Technetium-99m labeled antisense oligonucleotide-noninvasive tumor imaging in mice

RESUMO

The 15-mer oligonucleotide sequence was synthesized, aminolinked (sense and antisense phosphodiester) and conjugated with S-Acetyl-NHS-MAG3 by a N-hydroxy-succinimide derivative. The purified MAG3-DNA was radiolabeled with 99mTc by transchelation from sodium tartrate and free 99mTc was separated by gel filtration. The radiolabeled antisense and sense probes were injected intravenously in mammary tumor-bearing KM mice(1×106 cells,6 days post inoculation).Biodistribution was studied and the mice were imaged.Essentially complete conjugation was achieved by reverse-phase Sep-Pak C18 chromatography analysis.The MAG3-DNA was labeled with 99mTc at room temperature and neutral pH, with a mean labeling efficiency of 80.11 percent (s.d=2.96 percent , N=4). After labeling, the stability of the DNA in saline or serum was retained as determined by reverse-phase Sep-Pak C18 chromatography analysis, except a shift at 30 min in serum incubation which might suggest a short time serum protein binding. The labeled antisense DNA still remained the ability to hybridize with its complementary DNA. The pharmacodynamics of 99mTc labeled c-myc probes (antisense and sense) in mammary tumor-bearing KM mice did not change with the time postinjection. The highest accumulation of label was in the liver first, with the kidney and small bowel next. The injected activity localized in the lesion as early as 30min and reached a saturation value at 4hr. The accumulation of radioactivity in the tumor was lower at all time points in animals receiving the blocking oligonucleotides or sense probes. All images obtained with 99mTc-MAG3-c-myc antisense probes showed specific accumulation of radioactivity at the site of tumor. Radiolabel rapidly accumulates at the site of tumor and remains associated with the site even though circulation levels of radioactivity have greatly diminished. The tumor was readily evident since 45min and reached the highest tumor-to-muscle ratio at 4hr. The quite encouraging result was obtained at 20hr to 22hr when the background activity was diminished sufficiently. Positive imaging was not obtained in case of control group (in which non-conjugated, non-labeled antisense oligonucleotides were administered 2hr before the radiolabeled antisense probes were injected) and of sense group. Conclusion The 99mTc labeled antisense probe may provide a sensible and specific tool for noninvasive imaging of c-myc oncogene mRNA for a variety of malignant tumors at an earlier stage