Pharmacokinetics-pharmacodynamics and biodistribution of a novel nucleotide-based thrombin inhibitor

Synthetic single-stranded oligonucleotides are novel pharmaceutical entities designed to interfere with the major functions of proteins or nucleic acids. Thrombin aptamer (GS522, a phosphodiester oligonucleotide, GGTTGGTGTGGTTGG) was chosen to investigate the pharmacokinetic-pharmacodynamic profile and biodistribution of these types of compounds following intravenous administration of 10 mg/kg, 20 mg/kg and 30 mg/kg in rats. A specific, sensitive and reproducible capillary gel electrophoresis method with laser induced fluorescence detection was developed to quantitatively determine pharmacodynamic response. The plasma concentration of GS522 after intravenous administration declines in a multiexponential manner, exhibiting extremely fast distribution and elimination {dollar}(tsb{lcub}1/2{rcub}=7.63{dollar}-9.02 min, {dollar}Vsb{lcub}SS{rcub}=83.96-115.09{dollar} ml/kg, {dollar}CL=22.02-27.83{dollar} ml/min). GS522 follows linear pharmacokinetics, with area under the plasma concentration curve being proportional to the administered dose (Rsq = 0.9744). GS522 prolongs coagulation via binding to prothrombin and thrombin. Thus, binding to prothrombin was included in the pharmacokinetic model to describe the plasma-concentration-time profile of GS522. The dissociation constant for the GS522-prothrombin binding was estimated to be 5{dollar}mu{dollar}M. The pharmacodynamic response was found to be short-lived (1-2 min) and directly proportional to GS522 plasma concentration. The magnitude of the pharmacodynamic response was shown to depend on the GS522 dose and receptor saturation (GS522-prothrombin binding).; An in vivo distribution study was performed to identify the fate of tritiated GS522, following 10 mg/kg intravenous administration to rats. The oligonucleotide associated radioactivity was widely distributed in tissues, with the highest radioactivity levels detected in the kidneys and liver (39.69, 15.66 %dose/respective organ). Kidney, liver and muscle were the main sites of accumulation. The radiolabel was uniformly distributed among the brain regions (left and right lobes and cerebellum). However, it did not cross the blood brain barrier to any appreciable extent (0.26% of the dose). A statistically significant increase in radioactivity levels was observed in the brain, eyes, skin, liver, pancreas and vein six (6) hours following the dose administration. These results alert to the potential toxicity due to the reuptake of CS522 and/or its metabolites in these tissues.