The goals of this study were to systematically compare the pharmacokinetics and tissue distribution of phosphorothioate (PS), methylphosphonate (MP), and phosphorodithioate (PS2) oligonucleotide analogs; 15-mers of sequence d-TAC GCC AAC AGC TCC (5'-3') complementary to the AUG region of K-ras were radiolabeled with carbon-14. Oligomers were administered as a single dose in the tail vein of nude mice harboring a K-ras-dependent human pancreatic tumor (CFPAC1). The kinetics of PS, PS2, and MP oligomer availability in the bloodstream was followed. Concentration versus time profiles for all oligomers were biphasic, indicative of a two-compartment model. A rapid distribution phase with t1/2 alpha values of 1 minute or less and an elimination phase with average t1/2 beta values of 24-35 minutes were observed. Volumes of distribution (Vd) were 3.2, 4.8, and 6.3 ml for PS2, MP, and PS, respectively, in comparison to 3.6 ml for sucrose, a fluid-phase marker. Relative tissue drug levels obtained at 1 and 24 hours after administration were kidney > liver > spleen > tumor > muscle. Total kidney and liver oligonucleotide accumulation was approximately 7%-15% of the initial dose, with tumor accumulating 2%-3%. Intact compound was recovered from all tissues, including tumor, as assessed by high-pressure reversed-phase HPLC coupled to radiometric detection. Integrity of the oligonucleotides ranged from 73% in blood to 43%-46% in kidney and liver. Kidney and liver appear to be the primary sites of metabolism. These results demonstrate widespread tissue availability of these compounds and suggest their development as potential antitumor agents.