Technetium-99m labeling of DNA oligonucleotides

Single-stranded RNA and DNA oligonucleotides may be useful as radiopharmaceuticals for antisense and other in vivo applications if convenient methods for stably attaching radionuclides such as ^99mTc can be developed. Methods: To radiolabel DNA with ^99mTc, we have used the hydrazino nicotinamide (SHNH) moiety developed elsewhere. The diethylenetriaminepentaacetic acid (DTPA) chelate was used to label DNA with ¹¹¹In for comparison. Complementary 22-base, single-stranded oligonucleotides were obtained, each with a primary amine attached to either the 3' or 5' end and with a biotin moiety on the opposite end. The DNA was conjugated with SHNH by a N-hydroxysuccinimide derivative and with DTPA by the cyclic anhydride. Results: Reversed-phase HPLC analysis showed that essentially complete conjugation was achieved in both cases. The purified SHNH-DNA was radiolabeled with ^99mTc by transchelation from glucoheptonate at labeling efficiencies of up to 60% and DTPA-DNA with ¹¹¹In acetate at up to 100% efficiency. After labeling, the ability of the DNAs to bind to streptavidin through the biotin moieties and to hybridize with their complementary DNA in saline was retained for both radiolabels as determined by size-exclusion HPLC analysis. HPLC radiochromatograms of serum incubates showed a shift of ^99mTc, but not ¹¹¹In, to a high molecular weight, strongly suggesting serum protein binding in the former case only. Low- molecular weight degradation products were seen with ¹¹¹In, but not with ^99mTc and may be related to the use of phosphodiester-linked oligonucleotides. As a further measure of label stability, the DNAs were bound to streptavidin-conjugated magnetic beads and incubated in fresh 37°C human serum. Less than 4% of ^99mTc and 14% of ¹¹¹In was lost in 24 hr. Conclusion: Amino-modified, single-stranded DNA can be stably radiolabeled with ^99mTc by the SHNH moiety without loss of function.