To construct the strategy for delivery systems that can controlin vivodisposition of antisense oligonucleotides, we studied the stability and basic pharmacokinetic characteristics of oligonucleotides. Decathymidylic acid (T10), a model oligodeoxynucleotide, and its derivatives, 5′-biotin-T10(5′B-T10) and 3′-methoxyethylamine 5′-biotin-T10(3′M5′B-T10), containing phosphoroamidate modification at 3′- and/or 5′-terminal internucleoside linkages, were synthesized. In phosphate-buffered saline (PBS,pH 7.4) containing 10% mouse serum, unmodified T10was degraded with a half-life of 45 minutes; the degradation half-lives of 5′B-T10and 3′M5′B-T10were 11 and 30 h, respectively. In mouse whole blood, 3′M5′B-T10was relatively stable, and 45% remained intact after 1 h incubation. After intravenous injection of [3H]3′M5′B-T10into mice at a dose of 1 mg/kg, the radioactivity was rapidly cleared from plasma with a half-life of 2 minutes and accumulated in the kidney, liver, and gallbladder. About 30% of the dose was excreted in the urine within 60 minutes. A much more rapid degradation of [3H]3′M5′B-T10was observedin vivothan expected fromin vitroexperiments: more than 90% of the radioactivity in plasma was degradation product at 2 minutes after injection. These results suggested that enzymatic degradation occurred in some compartments in addition to the blood pool. The apparent urinary excretion clearance of [3H]3′M5′B-T10was close to that of inulin, whereas the apparent hepatic uptake clearance was much greater than that of inulin and comparable to that of dextran sulfate, which is taken up by the liver by scavenger receptors for polyanions. Thus, the present study demonstrated that the disposition processes to be controlled involve stability, urinary excretion, and hepatic uptake for the developmen