|
Introduction
Specific diagnosis of malignant tumors in an early stage is still a problem to be solved. Though X ray CT, MRI, Ultrasonagraphy and conventional nuclide imaging are broadly introduced in clinic, none of them can give a specific diagnose in early stages, especially to mammary and lung cancers. Nowadays, this difficulty might be overcome by means of antisense imaging. With the development of modern molecular biology, malignant tumors are recognized as the results of oncogene activation and tumor suppressor gene depression. In cancer, oncogenes were amplified so that multiple copies of mRNAs and hybridization sites would be available for binding and retention of radiolabled antisense probes for noninvasive imaging with the gamma camera (1).
Antisense oligonucleotides are synthetic single-strand deoxyribonuclic acids(DNAs) or oligoribonuclic acids(RNAs) designed to have a base sequence complementary to that of the targeted gene or mRNAs. These short base sequences are supposed to bind in a base-specific manner with the targeted gene or mRNAs by some antisense mechanisms, thus to interfere with the process of gene transcription or translation. As being restricted by various roadblocks such as nuclear membrane crossing, genotoxicity suspicion and unclear binding rules of Hoogsteen triplet formation, antisense strategies usually choose to interfere with the process of translation. Several theories were promoted to interpret the mechanism of translation arrest. It was originally considered that when hybridized with the sense region of the targeted mRNA, the antisense DNA might hinder the protein synthesis by physically blocking the mRNA migration through the ribosome. It is now known that the ribosomal complex can unwind the DNA/RNA duplex and might permit unhindered translation. So in many and possibly most cases, the mechanism at work is more likely to be mRNA degradation by RNase H enzymes that recognize the antisense DNA/RNA duplex. Whatever mechanism acts is most likely concerned by antisense therapy industries. For antisense imaging studies in which scintigraphy is performed after administration of antisense DNAs radiolabeled with imaging radionuclide, an increased, detectable amount of mRNA transcript together with specific retain of antisense probes in the cytoplasm are of the factors most concerned.
In application of antisense imaging, antisense DNAs must survive in plasma and in the cytosol long enough to locate and bind to their target. The main source of in vivo instability of native phosphodiester DNAs, especially single-strand DNAs, is enzyme degradation. Exo- and endonucleases (which attack DNA from its end and interior regions, respectively) are ubiquitous and are responsible for the rapid in vivo degradation of single-strand phosphodiester DNAs. For this reason, native phosphodiester DNAs are almost considered unsuitable and useless in antisense imaging. Few investigations were reported in antisense imaging using antisense phosphodiester DNAs. But in 1994, with Indium-111-DTPA-antisense phosphodiester probes, Dewanjee and his colleagues did make successful imaging in a mammary tumor-bearing mouse model. It seems that more investigations are necessary before a final validation of the phosphodiester DNAs is made. In this study, a c-myc mRNA antisense oligonucleotide (phosphodiester) was radiolabeled with 99mTc via the bifunctional chelator S-Acetyl-NHS-MAG3, with permission by Dr Donald J. Hnatowich, who along with Dr P. Winnard et al provided this method (2). The labeled antisense probe was injected into a mammary tumor-bearing mouse model to which the gamma imaging was then performed.
|
|
