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 diethylenetriaminepentacetic 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 3' or 5' end with a biotin moiety on the opposite end. The DNA was conjugated with SHNH by a N-hydroxysuccinimide derivative 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 to 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 degrees C human serum. Less than 4% of 99mTc and 14% of 111In was lost in 24 hr. CONCLUSIONS Amino-modified, single-stranded DNA can be stably radiolabeled with 99mTc by the SHNH moiety without loss of function.