Enhanced P-glycoprotein expression under high-altitude hypoxia contributes to increased phenytoin levels and reduced clearance in rats.

To study the effect of plateau hypoxia on the concentration of P-glycoprotein (P-gp) substrate phenytoin, Wistar rats are randomly divided into the control group and the hypoxic group, including P-gp inhibited groups respectively. Blood, cerebrospinal fluid, brain tissue, and blood-brain barrier were collected in plain areas at an altitude of 1500 m and plateau areas at an altitude of 4010 m. Evans Blue exclusion was used to assess the integrity of the blood-brain barrier. Western blot and qPCR were used to detect changes in P-gp expression. LC-MS/MS was used to determine the concentration of phenytoin in plasma and cerebrospinal fluid. In the high-altitude plateau group, phenytoin AUC0-t, MRT0-t and t1/2 increased significantly by 60%, 48%, and 61%, respectively, and clearance decreased by 67% (p <0.05 for all parameter). The protein expression of P-gp in the blood-brain barrier of the plateau group was up-regulated 1.84 times and the gene expression was up-regulated 2.21 times. The concentrations of phenytoin in the CSF of rats in the plain and high-altitude groups were 864.7 ± 348.3 and 1000±273.9 ng•mL-1, respectively. However, after inhibiting P-gp, the concentration of phenytoin in the CSF decreased significantly. It indicates that the increased expression of P-gp on the blood-brain barrier may lead to an increase in the amount of phenytoin excreted from the blood into the CSF, which may cause neurotoxic side effects. These results demonstrate significant changes in the pharmacokinetics of phenytoin under hypoxic conditions, supporting the need for careful dose titration for drugs with a narrow therapeutic range under high-altitude conditions.