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

Non-hybridization saturable mechanisms play a role in the uptake of (68)Ga-Labeled LNA-DNA mixmer antisense oligonucleotides in rats

Oligonucleotides (ODN) are key molecules for the aim of preventing translation of a gene product or monitoring gene expression in tissues. However, multiple methodological and biological hurdles need to be solved before in vivo application in humans will be possible. For positron emission tomography (PET) investigations, a 20-mer DNA-locked nucleic acid (LNA) mixmer ODN specific for rat chromogranin-A mRNA was labeled with (68)Ga and its uptake was examined in vivo in rats with and without blocking of scavenger receptors by polyribonucleotides. In addition, uptake studies of (68)Ga-LNA were performed with respect to time and concentration in human and rat cell lines. The human cell lines did not express the target mRNA. Both polyinosinic acid (poly-I) and polyadenylic acid (poly-A) reduced the uptake in rat tissues and in human cell lines. Poly-I was found to be more effective in the liver whereas poly-A was more effective in the kidney. In addition, the blockade by poly-I was statistically significant in the pancreas, adrenal gland, bone marrow, intestine, testis, urinary bladder, muscle, parotid gland, and heart, whereas poly-A also caused significant reduction in pancreas, adrenal gland, and bone marrow but not as much as in kidney. Cell culture study showed a 2-phase dose-dependent uptake characteristic with a saturable and a passive diffusion-like phase; however, these 2 phases were not so well expressed in the rat cell line. The results suggest that scavenger receptors or other saturable processes unrelated to hybridization may be involved in the tissue uptake of (68)Ga-LNA and in the clearance of antisense ODN through the liver, kidney, spleen, and bone marrow. The fact that these processes may be sequence-dependent suggests that proof of in vivo hybridization through imaging may not be obtained by only comparing sense and antisense sequences and proving dose-dependency.

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.