messenger RNAs (mRNAs) and induce translational repression or mRNA degradation. Although numerous studies have reported that miRNAs are of potential use for disease diagnostics and gene therapy, little is known about their fates in vivo. This study elucidated the whole-body distributions and kinetics of intravenously administered miRNA-targeting molecules in vivo by positron emission tomography (PET) imaging. A 22-mer sequence targeting miR-15b was conjugated with three different chelators and labeled with gallium-68 (⁶⁸Ga). These tracers were compared with a scrambled 22-mer sequence; 22-mer with two single base substitutions; anti-miR-34 22-mer; hexathymidylate (T₆), a 6-mer sequence; and an unconjugated chelator. miR-15b was chosen as a target because it is important for bone remodeling. All three ⁶⁸Ga-labeled anti-miR-15b molecules had similar biodistributions and kinetics, and they all accumulated in the bones, kidneys, and liver. The bone accumulation of these tracers was the highest in the epiphyses of long tubular bones, maxilla, and mandible. By contrast, the scrambled 22-mer sequence, the 6-mer, and the unconjugated chelator did not accumulate in bones. PET imaging successfully elucidated the distributions and kinetics of ⁶⁸Ga-labeled chelated miRNA-targeting molecules in vivo. This approach is potentially useful to evaluate new miRNA-based drugs.

Aptamers as radiopharmaceuticals for nuclear imaging and therapy

Today, radiopharmaceuticals belong to the standard instrumentation of nuclear medicine, both in the context of diagnosis and therapy. The majority of radiopharmaceuticals consist of targeting biomolecules which are designed to interact with a disease-related molecular target. A plethora of targeting biomolecules of radiopharmaceuticals exists, including antibodies, antibody fragments, proteins, peptides and nucleic acids. Nucleic acids have some significant advantages relative to proteinaceous biomolecules in terms of size, production, modifications, possible targets and immunogenicity. In particular, aptamers (non-coding, synthetic, single-stranded DNA or RNA oligonucleotides) are of interest because they can bind a molecular target with high affinity and specificity. At present, few aptamers have been investigated preclinically for imaging and therapeutic applications. In this review, we describe the use of aptamers as targeting biomolecules of radiopharmaceuticals. We also discuss the chemical modifications which are needed to turn aptamers into valuable (radio-)pharmaceuticals, as well as the different radiolabeling strategies that can be used to radiolabel oligonucleotides and, in particular, aptamers.

In Vitro Imaging Techniques in Neurodegenerative Diseases

Neurodegeneration induces various changes in the brain, changes that may be investigated using neuroimaging techniques. The in vivo techniques are useful for the visualization of major changes, and the progressing abnormalities may also be followed longitudinally. However, to study and quantify minor abnormalities, neuroimaging of postmortem brain tissue is used. These in vitro methods are complementary to the in vivo techniques and contribute to the knowledge of pathophysiology and etiology of the neurodegenerative diseases. In vitro radioligand autoradiography has given great insight in the involvement of different neuronal receptor systems in these diseases. Data on the dopamine and cholinergic systems in neurodegeneration are discussed in this review. Also, the amyloid plaques are studied using in vitro radioligand autoradiography. Using one of the newer methods, imaging matrix-assisted laser desorption ionization mass spectrometry, the distribution of a large number of peptides and proteins may be detected in vitro on brain cryosections. In this overview, we describe in vitro imaging techniques in the neurodegenerative diseases as a complement to in vivo positron emission tomography and single photon emission computed tomography imaging.

Production of non-standard PET radionuclides and the application of radiopharmaceuticals labeled with these nuclides

The field of positron emission tomography (PET) has expanded dramatically over recent years. In spite of this expansion the large majority of clinical studies are carried out utilizing one radiopharmaceutical-2-fluoro-2-deoxyglucose. Many research groups are developing novel radiopharmaceuticals. A major emphasis is on other agents labeled with 18F. Several other positron emitting radionuclides can be prepared in high yields in small biomedical cyclotrons. Some of these have half-lives that make delivery significantly easier than the delivery of 18F compounds. These radionuclides include: 64Cu (half life 12.7 h), 76Br (half life 16.2 h), 86Y (half life 14.74 h) and 124I (half life 4.2 days). The method of production of these and other 'non-standard' PET radionuclides will be discussed and the method of labeling radiopharmaceuticals with these radionuclides described. Several of these radiopharmaceuticals have been studied in animal models as well and a limited number translated to the human situation.

Biodistribution of 68Ga-labelled phosphodiester, phosphorothioate, and 2'-O-methyl phosphodiester oligonucleotides in normal rats

Antisense oligonucleotides may hybridise with high selectivity to mRNA sequences allowing monitoring of gene expression or inhibition of the manifestation of altered genes inducing diseases. As part of the development of positron emission tomography methods, 17-mer antisense phosphodiester (PO), phosphorothioate (PS) and 2'-O-methyl phosphodiester (OMe) oligonucleotides specific for point mutationally activated human K-ras oncogene were labelled with 68Ga radionuclide via a chelator coupled to the probe. Hybridisation in solution and non-denaturing polyacrylamide gel electrophoresis (PAGE) with a subsequent exposure of the gels was performed to verify the hybridisation ability after labelling. The biodistribution was studied in male Sprague-Dawley rats by injecting 2MBq of 68Ga-oligonucleotides via the tail vein and measuring the organ radioactivity concentration after 20, 60 and 120 min or using whole-body autoradiography with 10 MBq 68Ga-oligonucleotide and 20 min incubation time. Control experiments were performed with 68GaCl3 and 68Ga-chelator complex. The results revealed that 68Ga-labelling did not change the hybridisation abilities of the oligonucleotides. The biodistribution pattern depended on the nature of the oligonucleotide backbone. Bone marrow, kidney, liver, spleen and urinary bladder were the five organs of highest uptake with each oligonucleotide. The PO accumulated highly in the liver, whereas high kidney uptake dominated the PS and OMe patterns. Intact PS and OMe were detected in plasma samples taken 20 and 60 min after injection. This study supplies a base for the further development of 68Ga-labelled oligonucleotides as pharmacokinetic tools and a potential future use for in vivo imaging of gene expression.

Small animal positron emission tomography in food sciences

Positron emission tomography (PET) is a 3-dimensional imaging technique that has undergone tremendous developments during the last decade. Non-invasive tracing of molecular pathways in vivo is the key capability of PET. It has become an important tool in the diagnosis of human diseases as well as in biomedical and pharmaceutical research. In contrast to other imaging modalities, radiotracer concentrations can be determined quantitatively. By application of appropriate tracer kinetic models, the rate constants of numerous different biological processes can be determined. Rapid progress in PET radiochemistry has significantly increased the number of biologically important molecules labelled with PET nuclides to target a broader range of physiologic, metabolic, and molecular pathways. Progress in PET physics and technology strongly contributed to better scanners and image processing. In this context, dedicated high resolution scanners for dynamic PET studies in small laboratory animals are now available. These developments represent the driving force for the expansion of PET methodology into new areas of life sciences including food sciences. Small animal PET has a high potential to depict physiologic processes like absorption, distribution, metabolism, elimination and interactions of biologically significant substances, including nutrients, 'nutriceuticals', functional food ingredients, and foodborne toxicants. Based on present data, potential applications of small animal PET in food sciences are discussed.

Hybridisation of [76Br]-labelled antisense oligonucleotides to Chromogranin A mRNA verified by RT-PCR

Methods have been developed to label oligonucleotides (ODNs) in the 5'-position with (76)Br via a prosthetic group on a hexylamino-linker. The purpose of the study was to explore whether the labelling procedure would prevent specific hybridisation by using reverse transcription-polymerase chain reaction (RT-PCR) followed by sequencing of the PCR product. Antisense ODNs (30 mer, specific for rat Chromogranin A [CgA] mRNA) with phosphodiester (O-ODN) or phosphothioate (S-ODN) backbone, either unlabelled or labelled with (76)Br, served as one of the primers in individual PCR reactions. Using O-ODN as a primer, irrespective of being labelled or not, a selected 225-bp PCR fragment was successfully amplified. However, no amplification was obtained using S-ODN as a primer. The proper PCR products were only detected in the sample prepared from the adrenal gland, but not in that from the heart, liver or kidney. Autoradiographic recording of the gel, after gel electrophoresis, revealed radioactive signals corresponding to the amplified PCR products. The sequence of the PCR product matched the rat CgA mRNA sequence obtained from the EMBL database. RT-PCR is an attractive method to identify the selective binding of modified ODNs to target mRNA. This method confirmed that the labelling with (76)Br did not change the hybridisation ability of antisense O-ODN.

Antisense imaging: and miles to go before we sleep?

Labeled oligonucleotide analogues for antisense imaging of messenger RNA (mRNA) have great potential for detection of endogenous gene expression in vivo. Successful antisense imaging may be useful for detecting cellular gene expression patterns and early molecular changes in disease. Conclusive demonstration of this technique has been hindered by formidable challenges in surmounting biological barriers and detecting low concentrations of target mRNA. Recent advances in the development of novel antisense molecules, high specific activity radiolabeling chemistry, sophisticated drug targeting technology, and complementary molecular imaging modalities make it quite possible that true antisense imaging will be realized in the near future.

Quantitative imaging of bromine‐76 and yttrium‐86 with PET: A method for the removal of spurious activity introduced by cascade gamma rays

Positron Emission Tomography of bromine-76 and yttrium-86 results in the detection of coincident events that are not strictly associated with annihilation photon pairs. Instead, these coincidences occur because prompt gamma rays emitted by these nuclides result in cascades of photons that are emitted within the timing window of the PET scanner. Pairs of detected photons from these cascades are not angularly correlated and therefore contain little information regarding the location of their source. Furthermore, these coincidences are not removed by correction procedures (e.g., randoms, scatter) routinely applied to PET data. If left uncorrected, the cascade coincidences will result in spurious apparent activity within the PET images. A correction, applied within projection space, that removes the cascade coincidence signal from septa-in (i.e., two-dimensional) datasets is proposed and tested on phantom data.

Cytosine residues influence kidney accumulations of 99mTc-labeled morpholino oligomers

Watson-Crick pairing between complementary oligomers is proving to be an effective means for rapidly directing radioisotopes specifically to the exterior surface of cancer cells in vivo. In such pretargeting applications, it is highly desirable that the excess of isotopically labeled oligomers, which do not bind to the cancer cells, be rapidly cleared from the body. In this context, understanding the influence of chain length and base sequence of the radiolabeled oligomers is critical. We had earlier determined that the kidneys are the principal targets of short-chain radiolabeled morpholino oligomers (MORFs). To explain these observations, MORFs consisting of uniform cytosines (Cs), uniform thymines (Ts), uniform adenines (As), and uniform AAG repeat were labeled with technetium-99m (99mTc) and studied in normal mice. In a limited investigation of the influence of oligomer backbone, a 20-mer MORF (MORF20) with a base sequence rich in Cs was compared with a phosphoromonothioate DNA (S-DNA20) of the same sequence. The in vivo behavior of the labeled MORFs was nearly identical in all organs, with the exception of kidneys. The kidney accumulations were about 25- to 80-fold higher for the uniform Cs relative to the other three uniform MORFs at 3 hours. The S-DNA20 rich in Cs showed only modest kidney accumulations compared with the equivalent MORF20, presumably because of preferential clearance of the S-DNA20 through the liver. Urine analysis showed no evidence of intact labeled S-DNA20 in contrast to fully intact labeled MORF20. We conclude that the high kidney levels observed by us previously for MORFs are most likely due largely to the C residues in the base sequence. In the case of S-DNAs, this phenomenon is partly disguised by the increased hepatic excretion and degradation. These results show that the base sequences of MORFs, and probably other oligomers as well, are an important determinant of biodistribution.

Determination of specific radioactivity for (76)Br-labeled compounds measuring the ratio between (76)Br and (79)Br using packed capillary liquid chromatography mass spectrometry

Packed capillary liquid chromatography with electrospray mass spectrometry was used for direct determination of the specific radioactivity by calculation of isotope ratios between the (76)Br- and (79)Br-labeled analogues of N-((3-aminomethyl)benzyl)-4-bromobenzamide. Using 20 microL injections on packed capillary columns, sufficient mass sensitivity was attained for the determination on an injected amount of radioactivity corresponding to approximately 2 MBq (0.3 pmol of the (76)Br isotopic analogue).