The potential influence of CO2, as an agent for euthanasia, on the pharmacokinetics of basic compounds in rodents

Rodent tissue distribution and pharmacokinetic studies were performed on basic compounds Org A and Org B in support of central nervous system drug discovery programs. A consistent observation from these studies was that drug concentrations in plasma obtained by cardiac puncture after CO2 euthanasia were markedly higher compared with those from other sampling methods ( serial sampling, isoflurane anesthesia, or cervical dislocation). Further investigations demonstrated that CO2 euthanasia led to a reduction in blood pH in both rats and mice, which was not observed with the other sampling methods. The use of CO2 euthanasia resulted in a decrease in the brain/plasma ratio of Org B, largely as a result of increased plasma concentrations. The pharmacokinetics of a basic drug, raloxifene, in rat were also influenced by sampling technique. CO2 euthanasia before sampling, resulted in a 2- to 3-fold increase in the area under the drug concentration-time curve, a decrease in plasma clearance, and a decrease in the steady-state volume of distribution compared with isoflurane anesthesia. It is proposed that a decrease in the pH of blood relative to that of other tissues, as a consequence of CO2 exposure, results in a redistribution of basic compounds out of the tissues, leading to higher concentrations in plasma.